Normal B and T lymphocyte development

INTRODUCTION — Lymphocyte development involves a complex series of tightly choreographed events. The process is orchestrated by genes that act at specific stages of B or T cell differentiation that encode lineage-specific transcription factors, growth factors, and chemokines; deoxyribonucleic acid (DNA) recombinases (RAG1 and RAG2) and terminal deoxytransferase (TdT), which directs the rearrangement and diversification of B and T cell antigen receptor genes, respectively; and the DNA modifying enzyme activation-induced cytosine deaminase (AID), which is needed for immunoglobulin class-switching and somatic hypermutation, a phenomenon that is required for the production of high-affinity antibodies.

Of clinical importance, many lymphoid malignancies appear to be the neoplastic counterparts of cells "arrested" at particular stages of lymphoid differentiation, as judged by cytologic appearance, patterns of growth, immunophenotype, and genetic features. This insight serves as the organizing theme for the classification of lymphoid malignancies, which sort lymphoid tumors according to their apparent cell of origin [1,2].

This topic focuses on early events of B and T cell development and describes markers that define early and later stages of B and T cells.

NORMAL LYMPHOID TISSUES — Lymphoid tissues are subdivided into primary and secondary lymphoid organs.

●The primary lymphoid tissues responsible for the initial generation of B and T lymphocytes are the bone marrow and thymus, respectively.

●Secondary lymphoid tissues include lymph nodes, spleen, tonsils, and the aggregations of lymphoid tissue located in the gastrointestinal and respiratory tracts. Within these lymphoid organs, B and T lymphocytes tend to home to different domains, leading to the segregation of B and T cells. Specifically, B cells mainly localize to follicles, whereas T cells mainly localize to interfollicular areas (figure 1). Non-lymphoid cells (eg, dendritic cells, monocytes/macrophages, endothelial cells, pericytes, fibroblasts, and follicular dendritic cells) contribute to the formation of these distinct microenvironments, within which specific cell-cell interactions occur that are required for the generation of cellular and humoral immune responses [3]. (See "The adaptive cellular immune response: T cells and cytokines" and "The adaptive humoral immune response".)

ORIGIN OF LYMPHOID CELLS — All white blood cells are derived from hematopoietic stem cells (HSCs). The embryonic origins of human HSCs are complex and are largely inferred from studies conducted in mice [4]. Blood forming cells first appear in the yolk sac around day 16 of gestation, but yolk sac hematopoiesis is restricted to the production of primitive red cells, platelets, and monocyte/macrophage-like progenitors that home to tissues and give rise to specialized long-lived phagocytes such as Kupffer cells in the liver and microglia in the brain [5,6]. Definitive HSCs arise in the dorsal aorta around the end of the third week of gestation. These cells bud off from "hemogenic endothelium" but are clonally distinct from aortic endothelial cells [7-9]. Work in mice has shown that lymphoid potential depends on the transcription factors Pu.1, Ikaros (Ikzf1), Bcl11a, and Tcf3 [10]. These HSCs appear to migrate to the liver, which becomes the main site of hematopoiesis after six weeks of gestation, and the placenta, which is the source of the HSCs that are found in cord blood. Progenitor cells from these tissues populate the thymus by seven to eight weeks of gestation. Bone marrow hematopoiesis commences by approximately five months of gestation and under normal circumstances is the primary source of hematopoietic cells after birth. (See "Overview of hematopoietic stem cells".)

The earliest committed hematolymphoid progenitors are derived through asymmetric cell divisions, in which undifferentiated HSCs give rise to daughter cells with greater proliferative potential but lacking the property of self-renewal. (See "Overview of stem cells", section on 'What defines a stem cell?'.)

The events that lead to the appearance of cells that are committed to lymphoid differentiation are not completely understood. It is generally agreed that HSCs first give rise to multipotent progenitors (MPPs), a subset of which retain the capacity to produce either myeloid or lymphoid cells [11,12]. In the bone marrow, the progeny of some of these "lymphoid-competent" MPPs become committed to the B cell lineage, an event that requires the transcription factors Ebf1 and Pax5 and the suppression of Notch signaling (which otherwise overrides B cell differentiation programs and directs MPPs towards the T cell lineage). Other early progenitors in marrow (sometimes known as early thymic progenitors, or ETPs) migrate to the thymus, an organ rich in stromal cells expressing ligands that activate Notch signaling, which induces these cells to undergo T cell differentiation.

B CELL DEVELOPMENT — New B cells are generated throughout life in humans [13], first in the liver during gestation and thereafter in the bone marrow. Early B cell development constitutes the steps that lead to B cell commitment and the expression of surface immunoglobulin, which is essential for B cell survival and function. These events culminate in the production of mature B cells, which leave the marrow and migrate to secondary lymphoid tissues, such as the lymph nodes, the spleen, and the Peyer's patches of the gut. Subsequent steps occur after B cells interact with exogenous antigen and/or T helper cells; these constitute the "antigen-dependent phase" that results in the generation of humoral immune responses. (See "The adaptive humoral immune response".)

The nomenclature used to describe these steps varies somewhat among authors, and depends in part on how finely one wishes to make distinctions between "discrete" developmental stages [14]. The earliest recognizable stage is called the B progenitor cell or pre-pro-B cell. Subsequent stages include the pro-B cell; the pre-B cell; the naive or mature B cell; several different types of antigen-activated B cells; the plasma cell; and the memory B cell.

Even at the pro-B cell stage of development, lineage commitment is not complete, as these cells can be "reprogrammed" along a different pathway by combinations of cytokines and extracellular signals. As an example, progenitor B cells develop into dendritic cells when cultured with IL-1beta, IL-3, and IL-7, tumor necrosis factor, stem cell factor, and Flt-3 ligand [15]. The relevance of this finding to in vivo B cell or dendritic cell development is unclear.

As a further complication, there is evidence that the program of B cell development that occurs early in life (prenatally and during the first few post-natal years of life) differs from the program of B cell development that predominates in adults [16]. The section that follows will focus on the stepwise events of early B cell development, acknowledging that there is uncertainty about some of the details.

Role of immunoglobulin gene rearrangements in B cell development — Productive rearrangement of the Ig heavy chain and light chain genes is essential for the earliest stages of B cell development and for later mature B cell maturation. The germline DNA contains segments coding for different portions of the immunoglobulin molecule. For the immunoglobulin heavy chain, these consist of roughly 40 functional variable (V) segments, 9 diversity (D) segments, 6 joining (J) segments, and a constant region for each subclass of immunoglobulin heavy chains (mu, gamma, alpha, delta, and epsilon) [17]. Light chain genes have a similar organization but lack D segments. During early stages of B cell development functional rearrangement of the heavy chain gene locus (IgH) allows for assembly of the pre-B cell receptor complex. This complex transmits signals that lead to cessation of IgH rearrangements (a process termed allelic exclusion) and initiation of light chain rearrangements, first in the kappa light chain locus and then (if these are non-productive) the lambda light chain locus. Production of a complete immunoglobulin molecule comprised of two heavy chains and two light chains allows for assembly of the mature B cell receptor on the cell surface, which transmits signals that cause the cessation of light chain gene rearrangement and further maturation to the mature B cell stage. A detailed summary of this process is discussed separately. (See "Immunoglobulin genetics".)

Pro-B cells — Extrinsic factors such as interleukin (IL)-3 and IL-7, insulin-like growth factor 1, and stem cell factor (SCF) promote the development of pro-B cells from earlier progenitors. The pro-B cell stage is denoted by the expression of Pax5, a master transcription factor that is essential for not only the generation of pro-B cells, but also all subsequent stages of B cell development up to the plasma cell stage. Other events associated with this stage are expression of RAG1 and RAG2, which catalyze D-JH rearrangements within the immunoglobulin heavy chain gene loci, and the surface expression of CD19 [18] (table 1). (See "Immunoglobulin genetics".)

CD19 is a component of a multimolecular surface complex involved in signaling in response to antigen and T cell help [19]. Pro-B cells also express HLA-DR (class II histocompatibility antigen) and CD34, a 115 kD type I transmembrane glycoprotein present on other very early hematopoietic progenitors that is probably involved in cytoadhesion [20]. (See "Overview of hematopoietic stem cells".)

Pre-B cells — During this stage of B cell development, immunoglobulin (Ig) heavy chain genes complete V-D-J recombination, allowing the surface expression of a complex consisting of an Ig heavy chain and molecules called surrogate light chains [21,22] that show homology to Ig light chain V and C regions. (See "Immunoglobulin genetics".)

Several of these genes have been identified in humans. VpreB is a non-rearranging gene encoding most of an Ig V domain. (See "Structure of immunoglobulins".)

The other genes encoding surrogate light chains belong to a family called lambda 5 or lambda 5-like. The surrogate light chain genes are expressed only in pre-B cells, and the complex containing the Ig heavy chain and the surrogate light chain is called the pre-B cell receptor. Genetic defects that prevent expression of the pre-B cell receptor, or that prohibit the transduction of signals via the receptor, lead to an absence of B cells and congenital agammaglobulinemia. Loss-of-function mutations in genes that encode components of the pre-B cell receptor or downstream signaling molecules that lead to agammaglobulinemia include:

●IgM [23]

●lambda 5 surrogate light chain [24]

●CD79a [25] (see below)

●CD79b [26]

●Bruton tyrosine kinase (BTK) [27]

●B cell linker protein (BLNK) [28]

These disorders are discussed separately. (See "Primary humoral immunodeficiencies: An overview".)

The requirement of BTK for the survival of mature B cells led to development of BTK inhibitors, such as ibrutinib, which are widely used in the treatment of B cell malignancies. (See "Treatment of relapsed or refractory chronic lymphocytic leukemia", section on 'Ibrutinib' and "Treatment of relapsed or refractory mantle cell lymphoma", section on 'Bruton tyrosine kinase inhibitors'.)

ZAP-70, a 70-kDa Syk-family protein tyrosine kinase that is also expressed on T cells and natural killer cells, may also be required for pre-B cell receptor signaling, based on defects observed in ZAP-70 deficient mice [29].

The assembled pre-B cell receptor transmits pro-growth signals that result in a 32- to 64-fold expansion (five or six cell divisions) in cell numbers. It is not clear whether pre-B cell signaling is autonomous or follows receptor activation by a currently unknown ligand.

A number of markers first appear during the pre-B cell stage.

●CD79a and CD79b (Ig-alpha and Ig-beta, respectively), which associate non-covalently with surface Ig and constitute the signal transducing components of the pre-B cell receptor. Ig-alpha and Ig-beta are also components of the Ig receptors on the surface of mature B cells.

●CD10 (also known as common acute lymphoblastic leukemia antigen [CALLA], neutral endopeptidase 24.11, or enkephalinase [30-33]) is a zinc metalloprotease capable of inactivating a variety of peptide hormones including glucagon, enkephalin, atrial natriuretic peptide, substance P, oxytocin, bradykinin, neurotensin and angiotensins I and II, as well as f-Met-Leu-Phe.

●CD20 is a tetraspan phosphoprotein that resides in lipid raft domains [34]. CD20 functions as a Ca⁺⁺ conductive ion channel that may regulate B cell activation and cell cycle progression [35].

●CD29/49d is a member of the beta-1 integrin family of cell adhesion molecules [36]. It is involved in the adhesion of pre-B cells to bone marrow stromal cells via its ligands VCAM-1 and the extracellular matrix protein fibronectin [37].

●CD40, a member of the TNF receptor family, is a 50 kD phosphoprotein expressed on a subset of pre-B cells. CD40 is involved in growth regulation of B cells and immunoglobulin class switching [38]. CD154, the ligand for CD40, is expressed on activated CD4+ T cells and is a member of the tumor necrosis alpha family [39]. The CD40-CD154 interaction is central to the generation of humoral responses to T-cell dependent antigens. (See "The adaptive humoral immune response".)

●CD22 is a member of the Ig superfamily that binds to sialic acid-bearing molecules on other hematopoietic and non-hematopoietic cells. CD22 down-regulates signaling by antigen receptors by recruiting antagonistic signaling molecules, including the tyrosine phosphatase SHP-1 [40].

●TdT (terminal deoxytransferase) is a specialized DNA polymerase that adds random nucleotides to double-stranded DNA breaks created by RAG1 and RAG2, thereby contributing to the diversification of Ig genes. TdT is a sensitive marker for benign and malignant pre-B cells, but is not specific, as benign and malignant pre-T cells also express TdT.

Immature B cells — Transition from the pre-B cell stage to the immature B cell stage is marked by successful rearrangement of either a kappa or lambda Ig light chain gene, which permits the expression of a complete IgM antibody molecule on the cell surface. If surface IgM interacts strongly with membrane-bound self-antigen (such as MHC) on other cells, then that B cell either dies by apoptosis or undergoes further Ig gene rearrangement (a process called receptor editing) to create a new Ig with a different specificity (see "Immunoglobulin genetics"). Immature B cells continue to express HLA-DR, CD19, CD20, and CD40, but no longer express CD10, CD34, RAG1, RAG2, or TdT.

Rearrangement of kappa light chain genes generates a by-product called a "kappa recombination excision circle" (KREC) [41]. This is an episomal circular DNA fragment derived from the DNA that lies between the Vkappa and Jkappa gene segments prior to rearrangement (see "Immunoglobulin genetics", section on 'Immunoglobulin gene organization'). KRECs are not replicated but are carried in the nucleus of one of the daughter cells arising from a round of B cell division. As a result, KRECs are readily detectable in the peripheral blood of healthy individuals, and an abnormally low level of KRECs indicates some process that impedes B cell development. KREC measurement is undergoing development as a potential screening tool for defective B cell development in newborns or older individuals. (See "Newborn screening for inborn errors of immunity", section on 'Screening for B cell defects'.)

Mature B cells — After exiting the bone marrow, immature B cells migrate to secondary lymphoid organs, principally the spleen and lymph nodes. Cells in the spleen represent an intermediate stage of B cell development and are called "transitional B cells." Within the splenic white pulp, these cells can further differentiate in follicular B cells or splenic marginal zone B cells, a fate choice that is influenced by Notch signaling, which enforces marginal zone fate at the expense of follicular fate [42]. Splenic marginal zone B cells appear to differentiate into short-lived plasma cells that make polyreactive antibodies that protect against certain bacterial infections [43].

Other B cells migrate to other secondary lymphoid organs, principally the lymph nodes, tonsils, spleen, Peyer's patches, and less well-organized collections of lymphocytes that are found in the respiratory tract, gastrointestinal tract, and the skin. These B cells enter tissue by binding to a specialized endothelium within small vessels known as high endothelial venules [44]. Two surface molecules, CD44 [45-48] and CD62-L (L-selectin) [49-52], play important roles in lymphocyte extravasation. Once within organized secondary lymphoid tissues, mature B cells express both surface IgM and IgD, and well as other molecules that mediate cell-cell and cell-extracellular matrix adhesive interactions, including CD11a/CD18 (LFA-1) and CD29/49d (VLA-4) [53,54]:

●Surface immunoglobulin (sIg) mediates antigen binding and internalization for antigen presentation and is also linked to signal transduction pathways that are triggered by receptor aggregation. The receptor complex consists of a transmembrane form of a complete Ig molecule and two heterodimers comprised of Ig-alpha (CD79a) and Ig-beta (CD79b) [18]. (See "Structure of immunoglobulins".)

The cytoplasmic portions of Ig-alpha and beta proteins contain short sequences called "immunoreceptor tyrosine-based activation motifs" (ITAMs). Similar sequences are found in the cytoplasmic portions of molecules constituting the T cell antigen receptor complex and the high affinity IgE Fc receptor. These motifs participate in protein-protein interactions that link Ig-alpha and Ig-beta to signaling pathways in the cytoplasm [18]. As mentioned above, Ig-alpha and Ig-beta are also required for pre-B cell receptor signaling in pre-B cells, and mutations in Ig-alpha or Ig-beta in humans are associated with a lack of B cell development and agammaglobulinemia [25]. (See 'Pre-B cells' above.)

●CD11a (integrin alpha-L) is non-covalently associated with CD18 (integrin beta-2) [50]. Within secondary lymphoid tissues, CD11a/CD18 mediates B cell binding to B cells, T cells, monocytes, and follicular dendritic cells via its ligand ICAM-1 (CD54). (See "Leukocyte-endothelial adhesion in the pathogenesis of inflammation".)

●CD20 is present on more than 95 percent of B cells in both peripheral blood and in secondary lymphoid organs. As mentioned previously, this molecule may function as a membrane ion channel that plays a role in B cell activation. Loss-of-function mutations in CD20 are associated with defects in T cell independent B cell responses to antigen [55].

●CD19 and CD20 are the markers most often used to enumerate B cells in routine clinical applications of flow cytometry [56].

●In addition, the anti-CD20 antibody, rituximab, is a widely used therapy for several malignant and autoimmune diseases; this antibody kills B cells, both by fixing complement and initiating antibody-dependent cell-mediated cytotoxicity, and its use leads to a transient reduction in B cell numbers and hypogammaglobulinemia.

●CD19 and CD21 are present on the B cell membrane in a complex with CD81 (TAPA-1) and interferon induced transmembrane protein 1 (IFITM1, also known as CD225) [19]. This complex has an important role in lowering the signaling threshold of the Ig receptor complex.

●CD21 is also the receptor for the C3d complement fragment and for the Epstein-Barr virus (EBV). CD21 (also known as CR2) is found on the majority of resident B cells within secondary lymphoid tissues but is infrequently expressed on B cells in the peripheral blood. Signaling via CD21 is important in B cell activation, in the development of B cell memory, and in regulating the production of autoantibodies [57].

●EBV can infect B cells via CD21, and this property has been exploited to transform cultured peripheral blood mononuclear cells into cell lines that will grow indefinitely, providing useful tools to study a variety of aspects of B cell biology and other genetic disorders.

●In addition to CD21, B cells also express CD35, which is the C3b complement receptor 1. B cells can utilize these complement receptors to bind immune complexes, an event that can lead to B cell activation. (See "Regulators and receptors of the complement system".)

●CD29 (integrin beta-1) associates with CD49d (integrin alpha-4) to form VLA-4, which mediates adhesion of activated B cells to follicular dendritic cells in germinal centers through binding to its ligand VCAM-1 (CD106) [58]. Follicular dendritic cells in turn stimulate B cell growth [59] and modulate the levels of factors that regulate germinal center B cell migration [60].

●CD40 is a receptor for CD154 (CD40 ligand), which is expressed on activated T cells. CD40 plays a central role in antigen-dependent B cell development in lymphoid tissues [61]. (See "The adaptive humoral immune response".)

Mature B cells express many other surface molecules as well, some of which are restricted to the B cell lineage (table 2A-B). This table also shows the temporal sequence of the expression of some of the markers used to follow B cell development.

B cell subsets — All mature B cells have immunoglobulin receptors and express CD19 and CD20, but can be divided into subgroups based on expression of other surface markers (table 3). These subsets are functionally distinct and may have different roles in the immune system. There is some evidence that these subsets diverge prior to immunoglobulin gene rearrangement at very early stages of B cell development.

B cells have been classified into two major types:

●CD5- (or "conventional" B cells, also called B-2 cells)

●CD5+ (also called B-1 cells, which are further subdivided into B-1a and B-1b)

CD5 is expressed by T cells and some B cells. A ligand for CD5 on human B cells is CD72, a cell surface molecule of unknown function that is also expressed on B cells [62]. The functions of CD5 and CD72 on B cells are not known. There are data from mice suggesting that CD5 acts to suppress signaling via the Ig receptor [63].

Each of these B cell populations is distinct with respect to its immunophenotype, requirements for activation, and responses to ligation of certain receptors. However, B-1b cells have some characteristics in common with both B-1a and B-2 cells (table 3) [64].

The CD5+ (B-1a) subset is prominent early in ontogeny, as CD5 is expressed on the following cells:

●Almost all fetal liver B cells

●40 to 60 percent of fetal spleen B cells

●60 to 80 percent of umbilical cord blood B cells

In adults, 5 to 30 percent of B cells in the peripheral blood are B-1a cells, 4 to 6 percent are B1-b cells, and 65 to 89 percent are B-2 cells. CD5+ B cells are also prominent in the peritoneal cavity (19 to 76 percent), and can be found in the lymph nodes and in the spleen [65-68].

B-1a cells appear to be a self-renewing population of cells that is primed to produce polyreactive low affinity antibodies ("natural antibody") that interact with a wide variety of foreign and self-antigens. These include IgG Fc, single-stranded DNA, thyroglobulin, insulin, E. coli beta-galactosidase, and tetanus toxoid [69]. These self-reactive antibodies are mainly encoded by rearranged immunoglobulin genes that carry few, if any, somatic mutations, consistent with the view that B-1a cells fail to pass through germinal centers and do not experience a germinal center reaction (see "Immunoglobulin genetics"). Most B-1a cells produce IgM antibodies. Despite having some self-reactivity, these antibodies occur in healthy individuals and are not themselves pathogenic. They may have a beneficial role in host defense through their IgG-binding (rheumatoid factor) activity, which could enhance immune complex formation, complement lysis, or opsonization and phagocytosis [69-71]. These antibodies may also assist in homeostasis by binding to aging and apoptotic cells and cellular debris, which if not cleared, might be proinflammatory and immunogenic [72].

It has also been suggested that CD5+ B cells have a relatively greater role in the primary immune response to antigen [73]. This stems from their capacity to rapidly differentiate into antibody-secreting plasma cells following activation (described below), thus providing some measure of humoral immunity during the week or so lag between initial exposure to antigen and the appearance of specific high-affinity antibodies [70].

Mature B cells circulate as resting cells for days or weeks, but if they are not activated within that period of time, they die by apoptosis and are replaced by new B cells. The antigen-dependent phase of B cell development is synonymous with the generation of the humoral immune response. (See "The adaptive humoral immune response".)

In addition to these two B cell subsets, there is a small population of circulating regulatory B cells (Bregs), which secrete IL-10 and suppress T cell function [74]. These cells are enriched within a B cell population that expresses CD24 and CD1d and appear to contribute to immune tolerance by inhibiting Th1 responses, inhibiting Th17 differentiation, and favoring Treg differentiation [75,76]. Breg dysfunction may be linked to disorders of altered immunity, including autoimmune disease, allergy, infection, and cancer, all of which are active areas of investigation.

B cell activation — Exposure to antigen or various polyclonal mitogens activates resting B cells and stimulates their proliferation. Activated B cells lose expression of sIgD and CD21, and acquire expression of other proteins termed activation antigens, which fall into several classes:

●Growth factor receptors

●Structures involved in cell-cell interaction

●Molecules that play a role in the localization and binding of activated B cells within various microenvironments

Specific activation antigens include the following proteins [77]:

●CD23, a 45 kD glycoprotein, is the low affinity receptor for IgE; a soluble form is reported to have B cell growth promoting activity [78] (see "The biology of IgE"). Monoclonal antibodies against CD23 inhibit IgE production [79].

●CD25, the 55 kD Tac Ag, is the alpha chain of the IL-2 receptor (high affinity); it is present on activated B cells, T cells and monocytes.

●CD27 (also called TNF receptor superfamily member 7 or TNFRSF7) is a ligand for CD70 (TNFSF7); these are markers of both B cell and T cell activation. Surface CD27 is often used as a marker for the enumeration of memory versus naïve B cells. (See 'Memory B cells' below and "Pathogenesis of common variable immunodeficiency".)

●CD71, the transferrin receptor, which is expressed on all proliferating cells.

●CD80 and CD86 are glycoprotein members of the Ig gene superfamily [80]. These molecules are ligands for the T cell antigen CD28 and provide a costimulatory signal that is critical for optimal T cell activation and proliferation following presentation of antigen in the context of MHC to the T cell receptor (see "Major histocompatibility complex (MHC) structure and function"). This signal leads to augmented cytokine secretion, including IL-2, which optimizes T cell help to B cells (see "The adaptive humoral immune response"). Subsequent to CD28 expression, T cells express CTLA-4, which interacts with CD80 and CD86 to turn off the T cell response.

●Following activation, B cells also express Fas (CD95), another member of the tumor necrosis factor (TNF) receptor family. Binding of Fas to its natural ligand (Fas ligand, FasL, CD95L), which is expressed on activated T cells, triggers apoptotic cell death. Thus, Fas regulates clonal expansion of autoreactive B cells. Fas is present on germinal center B cells and B cells in sinusoids of lymphoid tissues but is expressed at much lower levels on precursor B cells, mature sIgD+ naive B cells, and plasma cells. (See "Apoptosis and autoimmune disease".)

Maturation of activated B cells — The paths that B cells follow after activation varies depending on whether they concomitantly receive co-signals from antigen-activated T cells. B-1b and B-2 cells are preferentially activated by T cells. T cell-activated B cells migrate under the influence of chemokines to the mantle zone of germinal centers where they acquire CD5 expression transiently, and then enter the germinal center.

Activated germinal center B cells are large rapidly dividing cells that express activation-induced cytosine deaminase (AID) and uracil nucleoside glycosylase (UNG), DNA modifying enzymes that mediate somatic mutation of the Ig genes (somatic hypermutation). However, AID is error-prone and germinal center B cells have a higher than normal mutation rate involving other genomic loci as well [81]. One gene that is often mutated at a high rate in normal germinal center B cells encodes the transcription factor Bcl-6 [81], a master regulator that is required for the formation of B cell follicles. Follicular B cells re-express CD10 and also turn on expression of several other antigens, such as GCET1, which has been used to sub-classify tumors of follicular B cell origin [82].

Most follicular B cells die, but a few that express antibodies that recognize antigen with high affinity receive survival signals through the Ig receptor. These cells next undergo Ig class switching, an event also requiring AID that permits B cells to express Ig molecules other than IgM. Class switching is influenced by cytokines; for example, in the presence of high levels of TGF-beta, B cells preferentially switch to IgA heavy chain [83]. The hyper-IgM syndromes are rare human immunodeficiency states characterized by a failure of Ig class switching and a marked hyperplasia of germinal center B cells (see "Hyperimmunoglobulin M syndromes"). They are caused by loss-of-function mutations in AID, UNG, CD40, or CD40L [84]. Following class switching, B cells leave the germinal center and either undergo terminal differentiation into plasma cells or enter into the long-lived memory B cell pool.

In the absence of T cell help, most activated B cells rapidly mature into short-lived plasma cells without undergoing somatic hypermutation or class switching; as a result, these cells secrete IgM antibodies of low affinity. B-1 B cells may preferentially follow this "non-follicular" differentiation pathway, as they appear to be much less dependent on T cell help for antibody production [64]. A subset of memory B cells can develop without interactions with T cells [85]. These T-independent memory B cells provide protection from microbes expressing thymus-independent antigens (eg, pneumococcal capsular polysaccharides), but these memory cells persist only for several years, whereas T-dependent memory B cells (eg, measles antibody) may persist for decades.

Plasma cells — The plasma cell is large, up to 20 microns in diameter, and secretes antibodies prodigiously, although it has few antibody molecules on its surface [86]. Plasma cells that arise from follicular B cells are found mainly in the bone marrow and are long-lived (months to years), whereas plasma cells arising from non-follicular B cells have life spans of only days to a few weeks. Plasma cell differentiation is accompanied by the loss of Bcl-6, Pax-5, CD19, CD20, and B cell activation antigens, and the appearance of the transcription factors X-box binding protein 1 (XBP-1) and PR domain-containing protein 1 (PRDM1, also called B-lymphocyte-induced maturation protein 1 [BLIMP-1] and positive regulatory domain I-binding factor [PRDIBF1]); cytoplasmic Ig (cIg); and the antigens CD38 and syndecan-1 (CD138) [87-89].

●CD38 is a cell surface glycoprotein that also is present on thymocytes, immature myeloid cells, and subsets of hematopoietic stem cells. CD38 is ADP-ribosyl cyclase whose substrate is nicotinamide adenine dinucleotide (NAD). In addition to its enzymatic function, CD38 is involved in Ca⁺⁺ mobilization, signal transduction, and adhesion [90].

●CD138, also called syndecan-1, is a heparan sulfate proteoglycan that acts as an adhesion molecule; it serves as a useful diagnostic marker for plasma cells [91].

Memory B cells — The memory B cell is identical in appearance to resting mature B cells. The factors that influence a B cell to become a memory cell rather than a plasma cell are incompletely understood; mouse models suggest that CD40 signaling, cytokines such as IL-21, and signals transmitted by interactions with T follicular helper cells (Tfh cells) all have a role [85]. Memory cells are more sensitive to antigen stimulation and proliferate and generate plasma cells more rapidly than do mature B cells. These plasma cells are responsible for the rapid appearance of high affinity antibodies in secondary immune responses. The life span of the memory cell is not precisely known, but they may persist for years, if not decades [92]. (See "The adaptive humoral immune response".)

Recall that mature naïve B cells express surface IgM and IgD. After activation, B cells no longer express IgD. If a memory B cell underwent class-switching in a germinal center it will express either IgG, IgA, or IgE on its surface; if not, it will retain expression of IgM and IgD ("unswitched"). Memory B cells also express CD27 (see 'B cell activation' above). This interaction modulates memory B cell activation. (See "The adaptive humoral immune response".)

Using these markers, one may distinguish several subsets of B cells:

●Mature naïve B cells – IgM+IgD+CD27-

●Unswitched memory B cells – IgM+IgD+CD27+

●Switched memory B cells – IgM-IgD-CD27+

Alterations in these subsets may have significant clinical associations in some forms of humoral immunodeficiency [93,94]. (See "Pathogenesis of common variable immunodeficiency".)

More information on antigen-dependent B cell development is available elsewhere. (See "The adaptive humoral immune response".)

T LYMPHOCYTES

T cell development

T cell lineages — The majority (usually >95 percent) of circulating T cells express the T cell receptor (TCR; CD3) containing chains designated alpha and beta (TCR alpha/beta, also called TCR2). The remainder of cells express receptors that include the homologous gamma and delta chains (TCR gamma/delta, or TCR1) (figure 2). CD3 is the marker most widely used to identify total T cells on routine flow cytometry, including CD4 and CD8 cells.

Most T cells develop in the thymus, while a distinct population develops in the intestinal lamina propria. An overall scheme of thymic T cell development, based on work performed in the mouse, is shown in the figure (figure 3). The details of extrathymic T cell development and the relationship of extrathymic T cells to their more numerous thymus-derived counterparts are still obscure. T cell development is more completely understood in the mouse due to its widespread use in experimental immunologic studies, but a very similar set of events is believed to occur during the intrathymic development of human T cells.

Gamma-delta and alpha-beta T cell development diverge at early stages of intrathymic development, and mature alpha-beta and gamma-delta T cells differ in their tissue distribution, mechanisms of antigen recognition, requirements for activation, and roles in immune responses. (See "The adaptive cellular immune response: T cells and cytokines".)

TCR genetics; discussion of the alpha, beta, gamma, and delta genes; and cellular interactions in T cell activation and effector mechanisms are discussed separately. (See "T-B-NK+ SCID: Pathogenesis, clinical manifestations, and diagnosis", section on 'T cell receptor generation'.)

T cell development — During human fetal development, lymphoid progenitor cells capable of becoming T cells initially appear in the liver by approximately six weeks of gestation, and then shift by approximately five months of gestation to the bone marrow, which is the major source of T cell progenitors throughout the remainder of life. T lymphoid priming of marrow progenitors may require exposure to marrow stromal cells that express the Notch ligand DLL4; if so, such interactions must be transient or weak to avoid ectopic T cell development in the marrow [95] Progenitor cells reach the thymus via the blood, entering into the thymus through venules near the corticomedullary junction and then migrating to the outer cortex. In humans, T cell progenitors are detected in the thymus by nine weeks of gestation, and mature T cells appear in the peripheral lymphoid organs by 24 weeks of gestation [96].

As a result of robust T cell development in utero, humans (unlike mice) are born with a full complement of T cells, including T effector cells and regulatory T cells [97]. This distinction may explain the observation that neonatal thymectomy in humans (eg, during cardiac surgery) causes only subtle defects in T cell function well into adult life. Imaging studies also suggest that partial regeneration of the thymus is common in thymectomized individuals [98], which may also mitigate the deleterious effects of thymectomy. Thymopoiesis in humans begins to decline early in life, particularly after puberty, and is minimal in most individuals >40 years of age, as assessed by studies of organ donors and by measuring markers of T cell production in the peripheral blood. The latter include CD31, a marker found on new thymic emigrants [99], and quantification of T cell receptor excision circles (TRECs; described below). (See 'The mature phase' below.)

Certain cell-surface molecules serve as markers of T cell differentiation (figure 3). The marker that is expressed earliest in bone marrow progenitors is CD7. As with very early B cell progenitors, early cells expressing CD7 are not yet committed to the T cell lineage. The role of CD7 in T cell differentiation is unknown. It may be a ligand for a complex family of glycoproteins called galectins, whose physiologic functions are still unclear [100]. Some CD7+ cells also express CD34, a marker of hematopoietic stem cells and early T, B, and myeloid cell progenitors [101].

Intrathymic regulatory factors — A variety of highly specialized types of epithelial cells play critical roles in thymic T cell development. They are classified based upon location within the thymus, physical appearance, surface staining characteristics, and secreted products (table 4) [102,103]. The biology of these cells is still poorly understood, although common progenitors have been identified in mice. If the same is true in humans, it may be possible to use these cells in thymic culture systems to create therapeutic T cells from undifferentiated progenitors [104,105].

Like B cell development, T cell development depends on intrinsic functions provided by transcription factors and extrinsic signals provided by soluble factors and cell-cell interactions. Transcription factors required at very early stages of T cell development include GATA-3, TCF-1, and Bcl11b. Of these factors, TCF-1 appears to be particularly important in establishing chromatin states that are permissive for T cell potential and for induction of differentiation by Notch signaling [106]. Interactions between thymic epithelial cells (TEC) and developing T cells (thymocytes) are mediated by specific membrane ligands, and by secreted factors (a broad array of cytokines and neuropeptides). Among the most important ligands are DLL4 and Jagged2, which activate the Notch1 receptor on early T cell progenitors. In the absence of Notch signals, commitment of early progenitors to T cell fate fails to occur [107]. Conversely, constitutive activation of Notch1 in the bone marrow compartment leads to a failure of B cell development and ectopic T cell development. The cytokine interleukin-7 (IL-7) is also produced by thymic epithelial cells and contributes to the proliferative burst that occurs early in intrathymic T cell development (figure 3) [108]. In fact, both B and T cell development is completely blocked in the absence of functional IL-7 receptor, which leads to a form of severe combined immunodeficiency [109]. TECs and thymocytes also respond to endocrine and neural signals [110]. Thymic development depends upon intact pituitary and thyroid function, while overproduction of gonadal or adrenal steroids may lead to thymic involution due to the steroid sensitivity of developing T cells.

Thymic epithelial cells also produce a variety of thymus-specific hormones and cytokines required for T cell differentiation.

●Thymopoietin (TP) refers to three protein isoforms generated by differential mRNA splicing of transcripts expressed from a single gene [111]. TP promotes T cell development, inhibits B cell development, interacts with acetylcholine receptors, and stimulates release of adrenocorticotropic hormone and beta-endorphin by the pituitary gland.

●Thymosins are a group of low molecular weight (3-4 kD) proteins that also promote thymic T cell differentiation.

●Thymulin is a zinc-binding nonapeptide that potentiates signaling through the IL-2 receptor, and promotes development of IL-2 receptor-positive thymocytes [103].

The roles of all of these molecules in T lymphocyte differentiation still require clarification.

Intrathymic T cell development — A general scheme of T cell development in the thymus is shown in the figure (figure 3). Once early T cell progenitors take up position in the outer cortex, they enlarge and begin to proliferate rapidly. These immature precursors represent only 2 to 3 percent of the total number of cells within the thymus [112]. After the proliferative phase, cells take on the appearance of small resting lymphocytes, and migrate through the cortex toward the medulla. In addition to Notch and IL-7, a broad array of additional cytokines and chemokines as well as adhesion molecules and components of the extracellular matrix act in concert to guide thymocyte development and migration through the thymus, a complex process that is still being unraveled [113]. Importantly, approximately 95 percent of cortical cells die as a result of selective processes (see below) before they reach the medulla.

Thymocytes at different stages of development can be identified by the expression of various surface molecules. As mentioned above, the most immature thymocytes are CD7+. In addition to these markers (figure 3), early T cell precursors express CD2, an adhesion molecule important in intercellular contacts between T cells and other cells participating in immune responses. As thymocytes mature further they begin to express CD1, a molecule homologous to MHC class I, which may interact with glycolipid antigens and present them to gamma-delta T cells [114]; its role in T cell development is unknown.

In the early stages of development, thymocytes are called double negative (DN) since they express CD3 but neither CD4 nor CD8. During this time, gamma/delta and beta chain gene rearrangement is occurring (see "T-B-NK+ SCID: Pathogenesis, clinical manifestations, and diagnosis", section on 'T cell receptor generation'). There are four DN stages (DN1-DN4) distinguished by expression of CD44 and CD25 [112] (figure 3).

CD44 is a hyaluronan receptor and ligand for E-selectin [115], while CD25 is the alpha chain of the receptor for IL-2 (IL-2R) [116]. The DN3 stage is a critical developmental checkpoint during alpha/beta T cell development. At this stage the cells express the RAG1/RAG2 DNA recombinase and rearrange their TCR beta genes (see "T-B-NK+ SCID: Pathogenesis, clinical manifestations, and diagnosis", section on 'T cell receptor generation' and "Immunoglobulin genetics"). Cells with productive rearrangements express TCR beta chains in complex with pre-TCR-alpha, a pseudo-alpha chain that functions much like surrogate light chains during pre-B cell development (figure 2). The pre-TCR complex associates with the CD3 complex and transmits signals that drive proliferation and promote cell survival [112,113]. Cells that fail to successfully rearrange their TCR-beta genes arrest and die at the DN3 stage. There are also data to suggest that the presence of an invariant chain of CD3, the delta chain, is essential for T cell development [117].

Signaling via the pre-TCR triggers the expression of CD4 and CD8 (ie, the transition to double-positive [DP] stage), allelic exclusion of the TCR beta chain gene locus, rearrangement of the TCR alpha genes, and cellular expansion [112,113]. No ligand for the pre-TCR has been identified, and T cells can develop with a pre-TCR that has no external domains [118], suggesting that pre-TCR signaling is cell autonomous. (See "T-B-NK+ SCID: Pathogenesis, clinical manifestations, and diagnosis", section on 'T cell receptor generation'.)

If a thymocyte does not produce any functional alpha chains, it cannot develop further, and dies via apoptosis after three to four days. If alpha gene rearrangement is successful, alpha chains assemble with beta chains and are expressed on the surface as a complete alpha-beta TCR-CD3 receptor complex. The CD4 and CD8 antigens next appear at low levels on the surface, and the cell has the phenotype TCRloCD3loCD4loCD8lo.

T cell co-receptors CD4 and CD8 — The surface antigens CD4 and CD8 distinguish two major subsets of CD3-positive T cells. Normal mature cells express either one of these molecules, but not both. The CD8 antigen is required for T cell interaction with cells expressing MHC class I molecules, while CD4 is necessary for interaction with cells expressing MHC class II. (See "Major histocompatibility complex (MHC) structure and function".)

●CD4 is a 55 kD transmembrane glycoprotein encoded by a gene on chromosome 12. It is a member of the immunoglobulin (Ig) superfamily and has four extracellular Ig-like domains. The outermost two domains of CD4 interact with a monomorphic determinant of MHC class II molecules. CD4 is also a receptor for interleukin-16, which acts as a chemoattractant factor for cells that express CD4 [119].

●CD8 is made up of two polypeptides designated alpha and beta. Both chains have a Mr of 32 kD and are encoded by genes on chromosome 2. CD8 binds to the membrane-proximal domain of MHC class I molecules. Most CD8 on T cells is in the form of alpha-beta heterodimers, but it also occurs as an alpha2 homodimer. Both forms are functional, but they behave differently with respect to signal transduction and their roles in T cell development.

The cytoplasmic domains of both CD4 and CD8 are associated with the protein tyrosine kinase Lck, a member of the src family of tyrosine kinases. CD4 and CD8 associate with Lck in slightly different fashions, but both molecules are important co-stimulators in T cell activation. (See "The adaptive cellular immune response: T cells and cytokines".)

The mechanism whereby gamma-delta or alpha-beta lineage commitment is determined is unknown. There are data to suggest that expression of a functional gamma-delta receptor precludes further development along the alpha-beta path [120].

Late thymic phase — Cells that fail to interact with self-MHC die by apoptosis. Following positive selection (see below), the T cells become DP cells (ie, TCRhiCD3hiCD4hiCD8hi). At this stage, alpha gene rearrangement ceases (see "The adaptive cellular immune response: T cells and cytokines"). DP cells are the targets of the selective processes that establish the repertoire of TCR specificities. In the final phase of thymic T cell development, cells become single-positive (SP), ie, they express only CD4 (MHC class II-restricted) or CD8 (MHC class I-restricted) (figure 3). What regulates the transition of thymocytes from DP to SP is not known. The process may be either random or controlled by specific cellular interactions. Neither the random nor the instructive models have been firmly established.

T cell repertoire — The concept of the T cell repertoire is identical to that of the B cell repertoire. (See 'B cell development' above.)

However, due to the different ways in which B cells and T cells interact with antigen, the set of antigenic determinants recognized by each cell type is different (although not necessarily mutually exclusive, as some epitopes may be recognized by both) [121]. In order for the immune system to be effective, T cells must be able to recognize the multitude of foreign molecules the body encounters without attacking self.

Positive selection — The MHC molecules expressed in the thymus determine the spectrum of antigens that T cells can recognize, a limitation referred to as MHC restriction. This same restriction extends to the ability of T cells to cooperate in immune responses, for example, in providing help to B cells. (See "The adaptive humoral immune response".)

MHC restriction results from the process of positive selection in the thymus (positive because T cells live as a result). Positive selection is mediated mainly through weak interactions between TCRs expressed on thymocytes and self-peptides presented through MHC antigens expressed on thymic epithelial cells [122], many of which are of neural crest origin. Positive selection rescues thymocytes from apoptosis, which is the fate of the vast majority of developing T cells.

Negative selection — Negative selection results from high affinity interactions of developing T cells with self-peptides presented on MHC antigens. This process prevents the survival of T cells that, if released into the periphery, could activate autoimmune responses. Bone marrow-derived antigen-presenting cells (eg, thymic dendritic cells, macrophages) appear to be important in negative selection.

As a result of positive and negative selection, only 1 to 5 percent of thymocytes exit the thymus as mature T cells; the remainder die [123].

The mature phase — Mature thymocytes exit the thymus through venules in the medulla and recirculate through the blood, secondary lymphoid organs and lymph, surveying the body for foreign antigens.

Thymic output of T cells can be evaluated with T cell-receptor rearrangement excision circle (TREC) analysis. These are analogous to the kappa rearrangement excision circles (KRECs) described above. (See 'Immature B cells' above.)

TRECs are formed during TCR rearrangement in the thymus when loops of DNA are excised from the chromosome. These excision circles are stable and are not replicated during subsequent mitoses; hence, they are diluted out in the peripheral blood T cell compartment with each cell division. Cell populations with high numbers of TRECs are recent thymic emigrants [124,125]. TREC measurement in dried blood spots is an established method of newborn screening for severe autologous T cell lymphopenia (severe combined immunodeficiency, or SCID) and is available in the United States [126]. (See "Newborn screening for inborn errors of immunity", section on 'Overview of TREC screening test' and "Severe combined immunodeficiency (SCID): An overview", section on 'Newborn screening'.)

T cells have a long life span. Upon antigen stimulation, T cells enlarge and undergo rapid proliferation. After stimulation, T cells become effector cells. Some T cells also become memory cells, analogous to memory B cells. These aspects of the antigen-dependent development of T cells are discussed separately. (See "The adaptive cellular immune response: T cells and cytokines".)

Development of gamma-delta cells — Gamma-delta receptor-bearing T cells are the first to appear during ontogeny in all species [120]. Mature gamma-delta T cells are mainly DN cells (expressing neither CD4 nor CD8) and do not pass through the same DP stage of development as alpha-beta T cells. As with alpha-beta T cells, some speculate that the TCRs of gamma-delta T cells generate autonomous signals that drive cells through certain developmental checkpoints. While the development of gamma-delta T cells clearly requires signals mediated by gamma-delta TCRs, these signals appear to be qualitatively very different from those that drive alpha-beta cell development.

Gamma-delta T cells undergo little cellular expansion within the thymus but may expand considerably in the periphery. The extent to which gamma-delta T cells undergo positive and/or negative selection is unknown [120]. Some reports support the existence of versions of these processes that are involved in gamma-delta T cell development, but the experimental systems are not clearly physiologic in many instances. One study strongly suggests that an autosomal dominant gene variant expressed in the thymus can mediate positive selection of a population of murine cutaneous gamma-delta T cells [127].

Characteristics of mature T cells — This section applies principally to cells expressing alpha-beta TCRs. Mature T cells are small lymphocytes with an appearance identical to other lymphocyte subpopulations. They recirculate continually from blood to lymphoid tissues to lymph, awaiting contact with antigen presenting cells that bear a complementary peptide-MHC complex. Most functional studies of mature T cells have been conducted in mice; these studies are largely believed to be relevant to humans, but there are important differences in the origins and maintenance of T cells in mice and humans with aging [128]. Even in aged mice, most naïve T cells are of thymic origin, whereas in humans, even before thymic involution, most naïve T cells are derived from proliferation of T cells in peripheral tissues. Furthermore, the lifespan of naïve T cells in mice is 6 to 10 weeks, whereas in humans naïve T cells can live for 5 to 10 years [128].

All mature thymocytes and peripheral blood T cells bear the CD2, CD3 T cell receptor complex, CD5, and CD28 antigens (table 5). Other surface molecules such as CD4 and CD8, as well as cytokine and chemokine receptors serve to differentiate several functionally distinct subpopulations of mature T cells. As described above, these subsets may differ in their interactions with various classes of self-histocompatibility proteins. In addition, they may have very different functions in immune responses. Subpopulations of mature T cells are similar morphologically; markers are required for their distinction.

There are two principal categories of CD4 T cells: T cells that modulate the activity of B cells and other T cells, either by stimulating (helper T cells) or suppressing (regulatory T cells or Tregs) immune responses, and effector cells mediating cellular immune responses (cytotoxic T cells, Tc; or cytotoxic T lymphocytes, CTLs), but other subpopulations are also recognized. While most regulatory T cells express CD4, and most cytotoxic T cells express CD8, there are exceptions. Cytotoxic T cells expressing CD4 are prominent in graft rejection and have also been observed in tumor immune responses [129]. Cytokine producing cells expressing CD8 may also be seen in certain normal or pathologic immune responses [130,131].

Origins of T subsets — Naïve T cells of three major types develop in the thymus: MHC class II restricted CD4+ T helper cells, MHC class II restricted CD4+ Tregs (defined by expression of the transcription factor FoxP3 and the high affinity IL-2 receptor), and MHC class I restricted CD8+ T cells. Naïve CD4+ T helper cells, once stimulated by antigen expressed on activated antigen presenting cells (eg, dendritic cells or B cells) in the context of self-MHC class II molecules, can acquire several different effector functions, each associated with expression of particular cytokines and other proteins, as described below. How antigen-stimulated CD4+ T helper cells "choose" what type of effector cells they become in response to different immune stimuli remains an area of ongoing research. Certain key transcription factors contribute to fate selection; for example, Tbet is required for the production of Th1 cells. However, Tbet also influences the differentiation of CD8+ T cells [132], and even within particular CD4+ T cell subsets there appears to be substantial plasticity, particularly in humans [133]. Thus, rather than being terminally differentiated cell fates, different T helper cell types may be better thought of as cell states that can be modulated during the course of immune responses.

Cytotoxic T cells — Cytotoxic T cells (Tc) express mainly CD8, and lyse autologous cells bearing foreign antigen molecules associated with class I histocompatibility proteins or cells that are allogeneic with respect to MHC class I. CD4+ cytotoxic T cells are recognized [134], but the function of these cells is less certain; they may participate in graft rejection and in tumor immune responses in which antigen is presented in the context of allogeneic MHC class II molecules [129].

A discussion of alloreactivity can be found elsewhere. (See "The adaptive cellular immune response: T cells and cytokines".)

T helper cells — T helper cells generally express CD4 and recognize antigens associated with MHC class II molecules. Subtypes of CD4+ T cells have been defined, including Th1 and Th2 T helper cells, T follicular helper (Tfh) cells, Th9 cells, and Th17 cells. These cells are distinguished by the cytokines they produce and by certain surface markers. An "undifferentiated" mature T cell, referred to as Th0, may develop into various kinds of effector cells, depending upon the cytokines they are exposed to during antigen stimulation.

Th1 cells — Th1 cells predominantly generate delayed hypersensitivity reactions, although they also can provide B cell help. (See "The adaptive cellular immune response: T cells and cytokines".)

Th2 cells — Th2 are prominent helpers for antibody production, especially IgE responses, and promote eosinophil development and activity. (See "The biology of IgE", section on 'Regulation of synthesis'.)

Regulatory T cells (Tregs) — Tregs are distinguished by expression of the transcription factor FoxP3, which appears to be a "master" gene for the development and function of this type of T cell. Tregs also express CD25 (the IL-2 receptor alpha chain) and are dependent on IL-2. CD4+CD25+FoxP3+ Tregs are important in countering the development of allergic and autoimmune disease, as FoxP3 mutant mice and humans develop airway inflammation, eosinophilia, and elevated serum IgE levels. (See "IPEX: Immune dysregulation, polyendocrinopathy, enteropathy, X-linked".)

Tregs have also been implicated in the prevention of graft rejection and graft versus host disease (GVHD) [135]. (See "Biology of the graft-versus-tumor effect following hematopoietic cell transplantation", section on 'Regulatory T cells'.)

Follicular T cells (Tfh cells) — T cell help is required for germinal center B cells to undergo somatic hypermutation and mature into cells expressing high affinity antibodies. This help is provided by specialized follicular helper T cells (so-called Tfh cells) [136]. Like germinal center B cells, the development of Tfh cells depends on the transcription factor Bcl6, and also appears to require Notch signaling triggered by the Notch ligand DLL4, which is expressed on follicular dendritic cells [137]. Like other T helper cells, Tfh cells express CD4 and several other distinguishing markers, including CXCR5, CXCR13, PD-1, SAP (SH2D1A), IL-21, and ICOS. Tfh cells are not only important for the formation of germinal centers, but also maintenance of germinal centers as well as the differentiation of germinal center B cells into plasma cells and memory B cells. In addition, a circulating pool of memory Tfh cells has also been described that may contribute to secondary immune responses following re-exposure to antigen [85].

Th9 cells — Th9 cells express IL-9 and have been implicated in tumor immunity, but their functions are not well-characterized [138]. In some mouse models, Th9 cells have strong anti-tumor activity even after depletion of CD8+ cytotoxic T cells [139].

Th17 cells — Th17 cells are proinflammatory T cells implicated in the cell-mediated cytotoxicity of some autoimmune diseases, many of which were formerly thought to be Th1-mediated [140-142]. Th17 cells produce IL-17, IL-22, IL-26, IFN-gamma, and CCL20 and express IL-23 receptor and CD45RO. In humans, differentiation of Th0 cells into Th17 cells requires exposure to IL-1beta and either IL-23 or IL-6 [143-145]. ROR-gamma t is a transcription factor that regulates this T cell subset. (See "The adaptive cellular immune response: T cells and cytokines".)

Th17 cells may have some role in certain types of asthma, although patients with Th17-mediated asthma are not atopic and inflammatory cells in the airway are predominately neutrophils, rather than eosinophils [146]. Th17 cells may also be involved in allergic rhinitis and conjunctivitis, food protein induced enterocolitis, atopic dermatitis, contact dermatitis, and are essential for effective mucosal immunity against fungi (eg, Candida species) [145,147].

Naïve T cells — CD45, also called "leukocyte common antigen," is a protein tyrosine phosphatase that has a critical role in T cell activation via the dephosphorylation of the tyrosine kinase Lck (see "The adaptive cellular immune response: T cells and cytokines"). Different CD45 isoforms are expressed at different stages of lymphocyte development, making it a useful marker for lymphocyte subtyping. Naïve T cells express an isoform of CD45 called CD45RA, and are negative for a second isoform, CD45RO, that is expressed following activation (see below).

T cell activation markers — T cells express a variety of surface molecules after activation via the antigen receptor. These include CD69, CD40 ligand, CD28, CD25, MHC class II, and an isoform of CD45 called CD4RO.

Memory T cells — T cell immune responses in mice and humans occur in three phases: clonal expansion and differentiation of antigen-specific T cells; cessation, during which most effector T cells die by apoptosis; and memory, marked by the persistence of a subpopulation of antigen-primed T cells. Memory T cells express CD45RO and (like memory B cells) CD27. In humans, memory T cells against antigens such as viral vaccines are proportional to the extent of the initial immune response, typically constituting approximately 5 percent of the responding cells, and may persist for as much as 25 years or more after the initial exposure [128]. The proportions of naïve versus activated or memory T cells may have clinical applications in the diagnosis of autoimmune or inflammatory conditions and immunodeficiency [148].

As humans age and naïve T cell numbers decline, memory T cells constitute a larger fraction of the T cell complement. This appears to be particularly true for tissue-resident quiescent memory T cells that express CD69, which are found in many human tissues and have a unique transcriptional signature [149]. It is proposed that upon activation, these tissue resident memory cells have the potential to give rise to T cells with helper, cytotoxic, and regulatory functions.

SUMMARY

●Description – There are two major lymphocyte lineages:

•B lymphocytes

•T lymphocytes

Natural killer (NK) cells, innate lymphoid cells, and myeloid lineages that cooperate with B and T lymphocytes in the innate and adaptive immune systems are discussed separately. (See "An overview of the innate immune system".)

●Origin of lymphocytes – Highly proliferative lymphoid-committed progenitor cells arise from bone marrow hematopoietic stem cells. (See 'Origin of lymphoid cells' above.)

●Normal lymphoid tissues. (See 'Normal lymphoid tissues' above.)

•Primary lymphoid organs – B and T lymphocytes are initially generated in bone marrow and thymus, respectively.

•Secondary lymphoid organs – Lymph nodes, spleen, tonsils, and lymphoid tissue in the gastrointestinal and respiratory tracts. B cells are mainly located in follicles and T cells in interfollicular areas of these sites.

●B cells — New B cells are generated throughout life. After birth, lymphoid development occurs in bone marrow. Mature B cells leave the marrow and migrate to secondary lymphoid tissues where they interact with exogenous antigen and/or T helper cells; these steps constitute the "antigen-dependent phase" of B cell development. (See 'B cell development' above.)

•Immunoglobulin (Ig) rearrangement – Productive rearrangement of Ig heavy chain and light chain genes is essential for early stages of B cell development and for later mature B cell maturation.

•Maturation stages – B lymphocytes further mature as:

-Pro-B cells (see 'Pro-B cells' above)

-Pre-B cells (see 'Pre-B cells' above)

-Immature B cells (see 'Immature B cells' above)

-Mature B cells (see 'Mature B cells' above)

•B cell subsets – Include (see 'B cell subsets' above):

-CD5 negative ("conventional" B cells)

-CD5 positive (B-1 cells)

•Activation and maturation – B cell activation and maturation are described above. (See 'B cell activation' above and 'Maturation of activated B cells' above.)

•Plasma cells – Large cells that produce large quantities of Igs. (See 'Plasma cells' above.)

•Memory B cells – Long-lived cells that are sensitive to antigen stimulation and rapidly proliferate to generate plasma cells. (See 'Memory B cells' above.)

●T lymphocytes

•T cell lineages – T lymphocytes express CD3, the T cell receptor (TCR). More than 95 percent express TCR alpha and beta chains; others express TCR gamma and delta chains. (See 'T cell lineages' above.)

•T cell development – T cell progenitors migrate from bone marrow to thymus, where intrinsic functions provided by transcription factors and extrinsic signals provided by soluble factors and cell-cell interactions are required for maturation.

•T cell subsets – Defined by the following cell surface antigens (see 'T cell co-receptors CD4 and CD8' above):

-CD4

-CD8

•T cell repertoire – Positive selection and negative selection contribute to the diversity of T lymphocytes. (See 'T cell repertoire' above.)

•Cytotoxic T cells – CD8-expressing T cells that lyse autologous cells bearing foreign antigen molecules associated with class I histocompatibility proteins (MHC) or cells that are allogeneic with respect to MHC class I. (See 'Cytotoxic T cells' above.)

•T helper cells – T cells that express CD4 and recognize antigens associated with MHC class II molecules. (See 'T helper cells' above.)

●Naïve T cells – T cells that are not yet antigen primed. (See 'Naïve T cells' above.)

●Memory T cells – Antigen-primed T cells that can persist for decades. (See 'Memory T cells' above.)

ACKNOWLEDGMENTS — The UpToDate editorial staff acknowledges Francisco A Bonilla, MD, PhD, who contributed as an author, and E Richard Stiehm, MD, who contributed as a Section Editor to an earlier version of this topic review.