4. B-cell markers: the main molecules of the B-lymphoid lineage

The majority of human peripheral blood B-cells express on their surface IgM and IgD that have the same antigenic specificity. In addition, in humans, a large population of circulating cells expresses membrane lgG or IgA (but very little IgE). In some tissues, including intestinal mucosa, B-cells selectively express membrane IgA. In addition to the Igα/Igβ heterodimer marker, which is part of the BCR complex, there are also other molecules on the B-cell surface that play notable roles in various cellular functions, especially in B-cell regulation, such as human complement receptor type 2 (CR2, C3d), designed as CD21, that regulates B-cell proliferative responses, and serving as a receptor for the C3d, C3dg, and iC3b proteins of complement. B-cells also express receptor for complement component C3b (CR1, CD35) as well as for Fc fragments (FcR) of certain Ig isotypes, such as IgG (FcγRII-B1, CD32). Of note, only FcγRIIB1 and FcγRIIB2 have an immunoreceptor tyrosinebased inhibitory motif (ITIM) sequence among the FcγR receptor family and are therefore inhibitory FcRs; they do not induce phagocytosis and represent an important receptor-mediated feedback circuit regulator by circulating antigenspecific IgG [24, 37]. CR2/CD21 has also been described as a receptor for the envelope glycoprotein gp350/220 of the Epstein–Barr virus (EBV) [38]. Other molecules are frequently used in routine practice as primary markers for identifying human B-cells, including CD19, CD20, and CD22 markers. CD19 is expressed at all stages of B-cell lineage, including normal plasma cells. CD20, a molecule restricted to the B-cell population, has been shown to be implicated to form calcium channels in cell membrane [39], and as an effective target for immunotherapy in treatment of B-cell lymphoma, as well as in a number of autoimmune diseases, such as type 1 diabetes [40]. It occurs at the early pre-B-cell stage of development and remains throughout all stages of B-cell maturation [41], ranging from pre-B-cells in the bone marrow to short-lived plasmablasts [42]. CD22, a B-cell-restricted surface molecule that regulates BCR signaling in mature B-cells [43], is an early marker and persists at all stages of B-cell differentiation, which has allowed it to be a useful pan marker for all mature B-cell subsets [44]. Recognized as a B-cell-specific sialic acid binding Ig-like lectin 2 (Siglec-2; B-lymphocyte cell adhesion molecule [BL-CAM]), CD22 has been exploited as a therapeutic target for humanized anti-CD22 monoclonal antibody to treat B-cell leukemia [45]. Both immature and mature mouse B-cells as well as subsets of T-cells and NK cells and subset of abnormal T-cells involved in the pathogenesis of systemic autoimmunity in MRL-Faslpr and MRL-Fasgld mice express one of the five isoforms of the transmembrane tyrosine phosphatase CD45 (LCA) found on lymphocytes [46], i.e., CD45R (B220, CD45RABC), which has been known to play a major role in lymphocyte signaling and activation. Other B-cell surface molecules allow them to cooperate with T-cells. They include essentially MHC II antigens (DR, DP, and DQ, in humans, and IA and IE, in mice) and CD40 (a member of the TNF receptor superfamily, also expressed on B-cell precursors in the bone marrow), which is capable of transducing extremely potent B-cell activation signals. Mutations in the gene coding for CD40 ligand (CD40L/CD154), predominantly expressed by activated CD4+ T-cells, lead to X-linked hyper-IgM syndrome (HIGM1), a severe primary immune deficiency (PID) in humans, characterized by recurrent infections, associated with very low or absent IgG, IgA, and IgE levels, but normal or elevated serum levels of IgM [47]. B-cells can also be identified using the inhibitory co-receptor CD72 (Lyb-2 in mice), containing an ITIM in the cytoplasmic. CD72 specifically recognizes the RNA-containing endogenous TLR7 ligand Sm/ribonucleoprotein (RNP) by C-type lectin-like domain (CTLD) located in its extracellular region and specifically inhibits B-cell responses to Sm/RNP by recruiting SH2 domain-containing phosphatase 1 (SHP-1) to the

phosphorylated ITIM [48]. It has been reported to have a role in the regulation of systemic lupus erythematosus (SLE) development [49]. Its specific ligand is CD5 [50], which is usually expressed on most T-cells, but also on a subset of mature B-cells [51] and CD5+ B1 cells, which emerge early in development and play a major role in autoimmunity [52]. Other markers, including CD10 and CD23, are fond at some stages of B-cell differentiation. CD10, a neutral endopeptidase, also referred to as common acute lymphoid leukemia antigen (CALLA), enkephalinase, and neprilysin, is expressed in early B-cell immature stages in bone marrow, but also in certain stages of immunopoiesis and is also present on antigen-activated cells in GC, in humans [53]. Defined initially as the low-affinity receptor for IgE (FcεRII) and as EBV cell surface (EBVCS) activation antigen or BLAST-2, CD23 exists in two distinct protein isoforms, CD23α (FcεRIIa), which is constitutively expressed only in normal B-cells and B-cell lines, and CD23β (FcεRIIb), which is an inducible molecule detectable on various cells [54], including most mature B-cells, follicular mantle (FM) B-cells, T-cell subsets (at low levels), NK cells, platelets, FDCs, monocytes, eosinophils, Langerhans cells, etc. CD23, a 45 kD type II transmembrane glycoprotein, can be cleaved and found in plasma as a freely soluble receptor (sCD23). In contrast to cross-linking of FcεRI that leads to degranulation of mast cells to release vasoactive mediators, engagement of membrane-bound CD23 suppresses the production of IgE by B-cells. Moreover, CD23 participates in many regulatory processes, including cytokine release, cell–cell contact, and

receptor-mediated positive and negative feedback circuits, either as a membranebound glycoprotein or as sCD23 [55]. Finally, a new type of immune cell, so-called X lymphocyte, that is a dual expresser (DE) of TCR and BCR and key lineage markers of both B- and T-cells, has just recently been discovered in patients with

Antigen binding to mIg induces the BCR aggregation, which leads to the rapid transmembrane disulfide-linked heterodimer phosphoprotein Igα (CD79a)/Igβ (CD79b) ITAM phosphorylation through recruitment of Syk and SFKs (Fyn, Lyn). This process begins with the formation of a "signalosome" [57]. The signalosome activation leads to three main pathways [58], including Btk, PLC-γ2, and PI3K. BCR can transactivate the B-cell co-receptor CD19, which forms, on B-cell surface, a tetrameric co-receptor complex with CD21 and CD81 (target of anti-proliferative antibody 1 (TAPA-1), a tetraspanin family member tetraspanin 1 (Tspan1, NET-1), and Leu13 (CD225)) [59]. CD19 can also be BCR-independently activated but lacks intrinsic or associated tyrosine kinase activity [60]. As CD19 has a long cytoplasmic domain, it binds and amplifies the function of the SFKs and recruits a heterodimer p85/p110 class IA PI3K concurrently, which phosphorylates a membrane phospholipid, PIP2, leading to the production of a second messenger PIP3 [61], as well as promoting Btk and Akt, and a serine/threonine, kinase phosphorylation in

Main types of diseases related to B-cell abnormalities. AID, activation-induced cytidine deaminase; Btk, Bruton's tyrosine kinase; HIGM1, X-linked hyper-IgM syndrome type 1; CD40L, CD40 ligand (CD154); CSR, class switch recombination; GCs, germinal centers; Ig H, immunoglobulin heavy chain; BAFF, B-cell activating factor also known as B-lymphocyte stimulator (BLyS) or tumor necrosis factor (TNF) ligand superfamily member 13B (TNF-like molecule BAFF); BCL-2, B-cell lymphoma 2; EBV, Epstein–barr virus; CVID, common variable immunodeficiency; IVIg, intravenous immunoglobulin; SCID, severe combined immunodeficiency; SHM, somatic hypermutation; SLOs, secondary lymphoid organs; TACI, transmembrane activator and calcium modulator and cytophilin ligand interactor; XLA, X-linked agammaglobulinemia (Bruton's agammaglobulinemia); HIGM2, hyper-IgM syndrome type 2 (autosomal recessive).

5. Normal mature B-cell activation and signaling mechanisms

type 1 diabetes (For review see [56]).

DOI: http://dx.doi.org/10.5772/intechopen.90636

Introductory Chapter: B-Cells

B-cell [62] (Figure 7).

Figure 8.

17

#### Figure 7.

BCR signaling. As mentioned above, the final activation of mature B-cell occurs in the SLOs, where they migrate through the blood. Within the SLOs, they receive a constant supply of antigen through the circulating lymph. The activation of B-cell is initiated after binding of an appropriate antigen to its BCR, leading to phosphorylation of the non-covalently associated Igα/Igβ transmembrane (CD79a and CD79b). The signaling mechanisms triggered during the B-cell activation can be summarized in four main steps: (i) antigen binding and starting signaling cascade, (ii) phosphorylation of Igα/Igβ ITAM, (iii) signalosome complex formation, and (iii) signalosome activation. Akt, protein kinase B/PKB; BCR, B-cell antigen receptor; BLNK, B-cell linker; Btk, Bruton's protein tyrosine kinase/non-receptor kinase; CD, cluster of differentiation; ERK, extracellular signal-regulated kinase; Fyn, proto-oncogene tyrosine-protein kinase Fyn/non-receptor Fyn protooncogene; Grb2, growth factor receptor-bound protein 2; ITAM, immunoreceptor tyrosine-based activation motifs; JNK, c-Jun N-terminal kinase; Lyn, Lck/yes novel tyrosine kinase; mIg, membrane-bound immunoglobulin; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B-cells; PI3K, phosphoinositide-3 kinase; PIP2, phosphatidylinositol-4,5-bisphosphate; PIP3, phosphatidylinositol-3,4,5-trisphosphate; PLCy2, phospholipase C-γ2; PKCβ, protein kinase C β; SLOs, secondary lymphoid organs; SFKs, Src-family kinase/Src-proteintyrosine kinase or Src family tyrosine kinase; Src, proto-oncogene tyrosine-protein kinase c-Src (cellular Src kinase); Syk, spleen tyrosine kinase/non-receptor tyrosine kinases; Vav, proteins acting as guanine nucleotide exchange factors (GEFs) for small G proteins of the rho family.

#### Introductory Chapter: B-Cells DOI: http://dx.doi.org/10.5772/intechopen.90636

phosphorylated ITIM [48]. It has been reported to have a role in the regulation of systemic lupus erythematosus (SLE) development [49]. Its specific ligand is CD5 [50], which is usually expressed on most T-cells, but also on a subset of mature B-cells [51] and CD5+ B1 cells, which emerge early in development and play a major role in autoimmunity [52]. Other markers, including CD10 and CD23, are fond at some stages of B-cell differentiation. CD10, a neutral endopeptidase, also referred to as common acute lymphoid leukemia antigen (CALLA), enkephalinase, and neprilysin, is expressed in early B-cell immature stages in bone marrow, but also in certain stages of immunopoiesis and is also present on antigen-activated cells in GC, in humans [53]. Defined initially as the low-affinity receptor for IgE (FcεRII) and as EBV cell surface (EBVCS) activation antigen or BLAST-2, CD23 exists in two distinct protein isoforms, CD23α (FcεRIIa), which is constitutively expressed only in normal B-cells and B-cell lines, and CD23β (FcεRIIb), which is an inducible molecule detectable on various cells [54], including most mature B-cells, follicular mantle (FM) B-cells, T-cell subsets (at low levels), NK cells, platelets, FDCs, monocytes, eosinophils, Langerhans cells, etc. CD23, a 45 kD type II transmembrane glycoprotein, can be cleaved and found in plasma as a freely soluble receptor (sCD23). In contrast to cross-linking of FcεRI that leads to degranulation of mast cells to release vasoactive mediators, engagement of membrane-bound CD23 suppresses the production of IgE by B-cells. Moreover, CD23 participates in many regulatory processes, including cytokine release, cell–cell contact, and

Normal and Malignant B-cell

BCR signaling. As mentioned above, the final activation of mature B-cell occurs in the SLOs, where they migrate through the blood. Within the SLOs, they receive a constant supply of antigen through the circulating lymph.

phosphorylation of the non-covalently associated Igα/Igβ transmembrane (CD79a and CD79b). The signaling mechanisms triggered during the B-cell activation can be summarized in four main steps: (i) antigen binding and starting signaling cascade, (ii) phosphorylation of Igα/Igβ ITAM, (iii) signalosome complex formation, and (iii) signalosome activation. Akt, protein kinase B/PKB; BCR, B-cell antigen receptor; BLNK, B-cell linker; Btk, Bruton's protein tyrosine kinase/non-receptor kinase; CD, cluster of differentiation; ERK, extracellular signal-regulated kinase; Fyn, proto-oncogene tyrosine-protein kinase Fyn/non-receptor Fyn protooncogene; Grb2, growth factor receptor-bound protein 2; ITAM, immunoreceptor tyrosine-based activation motifs; JNK, c-Jun N-terminal kinase; Lyn, Lck/yes novel tyrosine kinase; mIg, membrane-bound immunoglobulin; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B-cells; PI3K, phosphoinositide-3 kinase; PIP2, phosphatidylinositol-4,5-bisphosphate; PIP3, phosphatidylinositol-3,4,5-trisphosphate; PLCy2, phospholipase C-γ2; PKCβ, protein kinase C β; SLOs, secondary lymphoid organs; SFKs, Src-family kinase/Src-proteintyrosine kinase or Src family tyrosine kinase; Src, proto-oncogene tyrosine-protein kinase c-Src (cellular Src kinase); Syk, spleen tyrosine kinase/non-receptor tyrosine kinases; Vav, proteins acting as guanine nucleotide

The activation of B-cell is initiated after binding of an appropriate antigen to its BCR, leading to

exchange factors (GEFs) for small G proteins of the rho family.

Figure 7.

16

receptor-mediated positive and negative feedback circuits, either as a membranebound glycoprotein or as sCD23 [55]. Finally, a new type of immune cell, so-called X lymphocyte, that is a dual expresser (DE) of TCR and BCR and key lineage markers of both B- and T-cells, has just recently been discovered in patients with type 1 diabetes (For review see [56]).
