Chapter 10 : B-Lymphocyte Activation and Antibody Production

Figure 10.1

Structure of the BCR. Schematic representation of the BCR complex, the TCR complex, and the Fc receptors for IgE (FcϵR1) and IgG (FcγRIII), emphasizing the areas they have in common. The thicker lines in the cytoplasmic regions of each ligand represent ITAM. Ti, idiotypic TCR (TCR α/β or TCR γ/δ). Based on Fig. 1 of C. M. Pleiman et al., Immunol. Today 15:393–399, 1994.

Figure 10.2

Development of the mIg on the B cell. As the B cell develops, the surface immunoglobulin is assembled. At first a complex consisting of Ig-α, Ig-β, and the protein calnexin appears on the plasma membrane. This complex is called the pro-BCR. After immunoglobulin H-chain rearrangement is completed, the pre-BCR is formed, consisting of Ig-α, Ig-β, the rearranged Ig H (µ) chain, and the surrogate L chain (made up of the VPre-B and λ5 proteins). As B-cell development progresses, the L chains are assembled and the mature IgM antibody appears on the cell surface, also in association with the Ig-α and Ig-β proteins. The isotype of the mIg on a mature B cell includes both IgM and IgD.

Figure 10.3

Signal transduction via the BCR. Once the BCRs are specifically bound by antigen (or cross-linked by antibodies to immunoglobulin), various PTK are recruited to the ITAM sequences of Ig-α and Ig- β. p72 syk , a member of the Syk/ZAP-70 family of kinases, appears to be constitutively associated with the cytoplasmic tail of the immunoglobulin heavy chain and upon BCR cross-linking may autophosphorylate itself (top left panel, green circle). p72 syk autophosphorylation allows Src-family kinases such as p53/56 lyn , p59 fyn , and p55 blk to bind p72 syk via their SH2 domains, but prior to the SH2-mediated binding to p72 syk , these Src-family kinases need to be tyrosine dephosphorylated by CD45 (B220) (top right panel, arrow 1). Upon association with p72 syk , these PTK are activated by phosphorylation by p72 syk (arrow 2). The activated kinase molecules in turn phosphorylate Ig-α and Ig-β in their ITAM sequences (top right panel, arrows 3). It is also possible that p72 syk tyrosine phosphorylates the ITAMs of Ig-α and Ig-β directly (top right panel, arrows 4); however, the mechanism by which p72 syk mediates this process is unknown. The latter mechanism may be due to the creation of a lattice-like network of BCRs upon cross-linkage, allowing trans-phosphorylation of the tyrosine residues on p72 syk and Ig-αand Ig-β and further recruitment and binding of Src-family kinases and p72 syk . It is possible that both of these scenarios occur. Src-family kinases are recruited and reoriented via binding through their SH2 domains to ITAMs (bottom left panel). In addition, an adapter protein with SH2 domains, Shc, also is recruited, undergoing tyrosine phosphorylation and allowing association with Grb2 and Sos1 (other SH2-containing adapter proteins) and activation of the p21 ras pathway (bottom right panel). p53/56 lyn and p72 syk are important for liberation of intracellular Ca2+ after antigen binding, but p53/56 lyn carries this out via the p21 ras system. p72 syk mediates Ca2+ mobilization through activation of PLCγ2, which cleaves phosphatidylinositol bisphosphate into inositol trisphosphate and diacylglycerol. The result of all these events is the transcription of gene products necessary for B-cell activation, differentiation, proliferation and antibody production. Based on Fig. 1 of C. M. Pleiman et al., Immunol. Today 15:393–399, 1994.

Figure 10.4

PLC breakdown of PIP2. The cascade of events is initiated by PLC breakdown of PIP2 into IP3 and DAG. PKC is then activated by DAG, leading to phosphorylation of a variety of cellular proteins, including MAPK. MAPK activation leads to nuclear translocation of various transcription activators, such as c-Jun, Fos, and Myc. Production of IP3 leads to release of intracellular Ca2+, which binds to and activates the regulatory protein calmodulin. Activated calmodulin increases the activity of different enzymes, especially the phosphatase calcineurin. Calcineurin, in turn, can dephosphorylate the transcription factor NF-AT, activating it in the process. Calcineurin is the molecular target of the immunosuppressive drugs cyclosporine and FK506.

Figure 10.5a

Effect of simultaneous binding of the antibody-antigen complex to the BCR and the Fc receptor (FcγRII) on B-cell activation and signaling events. (A) When antigen in an antibody-antigen immune complex binds to the surface immunoglobulin of the BCR and the Fc portion of the antibody binds to the inhibitory FcR present on B cells, FcγRII, these two molecules are brought into close proximity. This allows phosphorylation of the ITIM region of this FcR by p53/56 lyn kinase and recruitment of the phosphatase SHIP (SH2-containing inositol polyphosphate 5-phosphatase). There are two separate inhibitory activities of FcγRII based on recruitment of SHIP, inhibition of ITAM-triggered calcium mobilization, and ITAM-triggered cellular proliferation due to the effect of SHIP activity on two different signaling pathways. Inhibition of calcium mobilization requires the phosphatase activity of SHIP to hydrolyze PIP3 bound to PLCγ to PIP2, eliciting dissociation of Btk and PLCγ from the BCR complex like Btk and PLCγ (1). Prevention or arrest of proliferation in B cells can also be due to SHIP's activating the adapter protein Dok either directly or by allowing recruitment of Dok to the membrane to allow access to Lyn kinase. Dok activation can lead to subsequent inactivation of MAPK (2). Moreover, dephosphorylation by SHIP of PIP3 bound to Akt inactivates PIP3 and enhances susceptibility of the B cell to apoptosis (3). (B) There is a third B-cell inhibitory activity of FcγRII that is independent of ITIM and SHIP and that requires homoaggregation of the Fc receptor. Clustering of FcγRII, which can occur with immune complexes binding to B cells whose mIg does not recognize the antigen portion of the complex, elicits a proapoptotic signal through its transmembrane sequence. This requires the presence and activation of Btk, which further activates Jnk, allowing for induction of apoptosis. Interestingly, recruitment of SHIP blocks this proapoptotic signal, likely through induction of dissociation of Btk from the BCR cytoplasmic complex. Based on A. L. Defranco and D. A. Law, Science 268:263–264, 1995.

There is a third B-cell inhibitory activity of FcγRIIB that is independent of ITIMs and SHIP and requires aggregation of the Fc receptor ( Fig. 10.5B ). Clustering of FcγRIIB, which can occur with immune complexes binding to B cells whose mIg does not recognize any of the antigenic epitopes in the complex, promotes apoptosis. This requires the association of Btk with the BCR, which normally is part of an activating pathway. Recruitment of SHIP blocks this form of apoptosis, likely through induction of the dissociation of Btk from the BCR cytoplasmic complex. This process would prevent inappropriate activation of B cells that cannot recognize an antigen when the B cells bind immune complexes.

Figure 10.5b

Effect of simultaneous binding of the antibody-antigen complex to the BCR and the Fc receptor (FcγRII) on B-cell activation and signaling events. (A) When antigen in an antibody-antigen immune complex binds to the surface immunoglobulin of the BCR and the Fc portion of the antibody binds to the inhibitory FcR present on B cells, FcγRII, these two molecules are brought into close proximity. This allows phosphorylation of the ITIM region of this FcR by p53/56 lyn kinase and recruitment of the phosphatase SHIP (SH2-containing inositol polyphosphate 5-phosphatase). There are two separate inhibitory activities of FcγRII based on recruitment of SHIP, inhibition of ITAM-triggered calcium mobilization, and ITAM-triggered cellular proliferation due to the effect of SHIP activity on two different signaling pathways. Inhibition of calcium mobilization requires the phosphatase activity of SHIP to hydrolyze PIP3 bound to PLCγ to PIP2, eliciting dissociation of Btk and PLCγ from the BCR complex like Btk and PLCγ (1). Prevention or arrest of proliferation in B cells can also be due to SHIP's activating the adapter protein Dok either directly or by allowing recruitment of Dok to the membrane to allow access to Lyn kinase. Dok activation can lead to subsequent inactivation of MAPK (2). Moreover, dephosphorylation by SHIP of PIP3 bound to Akt inactivates PIP3 and enhances susceptibility of the B cell to apoptosis (3). (B) There is a third B-cell inhibitory activity of FcγRII that is independent of ITIM and SHIP and that requires homoaggregation of the Fc receptor. Clustering of FcγRII, which can occur with immune complexes binding to B cells whose mIg does not recognize the antigen portion of the complex, elicits a proapoptotic signal through its transmembrane sequence. This requires the presence and activation of Btk, which further activates Jnk, allowing for induction of apoptosis. Interestingly, recruitment of SHIP blocks this proapoptotic signal, likely through induction of dissociation of Btk from the BCR cytoplasmic complex. Based on A. L. Defranco and D. A. Law, Science 268:263–264, 1995.

There is a third B-cell inhibitory activity of FcγRIIB that is independent of ITIMs and SHIP and requires aggregation of the Fc receptor ( Fig. 10.5B ). Clustering of FcγRIIB, which can occur with immune complexes binding to B cells whose mIg does not recognize any of the antigenic epitopes in the complex, promotes apoptosis. This requires the association of Btk with the BCR, which normally is part of an activating pathway. Recruitment of SHIP blocks this form of apoptosis, likely through induction of the dissociation of Btk from the BCR cytoplasmic complex. This process would prevent inappropriate activation of B cells that cannot recognize an antigen when the B cells bind immune complexes.

Figure 10.6

Antigen-dependent B-cell maturation in the germinal center is a multistep process involving several regulatory checkpoints. Antigen is first encountered in the T-cell-rich paracortical region of the lymph node, when a DC presents antigen to a T cell on MHC class II. Antigen-specific T cells migrate to the B-cell-rich cortical region where the primary lymphoid follicles reside. Here, antigen-specific B cells present antigen to antigen-specific T cells, and the B cell receives survival signals from the T cells in the form of CD40-CD40-L interactions and T-cell-secreted cytokines (checkpoint 1). B cells activated in this way migrate into the primary follicle and initiate the germinal center reaction. B-cell centroblasts proliferate in the dark zone of the germinal center and during this time undergo the process of somatic hypermutation to diversify their antibody genes. B cells that stop proliferating (centrocytes) then enter the light zone of the germinal center. Here, B cells with mutated antibody genes must compete with each other for binding to antigen on the surface of FDCs. Centrocytes with the highest affinity for antigen receive survival signals from the FDCs (checkpoint 2) and migrate to the apical region of the light zone, where they must interact with antigen-specific T cells (checkpoint 3). B cells that receive help from T cells at checkpoint 3 continue their maturation to become memory or plasma cells and also undergo the process of isotype switching under the regulation of T-cell-derived cytokines.

Figure 10.7

Survival signals received by B cells in the germinal center can be mediated at the level of Fas-triggered apoptosis. (A) In a proliferating centrocyte, the apoptotic pathway is inhibited since the initiating caspase, caspase-8, is maintained in its inactive precursor form (pro-caspase-8) by the protein FLIP (FLIP, FLICE inhibitory protein; FLICE, FADD-like IL-1β converting enzyme). (B) Centrocytes that do not continue to receive survival signals diminish their expression of FLIP, allowing the conversion of pro-caspase-8 to its active form. (C) During the germinal center reaction, B cells with a high affinity for antigen can avoid apoptosis because of enhanced FLIP expression. FLIP expression is increased by mIg binding to antigen-antibody complexes on FDCs (checkpoint 2 in Fig. 10.6 ) or by CD40-CD40L interactions (checkpoints 1 and 3).

Figure 10.8

Primary and secondary antibody response. The number of IgM antibodies in the secondary response varies with different antigens. It can be higher than that of the primary response, lower, or in some instances, not detectable. However, in the secondary response, IgG, and sometimes IgA, predominates and accounts for most of the total specific antibody.

Figure 10.9

Recruiting T-cell help for polysaccharide (PS) antigens. The membrane immunoglobulin of the BCR-specific for PS can be cross-linked by the repeating epitopes of the PS component of a protein- PS conjugate vaccine. The conjugate is internalized by receptormediated endocytosis. The carrier protein is processed, and the oligopeptide T-cell epitopes form complexes with the MHC class II molecules, which are then transported to the cell surface. The peptide- MHC class II complex is presented to T cells specific for the carrier T-cell epitopes. The TH is activated and secretes cytokines that activate the B cells, thus stimulating the B cell to differentiate and giving rise to a clone of anti-polysaccharide-producing cells. Adapted from G. R. Siber, Science 265:1385–1387, 1994.

Figure 10.10

Schematic representation of B-cell costimulatory ligands CD80 (B7-1) and CD86 (B7-2). P, areas of tyrosine phosphorylation.

Figure 10.11

Effect of CD40-CD40L interaction on B cells. Binding of B-cell CD40 to CD40L on T cells elicits a set of events that are essential for efficient B-cell function and antibody production, including isotype switching to IgG and induction of memory B cells. Based on F. H. Durie et al., Immunol. Today 15:406–411, 1994.

Figure 10.12

Involvement of costimulatory ligands and CD40- CD40L in B- and T-cell activation. A schematic representation of the positive and, possibly, negative effects of the engagement of CD40 on expression of CD40L, B7-1, B7-2, CD28, and CTLA-4 on B cells and T cells. Lymphocyte activation requires two signals. For TD antigens, one signal is transmitted via antigen presentation by MHC class II to the TCR. For TI antigens, the separation of the two signals into distinct entities is less clear, as it appears that the antigen binding to the B cell provides both signals for B-cell activation.

Figure 10.13

Effect of cytokines on B-lymphocyte antibody production. Various cytokines can affect the B cells by priming them for activation and by directing the isotypes of the antibodies they produce. IL-2 can induce production of IgG2a (in mice) and IgG3 (in humans). IFN-γ has a negative effect on antibody production. IL-4 stimulates immunoglobulin production of isotypes IgG, IgG1, and IgE, whereas IL-5 induces IgM and IgG1 with no effect on IgE production. Based on Fig. 2 and 3 of R. F. Coffman et al., Immunol. Rev. 102:5–28, 1988.2

Figure 10.14

Summary of the outcome of B-cell interaction with antigen and B-cell activation.