Section E – The antibody response
Two B cell groups can be distinguished based on their requirement for T cell help in order to proliferate and differentiate. B1 cellsarise early in ontogeny, produce mainly IgM antibodies for secretion that are encoded by germline anti- body genes, and mature independently of the bone marrow. These cells gener- ally recognize multimeric sugar/lipid antigens of microbes and are T-cell-independent (T-I), that is, they do not require T cell help in order to proliferate and differentiate in response to antigen.
B2 cells are the conventionalB cells primarily responsible for the development of humoral (antibody)-mediated immunity. They are produced in the bone marrow, and are T-cell-dependent (T-D); that is, they require T cell help in order to proliferate and differentiate in response to antigen. B2 cells eventually give rise to plasma cells that produce IgG, IgA and IgE antibodies.
Although B cell responses to most antigens require T cell help, activation of B cells by certain antigens does not. For the most part, these B1 cells recognize and respond to T-independent (T-I) antigens and produce primarily IgM anti- bodies of low affinity, whereas T-dependent (T-D) antigens generate much higher-affinity antibodies of the other classes. T-I antigens are of two types.
Type 1 antigens
Bacterial polysaccharides have the ability, at high enough concentration, to activate the majority of B cells independently of their specific antigen receptors. They do this through a mitogenic component that bypasses the early biochemical pathways initiated through the antibody receptor. The B cell focuses the polysaccharide antigen and at sufficiently high concentrations drives its activation (Fig. 1).
Type 2 antigens
Some linear antigens that are not easily degraded and have epitopes spaced appropriately on the molecule, e.g. pneumococcus polysaccharide, can directly stimulate B cells in a T-cell-independent fashion. These antigens persist on the Thymus-
independent (T-I) antigens Two kinds of B cells
E2 – B cell activation 103
B Anergy
(a) Type I (T-I) (b) Type II (T-I)
IL-1 (2) (1) (1)
(2)
Soluble antigen (signal 1)
Fig. 1. Activation of B cells through T cell independent antigens. Soluble antigen interaction with the B cell antigen receptor (antibody) results in anergy (signal 1 only). Signal 2 is provided by a mitogenic component (arrow) of the type I antigen (a) and via autocrine activity of IL-1 (arrow) for type II antigen (b).
surface of splenic marginal zone and lymph node subcapsular macrophages and directly stimulate B cells through cross-linking of their surface receptors (Fig. 1). Although activation is independent of T cells, cytokines produced by T cells can amplify these responses.
The production of antibody to most antigens requires the participation of T cells. In particular, Th cells induce B2 cells to proliferate, differentiate and produce antibodies. In addition, Th cells induce switching of the class of anti- body being produced and affinity maturation. To accomplish this, Th cells produce critical cytokines and directly engage the cognateB cell and trigger its activation through cell surface receptors. This T cell collaborationwith B cells is necessary since binding of most (non-multimeric) antigens to antigen receptors on most B cells provides one signal that, in the absence of a second signal, is an anergic signal, i.e. turns off the B cell. The cytokines produced by the Th cells and the engagement of complementary surface molecules provide the essential second signals to the B cell resulting in its activation (Fig. 2).
More specifically, Th cells recognize antigenic peptides on the surface of antigen-specific B cells because the B cells are able to capture antigen specifi- cally via membrane (m)IgM and mIgD. This feature of B cells, to capture, process and present specific antigen, makes them unique amongst the antigen- presenting cells which normally take up antigen via scavenger and other recep- tors (Topic B3). The antigen is then endocytosed, degraded via the exogenous processing pathway and peptides are associated with class II MHC molecules (Fig. 4; Topic F2). Th cells whose TCR are specific for that peptide–MHC complex, recognize and bind to B cells via TCR–MHC interactions and through engagement of adhesion molecules.
Thymus-
dependent (T-D) antigens
104 Section E – The antibody response
Th B
(1) TCR
MHC class II (2)
Cytokines (2)
CD154 CD40
Fig. 2. T cell activation of B cells. T cells provide the 2nd signal to B cells via ligation of CD40 by CD 154 (CD40 ligand) but also via cytokines.
Th B
MHC II
Antigen capture Processing Presentation
Fig. 3. Activation of B cells through T cell help. Captured soluble antigens are processed and presented to Th cells which provide the 2nd signal required for B cell activation.
Once triggered via the TCR, the Th cell expresses CD40 ligand (CD40L), the ligand for the B cell surface molecule CD40. This Th cell now triggers the acti- vation of the B cell via the CD40 surface receptor (Fig. 4). As a result, the acti- vated B cell reciprocally co-stimulates the Th cell via CD28. At this time, both the T cell and B cell are stimulated. T cells then produce cytokines including IL- 2 (autocrine growth factor for the Th cells) and IL-4 and IL-5 (growth and differentiation factors for the activated B cells). As a result, both the T cell and B cell clonally expand and differentiate. Ligation of B cell CD40 by CD40L on T cells is also important in that it rescues B cells from death in germinal centers (Topic E3).
E2 – B cell activation 105
Th B
B7 CD28
CD40 CD40L
IL-2
, IL-4, IL-5,IL-10, I FN
-γ
Cytokines Cytokine
receptors
T cell signaling of the B cell B cell signaling
of the T cell
B cell growth and differentiation T cell growth and differentiation Fig. 4. Reciprocal activation of T and B cells.
The transmembrane surface immunoglobulin antigen receptors on B cells, like the TCR on T cells have short intracytoplasmic tails unable to transduce signals themselves. Therefore, on engagement of the B cell antigen receptor, other molecules associated with these receptors mediate signaling. In particular, CD79a and b of the B cell receptor complex (Topic E1) contain ITAMs, which are phosphorylated during the early stages of activation and initiate the B cell signaling cascade. Other members of the B cell receptor complex (Fig. 5) modu- late the initial signals mediated through antigen binding and enhance the strength of cell–cell interaction. For example, CD21, a complement receptor that binds C3d, may provide an additional positive signal in B cell activation as a result of binding to C3d associated with antigen. As with T cell activation, these processes in B cells are mediated through phosphatases and kinases. The impor- tance of one kinase, btk, is indicated by the observation that mutation of the gene encoding it results in the absence of B cells (Bruton’s agammaglobulin- emia: Topic J2).
B cells like T cells require two signals for their activation. Binding of soluble antigen to the antibody receptor alone (signal 1) results in apoptosis. This is seen experimentally using antibodies to the sIgM on B cells. Proliferation is induced in the presence of a second signal that for B cell activation is provided by interaction of Th cell CD40L with CD40 on the B cell surface. This is also a requirement for class switching. Other cytokines produced by Th cells induce appropriate signals important to differentiation of the B cells into plasma or memory cells (Fig. 6). After initial B cell activation and following class switch- Biochemical
events leading to B cell activation
106 Section E – The antibody response
Nucleus Antigen Antigen
IgG
C3d (breakdown product of C3b)
CD21 CD19
CD32
CD79b CD79a
Kinases (tyrosine/serine)
‘Signaling cascade’
Positive
(Enhanced activation) Negative
(Reduced response)
Fig. 5. Activation of B cells via the BCR and co-receptor complex. Attachment of antigen results in activation of several kinases and the intracytoplasmic chains of Igaand Igbare phosphorylated on their ITAMs. Binding of CD21 to antigen bound to complement (C3d) regulates signaling via CD19 in a positive way, whilst interaction of CD32 with IgG antibody bound to antigen and the antigen receptor, provides a negative signal.
Th B
B
B
B Memory cell
Plasma cell
?
IL-2/IL-4 IL-6 Class
switching Cytokines
Fig. 6. The roles of cytokines in maturation of B cells into memory and plasma cells.
ing to IgG, B cells are susceptible to regulation by concomitant binding of FcγRII (CD32) and the B cell antigen receptor. In particular, further activation of these cells can be inhibited by their binding of specific antigen attached to IgG.
As a result, CD32 transmits a negative signal to the B cell and prevents its activation (‘negative feedback’: Topic G4).
T-I antigens, which do not induce IgG responses (since their CD40 molecules are not ligated), receive their second signal via the ‘mitogenic’ component of T-I type I antigens. The second signal via T-I type 2 antigens is through binding of repeating antigenic units by BCR, which leads to clustering of co-receptors. In these cases, the signals transduced are quite different from those resulting in activation of T-D B cells.
E2 – B cell activation 107
Section E – The antibody response