Structure and Function of the B Cell Receptor

The IgG or Antibody molecule is a large, roughly ‘Y’ shaped molecule, composed of two types of polypeptide chains, a 50KDa heavy (H) chain and a 25KDa light (L) chain. Each IgG molecule consists of two heavy chains and two light chains. The heavy chains are linked to each other by disulphide bonds and each heavy chain is linked to a light chain by disulphide bonds. The two heavy chains and two light chains of each IgG molecule are identical to each other.

The light chain is made up of two domains, each folded into a structure known as the immunoglobulin fold. The immunoglobulin fold is a structure that is mimicked by many other molecules. It essentially consists of two sheets of anti-parallel β-strands folded in a Greek-Key motif and sandwiched together.

The variability in the sequence of the light chain is mainly limited to the first domain, also known as the Light chain Variable domain (VL). The other domain is known as the Light chain Constant domain (CL). The heavy chain is made up of one Variable domain (VH) and three Constant domains (CH1, CH2 and CH3).

The Variable Domains of the Light and Heavy chains (VL and VH) fold together to make up the Variable region of the IgG molecule and confer on it its antigen binding specificity. The VL and VH and CL and CH1 domains fold together to make up the Fab or antibody binding fragment of the molecule. The CH2 and CH3 domains of the two heavy chains fold together to form the Fc or crystallizable fragment of the molecule.

Sequence variability is not distributed throughout the Variable domain but is concentrated instead into three hypervariable regions each of the VL and VH domains. These regions are named Hypervariable regions 1 – 3 (HV1, HV2 and HV3). Together, these Hypervariable regions code for hypervariable loops at the tips of the Variable domains. These loops determine the specificity of the IgG molecule and are therefore known as Complementarity Determining Regions (CDR1, CDR2 and CDR3).

IgG has several effector functions including:

• The neutralization of pathogen by coating the pathogen and occupying its bind sites so that it cannot affect cells
• The opsonization of pathogen by marking it for phagocytoses by phagocytes including macrophages
• The recruitment of Complement to lyse pathogen cells

Generation of B Cell Receptor Diversity

The total number of antibodies that humans can potentially produce exceeds 10¹¹. Before the sequences of the immunoglobulin genes were known, there were two theories put forward to explain how this level of diversity could be obtained. The germline theory held that all the possible variations were coded for by separate genes, whilst the somatic diversification theory held that immunoglobulin was coded for by a limited number of sequences which were rearranged throughout an individual’s lifetime. The somatic diversification has proved to be essentially correct.

The vast majority of the diversity of an immunoglobulin molecule lies in the variable domain. The variable domain is coded for by a set of gene segments which are randomly selected and rearranged in a process known as somatic recombination. Further diversification is later introduced when the B cell is activated. This consists of point mutations and is known as somatic hypermutation.

The immunoglobulin molecule is made up of a light and heavy chain. The light chain variable domain is coded for by a set of V gene segments (VL) and a set of joining or J gene segments (JL). The heavy chain variable domain is coded for by a set of V gene segments (VH), a set of diversity of D gene segments (DH) and a set of J gene segments (JH).

The somatic recombination process of the light chain involves the random selection of a VL gene segment which is then joined to a randomly selected JL gene segment. This is catalysed by recombination activation genes (RAGs). VL genes are prevented from accidentally joining to other VL gene segments by the use of recombination signalling sequences (RSS) which consist of a heptamer, a 12 or 23 nucleotide spacer and a nonamer. This is the 12/23 rule and it prevents gene segments with a 12 nucleotide spacer being joined to other 12 spacer segments and gene segments with a 23 nucleotide spacer being joined to other 23 spacer segments. This same process prevents J, D and VH gene segments from self joining. The joining process is not precise and introduces further Junctional diversification.

The variable heavy chain is created by a DH gene segment, randomly selected, joining to a JH segment and then the combination joining to a randomly selected VH gene segment. Junctional diversity is introduced during these joins.

Further combinational diversity is introduced by the different combinations of heavy and light chain variable regions that pair to form the antigen binding site.

Finally upon activation, further diversification is introduced in a process known as somatic hypermutation.

Production of Allogeniec Antibodies

Allogeneic HLA antibodies can be produced in response to exposure to foreign HLA through blood transfusion, pregnancy or transplantation. Allogeneic antibodies are produced in response to foreign HLA when B cell surface immunoglobulin receptors specifically bind the foreign HLA and the B cell is triggered by armed CD4+ T cells into activation and differentiation into antibody secreting plasma cells. The antibodies secreted have the same specificity as the surface immunoglobulin of the activated B cell.

When a B cell immunoglobulin receptor binds to allogeneic HLA, the bound HLA is internalised, processed and presented on the cell surface as peptides bound to HLA class II molecules. The peptide-MHC class II complex can be recognised by antigen specific armed T cells, resulting in activation of the B cells, leading to proliferation and differentiation into plasma cells. The T cells are armed in the direct and indirect pathways of allorecognition by presentation of peptide from the same allogeneic HLA by professional antigen presenting cells. The direct pathway involves donor derived dendritic cells presenting allogeneic HLA peptides to host CD4+ helper T cells. The indirect pathway involves allogeneic HLA being internalised, processed and presented to host CD4+ helper T cells as peptide-MHC complexes by host dendritic cells. Both the direct and indirect routes result in antigen specific activated CD4+ T cells which are capable of providing the second signal that the B cell requires, along with binding of antigen, in order to become activated. The trapping of B cells in the T cell zone of secondary lymphoid tissues raises the probability that the otherwise low frequency of armed T cells of the right specificity would make such as encounter.

T cell are triggered to synthesis and secrete a number of cytokines such as IL-4 and other effector molecules including CD40 ligand which binds the B cell CD40 receptor and helps drive the resting B cell into the cell cycle, by the recognition of the allogeneic HLA peptide in the context of HLA class II on the B cell by the armed T cell triggers.

The activated B cells proliferate for several days before eventually differentiating into Plasma cells capable of secreting antibodies. Plasma cells can have a wide range of life span with some living for only days to weeks but others are long lived and result in persist antibody production.

Allogeneic antibodies produced following transfusion can be IgG or IgM and are not generally long lasting. Antibodies developed as a result of pregnancy or transplantation are however generally IgG and are long lasting.

Other allogeneic antibodies that be produced include anti-HPA antibodies, anti-endothelial cell antibodies and anti-MICA antibodies. Female transplant recipients who subsequently have children are at risk of developing anti-paternal antibodies and need to be carefully managed if these antibodies are also donor specific.