Antibody diversity. New mechanism revealed

Epigenetic Codes Programing Class Switch Recombination

Class switch recombination imparts B cells with a fitness-associated adaptive ­advantage during a humoral immune response by using a precision-tailored DNA excision and ligation process to swap the default constant region gene of the antibody with a new one that has unique effector functions. This secondary diversification of the antibody repertoire is a hallmark of the adaptability of B cells when confronted with environmental and pathogenic challenges. Given that the nucleotide sequence of genes during class switching remains unchanged (genetic constraints), it is logical and necessary therefore, to integrate the adaptability of B cells to an epigenetic state, which is dynamic and can be heritably modulated before, after, or even during an antibody-dependent immune response. Epigenetic regulation encompasses heritable changes that affect function (phenotype) without altering the sequence information embedded in a gene, and include histone, DNA and RNA modifications. Here, we review current literature on how B cells use an epigenetic code language as a means to ensure antibody plasticity in light of pathogenic insults.

Analysis of the structural diversity of monoclonal antibodies to cyclosporine

The immunosuppressive cyclic undecapeptide cyclosporine (Cs) represents a useful model for studying the molecular basis of antibody-antigen interactions. The three-dimensional structure of the Cs molecule is known and a large panel of monoclonal antibodies (mAbs) to Cs has been well characterized by cross-reactivity studies with numerous Cs analogs. In the present study, the sequences of the variable regions of seven mAbs to Cs were determined and a striking relationship was found between the expressed variable region genes and the Cs recognition pattern. An analysis of the length and hydrophobic content of the hypervariable regions and sequence similarities suggested that the heavy chain plays a major role in Cs recognition. Different fine specificities were observed for mAbs exhibiting identical light chains, while two antibodies differed by only a single amino acid located in the heavy chain. The presence of a duplication of 12 nucleotides within the heavy chain third hypervariable region of two antibodies suggests the existence of an additional mechanism for creating antibody diversity.

Point mutations cause the somatic diversification of IgM and IgG2a antiphosphorylcholine antibodies

The genetic mechanism responsible for the somatic diversification of two mAbs was determined. The two PC-binding hybridomas were representative of events early and late in the immune response. The P28 cell line that produces an IgM antibody and thus represents events early in the immune response, was found to have 3 bp changes in its heavy chain variable (VH) region, with some changes in antibody affinity or specificity. The RP93 cell line that produces an IgG2a antibody and thus represents later events in the immune response, was found to have 9 bp changes in its VH region resulting in decreased affinity for PC and altered specificity. Oligonucleotides specific for linked base changes in the second hypervariable regions of both of these antibodies were used to look for previously undescribed V regions or other donor sequences that could have been responsible for these base changes. Since no donor sequences were found, we have concluded that somatic point mutation rather than gene conversion, V region replacement or the expression of an unidentified germline VH region gene is truly responsible for at least some of the somatic diversification of these antibodies.

Theoretical and practical aspects of B-cell activation: murine and human systems

We have reviewed observations which were made during studies of murine and human B-cell responses in vitro. One currently faces difficulties in drawing any clear schema as to which external signals elicit which responses (activation, proliferation, differentiation) in B cells. However, the most potent antigen-dependent or polyclonal B-cell responses in vitro occur when, in addition to various cytokines, accessory cells, serum etc., the cultures contain either a) intact T-helper cells which enter into cell-to-cell contact with B cells, or b) some B-cell "mitogen" (T-independent antigen). Murine B cells activated with LPS and anti-Ig antibodies represent a model for the study of IL2 receptor expression and function. LPS does not act on human B cells. Certain mutant EL-4 thymoma cells are potent activators of murine and human B cells via a direct cell-to-cell interaction. The majority of human B cells can be induced to proliferate and generate a few hundred antibody-secreting cells each in the presence of such thymoma cells and a mixture of cytokines. From a practical point of view, this observation should be useful in a variety of investigations such as the analysis of the human B-cell specificity repertoire.

Limiting dilution assay for human B cells based on their activation by mutant EL4 thymoma cells: total and antimalaria responder B cell frequencies

The analysis of human B cell responses at the clonal level (limiting dilution assay) is still technically difficult. In the present study we report on a culture system that leads to activation, proliferation and differentiation into antibody-secreting cells (ASC) of about 90% of B cells from peripheral blood or spleen. In this system, B cells are cultured in the presence of a mutant subclone of the mouse thymoma EL4 for B cell activation and human T cell plus macrophage supernatant as source of proliferation and differentiation factors. ASC precursors generating clonal responses of IgM only, IgM plus IgG, or IgG only occurred at a ratio of about 6:3:1. The mean clone size was 380 cytoplasmic Ig+ cells; the mean amount of Ig secreted per clone was 20 ng. Furthermore, it has been found using this system that a considerable proportion of peripheral blood B cells from individuals with a history of malaria infection could generate clones of anti-malaria (Plasmodium falciparum) ASC (range of 0.1 to 1%, n = 6). In a control group of blood donors the corresponding frequencies were 10 times lower (range of 0.01 to 0.1%, n = 9). These results show that the EL4 culture system can be applied to the investigation of the human B cell specificity repertoire and of priming effects such as result from infectious disease.