Immunoglobulin somatic variation; studies of receptor editing in a human B cell lymphoma

Self-antigen recognition by follicular lymphoma B-cell receptors

Follicular lymphoma is a monoclonal B-cell malignancy with each patient's tumor expressing a unique cell surface immunoglobulin (Ig), or B-cell receptor (BCR), that can potentially recognize antigens and/or transduce signals into the tumor cell. Here we evaluated the reactivity of tumor derived Igs for human tissue antigens. Self-reactivity was observed in 26% of tumor Igs (25 of 98). For one follicular lymphoma patient, the recognized self-antigen was identified as myoferlin. This patient's tumor cells bound recombinant myoferlin in proportion to their level of BCR expression, and the binding to myoferlin was preserved despite ongoing somatic hypermutation of Ig variable regions. Furthermore, BCR-mediated signaling was induced after culture of tumor cells with myoferlin. These results suggest that antigen stimulation may provide survival signals to tumor cells and that there is a selective pressure to preserve antigen recognition as the tumor evolves.

Known components of the immunoglobulin A:T mutational machinery are intact in Burkitt lymphoma cell lines with G:C bias

The basis for mutations at A:T base pairs in immunoglobulin hypermutation and defining how AID interacts with the DNA of the immunoglobulin locus are major aspects of the immunoglobulin mutator mechanism where questions remain unanswered. Here, we examined the pattern of mutations generated in mice deficient in various DNA repair proteins implicated in A:T mutation and found a previously unappreciated bias at G:C base pairs in spectra from mice simultaneously deficient in DNA mismatch repair and uracil DNA glycosylase. This suggests a strand-biased DNA transaction for AID delivery which is then masked by the mechanism that introduces A:T mutations. Additionally, we asked if any of the known components of the A:T mutation machinery underscore the basis for the paucity of A:T mutations in the Burkitt lymphoma cell lines, Ramos and BL2. Ramos and BL2 cells were proficient in MSH2/MSH6-mediated mismatch repair, and express high levels of wild-type, full-length DNA polymerase eta. In addition, Ramos cells have high levels of uracil DNA glycosylase protein and are proficient in base excision repair. These results suggest that Burkitt lymphoma cell lines may be deficient in an unidentified factor that recruits the machinery necessary for A:T mutation or that AID-mediated cytosine deamination in these cells may be processed by conventional base excision repair truncating somatic hypermutation at the G:C phase. Either scenario suggests that cytosine deamination by AID is not enough to trigger A:T mutation, and that additional unidentified factors are required for full spectrum hypermutation in vivo.

Receptor revision of immunoglobulin heavy chain genes in human MALT lymphomas

BACKGROUND/AIMS

Rearrangement of immunoglobulin gene segments, leading to B cells with functional receptors, is thought to be largely restricted to developing immature B cells in bone marrow. However, accumulating evidence suggests that mature B cells occasionally modify their antigen specificity by VH segment replacements during the germinal centre reaction to enhance antigen affinity, or to overcome self reactive antigen receptors. Although malignant B cells maintain the features of their normal counterparts in most instances, to date, such replacements have not been described for human B cell lymphomas.

METHODS

Rearranged immunoglobulin heavy chain genes from two extranodal marginal zone B cell lymphomas were amplified, cloned, and sequenced. Sequences with identical CDR3 regions were selected and aligned to each other and public databases.

RESULTS

VH replacements were seen in two extranodal marginal zone B cell lymphomas. In line with the hypothesis that in mature B cells these replacements are associated with active somatic hypermutation, in addition to VH replacement, different mutation patterns were seen in the revised VH portions. In the remaining common 3'-VH regions, these mutations could be used to establish a phylogenetic relation between the sequences, rendering the possibility of artefactual chimaeric polymerase chain reaction products very unlikely.

CONCLUSIONS

These results support the view that VH replacements are a further mechanism for reshaping antigen affinity and specificity, and indicate that these receptor modifications are not restricted to normal and reactive germinal centre B cells, but may also occur in close association with the development of malignant B cell lymphomas.

Receptor selection in B and T lymphocytes

The process of clonal selection is a central feature of the immune system, but immune specificity is also regulated by receptor selection, in which the fate of a lymphocyte's antigen receptor is uncoupled from that of the cell itself. Whereas clonal selection controls cell death or survival in response to antigen receptor signaling, receptor selection regulates the process of V(D)J recombination, which can alter or fix antigen receptor specificity. Receptor selection is carried out in both T and B cells and can occur at different stages of lymphocyte differentiation, in which it plays a key role in allelic exclusion, positive selection, receptor editing, and the diversification of the antigen receptor repertoire. Thus, the immune system takes advantage of its control of V(D)J recombination to modify antigen receptors in such a way that self/non-self discrimination is enhanced. New information about receptor editing in T cells and B-1 B cells is also discussed.