Molecular analyses of VH and VL genes used by 7 cases of Primary Effusion Lymphomas (PEL) disclose the presence of somatic mutations indicative of antigen selection.

Abstract

99 Background: PELs are an uncommon subset of AIDS related lymphomas that grow mainly in the body cavities as lymphomatous effusions. Because of the presence of a clonal rearrangement of the Ig genes these lymphomas are classified of B lineage origin although they lack of surface B cell antigens. To date no data are available on the sequences of these rearranged genes. Methods: We have sequenced the VH and VL genes used by 4 cell lines (HBL6, BC1, BC2, and BC3) derived from PEL and from 4 biopsy samples. HBL6 and BC1 were derived from the same patient though at different times. Results: VH gene usage among the samples was the following: 3 cases used a VH3 family gene (two the 3-23 gene and one the 3-73 gene), 2 cases employed a VH4 gene (both 4-39), 1 case used a VH1 gene (1-03) and 1 a VH5 gene (5-51). The malignant cells expressed a JH5 (2 cases), JH6 (4 cases) segments, and quite strikingly JH4 (the most common JH) was expressed only by one case. The D segments were not clearly assignable except than in two cases in which stretches deriving from the D2-15/D and from the D5-18 genes were identified. Indeed in these two cases the CDR3 length was longer than in the others (16 and 20 aa respectively against a general average of 13 aa). A productive rearrangement of a Lambda light chain was found in 6 out of 7 samples. The only sample having a productive rearrangement was the cell line HBL6 (and BC1). Among the Lambda genes the VL3 family was used predominantly (4 out of six samples). In all instances but one (BC3 cell line), the comparison of the VH and VL sequences with a database discloses significant nucleotide differences with the presumptive germline. We analyzed the pattern of these mutations using the classical R:S ratio and using a statistical analysis proposed by Chang and Casali. Both these analyses disclosed that the mutations present in the antigen receptors were compatible with antigen selection either at VH or VL level in all the samples. Conclusions: The presence and the pattern of the point mutations at VH and VL level in the cases reported suggest that these were "experienced" cells that had undergone stimulation/selection by antigen(s). Overall these data suggest that (i) PEL frequently derives from memory cells possibly of GC- or post-GC B cell and (li) antigen induced stimulation and selection may be part of the pathogenic origin of these lymphomas.