The leukemic cells from 95% of patients with B cell chronic lymphocytic leukemia (CLL) coexpress B cell differentiation antigens and the pan-T lymphocyte surface antigen CD5 (Leu 1). As such, CLL generally may be considered a malignancy of the CD5 B cell, a minor B cell subpopulation implicated in the production of autoantibodies. Recent data indicate that CD5+ leukemic cells may express autoantibody-associated V region genes with little or no somatic mutation. We examined the Heavy chain V genes expressed by an unusual CLL that secretes rheumatoid factor (RF) autoantibodies but does not express the CD5 surface Ag. Nucleic acid sequence analyses of the rearranged VH genes of three independent rDNA clones demonstrated intraclonal diversity not apparent in previously studied cases of CD5+ CLL. Comparison of the rearranged VH genes reveals that they belong to the VH4 gene subgroup and share highest homology with a rearranged VH gene (Ab44) that encodes a polyreactive autoantibody. That these productively rearranged VH genes encode the RF generated by this unusual CLL population is demonstrated by immunoblotting of the RF paraprotein using primary sequence dependent antipeptide antisera. These results indicate that CD5- CLL, like their CD5+ counterparts, may produce RF. However, unlike CD5+ CLL examined to date, CD5- CLL may have intraclonal diversity in their expressed Ig genes.

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