Variable region gene analysis of an isotype-switched (IgA) variant of chronic lymphocytic leukemia

Chronic lymphocytic leukemia of B-cell origin (B-CLL) is generally thought to arise by neoplastic transformation of B lymphocytes, which express CD5 and have features of an early stage of B-cell differentiation. To study isotype-switched B-CLL as a potentially more differentiated variant, we performed genetic and functional immunoglobulin (Ig) gene analysis in two cases of CD5+ B-CLL in which the peripheral blood mononuclear cells (PBMC) secreted predominantly IgA (CLL-249) or IgG (CLL-412) when stimulated with pokeweed mitogen in vitro. By cDNA sequencing and by studies of CLL-heterohybridomas, CLL- 249 expresses the heavy chain constant region C alpha as anticipated, while CLL-412 expresses C mu, not C gamma. In CLL-249, the expressed VH gene is 98% homologous to VH26, a germline VH3 gene that occurs frequently in the fetal repertoire, and which has been associated with anti-DNA specificity. The VL gene of CLL-249 is a lambda VL gene for which the germline sequence is not known. In CLL-412, the VH gene is 100% homologous to the VH1 gene of a published anti-DNA antibody (21/28), and is probably a germline gene sequence; the VL gene is 100% homologous to 15AVKI, also a germline gene. The supernatant antibody of the CLL-412 heterohybridoma is an IgM-kappa, which reacts with ssDNA and cardiolipin. The CLL-249 heterohybridoma secreted IgA-lambda, which bound none of the antigens tested, a finding that may be related to amino acid differences from the probable germline V genes. The demonstration of an in vivo isotype-switched variant, such as CLL-249, suggests that B-CLL may be a heterogeneous group of clonal disorders, of which less common variants may have features of more differentiated B-cell stages, such as isotype switching.