Polyreactive Monoclonal Antibodies Synthesized by Some B-CLL Cells Recognize Specific Antigens on Viable and Apoptotic T Cells.

B-cell chronic lymphocytic leukemia (CLL) is a disease characterized by uncontrolled clonal expansion of a CD5⁺/CD19⁺ B cell. Clones from different patients often express receptors/mAbs with remarkably similar VHDJH rearrangements, suggesting that the same antigen(s) or epitope(s) led to the initial clonal expansion. Although the specific antigens and epitopes remain to be identified, recent data indicate that B-CLL mAbs often bind autoantigens. In certain systemic autoimmune diseases, autoantibodies can react with cell surface molecules of viable lymphocytes (anti-lymphocyte mAbs) and with intracellular molecules of apoptotic cells that are transported to the cell membrane and function as autoantigens. Therefore, we expressed recombinant antibodies from B-CLL cells as IgG1 mAbs and tested them by flow cytometry for immunoreactivity with surface molecules on irradiated (apoptotic) and non-irradiated Jurkat cells, a human T-cell line. We chose to use T cells as our target for analysis to avoid difficulties that might arise from binding to FcgRIIb on B lymphocytes.

Initial screening indicated that 5 of 25 mAbs showed intense binding. Antibodies from CLL Nos. 141 and DO8 bound to live but not apoptotic cells; in contrast, CLL Nos. 014, 114, and DO13 bound to apoptotic but not live cells. These five antibodies express unmutated VH genes. CLL 141 and DO8 express VH4-34. CLL 014 expresses VH1-69, CLL 114 VH4-39 and DO13 VH1-02. Other antibodies expressing VH1-69 in its unmutated form also bound Jurkat cells, albeit weaker than CLL 014, whereas mAbs expressing different unmutated genes from the VH1 family did not react. Among the mAbs expressing VH1-02 that were tested, only DO13 showed reactivity. This indicates that the VH1-02 gene was not solely responsible for the observed binding, and that DJ and/or VLJL segments were also involved. Remarkably, none of the tested mAbs belonging to VH3 family recognize antigens on Jurkat cells; all of these VH3 mAbs were mutated. Only CLL 114 belongs to a previously identified stereotypic set. Of interest, although CLL 014 and 141 are polyreactive, binding ssDNA, dsDNA, insulin, LPS, and HEp-2 cell lysates, their reactivities with surface antigens were specific in that CLL 141 bound only live cells and CLL 014 reacted solely with apoptotic cells.

Our results indicate that, despite their inherent polyreactivity, certain B-CLL mAb bind specifically to antigenic epitopes on the surface of live as well as apoptotic Jurkat cells. The identity of these autoantigens is currently being investigated. These data highlight the importance of autoreactivity in the promotion and evolution of B-CLL.