T and B cell collaboration is essential for the autoantibody response to DNA topoisomerase I in systemic sclerosis.

To elucidate the mechanisms controlling anti-DNA topoisomerase I (topo I) antibody production in patients with systemic sclerosis (SSc), in particular the role of interactions between topo I-specific Th cells and B cells, we established an in vitro system for the analysis of anti-topo I antibody production. In vitro anti-topo I antibody synthesis in PBMC cultures was induced by recombinant topo I and PWM, and was measured by a topo I-specific ELISA. Anti-topo I antibody was detected in PBMC culture supernatants from 11 (61%) of 18 anti-topo I-positive SSc patients. In contrast, anti-topo I antibody was not detected in the PBMC culture supernatants from 4 anti-topo I-negative SSc patients or 10 healthy donors. Characterization of in vitro anti-topo I antibody production showed that 1) the anti-topo I antibody isotype produced was IgG; 2) the anti-topo I antibody levels in culture supernatants correlated with those in patients' sera; 3) CD4+ T cells were necessary for antibody synthesis; and 4) antibody synthesis was restricted by HLA-DR, but not by HLA-DQ or DP. In addition, separation of cultured T and B cells by a semipermeable membrane or culture with anti-CD40 ligand mAb blocked in vitro anti-topo I antibody production. These results indicate that a contact-mediated and HLA-DR-restricted collaboration between topo I-specific T and B cells is essential for in vitro anti-topo I antibody production in a subset of SSc patients.