The in vitro T cell proliferative response to DNA topoisomerase I (topo I) was examined in 26 systemic sclerosis (SSc) patients with anti-topo I antibody, 10 SSc patients without anti-topo I antibody, and 21 healthy donors. Using recombinant fusion proteins encompassing the entire human topo I amino acid sequence, a topo I-specific proliferative response was detected in PBMC cultures from 25 (96%) anti-topo I-positive SSc patients, 4 (40%) anti-topo I-negative SSc patients, and 13 (62%) healthy donors. Molecular typing at MHC class II loci revealed that all SSc patients and healthy donors having either DRB1*1501,2 (DR15), DRB1*1101,3,4 (DR11), or DRB1*07 (DR7) were responders. Characterization of the topo I-induced T cell proliferative response showed that (a) the responding cells were CD4+ T cells; (b) antigen-presenting cells were necessary for the response; (c) the response was restricted by HLA-DR, and to a lesser extent by HLA-DQ; and (d) the estimated frequency of the responding T cells determined by limiting dilution analysis was 1/9,277-1/24,853. PBMC cultures from anti-topo I-positive SSc patients showed a high T cell proliferative response after only 3 d of culture with topo I. Anti-topo I-negative SSc patients and healthy donors had no proliferative response after 3 d, but did respond after 7 d of culture. T cell proliferative responses to six truncated topo I fragments tested individually showed different patterns of T cell proliferation that were dependent upon the responder's HLA-DR alleles. These results indicate that T cells reactive with topo I are components of the normal T cell repertoire, and that the topo I-specific T cell proliferative response is not associated with the presence or absence of SSc or anti-topo I antibody, but is restricted by MHC class II alleles.