Autoreactive T cells to topoisomerase I in monozygotic twins discordant for systemic sclerosis

Objective. To examine T and B cell responses to topoisomerase I (topo I) in a monozygotic twin pair discordant for systemic sclerosis (SSc). Methods. The peripheral blood T cell proliferative responses induced by top o I and in vitro anti-topo I antibody production in cultures of T and B cel ls were examined in an SSc patient with serum anti-topo I antibody and in h er healthy monozygotic twin. Topo I-reactive T cell lines were generated fr om the twin pair and analyzed for antigenic specificity, major histocompati bility complex class II restriction, and T cell receptor (TCR) gene usage. Results. T cell proliferative responses to topo I were detected in both the SSc patient and her healthy twin, although the kinetics of the T cell resp onse were accelerated in the patient compared with the healthy twin. The es timated frequency of circulating topo I-reactive T cells was 1/6,700 in the SSc patient and 1/39,000 in the healthy twin. Anti-topo I antibody product ion was observed in cultures of T and B cells from the SSc patient, but not in those from the healthy twin. When the cells from the twins were mixed i n different combinations, T cells from the healthy twin did stimulate the S Sc patient's B cells to produce anti-topo I antibody, through a CD40-depend ent mechanism. Topo I-reactive T cell lines generated from the twins had si milar characteristics, including a CD4+ phenotype, restriction by HLA-DR, r ecognition of epitopes within amino acid residues 209-386 of topo I, and do minant usage of the TCR V(beta)20 gene segment. Conclusion. These results indicate that topo I-reactive T cells were activa ted and clonally expanded in the SSc patient. However, there were no substa ntial differences in either phenotypic or functional properties of topo I-r eactive T cells obtained from the SSc patient and those obtained from her h ealthy identical twin. It is likely, therefore, that the anti-topo I antibo dy response in the SSc patient is induced by in vivo activation of topo I-r eactive T cells derived from the normal T cell repertoire.