Distinct Autoreactive T Cell Responses to Native and Fragmented DNA Topoisomerase I: Influence of APC Type and IL-2

Abstract

Systemic sclerosis (SSc) is an autoimmune connective tissue disease of unknown etiology in which T cell responses to various autoantigens, including DNA topoisomerase I (Topo I), have been implicated. We investigated whether dendritic cells, generally considered to be the most potent APCs for the initiation of immune responses, would present either of two forms of Topo I to T cells more efficiently than PBMC APCs. Using cells from healthy controls and SSc patients, several important observations were made. First, neither APC type was able to initiate T cell proliferative responses to full-length native Topo I unless exogenous IL-2 was added. This is in contrast to vigorous T cell proliferation in response to Topo I polypeptide fragments presented by either APC type. Second, T cell responses to the full-length form of Topo I presented by dendritic cells were considerably lower than responses to Ag presented by PBMC APCs. Finally, no secondary T cell responses were observed unless the same Ag/APC combination as that used in the primary stimulation was maintained. These data indicate that different peptides are generated based upon the form of the Topo I and the APC that processes it. Taken together, these results suggest that a very specific combination of antigenic form and APC may be involved in breaking tolerance to Topo I in the early stages of development of SSc.

Critical Requirement for Professional APCs in Eliciting T Cell Responses to Novel Fragments of Histidyl-tRNA Synthetase (Jo-1) in Jo-1 Antibody-Positive Polymyositis

“…In fact, comparison of T cell proliferation induced by fragment A or full-length Jo-1 reveals that, in a number of responders, the absolute value of [ 3 H]thymidine incorporation for fragment A (after subtraction of the MBP background) greatly exceeds that of the full-length protein-but only when DCs are used as the APC source. These results parallel findings in topoisomerase I Ab-positive scleroderma in which the relative response to fulllength topoisomerase I and fragments thereof varies dramatically with the type of APC (32). More broadly, this finding is consistent with the concept that certain autoimmune diseases may be initiated by aberrant cleavage of proteins that are normally nonimmunogenic (33)(34).…”

Involvement of lysosomal cathepsins in the cleavage of DNA topoisomerase I during necrotic cell death

“…There is evidence that autoantibody responses to topo I in SSc are driven by cryptic determinants and that a very specific combination of fragmented forms of topo I and the type of APCs that process these forms may be involved in breaking tolerance to topo I in the early stages of development of this disease (1,2,7,41). It is conceivable that immunogenic fragmented forms of topo I could arise in vivo via caspase-or cathepsin-mediated cleavage of the protein during apoptosis or secondary necrosis.…”

The role of the acquired immune response in systemic sclerosis

“…Most studies pursuing the detection of topo-1-specific T cells in the peripheral blood of SSc patients have been limited by poor clinical characterization of involved subjects and by the existence of technical challenges to detect ex vivo lowfrequency antigen-specific T cells, particularly in patients receiving immunosuppression. In fact, in many cases, the detection of autoreactive T cells has shown poor specificity as they were found also in anti-topo-1-negative patients as well as healthy controls [58, 81,88,112]. More recently, a close temporal association between the onset of SSc and the detection of cancer has been described in a subset of patients positive for anti-RNA polymerase III antibodies [101].…”