This study represents the first direct comparison of SSc and SLE on the same platform. The results confirmed previously published data indicating possible similarities in increased expression of IFN-inducible genes in PB cells from SSc patients and SLE patients (4
). Importantly, we demonstrated that there appears to be a gradient effect, with the magnitude of IFN-inducible gene activation strongest and most prevalent among SLE patients, followed by SSc patients. Qualitatively, the overall pattern of IFN-inducible gene activation was strikingly similar between SSc and SLE.
Our data showed increased expression of IFN-inducible genes activated by type I and type II IFNs in PB cells. Based on the IFN score, it is not clear which type of IFN signaling predominates (8
). Further studies at the protein level would be required to validate this observation. Nonetheless, it is interesting to note that a randomized, placebo-controlled trial of subcutaneous IFNα in early SSc showed that IFNα treatment resulted in significant worsening of lung function and a trend toward skin deterioration (30
). Type I IFNs, in particular, have been implicated in the pathogenesis of SLE (31
). The plasmacytoid dendritic cells (PDCs) are thought to be one of the main sources of type I IFN (33
). IFNs lead to production of autoantibodies by their effects on B cells, and in the setting of antigen excess this leads to formation of immune complexes containing DNA or RNA. In vitro studies have shown that the nucleic acid–containing immune complexes can lead to self-perpetuating, excessive IFN production (26
). Though it has yet to be demonstrated in vivo, this process is thought to contribute to development of SLE in a susceptible host. Anecdotal reports of development of SLE and SSc in patients who were undergoing IFN treatment for other conditions lend some support to this notion (38
In our study, 89% of SLE patients demonstrated up-regulation of IFN-inducible genes compared with 77–93% in other studies (8
). Likely explanations for this high percentage are that the SLE patients were not receiving immunosuppressive agents and had high disease activity (average SLAM-R score 11.19) at enrollment. This is in agreement with observations that the presence of the IFN signature in SLE correlates with disease activity, renal involvement, and anti-dsDNA and complement levels (9
We did not observe differences in the PB cell transcript profiles or IFN score in relation to SSc disease duration. These observations are consistent with a previous report that the IFNα-inducing activity of SSc sera does not correlate with disease duration (41
). We also could not demonstrate any correlation of transcript profiles with various clinical manifestations of SSc or its disease activity. The latter is hampered by the lack of a standardized disease activity measure in SSc. The assessed clinical manifestations represent more accurately the irreversible disease damage rather than disease activity. This implies that a transient change in gene expression pattern correlating with the development of various clinical manifestations could have been missed in our cross-sectional design. Longitudinal studies are needed to investigate the predictive role of the IFN signature for development of various clinical complications of SSc.
Our data demonstrated that the presence of antitopoisomerase and anti–U1 RNP antibodies are associated with a higher IFN score. Furthermore, we were able to subgroup the SSc patients at the transcript level based on the presence of an IFN-inducible gene pattern. These data suggest that there is a gradient in the IFN signature among the serologic subtypes of SSc that could have implications for future studies of pathogenesis and therapeutic targets. Indeed, Kim et al demonstrated that >90% of antitopoisomerase-positive SSc sera had IFNα-inducing activity (41
). This activity was contained in the IgG fraction. Furthermore, the anti–topoisomerase I levels correlated with IFNα induction in that study. Moreover, another study showed that the serologically heterogeneous group of ACA-negative SSc patients had higher levels of IFN-inducible gene expression (7
We also observed a similar IFN signature in skin biopsy samples from SSc patients, but we did not find increased IFNα or IFNβ messenger RNA (mRNA) levels in PB cells from SSc or SLE patients, suggesting that cells may be responding to local IFNα at the target tissue level. In another study, increased levels of IFNα mRNA and PDCs were reported in the skin tissue of patients with diffuse SSc. An IFN-inducible gene pattern in the monocytes and T cells of a separate group of patients was also seen, but the corresponding sera did not have detectable IFNα that correlated with the IFN signature (6
). A study by Milano et al (42
) examining skin biopsy transcript profiles of SSc patients showed that a subgroup consisting of patients with limited and diffuse disease type showed an “inflammatory profile.” Although a correlation with different serologic subtypes of SSc was not conducted, it is notable that the samples classified as inflammatory had overexpression of multiple IFN-inducible genes as well.
We also found that the IFNAR2 rs7279064 SNP was significantly associated with a higher IFN score in the SSc patients, indicating that this polymorphism plays a role in the variance of IFN activity seen in this disease. Our genotype correlation study was hampered by our relatively small sample sizes and the difficulties in correcting for the effect of ethnicity on gene expression. Further studies are needed to investigate the effect of various SNPs on the expression of IFN-inducible genes.
In summary, our data show that SSc and SLE may belong in the same spectrum of IFN-mediated diseases. A subset of SSc patients has a “lupus-like” high IFN-inducible gene expression pattern that correlates with the presence of antitopoisomerase and anti–U1 RNP antibodies. The classification of SSc based on the presence of the IFN signature can provide opportunities for better understanding of its pathogenesis and for development of targeted therapeutic interventions.