Our study points to the critical role of TNFα in SSc pathology. First, it was the only cytokine higher in that disease; furthermore, all who developed cancer in 5-year follow-up had the highest levels of TNFα at baseline. Eventually, AG and GC rs361525 genotypes of TNFA gene polymorphism predicted higher serum TNFα and simultaneously a higher frequency of cancer.

TNFα is a pro-inflammatory cytokine released by various immune cells, including B and T lymphocytes, macrophages, neutrophils, fibroblasts, and NK cells. Its primary function is to protect against malignancies and infections [43]. It is also considered a fibrogenic cytokine, along with IL-1β, transforming growth factor beta, platelet-derived growth factor, and fibroblast growth factor. In SSc, elevated levels of those cytokines may contribute to the proliferation of fibroblasts and endothelial cells, leading to connective tissue and endothelium alterations, a primary hallmark of the disease [44]. Indeed, in our previous reports, we demonstrated that SSc patients present with endothelial dysfunction, prothrombotic state and increased oxidative stress [2, 45, 46].

Our results regarding TNFα are consistent with other research findings. Hagesawa et al. [30] conducted a study in 51 patients with limited cutaneous SSc and 30 patients with diffuse cutaneous SSc, showing increased serum TNFα levels associated with the diagnosis of pulmonary fibrosis. Kantor et al. reported similar results in 30 patients with definite SSc, noting higher spontaneous release of TNFα in patients with early diffuse cutaneous disease [31]. Our study found no relationship between TNFα and organ involvement, possibly due to the limited number of patients.

However, we found an association between the TNFA polymorphisms rs1799964 and rs361525 and cancer susceptibility in SSc patients without any other correlations to the laboratory and clinical characteristics. In turn, as we found in this report, AG and GC rs361525 genotypes may be related to higher serum TNF alpha, leading to increased cancer risk in this mechanism.

TNF α has a dualistic relation to malignancies. On the one hand, it acts as an antitumoral cytokine, but on the other, chronically elevated levels in low concentrations can promote tumor growth [47], which is consistent with our findings. Laboratory murine cancer models suggest that TNFα promotes malignancy by influencing both initiated cancer-developing cells and inflammatory cells in the microenvironment [48]. In clinical studies, elevated TNFα levels have been found in the blood of patients with renal, pancreatic, breast, and prostate cancers, interestingly in correlation with disease advancement and poor survival [43, 49]. Furthermore, it has been shown that high doses of recombinant TNFα are effective for treating melanoma [50] and soft tissue sarcomas [51] in isolated limb perfusion, where the agent penetrates local metastases but not the circulation. On the other hand, systemic administration of high TNFα doses has been associated with significant side effects, including hypotension and multiple organ failure [52]. On the contrary, an ongoing clinical trial exploring the removal of soluble TNFα receptors from the plasma of patients with advanced, refractory breast cancer using immunopheresis provides encouraging results [53].

TNFα inhibitors, such as infliximab, adalimumab, certolizumab, and etanercept are approved for treating chronic inflammatory diseases [54]. In experimental settings, those agents reduced collagen production and fibrosis and decreased vascular endothelial growth factor (VEGF) release, a proangiogenic factor associated with pulmonary arterial hypertension [54]. However, the clinical use of TNFα inhibitors in SSc needs to be evaluated in randomized clinical trials, as no ongoing trials have been reported to date. Encouragingly, there are case reports of successful infliximab administration in SSc patients who were refractory to conventional therapies [55, 56].

Another issue that merits comment in our data is the role of IL-17 in SSc prognosis. Surprisingly, patients with higher IL-17 in serum had an increased risk of death in a 5-year follow-up, representing a crucial finding of our study. Remarkably, levels of IL-17 and FAP were higher in patients on oral steroids, representing likely the more severe cases. Furthermore, higher IL-17 concentrations were associated with lower LVEF values, which also independently predicted, together with elevated CRP and PASP, increased mortality rate in multiple regression models.

Regarding IL-17, this pro-inflammatory cytokine is primarily produced by the Th17 cell subset in response to TGF-β and IL-23 and plays an essential role in host-defense mechanisms [25, 57, 58]. In SSc, IL-17 stimulates endothelial and epithelial cells and fibroblasts to secrete immunomodulatory factors [13, 25, 57]. IL-17 may play a role in autoimmune inflammatory diseases, such as systemic lupus erythematosus (SLE) [59]; however, data on its inhibition by mononuclear antibodies in that disease were ambiguous [60, 61].

Nevertheless, our results suggest that in SSc, IL-17 overproduction might worsen clinical prognosis, which has also been documented by Wei L et al. [25]. So far, no other studies with similar conclusions could support our findings. However, secukinumab, an approved IL-17 inhibitor for treating chronic inflammatory diseases, potentially ameliorated dermal fibrosis in mouse models of bleomycin-induced fibrosis [62]. It is noteworthy that IL-17 inhibitors in SSc are not, so far, approved by regulatory agencies, but a randomized clinical trial involving brodalumab has been undertaken (currently no longer recruiting) [13]. Nonetheless, a lack of published data exists regarding administering IL-17 inhibitors in SSc patients.

Our data indicate that the CT variant of rs1799964 in the TNFA gene may be associated with a predisposition to higher serum IL-17 levels, suggesting possible cross-talk between TNFα and IL-17 regulation. That is also a novel and exciting finding of our study. Additionally, an intriguing observation regards the potential influence of steroid administration on elevated IL-17. That raises an important question on the impact of chronic steroid therapy on the SSc prognosis. However, we are not able to exclude that steroid-treated patients were more severely ill and mutually had higher IL-17 levels.

The main limitation of our study is the low number of SSc patients, which may impact genetics the most. However, we used Hardy–Weinberg equilibrium with linkage disequilibrium to overcome that limitation. Furthermore, SSc is a rare disease, and each observation is valuable.