The objective of the current study was to identify distinct cytokine profiles that can discriminate SSc patients based on PAH status. Our data suggest that cytokine profiles can differentiate SSc patients who are at high-risk for or have PAH from SSc patients at low risk for PAH and healthy controls. However, high-risk and PAH patients had very similar cytokine profiles, suggesting that these patients are on a disease continuum.
Similar to prior studies, we found elevated levels of inflammatory mediators in SSc patients with PAH and high-risk for PAH such as TNF-alpha and IL-6 compared to healthy controls.[7,9] There was no difference in TNF-alpha levels when comparing low-risk and healthy patients. However, in the current study, these cytokines did not distinguish PAH and high-risk patients from low-risk patients (Figure 2A and Supplementary Figure 2A). Our study extended these findings and noted prominent increases in pro-inflammatory cytokines including RANTES, IL-12p40, and IFN-Beta, in patients with SSc overall, but most prominently in patients on the PAH spectrum when compared to healthy controls. Changes in these cytokines have been associated with pulmonary hypertension previously.[10,11] Consistent with prior studies, our data also show evidence of vascular dysregulation in the significantly increased levels of VEGF-D in the high-risk and PAH groups compared to low risk and HC groups.[7,8]
A major strength of this study is the comparison of cytokine profiles between an SSc group at low-risk for PAH based on well-established clinical parameters with patients at high risk for or with PAH. Being able to use biomarkers to predict the devastating complication of PAH has the potential to identify patients who may benefit from aggressive screening and early therapy. Although it is unknown whether these low-risk patients may ultimately develop PAH over time, they did not have evidence of disease three years after serum samples were drawn and had similar disease duration compared to SSc patients on the PAH spectrum. Cytokines most prominently differentiating high-risk and PAH patients from low-risk SSc patients were PAI-1, sICAM-1, BDNF, and VEGF-D. PAI-1 and sICAM-1 are cytokines implicated in modulating fibrosis and endothelial cell function and have been shown to have abnormal levels in patients with SSc. BDNF may play an important role in modulating angiogenesis and sympathetic function and there is evidence that hypoxia can increase the levels of BDNF production from pulmonary artery endothelial cells. Consistent with this, we found that BDNF levels were elevated in high-risk and PAH patients compared to low-risk patients. We also found that BDNF levels were lower in low-risk patients compared to healthy controls. This is in line with prior studies that showed BDNF levels in an SSc population enriched with those without PAH was lower than controls.
It is notable that PAI-1, BDNF, and sICAM-1 were significantly lower in the low-risk group compared to the healthy controls and significantly elevated in the high-risk and PAH populations compared to low-risk patients. Dysregulation of these cytokines may occur with the development of SSc, perhaps related to hypoxic injury. With the development of pulmonary vascular disease, persistent hypoxia may induce a negative feedback loop resulting in pronounced elevation of these cytokines.
A major strength of this study is that samples and clinical data were obtained from a large prospective multi-center registry of SSc patients enrolled at scleroderma centers that routinely screen their patients for PAH. However, the number of low-risk patients in the comparator group was limited as they were recruited from a single center. The patients with PAH were diagnosed using the gold standard of RHC. However, the high-risk patients underwent RHC only if deemed clinically appropriate by the treating physician. Furthermore, since the closure of this registry, criteria for the diagnosis of PAH have been revised with a lower mPAP (>20mmHg). As such, many of the high-risk PAH patients included in the study may in fact meet criteria for PAH. Regardless of the mPAP cut-off for the diagnosis of PAH, our findings of similar cytokine profiles observed between the high-risk and incident PAH groups supports a continuum of disease.
Although we assessed many cytokines, there are some cytokines that have distinguished SSc-PAH patients in other studies that were not included based on available data at the time of assay preparation and limitations of the assays themselves. Another limitation of this study is that we are unable to make any conclusions about the unique cytokine milieu of the pulmonary vascular bed that may contribute to PAH given the results are based on serum studies. Finally, these results need to be validated in an independent cohort and with a larger low-risk SSc patient group.
Arthritis Res Ther. 2022;24(39) © 2022 BioMed Central, Ltd.
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