A PD-1^highCD4⁺ T Cell Population With a Cytotoxic Phenotype is Associated With Interstitial Lung Disease in Systemic Sclerosis

Objective: T cells contribute to tissue injury in systemic sclerosis (SSc), yet the specific T cell subsets expanded in patients with SSc remain incompletely defined. Here we evaluated specific phenotypes and functions of peripheral helper T (Tph) and follicular helper T (Tfh) cells, which have been implicated in autoantibody production, and assessed their associations with clinical features in a well-characterized cohort of patients with SSc. Methods: Mass cytometry of T cells from peripheral blood mononuclear cells of patients with SSc and controls were evaluated using t-distributed stochastic neighbor embedding visualization, biaxial gating, and marker expression levels. Findings were validated with flow cytometry and in vitro assays. Results: The frequencies of PD-1^highCXCR5⁺ Tfh cells and PD-1^highCXCR5⁻ Tph cells were similar in patients with SSc and controls. t-distributed stochastic neighbor embedding visualization (tSNE) revealed distinct populations within the PD-1^highCXCR5⁻ cells distinguished by expression of HLA–DR and inducible costimulator (ICOS). Among PD-1^highCXCR5⁻ cells, only the HLA–DR⁺ICOS⁻ cell population was expanded in patients with SSc. Cytometric and RNA sequencing analyses indicated that these cells expressed cytotoxic rather than B cell helper features. HLA–DR⁺ICOS⁻ PD-1^highCXCR5⁻ cells were less potent in inducing B cell plasmablast differentiation and antibody production than comparator T helper cell populations. HLA–DR⁺ICOS⁻PD-1^highCXCR5⁻ cells were significantly associated with the presence and severity of interstitial lung disease among patients with SSc. Conclusion: Among PD-1^highCXCR5⁻ T cells, a subset of HLA–DR⁺ICOS⁻ cells with cytotoxic features is specifically expanded in patients with SSc and is significantly associated with interstitial lung disease severity. This potential cytotoxicity appearing in the CD4 T cell population can be evaluated as a prognostic disease biomarker in patients with SSc.