Patterns of T-Cell Phenotypes in Rheumatic Diseases From Single-Cell Studies of Tissue

High-dimensional analyses of tissue samples from patients with rheumatic diseases are providing increasingly detailed descriptions of the immune cell populations that infiltrate tissues in different rheumatic diseases. Here we review key observations emerging from high-dimensional analyses of T cells within tissues in different rheumatic diseases, highlighting common themes across diseases as well as distinguishing features. Single-cell RNA sequencing analyses capture several dimensions of T-cell states, yet surprisingly, these analyses generally have not demonstrated distinct clusters of paradigmatic T-cell effector subsets, such as T helper (Th) 1, Th2, and Th17 cells. Rather, global transcriptomics robustly identify both proliferating T cells and regulatory T cells and have also helped to reveal new effector subsets in inflamed tissues, including T peripheral helper cells and granzyme K⁺ T cells. Further characterization of the T-cell populations that accumulate within target tissues should enable more precise targeting of biologic therapies and accelerate development of more specific biomarkers to track activity of relevant immune pathways in patients with rheumatic diseases.