PD-1 signalling restrains pathogenic T peripheral helper and effector CD4⁺ T cell functions in rheumatoid arthritis

Inhibitory receptor signalling represents a potential therapeutic strategy in rheumatoid arthritis (RA), but its effects across pathogenic CD4⁺ T cell subsets and T-B cell interactions remain incompletely defined. We aimed to delineate the expression and function of major inhibitory receptors, including PD-1, BTLA, and TIGIT, in RA synovial CD4⁺ T cells and to determine how receptor agonism modulates T cell effector functions and T-B cell interactions.

CD4⁺ T cells were isolated from RA synovial fluid or from tonsils and blood of controls. Cells were cocultured with artificial antigen-presenting cells (murine fibroblast L cells) expressing PD-L2, HVEM, or CD155, and cytokine production was measured by ELISA (Enzyme-Linked Immunosorbent Assay). Three-dimensional organoid cultures were employed to assess PD-L2-mediated regulation of age-associated B cell (ABC) induction by flow cytometry. Single-cell RNA sequencing profiled inhibitory receptor and ligand expression across T cell subsets.

PD-1, BTLA, and TIGIT were preferentially expressed on T peripheral helper (Tph) and T follicular helper (Tfh) cells in RA synovium. Engagement of PD-L2 broadly suppressed pathogenic cytokine production across multiple CD4⁺ T cell subsets, including IL(Interleukin)-21 and CXCL13 in Tph cells and TNF (Tumor Necrosis Factor), IFN-γ (Interferon-γ), and IL-2 in PD-1⁺CXCR5⁻ non-Tph cells, with effects abolished by CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)-mediated PD-1 disruption. PD-L2 also inhibited cytokine production by tonsillar Tfh and non-Tfh cells. In 3-dimensional organoids, PD-L2 significantly reduced the induction of CD11c⁺CD21⁻T-bet⁺ ABCs.

PD-1 activation suppresses multiple T cell populations from diverse sites, including Tph cells in RA synovium. These results support the broad potential of PD-1 agonism to suppress pathologic T cell responses in RA.