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A post hoc analysis of a phase 2b trial shows rosnilimab, an investigational PD-1-targeting antibody, dramatically reduces pathogenic T and B cells in rheumatoid arthritis patients — both in the bloodstream and inflamed joint tissue. The drug also demonstrated a clean safety profile with no deaths, malignancies, or opportunistic infections. Researchers say it may offer durable disease control by treating RA at its immunological source.
A new analysis of a phase 2b trial presented at ACR Convergence 2026 offers a deeper look at how rosnilimab works in rheumatoid arthritis (RA) — and the immune-level findings are striking. The investigational monoclonal antibody targets the PD-1 receptor, selectively depleting pathogenic activated T cells while leaving protective regulatory T cells intact. Researchers examined blood and inflamed joint tissue samples before and after treatment, finding that rosnilimab reduced PD-1-positive activated T cells by ~50% in the blood — and by more than 90% among high-PD-1 T cells, including follicular helper and effector T cells.
In inflamed joint tissue, the effects were even more dramatic: a ~90% reduction in PD-1-positive T cells and an ~80% drop in B cells, compared to little change in the placebo group. Inflammatory markers like CRP, IL-6, and CXCL13 also fell significantly. Patients who responded clinically at 12 weeks showed greater reductions in T- and B-cell activation pathways than non-responders.
By the Numbers:
90% reduction in high-PD-1 T cells (follicular helper, peripheral helper, effector)
Why it matters: Rosnilimab's ability to target RA at its immunological root — with durable off-drug effects and a favorable safety profile — could position it as a meaningful advance over existing therapies, potentially offering longer-lasting disease control for patients.
