Objective
Rheumatoid arthritis (RA) is a major autoimmune disease that causes synovitis and joint damage. Although clinical trials using interleukin‐10 (IL‐10), an anti‐inflammatory cytokine, have been performed as a potential treatment of RA, its therapeutic effects have been limited, potentially due to insufficient residence in lymphoid organs, where antigen recognition primarily occurs. Here, we engineered IL‐10 as a fusion with serum albumin (SA).

Methods
SA‐fused IL‐10 was recombinantly expressed. After intravenous injection to mice, retention of SA‐IL‐10 at lymph node (LN), immune cell compositions at paws, and therapeutic effect on arthritis model mice were assessed.

Results
SA fusion to IL‐10 led to enhanced LN accumulation compared with unmodified IL‐10. Intravenous SA‐IL‐10 treatment restored immune cell composition in the paws to a normal status, elevated the frequency of suppressive M2 macrophages, reduced IL‐17A amount in the paw‐draining LN, and protected joint morphology. Intravenous SA‐IL‐10 treatment showed similar efficacy as treatment with an anti‐TNF‐α antibody. SA‐IL‐10 was equally effective when administered intravenously, locally or subcutaneously, which benefits clinical translation of this molecule.

Conclusion
SA fusion to IL‐10 is a simple but effective engineering strategy for RA therapy and holds clinical translational potential.