CD4+CD25+FoxP3+ regulatory T (Treg) cells are critical regulators of autoimmunity. Yet the number of Treg cells is paradoxically increased in rheumatoid arthritis (RA) patients, and Treg cells show variable activity in human studies. The objective of this study was to characterize the expansion and function of Treg cells during the initiation and progression of experimental arthritis.
To unequivocally identify Treg cells, we crossed FoxP3gfp mice with K/BxN mice to generate arthritic mice in which Treg cells express green fluorescence protein. We examined the expansion and function of Treg cells and effector T (Teff) cells during different stages of arthritis, using flow cytometry and cell proliferation analyses.
In K/BxN mice, thymic selection of KRN T cells resulted in an enrichment of forkhead box P3 (FoxP3)–positive Treg cells. Treg cell numbers increased during arthritis, with significant increases in spleens and draining lymph nodes, indicating selective tropism to sites of disease. In contrast to the in vitro unresponsiveness of Treg cells when cultured alone, substantial proportions of Treg cells proliferated in both nonarthritic and arthritic mice. However, they also underwent greater apoptosis, thereby maintaining equilibrium with Teff cells. Similarly, enhanced Treg cell–suppressive activity during arthritis was offset by greater resistance by their Teff cell counterparts and antigen-presenting cells.
In this well-established model of RA, the interplay of Teff cells and Treg cells in K/BxN mice recapitulated many features of the human disease. We demonstrated an ordered expansion of Treg cells during arthritis and dynamic changes in Treg cell and Teff cell functions. By elucidating factors that govern Treg cell and Teff cell development in K/BxNgfp mice, we will gain insight into the pathophysiology of and develop novel therapeutics for human RA.