Accessory cells (ACs) of immunised IFN-γR KO mice are required for their defective Treg activity. Treg cells, Teff cells and ACs were isolated from lymph nodes and spleen of IFN-γR KO and wild-type DBA/1 mice 21 days after immunisation with collagen type II in complete Freund's adjuvant. Mixing experiments were performed as indicated. In each set, 5 × 104 CD4+CD25- Teff cells were incubated with anti-CD3 antibody in the presence of ACs and the indicated number of CD4+CD25+ Treg cells. The percentage inhibition of the proliferation of Teff cells (CD4+CD25-) by increasing numbers of CD4+CD25+ Treg cells is shown. The results are representative of two independent experiments.