CD4 and Arthritis: MHC IIΔ/Δ and NRG mice show clearly reduced foot swelling when compared with C57BL/6 mice (Fig. 4, C57BL/6, MHC IIΔ/Δ and NRG), consistent with previous data showing that CD4 T cells are important for driving CHIKV arthritis [19], [20].