T cell production of IL-21 is required for arthritis development. (A) 1×106 CD4+ cells purified from KRN/B6 (K), KRN IL-21R−/− (K/IL-21R−/−), or KRN IL-21−/− (K/IL-21−/−) mice were transferred into Cα−/− BxN hosts and ankle thickening was measured over time. Results shown are from two experiments (n = 6 - 8 mice per group). Mean and standard deviation are shown. (B) Anti-GPI IgG titers were measured by ELISA from serum collected 8 and 29 - 31 days post-transfer. Mean is shown; each symbol represents an individual mouse. Day eight results are from four experiments (n = 5 - 10 mice per group). Results for days 29-31 are from the experiment described in A. (C) 8×106 splenocytes from KRN/B6 (K) or KRN IL-21−/− (K/IL-21−/−) mice were transferred into Cα−/− BxN hosts and serum was collected on the days indicated. Anti-GPI IgM and IgG titers were measured by ELISA. Mean is shown; each symbol represents an individual mouse (n = 4-5 mice per group). ND (not detectable). Student’s t-test: ns, p > 0.05; *, p < 0.05; **, p < 0.01; ***, p < 0.001.