ICOS and systemic lupus erythematosus: To ascertain whether the reduction in CD4+ TFH cell numbers required the autosomal SLE predisposition background of BXSB, we determined the effects of an IL6 deficiency in B6.Yaa mice, which develop a late onset autoimmune disease due to Yaa. We found that B6.Yaa Il6-/- mice analyzed at 40 weeks of age had reduced frequencies of CD4+ TFH cells (S5A and S5B Fig) as evidenced by the reduced expression of ICOS and PD1 on CD4+ T cells of B6.YaaIl6-/- mice when compared with age matched WT controls (S5C and S5D Fig).