Lack of T cells in Act1-deficient mice results in elevated IgM-specific autoantibodies but reduced lupus-like disease

Act1 is a negative regulator of B-cell activation factor of the TNF family (BAFF) and CD40L-induced signaling. BALB/C mice lacking Act1 develop systemic autoimmunity resembling systemic lupus erythematosus (SLE) and Sjogren's syndrome (SjS). SLE and SjS are characterized by anti-nuclear IgG autoantibody (ANA-IgG) production and inflammation of peripheral tissues. As autoantibody production can occur in a T-cell dependent or T-cell independent manner, we investigated the role of T-cell help during Act1-mediated autoimmunity. Act1-deficiency was bred onto C57Bl/6 (B6.Act1-/-) mice and B6.TCR beta-/-TCRd-/-Act1-/- (TKO) mice were generated. While TCR beta/d-sufficient B6.Act1-/- mice developed splenomegaly and lymphadenopathy, hypergammaglobulinemia, elevated levels of ANA-IgG, and kidney pathology, TKO mice failed to develop any such signs of disease. Neither B6.Act1-/- nor TKO mice developed SjS-like disease, suggesting that epigenetic interactions on the BALB/C background are responsible for this phenotype in BALB/C.Act1-/- mice. Interestingly, BAFF-driven transitional B-cell abnormalities, previously reported in BALB/C.Act1-/- mice, were intact in B6.Act1-/- mice and largely independent of T cells. In conclusion, T cells are necessary for the development of SLE-like disease in B6.Act1-/- mice, but not BAFF-driven transitional B-cell differentiation.