Reduced CTLA-4 expression in Th17 cells or Tregs induces T cell activation: implications for sarcoidosis

<bold>Objective:</bold> Sarcoidosis is a granulomatous disease in which CD4⁺ T cells are central to disease pathogenesis. We previously found decreased expression of the co-inhibitory marker cytotoxic T-lymphocyte antigen 4 (CTLA-4) in Th17 cells and regulatory T cells (Tregs) in lymph nodes (LN) of patients. However, the effects of low CTLA-4 expression in Th17 cells or Tregs are unknown. <bold>Methods:</bold> Using the Cre-LoxP system, we generated mice in which the Ctla4 gene was specifically targeted in IL-17-producing T cells on one (17-Ctla4^+/-) or both alleles (17-Ctla4^−/-), as well as mice haplo-insufficient for CTLA4 in Tregs (Treg-Ctla4^+/-). The immune status of the mice was evaluated at 8-30 weeks, using immunohistochemistry and multi-color flow cytometry. <bold>Results:</bold> We did not find evidence for immune dysregulation in 17-Ctla4^+/- mice. In contrast, spleen and LN of 17-Ctla4^−/- mice showed a significant increase in the proportions of CCR6⁺ T cells, increased fractions of cells expressing the ICOS activation marker within total CD4⁺ memory T (Tm) cells, CCR6⁺ T cells and Tregs, and signs of substantial B cell activation, but no granuloma formation. Upon collagen immunization, splenic CD4⁺ T cells exhibited increased cytokine production, but no enhanced arthritis symptoms or granuloma formation. In Treg-Ctla4^+/- mice, Tregs were expanded and, interestingly, the fractions of ICOS⁺ cells were increased in the Tm, CCR6⁺ T cell and Treg populations. <bold>Conclusions:</bold> Our results indicate that expression levels of CTLA-4 in Tregs are critical for the maintenance of immune homeostasis. In IL-17-producing T cells CTLA-4 expression appears less critical and only a complete deficiency induces immune dysregulation.