CTLA-4 Ablation and Interleukin-12–Driven Differentiation Synergistically Augment Cardiac Pathogenicity of Cytotoxic T Lymphocytes

CD8 + cytotoxic T lymphocytes contribute to viral and autoimmune myocarditis and cardiac allograft rejection. The role of cytotoxic T-lymphocyte–associated antigen (CTLA)-4 as a negative regulator of CD4 + T cells is well defined, yet CTLA-4 regulation of CD8 + T cells is less clear. We studied CTLA-4 regulation of cytotoxic T lymphocytes in a transgenic model of CD8 + T-cell–mediated myocarditis. We generated CTLA-4 −/− Rag 2 −/− OT-1 mice, the CD8 + T cells of which express an ovalbumin (OVA) peptide-specific, class I major histocompatibility complex–restricted T-cell receptor. CTLA-4 −/−Tc12 OT-1 effectors, differentiated with interleukin-12 present, are hyperproliferative in vitro, compared with CTLA-4 +/+ Tc12 OT-1 controls. Transfer of low doses of CTLA-4 −/−Tc12 OT-1 cells to cMy-mOVA mice, which express OVA on cardiac myocytes, causes severe myocarditis, with 99% mortality, compared with no mortality after transfer of low doses of CTLA-4 +/+ Tc12 OT-1 cells. High doses of CTLA-4 +/+ Tc12 cells cause lethal myocarditis in cMy-mOVA mice, but high doses of CTLA-4 +/+ Tc0 CTL, generated without interleukin-12, are hypoproliferative within the cardiac-draining lymph node and do not significantly infiltrate the heart. In contrast, CTLA-4 −/−Tc0 cytotoxic T lymphocytes do proliferate in the cardiac-draining lymph node and diffusely infiltrate the heart. Nonetheless, high doses of CTLA-4 −/−Tc0 cells cause only limited tissue damage, and the disease is not lethal. These data show that CTLA-4 regulates myocarditic CD8 + T cell responses and that CTLA-4 deficiency partly overcomes a differentiation block that exists when naïve CD8 + T cells are stimulated without interleukin-12. Therefore, targeting CTLA-4 solely or in conjunction with interleukin-12 could influence effector CD8 + T cell responses in therapeutically beneficial ways.