Inhibition of Histone Methylation Induces Long-Term Survival of Vascularized Cardiac Allografts and Preserves the Third-Party T Cell Immunity.

Histone methylation plays important roles in orchestrating the expression of genes critical for antigen-driven T cell responses. We have recently shown that inhibition of histone methylation caused selective apoptosis of alloantigen-activated T cells. Here we report that a short-term treatment with the histone methylation inhibitor 3-deazaneplanocin A (DZNep) induces long-term acceptance of cardiac allografts in mice. Inductive DZNep treatment caused a dramatic reduction of T helper (Th)1 cells, decreased production of donor-specific antibodies, and long-term allograft survival. Conditional deletion of T cell Ezh2, which catalyzes trimethylation of histone H3 at lysine 27 and is a major target of DZNep, led to a similar long-term allograft survival as DZNep treatment. TGFβ inhibits effector T cell differentiation and survival. Mice with abrogated T cell TGFβ signaling failed to accept allografts after withdrawal of the histone methylation inhibitor. Ten days after discontinuation of DZNep treatment, alloreactive T cells producing high levels of granzyme B and IFN-g recurred in the cardiac graft, causing their rejection. This suggests that DZNep treatment requires T cell TGFβ signaling to facilitate long-term acceptance. Intriguingly, in recipients lacking T-bet, which is essential for promoting Th1 cell responses, allografts were rejected ten days following discontinuation of the therapy. This was associated with the occurrence of alloreactive T cells producing high levels of IL-17 and CXCL1 (which are derived from Th17 cells). Depleting CD8 T cells attenuated the graft rejection in T-bet-deficient mice after withdrawal of DZNep treatment. Thus, in normal recipients, T cell expression of functional T-bet is critical for inductive DZNep treatment to sustain the repression of Th17 cell-mediated graft rejection. Importantly, inductive DZNep treatment preserved T cell immunity against third party antigens, such as allograft and viral antigens. Our findings identify that inhibition of histone methylation and Ezh2 induce long-term suppression of graft-reactive T and B cells, and may lead to new methods to improve the outcome of transplanting vascularized allografts without the requirement for continual immunosuppression.