Calcineurin Inhibitors Suppress Acute Graft-Versus-Host Disease Via Nfat-Independent Inhibition Of T Cell Receptor Signaling

Inhibitors of calcineurin phosphatase activity (CNIs) such as cyclosporin A (CsA) are widely used to treat tissue transplant rejection and acute graft-versus-host disease (aGVHD), for which inhibition of gene expression dependent on nuclear factor of activated T cells (NFAT) is the mechanistic paradigm. We recently reported that CNIs inhibit TCR-proximal signaling by preventing calcineurin-mediated dephosphorylation of Lck(S59), an inhibitory modification, raising the possibility of another mechanism by which CNIs suppress immune responses. Here we used T cells from mice that express Lck(S59A), which cannot accept a phosphate at residue 59, to initiate aGVHD. Although CsA inhibited NFAT-dependent gene upregulation in alloaggressive T cells expressing either LckWT or Lck(S59A), it was ineffective in treating disease when the T cells expressed Lck(S59A). Two important NFAT-independent T cell functions were found to be CsA-resistant in Lck(S59A) T cells: upregulation of the cytolytic protein perforin in tissue-infiltrating CD8(+) T cells and antigen-specific T/DC adhesion and clustering in lymph nodes. These results demonstrate that effective treatment of aGVHD by CsA requires NFAT-independent inhibition of TCR signaling. Given that NFATs are widely expressed and off-target effects are a major limitation in CNI use, it is possible that targeting TCR-associated calcineurin directly may provide effective therapies with less toxicity.