VSIR and leukemia: In addition, V-domain Ig suppressor of T cell activation (VISTA), a recently defined negative regulator mediating immune evasion in tumors, was highly expressed on these MDSCs in AML patients; knockdown of VISTA by specific siRNA potently reduced the MDSCs-mediated inhibition of CD8 T cell activity in AML, suggesting a suppressive effect of VISTA on anti-leukemia T cell response [51].