CD4 and neoplasm: NS398 also inhibits arginase activity and downregulates Arg-1 expression by splenic MDSCs, thereby enhancing proliferation while inhibiting apoptosis in CD4 + T cells.112 Modulation of chemokine CXCL12 and its receptor CXCR4 by PGE-2 attracts MDSC into the tumor microenvironment.