PDCD1 and neoplasm: Furthermore, tumor cell-extrinsic STAT3 activation also regulates the immune tolerance and suppression of anti-tumor immunity by upregulating the expression of immune checkpoint proteins (e.g., PD1, CTLA4) on the surface of tumor-associated myeloid cells [92], B-cells [123], Tregs [134], and CD4 and CD8 T-cells [144,145].