PDCD1 and neoplasm: In addition, PD-L1-expressing DCs may also drive the progression of naïve, PD-1-expressing CD4 Tregs to the mature, immunosuppressive phenotype, favoring co-operative impairment of anti-tumor host defenses due to co-expression of CTLA-4 and PD-1 by Tregs (5, 18, 19).