CD4 and head and neck squamous cell carcinoma: Like ATRA, vitamin D3 can induce the differentiation of MDSCs and improve the antitumor immune responses.28,232 Treatment with 1α,25(OH)2D3 in HNSCC patients before surgery reduced the frequency of immune inhibitory CD34+ progenitor cells while increased the maturation of DCs in tumor tissues.233 In another study, HNSCC patients administrated with 1α,25(OH)2D3 had increased intra-tumoral CD4+ and CD8+ T cells and a lengthier tumor progression free time compared to untreated patients.234