SEMA4D and head and neck squamous cell carcinoma: Studies from our lab have shown that HNSCC-associated Sema4D leads to immune suppression through the induction of myeloid derived suppressor cells (MDSC) (CD33+, CD11b+, HLA-DR -/low), with subsequent regulatory T cell (Treg) differentiation, and downregulation of T-helper cells (Th1) and cytotoxic T lymphocytes (CTLs) [21].