PDCD1 and neoplasm: Although PD-1 has been mostly studied in T cells, it is increasingly being appreciated that is also upregulated in other immune cell types including innate immune cells such as macrophages [69–71], tumor-associated macrophages (TAMs) [72], monocytes [73,74] myeloid progenitors and myeloid-derived suppressor cells (MDSC) [75] and innate lymphoid cells (ILCs) such as natural killer (NK) cells [76], and regulates their differentiation and function as previously extensively reviewed [11,77–79].