CXCR4 and neoplasm: These are the immunosuppressive cells (e.g., regulatory T cells, macrophages, myeloid-derived suppressor cells, tumor-associated neutrophils) and tumor-derived immunosuppressive factors, such as cytokines (e.g., IL-6, IL-10, TGF-β, TNFα), chemokines and their receptors (e.g., CXCL12, CXCR4), tumor-secreted factors (e.g., VEGF, prostaglandin E2), or enzymes (e.g., IDO, Arg1) that are engaged in suppressing the responses of the immune cells [40].