Fig. 3

Microbial immunomodulation mediated by pattern recognition receptors in the tumor microenvironment. TLRs and NLRs are the major subtypes of pattern recognition receptors. Some microbes activate TLR4 to promote MDSCs infiltration and M2-like TAM polarization, resulting in an immunosuppressive tumor microenvironment. On the contrary, some microbes activate TLR2 to promote M1-like TAM polarization and suppress MDSCs function. TLR agonist synergizes with interferon-γ to increase pro-inflammatory cytokines TNF-α, IL-12 and decrease IL-10, forming an inflammatory tumor microenvironment. Nod1, a member of the NLRs family, promotes MDSCs proliferation and arginase-1 expression and thereby leading to M2-like TAM repolarization. Abbreviations: CXCL = The chemokine (C-X-C motif) ligand, CXCR = The chemokine (C-X-C motif) receptor, MDSC = myeloid-derived immunosuppressive cell, NLRs = Nucleotide-binding domain and leucine-rich repeat–containing receptors, Nod = Nucleotide-binding domain, TAM = tumor-associated macrophage, TLR = Toll-like receptor, TNF = tumor necrosis factor