TLR4 and major depressive disorder: Three types of PRRs have been identified, namely, Toll-like receptors (TLRs), NOD-like receptors (NLRs), and C-type lectin receptors (CLRs) [9]; of these, TLR4 and the inflammasome-associated NOD-like receptor pyrin domain containing 3 (NLRP3) are most closely linked to depression.