Inhibitory MHC Class I Receptors on Myeloid Cells

Inhibitory MHC class I receptors are mainly expressed on NK cells. NK cells bear various kinds of inhibitory MHC class I receptors, Killer cell immunoglobulin (Ig)-like receptors, CD94/NKG2A hetrodimer, and murine Ly49 receptor family for monitoring the expression of MHC class I on surrounding cells. Leukocyte Ig-like receptors (LILRs), which bind MHC class I molecules, are expressed on other immune cells, such as B cells, macrophages, dendritic cells (DCs) and mast cells. A murine LILR relative, paired Ig-like receptor (PIR), which is expressed on B cells and myeloid cells but not on NK and T cells, also recognizes MHC class I molecules as its ligand. Recent studies have revealed that some of these inhibitory receptors associate with MHC class I on the same cell surface (in cis). Moreover, the cisinteraction has been verified to regulate effector functions of NK cells or myeloid cells. In this review, we summarize recent discoveries on the functions of inhibitory MHC class I receptors, and discuss their regulatory roles in immune responses.