A novel family of Ig-like receptors for HLA class I molecules that modulate function of lymphoid and myeloid cells

We review what is presently known about structure, cellular distribution, b iochemical characteristics, and function of a new family of human cell-surf ace receptors referred to as immunoglobulin-like transcripts (ILTs), leukoc yte Ig-like receptors (LIRs), or monocyte/macrophage Ig-like receptors (MIR s), These receptors are genetically, structurally, and functionally related to a group of natural killer (NK) cell receptors for HLA class I molecules known as killer cell Ig-like receptors (KIRs), Distinct ILT/LIR/MIR isotyp es are differentially expressed on lymphocytes, monocytes, macrophages, den dritic cells, and granulocytes; at least some of them recognize HLA class I molecules. Whereas some isotypes either inhibit or induce cell activation, others may be secreted as soluble receptors. ILT/LIR/MIR receptors may all ow all immune cells to monitor class I expression on other cells and to res pond in its absence, just as NK cells do. In addition, they may contribute to homeostasis by establishing activation thresholds that can be overcome o nly by relevant triggering stimuli and not by bystander cells.