Leukocyte-Associated Immunoglobulin-Like Receptor 1 (LAIR1) is a single-pass type I membrane protein. LAIR1 expressed on the majority of peripheral mononuclear cells, including natural killer (NK) cells, T-cells, B-cells, monocytes, and dendritic cells, highly in naive T-cells and B-cells. As an inhibitory receptor, LAIR1 plays a constitutive negative regulatory role on cytolytic function of natural killer (NK) cells, B-cells and T-cells. Activation by Tyr phosphorylation results in recruitment and activation of the phosphatases PTPN6 and PTPN11. It also reduces the increase of intracellular calcium evoked by B-cell receptor ligation. LAIR1 plays inhibitory role independently of SH2-containing phosphatases and modulates cytokine production in CD4+ T-cells. It down-regulates IL2 and IFNG production while inducing secretion of transforming growth factor beta, also down-regulates IgG and IgE production in B-cells as well as IL8, IL10 and TNF secretion. LAIR1 inhibits the differentiation of peripheral blood precursors towards dendritic cells. It also restrains proliferation and induces apoptosis in myeloid leukemia cell lines as well as prevents nuclear translocation of NF-kappa-B p65 subunit/RELA and phosphorylation of I-kappa-B alpha/CHUK in these cells.
Leukocyte-Associated Immunoglobulin-Like Receptor 2 (LAIR2) is a secreted, 131 amino acid protein that contains one Ig-like C2 type domain, making it a member of the Ig superfamily. When compared to LAIR-1, its transmembrane counterpart, it shares 83% amino acid identity across the signal sequence and extracellular domains; although one is secreted and one is membrane-bound, the two LAIR proteins are thought to have arisen from a common gene ancestor and appear to share similar adhesion profiles. This suggests that LAIR-2 may compete with LAIR-1 for ligand binding. A 114 amino acid alternate splice form of LAIR-2 is truncated at the C terminus, but retains the entire Ig domain. The expression profile of these splice forms, and the presence of orthologs in other species, have not been reported.
Leukocyte immunoglobulin-like receptor subfamily A member 3 is also known as CD85 antigen-like family member E, Immunoglobulin-like transcript 6, ILT-6, Leukocyte immunoglobulin-like receptor 4, LIR-4 and Monocyte inhibitory receptor HM43/HM31. In humans, it is encoded by the LILRA3 gene. It acts as soluble receptor for class I MHC antigens. Binds both classical and non-classical HLA class I molecules but with reduced affinities compared to LILRB1 or LILRB2.It is detected in B-cells, natural killer (NK) cells, peripheral blood monocytes and lung.
Leukocyte Immunoglobulin-like Receptor Subfamily A Member 5（LILRA5）is a member of the leukocyte immunoglobulin-like receptors (LILR), comprise a family of activating and inhibitory type immunoreceptors. LILRA5 consists of a 227 amino acid (aa) extracellular domain (ECD), a 21 aa transmembrane segment, and a 10 aa cytoplasmic tail. The ECD contains two Ig-like domains and the transmembrane segment contains a positively charged aspartic acid residue which may mediate its association with the signaling molecule, FcR common gamma chain. LILRA5 is expressed by monocytes, macrophages, and neutrophils. Cross-linking of LILRA5 on monocytes induces the expression of pro-inflammatory cytokines (IL-1beta, IL-6, TNF-alpha) as well as the anti-inflammatory IL-10. It can be detected in tissues of the hematopoietic system, including bone marrow, spleen, lymph node and peripheral leukocytes. Crosslink of ILT-11 on the surface of monocytes has been shown to induce calcium flux and secretion of several proinflammatory cytokines, which suggests the roles of this protein in triggering innate immune responses.
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