|Detection of PILR‑ alpha in J774A.1 Mouse Cell Line by Flow Cytometry. J774A.1 mouse reticulum cell sarcoma macrophage cell line was stained with Goat Anti-Mouse PILR‑ alpha Alexa Fluor® 488‑conjugated Antigen Affinity-purified Polyclonal Antibody (Catalog # FAB4318G, filled histogram) or isotype control antibody (Catalog # IC108G, open histogram). View our protocol for Staining Membrane-associated Proteins.|
PILR-alpha (Paired Immunoglobulin-like type 2 Receptor-alpha), also named FDF03, is one of two members of a small family of immunoregulatory Ig-superfamily receptors (1, 2). It is a counterpart to PILR-beta and it likely gave rise to PILR-beta through gene duplication and rearrangement (1). The PILRs represent one of many pairs of Ig-like domain-containing receptors that participate in immune regulation. PILR-alpha and -beta should not be confused with the similarly named PIRs (also paired immunoglobulin‑like receptors), or the functionally-related SIRP and ILT/LILR/CD85/LIR family of receptors (2). While PIRs, ILTs and SIRPs contain three to six Ig‑like domains in their extracellular region, PILR-alpha and -beta show only one Ig-like region in their extracellular domain (ECD) (1, 2). Mouse PILR-alpha is a monomeric, 271 amino acid (aa) type I transmembrane (TM) protein (3). It contains a 167 aa ECD, a 21 aa TM segment, and a long, 83 aa cytoplasmic region. The ECD shows one V‑type Ig-like domain between aa 40-134, while the cytoplasmic region contains two ITIMs (immunoreceptor Tyr-based inhibitory motifs) between aa 265-270 and 294‑299. Given that ITIMs are known to interact with phosphatases such as PTPN6 and PTPN11, the presence of these motifs makes mouse PILR-alpha an inhibitory receptor. In human, activation of PILR-alpha inhibits CD32/Fc gamma RII-induced calcium mobilization (3). Although CD99 is a known ligand for both PILR-alpha and -beta (4), highest affinity binding seems to occur between CD99 and PILR-alpha (4). Mouse PILR-alpha is found on neutrophils and macrophages (4). The additional ligands for PILR-alpha have been reported. One is PANP that is expressed in neural tissue, while two others are NPDC1 and Collectin-12. In all cases, O-linked carbohydrates on the ligands are central to binding (5,6). Mouse PILR-alpha ECD is 43% and 69% aa identical to human and rat PILR-alpha ECD, respectively; it is 75% aa identical to the ECD of mouse PILR-beta (3).