TMD8-FCRL5-KO Cell Line

Fc receptor-like 5 (FCRL5), also known as IRTA2 or CD307e, is a single-pass type I transmembrane glycoprotein belonging to the immunoglobulin receptor superfamily. Encoded by a gene located on human chromosome 1q23.1, FCRL5 contains eight to nine immunoglobulin-like C2-type extracellular domains and cytoplasmic regions harboring both immunoreceptor tyrosine-based activating (ITAM) and inhibitory (ITIM) motifs, enabling its dual immunoregulatory functions. FCRL5 is predominantly expressed on B cells, with particularly high levels on naive B cells, memory B cells, and plasma cells. It functions as a bidirectional regulator of B-cell receptor (BCR) signaling: in the absence of co-stimulation, FCRL5 recruits SHP-1 phosphatase via its ITIMs to inhibit BCR signaling, whereas upon engagement with complement receptor 2 (CR2) and BCR, it enhances calcium flux and promotes B-cell activation. Notably, recent structural studies have elucidated that FCRL5 recognizes IgG in a unique avidity-dependent manner, simultaneously engaging two IgG-Fc molecules through its D1-D3 domains, thereby enabling B cells to sense the density of IgG within immune complexes. This distinctive recognition mode distinguishes FCRL5 from classical Fcγ receptors. FCRL5 overexpression has been implicated in the pathogenesis of systemic autoimmunity, as demonstrated by B-cell-specific Fcrl5 transgenic mice that develop age-related lupus-like disease associated with disrupted B-cell anergy. Furthermore, FCRL5 is highly expressed in various B-cell malignancies, including multiple myeloma, hairy cell leukemia, and chronic lymphocytic leukemia, making it an attractive therapeutic target. Antibody-drug conjugates and chimeric antigen receptor (CAR)-T cells targeting FCRL5 are currently under preclinical and clinical development. Thus, FCRL5 represents a promising biomarker and immunotherapeutic target for both autoimmune diseases and B-cell malignancies.