Human NTB-A/SLAMF6 Fluorescein-conjugated Antibody

NTB-A (NK-T-B-antigen), also known as Ly108 and SLAMF6, is a 60 kDa type I transmembrane glycoprotein that belongs to the SLAM subgroup of the CD2 family (1). Mature human NTB-A contains a 205 amino acid (aa) extracellular domain (ECD) with one Ig-like V-set and one Ig-like C2-set domain. It also contains a 21 aa transmembrane segment and an 84 aa cytoplasmic domain with two immunoreceptor tyrosine-based switch motifs (ITSMs) (2, 3). An alternately spliced isoform is truncated in the cytoplasmic domain and lacks the two ITSMs. Within the ECD, human NTB-A shares 48% aa sequence identity with mouse and rat NTB-A. The ECD of human NTB-A shares 19%‑34% aa sequence identity with comparable regions of human 2B4, BLAME, CD2F-10, CD84, CD229, CRACC, and SLAM. NTB-A is expressed on the surface of NK, T, and B lymphocytes as well as eosinophils (2, 4, 5). It interacts homophilically through weak associations between the Ig-V domains (2, 5-7). NTB-A functions as an activating coreceptor on NK and T cells (2, 5, 6, 8). Tyrosine phosphorylation in the membrane proximal ITSM enables specific association with EAT-2, an interaction that is required for NTB-A mediated cytotoxicity of NK cells (9). Phosphorylation-dependent NTB-A association with SAP is required for full production of IFN-gamma by NK cells (5, 9). This interaction is independent of EAT-2 binding and appears to involve the membrane distal ITSM (5, 9). NTB-A deficient mice show weakened Th2 responses and elevated levels of neutrophil-derived inflammatory mediators (10). On B cells, NTB-A modulates immunoglobulin class switching and the balance between tolerance and autoimmunity (5, 11).