SECTM1 (secreted and transmembrane 1), also called K12, is either found as an approximately 27 kDa intracellular type I transmembrane protein that shows a perinuclear, Golgi like staining pattern, or as a 20 kDa soluble, secreted form. SECTM1 is expressed on thymic epithelial and fibroblast cells, breast cancer and leukemia cell lines, and neutrophils but not in peripheral lymphocytes. SECTM1 is expressed on cytomembrane, which is identified as a CD7 ligand.CD7 is expressed in T and natural killer (NK) cells, which could be activated by its ligand SECTM1, thus promoting the proliferation of T and NK cells. SECTM1 strongly costimulates CD4 and CD8 T cell proliferation and induces IFN-γ production, likely via a CD7-dependent mechanism. In addition, SECTM1 synergizes with suboptimal anti-CD28 to strongly augment T cell functions. A robust induction of IL-2 production when SECTM1 and anti-CD28 signals were present with TCR ligation.
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