Human CD68, also known as GP110, LAMP4, Scavenger Receptor D1 (SR-D1) or macrosialin in mouse, encodes a 110-kD transmembrane glycoprotein that belongs to the lysosomal/endosomal-associated membrane glycoprotein (LAMP) family. Members of the LAMP family include LAMP-1, LAMP-2, dendritic cell (DC)-LAMP (aka CD208), and brain and dendritic cell-associated (BAD)-LAMP (aka LAMP-5). Unlike the two LAMP domains facing the lysosomal lumen in LAMP-1 and LAMP-2, CD68 has a single LAMP domain containing four cystines spaced 36-37 residues apart along with an N-terminal Mucin-like domain. The 354 amino acid (a.a.) human CD68 and 326 a.a. murine ortholog share 80.6% a.a. sequence identity (1).
CD68 is highly expressed in cells of the mononuclear phagocyte system such as macrophages, microglia, osteoclasts, and myeloid dendritic cells (DCs); and is expressed to a lesser extent in lymphoid cells (CD19+ B lymphocytes and CD4+ T lymphocytes), human umbilical cord mesenchymal stem cells (MSCs), fibroblasts, endothelial cells, multiple non-hematopoietic cancer cell lines, and human arterial intimal smooth muscle cells (SMCs). Expression has been also observed in diseased states for granulocytes and neutrophils, in particular basophils from myeloproliferative disorders and intestinal neutrophils from inflammatory bowel disease (IBD), respectively (1).
Although the function of CD68 has yet to be established, it has often been used as an immunohistochemistry (IHC) marker of inflammation and for granular cell tumors (GCTs). CD68+ tumor associated macrophages (TAMs) has been suggested to be a predictive marker for poor cancer prognosis, but a meta-analysis showed the presence of CD68 is not correlated with survival (2). In addition, a role in hepatic malaria infection has been reported based on the finding that peptide P39 binds CD68, considered a receptor for malaria sporozoite, and inhibits parasite entry into Kupffer cells. CD68 was deemed a member of the Scavenger receptor family due to its upregulation in macrophages following inflammatory stimuli, ability to bind modified LDL, phosphatidylserine, and apoptotic cells, as well as shuttling between the plasma membrane and endosomes. CD68 has been linked to atherogenesis based on binding and internalization of its ligand, oxLDL (1).
1. Chistiakov, DA, Killingsworth, MC, Myasoedova, VA. Orekhov AN, Bobryshev YV. (2017) CD68/macrosialin: not just a histochemical marker. Lab Invest. 97:4-13. PMID: 27869795
2. Troiano G, Caponio VCA, Adipietro I, Tepedino M, Santoro R, Laino L, Lo Russo L, Cirillo N, Lo Muzio L. (2019) Prognostic significance of CD68+ and CD163+ tumor associated macrophages in head and neck squamous cell carcinoma: A systematic review and meta-analysis. Oral Oncol. 93:66-75. PMID: 31109698.