CD68 / MACROSIALIN

CD68 Overview

Human monocytes and tissue macrophages express a 110-kD transmembrane glycoprotein encoded by this gene. It belongs to the LAMP (lysosomal/endosomal-associated membrane glycoprotein) family of proteins. The protein is mostly found in lysosomes and endosomes, with only a little amount circulating to the cell surface. It's a type I integral membrane protein with a highly glycosylated extracellular domain that binds to lectins or selectins that are specific to different tissues and organs. The protein belongs to the family of scavenger receptors. Scavenger receptors are responsible for clearing cellular debris, promoting phagocytosis, and mediating macrophage recruitment and activation.

CD68 is involved in tissue macrophage phagocytic activities, as well as intracellular lysosomal metabolism and extracellular cell-cell and cell-pathogen interactions. Binds to tissue and organ-specific lectins or selectins, allowing macrophage subsets to be directed to specific locations. Macrophages may be able to crawl over selectin-bearing substrates or other cells if CD68 is rapidly recirculated from endosomes and lysosomes to the plasma membrane.

CD68 Expression

Expression of CD68 and other macrophage-specific genes is enhanced in common myeloid progenitors in the bone marrow by macrophage/granulocyte-macrophage colony-stimulating factors (M-CSF/GM-CSF) during the transition between lymphoid and myeloid lineages. CD68+lin plasmacytoid monocytes in mice have been found to express high levels of CD68 but lack other myeloid surface (CD14, CD33, CD13, CD11b, CD11c) and lysosomal (myeloperoxidase and lysozyme) markers. These cells are primary dendritic cell progenitors, as they can develop into conventional DCs after being stimulated with IL-3 and CD40 ligand.

During the transition between lymphoid and myeloid lineages, macrophage/granulocyte-macrophage colony-stimulating factors (M-CSF/GM-CSF) increases the expression of CD68 and other macrophage-specific genes in common myeloid progenitors in the bone marrow. CD68+lin plasmacytoid monocytes in mice expressed high amounts of CD68 but lacked other myeloid surface (CD14, CD33, CD13, CD11b, CD11c) and lysosomal (myeloperoxidase and lysozyme) markers. After being activated with IL-3 and CD40 ligand, these cells can grow into conventional DCs, indicating that they are primary dendritic cell progenitors. CD68 expression was shown to be low in a variety of non-hematopoietic cell types, including human umbilical cord mesenchymal stem cells, fibroblasts, endothelial cells, and numerous tumor cell lines15, as well as in human artery intimal smooth muscle cells. Even while CD68 expression is significantly greater in myeloid cells, the discovery of CD68 in several non-myeloid cell types calls into doubt the mononuclear phagocyte lineage specificity of this surface marker

CD68 has a lot of glycosylation. In both human and murine molecules, there are nine potential N-glycosylation sites, four of which are in the LAMP domain. O-glycosylation sites are substantially more numerous (up to 29 in human CD68 and 26 in mouse macrosialin). The mucin-like domain contains all O-glycosylation sites. 1 The polypeptide backbone of murine macrosialin has a molecular mass of only 35 kDa, but the whole mass of mature glycoprotein ranges from 90 to 120 kDa. Both O- and N-linked sugars contribute the most mass to mature macrosialin (40 percent and 20–25 percent, respectively), with just about a third coming from the polypeptide backbone.

During macrophage activation, CD68 is differentially glycosylated, resulting in the accumulation of many O-linked terminal sialic acid residues and N-linked polylactosaminoglycans. CD68 expression is modest in rat resident peritoneal macrophages at rest, but it can be dramatically increased in response to inflammatory stimuli. As a result, during proinflammatory activation, CD68 glycosylation is significantly modified, resulting in both O-linked and N-linked glycosylation. Phagocytosis enhances the complexity and length of O-linked chains in mouse CD68, resulting in an increase in peritoneal macrophage ligand-binding capability (i.e., binding of plant agglutinins).

Due to their major role in the clearance of infections, cytotoxic chemicals, and dead cells, macrophages have established a repertoire of scavenger receptors (SRs) that can detect a wide range of ligands. CD68 was also classified as a scavenger receptor type D (SCARD)46 because it can be significantly upregulated in macrophages in response to inflammatory stimuli; CD68 can bind modified LDL, phosphatidylserine, and apoptotic cells; and CD68 can rapidly shuttle between the plasma membrane and endosomes. Indeed, CD68's inherent capacity to bind and internalize oxLDL allowed researchers to speculate about its possible role in atherogenesis and intracellular lipid buildup. Indeed, CD68's inherent capacity to bind and internalize oxLDL allowed researchers to speculate about its possible role in atherogenesis and intracellular lipid buildup.

In APA hamsters that develop atheromatous plaques in streptozotocin-induced diabetes, in vivo investigations were conducted to determine the role of CD68 in atherosclerosis. Rabbits develop early lesions (fatty streaks) and hyperlipidemia 6 weeks after streptozotocin treatment in this diabetic atherosclerosis model.

CD68 is commonly utilized as a histopathological marker of inflammation when monocytes/macrophages are involved. Cells of the macrophage lineage, such as tissue histiocytes, multinucleated giant cells, Kupffer cells, and osteoclasts, also are identified using CD68. To distinguish M1 from M2 macrophage polarization in vivo, CD68 can be combined with transcription factor markers such as pSTAT1, RBP-J, and CMAF. CD68 is predominantly found in lysosomes, according to ultrastructural studies of human atherosclerotic lesions.

CD68 is not expressed by dendritic cells (DCs), unlike macrophages, and this helps immunohistochemically identify these two cell types, which both have antigen-presenting capabilities. Several studies have shown that inflammatory stimuli such as oxLDL exposure and chronic stimulation with bacterial lipopolysaccharide (LPS) or the inflammatory cytokine interferon- (IFN-) cause considerable elevation of CD68 expression in macrophages.

CD68 is a cancer-related diagnostic and prognostic marker that is widely used. It is used to identify tumors of the macrophage lineage, however tumor cells of other lineages may also express it. It's not unexpected that tumor cells express CD68 because metastatic tumor cells use immunological markers to avoid macrophage-mediated phagocytosis and cell-damaging effects from cytotoxic CD8+ T cells during invasion of a normal, non-tumor tissue environment. Macrophage molecular characteristics, including expression of antigens such as CD14, CD47, CD68, MAC387, CD163, and DAP12, appear to be the most often employed immune antigens by emigrated tumor cells.

Macrophages are the most prevalent form of immune cell that infiltrates tumors. The tumor microenvironment, on the other hand, is anti-inflammatory and immunosuppressive. This milieu causes macrophages to convert from a proinflammatory (M1) to an anti-inflammatory (M2) phenotype (M2). Tumor-associated macrophages are CD68-positive macrophages that are seen in tumors (TAMs). TAMs develop immunosuppressive qualities and limit CD8+ T cell cytotoxicity in malignancies. In glioma and squamous cell carcinoma tumors, researchers discovered a population of CD68-positive immature myeloid cells dubbed myeloid-derived suppressor cells (MDSCs) that restrict T cell growth and cytokine production. The tumor microenvironment can make these cells from circulating monocytes.