Complement regulator CD46: Genetic variants and disease associations

Membrane cofactor protein (MCP; CD46) is an ubiquitously expressed complement regulatory protein that protects host cells from injury by complement. This type-I membrane glycoprotein serves as a cofactor for the serine protease factor I to mediate inactivation of C3b and C4b deposited on host cells. More than 60 disease-associated mutations in MCP have now been identified. The majority of the mutations are linked to a rare thrombotic microangiopathic-based disease, atypical hemolytic uremic syndrome (aHUS), but new putative links to systemic lupus erythematosus, glomerulonephritis, and pregnancy-related disorders among others have also been identified. This review summarizes our current knowledge of disease-associated mutations in this complement inhibitor

Complement factor H and the hemolytic uremic syndrome

Immune recognition is coupled to powerful proinflammatory effector pathways that must be tightly regulated. The ancient alternative pathway of complement activation is one such proinflammatory pathway. Genetic susceptibility factors have been identified in both regulators and activating components of the alternative pathway that are associated with thrombotic microangiopathies, glomerulonephritides, and chronic conditions featuring debris deposition. These observations indicate that excessive alternative pathway activation promotes thrombosis in the microvasculature and tissue damage during debris accumulation. Intriguingly, distinct genetic changes in factor H (FH), a key regulator of the alternative pathway, are associated with hemolytic uremic syndrome (HUS), membranoproliferative glomerulonephritis (dense deposit disease), or age-related macular degeneration (AMD). A mouse model of HUS designed to mirror human mutations in FH has now been developed, providing new understanding of the molecular pathogenesis of complement-related endothelial disorders

Methane Distribution and Production in the Georgia Salt Marsh

Bacterial action in the anaerobic sediments of Spartina salt marsh produce large quantities of methane. Water in contact with the marsh has methane concentrations 200-300 times normal oceanic concentrations. The methane content of the water varies from maximum of 1.2 mu l CH sub(4)/ super(l) super(-1) in the summer to a minimum of 0.005 mu l CHl

CD46 and oncologic interactions: Friendly fire against cancer

One of the most challenging aspects of cancer therapeutics is target selection. Recently, CD46 (membrane cofactor protein; MCP) has emerged as a key player in both malignant transformation as well as in cancer treatments. Normally a regulator of complement activation, CD46 is co-expressed as four predominant isoforms on almost all cell types. CD46 is highly overexpressed on a variety of human tumor cells. Clinical and experimental data support an association between increased CD46 expression and malignant transformation and metastasizing potential. Further, CD46 is a newly discovered driver of metabolic processes and plays a role in the intracellular complement system (complosome). CD46 is also known as a pathogen magnet due to its role as a receptor for numerous microbes, including several species of measles virus and adenoviruses. Strains of these two viruses have been exploited as vectors for the therapeutic development of oncolytic agents targeting CD46. In addition, monoclonal antibody-drug conjugates against CD46 also are being clinically evaluated. As a result, there are multiple early-phase clinical trials targeting CD46 to treat a variety of cancers. Here, we review CD46 relative to these oncologic connections

TREX1 is expressed by microglia in normal human brain and increases in regions affected by ischemia

BACKGROUND: Mutations in the three-prime repair exonuclease 1 (TREX1) gene have been associated with neurological diseases, including Retinal Vasculopathy with Cerebral Leukoencephalopathy (RVCL). However, the endogenous expression of TREX1 in human brain has not been studied. METHODS: We produced a rabbit polyclonal antibody (pAb) to TREX1 to characterize TREX1 by Western blotting (WB) of cell lysates from normal controls and subjects carrying an RVCL frame-shift mutation. Dual staining was performed to determine cell types expressing TREX1 in human brain tissue. TREX1 distribution in human brain was further evaluated by immunohistochemical analyses of formalin-fixed, paraffin-embedded samples from normal controls and patients with RVCL and ischemic stroke. RESULTS: After validating the specificity of our anti-TREX1 rabbit pAb, WB analysis was utilized to detect the endogenous wild-type and frame-shift mutant of TREX1 in cell lysates. Dual staining in human brain tissues from patients with RVCL and normal controls localized TREX1 to a subset of microglia and macrophages. Quantification of immunohistochemical staining of the cerebral cortex revealed that TREX1 CONCLUSIONS: TREX1 is expressed by a subset of microglia in normal human brain, often in close proximity to the microvasculature, and increases in the setting of ischemic lesions. These findings suggest a role for TREX

A C3(H20) recycling pathway is a component of the intracellular complement system

An intracellular complement system (ICS) has recently been described in immune and nonimmune human cells. This system can be activated in a convertase-independent manner from intracellular stores of the complement component C3. The source of these stores has not been rigorously investigated. In the present study, Western blotting identified a band corresponding to C3 in freshly isolated human peripheral blood cells that was absent in corresponding cell lines. One difference between native cells and cell lines was the time absent from a fluid-phase complement source; therefore, we hypothesized that loading C3 from plasma was a route of establishing intracellular C3 stores. We found that many types of human cells specifically internalized C3(H(2)O), the hydrolytic product of C3, and not native C3, from the extracellular milieu. Uptake was rapid, saturable, and sensitive to competition with unlabeled C3(H(2)O), indicating a specific mechanism of loading. Under steady-state conditions, approximately 80% of incorporated C3(H(2)O) was returned to the extracellular space. These studies identify an ICS recycling pathway for C3(H(2)O). The loaded C3(H(2)O) represents a source of C3a, and its uptake altered the cytokine profile of activated CD4(+) T cells. Importantly, these results indicate that the impact of soluble plasma factors should be considered when performing in vitro studies assessing cellular immune function

Down-regulation of CD46 by Piliated Neisseria gonorrhoeae

Human membrane cofactor protein (CD46) protects host cells against complement attack and may function as a receptor for pathogenic Neisseriae. We assessed CD46 expression in the human cervical cell line ME-180 after exposure to Neisseria gonorrhoeae. Piliated but not nonpiliated gonococci adhered to cells and produced up to an 80% reduction in CD46 surface expression by 6 h that persisted for at least 24 h. This response required a minimum multiplicity of infection of 10 and was not prevented by antibodies to CD46. CD46 down-regulation was not attributable to intracellular retention or a global or specific shutdown of mRNA or protein synthesis. Substantial quantities of CD46 were found in the supernatants, indicating a specific shedding of this protein. Adherent gonococci lacking the pilus retraction protein PilT did not down-regulate CD46 but de-repression of pilT expression restored CD46 down-regulation. After experimental infection of human volunteers with a gonococcal variant incapable of inducing CD46 down-regulation, variants of this strain were reisolated that exhibited CD46 down-regulation. Pilus-mediated interactions of gonococci with human epithelial cells results in a pathogen-induced manipulation of the host cell environment in which a membrane protein is removed from epithelial cells by liberation into the surrounding milieu