Peripheral nerve CR1 express in situ cofactor activity for degradation of C3b

CR1 in Alzheimer's disease

The complement component receptor 1 gene (CR1), which encodes a type-I transmembrane glycoprotein, has recently been identified as one of the most important risk genes for late-onset Alzheimer's disease (LOAD). In this article, we reviewed the recent evidence concerning the role of CR1 in LOAD. First, we introduced the structure, localization and physiological function of CR1 in humans. Afterward, we summarized the relation of CR1 polymorphisms with LOAD risk. Finally, we discussed the possible impact of CR1 on the pathogenesis of AD including amyloid-β pathology, tauopathy, immune dysfunction and glial-mediated neuroinflammation. We hope that a more comprehensive understanding of the role that CR1 played in AD may lead to the development of novel therapeutics for the prevention and treatment of AD.

What does complement do in Alzheimer's disease? Old molecules with new insights

Increasing evidence suggests that inflammatory and immune components in brain are important in Alzheimer's disease (AD) and anti-inflammatory and immunotherapeutic approaches may be amenable to AD treatment. It is known that complement activation occurs in the brain of patients with AD, and contributes to a local inflammatory state development which is correlated with cognitive impairment. In addition to the complement's critical role in the innate immune system recognizing and killing, or targeting for destruction, complement proteins can also interact with cell surface receptors to promote a local inflammatory response and contributes to the protection and healing of the host. On the other hand, complement activation also causes inflammation and cell damage as an essential immune function to eliminate cell debris and potentially toxic protein aggregates. It is the balance of these seemingly competing events that influences the ultimate state of neuronal function. Our mini review will be focusing on the unique molecular interactions happening in the AD development, the functional outcomes of those interactions, as well as the contribution of each element to AD.

Yin and Yang: complement activation and regulation in Alzheimer's disease

The spectrum of inflammatory diseases is nowadays considered to include diverse diseases of the central nervous system (CNS). Current evidence suggests that syndromes such as Alzheimer's disease (AD) have important inflammatory and immune components and may be amenable to treatment by anti-inflammatory and immunotherapeutic approaches. Compelling evidence has been reported that complement activation occurs in the brain with Alzheimer's disease, and that this contributes to the development of a local inflammatory state that is correlated with cognitive dysfunction. The complement system is a critical element of the innate immune system recognizing and killing, or targeting for destruction, otherwise pathogenic organisms. In addition to triggering the generation of a membranolytic complex, complement proteins interact with cell surface receptors to promote a local inflammatory response that contributes to the protection and healing of the host. Complement activation causes inflammation and cell damage, yet it is an essential component in trying to eliminate cell debris and potentially toxic protein aggregates. It is the balance of these seemingly competing events--the "Yin" and the "Yang"--that influences the ultimate state of neuronal function. Knowledge of the unique molecular interactions that occur in the development of Alzheimer's disease, the functional consequences of those interactions, and the proportional contribution of each element to this disorder, should facilitate the design of effective therapeutic strategies for this disease.

The expression of CD59 in experimental allergic neuritis

Complement is implicated as an effector in inflammatory demyelination occurring in Guillain-Barré syndrome (GBS) and in experimental allergic neuritis (EAN). CD59, a potent complement regulatory protein that inhibits the formation of the terminal cytolytic membrane attack complex (MAC), is expressed on human and rat Schwann cells. In EAN the expression of CD59 was increased on Schwann cells during demyelination and axonal degeneration, evaluated by immunostaining of nerve sections and teased fibres. Mac-1 (CD11b) positive leukocytes were localized close to the Schwann cells showing enhanced CD59 staining. The increased CD59 expression in EAN could therefore be due to the release of cytokines or other immunoregulatory molecules from the inflammatory cells. However, interferon gamma (IFN-gamma) or tumor necrosis factor alfa (TNF-alpha) did not upregulate the expression of CD59 on rat Schwann cells in culture. The increased expression of CD59 in EAN is likely to be important in the protection of Schwann cells from MAC.

Complement regulation

Because of its strong potential for generating inflammation and causing tissue destruction the complement system has to be kept strictly under control. Cells of the host need special protection against the cytolytic complement system. This paper will describe how inappropriate activation of complement in the fluid phase is prevented and how viable human blood cells defend themselves against being destroyed and cleared away by the complement system. Since disturbances in complement regulation occasionally result in disease a brief reference will be made to two of the syndromes caused by complement regulator deficiency, hereditary angioedema (HAE) and paroxysmal nocturnal hemoglobinuria (PNH).

Immunopathogenesis and treatment of the Guillain-Barré syndrome--Part I

The etiology of the Guillain-Barré syndrome (GBS) still remains elusive. Recent years have witnessed important advances in the delineation of the mechanisms that may operate to produce nerve damage. Evidence gathered from cell biology, immunology, and immunopathology studies in patients with GBS and animals with experimental autoimmune neuritis (EAN) indicate that GBS results from aberrant immune responses against components of peripheral nerve. Autoreactive T lymphocytes specific for the myelin antigens P0 and P2 and circulating antibodies to these antigens and various glycoproteins and glycolipids have been identified but their pathogenic role remains unclear. The multiplicity of these factors and the involvement of several antigen nonspecific proinflammatory mechanisms suggest that a complex interaction of immune pathways results in nerve damage. Data on disturbed humoral immunity with particular emphasis on glycolipid antibodies and on activation of autoreactive T lymphocytes and macrophages will be reviewed. Possible mechanisms underlying initiation of peripheral nerve-directed immune responses will be discussed with particular emphasis on the recently highlighted association with Campylobacter jejuni infection.

Expression of genes encoding receptors for IgG (FcRIII) and for C3b/C4b (Crry) in rat sciatic nerve during development and Wallerian degeneration

Northern blots were used to examine the expression of genes encoding receptors for IgG (FcRIII) and for C3b/C4b (Crry) in rat sciatic nerve during development and Wallerian degeneration. Steady state levels of FcRIII (1.4 kb) and Crry (1.9 and 2.1 kb) mRNAs were higher in adult rat nerves than in 6 day and 21 day postnatal rat nerves, indicating that the expression of these receptors is developmentally regulated. The FcRIII and Crry cDNA probes also hybridized with total RNA from 3 day old rat Schwann cells and from adult rat peritoneal macrophages. The size of the FcRIII mRNA expressed by cultured Schwann cells (1.6 kb) differed from that expressed by peritoneal macrophages (1.4 kb); the two may be splice variants of one transcript or products of related genes. Peritoneal macrophages contained approximately 100 times higher FcRIII mRNA levels than Schwann cells. In contrast, steady state levels of both 1.9 and 2.1 kb Crry mRNAs were similar in cultured Schwann cells and macrophages. Nerve transection induced a generalized increase in the level of sciatic FcRIII mRNA (1.4 kb) 3 days post-surgery, whereas the level of Crry mRNA was increased only in the nerve segment immediately to the cut. The increase of FcRIII mRNA that occurred in Wallerian degeneration was most likely due to infiltration of macrophages, as FcRIII mRNA-positive macrophages were demonstrated in the degenerating nerves by in situ hybridization. FcRIII mRNA-positive macrophages were not found in normal nerve. The functions of FcRIII and Crry in peripheral nerves are uncertain, but they may be of significance in phagocytosis, antibody-dependent cellular cytotoxicity, and in local immune regulation.

Peripheral nerve CR1 limit complement-mediated haemolysis

CR1 were purified from sciatic nerve extracts by monoclonal antibody affinity chromatography. The effect of CR1 on complement-mediated haemolysis was studied by adding purified CR1 to a mixture of human serum as source of complement and sheep erythrocytes sensitized with rabbit IgG. A dose-dependent inhibition of the haemolysis occurred. There was no effect on the haemolysis when phosphate-buffered saline, elution buffer or run-through fraction not containing CR1 was added. Addition of a polyclonal anti-CR1 antibody to purified CR1 suppressed the inhibiting activity, strongly indicating that peripheral nerve CR1 inhibit complement-mediated haemolysis. This may be of great importance in vivo since CR1 present on the Schwann cell membrane may prevent the formation of terminal lytic complexes.

Plasma exchange in Guillain-Barré syndrome: effect on anti-peripheral nerve myelin antibodies

Increased levels of anti-peripheral nerve myelin (anti-PNM) antibodies were demonstrated by ELISA in serum from 4 of 7 patients with Guillain-Barré syndrome (GBS). Treatment with plasma exchange (PE) was performed and 6 of the 7 patients showed clinical improvement with marked increase in muscular strength. One patient, however, continued to deteriorate during the treatment. No correlation between clinical improvement and levels of anti-PNM antibodies was observed. Whether the antibodies are of pathogenetic importance in GBS is therefore still unknown.

The ontogenesis of Fc gamma receptors and complement receptors CR1 in human peripheral nerve

The ontogenesis of Fc gamma receptors (FcR) and C3b/C4b receptors (CR1) was studied in peripheral nerves from ten fetuses aged from 20 to 38 weeks using immunohistochemical and functional assays. Monoclonal antibodies (mAbs) against FcR and CR1 stained nerve fibers at 10 weeks of gestation and the staining intensity increased during nerve maturation. FcR and CR1 are probably expressed on Schwann cells and are early markers during the development of peripheral nerves. Functional FcR activity was detected in nerve sections before initiation of myelination, which occurs at approximately 18-19 weeks, whereas functional CR1 activity was found in the sections after myelination. Functional CR1 activity may, therefore, be related to myelin. The ontogenesis of FcR and CR1 was also studied on Schwann cells in culture from three fetuses aged 14, 16 and 19 weeks, using immunofluorescence technique with mAbs. The FcR and CR1 are lost on cultured Schwann cells. This suggests that the receptors are not intrinsic to the cells or that Schwann cells require axonal contact for the expression of FcR and CR1.