Isolation and characterization of complement receptors CR1 from human peripheral nerves

Relation between complement and the febrile response of guinea pigs to systemic endotoxin

We reported recently that the complement (C) system may play a role in the febrile response of guinea pigs to intravenous lipopolysaccharide (LPS) administration because C depletion abolished the LPS-induced rise in core temperature (T(c)). The present study was designed to investigate further the relation between C reduction [induced by cobra venom factor (CVF); 20, 50, 100, and 200 U/animal iv] and the fever of adult, conscious guinea pigs produced by LPS injected intravenously (2 microg/kg) or intraperitoneally (8, 16, 32 microg/kg) 18 h after CVF; control animals received pyrogen-free saline. Serum C levels were measured as total hemolytic C activity before and 18 h after CVF injection and expressed as CH(100) units. In other experiments, serum C levels were determined at various intervals after the intravenous and intraperitoneal injections at different doses of LPS alone. LPS produced fevers generally of similar heights but of different onset latencies and durations, depending on the dose and route of administration. CVF caused dose-related reductions in serum C, from approximately 1,136 U to below detection. These reductions proportionately attenuated the fevers induced by intraperitoneal LPS, but not by intravenous LPS. Intravenous and intraperitoneal LPS per se caused reductions in serum C of 25 and 40%, respectively, indicating activation of the C cascade. These decreases were transient, however, occurring early during the febrile rise approximately 30 min after LPS injection. These data thus support the notion that the C system may be critically involved in the febrile response of guinea pigs to systemic, particularly intraperitoneal, LPS.

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 biosynthesis in the central nervous system

Complement is an important effector arm of the human immune response. Binding of proteolytic fragments derived from activation of complement by specific receptors leads to responses as diverse as inflammation, opsonization, and B-cell activation. The importance of characterizing the expression and regulation of complement in the CNS is highlighted by growing evidence that complement plays a significant role in the pathogenesis of a variety of neurological diseases, such as multiple sclerosis and Alzheimer's disease. In vitro studies have demonstrated that astrocytes, the predominant glial cell type in the brain, are capable of expressing or producing a majority of the components of the complement system. Expression of many complement proteins synthesized by astrocytes is regulated by both pro- and anti-inflammatory cytokines, many of which are also produced by several cell types in the CNS. In addition to astrocytes, ependymal cells, endothelial cells, microglia, and neurons have recently been shown to synthesize various complement proteins or express complement receptors on their cell surfaces. Together, these studies demonstrate that several cell types throughout the brain have the potential to express complement and, in many cases, increase expression in response to mediators of the acute phase response. These studies suggest that complement may play a greater role in CNS immune responses than previously thought, and pave the way for better understanding of the dynamics of complement expression and regulation in vivo. Such understanding may lead to therapeutic manipulation of complement host defense functions in a variety of inflammatory and degenerative diseases in the CNS.

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.

Soluble complement receptor type 1 (CD35) is released from leukocytes by surface cleavage

The soluble form of complement receptor type 1 in human plasma (sCR1) might correspond to the shedding of the receptor by proteolytic cleavage at the cell surface. A new enzyme-linked immunosorbent assay (ELISA) was established to specifically measure membrane-bound CR1 using a rabbit polyclonal antibody against a 19-amino acid peptide corresponding to the C-terminal sequence of the intracellular domain of CR1 (mCR1-ELISA). This ELISA measured CR1 from solubilized erythrocyte membranes, polymorphonuclear leukocytes (PMN), a B lymphocyte cell line and renal podocyte-derived urinary vesicles in a dose-dependent manner. In contrast, and similarly to recombinant soluble CR1 which lacks the intracellular domain of CR1, plasmatic sCR1 was not recognized, suggesting that sCR1 corresponds to an extracellular fragment of whole CR1. In vitro, PMN were shown to release a soluble form of CR1 which was also not recognized in the mCR1-ELISA, and whose size was smaller (5 kDa) than the CR1 of PMN cell membranes. The release of soluble CR1 was highest for PMN and HL60 cells, followed by U937 cells and three different B lymphocyte cell lines, whereas T lymphocyte cell lines did not release soluble CR1. The levels of CR1 gene expression were also higher in PMN compared to remaining blood leukocytes and the different cell lines tested above. Incubation of PMN with formyl-methionyl-leucyl-phenylalanine, tumor necrosis factor-alpha or lipopolysaccharide accelerated the release of soluble CR1, and incubation with granulocyte/macrophage colony-stimulating factor resulted in sustained CR1 gene expression and higher total soluble CR1 release. Our results suggest that soluble CR1 is produced by cleavage of cell surface CR1, and that a large fraction of human plasma sCR1 is cleaved from PMN. The release of sCR1 by leukocytes may play a role in the control of complement activation at sites of inflammation.

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.

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

Adsorption of sheep erythrocytes (E) sensitized with IgM antibodies (A) and C3b (EAC3b) to C3b/C4b receptors (CR1) in cryostat sections of human myelinated nerves was studied using the closed chamber technique. The adsorption was stable for at least 3 h at 37 degrees C. In the presence of purified factor I, the indicator cells detached from the sections after 40 min at 37 degrees C. Factor H was not required. The release was not due to loss of CR1 activity in the sections. The detached indicator cells were negative in the immune adherence test and were agglutinated by antibody to C3d, but not by antibody to C3c. Western blot of the detached indicator cells revealed the presence of C3d and C3c was found in the chamber fluid. Accordingly, detachment of the indicator cells was due to degradation of C3b to C3d with the release of C3c into the chamber fluid. Protease inhibitors did not prevent the detachment of the indicator cells. EAC3b incubated with sections of myelinated nerves pre-incubated with anti-CR1 antibody or with sections of unmyelinated nerves which contain functionally inactive CR1 were not degraded. The results therefore indicate that CR1 in situ in myelinated nerves can provide the necessary cofactor activity for factor I-mediated degradation of C3b to C3d and C3c.