Plasma levels of soluble membrane attack complex are elevated despite viral suppression in HIV patients with poor immune reconstitution

The complement system is now a therapeutic target for the management of serious and life-threatening conditions such as paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, glomerulonephritis and other diseases caused by complement deficiencies or genetic variants. As complement therapeutics expand into more clinical conditions, monitoring complement activation is increasingly important, as is the baseline levels of complement activation fragments in blood or other body fluid levels. Although baseline complement levels have been reported in the literature, the majority of these data were generated using non-standard assays and with variable sample handling, potentially skewing results. In this study, we examined the plasma and serum levels of the soluble membrane attack complex of complement (sMAC). sMAC is formed in the fluid phase when complement is activated through the terminal pathway. It binds the regulatory proteins vitronectin and/or clusterin and cannot insert into cell membranes, and can serve as a soluble diagnostic marker in infectious disease settings, as previously shown for intraventricular shunt infections. Here we show that in healthy adults, serum sMAC levels were significantly higher than those in plasma, that plasma sMAC levels were similar between in African Americans and Caucasians and that plasma sMAC levels increase with age. Plasma sMAC levels were significantly higher in virally suppressed people living with HIV (PLWH) compared to non-HIV infected healthy donors. More specifically, PLWH with CD4⁺ T cell counts below 200 had even greater sMAC levels, suggesting diagnostic value in monitoring sMAC levels in this group.

Membrane attack complex generation increases as a function of time in stored blood

OBJECTIVE

To determine if the complement system, a potent mediator of inflammation, contributes to haemolysis during red blood cell (RBC) storage.

BACKGROUND

RBCs in storage undergo structural and biochemical changes that may result in adverse patient outcomes post-transfusion. Complement activation on leukodepletion and during storage may contribute to the RBC storage lesion.

METHODS/MATERIALS

We performed a cross-sectional analysis of aliquots of leukoreduced RBC units, stored for 1-6 weeks, for the levels of C3a, C5a, Bb, iC3b, C4d and C5b-9 [membrane attack complex (MAC)] by enzyme-linked immunosorbent assay (ELISA).

RESULTS

We observed that only MAC levels significantly increased in RBC units as a function of storage time. We also observed that the level of C5b-9 bound to RBCs increased as a function of storage time.

CONCLUSION

MAC levels increased over time, suggesting that MAC is the primary complement-mediated contributor to changes in stored RBCs. Inhibition of the terminal complement pathway may stabilise RBC functionality and extend shelf life.

Deletion of the complement phagocytic receptors CR3 and CR4 does not alter susceptibility to experimental cerebral malaria

Complement receptors for C3-derived fragments (CR1-4) play critical roles in innate and adaptive immune responses. Of these receptors, CR3 and CR4 are important in binding and phagocytosis of complement-opsonized pathogens including parasites. The role of CR3 and CR4 in malaria or in cerebral malaria (CM) has received little attention and remains poorly understood in both human disease and rodent models of malaria. CR3 and CR4 are members of the β(2) -integrin family of adhesion molecules and are expressed on all leucocytes that participate in the development of CM, most importantly as it relates to parasite phagocytosis (monocytes/macrophages) and antigen processing and presentation (dendritic cells). Thus, it is possible that these receptors might play an important role in disease development. To address this question, we examined the role of CR3(-/-) and CR4(-/-) in experimental cerebral malaria (ECM). We found that both CR3(-/-) and CR4(-/-) mice were fully susceptible to ECM and developed disease comparable to wild-type mice. Our results indicate that CR3 and CR4 are not critical to the pathogenesis of ECM despite their role in elimination of complement-opsonized pathogens. These findings support recent studies indicating the importance of the terminal complement pathway and the membrane attack complex in ECM pathogenesis.

Deletion of carboxypeptidase N delays onset of experimental cerebral malaria

Complement contributes to inflammation during pathogen infections; however, less is known regarding its role during malaria and in the severest form of the disease, cerebral malaria. Recent studies have shown that deletion of the complement anaphylatoxins receptors, C3aR and C5aR, does not alter disease susceptibility in experimental cerebral malaria (ECM). This does not, however, preclude C3a- and C5a-mediated contributions to inflammation in ECM and raises the possibility that carboxypeptidase regulation of anaphylatoxin activity rapidly over rides their functions. To address this question, we performed ECM using carboxypeptidase N-deficient (CPN(-/-)) mice. Unexpectedly, we found that CPN(-/-) mice survived longer than wild-type mice, but they were fully susceptible to ECM. CD4(+) and CD8(+) T cell infiltration was not reduced at the peak of disease in CPN(-/-) mice, and there was no corresponding reduction in pro-inflammatory cytokine production. Our results indicate that carboxypeptidases contribute to the pathogenesis of ECM and that studies examining the contribution of other carboxypeptidase families and family members may provide greater insight into the role these enzymes play in malaria.

Excision of the nifD element in the heterocystous cyanobacteria

Heterocyst differentiation in cyanobacteria is accompanied by developmentally regulated DNA rearrangements that occur within the nifD, fdxN, and hupL genes. These genetic elements are excised from the genome by site-specific recombination during the latter stages of differentiation. The nifD element is excised by the recombinase, XisA, located within the element. Our objective was to examine the XisA-mediated excision of the nifD element. To accomplish this, we observed the ability of XisA to excise substrate plasmids that contained the flanking regions of the nifD element in an E. coli host. Using PCR directed mutagenesis, nucleotides in the nifD element flanking regions in substrate plasmids were altered and the effect on recombination was determined. Results indicate that only certain nucleotides within and surrounding the direct repeats are involved in excision. In some nucleotide positions, the presence of a purine versus a pyrimidine greatly affected recombination. Our results also indicated that the site of excision and branch migration occurs in a 6 bp region within the direct repeats.

Expression of the anaphylatoxin C5a receptor in the oligodendrocyte lineage

Expression of the C5a receptor in the central nervous system has been demonstrated on microglia, astrocytes and neurons. In the present study, we demonstrate C5aR expression in vitro by rat and murine O2-A progenitor cells and oligodendrocytes. We also observed that in vitro differentiation of O2-A progenitors into mature oligodendrocytes is accompanied by down-regulation of C5aR mRNA expression. These results suggest that the C5aR may be a marker for oligodendroglial differentiation and play a role in oligodendrocyte function.

Attenuation of experimental autoimmune demyelination in complement-deficient mice

The exact mechanisms leading to CNS inflammation and myelin destruction in multiple sclerosis and in its animal model, experimental allergic encephalomyelitis (EAE) remain equivocal. In both multiple sclerosis and EAE, complement activation is thought to play a pivotal role by recruiting inflammatory cells, increasing myelin phagocytosis by macrophages, and exerting direct cytotoxic effects through the deposition of the membrane attack complex on oligodendrocytes. Despite this assumption, attempts to evaluate complement's contribution to autoimmune demyelination in vivo have been limited by the lack of nontoxic and/or nonimmunogenic complement inhibitors. In this report, we used mice deficient in either C3 or factor B to clarify the role of the complement system in an Ab-independent model of EAE. Both types of complement-deficient mice presented with a markedly reduced disease severity. Although induction of EAE led to inflammatory changes in the meninges and perivascular spaces of both wild-type and complement-deficient animals, in both C3(-/-) and factor B(-/-) mice there was little infiltration of the parenchyma by macrophages and T cells. In addition, compared with their wild-type littermates, the CNS of both C3(-/-) and factor B(-/-) mice induced for EAE are protected from demyelination. These results suggest that complement might be a target for the therapeutic treatment of inflammatory demyelinating diseases of the CNS.

Intracerebral complement C5a receptor (CD88) expression is regulated by TNF and lymphotoxin-alpha following closed head injury in mice

The anaphylatoxin C5a is a potent mediator of inflammation in the CNS. We analyzed the intracerebral expression of the C5a receptor (C5aR) in a model of closed head injury (CHI) in mice. Up-regulation of C5aR mRNA and protein expression was observed mainly on neurons in sham-operated and head-injured wild-type mice at 24 h. In contrast, in TNF/lymphotoxin-alpha knockout mice, the intracerebral C5aR expression remained at low constitutive levels after sham operation, whereas it strongly increased in response to trauma between 24 and 72 h. Interestingly, by 7 days after CHI, the intrathecal C5aR expression was clearly attenuated in the knockout animals. These data show that the posttraumatic neuronal expression of the C5aR is, at least in part, regulated by TNF and lymphotoxin-alpha at 7 days after trauma.

HSP16.6 is involved in the development of thermotolerance and thylakoid stability in the unicellular cyanobacterium, Synechocystis sp. PCC 6803

The low molecular weight (LMW) heat shock protein (HSP), HSP16.6, in the unicellular cyanobacterium, Synechocystis sp. PCC 6803, protects cells from elevated temperatures. A 95% reduction in the survival of mutant cells with an inactivated hsp16.6 was observed after exposure for 1 h at 47 degrees C. Wild-type cell survival was reduced to only 41%. HSP16.6 is also involved in the development of thermotolerance. After a sublethal heat shock at 43 degrees C for 1 h and subsequent challenge exposure at 49 degrees C for 40 min, mutant cells did not survive, while 64% of wild-type cells survived. Ultrastructural changes in the integrity of thylakoid membranes of heat-shocked mutant cells also are discussed. These results demonstrate an important protective role for HSP16.6 in the protection of cells and, in particular, thylakoid membrane against thermal stress.

Detection of activated complement complex C5b-9 and complement receptor C5a in skin biopsies of patients with systemic sclerosis (scleroderma)

OBJECTIVE

Upregulated matrix synthesis is a hallmark of systemic sclerosis (SSc). There are indications that growth factors such as platelet derived growth factor (PDGF) are involved in proliferative pathways in SSc lesions. As activated complement releases PDGF from endothelial cells, we searched for activated complement and the complement receptor for C5a (C5aR) in skin biopsies of patients with SSc.

METHODS

Snap frozen sections of 8 patients with early SSc and 5 patients with longterm SSc were examined. Using monoclonal antibodies against activated complement complex C5b-9 and the C5aR, skin biopsies derived from both clinically involved and non-involved skin were examined by APAAP immunohistochemistry.

RESULTS

A pattern of activated complement C5b-9 and the CSaR could be detected in SSc microvasculature. Eleven of the 13 patients (7/8 patients with early SSc) showed positive staining for C5b-9. The CSaR was detected in 6 of the 8 patients with early SSc. In 3 patients with longterm disease, C5aR expression could also be detected in non-involved skin.

CONCLUSION

Activated complement and complement receptors could be detected in early and late stages of SSc skin lesions. The presence of complement receptors in non-involved skin may indicate preclinical activation of pathways resulting in growth factor dependent matrix synthesis.

Central nervous system-targeted expression of the complement inhibitor sCrry prevents experimental allergic encephalomyelitis

Although generally thought of as a T cell-driven autoimmune disease, recent studies in experimental allergic encephalomyelitis (EAE), the animal model of multiple sclerosis, suggest a significant role for innate immune mechanisms. To address the possibility that the complement system plays a central role in these diseases, we developed a transgenic mouse with astrocyte-targeted production of a soluble inhibitor of complement activation, complement receptor-related protein y (sCrry). Here, we show that sCrry transgenic mice are either fully protected against EAE or develop significantly delayed clinical signs. These results indicate that complement activation may have an essential role in the pathogenesis of the disease and that complement-mediated events may occur early during the effector phase of EAE. Furthermore, this work underscores the importance of humoral immunity in amplifying a T cell-initiated pathogenic process.

Complement anaphylatoxin receptors on neurons: new tricks for old receptors?

Activation of the complement system has been reported in a variety of inflammatory diseases and neurodegenerative processes of the CNS. Recent evidence indicates that complement proteins and receptors are synthesized on or by glial cells and, surprisingly, neurons. Among these proteins are the receptors for the chemotactic and anaphylactic peptides, C5a and C3a, which are the most-potent mediators of complement inflammatory functions. The functions of glial-cell C3a and C5a receptors (C3aR and C5aR) appear to be similar to immune-cell C3aRs and C5aRs. However, little is known about the roles these receptors might have on neurons. Indeed, when compared with glial cells, neurons display a distinct pattern of C3aR and C5aR expression, in either the normal or the inflamed CNS. These findings suggest unique functions for these receptors on neurons.

Expression of the murine complement regulatory protein crry by glial cells and neurons

The expression of the murine complement regulatory protein, Crry, in the CNS remains largely unexplored. In this study, we examined murine astrocytes and microglia purified from neonatal brain and sections of adult murine brain for the expression of Crry. Using RT-PCR, immunohistochemistry, in situ hybridization, flow cytometry, and Western blot analysis, we demonstrated that astrocytes and microglia express Crry protein and RNA. Crry expression is greater on microglia than astrocytes and, as determined by Western blot analysis, each cell type expresses a Crry protein of different molecular weight. Interestingly, neuronal expression of Crry was seen only at the RNA level. These data demonstrate Crry expression by astrocytes, microglia, and neurons in the murine CNS and suggest that Crry may play an important role in protecting the CNS against complement-mediated damage.

Receptor for the C3a anaphylatoxin is expressed by neurons and glial cells

Little is known about the expression of the receptor for complement anaphylatoxin C3a (C3aR) in the central nervous system (CNS). In this study, we provide the first evidence that neurons are the predominant cell type expressing C3aR in the normal CNS. By using in situ hybridization (ISH) and immunohistochemistry, we found that C3aR is constitutively expressed at high levels in cortical and hippocampal neurons as well as in Purkinje cells. Moreover, we showed that primary culture of human astrocytes and microglia express the C3aR mRNA as assessed by RT-PCR. In situ hybridization performed on rat primary astrocytes confirmed the RT-PCR result demonstrating C3aR expression by astrocytes. In experimental allergic encephalitis (EAE), C3aR expression was elevated on microglia, infiltrating monocyte-macrophage cells and a few astrocytes, whereas neuronal expression remained unchanged during the course of the disease. These data demonstrate that the C3aR is expressed primarily by neurons in the normal CNS and that its neuronal expression is not dramatically upregulated under inflammation. This is in contrast to the increased neuronal expression of the C5aR in several inflammatory CNS conditions. The high constitutive expression of the C3aR by neurons suggests this receptor may play an important role in normal physiological conditions in the CNS.

Human T cells express the C5a receptor and are chemoattracted to C5a

The anaphylatoxin C5a is a potent mediator of inflammation that exerts a broad range of activity on cells of the myeloid lineage. In this study, we present the first evidence that human T cells express the C5a receptor (C5aR) and are chemotactic to C5a. Using FACS analysis, we found that the C5aR was expressed at a low basal level on unstimulated T cells and was strikingly up-regulated upon PHA stimulation in a time- and dose-dependent manner. CD3+ sorted T cells as well as Jurkat T cells were shown to express C5aR mRNA as assessed by RT-PCR. Moreover, semiquantitative RT-PCR analysis demonstrated that C5aR mRNA was down-regulated in purified T cells upon long-term PHA stimulation. To demonstrate that C5a was biologically active on T cells, we investigated the chemotactic activity of C5a and observed that purified CD3+ T cells are chemotactic to C5a at nanomolar concentrations. Finally, using a combination of in situ hybridization and immunohistochemistry, we showed that the T cells infiltrating the central nervous system during experimental allergic encephalomyelitis express the C5aR mRNA. In summary, these results suggest that C5a exerts direct effects on T cells and could be involved in the trafficking of T cells under physiological and pathological conditions, including inflammatory diseases of the central nervous system.

A 16.6-kilodalton protein in the Cyanobacterium synechocystis sp. PCC 6803 plays a role in the heat shock response

The low molecular weight (LMW) heat shock protein (HSP) gene hsp16.6 was identified and cloned from the unicellular cyanobacterium Synechocystis sp. PCC 6803 through comparisons of genomic sequences and conserved gene sequences of the LMW HSPs. Hsp16.6 was isolated using PCR and cloned into the pGEMT plasmid. Hsp16.6 showed a significant increase in transcription after heat shock at 42 degreesC that indicated hsp16.6 was a heat shock gene. To determine the role that hsp16.6 plays in the heat shock response, a mutant Synechocystis cell line was generated. Cell growth and oxygen evolution rates of wild type and mutant cells were compared after heat shock. Results showed significantly decreased cell growth rates and a 40% reduction in oxygen evolution rates in mutants after heat shock treatments. These data indicate a protective role for hsp16.6 in the heat shock response.

Kinetics of anaphylatoxin C5a receptor expression during experimental allergic encephalomyelitis

In this study, we investigated the expression of the C5aR in spinal cords of Lewis rats with experimental allergic encephalomyelitis (EAE). Using in situ hybridization (ISH) we analyzed the kinetics of C5aR at different time points of EAE (preclinical stage, clinical peak, remission phase). We observed that C5aR mRNA was readily detected in the CNS of EAE rats at all the stages of the disease. Using a combination of ISH and immunohistochemistry, we formally demonstrated that C5aR is strongly expressed on microglial cells and hypertrophic astrocytes during EAE. The potential involvement of C5a receptor in EAE physiopathology is discussed.

Elevated complement C5a receptor expression on neurons and glia in astrocyte-targeted interleukin-3 transgenic mice

Evidence from several central nervous system (CNS) inflammatory disease models suggests that intrathecal complement synthesis may contribute to early inflammatory events in the brain. In this study, we examined the expression of the receptor for C5a (C5aR), a potent inflammatory and chemotactic factor, in the brains of transgenic mice with constitutive astrocyte expression of interleukin-3 (IL-3), a hematopoietic and immunomodulatory cytokine. By in situ hybridization, we demonstrated that cells infiltrating the cerebellar meninges, the cerebellum, and demyelinating lesions in the cerebellum were strongly positive for C5aR mRNA. By immunohistochemistry, the infiltrating cells expressing the C5aR were identified as macrophages based on staining with antibodies to the complement receptor type 3 and F4/80, a mouse macrophage-specific marker. In addition, some of the cells in cerebellar lesions were positive for the astrocyte-specific marker, glial fibrillary acidic protein, suggesting that a subpopulation of astrocytes in these lesions express elevated levels of the C5aR. Increased C5aR expression was also observed in cortical neurons in the occipital cortex and in pyramidal neurons in the cornu ammonis and subiculum of the hippocampus, at both the protein and mRNA levels. These data suggest that IL-3 may play an immunomodulatory role in C5aR expression on several cell types in the brain and that increased C5aR expression correlates with the pathology seen in this model. The transgenic mice used in this study provide a useful tool for characterizing the mechanism of regulation of the C5aR expression and for examining the functions of this chemotactic receptor in CNS inflammation.

Transforming growth factor-beta2 regulates C3 secretion in monocytes through a protein kinase C-dependent pathway

Previously, we reported that TGF-beta2 regulates the C3 gene expression in a dose- and time-dependent manner in monocytes. To extend these studies, we examined the role of PKC in the TGF-beta2-mediated induction of C3 expression by the human monocyte cell line, U937. Treatment of U937 cells with the PKC inhibitors, H7 and calphostin C, suppressed TGF-beta2-mediated induction of C3 protein levels, but not mRNA levels, in a dose-dependent manner. At the highest concentrations of H7 and calphostin C, C3 protein levels were inhibited 50% and 93%, respectively, compared to control levels. Treatment of U937 cells with HA1004, a weak PKC inhibitor used as a control for H7, did not inhibit induction of C3 protein levels. Down-modulating PKC with a prolonged exposure of U937 cells to PMA also suppressed TGF-beta2-mediated C3 protein induction by as much as 82%. Incubating cell extracts isolated from TGF-beta2-treated U937 cells with the PKC substrate, MIBP(4-14), resulted in increased substrate phosphorylation compared to cell extracts isolated from untreated cells. Addition of calphostin C suppressed the increased substrate phosphorylation by TGF-beta2. Furthermore, biosynthetic labeling of U937 cells treated with TGF-beta2 and calphostin C demonstrated an accumulation of C3 protein within cell lysates compared to controls. Collectively, these studies suggest a role for PKC in the secretion of C3 protein during TGF-beta2-mediated regulation of C3 expression in U937 cells.

Bacterial meningitis: complement gene expression in the central nervous system

Inflammation in the subarachnoid space represents the pathological hallmark of bacterial meningitis. The intrathecal accumulation of leukocytes, in response to bacterial pathogens, and the subsequent release of endogenous inflammatory mediators are associated with a breakdown of the blood-brain barrier function and poor prognosis. Complement has been shown to play a major role in the inflammatory response within the intrathecal compartment in bacterial meningitis. In the present review, we provide an outline of the current understanding of the involvement of the complement system in the pathophysiology of bacterial meningitis, and propose future directions of investigation.

Experimental diffuse axonal injury induces enhanced neuronal C5a receptor mRNA expression in rats

Several studies suggest the involvement of the complement system in the pathophysiology of traumatic brain injury (TBI). Since the intrathecal generation of anaphylatoxin C5a has been shown to mediate inflammatory effects within the central nervous system, we sought to characterize the cellular expression of the mRNA for the C5a receptor (C5aR, CD88) in brains of rats with experimental diffuse axonal injury (DAI) by in situ hybridization. Infiltrating leukocytes expressing C5aR mRNA were seen in meninges and lateral ventricles as early as 4 h after induction of DAI. The number of infiltrating C5aR-positive cells increased gradually up to 24 h after trauma. Within the brain parenchyma, up-regulation of C5aR mRNA expression was first seen in cerebellar Purkinje cells within 8 h. At 24 h after TBI, expression of C5aR mRNA was widespread bilaterally throughout the cortex and cerebellum, the cellular expression being restricted to pyramidal neurons and Purkinje cells. The intensity of C5aR transcript signals on neurons increased further up to 96 h after trauma. Ligand binding of C5a to its receptor on neurons might mediate previously unknown functions, thus possibly leading to neurotoxicity and secondary neuronal damage after TBI.

Evidence for intrathecal synthesis of alternative pathway complement activation proteins in experimental meningitis

Complement has been shown to contribute to intrathecal inflammation in bacterial meningitis. However, the cellular source of complement in the infected central nervous system has not been determined. In this study, we analyzed protein and mRNA expression of two alternative pathway complement activation proteins, C3 and factor B, in the brains of mice with Listeria monocytogenes meningitis. Complement protein levels were found elevated in the cerebrospinal fluid of infected mice, compared with mock-infected animals. In the course of the disease, enhanced C3 and factor B mRNA expression was detected on pyramidal neurons and Purkinje cells within 6 hours, peaking at 12 hours and then gradually decreasing by 72 hours after infection. In addition, leukocytes infiltrating the subarachnoid space, within 12 to 24 hours, expressed mRNA for C3 and factor B. The cellular infiltration increased dramatically up to 72 hours. Intraperitoneal injection of tumor necrosis factor (TNF)-alpha up-regulated C3 and factor B mRNA expression on neurons in normal mice, suggesting that TNF-alpha may represent one cytokine regulating complement expression in this model of bacterial meningitis. However, additional mediators may be involved in regulation of intrathecal complement expression, as infected mice deficient of TNF/lymphotoxin-alpha genes did not demonstrate attenuated complement expression in the brain.

TNF-alpha-mediated expression of the receptor for anaphylatoxin C5a on neurons in experimental Listeria meningoencephalitis

The anaphylatoxin C5a has been implicated in the pathogenesis of bacterial meningitis as a potent mediator of inflammation in the subarachnoid space. We investigated the expression of the receptor for C5a (C5aR) in brains of mice with experimental Listeria monocytogenes (LM) meningoencephalitis. In the course of the disease, infiltrating cells in the meninges and the ventricles were found to express C5aR mRNA and protein. In the brain parenchyma, very low constitutive C5aR expression was seen on pyramidal neurons and Purkinje cells. However, in LM-infected mice, a dramatic increase in C5aR expression occurred on neurons starting 6 h after infection and was maximal between 24 and 36 h. TNF-alpha was identified as an essential mediator of neuronal C5aR expression, since mice lacking the genes for TNF and lymphotoxin-alpha (TNF/lymphotoxin-alpha -/- mice) showed significantly attenuated C5aR expression after LM infection. Furthermore, i.p. injection of recombinant TNF-alpha induced enhanced C5aR expression in the brains of TNF/lymphotoxin-alpha -/- mice and in normal animals even in the absence of a bacterial infection. We also assessed the levels of anaphylatoxin C5a in the cerebrospinal fluid of patients with infectious meningitis. C5a was detected in all patients with bacterial meningitis (n = 9), in 6 of 18 patients with aseptic meningitis, and in 1 of 66 control patients. The finding of TNF-alpha-mediated C5aR expression on neurons in experimental Listeria meningitis and the detection of the ligand, C5a, in the cerebrospinal fluid of human patients with infectious meningitis present new directions in the investigation of the pathophysiologic sequelae leading to secondary brain damage.

TNF-alpha-mediated expression of the receptor for anaphylatoxin C5a on neurons in experimental Listeria meningoencephalitis

The anaphylatoxin C5a has been implicated in the pathogenesis of bacterial meningitis as a potent mediator of inflammation in the subarachnoid space. We investigated the expression of the receptor for C5a (C5aR) in brains of mice with experimental Listeria monocytogenes (LM) meningoencephalitis. In the course of the disease, infiltrating cells in the meninges and the ventricles were found to express C5aR mRNA and protein. In the brain parenchyma, very low constitutive C5aR expression was seen on pyramidal neurons and Purkinje cells. However, in LM-infected mice, a dramatic increase in C5aR expression occurred on neurons starting 6 h after infection and was maximal between 24 and 36 h. TNF-alpha was identified as an essential mediator of neuronal C5aR expression, since mice lacking the genes for TNF and lymphotoxin-alpha (TNF/lymphotoxin-alpha -/- mice) showed significantly attenuated C5aR expression after LM infection. Furthermore, i.p. injection of recombinant TNF-alpha induced enhanced C5aR expression in the brains of TNF/lymphotoxin-alpha -/- mice and in normal animals even in the absence of a bacterial infection. We also assessed the levels of anaphylatoxin C5a in the cerebrospinal fluid of patients with infectious meningitis. C5a was detected in all patients with bacterial meningitis (n = 9), in 6 of 18 patients with aseptic meningitis, and in 1 of 66 control patients. The finding of TNF-alpha-mediated C5aR expression on neurons in experimental Listeria meningitis and the detection of the ligand, C5a, in the cerebrospinal fluid of human patients with infectious meningitis present new directions in the investigation of the pathophysiologic sequelae leading to secondary brain damage.