The Role of Placental Trophoblast in the Pathophysiology of the Antiphospholipid Antibody Syndrome

PROBLEM: The antiphospholipid (aPL) antibody syndrome is characterized by severe pregnancy complications, the cause of which remains unknown. We hypothesized that the placental trophoblast is a target for aPLs. METHOD OF STUDY: The effects of monoclonal aPLs on trophoblast function, including the invasion of JAR into matrigel-coated filters and the effects of annexin V expression on BeWo, were investigated using choriocarcinoma models. RESULTS: aPLs against phosphatidylserine (PS) significantly (P < 0.001) decreased the migration of JAR across the membrane. In the annexin V studies, undifferentiated BeWo did not express surface annexin V. After differentiation, BeWo expressed surface annexin V, which was removed in the presence of aPLs, resulting in increased binding of prothrombin. CONCLUSIONS: PS is expressed on the trophoblast surface during differentiation and invasion of extracellular matrix. Our data suggest that aPLs against PS can directly affect trophoblast function by limiting the depth of decidual invasion and by concurrently creating a procoagulant surface on trophoblast exposed to the maternal circulation.     

The Role of Complement in Pregnancy and Fetal Loss

In the United States, between 1 and 3% of women suffer recurrent miscarriages; 50-70% of all conceptions fail. [1,2] Although in the majority of affected women the cause of recurrent miscarriages is unknown, an immune mechanism involving the inappropriate and subsequently injurious recognition of the conceptus by the mother's immune system has been proposed. Murine models have recently been developed that are relevant to this issue. We and others have identified a novel role for complement as an early effector in the pathway leading to pregnancy loss associated with placental inflammation. Indeed, it appears that inhibition of complement activation is an absolute requirement for normal pregnancy, and that in the antiphosphospholid syndrome overwhelming activation of complement triggered by antibodies (Ab) deposited in placenta leads to fetal injury. Identification of complement activation as a mediator of pregnancy loss and definition of the complement components necessary to trigger such injury is likely to lead to a better understanding of its pathogenesis and to new and improved treatments.     

The Role of Apoptosis Proteins and Complement Components in the Etiopathogenesis of Systemic Lupus Erythematosus

Systemic lupus erythematosus is a prototypical autoimmune disease characterized by the deregulation of T and B cells, tissue infiltration by mononuclear cells, tissue damage and the production of autoantibodies. There is a consensus that accelerated apoptosis of circulating lymphocytes and/or impaired clearance of apoptotic bodies may increase the amount of nuclear antigens presented to T lymphocytes. This process is accompanied by autoimmune responses that can lead to the development of lupus. The dysfunction of apoptosis may be a direct consequence of alterations in proteins/genes such as Fas, Bcl-2 and C1q. Increased expression of Fas antigen could intensify the exposure of hidden antigens. The overexpression of Bcl-2 protein might inhibit the removal of auto-reactive cells, and the lack of C1q could impair the clearance of self-antigens. The complete knowledge of the role of apoptosis components in the etiopathogenesis of lupus could lead to the development of new therapies targeting the apoptotic threshold, which could result in a more specific and effective disease response compared to global immunosuppression. This review summarizes the role of each component of the apoptotic process in the pathogenesis of lupus.     

Role of Complement in Immune Lysis of Trypanosoma cruzi

Studies were performed on the mechanism of immune lysis of culture forms of Trypanosoma (Schizotrypanum) cruzi. Antibody-mediated lysis is caused by complement, which is activated via the classical pathway. The properdin system is not required. The kinetics of the reaction is similar to that followed by immune lysis of sensitized sheep erythrocytes, as is the concentration of divalent cations required for optimal lysis (0.15 mm Ca²⁺ and 0.5 mm Mg²⁺), the occurrence of cell membrane uptake of the complement components C3 and C4, and the development of characteristic ultrastructural modifications on the cell membrane.     

The Role of Ultrastructural Pathology in the Diagnosis of Epithelial and Unusual Renal Tumors

The electron microscopic and immunohistochemical features of epithelial and unusual renal tumors are reviewed. Ultra-structural and immunohistochemical features of diagnostic value are highlighted. An attempt is made to address histoge-nesis/differentiation issues that would help in understanding morphologic findings, with an emphasis placed on the role that ultrastructural evaluation has played in demonstrating these.     

Role of Tissue Factor in the Maternal Immunological Attack of the Embryo in the Antiphospholipid Syndrome

Recurrent fetal loss affects 1–5% of women of childbearing age. Immunological mechanisms may account for 40% of recurrent miscarriages, and in particular, the antiphospholipid syndrome (APS) appears to be implicated in 7–25% of the cases. Because antiphospholipid (aPL) antibodies have thrombogenic properties, fetal loss in patients with APS has been ascribed to thrombosis of placental vessels. However, we have shown that inflammation, specifically activation of complement with generation of the anaphylotoxin C5a, is an essential trigger of fetal injury. Thrombosis and inflammation are linked in many clinical conditions. Tissue factor (TF), the major cellular initiator of the coagulation protease cascade, plays important roles in both thrombosis and inflammation, and its expression is increased in patients with APS. Here we describe how TF, acting as a proinflammatory molecule, induces trophoblast injury and fetal death in a mouse model of APS. Importantly, we will discuss how TF contributes to C5a-induced oxidative burst in neutrophils leading to trophoblasts and fetal injury in APS. The finding that TF is an important effector in aPL-induced inflammation may allow the development of new therapies to abrogate the inflammatory loop caused by tissue factor and improve pregnancy outcomes in patients with aPL antibodies. Statins downregulate TF-induced inflammation and rescued the pregnancies in aPL-treated mice, suggesting they may be a good treatment for women with aPL-induced pregnancy complications.     

Role of Late Complement Components in Experimental Autoimmune Thyroiditis

Availability of a line of rabbits deficient in the sixth complement component (C6-D) made it possible to evaluate the role of the terminal complement complex (TCC) in the development of experimental autoimmune thyroiditis (EAT) of the rabbit. Immunization with saline extract of homologous thyroid, known to be composed predominantly of thyroglobulin, led in normocomplementemic (NC) rabbits to severe thyroidiris, with cellular infiltrates occupying 50-95% of the thyroid, and to minimal or moderate thyroidiris, with 1-35% of thyroids infiltrated in C6-D rabbits. Cellular infiltrates consisted predominantly of mononuclear cells with appreciable numbers of granulocytes. Destruction of thyroid follicles was extensive and diffuse in NC rabbits, but it was only minimal and focal in C6-D rabbits. Immunohistology revealed in both groups of rabbits deposits of IgG and C3 along follicular basal laminae. In addition, NC rabbits showed deposition of C6 and MAC in thyroid follicles. These results suggested that TCC is necessary for the development of fully expressed, severe EAT; simultaneously, however, they showed that a significantly reduced EAT can develop without TCC. Administration of NC but not of C6-D rabbit serum to C6-D rabbits resulted in a significant increase in the severity of EAT. It was also shown that C6-D rabbits have "normal" T-cell activity, since they developed experimental autoallergic encephalomyelitis as readily as NC rabbits. Therefore, it is likely that development of EAT is indeed impaired by the C6 deficiency in rabbits. The requirement for TCC observed in this study may be relevant to the understanding of the pathogenesis of Hashimoto's thyroiditis, in which thyroid tissue was recently shown to contain TCC deposits.     

Role of Complement in Protection against Cryptococcus gattii Infection

Previous studies have shown that the alternative pathway of complement activation plays an important role in protection against infection with Cryptococcus neoformans. Cryptococcus gattii does not activate the alternative pathway as well as C. neoformans in vitro. The role of complement in C. gattii infection in vivo has not been reported. In this study, we used mice deficient in complement components to investigate the role of complement in protection against a C. gattii isolate from an ongoing outbreak in northwestern North America. While factor B-deficient mice showed an enhanced rate of death, complement component C3-deficient mice died even more rapidly, indicating that the alternative pathway was not the only complement pathway contributing to protection against disease. Both C3- and factor B-deficient mice had increased fungal burdens in comparison to wild-type mice. Histopathology revealed an overwhelming fungal burden in the lungs of these complement-deficient mice, which undoubtedly prevented efficient gas exchange, causing death. Following the fate of radiolabeled organisms showed that both factor B- and C3-deficient mice were less effective than wild-type mice in clearing organisms. However, opsonization of C. gattii with complement components was not sufficient to prolong life in mice deficient in complement. Killing of C. gattii by macrophages in vitro was decreased in the presence of serum from factor B- and C3-deficient versus wild-type mice. In conclusion, we have demonstrated that complement activation is crucial for survival in C. gattii infection. Additionally, we have shown that the alternative pathway of complement activation is not the only complement pathway contributing to protection.The complement system consists of a cascade of serum proteins that are involved in opsonization, membrane lysis, and chemotaxis. There are three pathways through which complement can be activated: classical, alternative, and lectin. Complement components C3 to C9 participate in all three complement cascade pathways. C1q is used only in the classical pathway, and factor B is used only in the alternative pathway. While C4 is used in both the classical and lectin pathways, recently it was reported that the lectin pathway can function in the absence of C2 and/or C4 if the alternative pathway is intact (23). Mannose binding lectin (MBL) (24), which is used only in the lectin pathway, exists in two forms in rodents (MBL-A and MBL-C) while only one form is found in humans (8). MBL-A and MBL-C have different levels in serum and differ in their affinity for d-glucose and α-methyl-d-glucose but have redundant function (24).Cryptococcus spp. are fungal pathogens that possess a polysaccharide capsule composed mainly of glucuronoxylomannan (GXM). The capsule is antiphagocytic and anti-inflammatory but is known to activate the alternative complement pathway (11). Previously, Cryptococcus spp. were identified serologically and were all considered as one species, Cryptococcus neoformans. More recently, two species have been designated, C. neoformans (serotypes A and D) and Cryptococcus gattii (serotypes B and C). A key difference between the two species is that C. neoformans tends to infect immunocompromised individuals while C. gattii generally infects apparently immunocompetent people (15). Cryptococcus spp. most commonly cause pulmonary and central nervous system infections in humans (15, 25) and mouse models (3).In 1999, an outbreak of C. gattii began on Vancouver Island, British Columbia, infecting people, companion animals, and porpoises. Strain A1MR265 is the major clinical reference isolate from the Vancouver Island outbreak. Recently, a case of cryptococcosis caused by the strain predominant in Vancouver Island was identified in Puget Sound, WA (27). A total of eight human cases have been reported in Washington State in the past 2 years; four of these individuals had not traveled out of state (29). Nine cases have been reported in Oregon (L. Hoang, presented at the 108th General Meeting of the American Society of Microbiology, Boston, MA, 1 to 5 June 2008). The potential spreading of a strain of Cryptococcus capable of infecting immunocompetent people is cause for concern, and C. gattii infection is now a reportable disease in Washington State (29).The Kozel laboratory and others have shown that Cryptococcus spp. strongly activate the alternative pathway of the complement cascade (5, 14) while the polysaccharide capsule blocks the activation of the classical pathway that occurs at the cell wall in nonencapsulated strains (13). The capsule serves as a site for activation and deposition of C3 fragments, mainly iC3b, which promote phagocytosis of the yeast (12). C. gattii does not appear to activate the alternative pathway as potently as C. neoformans (28, 31). One study found that C. gattii binds fewer C3 molecules than C. neoformans (28). A later report indicated that while the maximum amount of bound C3 did not differ significantly between species, there was more rapid accumulation of C3 on C. neoformans before a steady state was achieved (31). In the absence of C5 in mouse strains B10.D2/oSn, DBA/2, and A/J, C. neoformans infection proceeds to a fatal pneumonia with higher fungal burdens in the blood, brain, lungs, and liver than in complement-sufficient animals (6, 21, 22). Together, these studies indicate a significant role for the alternative pathway of complement in protection against C. neoformans. While C. gattii has been reported to not activate the alternative pathway as vigorously as C. neoformans in vitro, the role of complement in in vivo C. gattii infection has not been determined.In this study, we investigated the role of complement pathways in protecting against infection with C. gattii. Mice deficient in complement components C1q, C4, C3, and factor B have been generated on the C57BL/6J background (2, 16, 30). Using these mice and mice treated with cobra venom factor (CVF), which depletes C3 and prevents cascade progression from C3 to C9, we have now shown that complement activation plays an essential role in delaying disease progression in mice infected with C. gattii.     

Antiphospholipid Antibody-Mediated Reproductive Failure in Antiphospholipid Syndrome

The association of elevated titers of circulating antiphospholipid (anti-PL) Abs in antiphospholipid syndrome (APS) and reproductive failure is well established in the literature. The clinical features include recurrent abortions at various stages, including implantation, placentation in the first trimester, miscarriages in the second and third trimesters, intrauterine growth retardation, preeclampsia with placental insufficiency and growth restrictions, arterial and venous thrombosis, and possibly also infertility. APS-mediated recurrent pregnancy loss and other features of reproductive failure might result from diverse autoimmune factors, inflammation, involving different mechanisms, which encompass pathogenic anti-PL Abs. Herein, we discuss the association of anti-PL Abs with reproductive failure with special emphasis on antiphospholipid autoantibodies characterizing APS. This association is evident from either human studies or murine models.     

Macrophage Activation Syndrome-Associated Markers in Severe Dengue

Hemophagocytosis, a phenomenon of which activated macrophages phagocytosed hematopoietic elements was reportedly observed in severe dengue patients. In the present study, we investigated whether markers of macrophage activation syndrome (MAS) can be used as differential diagnostic markers of severe dengue. Two hundred and eight confirmed dengue patients were recruited for the study. Sandwich ELISA was used to determine serum ferritin, soluble CD163 (sCD163), and soluble CD25 (sCD25) levels. The population of circulating CD163 (mCD163) monocytes was determined using flow cytometry. Receiver operating characteristic (ROC) analysis was plotted to determine the predictive validity of the biomarkers. Serum ferritin and sCD163 were found significantly increased in severe dengue patients compared to dengue fever patients (P = 0.003). A fair area under ROC curves (AUC) at 0.72 with a significant P value of 0.004 was observed for sCD163. sCD25 and mCD163 levels were not significantly different between severe dengue and dengue fever patients. Our findings suggest that in addition to serum ferritin, sCD163 can differentiate severe dengue from that of dengue fever patients. Hence, sCD163 level can be considered for use as a predictive marker for impending severe dengue.     

Role of Electron Microscopy in Transplant Renal Pathology

The crucial role that electron microscopy plays in diagnostic renal pathology is undisputed. By allowing recognition of findings not identifiable by light microscopic evaluation, electron microscopy has contributed significantly to the understanding of renal diseases and has proven to be of unquestionable value in many diagnostic situations. However, the percentage of cases in which electron microscopic examination adds important information that is either key for establishing or confirming a diagnosis or provides valuable data that influence patient's management remains controversial. This figure depends on the renal biopsy service that is surveyed, but it is reported that on the average ultrastructural evaluation is of value in approximately 30 to 45% of the cases. Correct interpretation of a renal biopsy depends on the ability to correlate light, immunofluorescence, and ultrastructural findings. In contrast, the role of electron microscopy in the examination of renal transplant specimens remains controversial. Many centers do not use routine electron microscopy to examine these specimens and insist that there are only a few specific indications that require ultrastructural evaluation. There is general agreement among renal pathologists that electron microscopy is of importance in the evaluation of renal specimens from patients with proteinuria to distinguish between transplant glomerulopathy, recurrent or de novo glomerulonephritis in order to correctly manage these patients and predict survival of the graft. The other possible indications are much more controversial. This paper summarizes and critically reviews the literature available on this subject and defines recommendations based on the information available at the current time.     

Role of Complement in Host Defense against Pneumococcal Otitis Media

Strategies to limit complement deposition on Streptococcus pneumoniae are established as virulence features for invasive disease, but their role in respiratory tract infection requires further analysis. We evaluated complement C3 protein deposition on discordant S. pneumoniae isolates of the same serotype (6A) and their capacity to cause nasopharyngeal (NP) colonization and experimental otitis media (EOM) in an animal model. We compared C3 binding to five 6A isolates from asymptomatic NP carriers with five 6A strains that caused invasive disease, and we observed less C3 (∼10-fold less fluorescence) binding to invasive isolates. We selected two high-level C3-binding carriage and two low-level C3-binding invasive 6A isolates for further study. In the EOM model, 11/12 (92%) ears challenged with a low-level C3-binding 6A strain became infected. Only 2/8 (25%) ears challenged with the discordant high-level C3-binding 6A isolate developed disease (P = 0.005). Results with the second discordant 6A isolate pair were comparable. Cobra venom factor (CoVF) treatment, which depletes C3 and consumes complement, restored virulence of the high-level C3-binding strain; 8/8 (100%) ears in CoVF-treated animals developed EOM compared to only 25% of ears in naïve animals (P = 0.007). These studies demonstrate the critical role for complement evasion in pneumococcal EOM. Colonization with carriage isolates that bound high levels of C3 caused EOM in fewer animals compared to low-level C3-binding invasive strains. Thus, limiting C3 deposition on the surface of S. pneumoniae correlates with increased incidence of EOM following NP colonization and barotrauma in the animal model.The pathogenesis of Streptococcus pneumoniae infection involves initial colonization of the nasopharynx, followed by its spreading to the middle ear, sinus, or lower respiratory tract and, in some cases, invasion of the bloodstream. To successfully cause disease, the pneumococcus has evolved a number of mechanisms to avert complement-mediated opsonization and phagocytosis. Pneumococci possess a broad variety of specialized surface proteins, some of which are adapted to interact with host defenses during colonization or dissemination in humans. Being a gram-positive bacterium, it is resistant to the bactericidal activity of complement (24) because its rigid cell wall prevents lysis by the membrane attack complex. The capsular polysaccharide is critical for resistance to complement deposition (32) and may also mask cell wall-associated complement from being recognized by the complement receptors on phagocytes (6). Additionally, select surface proteins can degrade native C3 proteins, thereby preventing or diminishing binding of C3b and iC3b to the bacterial surface, which are necessary components for opsonization (3). Furthermore, an important role for complement is suggested by the association of increased risk for invasive infections in individuals (or animal models) with deficiencies of complement proteins such as C2 and C3 and of complement receptors such as CR3 (2, 16). Type-specific antibody formation is an important host defense mechanism against infections caused by S. pneumoniae. However, the efficacy of opsonization of pneumococci by either immunoglobulin M (IgM) or IgG is related to their ability to enhance complement deposition on the bacterial surface, thus making complement essential for recovery from pneumococcal disease (6, 9).Colonization of mucosal surfaces is often the first step in the development of disease. Studies of S. pneumoniae support recent acquisition as the critical event preceding the development of pneumococcal otitis media. S. pneumoniae has evolved specific characteristics that are critical in dictating initial success for establishing colonization within a competitive niche of the mucosal surface of the nasopharynx. Often the success of an organism in establishing carriage depends on its ability to resist innate clearance mechanisms generated in the setting of polymicrobial stimulation. Lysenko and colleagues have demonstrated that complement and polymorphonuclear leukocytes are necessary host defenses for the elimination of S. pneumoniae from the nasopharynx in the presence of nontypeable Haemophilus influenzae (NTHi) (29).Prior work from our laboratory demonstrated that complement was an important host defense mechanism against protection of the middle ear from infection with NTHi (15). We hypothesized that complement would also be relevant for protection against S. pneumoniae respiratory tract infection (RTI). We evaluated the role of complement by comparing the capacities of four isolates of S. pneumoniae, all belonging to serotype 6A but differing in their abilities to bind complement to their surface to cause otitis media following nasopharyngeal (NP) colonization. Our goal was to determine whether evasion of complement deposition was an important virulence feature in the pathogenesis of RTI, using a model for experimental otitis media (EOM). The model requires initial NP colonization followed by ascension through the Eustachian tube after barotrauma for establishing middle ear infection.     

The Nature of Experimental Second-Set Kidney Transplant Rejection: 3. The Role of Complement

Second-set kidney transplant reactions occur in decomplemented dogs. The microscopic features of the reaction under such conditions indicate that the typical acute inflammatory reaction has occurred. Since the factors (presumably antibodies) mediating the second-set reaction are not dependent on complement fixation it is unlikely that HL-A antibodies, which are dependent on complement fixation for cytotoxicity, can be implicated in so-called hyperacute rejection (i.e. second-set rejection) in humans. The microscopic features of a genuine second-set reaction in humans are identical to those originally described in the dog. Since the first observable sign of the second-set kidney reaction is vasoconstriction of the outer cortical vessels, it is essential to establish how a severe antigen-antibody reaction can evoke such a rapid destruction of a kidney allotransplanted to a previously sensitized recipient. It is glomerular basement membrane which first bears the brunt of the second-set reaction. Tubular damage appears to follow gross ischaemia resulting from afferent renal vasoconstriction. Since the antigens of renal basement membrane are not represented on the lymphocytes, serious problems arise in attempting to assess renal histocompatibility by lymphocyte markers.     

Antiphospholipid Antibodies are Associated with Low Levels of Complement C3 and C4 in Patients with Systemic Lupus Erythematosus

Complement activation and low complement levels are common in systemic lupus erythematosus (SLE). Antiphospholipid antibodies (aPL) are found in about 30‐40% of patients with SLE. This study aimed to investigate the association between aPL and complement levels in patients with SLE. Serum samples were collected from 269 patients with SLE enrolled in the Norwegian Systemic Connective Tissue and Vasculitis Registry (NOSVAR) during 2003‐2009, and from 353 controls. All samples were analysed for anti‐β2 glycoprotein 1 (anti‐β2GP1) and anticardiolipin antibodies (aCL), C‐reactive protein (CRP) and complement components C3 and C4. Median CRP level was significantly higher in cases than in controls (2.06 versus 0.90 mg/l; P < 0.0001). No significant difference in CRP was found between SLE patients with or without aPL (2.09 versus 1.89; P = 0.665). Median C3 levels were similar in cases (1.03 g/l) and controls (1.00 g/l), whereas median C4 levels were 0.16 g/l in cases versus. 0.19 in controls (P < 0.0001). However, aPL‐positive SLE patients had significantly lower median C3 levels (0.92 versus. 1.07 g/l; P = 0.001) and C4 levels (0.11 versus 0.16 g/l; P < 0.0001) compared to aPL‐negative patients. Lower C3 and C4 values in aPL‐positive SLE patients may reflect a higher consumption of C3 and C4 due to more pronounced complement activation in aPL‐positive SLE patients compared to SLE patients without aPL.     

Complement C3 Plays an Essential Role in the Control of Opportunistic Fungal Infections

The innate recognition of fungal pathogens is a crucial first step in the induction of protective antifungal immunity. Complement is thought to be one key component in this process, facilitating fungal recognition and inducing early inflammation. However, the roles of the individual complement components have not been examined extensively. Here we have used mice lacking C3 to examine its role in immunity to opportunistic fungal pathogens and show that this complement component is essential for resistance to infections with Candida albicans and Candida glabrata. We demonstrate that the absence of C3 impairs fungal clearance but does not affect inflammatory responses. We also show that the presence of C3 contributes to mortality in mice challenged with very high doses of Saccharomyces cerevisiae, although these effects were found to be mouse strain dependent.Over the last few decades, modern medical practice and acquired immunodeficiencies have contributed to a substantial increase in the incidence of infections with normally commensal or nonpathogenic fungi (22), prompting a renewed interest in expanding our understanding of the mechanisms underlying protective host immunity. Identification and characterization of the receptors involved in the innate recognition of these organisms are of particular importance, as these “pattern recognition receptors” (14) not only are responsible for mediating the recognition and uptake of fungi by phagocytes but also initiate and direct the resultant immune response (17). While numerous nonopsonic pattern recognition receptors for fungi have been well characterized, including several members of the C-type lectin and Toll-like receptor families (for recent reviews, see references 20, 21, and 32), less is known about the opsonic mechanisms of fungal recognition, such as those mediated by complement, despite the essential role of these systems in antifungal immunity (24).The complement system consists of more than 30 serum and cellular surface proteins and is activated through three main routes: the classical, alternative, and lectin pathways. Triggering of these three pathways initiates enzymatic cascades which converge on the third complement component, C3, whose activation leads ultimately to microbial opsonization, the release of chemotactic factors, including C3a and C5a, and the generation of a membrane attack complex (MAC) (18). Activation of the classical pathway occurs primarily following the binding of C1q to antibody-antigen complexes, and although involved in the adaptive arm of the immune system, this pathway is also triggered by natural antibodies, thereby contributing to the innate recognition of fungi (16). Initiation of the alternative pathway occurs through the spontaneous activation of C3 on microbial surfaces, and the lectin pathway is initiated by the binding of the mannose-binding lectin to carbohydrates on microbial surfaces (24).All three of these pathways are induced by opportunistic fungal pathogens, including Candida albicans (13, 15, 24), although the alternative pathway may be the most critical (7, 15). The protective role of complement in immunity to these pathogens has largely been determined in mice treated with cobra venom factor (CVF), which depletes serum complement by forming a potent C3 convertase, or in C5-deficient mouse strains, such as A/J or DBA2, where fungal opsonization remains intact but the animals lack the ability to generate C5a or the MAC (6, 7, 9, 10, 19). Why C5-deficient mice are susceptible is still unclear, but is likely to be due to aberrant inflammatory responses, as the MAC has little effect on the viability of fungal pathogens (15, 19). Although the roles of each of the pathways of complement activation have been relatively poorly examined, recent studies have started to address this issue, looking at the role of factor B, C2, C1q and mannose-binding lectin in the control of systemic candidiasis (11).C3 is a central component in all of the complement pathways, as described above, but it has not been studied in isolation with respect to fungal infection. Studying the role of this complement component in isolation is particularly relevant, given the alternative route of C5 activation by thrombin, which can substitute for C3-dependent C5 convertase activity (12, 30). Therefore, we undertook to specifically examine the role of C3 in the control of opportunistic fungal infections using C3-sufficient and C3-deficient mice.