Native properdin binds to Chlamydia pneumoniae and promotes complement activation

Activation of complement represents one means of natural resistance to infection from a wide variety of potential pathogens. Recently, properdin, a positive regulator of the alternative pathway of complement, has been shown to bind to surfaces and promote complement activation. Here we studied whether properdin-mediated complement activation occurs on the surface of Chlamydia pneumoniae, an obligate intracellular Gram-negative bacterium that causes 10 to 20% of community-acquired pneumonia. We have determined for the first time that the physiological P₂, P₃, and P₄ forms of human properdin bind to the surface of Chlamydia pneumoniae directly. The binding of these physiological forms accelerates complement activation on the Chlamydia pneumoniae surface, as measured by C3b and C9 deposition. Finally, properdin-depleted serum could not control Chlamydia pneumoniae infection of HEp-2 cells compared with normal human serum. However, after addition of native properdin, the properdin-depleted serum recovered the ability to control the infection. Altogether, our data suggest that properdin is a pattern recognition molecule that plays a role in resistance to Chlamydia infection.Chlamydiae are obligate intracellular Gram-negative bacteria that develop in a host cell within a membrane-bound compartment termed an inclusion. In humans, Chlamydia pneumoniae causes diseases of the respiratory tract, e.g., bronchitis, sinusitis, or pneumonia, with potentially severe sequelae that include atherosclerosis and chronic obstructive pulmonary disease (2, 8, 20).Chlamydia''s unique developmental cycle starts when the infectious form of the bacterium, the elementary body (EB), targets the mucosal respiratory epithelium and remains within a nonacidified vacuole known as an inclusion (13). Soon after the infection, the EB differentiates into a noninfectious, but metabolically active, reticulate body (RB), which proliferates by binary fission within the expanding inclusion. The generated progeny differentiates back into EBs that are released upon host cell lysis to infect other cells. During the infection process, Chlamydia does not necessarily remain confined to these primary sites; it also has the propensity to disseminate to extramucosal tissues. For instance, it has been suggested that C. pneumoniae multiplies and survives within macrophages (16, 35, 40) and polymorphonuclear neutrophils (PMN) (42, 50) in order to propagate to the rest of the body and reach endothelial cells. During this journey, from circulating cells to endothelial cells, C. pneumoniae encounters the innate immune system, in which the complement system may play a fundamental role in controlling Chlamydia infection.The complement system is a central component of the innate immune response and is involved in many functions, including recognition, opsonization, phagocytosis, and destruction of foreign cells, as well as generation of chemotactic fragments (C3a and C5a) and activation of adaptive immunity (30, 32, 51) Three pathways of complement activation are known: the classical, lectin, and alternative pathways. Although each uses its own unique mechanism for target versus host discrimination, all pathways result in covalent attachment of C3b to the target and can potentially assemble pores in the bilipid layer of the cell being attacked. The alternative pathway initiates when spontaneously hydrolyzed C3 binds to activating surfaces (i.e., certain bacteria and viruses). Therefore, this pathway does not require a specific antibody response for activation and may play an important role in controlling primary infections with pathogens. Although it is has been described that Chlamydia trachomatis activates the alternative pathway (21, 31), little is known about the effect of this pathway on C. pneumoniae. Moreover, the mechanisms involved in alternative pathway-mediated complement activation or its consequences on infection in C. pneumoniae remain unknown.Properdin is the only positive regulator of the alternative pathway. It is a plasma protein synthesized by monocytes, hepatocytes, mast cells, T cells (45, 46, 49), and shear-stressed endothelial cells (6) and is also a component of the secondary granules in neutrophils (PMN) (53). Patients with properdin deficiency have a higher risk of meningococcal disease than the general population (14), and an association has also recently been found with recurrent otitis media and pneumonia (44). Properdin facilitates alternative pathway complement activation and amplification by extending the half-life of the C3b,Bb convertase (11). The stabilized C3bBb convertase then rapidly cleaves more C3 to C3b, which acts as an opsonin and can reinitiate the pathway in an amplification loop that proceeds on the pathogenic cell.Properdin is composed of cyclic dimers (P₂), trimers (P₃), and tetramers (P₄) of a 53-kDa monomeric subunit (37, 47). Biochemical studies of purified properdin indicate that this protein can form nonphysiological higher-level polymers during events such as long-term storage and freeze-thawing (10, 37). This form, also known as “activated properdin or P_n,” has the abnormal ability to activate complement in solution (37). Recent studies using purified properdin have reported that properdin can act as a pattern recognition molecule and bind directly to surfaces such as dying cells and Neisseria gonorrhoeae in the absence of C3b, serving as a platform for de novo C3bBb assembly (15, 25, 26, 48, 56). Although the data from these studies are consistent with the complement initiation function proposed over 50 years ago (38), we have recently shown that physiological forms of properdin in the absence of artifactual aggregates do bind to late apoptotic and necrotic cells (12, 56) but do not bind to Neisseria spp. (1). Therefore, properdin is likely very selective in its recognition of surfaces.Considering the importance of the recent findings that implicate properdin as a complement initiator on surfaces, we sought to determine whether the physiological forms of properdin (P₂, P₃, and P₄) have the ability to bind to and promote complement activation on the C. pneumoniae surface. Herein, we provide evidence that properdin plays an active role in alternative pathway activation on the C. pneumoniae surface and in controlling infection, suggesting a role for properdin as a specific pattern recognition molecule.     

Alternative complement pathway activation by HIV infected cells: C3 fixation does not lead to complement lysis but enhances NK sensitivity

HIV infected T and monocytic cell lines could activate and fix C3 fragments when incubated in human serum under conditions allowing for activation of the alternative complement pathway. Normal T lymphocytes incubated with HIV could also activate and fix C3. This activity was, at least in part, the property of the virus itself since cell-free HIV could efficiently activate C3. The C3 activating HIV infected cells were resistant to complement-mediated lysis, even after prolonged incubation periods. However, their sensitivity to cell-mediated natural killing increased, presumably due to their interaction with complement receptor bearing NK lymphocytes. The results suggest that the alternative complement pathway may contribute to the depletion of CD4+ T lymphocytes during HIV infection in vivo.     

Covalent linkage of C3 to properdin during complement activation

Activation of the alternative complement pathway of serum produces complexes of properdin (P) and C3 as measured in a double antibody enzyme-linked immunosorbent assay. When purified from serum, these complexes decrease factor B hemolytic activity in serum and do not restore the alternative pathway hemolytic activity of serum deficient in P. Sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of activated serum containing biotinylated P, followed by blotting to nitrocellulose and development with streptavidin-alkaline phosphatase revealed a band at 53 kDa for monomeric P and an additional band at 160 kDa. P samples eluted from zymosan and purified from activated serum revealed a band at 116 kDa for C3α and 74 kDa for C3β, and an additional band at 160 kDa when analyzed by SDS-PAGE, Western blotting and development with antibody to C3. The appearance of a 160 kDa band containing P and C3 indicates that these proteins are contained in a complex formed during activation of the alternative pathway. Activation of a purified reagent mixture containing factors B, D, and H, and ¹²⁵I-labeled P or ¹²⁵I-labeled C3, followed by SDS-PAGE and autoradiography confirmed the presence of a 160-kDa band which disappeared following hydroxylamine treatment of the sample. These data are consistent with a covalent linkage of C3 to P via the C3α chain, producing the 160-kDa complex.     

Human complement C3 deficiency: Th1 induction requires T cell-derived complement C3a and CD46 activation

Human T helper type 1 (Th1) responses are essential in defense. Although T cell receptor (TCR) and co-stimulator engagement are indispensable for T cell activation, stimulation of additional receptor pathways are also necessary for effector induction. For example, engagement of the complement regulator CD46 by its ligand C3b generated upon TCR activation is required for IFN-γ production as CD46-deficient patients lack Th1 responses. Utilizing T cells from two C3-deficient patients we demonstrate here that normal Th1 responses also depend on signals mediated by the anaphylatoxin C3a receptor (C3aR). Importantly, and like in CD46-deficient patients, whilst Th1 induction are impaired in C3-deficient patients in vitro, their Th2 responses are unaffected. Furthermore, C3-deficient CD4⁺ T cells present with reduced expression of CD25 and CD122, further substantiating the growing notion that complement fragments regulate interleukin-2 receptor (IL-2R) assembly and that disturbance of complement-guided IL-2R assembly contributes to aberrant Th1 effector responses. Lastly, sustained intrinsic production of complement fragments may participate in the Th1 contraction phase as both C3a and CD46 engagement regulate IL-10 co-expression in Th1 cells. These data suggest that C3aR and CD46 activation via intrinsic generation of their respective ligands is an integral part of human Th1 (but not Th2) immunity.     

In vivo complement activation and binding of C3 to encapsulated Cryptococcus neoformans.

Tissues from mice infected with Cryptococcus neoformans were examined by immunofluorescence to determine the extent of deposition of complement component C3 on encapsulated cryptococci. The relative percentages of cryptococci in each tissue having readily visible C3 were greatest for liver and lung tissues, with the kidney tissue having the next highest percentage and the spleen having the lowest percentage. Binding of C3 fragments to cryptococci in brain tissue was essentially absent.     

Plasma C3d/C3 quotient as a parameter for in vivo complement activation

Plasma levels of C3 and C3d in 30 healthy donors and 58 patients with systemic lupus erythematosus were determined by rocket immunoelectrophoresis and Con-A affinoimmunoelectrophoresis. Significantly decreased levels of C3 were found in approximately 50% of the SLE sera. Slight increases of free C3d were found in 55%, and marked increases in 9% of the SLE sera; the remaining SLE sera did not show increased C3d levels. When results were expressed as quotients of plasma C3d/C3, a significant pattern emerged and 70% of the SLE sera clearly fell outside the normal range. 10% of the SLE sera had normal C3d/C3 indices but had markedly depressed C3 levels. We conclude that in cases where total serum C3 is not markedly reduced (less than 60% of normal), the C3d/C3 quotient may be a more sensitive parameter for assessing in vivo complement activation than C3 or C3d determinations alone.     

Human astrovirus coat protein binds C1q and MBL and inhibits the classical and lectin pathways of complement activation

Human astroviruses (HAstVs) constitute a family of non-enveloped, RNA viruses which cause infantile gastroenteritis. We have previously demonstrated that purified HAstV coat protein (CP), multiple copies of which compose the viral capsid, bind C1q resulting in inhibition of classical complement pathway activity. The objective of this study was to further analyze the mechanism by which CP inhibits C1 activation. CP inhibited C1 activation, preventing cleavage of C1s to its active form in the presence of heat-aggregated IgG, a potent classical pathway activator. CP also inhibited generation of the potent anaphylatoxin C5a. CP dose-dependently bound to C1q, the isolated globular heads and the collagen-like regions of the C1q molecule. When CP was added to C1, C1s dissociated from C1q suggesting that CP functionally displaces the protease tetramer (C1s–C1r–C1r–C1s). Given the structural and functional relatedness of C1q and MBL, we subsequently investigated the interactions between CP and MBL. CP bound to purified MBL and was able to inhibit mannan-mediated activation of the lectin pathway. Interestingly, CP did not bind to a variant of MBL that replaces a lysine residue (Lys55) critical for binding to MASP-2, a functional homolog of C1s. Finally, CP was shown to cross the species barrier to inhibit C3 activation and MAC formation in rat serum. These findings suggest CP inhibits C1 and MBL activation via a novel mechanism of interference with the normal interaction of the recognition molecule with its cognate serine proteases.     

Compstatin inhibits complement activation by binding to the beta-chain of complement factor 3

Compstatin is a peptidic complement inhibitor that prevents the cleavage of complement factor 3 (C3) by C3 convertase. Compstatin differs from other C3-regulatory proteins, such as complement receptor (CR) 1 and decay-accelerating factor (DAF), in that it binds native as well as activated C3 fragments and acts through mechanisms that do not involve the destabilization of the C3 convertase or the accelerated degradation of C3b. Compstatin's activity most likely relies on its affinity for native C3 and the conformational change that results upon binding with C3. Although the intermolecular interactions between compstatin and C3 have been studied, the identity of the targeted region on C3 is still elusive. To address this issue, we synthesized a photo-crosslinking compstatin analog and used it to probe C3 for sites of interaction. We identified a 40-kDa region at the C-terminus of the beta-chain of C3 that included the binding site of the compstatin analog. The specificity of the binding was confirmed by inhibition studies, which showed reduced crosslinking signal after pre-incubation of C3 with compstatin but not with various inactive analogs. Binding studies performed with a recombinant homolog of the 40-kDa region confirmed these findings. Five smaller recombinant proteins corresponding to various overlapping regions of the 40-kDa fragment did not bind compstatin, suggesting that a proper protein conformation, only found in larger fragments, is required for compstatin binding. The identified region on the beta-chain has, thus far, not been implicated in C3 cleavage or interactions with other proteins. Therefore, further research on this part of the C3 molecule may have implications for studies on the regulation of C3 cleavage, as well as for complement-based drug design.     

S protein binds to serum-treated agarose beads independently of complement activation and the formation of the terminal complement complex on the beads.

Comparison of initial (early-phase) and terminal (late-phase) sequence activation of complement by agarose beads and endotoxin was evaluated in an enzyme immunoassay (EIA) of serum levels of C3c and C9 neoepitopes, respectively. EIA and Western blotting with anti-S protein monoclonal antibody revealed lower S protein values and weaker S protein bands in serum activated by agarose beads than by endotoxin, implying that S protein was removed from serum by binding to agarose. The binding of S protein to the beads was confirmed by radioimmunoassay and was found to be equal in normal and heat-inactivated serum. In contrast, the terminal complement complex was formed only on agarose beads incubated with normal serum and not with inactivated serum.     

Classical complement pathway activation by antipneumococcal antibodies leads to covalent binding of C3b to antibody molecules

We have examined whether or not a physical relationship exists between antipneumococcal antibodies (Ab) and C3b when Ab activate the classical complement pathway on the surface of pneumococci (Pn). After sensitization with 125I-labeled Ab, Pn were sequentially incubated with purified C1, C4, C2, and biotinylated C3. Ab molecules were then eluted from Pn, and C3b-associated molecules were purified on avidin-Sepharose. Both 125I-labeled immunoglobulin G (IgG) and [125I]IgM bound to C3b; the association was stable to incubation in 1% sodium dodecyl sulfate at 37 degrees C. The association was only partially reversed by incubation in 1 M hydroxylamine-0.5% sodium dodecyl sulfate (pH 10.5), implying that Ab and C3b were linked by amide as well as ester bonds. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of dithiothreitol and NH2OH eluates from the avidin-Sepharose showed that C3b bound to both heavy and light chains of the Ab. Moreover, the ability to bind to erythrocyte C3b receptors could be transferred to unsensitized Pn by eluates from Pn on which Ab had activated the classical pathway. These covalent complexes of Ab and C3b may be especially important in the opsonization of Pn by Ab and complement.     

Mediation of a non-proteolytic activation of complement component C3 by phospholipid vesicles

Liposomes are becoming increasingly important as drug delivery systems, to target a drug to specific cells and tissues and thereby protecting the recipient from toxic effects of the contained drug. Liposome preparations have been described to activate complement. In this study, we have investigated complement activation triggered by neutral dimyristoyl-phosphocholine (DMPC) liposomes in human plasma and whole-blood systems. Incubation in plasma led to the generation of complement activation products (C3a and sC5b-9). Unexpectedly, investigations of surface-bound C3 revealed contact activated, conformationally changed C3 molecules on the liposomes. These changes were characterized by Western blotting with C3 monoclonal antibodies, and by incubating liposomes with purified native C3 and factors I and H. Quartz crystal microbalance analysis confirmed binding of C3 to planar DMPC surfaces. In addition, we demonstrated that DMPC liposomes bound to or were phagocytized by granulocytes in a complement-dependent manner, as evidenced by the use of complement inhibitors. In summary, we have shown that C3 is activated both by convertase-dependent cleavage, preferentially in the fluid phase, by mechanisms which are not well elucidated, and also by contact activation into C3(H₂O) on the DMPC surface. In particular, this contact activation has implications for the therapeutic regulation of complement activation during liposome treatment.     

Regulation of complement activation at the C3-level by serum resistant leptospires

Leptospires are spirochetes that are transmitted to humans through contacts with wild or domestic animals or via an exposure to contaminated soil or water. In this study we have compared the serum-sensitivity of five pathogenic strains of leptospires (L. interrogans) to an environmental isolate (strain Patoc). Different levels of sensitivities to human serum were seen. Interestingly, the most sensitive strain was the non-pathogenic Patoc strain. The fully and intermediately resistant strains have been isolated from human patients. Testing was performed in the absence of specific antibodies, and killing was found to be dependent on the complement system. The serum sensitive Patoc strain was killed in human serum within minutes, whereas the most resistant strains tolerated serum up to 4h. We also tested the deposition of the complement components C3, C5, C6, C8 and C5b-9 to the surfaces of the sensitive and resistant strains of Leptospira by immunofluorescence microscopy and ELISA. C3 was deposited on both the sensitive and resistant strains, but the terminal complement components were detected only on the surface of the complement-sensitive strain. The complement resistant and intermediate strains were found to bind more factor H from human serum than the complement sensitive strain. Thus, binding of this major alternative complement pathway inhibitor is related to serum resistance in Leptospira spirochetes.     

Complement in asthma: sensitivity to activation and generation of C3a and C5a via the different complement pathways

Studies in rodent models suggested that complement may play a critical role in susceptibility to airway hyperresponsiveness (AHR) and as a mediator of bronchial obstruction and inflammation in asthma. Complement may participate in susceptibility to asthma because of an intrinsic abnormality in complement activation and generation of C3a, C5a, or other products that affect cellular responses, resulting in T(H)2 predominance and asthma susceptibility. Alternatively, an intrinsic abnormality in the cellular response to complement activation products could determine susceptibility to asthma. In this study, the authors investigated whether complement in patients with atopic asthma versus nonatopic controls possesses an increased propensity to become activated. Despite reports that total complement plasma levels in unchallenged asthmatics are normal, an abnormal sensitivity of complement to activation may exist if an isoform or a polymorphic variant of a complement protein was present and resulted in gain or loss of function without associated changes in total complement levels. Therefore, complement activation was induced in vitro in plasma of asthmatics and controls using activators of the classical, alternative, and lectin pathways and measured C3a, other C3 fragments, and C5a. For each pathway, similar amounts of generated fragments, as well as C3a/C3 and C5a/C5 ratios, were found in asthmatics and controls. Also, similar basal plasma levels of C3a and C5a were found in both groups; however, mannan-binding lectin (MBL) levels were moderately elevated in asthmatics. In conclusion, the results suggest that, in asthmatic patients, complement activation does not exhibit an abnormal sensitivity to activation by any of the known activation pathways.     

Evidence for non-traditional activation of complement factor C3 during murine liver regeneration

Complement signaling has been implicated as important for normal hepatic regeneration. However, the specific mechanism by which complement is activated during liver regeneration remains undefined. To address this question, we investigated the hepatic regenerative response to partial hepatectomy in wildtype mice, C3-, C4-, and factor B-null mice, and C4-null mice treated with a factor B neutralizing antibody (mAb 1379). The results showed that following partial hepatectomy, C3-null mice exhibit reduced hepatic regeneration compared to wildtype mice as assessed by quantification of hepatic cyclin D1 expression and hepatocellular DNA synthesis and mitosis. In contrast, C4-null mice and factor B-null mice demonstrated normal liver regeneration. Moreover, animals in which all of the traditional upstream C3 activation pathways were disrupted, i.e. C4-null mice treated with mAb 1379, exhibited normal C3 activation and hepatocellular proliferation following partial hepatectomy. In order to define candidate non-traditional mechanisms of C3 activation during liver regeneration, plasmin and thrombin were investigated for their abilities to activate C3 in mouse plasma in vitro. The results showed that both proteases are capable of initiating C3 activation, and that plasmin can do so independent of the classical and alternative pathways. Conclusions: These results show that C3 is required for a normal hepatic regenerative response, but that disruption of the classical- or lectin-dependent pathways (C4-dependent), the alternative pathway (factor B-dependent), or all of these pathways does not impair the hepatic regenerative response, and indicate that non-traditional mechanisms by which C3 is activated during hepatic regeneration must exist. In vitro analysis raises the possibility that plasmin may contribute to non-traditional complement activation during liver regeneration in vivo.     

PEO enhancement of platelet deposition, fibrinogen deposition, and complement C3 activation.

Whereas it has been commonly thought that adding polyethylene oxide PEO to a surface would diminish the capacity of the surface to cause deposition of platelets and of fibrinogen, and to activate complement C3, we present data showing exactly the opposite. These unexpected results are obtained with low molecular weight (2000) PEO, and are not found with higher molecular weight (20,000) PEO.     

Regulation of B lymphocyte activation by complement C3 and the B cell coreceptor complex

Complement is an essential innate immune mechanism that recognizes and eradicates microbes and associated toxins. In addition, complement receptors (CD21 and CD35) on B cells cooperate with the B-cell antigen receptor (BCR) to efficiently recognize and respond to antigens bearing complement C3d(g). Fixation of C3d(g) to antigen confers adjuvant properties and therefore its deposition may need to be carefully regulated to avoid autoreactivity. CD21 and/or CD35 engagement is nonmitogenic, and B-cell activation via BCR-CD21 coligation is enhanced through the recruitment of CD19. Recent efforts have sought a better understanding of the topological and biochemical properties of BCR and coreceptor (CD19-CD21-CD81) signaling, as well as the context for complement activation in the response to foreign and self antigens.     

Complexes between C1q and C3 or C4: novel and specific markers for classical complement pathway activation

Classical pathway activation is often assessed by measuring circulating levels of activated C4. However, this parameter does not discriminate between activation through the classical or the lectin pathway. We hypothesized that during classical pathway activation, complexes are formed between C1q and activated C4 or C3. Using ELISA, we investigated whether such complexes constitute specific markers for classical pathway activation. In vitro, C1q-C3d/C4d complexes were generated upon incubation of normal recalcified plasma with aggregated IgG or an anti-C1q mAb that activates C1 (mAb anti-C1q-130). In contrast, during incubation with C1s or trypsin, C1q-C3d/C4d complexes were not generated, which excludes an innocent bystander effect. Additionally, C1q-C3d/C4d complexes were not generated during activation of the alternative or the lectin pathway. Repeated freezing and thawing did not influence levels of C1q-C3d/C4d complexes in recalcified plasma. To measure C1q-complement complexes in plasma samples, we separated unbound complement proteins from C1q-C3d/C4d complexes in the samples prior to testing with ELISA. In samples from patients undergoing cardiopulmonary bypass surgery or suffering from rheumatoid arthritis, we found higher levels of C1q-C4 complexes than in samples from healthy individuals. We conclude that complexes between C1q and C4 or C3 are specific markers of classical complement pathway activation.     

Increased expression of the C3b receptor by neutrophils and complement activation during haemodialysis

Activation of complement and the relative number of C3b receptors expressed by neutrophils was assessed in patients undergoing haemodialysis with new and reused cellulosic membranes, and with polymethylmethacrylate (PMMA) membranes. Activation of complement was assessed by radioimmunoassay of plasma C3adesArg, and neutrophil C3b receptors were measured by fluorescent flow cytometry of cells indirectly stained with F(ab')2 anti-C3b receptor. During first use of cellulosic dialysis membranes by four patients, the mean expression of C3b receptors by neutrophils in blood taken from the afferent line of the extra-corporeal system after 10, 20, 60 and 120 min of dialysis increased to 127, 189, 255 and 296%, respectively. The mean plasma C3adesArg concentrations in the corresponding samples of blood were 225, 320, 236 and 160% of the pre-dialysis levels. During third and fifth use of the same membranes by these patients, the mean C3b receptor expression by neutrophils did not exceed 150% of the predialysis determination, and correspondingly minimal increases in plasma C3adesArg were observed. Analysis of blood taken simultaneously from the afferent and efferent lines of the first use cellulosic dialysis system indicated that the increase in C3b receptor expression by neutrophils and generation of C3adesArg occurred when blood came in contact with the dialysis membrane. Haemodialysis of four additional patients with the non-complement activating PMMA membrane caused only modest or no increases in neutrophil C3b receptors. Thus, complement activation in vivo is associated with up-regulation of neutrophilic C3b receptors, indicating that this cellular response previously described only in model, in vitro systems, is a physiological mechanism by which this cell can augment its capacity for responding to C3b opsonized material.     

In vitro C3 deposition on Cryptococcus capsule occurs via multiple complement activation pathways

Complement can be activated via three pathways: classical, alternative, and lectin. Cryptococcus gattii and Cryptococcus neoformans are closely related fungal pathogens possessing a polysaccharide capsule composed mainly of glucuronoxylomannan (GXM), which serves as a site for complement activation and deposition of complement components. We determined C3 deposition on Cryptococcus spp. by flow cytometry and confocal microscopy after incubation with serum from C57BL/6J mice as well as mice deficient in complement components C4, C3, factor B, and mannose binding lectin (MBL). C. gattii and C. neoformans activate complement in EGTA-treated serum indicating that they can activate the alternative pathway. However, complement activation was seen with factor B^−/− serum suggesting activation could also take place in the absence of a functional alternative pathway. Furthermore, we uncovered a role for C4 in the alternative pathway activation by Cryptococcus spp. We also identified an unexpected and complex role for MBL in complement activation by Cryptococcus spp. No complement activation occurred in the absence of MBL-A and -C proteins although activation took place when the lectin binding activity of MBL was disrupted by calcium chelation. In addition, alternative pathway activation by C. neoformans required both MBL-A and -C, while either MBL-A or -C was sufficient for alternative pathway activation by C. gattii. Thus, complement activation by Cryptococcus spp. can take place through multiple pathways and complement activation via the alternative pathway requires the presence of C4 and MBL proteins.