Plasmodium berghei: sporozoites are sensitive to human serum but not susceptible host serum

Plasmodium falciparum Gametes and Sporozoites Hijack Plasmin and Factor H To Evade Host Complement Killing

Plasmodium parasites are the etiological agents of malaria, a disease responsible for over half a million deaths annually. Successful completion of the parasite's life cycle in the vertebrate host and transmission to a mosquito vector is contingent upon the ability of the parasite to evade the host's defenses. The extracellular stages of the parasite, including gametes and sporozoites, must evade complement attack in both the mammalian host and in the blood ingested by the mosquito vector. Here, we show that Plasmodium falciparum gametes and sporozoites acquire mammalian plasminogen and activate it into the serine protease plasmin to evade complement attack by degrading C3b. Complement-mediated permeabilization of gametes and sporozoites was higher in plasminogen-depleted plasma, suggesting that plasminogen is important for complement evasion. Plasmin also facilitates gamete exflagellation through complement evasion. Furthermore, supplementing serum with plasmin significantly increased parasite infectivity to mosquitoes and lowered the transmission-blocking activity of antibodies to Pfs230, a potent vaccine candidate currently in clinical trials. Finally, we show that human factor H, previously shown to facilitate complement evasion by gametes, also facilitates complement evasion by sporozoites. Plasmin and factor H simultaneously cooperate to enhance complement evasion by gametes and sporozoites. Taken together, our data show that Plasmodium falciparum gametes and sporozoites hijack the mammalian serine protease plasmin to evade complement attack by degrading C3b. Understanding of the mechanisms of complement evasion by the parasite is key to the development of novel effective therapeutics. IMPORTANCE Current approaches to control malaria are complicated by the development of antimalarial-resistant parasites and insecticide-resistant vectors. Vaccines that block transmission to mosquitoes and humans are a plausible alternative to overcome these setbacks. To inform the development of efficacious vaccines, it is imperative to understand how the parasite interacts with the host immune response. In this report, we show that the parasite can co-opt host plasmin, a mammalian fibrinolytic protein to evade host complement attack. Our results highlight a potential mechanism that may reduce efficacy of potent vaccine candidates. Taken together, our results will inform future studies in developing novel antimalarial therapeutics.

Hijacking the human complement inhibitor C4b-binding protein by the sporozoite stage of the Plasmodium falciparum parasite

The complement system is considered the first line of defense against pathogens. Hijacking complement regulators from blood is a common evasion tactic of pathogens to inhibit complement activation on their surfaces. Here, we report hijacking of the complement C4b-binding protein (C4bp), the regulator of the classical and lectin pathways of complement activation, by the sporozoite (SPZ) stage of the Plasmodium falciparum parasite. This was shown by direct binding of radiolabeled purified C4bp to live SPZs as well as by binding of C4bp from human serum to SPZs in indirect immunofluorescence assays. Using a membrane-bound peptide array, peptides from the N-terminal domain (NTD) of P. falciparum circumsporozoite protein (CSP) were found to bind C4bp. Soluble biotinylated peptide covering the same region on the NTD and a recombinantly expressed NTD also bound C4bp in a dose-dependent manner. NTD-binding site on C4bp was mapped to the CCP1-2 of the C4bp α-chain, a common binding site for many pathogens. Native CSP was also co-immunoprecipitated with C4bp from human serum. Preventing C4bp binding to the SPZ surface negatively affected the SPZs gliding motility in the presence of functional complement and malaria hyperimmune IgG confirming the protective role of C4bp in controlling complement activation through the classical pathway on the SPZ surface. Incorporating the CSP-C4bp binding region into a CSP-based vaccine formulation could induce vaccine-mediated immunity that neutralizes this immune evasion region and increases the vaccine efficacy.

Deceiving and escaping complement - the evasive journey of the malaria parasite

During its life cycle, Plasmodium, the malaria parasite, is exposed to the human and mosquito complement systems. Early experiments demonstrated that activation of complement can pose a serious threat to parasites, but recent studies revealed complement-evasion mechanisms important for parasite survival. Blood-stage parasites and gametes recruit regulators to neutralize human complement activation, while ookinetes inhibit mosquito complement by disrupting epithelial nitration in response to midgut invasion. Here we provide an in-depth overview of the evasion mechanisms currently known and speculate on the existence of others not yet identified. Finally, we discuss how these mechanisms could provide novel targets for urgently needed malaria vaccines and therapeutics.

The impact of human complement on the clinical outcome of malaria infection

The tropical disease malaria remains a major cause of global morbidity. Once transmitted to the human by a blood-feeding mosquito, the unicellular malaria parasite comes into contact with the complement system and continues to interact with human complement during its intraerythrocytic replication cycles. In the course of infection, both the classical and the alternative pathway of complement are activated, leading to parasite opsonization and lysis as well as the induction of complement-binding antibodies. While complement activity can be linked to the severity of malaria, it remains to date unclear, whether human complement is beneficial for protective immunity or if extensive complement reactions may rather enhance pathogenesis. In addition, the parasite has evolved molecular strategies to circumvent attack by human complement and has even developed means to utilize complement factors as mediators of host cell infection. In this review, we highlight current knowledge on the role of human complement for the progression of malaria infection. We discuss the various types of interactions between malaria parasites and complement factors with regard to immunity and infection outcome and set a special emphasis on the dual role of complement in the context of parasite fitness.

Complement Factors in COVID-19 Therapeutics and Vaccines

Complement is integral to a healthy functioning immune system and orchestrates various innate and adaptive responses against viruses and other pathogens. Despite its importance, the potential beneficial role of complement in immunity to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been overshadowed by reports of extensive complement activation in severe coronavirus disease 2019 (COVID-19) patients. Here, we hypothesize that complement may also have a protective role and could function to enhance virus neutralization by antibodies, promote virus phagocytosis by immune cells, and lysis of virus. These functions might be exploited in the development of effective therapeutics and vaccines against SARS-CoV-2.

Complement has been implicated in playing some role in severe COVID-19 pathogenesis. However, the evidence to support this is largely inferred from case–control studies.

The potential protective role of complement has been largely ignored, which might contribute to innate and adaptive immunity against SARS-CoV-2 infection.

Immunity to many pathogens relies on complement to enhance antibody-mediated neutralization and mediate phagocytosis and lysis. These mechanisms might also contribute to immunity against SARS-CoV-2 infection, and complement might be potentially exploited in antibody-based therapeutics and vaccines.

Careful selection of vaccine adjuvants and epitopes included in vaccine constructs can influence whether vaccine-induced antibodies activate complement.

Mutations in monoclonal antibodies can be used to promote hexamer formation between antibodies, which can significantly improve complement binding and activation.

Complement in malaria: immune evasion strategies and role in protective immunity

The malaria parasite has for long been thought to escape host complement attack as a survival strategy. However, it was only recently that complement evasion mechanisms of the parasite were described. Simultaneously, the role of complement in antibody-mediated naturally acquired and vaccine-induced protection against malaria has also been reported. Such findings should be considered in future vaccine design, given the current need to develop more efficacious vaccines against malaria. Parasite antigens derived from molecules mediating functions crucial for parasite survival, such as complement evasion, or parasite antigens against which antibody responses lead to an efficient complement attack could present new candidates for vaccines. In this review, we discuss recent findings on complement evasion by the malaria parasites and the emerging role of complement in antibody-mediated protection against malaria. We emphasize that immune responses to vaccines based on complement inhibitors should not only induce complement-activating antibodies but also neutralize the escape mechanisms of the parasite.

Complement in malaria immunity and vaccines

Developing efficacious vaccines for human malaria caused by Plasmodium falciparum is a major global health priority, although this has proven to be immensely challenging over the decades. One major hindrance is the incomplete understanding of specific immune responses that confer protection against disease and/or infection. While antibodies to play a crucial role in malaria immunity, the functional mechanisms of these antibodies remain unclear as most research has primarily focused on the direct inhibitory or neutralizing activity of antibodies. Recently, there is a growing body of evidence that antibodies can also mediate effector functions through activating the complement system against multiple developmental stages of the parasite life cycle. These antibody‐complement interactions can have detrimental consequences to parasite function and viability, and have been significantly associated with protection against clinical malaria in naturally acquired immunity, and emerging findings suggest these mechanisms could contribute to vaccine‐induced immunity. In order to develop highly efficacious vaccines, strategies are needed that prioritize the induction of antibodies with enhanced functional activity, including the ability to activate complement. Here we review the role of complement in acquired immunity to malaria, and provide insights into how this knowledge could be used to harness complement in malaria vaccine development.

Immune Escape Mechanisms are Plasmodium's Secret Weapons Foiling the Success of Potent and Persistently Efficacious Malaria Vaccines

Despite decades of active research, an efficacious vaccine mediating long-term protection is still not available. This review highlights various mechanisms and the different facets by which the parasites outsmart the immune system. An understanding of how the parasites escape immune recognition and interfere with the induction of a protective immune response that provides sterilizing immunity will be crucial to vaccine design.

Complement 3d: from molecular adjuvant to target of immune escape mechanisms

C3d is a fragment of the complement factor C3 and is generated in the course of complement activation. When bound to antigen in single or multiple copies, the B cell receptor and complement receptor 2 become co-crosslinked resulting in decreased or increased B cell responses depending on the valence of the antigen-C3d construct. When antigen-C3d constructs are used for the purpose of generating a protective immune response (vaccines), they may either enhance the expected response or suppress it depending on the nature of the antigen. Various pathogens use C3d to evade the immune system by inhibiting complement activation, invading and homing in host cells or masking immunogenic areas of pathogen proteins. Therefore, future vaccination strategies for infectious diseases and cancer employing C3d as a molecular adjuvant need to be carefully evaluated before choosing a target antigen in order to take advantage of the adjuvant effect of the complement component while avoiding potential vaccine complications associated with immune escape mechanisms.

Was it there all the time?

The complement system is old, yet it may still have something new to teach us. For many years, research has existed which shows that C3d, in addition to its established role as an adjuvant, could have an immunosuppressive activity. Being true, it suggests that a common mechanism may be used both by organisms and by their pathogens to prevent unwanted immune responses.

Complement contributes to protective immunity against reinfection by Plasmodium chabaudi chabaudi parasites

We have studied the impact of deficiency of the complement system on the progression and control of the erythrocyte stages of the malarial parasite Plasmodium chabaudi chabaudi. C1q-deficient mice and factor B- and C2-deficient mice, deficient in the classical complement pathway and in both the alternative and classical complement activation pathways, respectively, exhibited only a slight delay in the resolution of the acute phase of parasitemia. Complement-deficient mice showed a transiently elevated level of gamma interferon (IFN-gamma) in the plasma at the time of the acute parasitemia compared with that of wild-type mice. Although there was a trend for increased precursor frequencies in CD4(+) T cells from C1q-deficient mice producing IFN-gamma in response to malarial antigens in vitro, intracellular cytokine staining of spleen cells ex vivo showed no difference in the numbers of IFN-gamma(+) splenic CD4(+) and CD8(+) cells. In contrast, C1q-deficient animals were significantly more susceptible to a second challenge with the same parasite. C1q-deficient animals showed a reduced level of anti-malarial immunoglobulin G2a (IgG2a) antibody 100 days after primary infection. However, following a significantly higher parasitemia, C1q-deficient mice had increased levels of IgM and IgG2a anti-malarial antibodies. In summary, this study indicates that while complement plays only a minor role in the control of the acute phase of parasitemia of a primary infection, it does contribute to parasite control in reinfection.

Study of the role of the htrB gene in Salmonella typhimurium virulence

We have undertaken a study to investigate the contribution of the htrB gene to the virulence of pathogenic Salmonella typhimurium. An htrB::mini-Tn10 mutation from Escherichia coli was transferred by transduction to the mouse-virulent strain S. typhimurium SL1344 to create an htrB mutant. The S. typhimurium htrB mutant was inoculated into mice and found to be severely limited in its ability to colonize organs of the lymphatic system and to cause systemic disease in mice. A variety of experiments were performed to determine the possible reasons for this loss of virulence. Serum killing assays revealed that the S. typhimurium htrB mutant was as resistant to killing by complement as the wild-type strain. However, macrophage survival assays revealed that the S. typhimurium htrB mutant was more sensitive to the intracellular environment of murine macrophages than the wild-type strain. In addition, the bioactivity of the lipopolysaccharide (LPS) of the htrB mutant was reduced compared to that of the LPS from the parent strain as measured by both a Limulus amoebocyte lysate endotoxin quantitation assay and a tumor necrosis factor alpha bioassay. These results indicate that the htrB gene plays a role in the virulence of S. typhimurium.