PfRh has been proposed as

PfRh4 has been proposed as the native merozoite ligand that binds to CR1. SA-dependent strains of P. falciparum are unable to use this ligand (Stubbs et al., 2005). Therefore, we hypothesized that SA-dependent strains may be more reliant on complement to use this invasion pathway. Although the invasion of NA-treated RBCs by SA-dependent strains is low, the percent enhancement of invasion in the presence of FS is significantly greater than SA-independent strains. These data demonstrate that, for some strains, C3 can act as the sole CR1 ligand. Lastly, our passive transfer experiments in mice confirm that anti-malaria antibody and complement can benefit parasite growth. Administration of anti-Pb IgG enhanced parasite growth in a reverse dose-dependent manner. At first look, these results stand in contrast to previous studies that have shown that antibody from immune rodents inhibits growth in vivo when given to naïve animals (Cavinato et al., 2001; Jarra et al., 1986; Rotman et al., 1998; Waki et al., 1995; Yoneto et al., 2001). However, the doses of antibody in those studies were much higher, usually given in milligram amounts and sometimes in several doses prior to and after infection. While mice do not express CR1 on their RBCs, they do express other complement receptors that can bind complement such as Crry (Molina, 2002). Several studies have also shown that passive transfer of total IgG from individuals living in endemic areas to acutely infected children can reduce parasitemia, but this was achieved at 5–10 fold higher doses per Kg than the ones we have used in our studies (Cohen et al., 1961; McGregor, 1964b; Sabchareon et al., 1991). It is unlikely that these levels of mps1 kinase can be achieved by natural infection or by immunization. Thus, it is possible that low doses of anti-merozoite antibodies enhance parasite growth in vivo while very high doses are required for true in vivo inhibitory activity. By contrast, the inhibitory activity of control polyclonal antibody may be due to its complement scavenging properties (Arumugam et al., 2007; Hartung, 2008). C3–/− mice consistently showed significant blunting of parasitemia relative to wild type mice. Yet, they still showed enhancement of parasite growth in response to anti-Pb IgG, probably due to residual C4b opsonizing activity. Previous studies of the role of complement in the growth of rodent malaria have been inconsistent. Use of cobra venom factor (CVF) as a complement depleting agent resulted in increased parasitemia (Ward et al., 1981). However, CVF exerts its action by inducing AP convertase formation and C3 activation which could result in parasite growth enhancement by increased deposition of C3 fragments on merozoites. Unlike our study, a previous study (Ramos et al., 2012) reported no change in P. berghei parasitemia in C3–/− mice compared to wild type mice, but in that study quantitation was done by microscopy and infection was by intraperitoneal injection. C1q and C2/fB deficiency resulted in minimal increase in Plasmodium chabaudi parasitemia (Taylor et al., 2001). Thus, further studies of these models are needed to fully understand the role of complement activation in parasite growth.
Conclusions
Funding Sources This work was funded by Grant P131040 from the Congressionally Directed Medical Research Program, PI José A. Stoute. The funding agency played no role in the collection or interpretation of the data, the preparation of the manuscript, and the decision to publish.
Conflict of Interest
Authorship Contributions
Acknowledgments
Introduction Infections that can trigger a systemic inflammatory response syndrome (SIRS) leading to sepsis due to the presence of pathogens or released toxins (Bone, 1992) are most commonly diagnosed using blood cultures in combination with characteristic physiological alterations (e.g., increased heart and respiratory rates, temperature, white blood cells counts) (Angus and van der Poll, 2013; Yealy et al., 2014; Singer et al., 2016). But only 15 to 30% of sepsis patients with documented infection produce positive blood cultures, (Tsalik et al., 2012; Gille-Johnson et al., 2013; Loonen et al., 2014; Knabl et al., 2016; Bacconi et al., 2014) and even then it can often take days before definitive results are obtained. With the absence of direct biomarkers for sepsis, blood proteins involved in the host inflammatory response, such as C-reactive protein (CRP) and procalcitonin (PCT), have been used to detect infection and predict outcome in septic patients (Pierrakos and Vincent, 2010; Chan and Gu, 2011). However, while rising levels of the CRP and PCT correlate with an increased likelihood of infection and sepsis severity, these tests produce a wide range of sensitivities and specificities (Pierrakos and Vincent, 2010; Carr, 2015). Moreover, none of these inflammatory biomarkers unequivocally distinguishes infection-induced SIRS from sterile SIRS caused by burn injury (Seoane et al., 2014), trauma (Wojtaszek et al., 2014) or surgery (Battistelli et al., 2014). Other groups (e.g. Spectral Diagnostics) have developed assays that measure pathogen-derived biomarkers (e.g., endotoxin), but they have not gained widespread use (Marshall et al., 2004) possibly because they only detect Gram negative bacteria. More rapid molecular diagnostic assays (e.g., based on PCR and mass spectrometry) can identify specific types of pathogens, but MALDI mass spectrometry requires a culture step (Idelevich et al., 2014) to expand the number of live pathogens to above 105 colony forming units (CFUs), which again limits its use to the minority of patients who are culture positive (Tsalik et al., 2012; Gille-Johnson et al., 2013). PCR-based methods, such as SeptiFast (Roche, Diagnostics GmbH), hybcell® Pathogens (Anagnostics Bioanalysis GmbH), and IRIDICA (Abbott Molecular) have been developed that can detect specific types of whole pathogens directly in blood within 4–6h. However, because they require the presence of the pathogen DNA in blood, they fail to detect suspected bloodstream infections in 60–90% of patients (Knabl et al., 2016; Bacconi et al., 2014). The T2Candida Panel (T2 Biosytems) has demonstrated improved high specificity and sensitivity for fungal infections, and it can identify invasive candidiasis in blood culture negative patients (Mylonakis et al., 2015). However, none of these tests can be used to detect a broad spectrum of pathogens (i.e., Gram negative and positive bacteria, fungi, viruses, and parasites) as well as the toxins they release (e.g., endotoxin, lipoteichoic acid) that are largely responsible for triggering the cytokine cascade that leads to SIRS and sepsis.