Identification of a Naegleria fowleri membrane protein reactive with anti-human CD59 antibody

Microbial evasion of the complement system: a continuous and evolving story

The complement system is a fundamental part of the innate immune system that plays a key role in the battle of the human body against invading pathogens. Through its three pathways, represented by the classical, alternative, and lectin pathways, the complement system forms a tightly regulated network of soluble proteins, membrane-expressed receptors, and regulators with versatile protective and killing mechanisms. However, ingenious pathogens have developed strategies over the years to protect themselves from this complex part of the immune system. This review briefly discusses the sequence of the complement activation pathways. Then, we present a comprehensive updated overview of how the major four pathogenic groups, namely, bacteria, viruses, fungi, and parasites, control, modulate, and block the complement attacks at different steps of the complement cascade. We shed more light on the ability of those pathogens to deploy more than one mechanism to tackle the complement system in their path to establish infection within the human host.

Biogenesis of extracellular vesicles in protozoan parasites: The ESCRT complex in the trafficking fast lane?

Extracellular vesicles (EVs) provide a central mechanism of cell-cell communication. While EVs are found in most organisms, their pathogenesis-promoting roles in parasites are of particular interest given the potential for medical insight and consequential therapeutic intervention. Yet, a key feature of EVs in human parasitic protozoa remains elusive: their mechanisms of biogenesis. Here, we survey the current knowledge on the biogenesis pathways of EVs secreted by the four main clades of human parasitic protozoa: apicomplexans, trypanosomatids, flagellates, and amoebae. In particular, we shine a light on findings pertaining to the Endosomal Sorting Complex Required for Transport (ESCRT) machinery, as in mammals it plays important roles in EV biogenesis. This review highlights the diversity in EV biogenesis in protozoa, as well as the related involvement of the ESCRT system in these unique organisms.

Host Invasion by Pathogenic Amoebae: Epithelial Disruption by Parasite Proteins

The epithelium represents the first and most extensive line of defence against pathogens, toxins and pollutant agents in humans. In general, pathogens have developed strategies to overcome this barrier and use it as an entrance to the organism. Entamoeba histolytica, Naegleriafowleri and Acanthamoeba spp. are amoebae mainly responsible for intestinal dysentery, meningoencephalitis and keratitis, respectively. These amoebae cause significant morbidity and mortality rates. Thus, the identification, characterization and validation of molecules participating in host-parasite interactions can provide attractive targets to timely intervene disease progress. In this work, we present a compendium of the parasite adhesins, lectins, proteases, hydrolases, kinases, and others, that participate in key pathogenic events. Special focus is made for the analysis of assorted molecules and mechanisms involved in the interaction of the parasites with epithelial surface receptors, changes in epithelial junctional markers, implications on the barrier function, among others. This review allows the assessment of initial host-pathogen interaction, to correlate it to the potential of parasite invasion.

Biology and pathogenesis of Naegleria fowleri

Naegleria fowleri is a protist pathogen that can cause lethal brain infection. Despite decades of research, the mortality rate related with primary amoebic meningoencephalitis owing to N. fowleri remains more than 90%. The amoebae pass through the nose to enter the central nervous system killing the host within days, making it one of the deadliest opportunistic parasites. Accordingly, we present an up to date review of the biology and pathogenesis of N. fowleri and discuss needs for future research against this fatal infection.

Glycosylphosphatidylinositol Anchor Deficiency Attenuates the Production of Infectious HIV-1 and Renders Virions Sensitive to Complement Attack

Human immunodeficiency virus type 1 (HIV-1) escapes complement-mediated lysis (CML) by incorporating host regulators of complement activation (RCA) into its envelope. CD59, a key member of RCA, is incorporated into HIV-1 virions at levels that protect against CML. Since CD59 is a glycosylphosphatidylinositol-anchored protein (GPI-AP), we used GPI anchor-deficient Jurkat cells (Jurkat-7) that express intracellular CD59, but not surface CD59, to study the molecular mechanisms underlying CD59 incorporation into HIV-1 virions and the role of host proteins in virus replication. Compared to Jurkat cells, Jurkat-7 cells were less supportive to HIV-1 replication and more sensitive to CML. Jurkat-7 cells exhibited similar capacities of HIV-1 binding and entry to Jurkat cells, but were less supportive to viral RNA and DNA biosynthesis as infected Jurkat-7 cells produced reduced amounts of HIV-1 RNA and DNA. HIV-1 virions produced from Jurkat-7 cells were CD59 negative, suggesting that viral particles acquire CD59, and probably other host proteins, from the cell membrane rather than intracellular compartments. As a result, CD59-negative virions were sensitive to CML. Strikingly, these virions exhibited reduced activity of virus binding and were less infectious, implicating that GPI-APs may be also important in ensuring the integrity of HIV-1 particles. Transient expression of the PIG-A gene restored CD59 expression on the surface of Jurkat-7 cells. After HIV-1 infection, the restored CD59 was colocalized with viral envelope glycoprotein gp120/gp41 within lipid rafts, which is identical to that on infected Jurkat cells. Thus, HIV-1 virions acquire RCA from the cell surface, likely lipid rafts, to escape CML and ensure viral infectivity.

Sequestration of host-CD59 as potential immune evasion strategy of Trichomonas vaginalis

Trichomonas vaginalis is known to evade complement-mediated lysis. Because the genome of T. vaginalis does not possess DNA sequence with homology to human protectin (CD59), a complement lysis restricting factor, we tested the hypothesis that host CD59 acquisition by T. vaginalis organisms mediates resistance to complement killing. This hypothesis was based on the fact that trichomonads are known to associate with host proteins. No CD59 was detected on the surface of T. vaginalis grown in serum-based medium using as probe anti-CD59 monoclonal antibody (MAb). We, therefore, infected mice intraperitoneally with live T. vaginalis, and trichomonads harvested from ascites were tested for binding of CD59. Immunofluorescence showed that parasites had surface CD59. Furthermore, as mouse erythrocytes (RBCs) possess membrane-associated CD59, and trichomonads use RBCs as a nutrient source, organisms were co-cultured with murine RBCs for one week. Parasites were shown to have detectable surface CD59. Importantly, live T. vaginalis with bound CD59 were compared with batch-grown parasites without surface-associated CD59 for sensitivity to complement in human serum. Trichomonads without surface-bound CD59 had a higher level of killing by complement than did parasites with surface CD59. These data show that host CD59 acquired onto the surface by live T. vaginalis may be an alternative mechanism for complement evasion. We describe a novel strategy by T. vaginalis consistent with host protein procurement by this parasite to evade the lytic action of complement.

First insight into CD59-like molecules of adult Fasciola hepatica

The present study focussed on investigating CD59-like molecules of Fasciola hepatica. A cDNA encoding a CD59-like protein (termed FhCD59-1) identified previously in the membrane fraction of the F. hepatica tegument was isolated. This homologue was shown to encode a predicted open reading frame (ORF) of 122 amino acids (aa) orthologous to human CD59 with a 25 aa signal peptide, a mature protein containing 10 cysteines and a conserved CD59/Ly-6 family motif "CCXXXXCN". An analysis of cDNAs from two different adult specimens of F. hepatica revealed seven variable types of FhCD59-1 sequences, designated FhCD59-1.1 to FhCD59-1.7, which had 94.3-99.7% amino acid sequence identity upon pairwise comparison. Molecular modeling of FhCD59-1.1 with human CD59 confirmed the presence of the three-finger protein domain found in the CD59 family and predicted three disulphide bonds in the F. hepatica sequence. The interrogation of F. hepatica databases identified two additional sequences, designated FhCD59-2 and FhCD59-3, which had only 23.4-29.5% amino acid identity to FhCD59-1.1. Orthologues of the inferred CD59 protein sequences of F. hepatica were also identified in other flatworms, including Fasciola gigantica, Fascioloides magna, Schistosoma haematobium, Schistosoma japonicum, Schistosoma mansoni, Clonorchis sinensis, Opisthorchis viverrini, Taenia solium, Echinococcus granulosus and the free living Schmidtea mediterannea. The results revealed a considerable degree of sequence complexity in the CD59-like sequence families in F. hepatica and flatworms. Phylogenetic analysis of CD59-like aa sequences from F. hepatica and flatworms showed that FhCD59-2 clustered with the known surface-associated protein SmCD59-2 of S. mansoni. Relatively well-supported clades specific to schistosomes, fasciolids and opisthorchiids were identified. The qPCR analysis of gene transcription showed that the relative expression of these 3 FhCD59-like sequences varied by 11-47-fold during fluke maturation, from the newly excysted juvenile (NEJ) to the adult stage. These findings suggest that different FhCD59-like sequences play distinct roles during the development of F. hepatica.

Genome-wide identification of pathogenicity factors of the free-living amoeba Naegleria fowleri

BACKGROUND

The free-living amoeba Naegleria fowleri is the causative agent of the rapidly progressing and typically fatal primary amoebic meningoencephalitis (PAM) in humans. Despite the devastating nature of this disease, which results in > 97% mortality, knowledge of the pathogenic mechanisms of the amoeba is incomplete. This work presents a comparative proteomic approach based on an experimental model in which the pathogenic potential of N. fowleri trophozoites is influenced by the compositions of different media.

RESULTS

As a scaffold for proteomic analysis, we sequenced the genome and transcriptome of N. fowleri. Since the sequence similarity of the recently published genome of Naegleria gruberi was far lower than the close taxonomic relationship of these species would suggest, a de novo sequencing approach was chosen. After excluding cell regulatory mechanisms originating from different media compositions, we identified 22 proteins with a potential role in the pathogenesis of PAM. Functional annotation of these proteins revealed, that the membrane is the major location where the amoeba exerts its pathogenic potential, possibly involving actin-dependent processes such as intracellular trafficking via vesicles.

CONCLUSION

This study describes for the first time the 30 Mb-genome and the transcriptome sequence of N. fowleri and provides the basis for the further definition of effective intervention strategies against the rare but highly fatal form of amoebic meningoencephalitis.

Binding to complement factors and activation of the alternative pathway by Acanthamoeba

Acanthamoeba can cause severe ocular and cerebral diseases in healthy and immunocompromised individuals, respectively. Activation of complement appears to play an important role in host defence against infection. The exact mechanism, however, is still unclear. The aim of the present study was to investigate the effect of normal human serum (NHS) and normal mouse serum (NMS) on Acanthamoeba trophozoites, the binding of different complement factors to Acanthamoeba and the activation of the complement system. Moreover, we aimed to work out any possible differences between different strains of Acanthamoeba. A virulent T4 strain, a non-virulent T4 strain and a virulent T6 strain were included in the study. It was shown that NHS, but not NMS clearly has amoebicidal properties. After 5min of incubation with NHS, amoebae showed plasma membrane disruption and extrusion of intracellular components. Cells were completely destroyed within 60min of incubation in NHS but stayed intact after incubation in heat-inactivated serum. The binding of human C3 and C9 to amoebae was established by immunoblotting. Although incubation with mouse serum did not result in lysis of Acanthamoeba trophozoites an immunofluorescence assay (IFA) demonstrated a strong deposition of mouse complement factor C3 activation products, moderate binding of C1q, but no binding of MBL-A and MBL-C. EDTA inhibited the binding of C3 to acanthamoebae. Binding of amoebae to C3b was observed with sera from C1qa-/- and MBL-A/C-/- mice, but not with serum from Bf/C2-/- mice demonstrating an activation of complement via the alternative pathway. There were no significant differences between the three Acanthamoeba strains investigated. Altogether, our results prove that NHS is amoebolytic and that Acanthamoeba binds to C3 and C9 and activates the complement system via the alternative pathway.

The good and evil of complement activation in HIV-1 infection

The complement system, a key component of innate immunity, is a first-line defender against foreign pathogens such as HIV-1. The role of the complement system in HIV-1 pathogenesis appears to be multifaceted. Although the complement system plays critical roles in clearing and neutralizing HIV-1 virions, it also represents a critical factor for the spread and maintenance of the virus in the infected host. In addition, complement regulators such as human CD59 present in the envelope of HIV-1 prevent complement-mediated lysis of HIV-1. Some novel approaches are proposed to combat HIV-1 infection through the enhancement of antibody-dependent complement activity against HIV-1. In this paper, we will review these diverse roles of complement in HIV-1 infection.

Recognition of Naegleriae ameba surface protein epitopes by anti-human CD45 antibodies

Phagocytosis is a highly conserved mechanism exhibited by both free-living amebas and mammalian blood cells. Similarities demonstrated by either cell type during engulfment of the same bacterial species may imply analogous surface proteins involved in receptor-mediated endocytosis. The increased availability of anti-human leukocyte antibodies or clusters of differentiation (CD) markers used in conjunction with flow cytometric (FCM) and/or immunohistochemical (IHC) analysis provides investigators with a relatively easy method to screen different cell populations for comparable plasma membrane proteins. In this study, we incubated Naegleria and Acanthamoeba amebas with several directly conjugated anti-human leukocyte monoclonal antibodies (mAb) for similarly recognized amebic epitopes. CD marker selection was based upon a recognized role of each mAb in phagocyte activation and/or uptake of bacteria. These included CD14, CD45, and CD206. In FCM, only one CD45 antibody demonstrated strong reactivity with both Naegleria fowleri and Naegleria gruberi that was not expressed in similarly tested Acanthamoeba species. Additional testing of N. gruberi by IHC demonstrated reactivity to a different CD45 antibody. Our results suggest a possible utility of using anti-human leukocyte antibodies to screen amebic cells for similarly expressed protein epitopes. In doing so, several important items must be considered when selecting potential mAbs for testing to increase the probability of a positive result.

A high-affinity inhibitor of human CD59 enhances complement-mediated virolysis of HIV-1: implications for treatment of HIV-1/AIDS

Many pathogenic enveloped viruses, including HIV-1, escape complement-mediated virolysis by incorporating host cell regulators of complement activation into their own viral envelope. The presence of complement regulators including CD59 on the external surface of the viral envelope confers resistance to complement-mediated virolysis, which may explain why human pathogenic viruses such as HIV-1 are not neutralized by complement in human fluids, even in the presence of high Ab titers against the viral surface proteins. In this study, we report the development of a recombinant form of the fourth domain of the bacterial toxin intermedilysin (the recombinant domain 4 of intermedilysin [rILYd4]), a 114 aa protein that inhibits human CD59 function with high affinity and specificity. In the presence of rILYd4, HIV-1 virions derived from either cell lines or peripheral blood mononuclear cells of HIV-1-infected patients became highly sensitive to complement-mediated lysis activated by either anti-HIV-1 gp120 Abs or by viral infection-induced Abs present in the plasma of HIV-1-infected individuals. We also demonstrated that rILYd4 together with serum or plasma from HIV-1-infected patients as a source of anti-HIV-1 Abs and complement did not mediate complement-mediated lysis of either erythrocytes or peripheral blood mononuclear cells. These results indicate that rILYd4 may represent a novel therapeutic agent against HIV-1/AIDS.

The immune response toNaegleria fowleriamebae and pathogenesis of infection

The genus Naegleria is comprised of a group of free-living ameboflagellates found in diverse habitats worldwide. Over 30 species have been isolated from soil and water but only Naegleria fowleri (N. fowleri) has been associated with human disease. Naegleria fowleri causes primary amebic meningoencephalitis (PAM), a fatal disease of the central nervous system. The pathogenesis of PAM and the role of host immunity to N. fowleri are poorly understood. Strategies for combating infection are limited because disease progression is rapid and N. fowleri has developed strategies to evade the immune system. The medical significance of these free-living ameboflagellates should not be underestimated, not only because they are agents of human disease, but also because they can serve as reservoirs of pathogenic bacteria.