Identification of Immunogenic Antigens of Naegleria fowleri Adjuvanted by Cholera Toxin

Advancing the understanding of Naegleria fowleri: Global epidemiology, phylogenetic analysis, and strategies to combat a deadly pathogen

Naegleria fowleri is a rare but deadly pathogen that has emerged as an important global public health concern. The pathogen induces primary amoebic meningoencephalitis (PAM), a rapidly progressive and almost always fatal life-threatening brain infection. The devastating impact of N. fowleri and the high mortality rate underscores a deeper understanding and the development of innovative strategies to tackle this issue. Despite various studies that have been conducted on N. fowleri, a comprehensive review that integrates recent findings and addresses critical gaps in understanding remains lacking. This review provides a detailed overview of N. fowleri epidemiology, transmission dynamics, phylogenetic diversity, state-of-the-art diagnostic techniques, therapeutic approaches, and preventive measures. We identified 488 PAM cases globally, reported since 1962, with the highest numbers in the US, Pakistan, and Australia. A phylogenetic analysis of 41 N. fowleri ITS-1, 5.8S, ITS-2 region-based sequences showed genotypic diversity, with genotypes II and III being the most prevalent in Asia, North America, and Europe. Effective approaches to preventing N. fowleri transmission include applying free chlorine to water in storage tanks, taking precautions while swimming, and performing ablution with sterilized water (e.g., boiled or distilled) while avoiding deep inhalation of water, especially in regions with high water contamination. This review highlights the global impact of N. fowleri, future surveillance strategies, prompt diagnosis, potential therapeutic options, and vaccine development to prevent PAM outbreaks. It highlights the importance of joint government and public health efforts to combat this deadly pathogen.

Identification of Naegleria fowleri antigens recognized by serum antibodies from people of Mexicali Valley, México

Naegleria fowleri is an amoeba that causes a fatal disease in the central nervous system known as primary amoebic meningoencephalitis (PAM) in humans. Most of the infections are acquired by people who practice recreational activities in water contaminated with trophozoites. Swimming and wading in irrigation channels of Mexicali are common practices for local people. Although there are some warning signposts in the surrounding sites, people continue using these channels for recreational purposes. In that region, cases of PAM have been reported; however, not everyone who comes into contact with contaminated water containing trophozoites becomes infected, and the factors influencing their immune response to N. fowleri remain unknown. We analyzed the levels of antibodies against N. fowleri in two groups: local individuals, including visitors who swam in the Mexicali channels, and a group from Mexico City (CDMX). In both groups, specific antibody responses were analyzed using immunoassays, including Western blot, ELISA, and cytochemistry. The highest levels of both IgG and IgA were found in samples from Mexicali, compared to those from CDMX. In both groups, IgG recognized polypeptide bands from N. fowleri at molecular weights of 100, 50, and 19 kDa, bands that we have already reported as immunogenic. Moreover, the IgG subjects recognized trophozoite structures such as membrane, pseudopodia, food cups, and even small like-vesicles. This antibody immune response directed against these polypeptide bands and trophozoite structures along with other factors could be participating in the defense of these people against PAM.

Analysis of virulence factors in extracellular vesicles secreted by Naegleria fowleri

Naegleria fowleri is the etiological agent of primary amebic meningoencephalitis (PAM), a rapidly progressive acute and fulminant infection that affects the central nervous system, particularly of children and young adults, which has a mortality rate greater than 95%, and its symptomatologic similarity with other meningitis caused by virus or bacteria makes it difficult to make a quick and timely diagnosis that prevents the progression of the infection. It is necessary to know the antigenic determinants as well as the pathogenicity mechanisms of this amoeba to implement strategies that allow for better antiamoebic therapeutic and diagnostic targets that directly impact the health sector. Therefore, the aim of this work was to analyze some virulence factors as part of extracellular vesicle (EV) cargo secreted by N. fowleri. The EV secretion to the extracellular medium was evaluated in trophozoites fixed and incubated with anti-N. fowleri antibody while molecular identification of EV cargo was performed by SDS-PAGE, Western blot, and RT-PCR. Our results showed that N. fowleri secretes a wide variety of vesicle sizes ranging from 0.2 to > 2 μm, and these EVs were recognized by antibodies anti-Naegleropore B, anti-19 kDa polypeptide band, anti-membrane protein Mp2CL5, anti-protease cathepsin B, and anti-actin. Furthermore, these vesicles were localized in the trophozoites cytoplasm or secreted into the extracellular medium. Specifically in relation to small vesicles, our purified exosomes were recognized by CD63 and Hsp70 markers, along with the previously mentioned proteins. RT-PCR analysis was made through the isolation of EVs from N. fowleri trophozoite culture by concentration, filtration, and ultracentrifugation. Interestingly, we obtained PCR products for Nfa1, NPB, Mp2CL5, and CatB genes as part of exosomes cargo. This suggests that the molecules identified in this work could play an important role in communication as well as in infectious processes caused by this amoeba. Therefore, the study and characterization of the pathogenicity mechanisms, as well as the virulence factors released by N. fowleri remains a key point to provide valuable information for the development of therapeutic treatments, vaccine design, or biomarkers for a timely diagnosis against infections caused by protozoa.

Electrochemical biosensors for the detection of protozoan parasite: a scoping review

The development of rapid, accurate, and efficient detection methods for protozoan parasites can substantially control the outbreak of protozoan parasites infection, which poses a threat to global public health. Idealistically, electrochemical biosensors would be able to overcome the limitations of current detection methods due to their simplified detection procedure, on-site quantitative analysis, rapid detection time, high sensitivity, and portability. The objective of this scoping review is to evaluate the current state of electrochemical biosensors for detecting protozoan parasites. This review followed the most recent Preferred Reporting Items for Systematic Review and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) recommendations. Using electrochemical biosensor and protozoan parasite keywords, a literature search was conducted in PubMed, Scopus, Web of Science, and ScienceDirect on journals published between January 2014 and January 2022. Of the 52 studies, 19 were evaluated for eligibility, and 11 met the review's inclusion criteria to evaluate the effectiveness and limitations of the developed electrochemical biosensor platforms for detecting protozoan parasite including information about the samples, biomarkers, bioreceptors, detection system platform, nanomaterials used in fabrication, and limit of detection (LoD). Most electrochemical biosensors were fabricated using conventional electrodes rather than screen-printed electrodes (SPE). The range of the linear calibration curves for the developed electrochemical biosensors was between 200 ng/ml and 0.77 pM. The encouraging detection performance of the electrochemical biosensors demonstrate their potential as a superior alternative to existing detection techniques. On the other hand, more study is needed to determine the sensitivity and specificity of the electrochemical sensing platform for protozoan parasite detection.

THE 72-KDA PROTEIN OF NAEGLERIA FOWLERI PLAYS AN IMPORTANT ROLE IN THE ADHESION OF TROPHOZOITES TO BALB/C MICE NASAL EPITHELIUM

Naegleria fowleri is a protozoan that causes primary amebic meningoencephalitis (PAM). The infection occurs when the trophozoites enter the nasal cavity, adhere to the nasal mucosa, invade the epithelium, and migrate until they reach the olfactory bulb. Like other pathogens, there is evidence that the adhesion of N. fowleri to host cells is an important factor in the process of cytopathogenicity and disease progression. However, the factors involved in the adhesion of the pathogen to the cells of the nasal epithelium have not been characterized. The objective of this study was to identify a protein on the surface of N. fowleri, which could act as adhesin to the mouse nasal epithelium in the PAM model. The interaction between proteins of extracts of N. fowleri and cells of the nasal epithelium of BALB/c mice was analyzed using overlay and Western blot assays. A 72-kDa band of N. fowleri interacted directly with epithelial cell proteins, this polypeptide band was purified and analyzed by mass spectrometry. Analysis revealed that polypeptide bands of 72 kDa contained peptides that matched the membrane protein, actin 1 and 2, and Hsp70. Moreover, the N. fowleri extracts resolved in 2D-SDS-PAGE showed that 72-kDa spot interacted with proteins of mouse epithelial cells, which include characteristics of the theoretical data of molecular weight and pH obtained in the analysis by mass spectrometry. Immunofluorescence tests showed that this protein is located on the surface of trophozoites and plays an important role in the adhesion of amoeba either in vitro or in vivo assays, suggesting that this protein contributes during the N. fowleri invasion and migration to the brain, causing primary amoebic meningoencephalitis.

In silico discovery of diagnostic/vaccine candidate antigenic epitopes and a multi-epitope peptide vaccine (NaeVac) design for the brain-eating amoeba Naegleria fowleri causing human meningitis

Naegleria fowleri, the brain-eating amoeba, is a free-living amoeboflagellate with three different life cycles (trophozoite, flagellated, and cyst) that lives in a variety of habitats around the world including warm freshwater and soil. It causes a disease called naegleriasis leading meningitis and primary amoebic meningoencephalitis (PAM) in humans. N. fowleri is transmitted through contaminated water sources such as insufficiently chlorinated swimming pool water or contaminated tap water, and swimmers are at risk. N. fowleri is found all over the world, and most infections were reported in both developed and developing countries with high mortality rates and serious clinical findings. Until now, there is no FDA approved vaccine and early diagnosis is urgent against this pathogen. In this study, by analyzing the N. fowleri vaccine candidate proteins (Mp2CL5, Nfa1, Nf314, proNP-A and proNP-B), it was aimed to discover diagnostic/vaccine candidate epitopes and to design a multi-epitope peptide vaccine against this pathogen. After the in silico evaluation, three prominent diagnostic/vaccine candidate epitopes (EAKDSK, LLPHIRILVY, and FYAKLLPHIRILVYS) with the highest antigenicities were discovered and a potentially highly immunogenic/antigenic multi-epitope peptide vaccine (NaeVac) was designed against the brain-eating amoeba N. fowleri causing human meningitis.

Two MP2CL5 Antigen Vaccines from Naegleria fowleri Stimulate the Immune Response against Meningitis in the BALB/c Model

Naegleria fowleri is an etiological agent that generates primary amoebic meningoencephalitis; unfortunately, no effective treatment or vaccine is available. The objective of this work was to determine the immunoprotective response of two vaccine antigens, as follows: (i) the polypeptide band of 19 kDa or (ii) a predicted immunogenic peptide from the membrane protein MP2CL5 (Smp145). Both antigens were administered intranasally in mice using cholera toxin (CT) as an adjuvant. The survival rate and immune response of immunized mice with both antigens and challenged with N. fowleri trophozoites were measured in the nose-associated lymphoid tissue (NALT) and nasal passages (NPs) by flow cytometry and enzyme-linked immunosorbent assay (ELISA). We also determined the immunolocalization of both antigens in N. fowleri trophozoites by confocal microscopy. Immunization with the polypeptide band of 19 kDa alone or coadministered with CT was able to confer 80% and 100% of protection, respectively. The immunization with both antigens (alone or coadministered with CT) showed an increase in T and B lymphocytes. In addition, there was an increase in the expression of integrin α4β1 and IgA in the nasal cavity of protected mice, and the IgA, IgG, and IgM levels were increased in serum and nasal washes. The immunolocalization of both antigens in N. fowleri trophozoites was observed in the plasma membrane, specifically in pseudopod-like structures. The MP2CL5 antigens evaluated in this work were capable of conferring protection which would lead us to consider them as potential candidates for vaccines against meningitis caused by N. fowleri.

Role of FcγRIII in the nasal cavity of BALB/c mice in the primary amebic meningoencephalitis protection model

Different mechanisms of the host immune response against the primary amebic meningoencephalitis (PAM) in the mouse protection model have been described. It has been proposed that antibodies opsonize Naegleria fowleri trophozoites; subsequently, the polymorphonuclear cells (PMNs) surround the trophozoites to avoid the infection. FcγRs activate signaling pathways of adapter proteins such as Syk and Hck on PMNs to promote different effector cell functions which are induced by the Fc portion of the antibody-antigen complexes. In this work, we analyzed the activation of PMNs, epithelial cells, and nasal passage cells via the expression of Syk and Hck genes. Our results showed an increment of the FcγRIII and IgG subclasses in the nasal cavity from immunized mice as well as Syk and Hck expression was increased, whereas in the in vitro assay, we observed that when the trophozoites of N. fowleri were opsonized with IgG anti-N. fowleri and interacted with PMN, the expression of Syk and Hck was also increased. We suggest that PMNs are activated via their FcγRIII, which leads to the elimination of the trophozoites in vitro, while in the nasal cavity, the adhesion and consequently infection are avoided.

Role of cathepsin B of Naegleria fowleri during primary amebic meningoencephalitis

Naegleria fowleri causes primary amoebic meningoencephalitis in humans and experimental animals. It has been suggested that cysteine proteases of parasites play key roles in metabolism, nutrient uptake, host tissue invasion, and immune evasion. The aim of this work was to evaluate the presence, expression, and role of cathepsin B from N. fowleri in vitro and during PAM. Rabbit-specific polyclonal antibodies against cathepsin B were obtained from rabbit immunization with a synthetic peptide obtained by bioinformatic design. In addition, a probe was designed from mRNA for N. fowleri cathepsin B. Both protein and messenger were detected in fixed trophozoites, trophozoites interacted with polymorphonuclear and histological sections of infected mice. The main cathepsin B distribution was observed in cytoplasm or membrane mainly pseudopods and food-cups while messenger was in nucleus and cytoplasm. Surprisingly, both the messenger and enzyme were observed in extracellular medium. To determine cathepsin B release, we used trophozoites supernatant recovered from nasal passages or brain of infected mice. We observed the highest release in supernatant from recovered brain amoebae, and when we analyzed molecular weight of secreted proteins by immunoblot, we found 30 and 37 kDa bands which were highly immunogenic. Finally, role of cathepsin B during N. fowleri infection was determined; we preincubated trophozoites with E-64, pHMB or antibodies with which we obtained 60%, 100%, and 60% of survival, respectively, in infected mice. These results suggest that cathepsin B plays a role during pathogenesis caused by N. fowleri mainly in adhesion and contributes to nervous tissue damage.

A 250-kDa glycoprotein of Naegleria fowleri induces protection and modifies the expression of α4β1 and LFA-1 on T and B lymphocytes in mouse meningitis model

The aims of this work were to evaluate the protective role of the 250-kDa polypeptide band of Naegleria fowleri. We designed an immunization strategy in Balb/c mice which were inoculated by i.n. route with an electrocuted 250-kDa polypeptide band of N. fowleri. We observed that the 250-kDa band induced 80% of protection, whereas the coadministration with Cholera Toxin induced 100% of protection. Moreover, high levels of IgA- and IgG-specific antibodies were detected by ELISA assay. We also analysed migration molecules (α4β1 and LFA-1) on T and B lymphocytes in nose-associated lymphoid tissue (NALT), cervical lymph nodes (CN) and nasal passages (NP) by flow cytometry. We observed that the percentage of B cells (B220/α4β1) and T cells (CD4/α4β1) in NP were higher in all immunized groups compared with the other compartments analysed. Finally, we detected by immunohistochemistry ICAM-1 and V-CAM-1 in the nasal cavity. The immunization with the 250-kDa polypeptide band, protect mice against N. fowleri challenge and modifies migration molecules and their ligands.