Cryptosporidium parvum sporozoite pellicle antigen recognized by a neutralizing monoclonal antibody is a beta-mannosylated glycolipid

Vaccination with Eimeria tenella elongation factor-1α recombinant protein induces protective immunity against E. tenella and E. maxima infections

Avian coccidiosis is caused by multiple species of the apicomplexan protozoan, Eimeria, and is one of the most economically devastating enteric diseases for the poultry industry worldwide. Host immunity to Eimeria infection, however, is relatively species-specific. The ability to immunize chickens against different species of Eimeria using a single vaccine will have a major beneficial impact on commercial poultry production. In this paper, we describe the molecular cloning, purification, and vaccination efficacy of a novel Eimeria vaccine candidate, elongation factor-1α (EF-1α). One day-old broiler chickens were given two subcutaneous immunizations one week apart with E. coli-expressed E. tenella recombinant (r)EF-1α protein and evaluated for protection against challenge infection with E. tenella or E. maxima. rEF-1α-vaccinated chickens exhibited increased body weight gains, decreased fecal oocyst output, and greater serum anti-EF-1α antibody levels following challenge infection with either E. tenella or E. maxima compared with unimmunized controls. Vaccination with EF-1α may represent a new approach to inducing cross-protective immunity against avian coccidiosis in the field.

Parasites and immunotherapy: with or against?

Immunotherapy is a sort of therapy in which antibody or antigen administrates to the patient in order to treat or reduce the severity of complications of disease. This kind of treatment practiced in a wide variety of diseases including infectious diseases, autoimmune disorders, cancers and allergy. Successful and unsuccessful immunotherapeutic strategies have been practiced in variety of parasitic infections. On the other hand parasites or parasite antigens have also been considered for immunotherapy against other diseases such as cancer, asthma and multiple sclerosis. In this paper immunotherapy against common parasitic infections, and also immunotherapy of cancer, asthma and multiple sclerosis with parasites or parasite antigens have been reviewed.

Elongation factor-1α is a novel protein associated with host cell invasion and a potential protective antigen of Cryptosporidium parvum

The phylum Apicomplexa comprises obligate intracellular parasites that infect vertebrates. All invasive forms of Apicomplexa possess an apical complex, a unique assembly of organelles localized to the anterior end of the cell and involved in host cell invasion. Previously, we generated a chicken monoclonal antibody (mAb), 6D-12-G10, with specificity for an antigen located in the apical cytoskeleton of Eimeria acervulina sporozoites. This antigen was highly conserved among Apicomplexan parasites, including other Eimeria spp., Toxoplasma, Neospora, and Cryptosporidium. In the present study, we identified the apical cytoskeletal antigen of Cryptosporidium parvum (C. parvum) and further characterized this antigen in C. parvum to assess its potential as a target molecule against cryptosporidiosis. Indirect immunofluorescence demonstrated that the reactivity of 6D-12-G10 with C. parvum sporozoites was similar to those of anti-β- and anti-γ-tubulins antibodies. Immunoelectron microscopy with the 6D-12-G10 mAb detected the antigen both on the sporozoite surface and underneath the inner membrane at the apical region of zoites. The 6D-12-G10 mAb significantly inhibited in vitro host cell invasion by C. parvum. MALDI-TOF/MS and LC-MS/MS analysis of tryptic peptides revealed that the mAb 6D-12-G10 target antigen was elongation factor-1α (EF-1α). These results indicate that C. parvum EF-1α plays an essential role in mediating host cell entry by the parasite and, as such, could be a candidate vaccine antigen against cryptosporidiosis.

Evolution of Cryptosporidium in vitro culture

This overview discusses findings from culturing Cryptosporidium spp. in cell and axenic cultures as well as factors limiting the development of this parasite in cultivation systems during recent years. A systematic review is undertaken of findings regarding the life cycle of the parasite, taking into account physiological, biochemical and genetic aspects, in the hope that this attempt will facilitate future approaches to research and developments in the understanding of Cryptosporidium biology.

Antibodies fused to innate immune molecules reduce initiation of Cryptosporidium parvum infection in mice

At present no completely effective treatments are available for Cryptosporidium parvum infections in humans and livestock. Based on previous data showing the neutralizing potential of a panel of monoclonal antibodies developed against C. parvum, and based on the fact that innate immune peptides and enzymes have anticryptosporidial activity, we engineered several of these antibodies into antibody-biocide fusion proteins. We hypothesized that the combination of high-affinity antibody targeting with innate immune molecule-mediated killing would result in a highly effective new antiprotozoal agent. To test this hypothesis, we expressed antibody-biocide fusion proteins in a mammalian cell culture system and used the resulting products for in vitro and in vivo efficacy experiments. Antibody-biocide fusion proteins efficiently bound to, and destroyed, C. parvum sporozoites in vitro through a membrane-disruptive mechanism. When antibody-biocide fusion proteins were administered orally to neonatal mice in a prophylactic model of cryptosporidiosis, the induction of infection was reduced by as much as 81% in the mucosal epithelium of the gut, as determined on the basis of histopathological scoring of infectious stages. Several versions of antibody fusion proteins that differed in antigen specificity and in the biocide used had strong inhibitory effects on the initiation of infection. The results lay the groundwork for the development of a new class of antimicrobials effective against Cryptosporidium.

Morphological characterization of Cryptosporidium parvum life-cycle stages in an in vitro model system

Cryptosporidium parvum is a zoonotic protozoan parasite that mainly affects the ileum of humans and livestock, with the potential to cause severe enteric disease. We describe the complete life cycle of C. parvum in an in vitro system. Infected cultures of the human ileocecal epithelial cell line (HCT-8) were observed over time using electron microscopy. Additional data are presented on the morphology, development and behavioural characteristics of the different life-cycle stages as well as determining their time of occurrence after inoculation. Numerous stages of C. parvum and their behaviour have been visualized and morphologically characterized for the first time using scanning electron microscopy. Further, parasite-host interactions and the effect of C. parvum on host cells were also visualized. An improved understanding of the parasite's biology, proliferation and interactions with host cells will aid in the development of treatments for the disease.

Active invasion and/or encapsulation? A reappraisal of host-cell parasitism by Cryptosporidium

Host-cell invasion by Cryptosporidium is a complex process that requires many different factors derived from both the parasite and the host cell. However, the exact natures of the processes have yet to be resolved. Here, research on different components of the invasion process is put in context, and the sequence of events and pathways associated with the establishment of Cryptosporidium in its unique niche is clarified. In addition, initial parasite-host contact, host-cell invasion and host-cell responses are described. The roles of parasite and host-cell-derived components in the invasion process are examined, as is the question of whether Cryptosporidium actively invades cells and to what extent host-cell responses are involved.

Molecular targets for detection and immunotherapy in Cryptosporidium parvum

Cryptosporidium parvum is an obligate protozoan parasite responsible for the diarrheal illness cryptosporidiosis in humans and animals. Although C. parvum is particularly pathogenic in immunocompromised hosts, the molecular mechanisms by which C. parvum invades the host epithelial cells are not well understood. Characterization of molecular-based antigenic targets of C. parvum is required to improve the specificity of detection, viability assessments, and immunotherapy (treatment). A number of zoite surface (glyco)proteins are known to be expressed during, and believed to be involved in, invasion and infection of host epithelial cells. In the absence of protective treatments for this illness, antibodies targeted against these zoite surface (glyco)proteins offers a rational approach to therapy. Monoclonal, polyclonal and recombinant antibodies represent useful immunotherapeutic means of combating infection, especially when highly immunogenic C. parvum antigens are utilized as targets. Interruption of life cycle stages of this parasite via antibodies that target critical surface-exposed proteins can potentially decrease the severity of disease symptoms and subsequent re-infection of host tissues. In addition, development of vaccines to this parasite based on the same antigens may be a valuable means of preventing infection. This paper describes many of the zoite surface glycoproteins potentially involved in infection, as well as summarizes many of the immunotherapeutic studies completed to date. The identification and characterization of antibodies that bind to C. parvum-specific cell surface antigens of the oocyst and sporozoite will allow researchers to fully realize the potential of molecular-based immunotherapy to this parasite.

The potential of antibody-mediated immunity in the defence against biological weapons

Antibody-mediated immunity (AMI) has been used for the treatment and prevention of infectious diseases for > 100 years, and has a remarkable record of safety, efficacy and versatility. AMI can be used for defence against a wide variety of biological weapons, and passive antibody (Ab) therapy has the potential to provide immediate immunity to susceptible individuals. Recent advances in the Ab field make it possible to generate Abs with enhanced antimicrobial functions. There are significant gaps in our understanding of Ab function, such that the development of Ab-based strategies remains a largely empirical exercise. Nevertheless, the advantages inherent in the therapeutic and prophylactic use of AMI provide a powerful rationale for continued development that will undoubtedly yield many new vaccines and therapeutic Abs.

Cryptosporidium excystation and invasion: getting to the guts of the matter

Cryptosporidium parvum excystation and host cell invasion have been characterized in some detail ultrastructurally. However, until recently, the biochemical and molecular basis of host-parasite interactions and parasite- and host-specific molecules involved in excystation, motility and host cell invasion have been poorly understood. This article describes our understanding of Cryptosporidium excystation and the events leading to host cell invasion, and draws from information available about these processes in other apicomplexans. Many questions remain but, once the specific mechanisms are identified, they could prove to be novel targets for drug delivery.

Cross-reactivities with Cryptosporidium spp. by chicken monoclonal antibodies that recognize avian Eimeria spp

In a previous study, we have developed several chicken monoclonal antibodies (mAbs) against Eimeria acervulina (EA) in order to identify potential ligand molecules of Eimeria. One of these mAbs, 6D-12-G10, was found to recognize a conoid antigen of EA sporozoites and significantly inhibited the sporozoite invasions of host T lymphocytes in vitro. Furthermore, some of these chicken mAbs showed cross-reactivities with several different avian Eimeria spp. and the mAb 6D-12-G10 also demonstrated cross-reactivities with the tachyzoites of Neospora caninum and Toxoplasma gondii. Cryptosporidium spp. are coccidian parasites closely related to Eimeria spp., and especially C. parvum is an important cause of diarrhea in human and mammals. In the present study, to assess that the epitopes recognized by these chicken mAbs could exist on Cryptosporidium parasites, we examined the cross-reactivity of these mAbs with Cryptosporidium spp. using an indirect immunofluorescent assay (IFA) and Western blotting analyses. In IFA by chicken mAbs, the mAb 6D-12-G10 only showed a immunofluorescence staining at the apical end of sporozoites of C. parvum and C. muris, and merozoites of C. parvum. Western blotting analyses revealed that the mAb 6D-12-G10 reacted with the 48-kDa molecular weight band of C. parvum and C. muris oocyst antigens, 5D-11 reacted the 155 kDa of C. muris. Furthermore, these epitopes appeared to be periodate insensitive. These results indicate that the target antigen recognized by these chicken mAbs might have a shared epitope, which is present on the apical complex of apicomplexan parasites.

Structural analysis of Cryptosporidium parvum

Cryptosporidium parvum (Apicomplexa, formerly Sporozoa) is the causative agent of cryptosporidiosis, an enteric disease of substantial medical and veterinary importance. C. parvum shows a number of unique features that differ from the rest of the class of coccidea in which it is currently grouped taxonomically. Differences occur in the overall structure of the transmission form and the invasive stages of the parasite, its intracellular location, the presence of recently described additional extracellular stages, the host range and target cell tropism, the ability to autoinfection, the nonresponsiveness to anticoccidial drugs, the immune response of the host, and immunochemical and genetic characteristics. These differences have an important impact on the infectivity, the epidemiology, the therapy, and the taxonomy of the parasite. The present article describes the structural analysis of the parasite using light and electron microscopy with an emphasis on structural details unique to C. parvum.

Biology, persistence and detection of Cryptosporidium parvum and Cryptosporidium hominis oocyst

Cryptosporidium parvum and Cryptosporidium hominis are obligate enteric protozoan parasites which infect the gastrointestinal tract of animals and humans. The mechanism(s) by which these parasites cause gastrointestinal distress in their hosts is not well understood. The risk of waterborne transmission of Cryptosporidium is a serious global issue in drinking water safety. Oocysts from these organisms are extremely robust, prevalent in source water supplies and capable of surviving in the environment for extended periods of time. Resistance to conventional water treatment by chlorination, lack of correlation with biological indicator microorganisms and the absence of adequate methods to detect the presence of infectious oocysts necessitates the development of consistent and effective means of parasite removal from the water supply. Additional research into improving water treatment and sewage treatment practices is needed, particularly in testing the efficiency of ozone in oocyst inactivation. Timely and efficient detection of infectious C. parvum and C. hominis oocysts in environmental samples requires the development of rapid and sensitive techniques for the concentration, purification and detection of these parasites. A major factor confounding proper detection remains the inability to adequately and efficiently concentrate oocysts from environmental samples, while limiting the presence of extraneous materials. Molecular-based techniques are the most promising methods for the sensitive and accurate detection of C. parvum and C. hominis. With the availability of numerous target sequences, RT-PCR will likely emerge as an important method to assess oocyst viability. In addition, a multiplex PCR for the simultaneous detection of C. parvum, C. hominis and other waterborne pathogens such as Giardia lamblia would greatly benefit the water industry and protect human health.

Characterization of a Low Molecular Weight Glycolipid Antigen from Cryptosporidium parvum

Cryptosporidium parvum, an Apicomplexan parasite of the mammalian gut epithelium, causes a diarrheal illness in a wide range of hosts and is transmitted by contamination of food or water with oocyst-laden feces from an infected animal. We have identified a glycosylinositol phospholipid from the sporozoite stage of the parasite that is frequently recognized by serum antibodies from human cryptosporidiosis patients. The humoral immune response is dominated by IgG1 subclass antibodies but can also include IgA and IgM antibodies. The glycosylinositol phospholipids were purified by butanol extraction of a Triton X-114-soluble fraction followed by octyl-Sepharose column chromatography and preparative high performance TLC and were shown to include at least 5 species. By using mass spectrometry and radiolabeled neutral glycan analysis, we found that the structure of the dominant glycosylinositol phospholipid antigen contained a C18:0 lyso-acylglycerol, a C16:0-acylated inositol, and an unsubstituted mannose3-glucosamine glycan core. Other diacyl species were also identified, most notably a series of glycosylinositol phospholipids having an acyl-linked C20:0 to C28:0 lipid on the inositol ring. Less abundant species having three acyl-linked fatty acids and species with an additional 1-3 hexoses linked to the mannose core were also observed. We are currently working to determine the role that these glycolipids may play in the development of disease and in the clearance of infection.

Recent advances in cryptosporidiosis: the immune response

An increased understanding of host immune responses to Cryptosporidium parvum which are responsible for clearance of primary infection and resistance to reinfection, and characterization of the parasite molecules to which they are directed, are essential for discovery of effective active and passive immunization strategies against cryptosporidiosis. In this article, recent advances in knowledge of humoral and cellular immune responses to C. parvum, their antigen specificities, and mechanisms of protection are briefly reviewed.

Cryptosporidium virulence determinants--are we there yet?

Exposure to Cryptosporidium parvum in healthy individuals results in transient infection that may be asymptomatic or can result in self-limited diarrhoea. In contrast, acquired immune deficiency syndrome patients with cryptosporidiosis can experience severe manifestations of disease. Volunteer studies have demonstrated that as few as 10 oocysts can cause infection in otherwise healthy adults and that isolates from geographically diverse regions differ in infectivity and, perhaps, virulence. Variability in isolate pathogenicity and infectivity has also been seen in bovine and murine models, respectively. Furthermore, isolate specific differences in protein composition and in host immunoreactivity have been observed. The molecular basis for differences in pathogenicity is not understood. Determining which factors are responsible for host selectivity and for the initiation, establishment, and perpetuation of infection with Cryptosporidium is key to rational drug design and vaccine development. To date, no specific virulence factors have been unequivocally shown to individually cause direct or indirect damage to host tissues nor have mutant strains been produced that could prove that particular deletions result in less virulent strains. Nevertheless, a number of candidate molecules have been identified by immunological and molecular methods. Here, we review the salient characteristics of some of these putative virulence determinants, including molecules that are involved in adhesion, protein degradation and the modulation of the host responses.

Expression of the highly polymorphic Cryptosporidium parvum Cpgp40/15 gene in genotype I and II isolates

The enteric protozoan Cryptosporidium parvum infects intestinal epithelial cells in a wide range of hosts, causing severe gastrointestinal disease. The invasive sporozoite stage most likely attaches to and invades host cells through multiple host receptor/parasite ligand interactions. Preliminary evidence suggests that the glycoprotein products of the Cpgp40/15 gene, gp40 and gp15, are involved in these interactions. In addition, the Cpgp40/15 gene that encodes these glycopeptides is highly polymorphic in genotype I isolates, suggesting that the gene products may be subject to immune selection. In this study, we characterized the Cpgp40/15 gene in a genotype I isolate and compared expression of the Cpgp40/15 gene in isolates of both genotype. Cpgp40/15 is a single copy gene in both TU502 (genotype I) and GCH1 (genotype II) isolates. However, Northern blot analysis revealed the presence of two transcripts, 2.3 and 1.5 kb in size, in mRNA from GCH1 as well as TU502-infected Caco-2A cells. Accumulation of the two Cpgp40/15 mRNAs peaked 12-24 h post-infection. Using 3'RACE analysis, three polyadenylation sites were identified 371, 978 and 1002 bp downstream of the GCH1 Cpgp40/15 stop codon. Two of these polyadenylation sites were also used in TU502. The sequences of the GCH1 Cpgp40/15 3'untranslated regions (3'UTRs) were identical to genomic sequence and shared 96.7% homology with TU502 3'UTRs. Actinomycin D treatment of GCH1-infected Caco-2A cells followed by Northern blot analysis, revealed that the stability of the 1.5 kb message was considerably greater than that of the 2.3 kb transcript.

Efficacy of monoclonal antibodies against defined antigens for passive immunotherapy of chronic gastrointestinal cryptosporidiosis

Cryptosporidium parvum is an important cause of diarrhea in humans and calves and can persistently infect immunocompromised hosts. Presently, there are no consistently effective parasite-specific drugs for cryptosporidiosis. We hypothesized that neutralizing monoclonal antibodies (MAbs) targeting the apical complex and surface antigens CSL, GP25-200, and P23 could passively immunize against cryptosporidiosis. We recently reported that a formulation of MAbs 3E2 (anti-CSL), 3H2 (anti-GP25-200), and 1E10 (anti-P23) provided significant additive prophylactic efficacy over that of the individual MAbs in neonatal ICR mice. In the present study, these MAbs were evaluated for therapeutic efficacy against persistent infection in adult gamma interferon-depleted SCID mice. 3E2 demonstrated the most significant and consistent therapeutic effect, reducing intestinal infection in two experiments. In one experiment, 3E2 plus 3H2 and 3E2 plus 3H2 plus 1E10 also significantly reduced infection; however, no significant increase in efficacy over 3E2 alone was apparent. The results indicate that anti-CSL MAb 3E2 has highly significant efficacy in reducing, but not eliminating, persistent C. parvum infection.

Characterization of an intestinal epithelial cell receptor recognized by the Cryptosporidium parvum sporozoite ligand CSL

The protozoan parasite Cryptosporidium parvum is a leading cause of diarrhea in humans and neonatal calves. The absence of approved parasite-specific drugs, vaccines, and immunotherapies for cryptosporidiosis relates in part to limited knowledge on the pathogenesis of zoite attachment and invasion. We recently reported that the C. parvum apical complex glycoprotein CSL contains a zoite ligand for intestinal epithelial cells which is defined by monoclonal antibody (MAb) 3E2. In the present study, the host cell receptor for CSL was characterized. For these studies, a panel of epithelial and mesenchymal cell lines was examined for permissiveness to C. parvum and the ability to bind CSL. Cells of epithelial origin were significantly more permissive and bound significantly greater quantities of CSL than cells of mesenchymal origin. Caco-2 intestinal cells were selected from the epithelial panel for further characterization of the CSL receptor. Immunoelectron microscopy demonstrated that CSL bound initially to the surface of Caco-2 cells and was rapidly internalized. The molecule bound by CSL was identified as an 85-kDa Caco-2 cell surface protein by radioimmunoprecipitation and CSL affinity chromatography. Sporozoite incubation with the isolated 85-kDa protein reduced binding of MAb 3E2. Further, attachment and invasion were significantly inhibited when sporozoites were incubated with the 85-kDa protein prior to inoculation onto Caco-2 cells. These observations indicate that the 85-kDa protein functions as a Caco-2 cell receptor for CSL. CSL also bound specifically to intestinal epithelium from calves, indicating receptor expression in a second important host species. Molecular characterization of the CSL receptor may lead to novel avenues for disrupting ligand-receptor interactions in the pathogenesis of C. parvum infection.

Mannose-binding lectin is a component of innate mucosal defense against Cryptosporidium parvum in AIDS

BACKGROUND & AIMS

Nonimmune mechanisms of mucosal defense seem to be biologically important and might explain the observed variability in the course of enteric infection in immunodeficiency. Mannose-binding lectin (MBL) deficiency is associated with persistent diarrhea in children. We found that genetic determinants of MBL deficiency appear to predispose to cryptosporidiosis in patients with the acquired immunodeficiency syndrome (AIDS), and went on to study interactions of MBL and complement on Cryptosporidium parvum sporozoites.

METHODS

This study involved cross-sectional study of MBL genotype and enteric infection in 72 Zambian AIDS patients with diarrhea, immunofluorescence analysis of MBL and C4 binding to C. parvum, and immunoblotting for MBL and complement in small intestinal fluid.

RESULTS

Individuals homozygous for MBL structural gene mutations were at increased risk of cryptosporidiosis (odds ratio, 8.2; 95% confidence interval, 1. 5-42; P = 0.02). Lectin-mediated and concentration-dependent binding of purified MBL was detected on sporozoites but not oocysts, and MBL activated C4 on sporozoites. MBL, C3, C4, and albumin were detected in small intestinal fluid in half the patients tested, suggesting transudation of serum components into the enteropathic gut.

CONCLUSIONS

The increased risk of cryptosporidiosis in MBL deficiency appears to include patients with AIDS. It may operate through MBL-mediated complement activation on sporozoites.

Cloning and sequence analysis of a highly polymorphic Cryptosporidium parvum gene encoding a 60-kilodalton glycoprotein and characterization of its 15- and 45-kilodalton zoite surface antigen products

The apicomplexan parasite Cryptosporidium parvum is a major cause of serious diarrheal disease in both humans and animals. No efficacious chemo- or immunotherapies have been identified for cryptosporidiosis, but certain antibodies directed against zoite surface antigens and/or proteins shed by gliding zoites have been shown to neutralize infectivity in vitro and/or to passively protect against, or ameliorate, disease in vivo. We previously used monoclonal antibody 11A5 to identify a 15-kDa surface glycoprotein that was shed behind motile sporozoites and was recognized by several lectins that neutralized parasite infectivity for cultured epithelial cells. Here we report the cloning and sequence analysis of the gene encoding this 11A5 antigen. Surprisingly, the gene encoded a 330-amino-acid, mucin-like glycoprotein that was predicted to contain an N-terminal signal peptide, a homopolymeric tract of serine residues, 36 sites of O-linked glycosylation, and a hydrophobic C-terminal peptide specifying attachment of a glycosylphosphatidylinositol anchor. The single-copy gene lacked introns and was expressed during merogony to produce a 60-kDa precursor which was proteolytically cleaved to 15- and 45-kDa glycoprotein products that both localized to the surface of sporozoites and merozoites. The gp15/45/60 gene displayed a very high degree of sequence diversity among C. parvum isolates, and the numerous single-nucleotide and single-amino-acid polymorphisms defined five to six allelic classes, each characterized by additional intra-allelic sequence variation. The gp15/45/60 single-nucleotide polymorphisms will prove useful for haplotyping and fingerprinting isolates and for establishing meaningful relationships between C. parvum genotype and phenotype.

Cryptosporidium parvum and mucosal immunity in neonatal cattle

Cryptosporidium parvum is an important zoonotic protozoan pathogen that causes acute infection and self-limiting gastrointestinal disease in neonatal calves. There are currently no consistently effective antimicrobials available to control cryptosporidiosis. Therefore, immunotherapeutic and vaccination protocols offer the greatest potential for long-term control of the disease. In order to devise effective control measures, it is important to better define mucosal immunity to C. parvum in young calves. This review summarizes the information that has accumulated over the last decade which helps to define the intestinal mucosal immune system in neonatal calves, and the events that occur in the intestinal mucosa after infection by C. parvum.

Characterization and formulation of multiple epitope-specific neutralizing monoclonal antibodies for passive immunization against cryptosporidiosis

The coccidian parasite Cryptosporidium parvum causes diarrhea in humans, calves, and other mammals. Neither immunization nor parasite-specific pharmaceuticals that are consistently effective against this organism are available. While polyclonal antibodies against whole C. parvum reduce infection, their efficacy and predictability are suboptimal. We hypothesized that passive immunization against cryptosporidiosis could be improved by using neutralizing monoclonal antibodies (MAbs) targeting functionally defined antigens on the infective stages. We previously reported that the apical complex and surface-exposed zoite antigens CSL, GP25-200, and P23 are critical in the infection process and are therefore rational targets. In the present study, a panel of 126 MAbs generated against affinity-purified CSL, GP25-200, and P23 was characterized to identify the most efficacious neutralizing MAb formulation targeting each antigen. To identify neutralizing MAbs, sporozoite infectivity following exposure to individual MAbs was assessed by enzyme-linked immunosorbent assay. Of 126 MAbs evaluated, 47 had neutralizing activity. These were then evaluated individually in oocyst-challenged neonatal mice, and 14 MAbs having highly significant efficacy were identified for further testing in formulations. Epitope specificity assays were performed to determine if candidate MAbs recognized the same or different epitopes. Formulations of two or three neutralizing MAbs, each recognizing distinct epitopes, were then evaluated. A formulation of MAbs 3E2 (anti-CSL [alphaCSL]), 3H2 (alphaGP25-200), and 1E10 (alphaP23) provided highly significant additive efficacy over that of either individual MAbs or combinations of two MAbs and reduced intestinal infection by 86 to 93%. These findings indicate that polyvalent neutralizing MAb formulations targeting epitopes on defined antigens may provide optimal passive immunization against cryptosporidiosis.

Cryptosporidium parvum apical complex glycoprotein CSL contains a sporozoite ligand for intestinal epithelial cells

Cryptosporidiosis, caused by the apicomplexan parasite Cryptosporidium parvum, has become a well-recognized diarrheal disease of humans and other mammals throughout the world. No approved parasite-specific drugs, vaccines, or immunotherapies for control of the disease are currently available, although passive immunization with C. parvum-specific antibodies has some efficacy in immunocompromised and neonatal hosts. We previously reported that CSL, an approximately 1,300-kDa conserved apical glycoprotein of C. parvum sporozoites and merozoites, is the antigenic species mechanistically bound by neutralizing monoclonal antibody 3E2 which elicits the circumsporozoite precipitate (CSP)-like reaction and passively protects against C. parvum infection in vivo. These findings indicated that CSL has a functional role in sporozoite infectivity. Here we report that CSL has properties consistent with being a sporozoite ligand for intestinal epithelial cells. For these studies, native CSL was isolated from whole sporozoites by isoelectric focusing (IEF) following observations that the approximately 1,300-kDa region containing CSL as seen by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was comprised of approximately 15 molecular species (pI 3 to 10) when examined by two-dimensional (2-D) electrophoresis and silver staining. A subset of six approximately 1,300-kDa species (pI 4.0 to 6.5) was specifically recognized by 3E2 in 2-D Western immunoblots of IEF-isolated CSL. Isolated native CSL bound specifically and with high affinity to permissive human intestinal epithelial Caco-2 cells in a dose-dependent, saturable, and self-displaceable manner. Further, CSL specifically bound to the surface of live Caco-2 cells inhibited sporozoite attachment and invasion. In addition, sporozoites having released CSL after incubation with 3E2 and occurrence of the CSP-like reaction did not attach to and invade Caco-2 cells. These findings indicate that CSL contains a sporozoite ligand which facilitates attachment to and invasion of Caco-2 cells and, further, that ligand function may be disrupted by CSL-reactive monoclonal antibody. We conclude that CSL is a rational target for passive or active immunization against cryptosporidiosis.