Gamma interferon functions in resistance to Cryptosporidium parvum infection in severe combined immunodeficient mice

Cryptosporidium infection of human small intestinal epithelial cells induces type III interferon and impairs infectivity of Rotavirus

Cryptosporidiosis is a major cause of severe diarrheal disease in infants from resource poor settings. The majority of infections are caused by the human-specific pathogen C. hominis and absence of in vitro growth platforms has limited our understanding of host-pathogen interactions and development of effective treatments. To address this problem, we developed a stem cell-derived culture system for C. hominis using human enterocytes differentiated under air-liquid interface (ALI) conditions. Human ALI cultures supported robust growth and complete development of C. hominis in vitro including all life cycle stages. Cryptosporidium infection induced a strong interferon response from enterocytes, possibly driven, in part, by an endogenous dsRNA virus in the parasite. Prior infection with Cryptosporidium induced type III IFN secretion and consequently blunted infection with Rotavirus, including live attenuated vaccine strains. The development of hALI provides a platform for further studies on human-specific pathogens, including clinically important coinfections that may alter vaccine efficacy.

Cytotoxic innate intraepithelial lymphocytes control early stages of Cryptosporidium infection

Background

Intraepithelial lymphocytes (IELs) are the first immune cells to contact and fight intestinal pathogens such as Cryptosporidium, a widespread parasite which infects the gut epithelium. IFN-γ producing CD4+ T IELs provide an efficient and a long-term protection against cryptosporidiosis while intraepithelial type 1 innate lymphoid cells limits pathogen spreading during early stages of infection in immunodeficient individuals. Yet, the role of T-cell like innate IELs, the most frequent subset of innate lymphocytes in the gut, remains unknown.

Methods

To better define functions of innate IELs in cryptosporidiosis, we developed a co-culture model with innate IELs isolated from Rag2-/- mice and 3D intestinal organoids infected with C. parvum using microinjection.

Results

Thanks to this original model, we demonstrated that innate IELs control parasite proliferation. We further showed that although innate IELs secrete IFN-γ in response to C. parvum, the cytokine was not sufficient to inhibit parasite proliferation at early stages of the infection. The rapid protective effect of innate IELs was in fact mediated by a cytotoxic, granzyme-dependent mechanism. Moreover, transcriptomic analysis of the Cryptosporidium-infected organoids revealed that epithelial cells down regulated Serpinb9b, a granzyme inhibitor, which may increase their sensitivity to cytolytic attack by innate IELs.

Conclusion

Based on these data we conclude that innate IELs, most likely T-cell-like innate IELs, provide a rapid protection against C. parvum infection through a perforin/granzymes-dependent mechanism. C. parvum infection. The infection may also increase the sensitivity of intestinal epithelial cells to the innate IEL-mediated cytotoxic attack by decreasing the expression of Serpin genes.

Cryptosporidium infection of human small intestinal epithelial cells induces type III interferon and impairs infectivity of Rotavirus

Cryptosporidiosis is a major cause of severe diarrheal disease in infants from resource poor settings. The majority of infections are caused by the human-specific pathogen C. hominis and absence of in vitro growth platforms has limited our understanding of host-pathogen interactions and development of effective treatments. To address this problem, we developed a stem cell-derived culture system for C. hominis using human enterocytes differentiated under air-liquid interface (ALI) conditions. Human ALI cultures supported robust growth and complete development of C. hominis in vitro including all life cycle stages. C. hominis infection induced a strong interferon response from enterocytes, likely driven by an endogenous dsRNA virus in the parasite. Prior infection with Cryptosporidium induced type III IFN secretion and consequently blunted infection with Rotavirus, including live attenuated vaccine strains. The development of hALI provides a platform for further studies on human-specific pathogens, including clinically important coinfections that may alter vaccine efficacy.

Cryptosporidium uses CSpV1 to activate host type I interferon and attenuate antiparasitic defenses

Cryptosporidium infects gastrointestinal epithelium and is a leading cause of infectious diarrhea and diarrheal-related death in children worldwide. There are no vaccines and no fully effective therapy available for the infection. Type II and III interferon (IFN) responses are important determinants of susceptibility to infection but the role for type I IFN response remains obscure. Cryptosporidium parvum virus 1 (CSpV1) is a double-stranded RNA (dsRNA) virus harbored by Cryptosporidium spp. Here we show that intestinal epithelial conditional Ifnar1^-/- mice (deficient in type I IFN receptor) are resistant to C. parvum infection. CSpV1-dsRNAs are delivered into host cells and trigger type I IFN response in infected cells. Whereas C. parvum infection attenuates epithelial response to IFN-γ, loss of type I IFN signaling or inhibition of CSpV1-dsRNA delivery can restore IFN-γ-mediated protective response. Our findings demonstrate that type I IFN signaling in intestinal epithelial cells is detrimental to intestinal anti-C. parvum defense and Cryptosporidium uses CSpV1 to activate type I IFN signaling to evade epithelial antiparasitic response.

Early immune and host cell responses to Cryptosporidium infection

Cryptosporidium spp. are opportunistic protozoan parasites that infect epithelial cells of the small intestine and cause diarrheal illness in both immunocompetent and immunodeficient individuals. These infections may be more severe in immunocompromised individuals and young children, especially in children under 2 in developing countries. The parasite has a global distribution and is an important cause of childhood diarrhea where it may result in cognitive impairment and growth deficits. Current therapies are limited with nitazoxanide being the only FDA-approved drug. However, it is not efficacious in immunocompromised patients. Additionally, there are no vaccines for cryptosporidiosis available. While acquired immunity is needed to clear Cryptosporidium parasites completely, innate immunity and early responses to infection are important in keeping the infection in check so that adaptive responses have time to develop. Infection is localized to the epithelial cells of the gut. Therefore, host cell defenses are important in the early response to infection and may be triggered through toll receptors or inflammasomes which induce a number of signal pathways, interferons, cytokines, and other immune mediators. Chemokines and chemokine receptors are upregulated which recruit immune cells such neutrophils, NK cells, and macrophages to the infection site to help in host cell defense as well as dendritic cells that are an important bridge between innate and adaptive responses. This review will focus on the host cell responses and the immune responses that are important in the early stages of infection.

A genetic screen identifies a protective type III interferon response to Cryptosporidium that requires TLR3 dependent recognition

Cryptosporidium is a leading cause of severe diarrhea and diarrheal-related death in children worldwide. As an obligate intracellular parasite, Cryptosporidium relies on intestinal epithelial cells to provide a niche for its growth and survival, but little is known about the contributions that the infected cell makes to this relationship. Here we conducted a genome wide CRISPR/Cas9 knockout screen to discover host genes that influence Cryptosporidium parvum infection and/or host cell survival. Gene enrichment analysis indicated that the host interferon response, glycosaminoglycan (GAG) and glycosylphosphatidylinositol (GPI) anchor biosynthesis are important determinants of susceptibility to C. parvum infection and impact on the viability of host cells in the context of parasite infection. Several of these pathways are linked to parasite attachment and invasion and C-type lectins on the surface of the parasite. Evaluation of transcript and protein induction of innate interferons revealed a pronounced type III interferon response to Cryptosporidium in human cells as well as in mice. Treatment of mice with IFNλ reduced infection burden and protected immunocompromised mice from severe outcomes including death, with effects that required STAT1 signaling in the enterocyte. Initiation of this type III interferon response was dependent on sustained intracellular growth and mediated by the pattern recognition receptor TLR3. We conclude that host cell intrinsic recognition of Cryptosporidium results in IFNλ production critical to early protection against this infection.

Commensal Cryptosporidium colonization elicits a cDC1-dependent Th1 response that promotes intestinal homeostasis and limits other infections

Cryptosporidium can cause severe diarrhea and morbidity, but many infections are asymptomatic. Here, we studied the immune response to a commensal strain of Cryptosporidium tyzzeri (Ct-STL) serendipitously discovered when conventional type 1 dendritic cell (cDC1)-deficient mice developed cryptosporidiosis. Ct-STL was vertically transmitted without negative health effects in wild-type mice. Yet, Ct-STL provoked profound changes in the intestinal immune system, including induction of an IFN-γ-producing Th1 response. TCR sequencing coupled with in vitro and in vivo analysis of common Th1 TCRs revealed that Ct-STL elicited a dominant antigen-specific Th1 response. In contrast, deficiency in cDC1s skewed the Ct-STL CD4 T cell response toward Th17 and regulatory T cells. Although Ct-STL predominantly colonized the small intestine, colon Th1 responses were enhanced and associated with protection against Citrobacter rodentium infection and exacerbation of dextran sodium sulfate and anti-IL10R-triggered colitis. Thus, Ct-STL represents a commensal pathobiont that elicits Th1-mediated intestinal homeostasis that may reflect asymptomatic human Cryptosporidium infection.

Evaluation of possible prophylactic and therapeutic effect of mefloquine on experimental cryptosporidiosis in immunocompromised mice

Cryptosporidiosis is an imperative global health concern. Unfortunately, Nitazoxanide (NTZ) (the nowadays drug of choice) is not effective in treatment of immunocompromised patients. We aimed to assess the possible anti-cryptosporidial prophylactic and therapeutic effects of Mefloquine (MQ) on infected immunosuppressed murine models. Mice were divided into five groups; GI: received Mefloquine (400 mg/kg/day), GII: received NTZ (100 mg/kg/bid), GIII: received a combination, half dose regimen of both drugs, GIV: infected untreated and GV: non-infected untreated. Each treated group was divided into three subgroups; Ga prophylaxis (PX), thereafter infection, Gb first and Gc second treatment doses. Assessment was done by parasitological, histopathological and serological techniques. A significant oocyst clearance was detected in all prophylactically treated groups. GIa showed 77% reduction of the mean oocyst count in stool while GIb and GIIIc showed100% oocyst clearance. Histopathologically, the ileocecal sections from GIV showed loss of brush borders with marked villous atrophy. GIa induced a moderate improvement of those pathological changes. Moreover, the villi in GIb and GIIIc retained their normal appearance with minimal inflammatory cells. Serum interferon gamma levels showed highly significant increases in GI&GIII compared to GIV while a non-significant increase was observed in GIIa only. On the contrary, serum interleukin-17 levels showed a highly significant down-regulation in all treated groups in comparison to GIV. This study proved a marvelous effect of MQ-PX on cryptosporidiosis in immunosuppressed mice and thus it could be introduced as one of the most promising re-purposed prophylactic and therapeutic anti-cryptosporidial agents.

Genomics and molecular epidemiology of Cryptosporidium species

Cryptosporidium is one of the most widespread protozoan parasites that infects domestic and wild animals and is considered the second major cause of diarrhea and death in children after rotavirus. So far, around 20 distinct species are known to cause severe to moderate infections in humans, of which Cryptosporidium hominis and Cryptosporidium parvum are the major causative agents. Currently, ssurRNA and gp60 are used as the optimal markers for differentiating species and subtypes respectively. Over the last decade, diagnostic tools to detect and differentiate Cryptosporidium species at the genotype and subtype level have improved, but our understanding of the zoonotic and anthroponotic transmission potential of each species is less clear, largely because of the paucity of high resolution whole genome sequencing data for the different species. Defining which species possess an anthroponotic vs. zoonotic transmission cycle is critical if we are to limit the spread of disease between animals and humans. Likewise, it is unclear to what extent genetic hybridization impacts disease potential or the emergence of outbreak strains. The development of high resolution genetic markers and whole genome sequencing of different species should provide new insights into these knowledge gaps. The aim of this review is to outline currently available molecular epidemiology and genomics data for different species of Cryptosporidium.

Cryptosporidium Priming Is More Effective than Vaccine for Protection against Cryptosporidiosis in a Murine Protein Malnutrition Model

Cryptosporidium is a major cause of severe diarrhea, especially in malnourished children. Using a murine model of C. parvum oocyst challenge that recapitulates clinical features of severe cryptosporidiosis during malnutrition, we interrogated the effect of protein malnutrition (PM) on primary and secondary responses to C. parvum challenge, and tested the differential ability of mucosal priming strategies to overcome the PM-induced susceptibility. We determined that while PM fundamentally alters systemic and mucosal primary immune responses to Cryptosporidium, priming with C. parvum (10⁶ oocysts) provides robust protective immunity against re-challenge despite ongoing PM. C. parvum priming restores mucosal Th1-type effectors (CD3⁺CD8⁺CD103⁺ T-cells) and cytokines (IFNγ, and IL12p40) that otherwise decrease with ongoing PM. Vaccination strategies with Cryptosporidium antigens expressed in the S. Typhi vector 908htr, however, do not enhance Th1-type responses to C. parvum challenge during PM, even though vaccination strongly boosts immunity in challenged fully nourished hosts. Remote non-specific exposures to the attenuated S. Typhi vector alone or the TLR9 agonist CpG ODN-1668 can partially attenuate C. parvum severity during PM, but neither as effectively as viable C. parvum priming. We conclude that although PM interferes with basal and vaccine-boosted immune responses to C. parvum, sustained reductions in disease severity are possible through mucosal activators of host defenses, and specifically C. parvum priming can elicit impressively robust Th1-type protective immunity despite ongoing protein malnutrition. These findings add insight into potential correlates of Cryptosporidium immunity and future vaccine strategies in malnourished children.

Cryptosporidium attributable morbidities in malnourished children are increasingly recognized. Exactly how malnutrition interferes with host mucosal immunity to diarrheal pathogens and mucosal vaccine responses remains unclear. Dissecting these interactions in an experimental model of cryptosporidiosis can uncover new insights into novel therapeutic approaches against a pathogen for which effective therapies and vaccines are currently unavailable. We demonstrate that although malnutrition diminishes baseline (primary) Th1-type mucosal immunity these deficits can be partially overcome via non-specific mucosal strategies (S. Typhi and CpG) and completely restored after a sub-clinical (low-dose) exposure to viable C. parvum. These results add insight into preventive strategies to help alleviate Cryptosporidium-specific diarrhea in children in low-resource settings and abrogate prolonged post-infection sequelae.

Modulation of the immune response by infection with Cryptosporidium spp. in children with allergic diseases

It has been demonstrated that the allergic response can be ameliorated by the administration of pathogen derivatives that activate Toll-like receptors and induce a Th1-type immune response (IR). Cryptosporidium is a parasite that promotes an IR via Toll-like receptors and elicits the production of Th1-type cytokines, which limit cryptosporidiosis. The aim of this study was to investigate allergy-related immune markers in children naturally infected with Cryptosporidium. In a cross-sectional study, 49 children with or without clinical diagnosis of allergies, oocysts of Cryptosporidium spp. in the faeces were screened microscopically. We microscopically screened for leucocytes, examined T and B cells for allergy-related activation markers using flow cytometry and evaluated serum for total IgE using chemiluminescence. Children with allergies and Cryptosporidium in the faeces had significantly lower levels of total IgE, B cells, CD19(+) CD23(+) and CD19(+) CD124(+) cells as well as a greater percentage of interferon-gamma (IFN-γ(+) ) and IL-4(+) CD4(+) cells than children with allergies without Cryptosporidium. This is the first description of the modulation of the IR in children with allergic diseases in the setting of natural Cryptosporidium infection. Our findings suggest the involvement of CD4(+) cells producing IL-4 and IFN-γ in the IR to Cryptosporidium in naturally infected children.

Host specificity and in vivo infectivities of the mouse coccidian parasites Eimeria krijgsmanni

In the present study, infection experiments of E. krijgsmanni using various hosts were conducted to elucidate the host specificity among some animals and the infectivity to mouse strains. According to the results, the infection was not found in most animals, except for rats, in which some oocyst shedding was detected, and there was no significant difference in infectivity among mouse strains. Additionally, oocyst shedding was hardly detectable in a secondary infection to immunocompetent mice, although it was found in immunodeficient mice. These results indicated that only immunocompetent mice could develop adaptive immunity against reinfection by stimuli of the primary infection. Furthermore, the infection experiments were performed with splenic macrophage (Mφ)-depleted mice with a reagent and Beige (Bg) mice known to be a strain of mice with low NK cell activity. No significant effect was found in primary or secondary infections in the Mφ-depleted mice, whereas the mortality rate was clearly increased in Bg mice inoculated with a large number of oocysts. Their oocyst shedding was similar to that of immunocompetent hosts. Taken together, these results suggested that Mφ has only a minor role in the immune response, but the NK cell has an important function in resistance to primary infection of E. krijgsmanni.

A protective role for interleukin 18 in interferon γ-mediated innate immunity to Cryptosporidium parvum that is independent of natural killer cells

Innate immunity against some intracellular parasitic protozoa involves interleukin 18 (IL-18)-mediated interferon γ (IFN-γ) production by natural killer (NK) cells, but the role of IL-18 in innate resistance to Cryptosporidium infection is unknown. Adult Rag2(-/-)γc(-/-) mice that lack NK cells, T cells, and B cells demonstrated resistance to Cryptosporidium parvum infection that was IFN-γ dependent. Treatment with anti-IL-18-neutralizing antibodies resulted in loss of resistance correlating with reduced intestinal IFN-γ expression. Intestinal mature IL-18 expression increased in vivo during infection and also in the intestinal epithelial cell line CMT-93 following combined IFN-γ treatment/infection. Peritoneal macrophages produced IFN-γ when stimulated with IL-18 combined with interleukin 12, and the latter was expressed in vivo during infection. Macrophage depletion in infected mice caused a rapid growth of infection with no increase in IFN-γ expression. These findings provide evidence of an NK cell-independent, IFN-γ-mediated innate immune pathway against C. parvum in which IL-18 and macrophages play prominent parts.

Cryptosporidiosis: host immune responses and the prospects for effective immunotherapies

Cryptosporidium spp. that develop in intestinal epithelial cells are responsible for the diarrhoeal disease cryptosporidiosis, which is common in humans of all ages and in neonatal livestock. Following infection, parasite reproduction increases for a number of days before it is blunted and then impeded by innate and adaptive immune responses. Immunocompromised hosts often cannot establish strong immunity and develop chronic infections that can lead to death. Few drugs consistently inhibit parasite reproduction in the host, and chemotherapy might be ineffective in immunodeficient hosts. Future options for prevention or treatment of cryptosporidiosis might include vaccines or recombinant immunological molecules, but this will probably require a better understanding of both the mucosal immune system and intestinal immune responses to the parasite.

The cell biology of cryptosporidium infection

Cryptosporidiosis remains a significant cause of enteric disease worldwide. Basic investigations of host: pathogen interactions have revealed the intricate processes mediating infection. The following summarizes the interactions that mediate infection and the host responses that both permit and ultimately clear the infection.

Host immune response to Cryptosporidium parvum infection

Species of the genus Cryptosporidium are protozoan parasites (Apicomplexa) that cause gastroenteritis in animals and humans. Of these Cryptosporidium parvum and Cryptosporidium hominis are the major causative agents of human cryptosporidiosis. Whereas infection is self-limiting in the immunocompetent hosts, immunocompromised individuals develop a chronic, life-threatening disease. As specific therapeutic or preventive interventions are not yet available, better understanding of the immune response to the parasite is required. This minireview briefly summarizes the factors involved in the innate and acquired immune response in this pathogen-host interaction with an emphasis on more recent data from mouse models of infection.

Human CD8(+) T cells clear Cryptosporidium parvum from infected intestinal epithelial cells

Intracellular protozoans of the genus Cryptosporidium are a major cause of diarrheal illness worldwide, especially in immunocompromised individuals. CD4(+) T cells and interferon-gamma are key factors in the control of cryptosporidiosis in human and murine models. Previous studies led us to hypothesize that CD8(+) T cells contribute to clearance of intestinal epithelial Cryptosporidium infection in humans. We report here that antigen expanded sensitized CD8(+) T cells reduce the parasite load in infected intestinal epithelial cell cultures and lyse infected intestinal epithelial cells. These effects are most likely mediated by the release of cytotoxic granules. Elimination of parasites seems to require antigen presentation through both human leukocyte antigen (HLA)-A and HLA-B. These data suggest that cytotoxic CD8(+) T cells play a role in clearing Cryptosporidium from the intestine, a previously unrecognized feature of the human immune response against this parasite.

Induced susceptibility of host is associated with an impaired antioxidant system following infection with Cryptosporidium parvum in Se-deficient mice

BACKGROUND

Susceptibility or resistance to infection with Cryptosporidium parvum (C.parvum) correlates with Selenium (Se) deficiency in response to infection. Both adult Se-adequate and Se-deficient mouse models of cryptosporidiosis were used to study the cell-mediated immune response during the course of C. parvum infection.

METHODOLOGY/PRINCIPAL FINDINGS

Blood samples from mouse models were used for Se status. The concentration of MDA, SOD, GPx and CAT in blood has revealed that lower Se level exist in Se-deficient mice. Mesenteric lymph node (MLN) lymphocytes from both mouse models were proliferated after ex vivo re-stimulation with C. parvum sporozoite antigen. The study of the cytokine profiles from the supernatant of proliferated MLN cells revealed that Se-adequate mice produced higher levels of Th1 (IFN-gamma and IL-2) and moderate amounts of Th2 (IL-4) cytokines throughout the course of infection. Whereas, MLN cells from Se-deficient mice produced lower levels of IFN-gamma, IL-2 and IL-4 cytokines. The counts of total white cell and CD3, CD4, CD8 cell in Se-adequate were higher than that in Se-deficient mice.

SIGNIFICANCE

These results suggest that Cell immunity is affected by Se status after infection with C. parvum from kinetic changes of different white cells and cytokine. In conclusion, induced susceptibility of host is associated with an impaired antioxidant system following infection with C. parvum in C57BL/6 Selenium deficient mice.

Cryptosporidium infection in patients with primary immunodeficiencies

BACKGROUND

Cryptosporidium species infection is usually self-limited in immunocompetent populations, but can be severe and life-threatening among immunocompromised individuals, particularly in patients with AIDS and in these patients with primary immunodeficiencies (PIDs).

PATIENTS AND METHODS

A group of 5 patients with genetically confirmed hyper-IgM syndrome type 1 (XHIM) and one patient with primary CD4 lymphopenia were enrolled in the study. At least 2 stool samples and a bile sample in one patient were examined for Cryptosporidium oocysts by a modified Ziehl-Neelsen technique, by immunofluorescence assay using a commercial kit, as well as by molecular analysis followed by genotyping. Immunological status at the time of PID diagnosis and the complex picture of disease are presented.

RESULTS

Chronic cryptosporidiosis was confirmed in 3 patients with XHIM and in one patient with primary CD4 lymphopenia. Molecular diagnosis showed the presence of C parvum, C hominis, and C meleagridis in analyzed specimens.

CONCLUSIONS

Cryptosporidium infection with serious clinical symptoms observed in patients with hyper-IgM syndrome calls for regular, repeated screening in this group of patients.

Mouse model of oral infection with virulent type A Francisella tularensis

Francisella tularensis is a gram-negative facultative intracellular pathogen and the causative agent of tularemia. Little is known about the immunopathogenesis of oral infection with this pathogen. Here, for the first time, we examined the susceptibility of mice to intragastric inoculation with virulent type A F. tularensis and characterized the course of infection and the associated host responses. Both immunocompetent and immunodeficient mice were relatively susceptible to intragastric inoculation of type A F. tularensis with a 50% lethal dose (LD(50)) of 10(6) organisms, which was 100,000-fold higher than the LD(100) for intradermal or respiratory routes of infection. Mice deficient in gamma interferon or tumor necrosis factor receptors 1 and 2 were more susceptible than wild-type controls to oral infection with a high dose of the pathogen. After oral inoculation, F. tularensis appeared first in the mesenteric lymph nodes (MLN) and then rapidly spread to the livers and spleens, where the organism multiplied to high numbers and induced marked neutrophilic infiltration and severe tissue necrosis. Infected mice showed rapid increases in tissue cytokine mRNA expression, which peaked in the MLN at 2 days postinfection (dpi) and in the liver and spleen at 3 dpi. The levels of gamma interferon, interleukin-1beta (IL-1beta), IL-6, tumor necrosis factor alpha, macrophage inflammatory protein 1alpha, KC, interferon-inducible protein 10, and monocyte chemotactic protein 1 were elevated from day 2 postinoculation onward. Moreover, mice intradermally immunized with the live vaccine strain of F. tularensis showed little survival advantage over naive mice after oral challenge with type A F. tularensis. These results suggest that type A F. tularensis is an effective oral pathogen that can cause fatal systemic infection and could pose a public health concern, particularly to immunocompromised individuals, if ingested in contaminated water and food.

Recombinant bovine interleukin-12 stimulates a gut immune response but does not provide resistance to Cryptosporidium parvum infection in neonatal calves

This study was undertaken to determine if administration of recombinant bovine interleukin-12 (rBoIL-12) could stimulate a cellular immune response that protected calves from an oral challenge inoculation with Cryptosporidium parvum oocysts. In a first experiment, rBoIL-12 intraperitoneally administered as a single dose 1 day before challenge inoculation, did not alter the course of infection. The percentage of immune competent cells and levels of cytokine gene expression in the ileo-cecal mucosa and in the draining lymph nodes of treated calves were similar to those of untreated control calves. However, when rBoIL-12 was subcutaneously administered daily from 2 days before infection to 2 days after infection, a consistent increase of T lymphocytes and an higher expression of interferon-gamma (IFN-gamma) was detected. Again, treatment did not alter the course of infection. Similar results were obtained when rBoIL-12 was administered daily for 4 days beginning 2 days after oral inoculation. These data indicate that although rBoIL-12 stimulated a strong immune response in the gut of neonatal calves, the response was not able to provide protection from challenge inoculation with C. parvum oocysts.

An early intestinal mucosal source of gamma interferon is associated with resistance to and control of Cryptosporidium parvum infection in mice

Resistance to and control of Cryptosporidium parvum infection in mice in the absence of adaptive immunity appears to be gamma interferon (IFN-gamma) dependent. Using an IFN-gamma-neutralizing antibody in a murine model, we demonstrated increased susceptibility to infection within 24 h. We correlated this early resistance and control with increased mucosal expression of IFN-gamma and demonstrate that CD8+ T-cell receptor alphabeta intestinal intraepithelial lymphocytes express and secrete this cytokine shortly after infection. The rapid kinetics of IFN-gamma expression and secretion by naive CD8+ T cells in response to a protozoan pathogen have not previously been demonstrated.

Kinetics of Cryptosporidium parvum-specific cytokine responses in healing and nonhealing murine models of C. parvum infection

Susceptibility or resistance to infection with Cryptosporidium parvum correlates with the ability of mice to produce characteristic panels of cytokines in response to infection. Both adult healing and nonhealing mouse models of cryptosporidiosis were used to study the cell-mediated immune response during the course of C. parvum infection. Mesenteric lymph node (MLN) lymphocytes from both mouse models were proliferated after ex vivo re-stimulation with C. parvum sporozoite antigen. Study of the cytokine profile from the supernatant of proliferated MLN cells revealed that healing mice produced greater levels of Th1 (IFN-gamma and IL-2) and moderate amounts of Th2 (IL-4, IL-5, IL-6, and IL-10) cytokines throughout the course of infection. Whereas, MLN cells from nonhealing mice produced no IFN-gamma, low levels of IL-2 and IL-4, and higher levels of IL-5, IL-6, and IL-10 cytokines. These results suggest that the capacity to produce both Th1 and Th2 cytokines, rather than the presence of Th2 cytokines alone, determines the effective immune response against C. parvum infection.

Cryptosporidium and host resistance: historical perspective and some novel approaches

Cryptosporidium parvum is recognized as a major cause of diarrheal disease in neonatal bovine calves. In addition, this protozoan parasite has emerged as an important cause of disease in both immunocompromised and immunocompetent humans. Despite years of research, no consistently effective means of prevention or treatment are readily available for cryptosporidiosis in any species. Infection through ingestion of contaminated water has been widely documented; C. parvum was reported to be responsible for the largest waterborne outbreak of infectious disease in US history. In addition to its role as a primary disease agent, C. parvum has potential to initiate or exacerbate other gastrointestinal disorders, such as inflammatory bowel disease. Thus, control of C. parvum infection in both animals and humans remains an important objective. Research in our laboratory has focused on understanding mechanisms of resistance to C. parvum. We have demonstrated that acquisition of intestinal flora increases resistance to C. parvum. Substances present in the intestinal mucosa of adult animals can transfer resistance when fed to susceptible infants. Both expression of intestinal enzymes and rate of proliferation of epithelial cells may be altered following C. parvum infection. These and other changes may have profound effects on host resistance to C. parvum.

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.

Exogenous interleukin-12 (IL-12) exacerbates Cryptosporidium parvum infection in gamma interferon knockout mice

Experimental infection of BALB/c- or C57BL/6-gamma-interferon-knockout (GKO) mice with Cryptosporidium parvum results in infection in both strains with different outcomes of disease. The BALB/c-GKO mice recover from infection, whereas the C57BL/6-GKO mice succumb to infection in less than 2 weeks. Differences in cytokine mRNA expression suggested that recovery may involve other cytokines. To determine whether the addition of either a Th1 or Th2 cytokine could alter the outcome of infection, we treated GKO mice with either recombinant (r)IL-4 or rIL-12 1 day before infection (DBI) or daily. No effect on the oocyst shedding patterns in either strain nor an increase in survival of the C57BL/6-GKO mice was observed in the rIL-4-treated mice. Whereas one dose of 0.5 microg rIL-12 given 1 DBI had no effect on oocyst shedding, we found that daily doses of rIL-12 administered intraperitoneally exacerbated C. parvum infection in both animal models. Administration of rIL-12 shortened the survival time in the C57BL/6-GKO mice and prevented BALB/c-GKO mice from recovering from infection. Specific proliferation of T cells to cryptosporidial antigen and Th1 and Th2 mRNA cytokine expression was markedly decreased in rIL-12-treated mice. Nitric oxide (NO) may have played a minor role in the decreased proliferation observed since levels of NO present in the splenocyte cultures from rIL-12-treated mice in response to parasite antigen stimulation were higher than those observed in controls. Thus, we propose that resistance to and recovery from C. parvum infections involves a fine balance in the amount and timing of Th1 and Th2 cytokines.

Cytotoxic T cells in AIDS colonic cryptosporidiosis

BACKGROUND/AIMS

It is not known how enteric cryptosporidiosis induces severe intestinal impairment despite minimal invasion by the parasite. The aim of this study was to analyse the histological features and locally implicated immune cells in colonic biopsies of AIDS related cryptosporidiosis.

PATIENTS/METHODS

Colonic biopsies from patients with AIDS related cryptosporidiosis (n = 10, group I), patients with AIDS but without intestinal infection (n = 9, group II), and human seronegative controls (n = 9, group III) were studied. Using immunohistochemistry the infiltrating mononuclear cells were analysed in both the epithelium and lamina propria for the expression of CD3, CD8, TiA1, granzyme B, and CD68 and for glandular expression of human major histocompatibility complex DR antigen (HLA-DR).

RESULTS

Severe histological changes, resulting in abundant crypt epithelial apoptosis and inflammatory infiltrate in the lamina propria, were seen in all biopsies from group I. A significant increase of CD8+, TiA1+, and granzyme B+ T cells in the lamina propria and HLA-DR glandular expression was noted in group I compared with groups II and III. However, the number of intraepithelial lymphocytes, lamina propria CD3+ T cells, and macrophages was not significantly increased in cryptosporidiosis specimens compared with controls.

CONCLUSION

Epithelial apoptosis mediated by granzyme B+ cytotoxic host T cells might play a major role in the development of colonic lesions in AIDS related cryptosporidiosis.

Cryptosporidium parvum-specific mucosal immune response in C57BL/6 neonatal and gamma interferon-deficient mice: role of tumor necrosis factor alpha in protection

Both neonatal and C57BL/6 gamma interferon (IFN-gamma) knockout (C57BL/6-GKO) mice are susceptible to Cryptosporidium parvum, but the course of infection is different. Neonatal mice are able to clear the parasite within 3 weeks, whereas C57BL/6-GKO mice, depending on age, die rapidly or remain chronically infected. The mechanism by which IFN-gamma leads to a protective immunity is yet poorly understood. In order to investigate the effect of IFN-gamma on other cytokines expressed in the intestinal mucosa during C. parvum infection, we studied cytokine mRNA expression in the neonates and GKO (neonatal and adult) mice by quantitative reverse transcription-PCR (RT-PCR) at 4 and 9 days after infection. IFN-gamma mRNA levels were quickly and strongly up-regulated in the mucosa of neonatal mice. In GKO mice, the Th1-type response was dramatically altered during the infection, whereas the mRNA expression levels of the Th2-type cytokines interleukin 4 (IL-4) and IL-10 were increased in both mouse models. In the absence of IFN-gamma, the adult knockout mice up-regulated the mRNA levels of inflammatory cytokines, such as IL-1beta, IL-6, and granulocyte-macrophage colony-stimulating factor, in the mucosa, but not tumor necrosis factor alpha (TNF-alpha), whereas all these cytokines were up-regulated in the infected neonatal mice. Further experiments indicated that injections of TNF-alpha into GKO adult mice significantly reduced oocyst shedding. The results of the present study indicate that the resolution of infection is dependent on the expression of Th1-type cytokines in the mucosa of C57BL/6 mice and that TNF-alpha may participate in the control of parasite development.

Host cell-mediated responses to infection with Cryptosporidium

The coccidian Cryptosporidium infects epithelial cells of a variety of vertebrate hosts and is the causative agent of cryptosporidiosis. In mammals, including humans and domestic animals, C. parvum infects the gastrointestinal tract producing an acute watery diarrhoea and weight loss. CD4+ T-cell-deficient hosts have increased susceptibility to infection with the parasite and may develop severe life-threatening complications. The host responses which induce protective immunity and contribute to pathogenesis are poorly understood. In the immunological control of infection, recent studies with murine infection models suggest that IFN-gamma plays a key role in a partially protective innate immunity against infection identified in immunocompromised mice and also in the elimination of infection mediated by CD4+ T-cells. At the mucosal level, CD4+ intraepithelial lymphocytes are involved in the control of cryptosporidial infection, acting at least in part through production of IFN-gamma which has a direct inhibitory effect on parasite development in enterocytes. Primary infection of ruminants induces an intestinal inflammatory response in which increased numbers of various T-cell subpopulations appear in the villi. In addition, infection results in increased intestinal expression of pro-inflammatory cytokines such as IL-12, IFN-gamma and TNF-alpha. Because these cytokines appear to be important in the aetiology of inflammatory bowel disease, it is possible that they are involved in the mucosal pathogenesis of cryptosporidiosis.

A 23-kDa recombinant antigen of Cryptosporidium parvum induces a cellular immune response on in vitro stimulated spleen and mesenteric lymph node cells from infected mice

In the present study, we focused on a 23-kDa antigen, Cp23, which has been shown to be a major target of humoral immune responses in Cryptosporidium parvum infections and is present in both the sporozoite and merozoite stages. Recombinant Cp23 antigen was shown to stimulate a specific proliferative response by splenocytes and mesenteric lymph node cells from infected interferon gamma knockout BALB/c mice. Cp23 stimulation also induced TNF-alpha, IL-2, and IL-5 mRNA production by spleen cells from infected animals. In contrast, IL-12 mRNA was decreased by Cp23 stimulation compared with unstimulated splenocytes. These data suggest that, as with humoral responses, Cp23 is an important target of cellular immune responses in experimental C. parvum infections. The potential role of this antigen in conferring protective immunity is also discussed.

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.

Eradication of Cryptosporidium parvum infection by mice with ovalbumin-specific T cells

CD154 is necessary for mice to clear a Cryptosporidium parvum infection, but whether this ligand has to be expressed on T cells with specificity for C. parvum has not been determined. We infected DO11.10 (ovalbumin specific) T-cell receptor transgenic mice that had been bred to a RAG(-/-) background with C. parvum and found that the infection was cleared within 6 weeks, while RAG(-/-) controls were unable to clear C. parvum infection. Recovery was accompanied by an increase in the number of splenic T cells with the CD44(high) phenotype that characterizes memory cells. To determine whether a C. parvum-infected environment sufficed to activate transgenic T cells, we reconstituted C. parvum-infected BALB/c SCID mice with DO11.10 RAG(-/-) splenocytes. Fecal excretion of C. parvum antigen ceased in the 12 weeks following the adoptive transfer, unless the mice were also injected with tolerizing doses of ovalbumin. DO11.10 T cells were found in the submucosa of C. parvum-infected, but not uninfected, BALB/c SCID hosts within 48 h of injection. The transferred DO11.10 T cells divided and acquired a CD44(high) memory phenotype in C. parvum-infected, but not uninfected, recipients. DO11.10 splenocytes from CD154 knockout donors failed to clear a C. parvum infection, confirming a requirement for CD154 in recovery. In vitro, the DO11.10 cells did not proliferate in response to C. parvum antigen, and a tBlast GenBank search revealed no matches between the ovalbumin peptide and C. parvum DNA sequences. C. parvum-infected SCID mice given RAG(-/-) CD8(+) T cells with a Listeria-specific transgene did not recover from C. parvum infection. Our data suggest that antigen-nonspecific CD4(+) T-cell effector mechanisms in combination with the innate arm of the immune system are sufficient for the eradication of C. parvum infection.

Nasal immunization of mice with Cryptosporidium parvum DNA induces systemic and intestinal immune responses

DNA immunization offers a novel approach to inducing humoral and cellular immunity against infectious pathogens. We examined whether such an approach could be used against cryptosporiodiosis, an intestinal disease caused by the protozoan parasite Cryptosporidium parvum. This infection is a major problem for young ruminants and immunosuppressed individuals in whom cryptosporidiosis causes life-threatening symptoms. The life cycle of C. parvum takes place in the enterocytes of the intestinal epithelium. We therefore focused our attention on a route of immunization that might induce a mucosal immunoglobulin (Ig)A response. Eight-week-old BALB/c mice were immunized intranasally with DNA encoding a 15-kDa C. parvum sporozoite antigen (CP15-DNA) cloned onto the plasmid pcDNA3. CP15-DNA-immunized mice developed specific and longlasting production of anti-CP15 Ig A in intestinal secretions and specific IgG in sera 3 months and 1 year after the first DNA inoculation. CP15-DNA-immunized mice also developed an antigen-specific T lymphocyte proliferative response in both spleen and mesenteric lymph nodes. Control mice that received the pcDNA3 plasmid alone did not develop specific humoral and cellular responses. These results indicate that plasmid DNA may provide a powerful means of eliciting intestinal humoral and cellular responses to C. parvum infections in mammals.

New insights into human cryptosporidiosis

Cryptosporidium parvum is an important cause of diarrhea worldwide. Cryptosporidium causes a potentially life-threatening disease in people with AIDS and contributes significantly to morbidity among children in developing countries. In immunocompetent adults, Cryptosporidium is often associated with waterborne outbreaks of acute diarrheal illness. Recent studies with human volunteers have indicated that Cryptosporidium is highly infectious. Diagnosis of infection with this parasite has relied on identification of acid-fast oocysts in stool; however, new immunoassays or PCR-based assays may increase the sensitivity of detection. Although the mechanism by which Cryptosporidium causes diarrhea is still poorly understood, the parasite and the immune response to it probably combine to impair absorption and enhance secretion within the intestinal tract. Important genetic studies suggest that humans can be infected by at least two genetically distinct types of Cryptosporidium, which may vary in virulence. This may, in part, explain the clinical variability seen in patients with cryptosporidiosis.

Speculation on whether a vaccine against cryptosporidiosis is a reality or fantasy

In this paper the authors question whether the development of a vaccine against cryptosporidiosis could be taken into consideration. The necessity and feasibility of such a vaccine for human and veterinary application is discussed. Developmental stages within the life cycle of the parasite that might act as possible targets for vaccine development are summarised, as well as the target antigens offered by molecular biology and immunology studies. Vaccination trials against cryptosporidiosis carried out so far, including the active and passive immunisation approach, are also overviewed. It seems that with respect to a Cryptosporidium vaccine two target groups can be considered: children of the developing world and neonatal ruminants. Antigens representing possible candidates for a subunit vaccine were identified based on their function, location and/or the immune response they evoke. While the active vaccination of newborn calves, lambs and goat kids has to face a number of important limitations, the passive immunisation approach, where dams were immunised to protect their progeny by colostral transfer, was proven to be a valuable alternative. Finally, a number of points of action for the near future are put forward.

Evidence of thymus-independent local and systemic antibody responses to Cryptosporidium parvum infection in nude mice

Differences in susceptibility to persistent cryptosporidial infection between two strains of adult athymic nude mice prompted us to investigate the immune mechanism(s) that may control resistance to infection in these T-cell-deficient mice. We studied fecal oocyst shedding, serum and fecal parasite-specific antibody responses, and fecal immunoglobulin levels in athymic C57BL/6J nude and athymic BALB/cJ nude mice following oral inoculation with Cryptosporidium parvum oocysts at 8 to 9 weeks of age. C57BL/6J nude mice had significantly higher fecal parasite-specific immunoglobulin A (IgA) (days 27, 31, 35, and 42 postinoculation) and IgM (days 10, 17, 24, 28, 31, 38, 42, and 48 postinoculation) levels than BALB/cJ nude mice (P < 0.05) and significantly higher serum parasite-specific IgA levels at 63 days postinoculation (P < 0.03). Moreover, C57BL/6J nude mice shed significantly fewer C. parvum oocysts than BALB/cJ nude mice from days 52 to 63 postinoculation (P < 0.05). In contrast, BALB/cJ nude mice had higher levels of non-parasite-specific IgA (days 38 to 63 postinoculation) and IgM (days 24, 35, 38, and 52 postinoculation) than C57BL/6J nude mice in feces (P < 0.05). These data suggest that parasite-specific fecal antibodies may be associated with resistance to C. parvum in C57BL/6J nude mice.

Liver and bile duct pathology following Cryptosporidium parvum infection of immunodeficient mice

Patients with acquired immune deficiency syndrome (AIDS) and boys with mutations of the CD154 gene (causing congenital X-linked immunodeficiency with hyper-IgM [XHIM]) are susceptible to chronic infections of the biliary tract with Cryptosporidium parvum (CP) that may lead to biliary sclerosis and ultimately to cholangiocarcinoma. To determine whether the CP infection and the consequent immune response contribute independently to this morbidity, we infected mice with severe combined immunodeficiency (SCID) or with disrupted genes for CD154, CD40, or interferon gamma (IFN-gamma) with CP. Even when CP infection persisted for 16 weeks, the SCID mice developed only mild triaditis, without apoptosis of biliary epithelial cells (BEC). Fifty percent of the CD154 knockout mice developed lobular hepatitis with acute and chronic triaditis. The CD40 knockout mice developed marked triaditis, and the IFN-gamma knockouts either succumbed to enteritis or survived to develop marked triaditis, portal fibrosis, biliary sclerosis, necrosis with dilation of duct-like structures within the porta hepatis, and dysplastic changes. CP-infected SCID mice reconstituted with T cells from IFN-gamma knockout donors either developed severe enteritis or survived to develop triaditis, cholangitis, lobular hepatitis with periductular sclerosis, and scarring. Mice with disruptions of both the CD40 and IFN-gamma genes remained infected by CP and developed bile duct and liver disease, but not enteritis. Our results suggest that T-cell cytokines are required for the inflammatory and sclerosing responses to CP infection in immunodeficient animals. The response of immunodeficient mice to CP infection may model at least the initial steps toward the development of sclerosing cholangitis or bile duct cancers in XHIM patients.

Pathogenesis of Cryptosporidium parvum infection

Cryptosporidium parvum can be regarded as a minimally invasive mucosal pathogen, since it invades surface epithelial cells that line the intestinal tract but does not invade deeper layers of the intestinal mucosa. Nonetheless, infection can be associated with diarrhea and marked mucosal inflammation. This article briefly reviews in vitro and in vivo models useful for studying the pathogenesis of C. parvum infection and explores the role of innate and acquired immune responses in host defense against this protozoan parasite.

IL-4 Protects Adult C57BL/6 Mice from Prolonged Cryptosporidium parvum Infection: Analysis of CD4+αβ+IFN-γ+ and CD4+αβ+IL-4+ Lymphocytes in Gut-Associated Lymphoid Tissue During Resolution of Infection

Resistance of adult C57BL/6 mice to severe Cryptosporidium parvum infection is dependent on CD4+αβ+ TCR lymphocytes. In this study, we demonstrated that treatment with anti-IFN-γ mAb extended oocyst excretion 18 days longer, and anti-IL-4 mAb extended oocyst excretion at least 11 days longer than isotype control mAb treatment. Analysis of the specific activity of anti-IFN-γ mAb present in treated mouse sera suggested that IFN-γ may have a limited role in the resolution phase of infection. Changes were also documented in numbers of CD4+αβ+IFN-γ+ and CD4+αβ+IL-4+ lymphocytes in Peyer’s patches and intraepithelium of adult C57BL/6 mice during resolution of C. parvum infection. Resistance to initial severe infection was associated with CD4+αβ+IFN-γ+ lymphocytes, and eventual resolution of infection was associated with CD4+αβ+IL-4+ lymphocytes. Analysis of cytokine expression following in vitro stimulation with C. parvum Ags during resolution of infection demonstrated consistent increases in CD4+αβ+IL-4+ lymphocytes, but not CD4+αβ+IFN-γ+ lymphocytes. The relevance of CD4+αβ+IL-4+ lymphocytes in protection against C. parvum was then evaluated in C57BL/6 IL-4 gene knockout mice (IL-4−/−). Adult IL-4−/− mice excreted oocysts in feces approximately 23 days longer than IL-4+/+ mice. Further, anti-IFN-γ mAb treatment increased the severity and the duration of infection in IL-4−/− mice compared with those in IL-4+/+ mice. Together, the data demonstrated that IFN-γ was important in the control of severity of infection, and either IFN-γ or IL-4 accelerated termination of infection. However, neither IL-4 nor IFN-γ was required for the final clearance of infection from the intestinal tract of adult mice.

Roles for tumor necrosis factor and gamma interferon in resistance to enteric listeriosis

Listeria monocytogenes normally infects the host by translocating from the intestinal lumen. Experiments were carried out to determine if, when, and where tumor necrosis factor (TNF) and gamma interferon (IFN-gamma) function in antibacterial resistance during enteric listeriosis. Groups of normal mice and severe combined immunodeficient (SCID) mice were injected with neutralizing monoclonal antibodies (MAb) specific for each cytokine and then inoculated intragastrically with L. monocytogenes. The course of infection was monitored by enumerating listeriae in gut-associated lymphoid tissues, livers, and spleens. By the third day of infection, bacterial numbers in infected tissues and organs were greatly exacerbated in all mice treated with anti-TNF MAb, whereas bacterial numbers in the organs of mice treated with anti-IFN-gamma MAb did not differ from those present in the respective organs of control mice. However, by the fifth day of infection, bacterial numbers in the organs of anti-IFN-gamma MAb-treated normal mice and SCID mice were much greater than in the corresponding organs of control mice. Experiments with Listeria-immune mice revealed that TNF and IFN-gamma are involved in the expression of anti-Listeria memory immunity; however, it was also found that the anti-IFN-gamma MAb was relatively ineffective in inhibiting the expression of anti-Listeria immunity, whereas a polyclonal anti-IFN-gamma was quite effective.

Susceptibility and serologic response of healthy adults to reinfection with Cryptosporidium parvum

Healthy adults are susceptible to infection with small numbers of Cryptosporidium parvum oocysts, resulting in self-limited infection. We investigated if infection of humans with C. parvum is protective 1 year after primary exposure. At 1 year after a primary challenge with 30 to 10(6) oocysts, 19 healthy immunocompetent adults were rechallenged with 500 oocysts and monitored for the development of infection and/or illness. Oocyst excretion was quantitated by direct immunofluorescence with a C. parvum-specific monoclonal antibody, and anti-C. parvum antibodies in serum were detected by an enzyme-linked immunosorbent assay. Fewer subjects shed oocysts after the second exposure (3 of 19; 16%) than after the first exposure (12 of 19; 63%) (P < 0.005). Although the rates of diarrhea were comparable after each of the two exposures, the clinical severity as determined by the mean number of unformed stools passed was lower after reexposure (11.25 versus 8.62; P < 0.05). The number of anti-Cryptosporidium immunoglobulin G and A seroconversions increased after secondary exposure. However, the C. parvum serum antibody response did not correlate with the presence or absence of infection.