Cryptosporidium are parasitic agents causing infection in a wide variety of animals including birds, reptiles and mammals. C. parvum is believed to be the major cause of disease in humans and domestic animals. The disease is acute and self-limited in immunocompetent humans. It, however, causes a more severe and potentially lethal disease in persons affected with AIDS. In spite of the morbidity of cryptosporidiosis in AIDS patients, no effective immunotherapy or chemotherapy is available at the present time to counter this disease. Moreover, the understanding of the biology and biochemistry of Cryptosporidium, as well as the pathophysiology of cryptosporidiosis is still at an early stage.
C. parvum has been classified based on its ultrastructural features as a member of the phylum Apicomplexa. "Zoites" such as sporozoites, merozoites, and tachyzoites are the invasive stages of the Apicomplexa. The zoites are extracellular and have a unique trilaminar membrane, called the pellicle, which appears to mediate the attachment of the parasite to the host cell membrane. The pellicle is associated with subpellicular structures that are involved in motility. The zoites also share an anterior apical complex composed of specialized secretory organelles (rhoptries, micronemes and dense granules). These organelles secrete products which appear to facilitate the entry of the zoite into the host cell and the generation of the parasitophorous vacuole.
The C. parvum infection is initiated by the ingestion of oocysts, the excystation of oocysts with release of sporozoites and the invasion of gut epithelial cells by sporozoites. Thereafter, the intracellular forms mature and release new daughter merozoites which reinvade the gut epithelial cells. C. parvum also has a sexual cycle. The sexual cycle of C. parvum also occurs in the gut and results in the production of sporulated oocysts, some of which may excyst before being shed. In persistent infection of an immunocompromised host, both the merozoite and the endogenously produced sporozoite may contribute to the ongoing invasion by C. parvum . The relative contribution of each stage, however, remains unclear.
Sporozoites and merozoites are the only stages of C. parvum which are free in the gut and, therefore, accessible to neutralization by luminal antibody. In other Apicomplexan zoites, e.g., Plasmodium, Eimeria, and Toxoplasma, the pellicle and apicle complex proteins are targets of invasion-inhibiting antibodies in vitro, and neutralization antibodies in vivo. Polypeptides localized in the pellicles and apicle complex of C. parvum zoites are likely targets of endogenous host immune responses. To date, the most promising treatment for cryptosporidiosis or infection by C. parvum, is the passive oral transfer of anti-C. parvum hyperimmune bovine colostral immunoglobulin (HBC Ig). HBC Ig has been shown to react with numerous oocyst and sporozoite proteins on Western blots and to be therapeutic in neonatal mice. Whole hyperimmune bovine colostrum (HBC) has also been reported to inhibit infection by C. parvum. Duodenal infusions of HBC have been reported to ameliorate C. parvum infection in AIDS or other immunocompromised patients.
GP15, GP20 and P23 are examples of sporozoite proteins or glycoproteins which are exposed on the surface of C. parvum. These antigens are examples of targets of monoclonal antibodies raised against the corresponding oocysts/sporozoites. These monoclonal antibodies have been shown to prevent or attenuate infection in studies using animals challenged with C. parvum. Monoclonal antibodies such as 17.41 and 18.44 have been reported to partially protect mice from C. parvum infection, although the sizes of the target antigens are still unclear. Monoclonal antibody 18.44 recognizes a non-peptide antigen. In some instances, the epitope recognized by the monoclonal antibodies has been found in both sporozoites and merozoites.
Thus, there is still a need for agents useful for the immunotherapy of cryptosporidiosis in both uncompromised and immunocompromised subjects, e.g. AIDS, patients, which would prevent or limit the disease's manifestations. There is also still a need for an agent useful for the detection of ongoing C. parvum invasion, particularly in its early stages.