Bovine antibody against Cryptosporidium parvum elicits a circumsporozoite precipitate-like reaction and has immunotherapeutic effect against persistent cryptosporidiosis in SCID mice

Abstract

Control of cryptosporidiosis is currently hampered by the absence of drugs or vaccines proven consistently effective against Cryptosporidium parvum. On the basis of observations that anti-C. parvum antibody has therapeutic effect against cryptosporidiosis, cows were immunized with C. parvum to produce hyperimmune colostral antibody. An antibody-rich fraction was prepared and differentiated from control (nonhyperimmune) antibody by enzyme-linked immunosorbent assay, immunofluorescence assay, immunoelectron microscopy, and in vitro neutralizing titer against DEAE-cellulose-isolated C. parvum sporozoites. Oocyst, purified sporozoite, and merozoite antigens recognized by hyperimmune antibody were defined by Western blot (immunoblot). Hyperimmune antibody recognized antigens common to oocysts, sporozoites, and merozoites, as well as stage-specific antigens. Upon incubation with hyperimmune antibody, sporozoites underwent distinct morphologic changes characterized by progressive formation and eventual release of membranous sporozoite surface antigen-antibody complexes, similar to the malaria circumsporozoite precipitate reaction. The infectivity of sporozoites having undergone this reaction was neutralized. The reaction was minimal or absent on sporozoites incubated with control antibody. To determine therapeutic effect in vivo, persistent C. parvum infection was established in adult severe combined immune-deficient (SCID) mice by oral inoculation with 107 oocysts. At 5 weeks postinfection, infected mice were treated for 10 days with hyperimmune or control antibody by inclusion in drinking water and daily gavage. Fecal oocyst shedding and infection scores in the gastrointestinal tract and gall bladder/common bile duct in hyperimmune antibody-treated mice were significantly lower than those in the control antibody-treated mice. Hyperimmune bovine antibody prepared against C. parvum may provide a first- generation therapy for control of cryptosporidiosis. Additionally, the defined antigens can be evaluated as subunit immunogens to produce better- characterized polyclonal antibody for control of cryptosporidiosis or as targets for monoclonal antibody-based immunotherapy.