Monoclonal and polyclonal, monospecific antibodies to the major surface antigen of Toxoplasma gondii (SAG-1, P30) inhibit infection of human fibroblasts and murine enterocytes. Fab prepared from polyclonal, monospecific antibody to P30 also have this inhibitory effect on invasion, which indicates that this antibody directly blocks parasite infection of host cells rather agglutinating the parasite. Antibodies to another surface protein (P22) did not alter in vitro infection. If the inhibitory effect of antibody to P30 was due to steric hindrance or complexing of surface epitopes contiguous to P30, antibodies to other surface epitopes would also be inhibitory and they are not. Urea treatment of antibody (which permits discrimination of high and low avidity antibody) did not alter the effect of anti-P30 antibody. This observation indicates that the effect of the antibody to P30 was not an artifact of differences in the avidity of the antibody to P22 and P30. Heat inactivated antisera from mice infected with either RH or PTg strain T. gondii (P30+) inhibit infection of fibroblasts when challenged with autologous wild-type parasites by 87 and 40%, respectively. In contrast, these antisera have little inhibitory effect (13 and 19%, respectively) against infection of human fibroblasts by a P30-deficient mutant (PTgB). Antisera raised to the P30-deficient mutant had no significant effect on infection of cells by wild-type strains that have surface P30. The neoglycoprotein, BSA-glucosamide, competitively blocks infection of human fibroblasts by P30+ tachyzoites with surface P30 in higher level than those without surface P30. This observation indicates that there is likely to be a glycosylated host cell receptor to which T. gondii’s major surface Ag SAG-1 (P30) binds. Mice infected perorally develop intestinal IgA antibody to the major 30-kDa epitope of T. gondii. Thus, the major surface epitope of T. gondii, SAG-1 (P30), has an important, functional role in infection of host cells by T. gondii and elicits an intestinal antibody response after peroral infection.

Considering that little is known about the epidemiology ofNeospora caninuminfection in humans, particularly in populations with highToxoplasma gondiiinfection rates, the present study aimed to investigate the presence of antibodies toN. caninuminT. gondii-seropositive and -seronegative individuals. A total of 256 serum samples divided into four groups (61 samples from human immunodeficiency virus [HIV]-positive patients, 50 samples from patients with neurological disorders, 91 samples from newborns, and 54 samples from healthy subjects) were assessed forN. caninumandT. gondiiserologies by indirect fluorescent-antibody test, enzyme-linked immunosorbent assay, and immunoblotting (IB). Immunoglobulin G antibodies toN. caninumwere predominantly detected in HIV-infected patients (38%) and patients with neurological disorders (18%), while newborns and healthy subjects showed lower seropositivity rates (5% and 6%, respectively). Seropositivity toN. caninumwas significantly associated with seropositivity toT. gondiiin both HIV-infected patients and patients with neurological disorders. Seroreactivity toN. caninumwas confirmed by IB, with positive sera predominantly recognizing the 29-kDa antigen ofN. caninum. The results of this study indicate the presence ofN. caninuminfection or exposure in humans, particularly in HIV-infected patients or patients with neurological disorders, who could have opportunistic and concurrent infections withT. gondii. These findings may bring a new concern for the unstable clinical health of HIV-infected patients and the actual role ofN. caninuminfection in immunocompromised patients.

SUMMARY: Toxoplasma gondii, Neosporaspp.,Sarcocystisspp.,Hammondiaspp. andBesnoitia besnoitiare genetically related cyst-forming coccidia. Serology is frequently used for the identification ofT. gondii, Neosporaspp. andB. besnoiti-exposed individuals. Serologic cross-reactions occur in different tests among animals infected withT. gondiiandH. hammondi,as well as among animals infected byT. gondiiandN. caninum. Infections caused byN. caninumandN. hughesiare almost indistinguishable by serology.Neospora caninum, B. besnoitiandSarcocystisspp. infections in cattle show some degree of serologic cross-reactivity. Antibody cross-reactivity betweenNeosporaspp. andH. heydorni-infected animals is suspected, but not proven to occur. We review serologic cross-reactivity among animals and/or humans infected withT. gondii, Neosporaspp.,Sarcocystisspp.,Hammondiaspp. andB. besnoiti. Emphasis is laid upon antigens and serological methods forN. caninumdiagnosis which were tested for cross-reactivity with related protozoa. Species-specific antigens, as well as stage-specific proteins have been identified in some of these parasites and have promising use for diagnosis and epidemiological surveys.