Resistance to avirulent Toxoplasma gondii in normal and vaccinated rats

Investigation of the innate resistance to infection with Toxoplasma gondii showed that the Toxoplasma-hostile factor present in rats has an important lytic effect on the parasites but is unable to prevent an infection alone. Vaccination experiments showed that innate resistance was also related to specific immunity, which appeared faster in rats than in rodents with less innate resistance. Complete resistance to toxoplasmosis was only present in rats vaccinated with live parasites. In rats vaccinated with killed parasites, avirulent parasites were able to encyst themselves almost as readily as in unvaccinated rats.

Toxoplasma antigens recognized by naturally occurring human antibodies

Sera of most adults have high agglutination test titers to Toxoplasma gondii whether or not the adults have other serological evidence of the infection. This finding has been attributed to the presence of naturally occurring antibodies to T. gondii. Consistent with this observation, we have recently noted that protein blots (PB) of sera of individuals not previously infected with T. gondii had immunoglobulin G (IgG) and IgM antibodies to antigens of the parasite. To further define the antigens recognized by these naturally occurring antibodies, we studied PB of sera of 44 adults and 9 children who had no serological evidence of the infection. Multiple antigens of T. gondii with molecular weights of 15,000 to greater than 205,000 were recognized by IgG and IgM natural antibodies of each of the sera. Although a relatively consistent pattern was noted on the IgM PB of the sera of the adults in the molecular weight range of 48,000 to 85,000, greater heterogeneity was noted on the IgG PB. The most common bands noted on the latter were of approximately 30,000 and 92,000 molecular weight. All of the PB obtained with the serial sera collected at yearly intervals from the children revealed bands; in some cases, new bands had appeared with time, and in others the pattern was constant. In children older than 8 years, the patterns of the PB were similar to those noted in PB of sera of the adults.