Naturally occurring immunoglobulin M antibodies: enhancement of phagocytic and microbicidal activities of human neutrophils against Toxoplasma gondii.

Naturally occurring immunoglobulin M antibodies to Toxoplasma gondii in human sera were examined for their ability to enhance toxoplasmacidal activities of human neutrophils to show a role of natural antibodies in immunity to the early stage of Toxoplasma infection in humans. Neutrophils were mixed with tachyzoites at a ratio of 1:5 in the presence or absence of antibodies and were cultured for 1-18 h for microscopical examination. The count of tachyzoites phagocytosed in 200 neutrophils within 1 h was significantly higher in the presence of sera with natural IgM antibody levels of greater than 0.8 than those of less than 0.1, with 2.4- to 2.9-fold differences (P less than 0.02). The total tachyzoite counts at 18 h decreased to 26-39% of those at 1 h in the presence of natural IgM antibody levels of greater than 0.8 (P less than 0.01), while the counts at 1 h consistently increased by 18 h in the absence of natural IgM antibodies. These results indicate that relatively high levels of natural IgM antibodies enhanced phagocytic and microbicidal activities of neutrophils against Toxoplasma. The enhancement was dose dependent, and was also weaker than that obtained with the same dilution of sera from individuals with chronic or acute infections. Live and dead tachyzoite counts in infected neutrophils suggested a quicker effect of natural IgM antibodies than IgG antibodies elicited by infection.