Differential cytokine gene expression and secretion after phagocytosis by a human monocytic cell line of Toxoplasma gondii compared with Mycobacterium tuberculosis.

Toxoplasma gondii infection may be clinically silent in immunocompetent individuals but may cause fatal disease in immunocompromised patients such as those with HIV infection. Proinflammatory cytokines are known to be important in murine resistance to T. gondii but there are no data from human models of infection. We have investigated whether phagocytosis of T. gondii, of Mycobacterium tuberculosis (a pathogen which elicits a granulomatous host immune response) and of inert latex particles by THP-1 cells, a human monocytic line, caused gene expression and secretion of tumour necrosis factor (TNF), IL-6 and IL-8. These cytokines are important in recruitment and activation of T lymphocytes, and both TNF and IL-6 may have direct antitoxoplasmacidal and antimycobacterial activity. Phagocytosis of T. gondii by THP-1 cells resulted in minimal gene expression and secretion of TNF, IL-6 and IL-8 similar to that following phagocytosis of inert latex particles. In contrast, phagocytosis of M. tuberculosis resulted in increased gene expression of TNF and IL-8 as well as increased secretion of all three cytokines, particularly IL-8. These observations may partially explain the frequency of non-inflammatory host responses to T. gondii in immunocompetent individuals.