In Vitro Investigation of Host Resistance to Toxoplasma gondii Infection in Microglia of BALB/c and CBA/Ca Mice

Toxoplasmic encephalitis (TE) is a life-threatening disease of immunocompromised individuals and has increased in prevalence as a consequence of AIDS. TE has been modeled in inbred mice, with CBA/Ca mice being susceptible and BALB/c mice resistant to the development of TE. To better understand the innate mechanisms in the brain that play a role in resistance to TE, nitric oxide (NO)-dependent and NO-independent mechanisms were examined in microglia from BALB/c and CBA/Ca mice and correlated with the ability of these cells to inhibit Toxoplasma gondii replication. These parameters were measured 48 h after stimulation with lipopolysaccharide (LPS) gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), or combinations of these inducers in T. gondii-infected microglia isolated from newborn mice. CBA/Ca microglia consistently produced less NO than did BALB/c microglia after stimulation with LPS or with IFN-γ plus TNF-α, and they inhibited T. gondii replication significantly less than did BALB/c microglia. Cells of both strains treated with IFN-γ alone significantly inhibited uracil incorporation by T. gondii, and N(G)-monomethyl-l-arginine (NMMA) treatment did not reverse this effect. In cells treated with IFN-γ in combination with other inducers, NMMA treatment resulted in only partial recovery of T. gondii replication. This IFN-γ-dependent inhibition of replication was not due to generation of reactive oxygen species or to increased tryptophan degradation. These data suggest that NO production and an IFN-γ-dependent mechanism contribute to the inhibition of T. gondii replication after in vitro stimulation with IFN-γ plus TNF-α or with LPS. Differences in NO production but not in IFN-γ-dependent inhibition of T. gondii replication were observed between CBA/Ca and BALB/c microglia.