Toxoplasma MIC2 Is a Major Determinant of Invasion and Virulence

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Abstract
Like its apicomplexan kin, the obligate intracellular protozoan Toxoplasma gondii actively invades mammalian cells and uses a unique form of gliding motility. The recent identification of several transmembrane adhesive complexes, potentially capable of gripping external receptors and the sub-membrane actinomyosin motor, suggests that the parasite has multiple options for host-cell recognition and invasion. To test whether the transmembrane adhesin MIC2, together with its partner protein M2AP, participates in a major invasion pathway, we utilized a conditional expression system to introduce an anhydrotetracycline-responsive mic2 construct, allowing us to then knockout the endogenous mic2 gene. Conditional suppression of MIC2 provided the first opportunity to directly determine the role of this protein in infection. Reduced MIC2 expression resulted in mistrafficking of M2AP, markedly defective host-cell attachment and invasion, the loss of helical gliding motility, and the inability to support lethal infection in a murine model of acute toxoplasmosis. Survival of mice infected with MIC2-deficient parasites correlated with lower parasite burden in infected tissues, an attenuated inflammatory immune response, and induction of long-term protective immunity. Our findings demonstrate that the MIC2 protein complex is a major virulence determinant for Toxoplasma infection and that MIC2-deficient parasites constitute an effective live-attenuated vaccine for experimental toxoplasmosis. Toxoplasma gondii is a protozoan parasite that infects a broad range of hosts including humans. In people with weakened immunity resulting from HIV/AIDS or immune-suppressive treatment following organ transplantation, reactivation of a chronic T. gondii infection represents a serious threat, potentially leading to lethal disease within the brain, heart, or lungs. As an intracellular parasite, invasion into a host cell is a critical first step in ensuring parasite survival during infection. By using a regulatable expression system, this study shows that an adhesive protein called MIC2 is a limiting component of the parasite's invasion system and that it is required for the corkscrew-like movement of the parasite. Moreover, infection of mice with parasites lacking MIC2 no longer resulted in an acute infection leading to death. Not only do mice survive infection, they are protected from infection with a lethal dose of wild-type parasites, indicating an induction of protective immunity. In addition to having implications for the development of live-attenuated vaccines, this work suggests that novel treatment strategies directed at MIC2 may limit the severity of Toxoplasma infections.