Immunity–related GTPases (IRG) constitute a powerful resistance system against the protozoa Toxoplasma gondii and its close relative Neospora caninum as well as against two strains of the bacteria Chlamydia in mice. However, it remains a great mystery why all other organisms tested so far are not restricted by the IRG system. IRG-mediated restriction correlates with accumulation of effector IRG subfamily, the GKS proteins, at the parasitophorous vacuoles leading to the breakdown of the membrane barrier and death of the parasite. It is not known how GKS proteins can specifically recognise and bind to the parasitophorous vacuolar membranes (PVM), which is derived from the invaginated host plasma membrane and block fusion with the endolysosomal compartments. The second subfamily of IRG proteins, GMS proteins, prevents premature activation of GKS proteins and seems to protect endomembranes from GKS-mediated destruction. The GMS protein Irgm1 has been alternatively proposed to directly mediate acidification and destruction of bacterial phagosomes. However, this theory is incompatible with the current model that IRG resistance system acts only on non-phagosomal vacuoles.
The present study demonstrated the predicted existence of two protein isoforms of Irgm1, which localised slightly different to subcellular endomembranes. Moreover, in striking contrast to earlier studies, Irgm1 could never be detected at listerial or mycobacterial phagosomes, arguing against the alterative proposed role of Irgm1 on phagosomes. In order to understand IRG target membranes, the present study showed that certain GKS proteins have an intrinsic property to bind liposomes in a GTP-dependent manner. However, the absence of residual host cell plasma membranes on the PVM of T. gondii did not trigger GKS accumulation. The non-fusogenic character of a vacuole was also not sufficient for a vacuole to be recognised by GKS proteins. Lastly, this study presented a novel role of the IRG system in resistance to the microsporidian Encephalitozoon cuniculi. Interferon-γ stimulation, inducing IRG proteins, suppresses meront development and spore formation in mouse fibroblasts in vitro, and effector GKS proteins cooperatively accumulate on the PVM of E. cuniculi. In addition, IFNγ-induced cells infected with E. cuniculi died by necrosis similar to T. gondii infection.
Thus, the IRG resistance system provides cell-autonomous immunity to specific parasites from three kingdoms of life: protozoa, bacteria and fungi. The phylogenetic divergence of these IRG target organisms strongly suggests that the IRG system does not recognise specific parasite components. The absence of certain host components on the vacuolar membrane, such as the protective GMS proteins, might act as missing-self motifs to trigger GKS protein accumulation on parasitophorous vacuoles
