IRGs

Immunity Related Guanosine Triphosphatases or IRGs are proteins activated as part of an early immune response. IRGs have been described in various mammals but are most well characterized in mice. IRG activation in most cases is induced by an immune response and leads to clearance of certain pathogens. Immunity Related Guanosine Triphosphatases or IRGs are proteins activated as part of an early immune response. IRGs have been described in various mammals but are most well characterized in mice. IRG activation in most cases is induced by an immune response and leads to clearance of certain pathogens. Interferon (IFN)-inducible GTPases encompass four families of proteins including myxovirus resistant proteins (Mx), guanylate-binding proteins (GBP), immunity-related GTPase proteins (IRGs), and very large inducible GTPase proteins (VLIG). IRGs confer resistance from vacuolar pathogens by localizing to and disrupting the phagocytic vacuole during infection. The activation of IRGs in mice is induced by interferon. IRG genes have been identified in various vertebrates and some invertebrates. They are involved in important immune defenses against intracellular pathogens and as a result have become a target for immune evasion by those pathogens. The intracellular protozoan parasite Toxoplasma gondii has been shown to target IRGs in mice allowing for resistance from the host immune response. IRGs Have Evolved From Invertebrates Studies to determine the evolutionary origins of vertebrates have led to understanding the development of immune system processes and furthermore answer the questions of how and why pathogens have learned to evade and shut down these selectable genetic traits. Eight functional and four pseudo IRG genes have been identified in the invertebrate Branchiostoma floridiae. Li et al. determined expression patterns of functional IFN-inducible GTPase genes in Branchiostoma japonicum at various immunologic sites when induced by pathogens and pathogenic substances. This evidence suggests that IRGs may function in an immune-related capacity in cephalochordates. The paradox remains that these IRGs function without induction by IFN activation pathways, since B. japonicum and other amphioxus species do not possess IFN and IFN receptor genes. It is possible that IRGs may have existed prior to the Cambrian Explosion as an innate immune mechanism and with the evolution of the adaptive immune system in vertebrates, IFN evolved to modulate IRG function. Vertebrates have evolved an array of IRG genes as a whole, potentially due to the evolution between variable pathogen interactions. The C57BL/6 mouse has 23 IRG genes of which 21 may be functional in resistance to pathogens (6 are well characterized), whereas humans have evolved only 1 functional IRG gene (IRGM) and one pseudogene. Studies in mice have characterized the importance of the type 2 effector molecule IFNγ in various cell types and gone on to determine the importance of these proteins in intracellular pathogen resistance. Orthologous Irgc (aka: Cinema) genes are found in humans and mice. These orthologs are not IFN inducible and are expressed only in the testis of both mammals. Multiple IRG genes have been identified in canines and zebrafish but few in the model organism Tetraodontidae (the pufferfish). IRG genes in humans are thought to have been lost in the divergence of primates. The variations between and within species suggest a high rate of evolutionary change for this particular element of host pathogen interaction and highlight the importance of understanding the limitations of using model systems to study human immunology. Dependency on the IRGs is best exemplified in mouse studies. Multiple studies have been done using mouse knockout models to determine IRG function. Pathogen clearance mechanisms via lysosome maturation and vacuole destruction have been determined. Additionally, IRGs are implicated in the control of hematopoietic balance during infection. Irg1 knockout mice infected with Mycobacterium resulted in pancytopenia as a result of inadequate hematopoietic stem cell expansion. The mouse genome encodes 23 IRGs, several of which have been demonstrated to be widely expressed (liver, heart, spleen, intestine, thymus, lung, testis, kidney, brain, skin) in a number of cell types, and are greatly up-regulated following exposure to the potent immune effector molecule interferon gamma, IFNγ. IRGs are subdivided into two additional classes based on the mode of activity and mechanism. The GSK class (Irga6, Irgb6, and Irgd) are considered the canonical grouping of GTPases, whereas a second grouping of GMS proteins, which have a lysine to methionine mutation in the active site, function to prevent premature activation by associating with the nucleotide binding motif in a manner similar to Guanosine nucleotide dissociation inhibitors (GDI's). The subcellular localization of IRGs are variable; Irga6 and Irgm3 are predominantly found within the endoplasmic reticulum, Irgm1 and Irgm2 have been localized to the Golgi apparatus, and at least two IRGs (Irgb6 and Irgd) have been found predominantly within the cytosol. Following cellular entry of Toxoplasma gondii, IRGs can quickly redistribute onto the parasitophorous vacuole membrane (PVM) within 2–30 minutes. The approximate order of decorating the PVM has been defined starting with the loading of Irgb6 and Irgb10 followed by Irga6, Irgd and Irgm2. Faint localization of Irgm3 on T. gondii vacuoles was also reported to occur in rare settings. Activation of the IRGs is thought to follow a GTP dependent cycle of IRG-IRG oligomerization. The loading of the 'pioneer' IRGs onto the vacuole is thought to greatly enhance the recruitment of additional IRGs in a cooperative fashion. Pathogens have co-evolved unique mechanisms to interfere with different steps leading up to the association of the full complement of IRGs needed to constitute a vacuolar destructive complex. One such example was elucidated by infection with virulent and recombinant, avirulent strains of T. gondii. The intricate mechanism demonstrates a co-evolving interaction between the two species. Type I T. gondii rhoptry effector molecule Rop18, a serine-threonine kinase, was recently shown to selectively phosphorylate and inactivate the 'pioneer' IRGs, thereby preventing their assembly, activation and destruction of the T. gondii vacuole within monocytes.