With the discovery of the IRG gene family��the IFN�� responsive p47 GTPases��a key factor determining immune resistance against T. gondii was identified [31]. Although the expression of this group of proteins is not conserved in humans, a homologous group of guanylate thereby binding p65kDa proteins (GBPs) is under discussion to confer this defence mechanism in humans [32, 33]. The mechanism of PV destruction by IRGs is one of the research focuses of the last years [34�C36]. Recently, Khaminets et al. discovered the complex interaction of different IRGs to accumulate at the PV with Irgb6 and Irgb10 apparently acting as leading GTPases in the process, while Irga6 accumulation at the PV is a downstream of this process and dependent on the presence of Irgb6 and/or Irgb10 [34].

These findings are in line with our data comparing the kinetics of the two GTPases Irgb6 and Irga6. The kinetic accumulation of Irgb6 was much faster than that of Irga6. Based on our data and the observations of Khaminets et al. it is most likely that once a PV is positive for the leading IRG, the second IRG can be recruited [34]. Eventually the leading IRG is no longer detectable. If this is a maturation process resulting in the dispensability of the leading IRG or if it is substituted by Irgb10 has to be determined in the future. However, it is striking that the defined local distribution of the IRGs revealed a patchy-like clustering of Irga6 and Irgb6 at an individual PV. The area of colocalization was restricted to a small area forming a ring in parallel to the attached astrocyte surface.

This ring was observable in different dimensions around most of the surfaces of the PV and seemed to divide the PV in two areas one facing the bottom of the cell/attachment site and the other facing toward the medium site of the astrocyte. It might be speculated that polarisation of the host cell induced by the attached surface on the one hand and the medium site on the other could be a reason for the differences in IRG accumulation. Till now it is not known if this is an astrocyte specific phenomenon or if this is detectable in other polarized cells growing on surfaces (i.e., epithelial cells). Accumulation of the IRG eventually leads to parasite destruction, after the PV membrane peels back the parasite which is exposed to the cytosol [37]. Interestingly, in astrocytes, in murine embryonic fibroblasts, and macrophages virulent T.

gondii strains are characterized by a reduced loading of Irgb6 on the PV correlating with reduced vacuolar disruption [34, 37, 38]. We could additionally demonstrate Cilengitide the significant growth reduction of avirulent strains in astrocytes, whereas virulent strains with a reduced Irgb6 and Irga6 loading replicated almost unaffectedly. The coinfection experiments in this study with avirulent and virulent parasites in the same astrocytic host cell addressed an important question.