Guanylate binding proteins (GBPs) are interferon (IFN)-inducible mediators of cell-autonomous host resistance against intracellular pathogens and potent activators of canonical and non-canonical inflammasome pathways. The murine GBP family comprises eleven protein members (mGBP1-11) that that are critically involved in the defense against protozoa, bacteria, and viruses. Upon infection, GBPs are hierarchically recruited towards pathogen-containing compartments, such as the parasitophorous vacuole (PV) of Toxoplasma gondii or the inclusion of Chlamydia trachomatis.

In vivo studies by our group have revealed an increased susceptibility of mGBP2- and mGBP7-deficient mice to T. gondii, highlighting their importance in parasite control. In depth, in vitro analyses showed that mGBPs accumulate in supramolecular complexes at the PV, disrupting its integrity and restricting Toxoplasma replication. In bacterial infection models, it has been demonstrated that GBP-mediated lysis of pathogen-containing vacuoles exposes bacteria to the host cell cytosol. Cytosolic release of bacterial antigens in turn leads to inflammasome activation, enabling IL-1β and IL-18 processing, followed by induction of an inflammatory form of cell death termed pyroptosis. Several studies suggest a relevance of mGBPs for inflammasome assembly, inspiring us to investigate their role in T. gondii- and C. trachomatis-induced pyroptosis.
We could show that mouse embryonic fibroblasts (MEFs) stably transduced with mGBP1-10 released higher amounts of LDH and secrete increased concentrations of IL-1β and IL-18 in response to T. gondii or C. trachomatis compared to control MEFs. Although mGBPs share a high sequence identity, we observed differences regarding their effector function. For instance, mGBP6-overexpressing MEFs show a decreased cell death rate, accompanied by high cytokine levels, termed 'hyperactivation'. By contrast, mGBP9 targets the inclusion of C. trachomatis at highest frequency, leading to reinforced cell death despite normal cytokine release. These functional differences need further investigation to understand the individual regulation of mGBPs during cell-autonomous immune responses.