IL-1 alpha and cell death in the brain: promoting CNS immunity to infection

Toxoplasma gondii is a protozoan parasite that infects large portions of the human and animal populations, and which persists in the brain of its host. Like many chronic CNS infections, T. gondii requires consistent immune pressure to prevent symptomatic disease. Large numbers of T cells and myeloid cells infiltrate the brain during this infection to control parasite replication. But the brain also harbors microglia, resident macrophages of the CNS, which are thought to be first-line defenders against CNS infections. Whether microglia and peripheral macrophages in the same environment respond to the infection in similar or distinct ways is still not fully understood. This is due, in part, to the fact that microglia and infiltrating macrophages have historically been difficult to distinguish. Now, however, there are newer tools available to discriminate the functions of these two populations. In order to identify the specific function of microglia in the brain during infection, we sorted microglia and infiltrating myeloid cells from infected microglia reporter mice. Using RNA-sequencing, we found that microglia and blood-derived macrophages differ in their inflammatory profiles during infection, with strong NF-kB signatures overrepresented in macrophages versus microglia. Interestingly, we also found that IL-1 alpha is overrepresented in microglia and IL-1 beta is enriched in macrophages. We confirmed at the protein level that microglia express IL-1 alpha but not IL-1 beta. These results were especially interesting since the role of IL-1 signaling in the brain during chronic T. gondii infection had not yet been addressed.
We went on to show that mice lacking IL-1R1 have impaired parasite control and immune infiltration specifically within the brain. Using bone marrow chimeras, we have shown that IL-1R1 expression on a radio-resistant cell population is necessary to recruit iNOS-expressing monocyte-derived macrophages into the brain to mediate protective inflammation. Our evidence suggests that this occurs through IL-1-dependent activation of the blood vasculature in the brain. Surprisingly, we found that the alarmin IL-1 alpha, not IL-1 beta, mediates the pro-inflammatory effects of IL-1 signaling in our model. Further, by sorting purified populations from infected brains, we were able to show that microglia, not peripheral cells, release IL-1 alpha ex vivo. This implicates microglia as the source of IL-1 alpha in the brain and illuminates a microglia-specific function during infection.
Using knockout mice as well as chemical inhibition, we have shown that ex vivo IL-1 alpha release is gasdermin D-dependent, and that mice lacking gasdermin D have an impaired response to chronic T. gondii infection. These results implicate pyroptosis, or at least inflammasome activation, as a mechanism of IL-1 alpha release. Indeed, we have demonstrated the presence of ASC specks in infected brain tissue, indicating inflammasome formation. We have also shown that caspase-1/11 double-deficient mice have an impaired response to chronic T. gondii infection. We have further used single knockout mice to show that our phenotype is likely driven by caspase-1 rather than caspase-11. In addition, we have ruled out NLRP3 and AIM2 as inflammasome sensors involved in IL-1 alpha release, or in the control of chronic T. gondii infection.
Together, our results have demonstrated that microglia and macrophages are differently equipped to propagate inflammation, and that in chronic T. gondii infection, microglia specifically can release the alarmin IL-1 alpha, driving neuroinflammation and parasite control. We have shown that this occurs in a manner dependent on gasdermin D, possibly downstream of an inflammasome and subsequent caspase-1 activation. This work has demonstrated a role for a single alarmin in promoting the CNS response to T. gondii infection.