Project Summary Cryptococcosis is an opportunistic fungal infection that is caused by Cryptococcus neoformans and often occurs in immunocompromised individuals including HIV/AIDS patients. Although the infection starts in the lung via inhalation of the pathogenic fungus, the most common manifestation of cryptococcosis is meningoencephalitis, which is a leading cause of mortality of HIV/AIDS patients and accounts for proximately 181,000 deaths worldwide annually. The high susceptibility of HIV/AIDS patients to C. neoformans infection is attributed to their impaired cellular immunity. Consequently, C. neoformans proliferates in the brain without control, leading to microbe-mediated brain damage. Antiretroviral therapy (ART) is a major advance in treating HIV/AIDS patients by restoring cellular immune responses. However, after initiation of ART, up to 30% of HIV/AIDS patients with cryptococcosis develop immune reconstitution inflammatory syndrome (IRIS), an aberrant excessive immune response leading to host-mediated brain damage. Furthermore, post-infectious inflammatory response syndrome (PIIRS) occurs frequently in HIV-negative patients with cryptococcal meningoencephalitis. Therefore, a robust immune response is required for controlling the fungal growth; however, the immune response must be tightly controlled to avoid host-mediated brain damage during cryptococcal meningoencephalitis. A critical gap in our understanding remains: what are the mechanisms of maintaining the balance between protective immune responses and immunopathology during cryptococcal meningoencephalitis-associated IRIS? IL-10 is an important cytokine with anti-inflammatory properties and its production was enhanced in the cerebrospinal fluid of HIV/AIDS patients during cryptococcal IRIS. Based on clinical data and our preliminary murine studies, we hypothesize that IL-10 is critically involved in balancing the immune responses during cryptococcal meningoencephalitis-associated IRIS. We will test the hypothesis in a murine model that closely mimics cryptococcal IRIS of HIV/AIDS patients, by addressing the following aims: (1) To determine the mechanism(s) involved in regulation of the immune responses by IL-10 during cryptococcal meningoencephalitis-associated IRIS; (2) To determine the mechanism(s) involved in induction of IL-10 in the brain during cryptococcal meningoencephalitis-associated IRIS. If successful, the findings from this study would be fundamental for understanding regulation of immune balance during cryptococcal meningoencephalitis-associated IRIS and provide scientific basis for targeting this cytokine aiming at controlling deleterious inflammation in the brain of cryptococcal IRIS/AIDS patients.