Cryptococcus exploits delayed microglial activation, and microglial osteopontin (OPN/Spp1) impairs peripheral host control

Cryptococcus, a neurotropic fungus and the World Health Organization (WHO) critical-priority pathogen, causes cryptococcal meningoencephalitis (CM), the second leading cause of death in HIV/AIDS patients. Despite the significance, host brain responses during CM remain underexplored. In a non-acute systemic mouse model, Cryptococcus infiltrates the brain within one day from circulation, yet full activation of microglia is delayed until 14 days post-infection. Microglia, regarded as sentinels of the central nervous system (CNS), exhibit slow responses, partly due to limited direct fungal recognition and reliance on T cells as a source of IFNg. Once activated, microglia upregulate Spp1, encoding osteopontin (OPN), a factor highly expressed in neurodegenerative diseases. Our findings demonstrate that microglia-derived OPN negatively impacts the host by altering peripheral immune responses. Thus, Cryptococcus exploits the delayed host response to establish brain infection, with activated microglia producing OPN in a host-detrimental manner. Overall design: Age-matched female B6 WT and GeneX-/-mice were retro-orbitally infected with 10^6 yeasts/mouse of Cryptococcus neoformans (deneoformans) strain 52D; brains were collected at 0, 4, and 14 days post infection. Samples from two mice were pooled for each time point, except 14-dpi which only had 1 sample. Brain cells were isolated using the Papain Dissociation Kit (Worthington Biochemical Corporation) and myelin was removed using Myelin Removal Beads II (Miltenyi Biotec). Live cells were enriched using the EasySep™ Dead Cell Removal (Annexin V) Kit (STEMCELL Technologies).