PDE4B deficiency aids macrophage differentiation and contributes to Cryptococcus neoformans brain infection

Cryptococcal meningitis is a fatal complication. Macrophages have been proposed to function as candidate "Trojan horse" cells, transferring Cryptococcus neoformans (C. neoformans) into the brain. The mechanisms of Trojan horses in cryptococcal meningitis are largely elusive. In this study, we performed scRNA-Seq on immune cells infiltrating the brain in a murine model of cryptococcal meningitis. Bioinformatics analysis revealed that phosphodiesterase 4B (PDE4B) is a candidate regulator associated with C. neoformans infected-macrophage. C. neoformans increases the total level of PDE4B in macrophages. However, virulent strains with increased production of melanin paradoxically decreased PDE4B expression in macrophages, implying that PDE4B in macrophages may be negatively associated with C. neoformans invasion. PDE4B inhibition increased Arg1, CXCR4 and CCR7 expression in macrophages, a process regulated by the cAMP/PKA signaling pathway. As expected, PDE4B inhibitors promote the ability of C. neoformans infected-macrophages to cross the blood-brain barrier (BBB) in vitro. Similarly, PDE4B inhibitors or PDE4B knockout increase the fungal burden in the brain, which is, at least partially, rescued by macrophage depletion, and adoptive transfer experiments further support macrophage-mediated fungal delivery to the brain. In contrast, PDE4B activation reduces fungal burden in the brain, including when administered after infection onset. Overall, this study revealed that PDE4B functions as an important regulator of macrophage functional programming during infection and supports a macrophage-mediated dissemination mechanism contributing to brain invasion, and is a potential therapeutic target for cryptococcal meningitis.