Macrophage M1/M2 Polarization Dynamically Adapts to Changes in Cytokine Microenvironments in Cryptococcus neoformans Infection

The outcome of cryptococcal pneumonia correlates with local macrophage polarization status, as M1 and M2 polarization marks protective and nonprotective responses, respectively. Overall, pulmonary macrophage polarization status changes over time during a cryptococcal infection. This could have been caused by repolarization of individual macrophages or by a replacement of M2-polarized cells by new M1-polarized cells. To explore the ability of macrophages to change between polarization states, we conducted a series of experiments using in vitro macrophages. Coculture of macrophages with Cryptococcus neoformans resulted in development of a weak M1-like phenotype, with modestly increased inducible nitric oxide synthase (iNOS) but lacking interleukin 6 (IL-6) induction. The C. neoformans-induced M1-like polarization state was plastic, as macrophages stimulated first with C. neoformans and then with gamma interferon (IFN-gamma) or IL-4 expressed mRNA polarization patterns similar to those stimulated with cytokines alone. To further evaluate macrophage polarization plasticity, cytokine stimulatory conditions were established which fully polarized macrophages. IFN-gamma and IL-4 stimulation differentially induced complete M1 and M2 polarization, defined by differential expression of marker mRNA panels, surface marker expression, and tumor necrosis factor alpha (TNF-alpha) protein production. Switching IFN-gamma- to IL-4-stimulating conditions, and vice versa, resulted in uniform changes in profiles of polarization marker genes consistent with the most recent cytokine environment. Furthermore, the ability of sequentially stimulated macrophages to inhibit C. neoformans reflected the most recent polarizing condition, independent of previous polarization. Collectively, these data indicate that M1/M2 macrophage polarization phenotypes are highly plastic to external signals, and interventions which therapeutically repolarize macrophages could be beneficial for treatment of cryptococcosis. IMPORTANCE Our studies reveal how a major opportunistic fungal pathogen, Cryptococcus neoformans, interacts with macrophages, immune cells which can ingest and kill invading pathogens. Macrophages play a crucial role in the pathogenesis of cryptococcal infection, as their polarization phenotype determines the outcome of the battle between the infected host and C. neoformans. This study suggests that dynamic changes in polarization of macrophages at the level of individual cells are an important characteristic of in vivo cryptococcosis, as they occur throughout the natural course of infection. We demonstrate that macrophages can rapidly and uniformly reverse their polarization phenotype in response to dynamic signaling conditions and lose or regain their fungicidal function. Demonstrating importance of these pathways may become a cornerstone for novel therapeutic strategies for treatment of cryptococcosis in both immunocompromised and immunocompetent patients.