Ingestion of acapsular Cryptococcus neoformans occurs via mannose and beta-glucan receptors, resulting in cytokine production and increased phagocytosis of the encapsulated form

Abstract

Cryptococcus neoformans is a pathogenic yeast and a major cause of opportunistic infection in AIDS patients. It is commonly found in an acapsular form in the environment, and infection is likely to occur by inhalation. The lung provides a suitable environment for capsule synthesis, and once encapsulated, C. neoformans becomes resistant to phagocytosis. A stable acapsular mutant of the organism is readily ingested by murine macrophages in vitro, indicating entry via constitutively competent receptors. We demonstrate in this report that this process is inhibitable by particles derived from Saccharomyces cerevisiae that are rich in mannan and beta-glucan, as well as more purified forms of these glycans. Furthermore, ingestion of the acapsular form of C. neoformans induces a range of proinflammatory cytokines, including tumor necrosis factor alpha and granulocyte-macrophage colony-stimulating factor, which, as we have previously shown, enhance ingestion of serum-opsonized encapsulated C. neoformans in vitro. We demonstrate that ingestion of the acapsular form of the organism also enhances ingestion of the pathogenic encapsulated form. This is dependent on the production of tumor necrosis factor alpha and granulocyte-macrophage colony-stimulating factor by the macrophages, since addition of neutralizing antibodies to both cytokines inhibited the observed increase in ingestion. Together, these data demonstrate that ingestion of acapsular C. neoformans is mediated via mannose and beta-glucan receptors on the macrophage surface and that this process activates macrophages for enhanced phagocytosis of the encapsulated form via production of macrophage-derived cytokines.