The Unappreciated Intracellular Lifestyle of Blastomyces dermatitidis

Abstract

Blastomyces dermatitidis, a dimorphic fungus and the causative agent of blastomycosis, is widely considered an extracellular pathogen, with little evidence for a facultative intracellular lifestyle. We infected mice with spores - the infectious particle - via the pulmonary route and studied intracellular residence, transition to pathogenic yeast and replication inside lung cells. Nearly 80% of spores were inside cells at 24 hours after infection with 104 spores. The majority of spores were located inside of alveolar macrophages, with smaller numbers in neutrophils and dendritic cells. Real time imaging showed rapid uptake of spores into alveolar macrophages, conversion to yeast, and intracellular multiplication during in vitro co-culture. The finding of multiple yeast in a macrophage was chiefly due to intracellular replication rather than multiple phagocytic events or fusion of macrophages. Depletion of alveolar macrophages curtailed infection in mice infected with spores, and lead to a 26-fold reduction in lung CFU by 6 days post-infection vs. non-depleted mice. Phase transition of the spores to yeast was delayed in these depleted mice over a time frame that correlated with reduced lung CFU. Spores cultured in vitro converted to yeast faster in the presence of macrophages than in medium alone. Thus, while advanced B. dermatitidis infection may exhibit extracellular residence in tissue, early lung infection with infectious spores reveals its unappreciated facultative intracellular lifestyle.

Helper T‐cell responses and pulmonary fungal infections

“…Innate immune cells play a critical role in regulating the pathogenesis of this infection. Neutrophil recruitment helps to limit the initial growth of the pathogen, as depletion of neutrophils yields an increase in the lung fungal burden by 2 days post infection . Recent studies by Hernandez‐Santos et al .…”

“…Several studies have interrogated host immunity in response to fungal spores, but most animal studies of immunity to dimorphic fungal infection to date have been performed with the yeast‐like form of the fungi. This is in large part due to technical difficulties in generating pure populations of spores and biosafety concerns of handling infectious spores when performing animal infections.…”

New Developments in Blastomycosis

“…Recent investigation has demonstrated that conversion to yeast is accelerated following phagocytosis of B. dermatitidis spores by alveolar macrophages. 55 In addition to intracellular spore germination, yeasts are able to survive and replicate within macrophages during the early stages of infection. 55 Thus, B. dermatitidis exhibits an intracellular lifestyle, which is similar to other dimorphic pathogens including H. capsulatum, Coccidioides spp., and Paracoccidioides brasiliensis.…”

Blastomyces Antigen Detection for Diagnosis and Management of Blastomycosis

“…Peak concentrations varied widely (0.01 ng/ml to above the limit of quantification). This may reflect the increased excretion of antigen after the death of fungal cells, the transient increase in fungal load from macrophage uptake after initiation of antifungal medication, or differences in location or severity of infection (30). Two patients converted to positive within 1 month of starting treatment, and 4 patients reverted to negative within 30 days of starting antifungal medications.…”