Infection with the environmental fungus Aspergillus fumigatus can result in invasive aspergillosis (IA) in the lungs of immunocompromised patients, causing substantial morbidity and mortality. The innate immune defense against IA is known to involve neutrophils, but the role of monocytes in this process is poorly defined. A recent study published in PLoS Pathogens by the Dr. Tobias Hohl Lab (Memorial Sloan-Kettering Cancer Center, formerly of the Vaccine and Infectious Disease Division) examined the mechanisms of the antifungal activity of CCR2-expressing inflammatory monocytes (CCR2+Mo). CCR2+Mo cells were shown to increase the antifungal activity of neutrophils. They were also observed to exhibit their own antifungal effect via direct uptake and killing of the fungal conidia (spores).
IA can result in substantial mortality for patients with leukemia or who have received hematopoietic cell transplants but rarely affects immunocompetent patients. "Neutrophils have long been considered as the key player in defense against inhaled A. fumigatus conidia," explains lead author Dr. Anupam Jhingran. The researchers were interested in the role of CCR2+ monocytes, however, and employed cell ablation strategies in mice to pinpoint their activity. "We found necessary function for monocytes in conidial clearance from the lung," he explains.
First, the researchers tested the effects that CCR2+Mo cells had on the antifungal activity of neutrophils during infection with A. fumigatus. Although a lack of CCR2+Mo cells did not result in reduced recruitment of neutrophils to the site of infection, neutrophil antifungal activity was impaired. The researchers used fluorescently labeled A. fumigatus conidia that emit different colors depending on whether they are alive or dead. They found that in CCR2+Mo-ablated mice, neutrophils did not exhibit reduced ability to take up the conidia, but did have less conidial killing capacity. Thus, the CCR2+Mo-mediated effect on the antifungal activity of neutrophils was linked to an influence on the direct killing of conidia by neutrophils.
Next, the researches wanted to look for any direct conidial killing by the CCR2+ monocytes themselves. By using green fluorescent protein-expressing CCR2+Mo cells, the researchers were able to detect the presence of either live or dead conidia inside the cells (see Figure). They found that, aside from assisting neutrophils in killing the fungus, the CCR2+Mo cells also engulfed and killed conidia. Moreover, the rate that these cells killed the conidia was comparable to the conidial killing capacity of neutrophils.
Naturally, these results raise new questions. For example, it is not known how exactly monocytes inactivate conidia following their uptake. Additionally, the mechanism behind the effect of monocytes on neutrophils resulting in their enhanced antifungal activity needs further examination. Dr. Jhingran hypothesizes that, "soluble factors such as granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor (TNF) secreted by CCR2+Mo act upon neutrophils to enhance their conidiacidal activity."
This research provides clear insight into the role of monocytes in immune system control of infections by A. fumigatus, which are 100% fatal if not treated. "These results are clinically relevant," explains Dr. Jhingran, "as a deeper understanding of anti-conidial mechanisms in monocytes could help devise immune enhancing strategies since monocytes can be harvested by leukopheresis, are relatively long-lived, and can be transferred to patients for potential therapeutic gain."
Espinosa V, Jhingran A, Dutta O, Kasahara S, Donnelly R, Du P, Rosenfeld J, Leiner I, Chen C-C, Ron Y, Hohl TM, Rivera A. 2014. Inflammatory monocytes orchestrate innate antifungal immunity in the lung. PLoS Pathog 10:e1003940.
See also: Jhingran A, Mar KB, Kumasaka DK, Knoblaugh SE, Ngo LY, Segal BH, Iwakura Y, Lowell CA, Hamerman JA, Lin X, Hohl TM. 2012. Tracing conidial fate and measuring host cell antifungal activity using a reporter of microbial viability in the lung. Cell Rep 2:1762-73