Neutrophil Interactions Stimulate Evasive Hyphal Branching by Aspergillus fumigatus

Abstract

Invasive aspergillosis (IA), primarily caused by Aspergillus fumigatus, is an opportunistic fungal infection predominantly affecting immunocompromised and neutropenic patients that is difficult to treat and results in high mortality. Investigations of neutrophil-hypha interaction in vitro and in animal models of IA are limited by lack of temporal and spatial control over interactions. This study presents a new approach for studying neutrophil-hypha interaction at single cell resolution over time, which revealed an evasive fungal behavior triggered by interaction with neutrophils: Interacting hyphae performed de novo tip formation to generate new hyphal branches, allowing the fungi to avoid the interaction point and continue invasive growth. Induction of this mechanism was independent of neutrophil NADPH oxidase activity and neutrophil extracellular trap (NET) formation, but could be phenocopied by iron chelation and mechanical or physiological stalling of hyphal tip extension. The consequence of branch induction upon interaction outcome depends on the number and activity of neutrophils available: In the presence of sufficient neutrophils branching makes hyphae more vulnerable to destruction, while in the presence of limited neutrophils the interaction increases the number of hyphal tips, potentially making the infection more aggressive. This has direct implications for infections in neutrophil-deficient patients and opens new avenues for treatments targeting fungal branching.

Real-time visualization of immune cell clearance of Aspergillus fumigatus spores and hyphae

“…While neutrophils are the dominant phagocyte against hyphae, the recent observation that A. fumigatus can undergo a hyperbranching phenotype upon neutrophil stimulation (Ellett et al, 2017) shows that this interaction is more complex than recruitment towards and extracellular killing of vulnerable hyphae. While the benefit of hyperbranching is dependent upon neutrophil number and function (Ellett et al, 2017), in vivo observations at the neutrophil-hyphal interface (Vid.…”

“…While the benefit of hyperbranching is dependent upon neutrophil number and function (Ellett et al, 2017), in vivo observations at the neutrophil-hyphal interface (Vid. 2) (Knox et al, 2014) show that macrophages are also tightly associated at this junction.…”

“…It is this hyphal form that penetrates host tissue, preceding dissemination and lethality, making containment of hyphal growth essential for host survival. While demonstrating ability to kill both conidial and hyphal forms (Gazendam et al, 2016), neutrophils are widely regarded as the dominant innate immune cell for hyphal killing because of their ability to mount a successful extra-cellular defense (Ellett et al, 2017; Knox et al, 2014). Indeed, neutrophils efficiently migrate towards and are able to kill A. fumigatus hyphae in vivo (Video 2).…”

“…Despite efficient neutrophil trafficking towards hyphae, A. fumigatus possesses galactosaminogalactan (GAG), an extracellular matrix component that partially masks pro-inflammatory carbohydrates and mediates adherence and penetration into host tissue and full virulence (Gravelat et al, 2013). However, after physical contact with neutrophils, A. fumigatus can switch to an evasive hyper- branching phenotype (Ellett et al, 2017). …”

Promising immunotherapy against fungal diseases

“…Aspergillus (A.) fumigatus and Rhizopus oryzae and efficiently keeping fungal invasion and dissemination under control [18,19]. Nevertheless, the risk of inducing [20,21].…”