A Second Stimulus Required for Enhanced Antifungal Activity of Human Neutrophils in Blood Is Provided by Anaphylatoxin C5a

Abstract

Polymorphonuclear neutrophilic granulocytes (PMN) as cellular components of innate immunity play a crucial role in the defense against systemic Candida albicans infection. To analyze stimuli that are required for PMN activity during C. albicans infection in a situation similar to in vivo, we used a human whole-blood infection model. In this model, PMN activation 10 min after C. albicans infection was largely dependent on the anaphylatoxin C5a. Most importantly, C5a enabled blood PMN to overcome filament-restricted recognition of C. albicans and allowed efficient elimination of nonfilamentous C. albicans cph1Δ/efg1Δ from blood. Major PMN effector mechanisms, including oxidative burst, release of secondary granule contents and initial fungal phagocytosis could be prevented by blocking C5a receptor signaling. Identical effects were achieved using a humanized Ab specifically targeting human C5a. Phagocytosis of C. albicans 10 min postinfection was mediated by C5a-dependent enhancement of CD11b surface expression on PMN, thus establishing the C5a-C5aR-CD11b axis as a major modulator of early anti-Candida immune responses in human blood. In contrast, phagocytosis of C. albicans by PMN 60 min postinfection occurred almost independently of C5a and mainly contributed to activation of phagocytically active PMN at later time points. Our results show that C5a is a critical mediator in human blood during C. albicans infection.

Real-Time Imaging of Interactions of Neutrophils with Cryptococcus neoformans Demonstrates a Crucial Role of Complement C5a-C5aR Signaling

“…These data are consistent with previously reported findings showing that blocking C5aR or neutralizing C5a inhibits phagocytosis of Escherichia coli or Candida albicans by neutrophils, associated with the abrogation of the enhanced surface expression of CD11b on neutrophils (34,35). Although phagocytosis of these organisms was almost completely abolished by blocking of CD11b (34,35), evidence that there is a direct link between enhanced CD11b expression and phagocytosis of the organisms has not been provided. This is because treatment of neutrophils with anti-CD11b Abs not only blocks the enhanced expression of CD11b but also blocks the original expression of CD11b on the surface of neutrophils.…”

“…Importantly, the present study showed that treatment of neutrophils with elevated expression of CD11b with DIDS did not affect their ability to phagocytose C. neoformans, further suggesting that the inability of DIDS-treated neutrophils to ingest yeast cells can be attributed to a failure to enhance the surface expression of CD11b. It is noteworthy that phagocytosis of C. albicans by neutrophils is dependent on C5a-C5aR signaling 10 min after infection but is independent of this signaling 60 min after infection (34). This is in contrast to the phagocytosis of C. neoformans; this discrepancy may reflect a differences in surface components and structures between the organisms.…”

“…Previously reported work showed that C5a-C5aR signaling can enhance the surface expression of CD11b on neutrophils (34,35). We therefore explored the role of CD11b in the killing of C. neoformans.…”

Bottom-up modeling approach for the quantitative estimation of parameters in pathogen-host interactions

“…The immune response against C. albicans in human blood comprises the interplay of various complex biological processes involving different immune mechanisms (Duggan et al, 2015b). Most importantly, the whole-blood infection assay allows multiple immune effector mechanisms to occur at the same time and thus modulate the overall outcome (Luo et al, 2013; Cunha et al, 2014; Hünniger et al, 2015). Applying a systems biology approach, we quantified individual processes and in this way revealed the main route of the immune response against C. albicans in human blood (Hünniger et al, 2014).…”

Antifungal Innate Immunity: A Perspective from the Last 10 Years

“…Mice deficient in JAGN1 could not mount an efficient neutrophil-dependent immune response against C. albicans , characterized by defective migration and impaired formation of cytotoxic granules [89]. Furthermore, C5a receptor signaling in human neutrophils has been linked to the regulation of killing mechanisms during Candida infection [90]. …”