Complementary roles of the classical and lectin complement pathways in the defense against Aspergillus fumigatus

Anne Rosbjerg,Ninette Genster,Kathrine Pilely,Mikkel-Ole Skjoedt,Gregory L Stahl,Peter Garred

doi:10.1016/j.imbio.2016.06.165

Anne Rosbjerg, Ninette Genster + Show 4 more

Open Access

https://doi.org/10.1016/j.imbio.2016.06.165

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Abstract

Aspergillus fumigatus infections are associated with a high mortality rate for immunocompromised patients. The complement system is considered to be important in protection against this fungus, yet the course of activation is unclear. The aim of this study was to unravel the role of the classical, lectin and alternative pathways under both immunocompetent and immunocompromised conditions to provide a relevant dual-perspective on the response against A. fumigatus. Conidia (spores) from a clinical isolate of A. fumigatus were combined with various human serum types (including serum deficient of various complement components and serum from umbilical cord blood). We also combined this with inhibitors against C1q, mannose-binding lectin (MBL) and ficolin-2 before complement activation products and phagocytosis were detected by flow cytometry. Our results showed that alternative pathway amplified complement on A. fumigatus, but required classical and/or lectin pathway for initiation. In normal human serum this initiation came primarily from the classical pathway. However, with a dysfunctional classical pathway (C1q-deficient serum), lectin pathway activated complement and mediated opsonophagocytosis through MBL. To model the antibody-decline in a compromised immune system, we used serum from normal umbilical cords and found MBL to be the key complement initiator. In another set of experiments serum from patients with different kinds of immunoglobulin insufficiencies showed that the MBL lectin pathway contribution was highest in the samples with the lowest IgG/IgM binding. In conclusion, lectin pathway appears to be the primary route of complement activation in the absence of anti-A. fumigatus antibodies, whereas in a balanced immune state classical pathway is the main activator. This suggests a crucial role for the lectin pathway in innate immune protection against A. fumigatus in immunocompromised patients.

Highlights

The fungus Aspergillus fumigatus has its natural habitat in soil where it decomposes organic debris and the fungus is usually non-pathogenic for immunocompetent humans
Based on initial experiments, screening the complement pattern-recognition molecule (PRM) C1q, mannose-binding lectin (MBL), ficolin-1, ficolin-2, and ficolin-3 for their ability to bind A. fumigatus (Figure S1 in Supplementary Material), the following PRMs were chosen as candidates for further studies: ficolin-2, MBL, and C1q
The results clearly showed that magnesium amplified C3b and terminal complement complex (TCC), suggesting an alternative pathway-driven response (Figures 2A,B)

Summary

Introduction

The fungus Aspergillus fumigatus has its natural habitat in soil where it decomposes organic debris and the fungus is usually non-pathogenic for immunocompetent humans. IPA can turn into systemic dissemination when conidia (spores) mature into fungal hyphae breaching the pulmonary epithelia and reaching the blood stream. This exposes other organs like kidney, heart, and brain to fungal attack [1]. With a mortality rate of 40–90%, IPA poses a serious threat to several patient groups suffering from immune demolishing diseases such as leukemia and AIDS or during immunosuppressive therapy used under organ transplantations [2]. Due to the small airborne conidia (2–3 μm), A. fumigatus is able to penetrate into the alveolar spaces and initiate an infection. Research covering new aspects of the immune response against A. fumigatus is important for future treatment alternatives

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