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

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.

MASP-1 and MASP-3 Bind Directly to <b><i>Aspergillus fumigatus</i></b> and Promote Complement Activation and Phagocytosis

“…We measured the binding of various recombinant and serum-purified MASPs to heat-inactivated A. fumigatus conidia strain 6871 by flow cytometry. We have previously seen that heat inactivation does not compromise the binding of complement components [13]. The results showed that both rMASP-1 and rMASP-3, and to a minor degree rMASP-1/-3 heavy chain, bound to A. fumigatus .…”

“…To further investigate the consequence of MASPs binding to the surface of A. fumigatus , we tested the effects in human serum. More specifically, in MBL-defect serum combined with a C1q inhibitory mAb to exclude the contribution from previously identified classical and lectin pathway activation on A. fumigatus [13]. Figure 8a and b shows a tendency that prebound rMASP-1 and MASP-3 increased the C4b deposition and that rMASP-1 was able to induce a higher C3b deposition than rMASP-3.…”

“…However, it has the disadvantage of oversimplifying the system. Hence, we also tested the effects in serum by eliminating the two PRM-activation sources on A. fumigatus – C1q/classical and MBL-driven lectin pathway [13]. Both rMASP-1 and -3 increased the deposition of C4b, but interestingly only rMASP-1 increased C3b (Fig.…”

“…Thus, the development of new types of therapeutics is needed and an increased understanding of the interplay between the pathogen and the immune system is therefore valuable. In terms of complement, in vitro studies have deciphered the paths to activation [13, 14] and in vivo studies have shown that complement is important for fungal clearance [15, 16]. Additionally, complement works in close collaboration with PTX3, a nonredundant innate immune protection molecule against A. fumigatus [17, 18].…”