Alternative Complement Pathway Inhibition Abrogates Pneumococcal Opsonophagocytosis in Vaccine-Naïve, but Not in Vaccinated Individuals.

Lukas Muri,Natasa Zamurovic,Christine Thorburn,Anna Schubart,Michael Kammüller,Thomas Holbro,Emma Ispasanie,Gerd Pluschke

doi:10.3389/fimmu.2021.732146

Abstract

To assess the relative contribution of opsonisation by antibodies, classical and alternative complement pathways to pneumococcal phagocytosis, we analyzed killing of pneumococci by human blood leukocytes collected from vaccine-naïve and PCV13-vaccinated subjects. With serotype 4 pneumococci as model, two different physiologic opsonophagocytosis assays based on either hirudin-anticoagulated whole blood or on washed cells from EDTA-anticoagulated blood reconstituted with active serum, were compared. Pneumococcal killing was measured in the presence of inhibitors targeting the complement components C3, C5, MASP-2, factor B or factor D. The two assay formats yielded highly consistent and comparable results. They highlighted the importance of alternative complement pathway activation for efficient opsonophagocytic killing in blood of vaccine-naïve subjects. In contrast, alternative complement pathway inhibition did not affect pneumococcal killing in PCV13-vaccinated individuals. Independent of amplification by the alternative pathway, even low capsule-specific antibody concentrations were sufficient to efficiently trigger classical pathway mediated opsonophagocytosis. In heat-inactivated or C3-inhibited serum, high concentrations of capsule-specific antibodies were required to trigger complement-independent opsonophagocytosis. Our findings suggest that treatment with alternative complement pathway inhibitors will increase susceptibility for invasive pneumococcal infection in non-immune subjects, but it will not impede pneumococcal clearance in vaccinated individuals.

Highlights

The complement system is a complex protein network with essential functions during immunological and inflammatory processes [1]
While membrane attack complex (MAC) insertion into the outer membrane of Gram-negative bacteria like meningococci (Neisseria meningitidis) can kill them efficiently within minutes [1,2,3], Gram-positive bacteria are generally resistant to MAC-induced bacteriolysis, as the terminal complement complex cannot penetrate through the thick peptidoglycan layer of the Gram-positive cell wall [4, 5]
Data from humans with homozygous C3 deficiencies further highlighted the importance of C3 during pneumococcal infections, as human C3 deficiency is associated with recurrent and life-threatening bacterial infections by encapsulated bacteria such as S. pneumoniae, N. meningitidis and Haemophilus influenza [12, 13]

Summary

Introduction

The complement system is a complex protein network with essential functions during immunological and inflammatory processes [1]. Complement activation induces direct killing of certain pathogens via insertion of the pore-forming membrane attack complex (MAC, C5b-9), and opsonizes invading bacteria for subsequent engulfment by phagocytes [1, 2]. While MAC insertion into the outer membrane of Gram-negative bacteria like meningococci (Neisseria meningitidis) can kill them efficiently within minutes [1,2,3], Gram-positive bacteria are generally resistant to MAC-induced bacteriolysis, as the terminal complement complex cannot penetrate through the thick peptidoglycan layer of the Gram-positive cell wall [4, 5]. Factor B (fB) deficiency has only been reported in two patients so far, which presented with recurring invasive pneumococcal and meningococcal disease [19, 20]. The role of the lectin pathway (LP) during pneumococcal infection remains inconclusive [21], with large studies demonstrating no association between human LP deficiencies and the risk of pneumococcal infection [22, 23], despite affecting disease severity [22]

Methods

Results

Conclusion