C3 Opsonization of Anthrax Bacterium and Peptidoglycan Supports Recognition and Activation of Neutrophils.

Narcis I Popescu,Ravi S Keshari,Jackie Cochran,K Mark Coggeshall,Florea Lupu

doi:10.3390/microorganisms8071039

Narcis I Popescu, Ravi S Keshari + Show 3 more

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https://doi.org/10.3390/microorganisms8071039

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Journal: Microorganisms	Publication Date: Jul 13, 2020
Citations: 7	License type: CC BY 4.0

Affiliation: Oklahoma Medical Research Foundation

Abstract

Neutrophils are the most abundant innate cell population and a key immune player against invading pathogens. Neutrophils can kill both bacterium and spores of Bacillus anthracis, the causative anthrax pathogen. Unlike interactions with professional phagocytes, the molecular recognition of anthrax by neutrophils is largely unknown. In this study, we investigated the role of complement C3 deposition on anthrax particles for neutrophil recognition of bacterium and/or its cell wall peptidoglycan, an abundant pathogen-associated molecular pattern that supports anthrax sepsis. C3 opsonization and recognition by complement receptors accounted for 70–80% of the affinity interactions between neutrophils and anthrax particles at subphysiologic temperatures. In contrast, C3 supported up to 50% of the anthrax particle ingestion under thermophysiologic conditions. Opsonin-dependent low affinity interactions and, to a lower extent, opsonin-independent mechanisms, provide alternative entry routes. Similarly, C3 supported 58% of peptidoglycan-induced degranulation and, to a lower extent, 23% of bacterium-induced degranulation. Interestingly, an opsonin independent mechanism mediated by complement C5, likely through C5a anaphylatoxin, primes azurophilic granules in response to anthrax particles. Overall, we show that C3 deposition supports anthrax recognition by neutrophils but is dispensable for pathogen ingestion and neutrophil degranulation, highlighting immune recognition redundancies that minimize the risk of pathogen evasion.

Highlights

Bacillus anthracis spores cause anthrax infections upon penetration of the respiratory, cutaneous or gastrointestinal barriers [1,2]
We found that the C3b deposition on bacteria and/or peptidoglycan is required for affinity receptor recognition at subphysiologic temperatures, but it is dispensable at 37 ◦C, which is where other low affinity interactions could come in play
Complement opsonization of anthrax bacteria and spores have been shown to mediate pathogen internalization by professional phagocytes such as macrophages and dendritic cells [12,14], and this predominantly centers around the recognition of C3b/iC3b decorating the pathogen surface [10]

Summary

Introduction

Bacillus anthracis spores cause anthrax infections upon penetration of the respiratory, cutaneous or gastrointestinal barriers [1,2]. Innate immune defenses are critical for the seeding and progression of anthrax, and may explain differences in infection kinetics observed with inhalational and cutaneous disease. Cutaneous anthrax is usually localized to sites of infection and rarely evolves to systemic disease [17]. Neutrophils are the primary innate immune cells recruited at sites of cutaneous infection, and have been proposed to limit anthrax dissemination [18]. Earlier inhalational anthrax studies recognized only a minor contribution from neutrophils to disease progression [19]. We previously reported rapid activation of neutrophils in a non-human primate model of late stage systemic anthrax [22], but specific neutrophil contributions to septic pathophysiology were harder to isolate in this model

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