Complement C3 and Activated Fragment C3a Are Involved in Complement Activation and Anti-Bacterial Immunity.

Meng Wu,Bei-Bei Jia,Mo-Fei Li

doi:10.3389/fimmu.2022.813173

Abstract

In the complement system, C3 is a central component in complement activation, immune defense and immune regulation. In all pathways of complement activation, the pivotal step is conversion of the component C3 to C3b and C3a, which is responsible to eliminate the pathogen and opsonization. In this study, we examined the immunological properties of C3 and its activated fragment C3a from Japanese flounder (Paralichthys olivaceus) (PoC3 and PoC3a), a teleost species with important economic value. PoC3 is composed of 1655 amino acid residues, contains the six domains and highly conserved GCGEQ sequence of the C3 family. We found that PoC3 expression occurred in nine different tissues and was upregulated by bacterial challenge. In serum, PoC3 was able to bind to a broad-spectrum of bacteria, and purified native PoC3 could directly kill specific pathogen. When PoC3 expression in Japanese flounder was knocked down by siRNA, serum complement activity was significantly decreased, and bacterial replication in fish tissues was significantly increased. Recombinant PoC3a (rPoC3a) exhibited apparent binding capacities to bacteria and Japanese flounder peripheral blood leukocytes (PBL) and induce chemotaxis of PBL. Japanese flounder administered rPoC3a exhibited enhanced resistance against bacterial infection. Taken together, these results indicate that PoC3 is likely a key factor of complement activation, and PoC3 and PoC3a are required for optimal defense against bacterial infection in teleost.

Highlights

The complement system is composed of approximately 35 proteins, which present in serum, tissue fluid and the surface of cell membranes [1, 2]
With an aim to gain more insights into the function of teleost C3 and C3a, we examined the biological role of Japanese flounder (Paralichthys olivaceus) C3 (PoC3) and C3a (PoC3a) in complement activation and antibacterial immune defense
The results showed that PoC3 exhibited a direct bactericidal activity to V. harveyi and S. iniae in a manner that depended on the dose of the protein (Figure 5), whereas it had no significant effect on the survival of E. tarda, E. coli, V. anguillarum and P. fluorescens

Summary

Introduction

The complement system is composed of approximately 35 proteins, which present in serum, tissue fluid and the surface of cell membranes [1, 2]. C3 is a center factor in the complement system, all three complement activation pathways converge in C3 and formation of C5 convertase [6]. Functions of C3 and C3a cleavage of C5 to C5a and C5b, and C5b is involved in assembly of the membrane-attack complexes (MAC) [7, 8]. MAC forms channels or pores on the cell surface of pathogens, leading to the destruction of infected cells and the death of foreign pathogens [6,7,8]

Methods

Results

Conclusion