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

Area of Science:

• Immunology
• Marine Biology
• Fish Health

Background:

• The complement system is vital for innate immunity.
• Complement component 3 (C3) is central to complement activation, pathogen elimination, and immune regulation.
• Japanese flounder (Paralichthys olivaceus) is an economically important teleost species.

Purpose of the Study:

• To investigate the immunological properties and functions of C3 and C3a from Japanese flounder (PoC3 and PoC3a).
• To understand the role of PoC3 and PoC3a in the immune response against bacterial infections in teleosts.

Main Methods:

• Characterization of Japanese flounder C3 (PoC3) structure and tissue expression.
• Analysis of PoC3 expression levels following bacterial challenge.
• Assessment of PoC3's serum binding and bactericidal activity.
• Investigation of PoC3 knockdown effects on complement activity and bacterial load.
• Evaluation of recombinant PoC3a (rPoC3a) binding, chemotaxis, and protective effects against bacterial infection.

Main Results:

• PoC3 expression was detected in nine tissues and upregulated by bacterial challenge.
• PoC3 exhibited broad-spectrum bacterial binding and direct pathogen killing capabilities.
• PoC3 knockdown significantly reduced serum complement activity and increased bacterial replication.
• rPoC3a demonstrated bacterial binding, induced leukocyte chemotaxis, and enhanced resistance to bacterial infection.

Conclusions:

• PoC3 plays a key role in complement activation in teleosts.
• Both PoC3 and PoC3a are essential for effective defense against bacterial infections in Japanese flounder.
• These findings highlight the importance of the complement system in fish immunity.