C3b covalently associated to tetanus toxin modulates TT processing and presentation by U937 cells

Area of Science:

• Immunology
• Complement System
• Cellular Biology

Background:

• Complement protein C3 fragments, like C3b, can bind covalently to antigens.
• These antigen-C3b complexes interact with complement receptors on antigen-presenting cells (APCs).
• Antigen-C3b complex formation is common at inflammatory sites during immune responses.

Purpose of the Study:

• To investigate how C3b binding influences the processing of tetanus toxin (TT) by U937 cells acting as APCs.
• To determine the effect of C3b association on TT uptake, intracellular trafficking, and subsequent T cell proliferation.

Main Methods:

• Preparation of covalent TT-C3b complexes using purified proteins.
• Uptake studies using U937 cells and complement receptor CR1.
• Analysis of intracellular trafficking and proteolysis of TT and TT-C3b complexes in endosomes and lysosomes.

Main Results:

• TT-C3b complexes showed significantly higher uptake by U937 cells compared to TT alone, mediated by CR1.
• TT-C3b complexes were retained in endosomes with limited proteolysis, unlike free TT which rapidly degraded in lysosomes.
• The ester linkage in TT-C3b complexes was hydrolyzed in lysosomes, leading to efficient proteolysis and enhanced TT-specific T cell proliferation.

Conclusions:

• C3b acts as an escort for antigens, influencing their uptake, intracellular routing, and proteolysis within APCs.
• C3b binding modulates antigen processing, thereby enhancing antigen-dependent T cell proliferation.
• The complement system, through C3b, plays a significant role in shaping adaptive immune responses.