Complement-dependent and -independent pathways of T cell-B cell cooperation

Area of Science:

• Immunology
• Complement System
• B-cell Activation

Background:

• The complement system, particularly C3, plays a role in immune responses.
• Complement receptors (CR) on B cells can interact with complement components.
• The precise role of C3 in T-dependent antibody production is not fully understood.

Purpose of the Study:

• To investigate the role of C3 in T-dependent antibody responses.
• To determine if complement receptor-mediated interaction with C3 or its fragments affects B-cell responses.
• To elucidate the pathways of T-cell B-cell cooperation involving C3.

Main Methods:

• Mice were treated with cobra venom (CoV) to deplete serum C3.
• T-dependent IgM antibody-producing cells (PFC) were measured after immunization.
• Experiments included using anti-C3 antibodies and analyzing B cells expressing complement receptors (CR+ and CR-).

Main Results:

• CoV treatment markedly reduced serum C3 levels but did not alter the frequency of splenic CR+ cells.
• CoV treatment significantly reduced the IgM PFC response to a T-dependent antigen.
• Anti-C3 antibody did not affect the T-dependent IgM response of CR- B cells, and residual responses were mainly from CR- B cells.

Conclusions:

• The inhibitory effect of CoV on T-dependent antibody responses is mediated by C3 depletion, not direct receptor interaction.
• These findings suggest at least two distinct pathways for T-B cell cooperation exist.
• One pathway requires C3, while another pathway functions independently of C3.