Novel function of complement C3d as an autologous helper T-cell target

Area of Science:

• Immunology
• Complement System
• T-cell Epitopes

Background:

• The C3d fragment of complement component C3 enhances immune responses to T-cell independent antigens like bacterial polysaccharides.
• C3d binding to B-cell complement receptor 2 (CR2/CD21) acts as a co-activation signal, lowering B-cell activation thresholds.
• Previous studies showed C3d enhances antibody titers and cytokine secretion even in CD21 knockout mice, suggesting a CR2-independent pathway.

Purpose of the Study:

• To investigate the hypothesis that C3d possesses T-cell epitopes, acting as a 'co-signal 2' in addition to its adjuvant 'co-signal 1' function.
• To explore a potential CR2-independent pathway for immune activation mediated by C3d.

Main Methods:

• Utilized the EpiMatrix T-cell epitope-mapping algorithm to identify putative T-cell epitopes within the C3d sequence.
• Synthesized identified C3d epitope candidates and assessed their binding to various human leukocyte antigen (HLA)-DR molecules.
• Evaluated the ability of C3d peptides to stimulate T-cell cytokine secretion (IFN-gamma) in vitro using peripheral blood mononuclear cells (PBMCs).

Main Results:

• Identified 11 putative T-cell epitopes in C3d, indicating a high epitope density.
• Eight synthesized C3d epitope candidates demonstrated binding to diverse HLA-DR molecules and stimulated T-cell pro-inflammatory cytokine secretion.
• Observed a C3d-peptide specific increase in CD4(+) intracellular IFN-gamma(+) T cells in PBMCs.

Conclusions:

• The discovery of autologous T cells reactive to C3d peptides supports the 'co-signal 2' hypothesis.
• C3d's ability to stimulate T cells via epitopes offers a novel explanation for the CD21 knockout paradox.
• These findings suggest C3d acts as a molecular adjuvant through both B-cell co-stimulation and T-cell epitope presentation.