C3d adjuvant effects are mediated through the activation of C3d‐specific autoreactive T cells

Abstract

Complement fragment C3d covalently attached to antigens enhances immune responses, particularly for antigens lacking T cell epitopes. Enhancement has been attributed to receptor cross-linking between complement receptor CR2 (CD21) and polysaccharide antigen to surface IgM on naïve B cells. Paradoxically, C3d has still been shown to increase immune responses in CD21 KO mice, suggesting that an auxiliary activation pathway exists. In prior studies, we demonstrated the CD21-independent C3d adjuvant effect might be due to T cell recognition of C3d T helper epitopes processed and presented by MHC class II on the B cell surface. C3d peptide sequences containing concentrated clusters of putative human C3 T cell epitopes were identified using the epitope-mapping algorithm, EpiMatrix. These peptide sequences were synthesized and shown in vitro to bind multiple HLA-DR alleles with high affinity, and induce IFNγ responses in healthy donor PBMCs. In the present studies, we establish further correlations between HLA binding and HLA-specific lymphocyte reactions with select epitope clusters. Additionally, we show that the T cell phenotype of C3d-specific reactive T cells is CD4+CD45RO+ memory T cells. Finally, mutation of a single T cell epitope residing within the P28 peptide segment of C3d resulted in significantly diminished adjuvant activity in BALB/c mice. Collectively, these studies support the hypothesis that the paradoxical enhancement of immune responses by C3d in the absence of CD21 is due to internalization and processing of C3d into peptides that activate autoreactive CD4+ T helper cells in the context of HLA class II.

The Dendritic Cell Major Histocompatibility Complex II (MHC II) Peptidome Derives from a Variety of Processing Pathways and Includes Peptides with a Broad Spectrum of HLA-DM Sensitivity

“…Following C3 immunization, a proliferative response to the immunizing peptide (Fig. 6e) could be detected, confirming the presence of autoreactive T cells specific for different C3 determinants, as shown previously (82,83).…”

Better Epitope Discovery, Precision Immune Engineering, and Accelerated Vaccine Design Using Immunoinformatics Tools

“…This peptide is located in the previously defined P28 "adjuvant" region of C3d which is completely conserved across species (43,44). Mutation of a single T-cell epitope in P28 resulted in significantly diminished adjuvant activity of the peptide in mice (45), which supports the hypothesis that the epitope activates autoreactive T-helper cells so as to bridge innate and adaptive immunity.…”

C3d regulates immune checkpoint blockade and enhances antitumor immunity

“…However, Knopf et al (33) and De Groot et al (34) propose that presentation of C3d-derived peptides by antigen-presenting cells in other systems might stimulate autoreactive T cells. We cannot exclude entirely the possibility that protective immunity evoked by C3d + tumor cells in our experiments reflected cross-reactivity with "normal" peptides and, hence, autoimmunity rather than tumor-specific immunity.…”