Antibody Responses to Melanoma Helper Peptide Vaccines May Enhance Antigen Opsonization Through Formation of Immune Complexes and are Modulated by Vaccine Adjuvants.

Background: Vaccines targeting melanoma antigens can elicit CD8 + T cell responses, but a growing body of work suggests CD4 + T cells also play a role in tumor control. Induction of CD4 + cells may also support B cells in producing tumor antigen-specific antibodies (Abs). We investigated Abs induced by vaccination with a cocktail of six class II MHC-restricted melanoma peptides (6MHP) and the effect of adjuvant type on Ab isotypes. We hypothesized that the vaccines would induce Abs that respond to different epitopes on individual peptides and that IgG subclass distribution varies with different vaccine adjuvants. Methods: Sera from patients who received a 6MHP vaccine were evaluated with enzyme-linked immunosorbent assays to map epitopes for polyclonal Ab responses to synthetic melanoma peptides (n=8). IgG subclasses of Ab responses to 6MHP were assessed in patients who received one of 4 adjuvants (Incomplete Freund's Adjuvant (IFA) alone, IFA + polyICLC, IFA + systemic mCy, or IFA + polyICLC + systemic mCy) to characterize IgG subclass distribution (n=2-14). Comparisons were evaluated using a Mann-Whitney rank sum test. Results: Epitope mapping revealed that at least 50% of patients exhibiting Ab responses to melanoma peptides had responses to two or more epitopes on the same peptide, suggesting polyclonal antibody responses. Serum evaluation for IgG isotypes showed predominant induction of IgG1 and IgG3. Mean total IgG was highest when IFA and polyICLC were used in combination. Patients who received TLR3 agonist polyICLC had significantly higher concentrations of total IgG, IgG1, and IgG3 compared to patients who did not receive polyICLC. Conclusions: These findings suggest that vaccine-induced Abs may respond to multiple epitopes within the same peptide, which may support creation of large antigen-Ab complexes, with promise to facilitate antigen uptake and presentation. Abs were predominantly IgG1 and IgG3, which are optimal for binding complement and supporting Ab-dependent cellular cytotoxicity. The findings also show that adding polyICLC to IFA can significantly enhance Ab responses. Collectively, this work underscores the immunologic potential of peptide-induced Abs and the importance of adjuvant selection in cancer vaccine design.