Abstract PR01: T cell responses to peptide-epitopes of choice can be boosted by immune complexes of circulating anti-tetanus toxoid antibodies

The ability of dendritic cells (DCs) to boost an antigen-specific immune response is utilized in several cancer immunotherapy strategies including therapeutic vaccination using long peptides. Water-based peptide vaccines are rapidly degraded and oil-based delivery strategies trap immune cells to unwanted sites or have inappropriate adjuvant properties. To enhance the uptake of synthetic long peptides (SLP) and activation of DCs we make use of circulating antibodies to mount cellular responses against tumor antigens of interest by conjugating a B cell epitope to a T cell epitope. The conjugation of the two improves the uptake of peptide/antibody complexes and concomitant activation of the same DC in contrast to SLPs which are not conjugated. Our identified B cell epitope of choice is derived from tetanus toxin (and named MTTE = minimal tetanus toxin epitope) and can be targeted by tetanus-specific antibodies that are present in most vaccinated individuals. SLPs harbor viral or tumor-derived epitopes specific for the disease of interest. We have applied a modified chandler loop model preserving intact cascade systems, to characterize how the vaccine is targeted to human immune cells. The B cell-T cell conjugate ([MTTE] 3 -SLP) is taken up by human monocytes and blood DCs in an antibody-dependent manner. Rather than FcγRs, the internalization of the antigen appears to be mediated through the classical pathway of the complement system as it is partly reduced by blocking C1q but not when blocking C3. A [MTTE] 3 -CMV conjugate, containing a T cell epitope from the pp65 protein of cytomegalovirus (CMV), strongly reactivates memory T cells when analyzed in blood from donors with CMV-specific T cells. The CMV-specific T cells rapidly produce IFNγ in response to the conjugate illustrating that the uptake of the conjugate leads to activation of antigen-specific T cells. Uptake as well as T cell activation occurs at low concentrations of the SLP conjugate, superior to a conjugate lacking the tetanus-sequence as well as to SLPs with or without added adjuvant (LPS). Additionally, when the B and T cell epitopes are separate entities but mixed, CMV-specific T cells are not activated, illustrating the requirement of conjugating the two. Our data show that we have a unique delivery system for peptide based vaccines that can aid induction of human T cell responses, and may potentiate immune responses in cancer patients. Citation Format: E. Fletcher, W. van Maren, R. Cordfunke, J. Dinkelaar, R. Castelli, Jdc Codee, G. van der Marel, CJM Melief, JW Drijfhout, F. Ossendorp, Sm Mangsbo. T cell responses to peptide-epitopes of choice can be boosted by immune complexes of circulating anti-tetanus toxoid antibodies [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr PR01.