Endoplasmic reticulum-targeting sequence enhanced the cellular immunity of a tumor-associated antigen L6-based DNA vaccine

Cancer vaccine design to effectively eliminate tumors requires triggering strong immune reactions to elicit long-lasting humoral and cellular immunity and DNA vaccines have been demonstrated to be an attractive immunotherapeutic approach. The tumor-associated antigen L6 (TAL6) is overexpressed on the surface of different cancer cells and promotes cancer progression; therefore, it could be a potential target for cancer treatment. We have revealed that a synthetic peptide containing HLA-A2-restricted cytotoxic T lymphocyte (CTL) and B cell epitope can induce cellular and humoral immunity against TAL6-expressing cancer. To enhance the efficacy of immunotherapy, in this report, we designed an endoplasmic reticulum (ER)-targeting sequence (adenovirus E3/19K protein) at the N-terminus of TAL6 to facilitate MHC class I antigen presentation to CD8⁺ T cells. Transfection of mammalian cells with the plasmid containing TAL6 fused with the ER-targeting sequence (pEKL6) resulted in higher levels of TAL6 antigens in the ER than transfection with the full-length TAL6 (pL6). The plasmid pEKL6 induced both TAL6-specific CTL responses and antibody titers after intramuscular (IM) immunization with electroporation and it elicited higher levels of antigen-specific CTLs in HLA-A2 transgenic mice. Immunization with pEKL6 induced higher levels of protective antitumor immunity against tumor growth than pL6 immunization in thymoma and melanoma tumor animal models. Notably, pEKL6 elicited long-term anti-tumor immunity against the recurrence of cancers. We found that CD4⁺ T, CD8⁺ T, and NK cells are all important for the effector mechanisms of pEKL6 immunization. Thus, cancer therapy using an ER-targeting sequence linked to a tumor antigen holds promise for treating tumors by triggering strong immune reactions.