Abstract 5373: Altering the composition of the proteasome of melanoma antigen RNA-transfected dendritic cells enhances vaccination-induced anti-melanoma immunity: results of a Phase I trial

Abstract

Abstract Background: Dendritic cells (DC) must be matured to maximally stimulate antigen-specific immune responses and avoid the induction of tolerance. During maturation, the proteasome, the multi-subunit complex that generates peptides presented in the context of HLA class I, is altered to exclusively consist of immunoproteasomes (iPs). Compared to the constitutive proteasome (cP), the iP has altered protease activities and generates a different repertoire of peptide epitopes. Vaccination with mature tumor-antigen loaded DC will thus stimulate immunity against iP-generated peptides. Since melanomas, and other cancers, are frequently defective in inducible iP expression, this anti-tumor immune response may be miss-targeted and not recognize melanomas presenting cP-derived peptides. We hypothesized that mature melanoma antigen-loaded DC expressing the cP, rather than the iP, would be superior inducers of anti-melanoma immunity in subjects with metastatic melanoma. Methods: 12 subjects with metastatic melanoma were immunized with 6 weekly intradermal injections of 10 million mature autologous monocyte-derived DC transfected with RNA encoding full-length MART-1, MAGE-3, gp100 and tyrosinase. The monocytes from which the DC were generated were either untransfected (Study Arm A, n=4), transfected with control siRNA (Study Arm B, n=3), or transfected with siRNAs targeting the 3 inducible iP subunits LMP-2, LMP-7 and MECL-1 (Study Arm C, n=5). Results: 10 subjects received a full course of vaccination, while only enough DC were available for 4 and 5 total vaccine doses in 2 subjects. All 12 subjects tolerated vaccination without toxicity. Vaccination stimulated melanoma antigen-specific CD4+ and CD8+ T cell responses detected in peripheral blood by IFN-γ ELISPOT in all subjects, which peaked after 3-4 vaccinations. These T cell responses decreased thereafter in subjects in Study Arms A and B, but remained elevated in subjects in Study Arm C. Melanoma cell levels in peripheral blood, as detected by qPCR, fell after vaccination in Study Arm C. Using autologous melanoma cells (and autologous normal skin fibroblasts as controls) derived from each subject, we found that vaccination in Study Arm C, but not A or B, induced peripheral T cells with lytic activity against autologous melanoma. Two subjects had active disease, both in Study Arm C. The one subject with diffuse extremity in-transit disease had a partial clinical response with transient liquefaction of dermal lesions. The other subject with diffuse dermal and soft tissue metastatic lesions had complete resolution of detectable metastasis by PET scanning. Conclusion: The efficacy of melanoma immunotherapy using antigen RNA-transfected mature DC is enhanced when DC express the cP rather than the iP. We feel that this novel approach warrants further clinical investigation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5373. doi:1538-7445.AM2012-5373