The efficacy of dendritic cell (DC) cancer vaccines is linked to poor immunogenicity of tumor-associated antigens and failure to elicit robust MHC class II-restricted CD4⁺ T-cell responses. Here, we introduce PROTEXI, a DC vaccine platform that optimizes tumor immunity by co-presenting tumor-specific CD8⁺ T-cell epitopes alongside CD4⁺ T helper epitopes from the SARS-CoV-2 Spike protein, leveraging widespread anti-viral immunity. In preclinical mouse models of melanoma and breast cancer, PROTEXI significantly reduces tumor growth and improves survival by promoting robust T cell infiltration into immune-cold tumors, increasing cytotoxic T cell responses via epitope spreading, and activating genes linked to optimal DC, NK cell, and T cell function. Furthermore, PROTEXI elicits superior responses when combined with other immunotherapy agents in models of therapy-resistant tumors. Finally, in a humanized mouse model of melanoma, PROTEXI vaccine co-presenting CD4⁺ T-specific Spike epitopes with CD8⁺ T cell-restricted PRAME and MAGE-A3 antigens, significantly reduces tumor burden. Thus, these data underscore the potential of harnessing pre-existing viral-specific immunity to enhance the efficacy of DC vaccines in immune-cold tumors.