Bright side of the dark genome: antigens for next-gen cancer vaccines

The discovery of cancer's hidden antigen landscape-comprising non-canonical 'dark matter' antigens-has unveiled a vast, untapped reservoir of immune targets for next-generation cancer immunotherapy. While most cancer vaccine strategies of the past decade have focused on mutation-derived neoantigens, studies applying sensitive mass spectrometry methods fail to identify the majority of predicted neoepitopes being presented by tumor human leukocyte antigen (HLA) molecules, potentially explaining negative results of several recent neoantigen vaccine trials. By contrast, peptides from non-canonical open reading frames, aberrant splice products, and non-coding RNAs that derive from short-lived proteins (SLiPs) are readily stabilized in class I HLA, and as a consequence of frequently being undetected in the thymus, have demonstrated strong immunogenicity. Early reports suggest some non-canonical immunopeptides are shared within and sometimes across multiple cancer histologies, with early evidence that some have tumor-promoting functions. Because these SLiPs are degraded so quickly and are stabilized in HLA-I, the intact proteins are postulated to not be accessible to antigen-presenting cells and are not efficiently processed and cross-presented-positioning this 'junk DNA'-derived antigen class as an attractive foundation for off-the-shelf vaccines. Here, we trace four phases of cancer vaccine evolution, review the technological advances that enabled the discovery of the dark immunopeptidome and discuss how these findings challenge established paradigms and reinvigorate interest in shared tumor antigens. By embracing this expanded antigenic universe, the field is poised to overcome key limitations of neoantigen-focused immunotherapy and move toward more universally effective cancer vaccines.