How Tumors Hide From T Cells and New Ways to Expose Them

T cell immunity depends on the presentation of antigenic peptides through major histocompatibility complex class I molecules (pMHC-I) on cell surfaces. This review provides a comprehensive analysis of how tumor cells manipulate these critical immune recognition complexes to evade destruction and how clinicians might counteract these mechanisms.

The authors detail three primary levels of pMHC-I regulation that tumors exploit. At the genetic level, tumors frequently develop mutations in antigen processing machinery genes like TAP1/TAP2 (found in 15-20% of various cancers) and lose heterozygosity at HLA loci. Epigenetically, tumors silence key genes through DNA methylation and histone modifications, with studies showing 30-60% of tumors having methylated promoters for antigen presentation genes. At the protein level, tumors alter proteasome composition and reduce expression of critical components like β2-microglobulin.

The review highlights that both quantity and quality of pMHC-I presentation matter for effective immunity. Tumors can globally reduce pMHC-I levels or selectively eliminate highly immunogenic neoantigens while maintaining self-peptides to avoid NK cell recognition. This selective editing represents a sophisticated immune evasion strategy that maintains some HLA expression while removing the most threatening epitopes.

Therapeutic strategies discussed include HDAC inhibitors and DNA methyltransferase inhibitors to reverse epigenetic silencing, proteasome inhibitors to alter peptide processing, and combination approaches with checkpoint inhibitors. The authors emphasize that successful interventions must account for tumor heterogeneity and the complex interplay between different regulatory mechanisms. Understanding these pathways offers new opportunities for precision immunotherapy approaches that could restore effective T cell recognition of cancer cells.