Innate immune reprogramming in the tumor microenvironment

Innate immune cells within the tumor microenvironment (TME) sense danger, drive inflammation and, depending on their activation state and local cues, can either suppress or support tumor growth. This thesis investigates phenotypic and functional reprogramming of two innate populations—natural killer (NK) cells and mast cells (MCs)—during tumor progression, and defines mechanisms by which the TME shapes their activity. We demonstrated that, during tumor progression, chronic stimulation of the activating NK receptor NKG2D induces an hypofunctional state characterized not only by NKG2D downregulation but also by impairment of DNAM 1, a receptor critical for tumor cell clearance. By using primary human NK cells exposed overnight to MICA presented by transfectants, or plate bound MICA, we showed increased expression of a checkpoint receptor able to counteract DNAM-1 activation. Stimulation with an agonist antibody directly inhibits DNAM-1-mediated signal transduction and cytotoxic function with a mechanism that required NKG2D endocytosis. Moreover, by focusing on colorectal cancer (CRC), analysis of fresh biopsies and matched healthy mucosa revealed that tumors are enriched in soluble mediators that sustain MC survival and reprogramming and show an overall increase in MC numbers accompanied by a clear phenotypic shift between healthy mucosa and tumor lesions. Finally, stratification by microsatellite status revealed a subtype-specific polarization of MCs depending on the tumor’s molecular background. Overall, our findings clarify the molecular mechanisms behind the often controversial roles of innate immune components in tumor progression and pave the way for new therapeutic strategies.