KIT promotes tumor stroma formation and counteracts tumor-suppressive TGFß signaling in colorectal cancer.

Expression profiling has identified four consensus molecular subtypes (CMS1-4) in colorectal cancer (CRC). The receptor tyrosine kinase KIT has been associated with the most aggressive subtype, CMS4. However, it is unclear whether, and how, KIT contributes to the aggressive features of CMS4 CRC. Here, we employed genome-editing technologies in patient-derived organoids (PDOs) to study KIT function in CRC in vitro and in vivo. CRISPR-Cas9-mediated deletion of the KIT gene caused a partial mesenchymal-to-epithelial phenotype switch and a strong reduction of intra-tumor stromal content. Vice versa, overexpression of KIT caused a partial epithelial-to-mesenchymal phenotype switch, a strong increase of intra-tumor stromal content, and high expression of TGFß1. Surprisingly, the levels of phosphorylated SMAD2 were significantly lower in KIT-expressing versus KIT-deficient tumor cells. In vitro analyses showed that TGFß signaling in PDOs limits their regenerative capacity. Overexpression of KIT prevented tumor-suppressive TGFß signaling, while KIT deletion sensitized PDOs to TGFß-mediated growth inhibition. Mechanistically, we found that KIT expression caused a strong reduction in the expression of SMAD2, a central mediator of canonical TGFß signaling. We propose that KIT induces a pro-fibrotic tumor microenvironment by stimulating TGFß expression, and protects the tumor cells from tumor-suppressive TGFß signaling by inhibiting SMAD2 expression.

Directing CAR T cells towards the tumor vasculature for the treatment of solid tumors.

For successful application of chimeric antigen receptor (CAR) T cell therapy in solid tumors, major hurdles have to be overcome. CAR T cells have to cross the vascular barrier, which is hampered by the anergic state of the tumor vasculature, characterized by suppressed levels of leukocyte adhesion molecules on the endothelium. Additional immunosuppressive mechanisms in the solid tumor microenvironment can affect infiltration, activity and persistence of CAR T cells. Redirecting CAR T cells towards the tumor vasculature poses a possible solution, as molecular targets of tumor endothelial cells can be directly engaged from within the blood. In this review, we discuss recent advances in CAR T cell therapy against solid tumors, with a focus on targeting the tumor vasculature. Furthermore, we discuss opportunities to overcome challenges and barriers through engineering of CAR T cells to enhance trafficking, safety and efficacy.