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Extracellular vesicles (EVs) mediate intercellular communication in many physiologic processes and can modulate immune responses in individuals with cancer. Most studies of EVs in cancer have focused on their tumour promoting properties. Whether and how EVs might mediate tumour regression besides carrying antigens has not been well characterized. Using a mouse model of highly immunogenic regressor versus poorly immunogenic progressor tumour cells, we have characterized the role of EVs in activating macrophages and promoting tumour rejection. We found that the signalling molecule MAP2K1 (MEK1) is enriched in EVs secreted by regressor relative to progressor cells. Progressor EVs engineered to have levels of MEK1 similar to regressor EVs could inhibit tumour growth by indirectly promoting adaptive immunity in both syngeneic and 3rd party tumours. This effect required MEK1 activity and could occur by activating macrophages to promote adaptive immune responses against the tumour via the cytokine interferon-gamma. Our results suggest that MEK inhibition may be deleterious to cancer treatment, since MEK1 plays an important cell-extrinsic, tumour-suppressive role within EVs. Moreover, the delivery of MEK1 to tumour-associated macrophages, either by EVs, nanoparticles, or some other means, could be a useful strategy to treat cancer via the activation of anti-tumour immunity., (© 2024 The Author(s). Journal of Extracellular Vesicles published by Wiley Periodicals LLC on behalf of International Society for Extracellular Vesicles.)

Mechanistic understanding of the immune checkpoint receptor PD1 is largely based on mouse models, but human and mouse PD1 orthologs exhibit only 59.6% identity in amino acid sequences. Here we show that human PD1 is more inhibitory than mouse PD1 due to stronger interactions with the ligands PDL1 and PDL2 and with the effector phosphatase Shp2. A novel motif highly conserved among PD1 orthologs in vertebrates except in rodents is primarily responsible for the differential Shp2 recruitment. Evolutionary analysis suggested that rodent PD1 orthologs uniquely underwent functional relaxation, particularly during the K-Pg boundary. Humanization of the PD1 intracellular domain disrupted the anti-tumor activity of mouse T cells while increasing the magnitude of anti-PD1 response. Together, our study uncovers species-specific features of the PD1 pathway, with implications to PD1 evolution and differential anti-PD(L)1 responses in mouse models and human patients.