ABSTRACT Gastrointestinal stromal tumor (GIST) is the most common human sarcoma and typically arises from the stomach or small intestine. The cancer generally has a single mutation, either in KIT or PDGFRA. Imatinib and other tyrosine kinase inhibitors that attack the oncoproteins associated with these mutated genes are effective, but resistance develops around 18 months and cure is rare. There are no other effective treatments so new therapeutic approaches are needed. Over the last 13 years, we have extensively studied the immune response to GIST in a genetically engineered “knockin” mouse model and over 300 fresh human GIST surgical specimens. To investigate the antitumor immune response in greater depth, we have just performed single-cell RNA sequencing on over 50,000 intratumoral immune cells from tumors of mice that were treated with vehicle or imatinib. We discovered that gdT cells are enriched in the tumor compared with blood and spleen. We have preliminary data that gdT cells suppress GIST growth in vivo, at least in part due to IL-17 secretion. Furthermore, imatinib therapy enhanced their IL-17 production. We also found that gdT cells infiltrate human GIST and they produce IL-17, similar to the mouse model. The importance of gdT cells in human GIST, or any sarcoma for that matter, is unknown. We have preliminary data that the presence of gdT cells correlates positively with overall survival in GIST patients. We hypothesize that gdT cells play a critical role in the antitumor immune response to GIST and can be manipulated for therapeutic efficacy. In this proposal, we will investigate the importance of gdT cells in mouse and human GIST by first defining the mechanism of gdT cell suppression of murine GIST. Next, we will determine how imatinib affects intratumoral gdT cells in murine GIST. Interestingly, unsupervised clustering of the single-cell RNA expression revealed 2 main subsets of intratumoral gdT cells in our mouse model. We will study their different function. We will establish the translational relevance of gdT cells in GIST. We will test several gdT cell activating agents in combination with imatinib in our mouse model. Furthermore, we will study the function of gdT cells isolated from fresh human GIST surgical specimens and correlate their presence to clinicopathologic variables and immune infiltrate. Lastly, we propose to perform scRNAseq on intratumoral immune cells from fresh human GISTs. Our single-cell RNA sequencing data have given us an unprecedented look into the complexity of the immune response to GIST and have formed the basis for numerous hypotheses in this proposal. While we are focused on GIST, we expect that our findings will have relevance to gdT cells in other human cancers.