OBJECTIVES/GOALS: Glioblastoma (GBM) is the most common and aggressive adult primary brain malignancy. Clinically, GBM is refractory to T cell immune checkpoint blockade (ICB), in part due to its dense immune suppressive myeloid stroma. Here we show that myeloid-targeting STING agonists can repolarize the GBM microenvironment to cure ICB-refractory GBM models. METHODS/STUDY POPULATION: Using the synthetic cyclic di-nucleotide STING agonist IACS-8803 (8803) we treated orthotopic ICB-refractory QPP8 orthotopic murine GBM tumors intratumorally. We then analyzed survival and performed high parameter flow cytometry profiling of the tumor immune microenvironment following STING agonist treatment. To assess the contribution of adaptive immunity to STING agonist therapeutic efficacy, we treated orthotopic QPP8 tumors implanted in RAG1 KO mice and monitored survival. RESULTS/ANTICIPATED RESULTS: We found that STING agonist therapy cured murine orthotopic QPP8 tumors, in contrast to ICB that showed no survival benefit. In RAG1-/- mice bearing QPP8 tumors STING agonist therapy extended survival, however, the curative effect observed in wild-type mice was lost in the absence of adaptive immunity. STING agonist-treated QPP8 tumors displayed increased counts of CD8 T cells and NK cells, and decreased CD8 T cell PD1 expression. Infiltration of STING-treated gliomas by Ly6C+ F4/80+ Mono-MDSC substantially increased; however, these cells expressed reduced CD206 and CD163, suggestive of reduced immuno-suppression. Finally, in the cervical LN of QPP8-treated mice the frequency and CD80/CD86 expression of cDC1 cells increased.​ DISCUSSION/SIGNIFICANCE: ICB has failed in GBM, and the suppressive myeloid stroma remains a major barrier to generating anti-GBM T cell responses. Our work shows that STING activation, which primarily targets innate immunity myeloid cells 'upstream' of T cells in the antitumor immunity cycle, can cure ICB-refractory GBM tumors in an adaptive immunity-dependent manner.