Background: KRAS G12C mutation occurs in approximately 1% of pancreatic ductal adenocarcinoma (PDA). KRAS G12C inhibitors have a reported response rate of 21-33% in patients with previously treated advanced PDA. KRAS is a known modulator of the tumor immune microenvironment (TME) and a greater understanding of the TME in KRAS G12C as compared to other mutations is crucial for developing rational therapeutic combination strategies in PDA. Methods: We performed multiplex fluorescent immunohistochemistry (mfIHC) on tissue obtained from patients with histologically confirmed PDA (KRAS G12C n=8; other mutations n=108) and normal pancreas (n=38). Serial staining was performed using antibodies against CD3, CD8, CD163, FoxP3 and PanCK for simple and complex phenotyping as well as spatial analyses in the TME. After tyramide based signal amplification, the slides were imaged at 20x magnification using the Mantra Quantitative Pathology workstation. Images were analyzed using inForm Cell Analysis software (Akoya Biosciences). All statistical analyses were performed using JMP Pro 13.2.0. Differences in phenotype, distances, and engagement were evaluated by 2-tailed Student’s t test or ANOVA. Results: Patients with KRAS G12C mutation had a median age of 70 (45-73) years of which majority (n=5, 62.5%) were men with advanced stage at diagnosis (n=6; 75%). The median overall survival is not yet reached (n=5, alive) after a median follow-up time of 14.4 months through reverse censoring methodology. When examining cellular infiltration, CD8+ cytotoxic T lymphocyte cells were significantly more abundant in the G12C TME compared to the cancer and non-cancer controls (p <0.01). Additionally, the relative frequency of CD4+ and Tregs (CD3+/CD8-/FoxP3+) were also significantly higher in G12C (p<0.01). The percentage of CD163+ antigen presenting cells (APCs) out of all cells was significantly higher in G12C (p<0.01) while the percentage of non-functional APCs (PDL1+/CD163+) of all APCs was significantly higher in G12C than cancer and non-cancer controls (p=0.03). In comparison, the PDL1+ epithelial cells were much lower in G12C TME (p<0.01). The cellular engagement and interaction data from the spatial analysis in the TME will be presented at the meeting. Conclusions: We identified increased infiltration of cytotoxic T lymphocytes and APCs in PDA patients with KRAS G12C mutations relative to other mutations and non-cancerous pancreata. Furthermore, while all PDAs had elevated PD-L1 expression on APCs, KRAS G12C tumors had minimal expression on epithelial cells suggesting a mutation specific impact and a potential role of immune checkpoint inhibitors in combination with G12C inhibitors.