003 In vitro genetic reprogramming increases MHC-I expression and ameliorates resistance to an antitumor immune response in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a rare but aggressive skin cancer with half of patients unresponsive to immune checkpoint inhibitors (ICIs). A primary resistance mechanism driving ICI resistance in MCC in particular and cancer immune evasion in general is downregulation of tumor MHC class I (MHC-I) expression, thereby limiting cytotoxic cellular immune responses. Assessment of three patient-derived MCC cell lines, MCC13, MCC26, and UISO, demonstrated low baseline MHC-I expression in 2 of the 3 cell lines (MCC13 and UISO), with higher expression in MCC26. We used biodegradable polymeric nanoparticles based on poly(beta-amino ester)s to co-deliver DNA plasmids encoding a co-stimulatory molecule (4-1BBL) and an immunostimulatory cytokine (IL-12) to reprogram the cancer cells, allowing them to interact productively with cytotoxic lymphocytes and elicit a targeted anti-tumor immune response. Following nanoparticle administration, 22, 12, and 42% of MCC13, MCC26, and UISO cells expressed surface-bound 4-1BBL and produced 1460, 14, and 1000 ng/mL secreted IL-12, respectively. This was accompanied by a 2.5- and 2.3-fold increase in MHC-I expression (mean fluorescence intensity) in MCC13 and UISO 3 days after transfection. Without nanoparticle-based reprogramming, co-culture of MCC13 and UISO with unselected human CD8+ T-cells did not result in T-cell activation or cancer cell death. Following nanoparticle transfection with 4-1BBL and IL-12, however, MCC13, MCC26, and UISO co-culture with CD8+ T-cells led to 1300-, 4.4-, and 86-fold increased IFNg production and 3.6-, 1.6-, and 2.8-fold CD8+ T-cell expansion, respectively, as well as significant cancer cell death in MCC13 and UISO (p