733 Potent immunogenic cell death (ICD) caused by UVA-Activated 8-Methoxypsoralen (8-MOPA)

Abstract

ICD is a prerequisite for induction of patient-specific protective immunity against tumor antigens. We hypothesize that the efficacy of Extracorporeal photochemotherapy (ECP) in initiating anti-lymphoma tumor specific immunity is traceable to its efficient induction of ICD in cutaneous T cell lymphoma (CTCL) malignant cells following their lethal exposure to photoactivated 8-methoxysporalen (8-MOPA). We tested that premise in a standard animal ICD vaccination assay, in three murine solid tumor models (sB16-OVA melanoma, YUMMER melanoma and MC38 colon cancer). Selective immunoprotection against subsequent challenge with viable relevant or control malignant cells was assessed after initial introduction of the same malignant cells rendered apoptotic by 8-MOPA. Vaccination with 8-MOPA pretreated tumor cells prevented B16-OVA melanoma in 77% (17% controls, n=60, p<0.0001), YUMMER 1.7 melanoma in 80% (0% controls, n=10, p<0.0016) and MC38 colorectal cancer in 90% (10% controls, n=20, p=0.0002). These high protection levels compare favorably to the most effective experimental ICD-inducing agents reported to date. Cell surface expression of calreticulin (CRT), a hallmark feature of ICD, was progressively increased on 8-MOPA exposed B16-OVA melanoma cells from background 2% to 19.7%, 44.8% and 46.8% by 24, 48 and 72 hrs post-treatment, as determined by flow cytometry. Proof of CRT’s role in the induction of this ICD was established by elimination of the immunoprotective effect through CRT knockdown in the “immunizing” 8-MOPA-treated B16-OVA line. These findings support the premise that 8-MOPA’s efficacy in initiating immunoprotective cancer cell ICD is a key element underlying ECP’s capacity to immunize against established tumors. The immunoprotective efficacy of 8-MOPA in three different experimental solid tumor systems, together with its established role in human lymphoma, suggest broad anti-cancer applicability of this approach.