786 Distinct efficacy and immunological responses to aPD-1, aPD-L1 and aPD-L2 immunotherapy in B16 melanoma in aged versus young hosts includes T-cell stem cell effects and PD-L2 expression differences

Background Aging is the biggest risk factor for cancer, yet little is known about cancer immunotherapy effects. ?PD-1 can block PD-L1 and PD-L2 while ?PD-L1 blocks PD-1 and CD80.1 A recent key finding in young hosts including humans is that melanoma response to aPD-1/aPD-L1 correlates with CD8+TCF-1+ T cell stem cell (TCSC) generation.2 Methods We tested aPD-1 (100 or 200 µg/mouse), aPD-L1 (100 µg/mouse) or aPD-L2 (200 µg/mouse) in aged (18-33 months) and young (3-8 months) mice challenged orthotopically with B16 (WT or PD-L1ko) melanoma (SQ) or ID8agg ovarian cancer (IP). Tumors were analyzed by flow. Bone marrow-derived DC were generated with GM-CSF. Results We reported that aPD-1 treats young and aged with B16 and aPD-L1 treats young not aged.3 aPD-L2 treated B16 in aged but, remarkably, not young, the first anti-cancer single agent immunotherapy exhibiting this property [figure 1]. Efficacy in young (aPD-1, aPD-L1) and aged (aPD-L2) correlated with increased tumor TCSC [figure 3], but TCSC differed by age and treatment (e.g., distinct CCR2, CXCR5, CXCR3) [figure 2]. aPD-L2 efficacy against B16 in aged mice required host IFN-g and IL-17 [figure 4]. IP ID8agg ovarian cancer did not respond to aPD-L2 in aged or young mice. Aged expressed up to 40-fold more PD-L2 versus young on various immune cells suggesting high PD-L2 helps aPD-L2 response [figure 6]. Host IFN-g contributed to aged PD-L2 expression, which did not appear cell-autonomous [figure 6]. PD-L1KO aged but not young mice challenged with PD-L1KO B16 responded to aPD-1 [figure 5], consistent with PD-L2 block as a mechanism. Conclusions Treatment differences in aged versus young could depend on immune checkpoint or TCSC differences, which could be related to CD8+ T-cell infiltration, including TCSC. aPD-1 efficacy in aged PD-L1KO mice challenged with PD-L1KO B16 suggests that aPD-1 efficacy is through PD-L2 block in aged. PD-L2 expression differences and anatomical compartment differences in tumor microenvironment may also contribute to treatment efficacy differences. We are now identifying mechanisms for increased PD-L2 and other mechanisms for aPD-L2 efficacy in aged, and testing TCSC effects. Our work can improve cancer immunotherapy in aged hosts and provides insights in treatment failures, including in young hosts. Acknowledgements South Texas MSTP training grant (NIH T32GM113896), TL1TR002647, Graduate Research in Immunology Program training grant (NIH T32 AI138944), R01 CA231325, Samuel Waxman Cancer Research Foundation Grant Ethics Approval The study was approved by UTHSA IACUC, approval number 20180021. References Schildberg FA, Klein SR, Freeman GJ, Sharpe AH. Coinhibitory pathways in the B7-CD28 ligand-receptor family. Immunity 2016;44(5):955-72. Im SJ, Hashimoto M, Gerner MY, Lee J, Kissick HT, Burger MC, et al. Defining CD8+ T cells that provide the proliferative burst after PD-1 therapy. Nature 2016;537(7620):417-21. Padron A, Hurez V, Gupta HB, Clark CA, Pandeswara SL, Yuan B, et al. Age effects of distinct immune checkpoint blockade treatments in a mouse melanoma model. Exp Gerontol 2018;105:146-54.