Abstract A110: Efficacy of anti-PD1 immune checkpoint blockade involves the cooperative interaction of myeloid and lymphoid subpopulations in the tumor microenvironment

Suppression of the immune system by solid malignancies has proven to be a driving force of tumor development and an effective target for therapeutic intervention. The suppression of cytolytic T-cells by inhibitory receptors like PD1 can be blocked by antagonistic antibodies, reinvigorating suppressed antitumor responses. Nonetheless, only a minority of patients show clinical benefit. It is becoming clear the efficacy of checkpoint blockade relies on many factors, including pretreatment conditions, collaboration between innate and adaptive immune cells and immune-architecture of the tumor microenvironment (TME). To investigate this, we studied the immune profiles of the PD1-unresponsive murine B16 melanoma and PD1-responsive MC38 colorectal carcinoma models, systemically and in the TME before and during treatment. By using high-dimensional flow cytometry and unsupervised clustering analyses based on immune checkpoints, we show comparable early establishment of heterogeneity of tumor-infiltrating CD8+ and CD4+ T-cells. However, PD1-responsive MC38 tumor shows correlations in abundance between specific CD8+ T-cell, NK cells and myeloid subsets before checkpoint blockade treatment, while the PD1-unresponsive B16 tumors do not show lymphoid-myeloid codependences. Interestingly, the abundance of monocyte-derived dendritic cells did not increase upon anti-PD1 treatment but instead showed abundance correlation with PD1 CD4+ and CD8+ T-cells, suggesting a putative interaction. The unresponsive B16 tumors showed increased correlation of cDC1 and cDC2 with Foxp3+ regulatory T-cells. We have visualized these putative interactions within the myeloid and lymphoid population in the changing immune-architecture of the TME, using multiplex fluorescence and confocal microscopy. We show heterogeneity and interactive hotspots during immune checkpoint blockade and postulate that successful anti-PD1 treatment requires the location-dependent cooperation of specific myeloid and lymphoid subsets in the TME. Citation Format: Sjoerd Schetters, Yvette Van Kooyk. Efficacy of anti-PD1 immune checkpoint blockade involves the cooperative interaction of myeloid and lymphoid subpopulations in the tumor microenvironment [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A110.