Expression of PD-L1 by dendritic cells (DCs) is critical for the efficacy of immune checkpoint inhibitor therapy. However, the mechanisms by which DCs regulate PD-L1 and PD-L2 remain poorly understood. We characterised the expression of these molecules in DCs under steady-state conditions, during inflammation, and in the context of adoptive T cell therapy. Under resting conditions, conventional DC2 (cDC2) exhibited higher PD-L1 expression compared to cDC1. Both subsets upregulated PD-L1 in response to all inflammatory stimuli tested. In contrast, PD-L2 was absent from resting splenic cDC1 and cDC2 and was only induced by a limited set of stimuli, and only in a proportion of cells. During adoptive T cell therapy in a murine model of B cell lymphoma, both cDC1 and cDC2 upregulated PD-L1, under all conditions of T cell therapy tested, while PD-L2 was not expressed at all. In the presence of a large tumour burden, independent of T cell transfer, PD-L2-expressing cells with a myeloid-derived suppressor cell-like phenotype were identified. To uncover novel regulators of PD-L1 and PD-L2 in DCs, we performed a genome-wide CRISPR/Cas9 knockout screen. This analysis identified Elf4 and Bcl6 as positive and negative regulators of PD-L1 expression, respectively, with differing roles in specific DC populations.
In summary, our research reveals unique expression dynamics of PD-L1 and PD-L2 expression in cDC1 and cDC2 during inflammation and tumour immunotherapy and identifies new genetic targets for modulating DC function in future immunotherapy strategies.