INTRODUCTION: The immunoinhibitory ligands PD-L1 and PD-L2 belong to the B7 family of co-stimulatory molecules. PD-L1 and PD-L2 are important checkpoint molecules that act to limit T cell immunity. Cancer cells use increased expression of PD-L1 and PD-L2 to suppress anti-tumour immunity. While PD-L1 is expressed by both hematopoietic and non-hematopoietic cells, PD-L2 expression is more restricted to antigen presenting cells including dendritic cells.
METHODS: Here we have used the murine-derived dendritic cell line MuTu DC to study cell surface expression of PD-L1 and PD-L2 in resting and activated dendritic cells under different stimulatory conditions. Using a mouse model the results were also confirmed in vivo. Expression patterns on cDC were further determined in the presence of tumour pre- and post- adoptive T cell transfer using a B cell lymphoma mouse model.
RESULTS: PD-L1 and PD-L2 show distinct expression on the surface of dendritic cells. First, basal levels of both ligands are low on resting DC, with PD-L1 exhibiting higher surface expression than PD-L2. Second, we tested changes in the expression of PD-L1 and PD-L2 following treatment of DC with stimuli including PMA/ionomycin, bacterial agonists (LPS, CpG) and inflammatory cytokines (IFNa, IFNg, IL-4, IL-2, IL-10). Expression of PD-L1 is significantly increased by all stimuli while in contrast, PD-L2 displays a more regulated pattern of expression. In settings of cancer we found that PD-L1 expression is increased upon adoptive T cell transfer while PD-L2 expression appears on a subset of cells once the tumour has reached a certain size independent of transferred T cells.
CONCLUSION:In summary, resting and activated dendritic cells display differential and regulated expression of PD-L1 and PD-L2. The mechanisms involved will be investigated. Knowledge of how PD-L1 and PD-L2 are expressed has important consequences for understanding how these immunoinhibitory molecules act to limit T cell immunity in settings of infection and/or tumours.