Regulation of PD-L1 expression is a novel facet of cyclic-AMP-mediated immunosuppression

Cyclic-AMP (cAMP) exerts suppressive effects in the innate and adaptive immune system. The PD-1/PD-L1 immune checkpoint down-regulates T-cell activity. Here, we examined if these two immunosuppressive nodes intersect. Using normal and malignant lymphocytes from humans, and the phosphodiesterase 4b (Pde4b) knockout mouse, we found that cAMP induces PD-L1 transcription and protein expression. Mechanistically, we discovered that the cAMP effectors PKA and CREB induce the transcription/secretion of IL-10, IL-8 and IL-6, which initiate an autocrine loop that activates the JAK/STAT pathway and ultimately increase PD-L1 expression in the cell surface. This signaling axis is disarmed at two specific nodes in subsets of diffuse large B cell lymphoma, which may help explain the variable PD-L1 expression in these tumors. In vivo, we found that despite its immunosuppressive attributes, the PDE4 inhibitor roflumilast did not decrease the clinical activity of checkpoint inhibitors, an important clinical observation given the approved use of these agents in multiple diseases. In summary, we discovered that PD-L1 induction is a part of the repertoire of immunosuppressive actions mediated by cAMP, defined a cytokine-mediated autocrine loop that executes this action and, reassuringly, showed that PDE4 inhibition does not antagonize immune checkpoint blockade in an in vivo syngeneic lymphoma model.