Co-inhibitory receptors such as programmed death receptor 1 (PD-1) are expressed on T cells after activation and are known to inhibit T cell responses. However, despite the establishment of PD-1-blockade as a potent cancer immunotherapy, the precise mechanisms by which PD-1 modulates T cell proliferative responses are not yet fully understood. Upon activation, T cells undergo a controlled division burst to form a pool of antigen-specific effector cells. Previous work using quantitative T cell assays has demonstrated that parameters including division entry, subsequent division rate, cell survival, and the number of times the cells divide before returning to quiescence (termed division destiny) determine the size and duration of the division burst (Heinzel et al. 2017). These key variables are independently controlled by the type and strength of the signals received upon activation via the T cell receptor (TCR), co-stimulatory and cytokine inputs (Marchingo et al. 2014). We applied this in-depth understanding to investigate the precise role of PD-1 signalling in naïve T cell proliferative responses.
We developed a quantitative dendritic cell−T cell co-culture assay for controlled delivery of co-stimulatory and co-inhibitory signals, including PD-1, to T cells in vitro. Using this system, we discovered that PD-1 signalling reduced proliferation of naïve T cells by specifically decreasing division destiny, with no effect on cell survival. Not only did we confirm a reduction in IL-2 production in response to PD-1 signalling, but we also uncovered a novel IL-2-independent pathway for PD-1-mediated inhibition. Furthermore, we show that interference with CD28 signalling is a major mechanism for the inhibitory function of PD-1, suggesting the reduction in division destiny and IL-2 production is a consequence of attenuated CD28 activity. These findings have uncovered a key mechanism for how PD-1 exerts its inhibitory function to control the proliferative potential of activated T cells.