To understand the full impact of checkpoint inhibitors on T cell functions and antitumor responses, Strazza et al. investigated the effects of PD-1 signaling in circulating CD4+ T cells. Adding PD-1 ligation to TCR stimulation increased the proportions of both highly proliferative and non-proliferative populations, which were detected, sorted, and subjected to single-cell RNA sequencing. In spite of PD-1 ligation, proliferating CD4+ T cells had a distinct activated transcriptional signature, whereas the non-proliferative population contained an increased number of follicular helper T cells that were more abundant in melanoma patients responsive to PD-1 blockade.
Contributed by Katherine Turner
ABSTRACT: Ligation of the inhibitory receptor PD-1 on T cells results in the inhibition of numerous cellular functions. Despite the overtly inhibitory outcome of PD-1 signaling there are additionally a collection of functions that are activated. We have observed that CD4(+) T cells stimulated through the T cell receptor and PD-1 primarily do not proliferate, however, there is a population of cells that proliferates more than T cell receptor stimulation alone. These highly proliferating cells could potentially be associated with PD-1-blockade unresponsiveness in patients. In this study, we have performed RNA sequencing and found that following PD-1 ligation proliferating and non-proliferating T cells have distinct transcriptional signatures. Remarkably, the proliferating cells showed an enrichment of genes associated with an activated state despite PD-1 signaling. Additionally, circulating follicular helper T cells were significantly more prevalent in the non-proliferating population, demonstrated by enrichment of the associated genes CXCR5, CCR7, TCF7, BCL6, and PRDM1 and validated at the protein level. Translationally, we also show that there are more follicular helper T cells in patients that respond favorably to PD-1 blockade. Overall, the presence of transcriptionally and functionally distinct T cell populations responsive to PD-1 ligation may provide insights into the clinical differences observed following therapeutic PD-1 blockade.