Functional differences between PD-1+ and PD-1- CD4+ effector T cells in healthy donors and patients with glioblastoma multiforme

Immune checkpoint inhibitors targeting programmed cell death protein 1 (PD-1) have been highly successful in the treatment of cancer. While PD-1 expression has been widely investigated, its role in CD4(+) effector T cells in the setting of health and cancer remains unclear, particularly in the setting of glioblastoma multiforme (GBM), the most aggressive and common form of brain cancer. We examined the functional and molecular features of PD1 (+)CD4(+)CD25(-)CD127(+)Foxp3(-) effector cells in healthy subjects and in patients with GBM. In healthy subjects, we found that PD-1(+)CD4(+) effector cells are dysfunctional: they do not proliferate but can secrete large quantities of IFN gamma. Strikingly, blocking antibodies against PD-1 did not rescue proliferation. RNA-sequencing revealed features of exhaustion in PD1 (+) CD4 effectors. In the context of GBM, tumors were enriched in PD-1(+)CD4(+) effectors that were similarly dysfunctional and unable to proliferate. Furthermore, we found enrichment of PD-1(+)TIM-3(+)CD4(+) effectors in tumors, suggesting that co-blockade of PD-1 and TIM-3 in GBM may be therapeutically beneficial. RNA-sequencing of blood and tumors from GBM patients revealed distinct differences between CD4(+) effectors from both compartments with enrichment in multiple gene sets from tumor infiltrating PD-1(-)CD4(+) effectors cells. Enrichment of these gene sets in tumor suggests a more metabolically active cell state with signaling through other co-receptors. PD-1 expression on CD4 cells identifies a dysfunctional subset refractory to rescue with PD-1 blocking antibodies, suggesting that the influence of immune checkpoint inhibitors may involve recovery of function in the PD-1(-)CD4(+) T cell compartment. Additionally, co-blockade of PD-1 and TIM-3 in GBM may be therapeutically beneficial.