CD4+ T cell help is required for the generation of CD8+ cytotoxic T lymphocyte (CTL) memory. Here, we use genome-wide analyses to show how CD4+ T cell help delivered during priming promotes memory differentiation of CTLs. Help signals enhance IL-15-dependent maintenance of central memory T (TCM) cells. More importantly, help signals regulate the size and function of the effector memory T (TEM) cell pool. Helped TEM cells produce Granzyme B and IFNγ upon antigen-independent, innate-like recall by IL-12 and IL-18. In addition, helped memory CTLs express the effector program characteristic of helped primary CTLs upon recall with MHC class I-restricted antigens, likely due to epigenetic imprinting and sustained mRNA expression of effector genes. Our data thus indicate that during priming, CD4+ T cell help optimizes CTL memory by creating TEM cells with innate and help-independent antigen-specific recall capacities.

Persistent antigen exposure in chronic infection and cancer has been proposed to lead to cytotoxic T lymphocyte (CTL) “exhaustion”, i.e., loss of effector function and disease control. Recent work identifies a population of poorly differentiated TCF-1 + PD-1 + CD8 + T cells as precursors of the terminally exhausted CTL pool. These “predysfunctional” CTLs are suggested to respond to PD-1 targeted therapy by giving rise to a pool of functional CTLs. Supported by gene expression analyses, we present a model in which lack of CD4 + T cell help during CD8 + T cell priming results in the formation of predysfunctional CTLs. Our model implies that predysfunctional CTLs are formed during priming and that the remedy for CTL dysfunction is to provide “help” signals for generation of optimal CTL effectors. We substantiate that this may be achieved by engaging CD4 + T cells in new CD8 + T cell priming, or by combined PD-1 blocking and CD27 agonism with available immunotherapeutic antibodies.

Immunotherapy targeting the Programmed Death (PD‐1) receptor/ligand (L) “checkpoint” rapidly gains ground in the treatment of many cancer types. To increase treatment scope and efficacy, predictive biomarkers and rational selection of co‐treatments are required. To meet these demands, we must understand PD‐1 function in detail. We here outline recent insights into the regulation of the CD8+ T cell response by PD‐1. The prevailing view has been that blockade of PD‐1/ligand (L) interaction “reinvigorates” cytotoxic T lymphocytes (CTL) that were rendered dysfunctional in the tumor microenvironment (TME). However, this review stresses that tumors continuously communicate with adjacent draining lymph nodes (LNs) and that the PD‐1 checkpoint also operates during T cell priming. We clarify the role of the PD‐(L)1 system at the T cell/DC interface, where it regulates T cell receptor (TCR) signaling and CD28 costimulation and thus controls activation of tumor‐specific T cells. We also highlight the importance of CD4+ T cell help during priming, which allows DCs to provide other costimulatory and cytokine signals required for optimal CTL differentiation and likely avoidance of a dysfunctional state. Therefore, we pose that PD‐(L)1 blockade should exploit LN function and be combined with “help” signals to optimize CTL efficacy.