Cancer immunotherapy aims to promote the activity of cytotoxic T lymphocytes (CTLs) within a tumour, assist the priming of tumour-specific CTLs in lymphoid organs and establish efficient and durable antitumour immunity. During priming, help signals are relayed from CD4 T cells to CD8 T cells by specific dendritic cells to optimize the magnitude and quality of the CTL response. In this Review, we highlight the cellular dynamics and membrane receptors that mediate CD4 T cell help and the molecular mechanisms of the enhanced antitumour activity of CTLs. We outline how deficient CD4 T cell help reduces the response of CTLs and how maximizing CD4 T cell help can improve outcomes in cancer immunotherapy strategies.

The production of cytokines such as interferon-γ and interleukin 17 by αβ and γδ T cells influences the outcome of immune responses. Here we show that most γδ T lymphocytes expressed the tumor necrosis factor receptor family member CD27 and secreted interferon-γ, whereas interleukin 17 production was restricted to CD27- γδ T cells. In contrast to the apparent plasticity of αβ T cells, the cytokine profiles of these distinct γδ T cell subsets were essentially stable, even during infection. These phenotypes were established during thymic development, when CD27 functions as a regulator of the differentiation of γδ T cells at least in part by inducing expression of the lymphotoxin-β receptor and genes associated with trans-conditioning and interferon-γ production. Thus, the cytokine profiles of peripheral γδ T cells are predetermined mainly by a mechanism involving CD27.

The Traf-linked tumor necrosis factor receptor family member CD27 is known as a T cell costimulatory molecule. We generated CD27-/- mice and found that CD27 makes essential contributions to mature CD4+ and CD8+ T cell function: CD27 supported antigen-specific expansion (but not effector cell maturation) of naïve T cells, independent of the cell cycle-promoting activities of CD28 and interleukin 2. Primary CD4+ and CD8+ T cell responses to influenza virus were impaired in CD27-/- mice. Effects of deleting the gene encoding CD27 were most profound on T cell memory, reflected by delayed response kinetics and reduction of CD8+ virus-specific T cell numbers to the level seen in the primary response. This demonstrates the requirement for a costimulatory receptor in the generation of T cell memory.

The clinical benefit for patients with diverse types of metastatic cancers that has been observed upon blockade of the interaction between PD-1 and PD-L1 has highlighted the importance of this inhibitory axis in the suppression of tumour-specific T-cell responses. Notwithstanding the key role of PD-L1 expression by cells within the tumour micro-environment, our understanding of the regulation of the PD-L1 protein is limited. Here we identify, using a haploid genetic screen, CMTM6, a type-3 transmembrane protein of previously unknown function, as a regulator of the PD-L1 protein. Interference with CMTM6 expression results in impaired PD-L1 protein expression in all human tumour cell types tested and in primary human dendritic cells. Furthermore, through both a haploid genetic modifier screen in CMTM6-deficient cells and genetic complementation experiments, we demonstrate that this function is shared by its closest family member, CMTM4, but not by any of the other CMTM members tested. Notably, CMTM6 increases the PD-L1 protein pool without affecting PD-L1 (also known as CD274) transcription levels. Rather, we demonstrate that CMTM6 is present at the cell surface, associates with the PD-L1 protein, reduces its ubiquitination and increases PD-L1 protein half-life. Consistent with its role in PD-L1 protein regulation, CMTM6 enhances the ability of PD-L1-expressing tumour cells to inhibit T cells. Collectively, our data reveal that PD-L1 relies on CMTM6/4 to efficiently carry out its inhibitory function, and suggest potential new avenues to block this pathway.