The opposing roles of CD4⁺ T cells in anti-tumour immunity

Cancer immunotherapy focuses mainly on anti-tumour activity of CD8⁺ cytotoxic T lymphocytes (CTLs). CTLs can directly kill all tumour cell types, provided they carry recognizable antigens. However, CD4⁺ T cells also play important roles in anti-tumour immunity. CD4⁺ T cells can either suppress or promote the anti-tumour CTL response, either in secondary lymphoid organs or in the tumour. In this review, we highlight opposing mechanisms of conventional and regulatory T cells at both sites. We outline how current cancer immunotherapy strategies affect both subsets and how selective modulation of each subset is important to maximize the clinical response of cancer patients.