Th2-mediated anti-tumour immunity: Friend or foe?

The concept that the immune system can recognise tumour cells and either eliminate them (tumour immune surveillance) or select for immunologically resistant variants (immunoediting) is gaining general acceptance by immunologists. In terms of an adaptive immune response to cancer, however, much of the research has focused on the response of cytotoxic CD8⁺ T lymphocytes to tumour-specific antigens and the production of Th1 cytokines by CD4⁺ and CD8⁺ T cells. In contrast, Th2-mediated immunity has traditionally been viewed as favouring tumour growth, both by promoting angiogenesis and by inhibiting cell-mediated immunity and subsequent tumour cell killing. While there is evidence that components of type 2 inflammation, such as B cells and interleukin-10, do promote tumour growth, there are also many studies demonstrating the anti-tumour activity of CD4⁺ Th2 cells, particularly in collaboration with tumour-infiltrating granulocytes, such as eosinophils. In this review, we examine all the components of type 2 immunity and their effects on tumour growth. Collectively, from this analysis, we conclude that there is a great potential for the development of Th2-mediated immunotherapies that harness the cytotoxic activity of eosinophils.