Bystander activation of CD4+ T cells

Bystander activation of T cells, i.e. the stimulation of unrelated (heterologous) T cells by cytokines during an Ag‐specific T‐cell response, has been best described for CD8⁺ T cells. In the CD8⁺ compartment, the release of IFN and IFN‐inducers leads to the production of IL‐15, which mediates the proliferation of CD8⁺ T cells, notably memory‐phenotype CD8⁺ T cells. CD4⁺ T cells also undergo bystander activation, however, the signals inducing this Ag‐nonspecific stimulation of CD4⁺ T cells are less well known. A study in this issue of the European Journal of Immunology sheds light on this aspect, suggesting that common γ‐chain cytokines including IL‐2 might be involved in bystander activation of CD4⁺ T cells.