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.