Bystander stimulation of T cells in vivo by cytokines

Immune responses to infectious agents, especially viruses, are often associated with extensive proliferation of T cells and transient enlargement of the lymphoid tissues. Since the precursor frequency of T cells for specific antigen is low, the bulk of the T cells proliferating in the primary response are presumably stimulated via non-antigen-specific mechanisms, e.g. via cytokines elicited by the infectious agent concerned. Such 'bystander' stimulation of T cells occurs in mice injected with agents that elicit production of type I interferon (IFN I). Induction of IFN I in vivo causes marked stimulation of the CD44hi subset of CD8+ T cells and is prominent after injection of live viruses or products of bacteria such as lipopolysaccharide. Cytokines elicited by infectious agents may act as adjuvants during the primary response and could serve to boost the survival of long-lived memory cells.