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From the Department of Immunology, The Scripps Research Institute, La Jolla, California 92037
Lipopolysaccharide (LPS) from gram-negative bacteria causes polyclonal activation of B cells
and stimulation of macrophages and other APC. We show here that, under in vivo conditions,
LPS also induces strong stimulation of T cells. As manifested by CD69 upregulation, LPS injection stimulates both CD4 and CD8+ T cells, and, at high doses, stimulates naive (CD44lo)
cells as well as memory (CD44hi) cells. However, in terms of cell division, the response of T cells after LPS injection is limited to the CD44hi subset of CD8+ cells. In contrast with B cells,
proliferative responses of CD44hi CD8+ cells require only very low doses of LPS (10 ng). Based
on studies with LPS-nonresponder and gene-knockout mice, LPS-induced proliferation of
CD44hi CD8+ cells appears to operate via an indirect pathway involving LPS stimulation of
APC and release of type I (
, 
) interferon (IFN-I). Similar selective stimulation of CD44hi
CD8+ cells occurs in viral infections and after injection of IFN-I, implying a common mechanism. Hence, intermittent exposure to pathogens (gram-negative bacteria and viruses) could
contribute to the high background proliferation of memory-phenotype CD8+ cells found in
normal animals.
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