Interleukin-15 may be responsible for early activation of intestinal intraepithelial lymphocytes after oral infection with Listeria monocytogenes in rats

Exogenous interleukin-15 (IL-15) stimulates intestinal intraepithelial lymp hocytes (i-IEL) from mice to proliferate and produce gamma interferon (IFN- gamma) in vitro. To determine whether endogenous IL-15 is involved in activ ation of i-IEL during intestinal infection, we examined IL-15 synthesis by intestinal epithelial cells (i-EC) after infection with Listeria monocytoge nes in rats. In in vitro experiments, invasion oft. monocytogenes into IEC- 6 cells, a rat small intestine epithelial cell line, evidently induced IL-1 5 mRNA expression coincident with nuclear factor kappa B (NF-kappa B) activ ation, which is essential for IL-15 gene expression. IL-15 synthesis was de tected in rat i-EC on day 1 after an oral inoculation oft. monocytogenes in vivo. The numbers of T-cell receptor (TCR) gamma delta(+) T cells, NKR.P1( +) cells, and CD3(+) CD8(+) oar cells in i-IEL were significantly increased on day 1 after oral infection. The i-IEL from infected rats produced large r amounts of IFN-gamma upon stimulation,vith immobilized anti-TCR gamma del ta or anti-NKR.P1 monoclonal antibodies. These results suggest that IL-15 p roduced by i-EC may stimulate significant fractions of I-IEL to produce IFN -gamma at an early phase of oral infection with L. monocytogenes.