Development of TCRαβ CD8αα intestinal intraepithelial lymphocytes is promoted by interleukin-15-producing epithelial cells constitutively stimulated by gram-negative bacteria via TLR4

The microbes present in the intestine have a strong influence on the development and maturation of lymphoid organs. The cross-talk mechanisms between intestinal intraepithelial lymphocytes (i-IEL) and noninvasive microbes are still poorly understood. The influence of microbes and lipopolysaccharides on the development of i-IEL, especially the TCRalphabeta(+) CD8alphaalpha subset, was investigated using the different TLR4-mutant mouse strains C3H/HeJ, BALB/IpS(d), and C57BL/10ScCr. Intestinal epithelial cells (i-EC) from TLR4-mutant strains did not express interleukin (IL)-15 mRNA, while IL-15 mRNA expression in i-EC from the corresponding wild-type, C3H/He, BALB/c, and C57BL/10ScSn mice was detected. The development of TCRalphabeta(+) CD8aa cells in WEL significantly decreased in TLR4-mutant mice compared with the corresponding wild-type mice, while other T cell subsets in i-IEL showed similar percentages in the TLR4-mutant and wild-type mice. Adult thymectomized (ATx-) and lethally irradiated C3H/HeJ mice reconstituted with T cell-depleted bone marrow cells from C3H/He mice showed a significantly lower percentage of TCRalphabeta CD8alphaalpha i-IEL than ATx-C3H/He mice after transfer of C3H/HeJ BM cells. The percentage of TCRalphabeta CD8alphaalpha HEL and IL-15 mRNA expression in i-EC from BALB/IpSd mice did not increase during Salmonella typhimurium infection but was significantly enhanced during Listeria monocytogenes infection. Our findings suggest that LPS induces IL-15 production by i-EC, resulting in the development of TCRalphabeta CD8alphaalpha WEL.