Influence of beta 2-microglobulin expression on gamma interferon secretion and target cell lysis by intraepithelial lymphocytes during intestinal Listeria monocytogenes infection

Numerous microbial pathogens, including Listeria monocytogenes, enter the host through the intestine. Although relatively little is known about the biological functions of intestinal intraepithelial lymphocytes (i-IEL), they are generally considered a first line of defense against intestinal infections. In the mouse, the vast majority of i-IEL express the CD8 coreceptor either as a CD8 alpha/alpha homodimer or as a CD8 alpha/beta heterodimer. The CD8 receptor of T-cell receptor TcR gamma/delta i-IEL is exclusively homodimeric, whereas the CD8-expressing TcR alpha/beta i-IEL segregate into equal fractions of CD8 alpha/alpha and CD8 alpha/beta cells. We infected beta 2-microglobulin (beta 2m)+/- mice (possessing all i-IEL populations) and beta 2m -/- mutant mice (lacking all CD8 alpha/beta + i-IEL and having few CD8 alpha/alpha + TcR alpha/beta i-IEL) with L. monocytogenes per os and determined their biological functions after TcR ligation with monoclonal antibodies. Cytolytic activities of TcR alpha/beta and TcR gamma/delta i-IEL from beta 2m +/- mice were not influenced by intestinal listeriosis. Cytolytic activities of TcR alpha/beta i-IEL were impaired in uninfected beta 2m -/- mice, but this reduction was reestablished as a consequence of intestinal listeriosis. Frequencies of gamma interferon (IFN-gamma)-producing TcR alpha/beta i-IEL in uninfected beta 2m -/- mice were reduced, compared with that in their heterozygous controls. Equally low frequencies of IFN-gamma-producing TcR gamma/delta i-IEL in beta 2M +/- and beta 2m-/- mutants were found. Listeriosis increased frequencies of INF-gamma-producing TcR alpha/beta and TcR gamma/delta i-IEL in both mouse strains. Most remarkably, the proportion of IFN-gamma-producing TcR gamma/delta i-IEL was elevated 10-fold in listeria-infected beta 2M -/- mice. Our findings show that the beta 2m-independent CD8 beta- i-IEL expressing either TcR alpha/beta or TcR gamma/delta are stimulated by intestinal listeriosis independent of regional beta 2m expression. We conclude that the three major CD8+ i-IEL populations are stimulated by intestinal listeriosis and that CD8 beta- i-IEL compensate for the total lack of CD8 beta+ i-IEL in beta 2M -/- mutant mice. Hence, in contrast to the peripheral immune system, which crucially depends on CD8 alpha/beta + TcR alpha/beta lymphocytes, the mucosal immune system can rely on additional lymphocytes expressing the CD8 alpha/alpha homodimer.