The expansion and selection of T cell receptor alpha beta intestinal intraepithelial T cell clones

In conventional mice, the T cell receptor (TCR)alpha beta(-) CD8 alpha alpha(+) and CD8 alpha beta(-) subsets of the intestinal intraepithelial lymphocytes (IEL) constitute two subpopulations. Each comprise a few hundred clones expressing apparently random receptor repertoires which are different in individual genetically identical mice (Regnault, A., Cumano, A., Vassalli, P., Guy-Grand, D. and Kourilsky, P., J. Exp. Med. 1994. 180: 1345). We analyzed the repertoire diversity of sorted CD8 alpha alpha and CD8 alpha beta TCR alpha beta(-) IEL populations from the small intestine of individual germ-free mice that contain ten times less TCR alpha beta(+) T cells than conventional mice. The TCR beta repertoire of the CD8 alpha alpha and the CD8 alpha beta IEL populations of germ-free adult mice shows the same degree of oligoclonality as that of conventional mice. These results show that the intestinal microflora is not responsible for the repertoire oligoclonality of TCR alpha beta(+) IEL. The presence of the microflora leads to an expansion of clones which arise independently of bacteria. To evaluate the degree of expansion of IEL clones in conventional mice, we went on to measure their clone sizes in vivo by quantitative PCR in the total and in adjacent sections of the small intestine of adult animals. We found that both the CD8 alpha alpha and the CD8 alpha beta TCR alpha beta IEL clones have a heterogeneous size pattern, with clones containing from 3 x 10(3) cells up to 1.2 x 10(6) cells, the clones being qualitatively and quantitatively different in individual mice. Cells from a given IEL clone are not evenly distributed throughout the length of the small intestine. The observation that the TCR alpha beta IEL populations comprise a few hundred clones of very heterogeneous size and distribution suggests that they arise from a limited number of precursors, which may be slowly but continuously renewed, and undergo extensive clonal expansion in the epithelium.