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Transfer of peripheral lymph node lymphocytes to SCID mice leads to the long term establishment of mucosal T lymphocytes within the epithelium and lamina propria of the small and large intestines. Analysis of engrafted intraepithelial lymphocytes (IEL) showed that they had acquired a surface phenotype that in several respects is typical of IEL. In addition, the functional profile of engrafted IEL derived from lymph node T cells was similar to that of normal IEL; as the donor-derived T cells exhibited a strong cytolytic activity, a poor proliferative response to mitogenic stimuli, and a tendency to home and expand specifically in the intestine upon transfer to secondary SCID recipients. Optimal engraftment of intestinal T cells required bacterial flora, as the number of lymphocytes was greatly reduced in SCID recipients with a reduced flora. These results demonstrate that mature, thymus-derived T cells can migrate to the intestine and become functionally specialized to the intestinal milieu. The acquisition of phenotypic markers characteristic of the intestinal microenvironment by engrafted cells suggests that T cell migration of lymphocytes to the SCID intestine is not aberrant, but it may reflect processes that are ongoing in immunocompetent mice. Furthermore, these data suggest that the homing and/or expansion of typical, thymus-derived T cells in the intestine may be driven by luminal Ags such as those derived from bacterial flora.

Transfer of specific T lymphocyte subsets isolated from the spleens of healthy donor mice into immunodeficient SCID mice leads to chronic intestinal inflammation with characteristics similar to those of human inflammatory bowel disease (IBD). CD4+, CD45RBhigh cells cause disease, whereas CD4+, CD45RBlow and CD8+, CD45RBhigh cells do not. Despite this difference, we demonstrate that all three T cell populations reconstitute the intraepithelial and lamina propria compartments of both small and large intestines of SCID recipients. Therefore, infiltration of lymphocytes alone is not sufficient for disease development. CD4+ lymphocytes that have trafficked to the SCID intestine exhibit a phenotype characteristic of normal mucosal lymphocytes. This includes high expression of alpha E integrin and CD69, expression of CD8 alpha alpha homodimers in some of the intraepithelial lymphocytes, as well as low expression of CD62L and CD45RB. The phenotype of the infiltrating mucosal cells is indistinguishable, with respect to the cell surface markers tested, regardless of whether the starting donor population is CD45RBhigh or CD45RBlow. Severe inflammation is restricted primarily to the colon despite lymphocyte infiltration throughout the length of the intestine. This suggests that some property of the colon microenvironment contributes to inflammation. Consistent with this, transfer of CD4+, CD45RBhigh cells to SCID mice that have significantly reduced numbers of enteric flora results in attenuation of the wasting and colitis. Fewer numbers of donor lymphocytes are recovered from the intraepithelial and lamina propria compartments of reduced flora SCID mice. We hypothesize that the ability of pathogenic cells to traffic to the intestine and mediate colitis may be driven by T cell reactivity to bacteria or bacterial products.

Neonatal treatment with a monoclonal antibody specific for the alpha beta TCR results in mice with a long term, severe depletion in the number of alpha beta T cells in the periphery. Significant numbers of T cells reappear in the periphery about age 65 days, but these cells tend to lack expression of CD4 or CD8. Splenocytes of antibody-treated mice are less sensitive to mitogen stimulation or stimulation with MHC allogeneic cells. The level of serum IgG but not IgM was decreased by the treatment. Anti-alpha beta TCR antibody treatment decreased single-positive T lymphocytes that express high levels of the CD3/alpha beta TCR complex from the thymus, suggesting that the treatment could act in part by affecting negative selection of alpha beta TCR+ thymocytes. This treatment does not, however, detectably affect either the homing or the numbers of gamma delta T cells which are abundant in the intestinal epithelium, but which remain a minor population in the spleen and lymph nodes. This supports the hypothesis that gamma delta T cells are developmentally autonomous from alpha beta T cells. These mice provide an excellent model system for assessing the developmental and functional role of gamma delta T lymphocytes in vivo.

Transfer of specific T lymphocyte subsets isolated from the spleens of healthy donor mice into immunodeficient SCID mice leads to chronic intestinal inflammation with characteristics similar to those of human inflammatory bowel disease (IBD). CD4+, CD45RBhigh cells cause disease, whereas CD4+, CD45RBlow and CD8+, CD45RBhigh cells do not. Despite this difference, we demonstrate that all three T cell populations reconstitute the intraepithelial and lamina propria compartments of both small and large intestines of SCID recipients. Therefore, infiltration of lymphocytes alone is not sufficient for disease development. CD4+ lymphocytes that have trafficked to the SCID intestine exhibit a phenotype characteristic of normal mucosal lymphocytes. This includes high expression of alpha E integrin and CD69, expression of CD8 alpha alpha homodimers in some of the intraepithelial lymphocytes, as well as low expression of CD62L and CD45RB. The phenotype of the infiltrating mucosal cells is indistinguishable, with respect to the cell surface markers tested, regardless of whether the starting donor population is CD45RBhigh or CD45RBlow. Severe inflammation is restricted primarily to the colon despite lymphocyte infiltration throughout the length of the intestine. This suggests that some property of the colon microenvironment contributes to inflammation. Consistent with this, transfer of CD4+, CD45RBhigh cells to SCID mice that have significantly reduced numbers of enteric flora results in attenuation of the wasting and colitis. Fewer numbers of donor lymphocytes are recovered from the intraepithelial and lamina propria compartments of reduced flora SCID mice. We hypothesize that the ability of pathogenic cells to traffic to the intestine and mediate colitis may be driven by T cell reactivity to bacteria or bacterial products.

Compared with T lymphocytes from other organs, intestinal intraepithelial lymphocytes (IEL) proliferate weakly in response to CD3/TCR ligation, and they do not respond at all to treatment with other mitogenic stimuli. These signals also failed to induce expression of the IL-2R alpha-chain on the surface of most IEL. IEL from germ-free mice, from V gamma 1.1-transgenic mice, and from beta 2-microglobulin-deficient mice also gave a weak proliferative response. Therefore, the low proliferative response is not linked to the level of exposure to gut bacterial flora, the V gamma region expressed by the TCR-gamma delta + IEL, or the presence of class I molecules that may be recognized by CD8+ IEL. The relatively small amount of proliferation in response to TCR signaling, therefore, is not likely to be the result of induction of anergy caused by previous contact with Ag. In contrast, ligation of the CD3/TCR complex could elicit a rapid cytotoxic response and serine esterase release by IEL. The unusual functional capabilities and the activation state of IEL are independent of the TCR isotype expressed by these cells. Freshly isolated IEL have a high intracellular microtubule-associated protein kinase-2 (MAP-2K) activity level, further suggesting that these cells are activated despite their weak proliferative response. Consistent with this, MAP-2K is tyrosine-phosphorylated in both untreated and PMA-treated IEL. In contrast, MAP-2K activation and tyrosine phosphorylation occur in other T cells only when they are activated by PMA or other treatments. MAP-2K activity also is elevated in IEL from germ-free mice, demonstrating that activation does not depend on normal levels of exposure to bacterial flora. The activation of protein kinases such as MAP-2K could reflect the differentiation state of IEL or Ag receptor stimulation of some of these cells by epithelial cells in the preparation.

Neonatal treatment with a monoclonal antibody specific for the alpha beta TCR results in mice with a long term, severe depletion in the number of alpha beta T cells in the periphery. Significant numbers of T cells reappear in the periphery about age 65 days, but these cells tend to lack expression of CD4 or CD8. Splenocytes of antibody-treated mice are less sensitive to mitogen stimulation or stimulation with MHC allogeneic cells. The level of serum IgG but not IgM was decreased by the treatment. Anti-alpha beta TCR antibody treatment decreased single-positive T lymphocytes that express high levels of the CD3/alpha beta TCR complex from the thymus, suggesting that the treatment could act in part by affecting negative selection of alpha beta TCR+ thymocytes. This treatment does not, however, detectably affect either the homing or the numbers of gamma delta T cells which are abundant in the intestinal epithelium, but which remain a minor population in the spleen and lymph nodes. This supports the hypothesis that gamma delta T cells are developmentally autonomous from alpha beta T cells. These mice provide an excellent model system for assessing the developmental and functional role of gamma delta T lymphocytes in vivo.