Lamina propria (LP) T cells of the human intestinal mucosa usually do not develop systemic immune responses despite permanent exposure to foreign antigens. The mechanisms maintaining this hyporeactivity in the normal gut are poorly understood. It is, at present, not clear what role the microenvironment of the mucosa plays for low T cell reactivity and in the pathogenesis of mucosal inflammation. Despite the importance of cytosolic Ca 2+ signals for T lymphocyte activation, intracellular Ca 2+ concentration measurements have so far only been performed in dissociated T cells, following disruption of the microenvironment. We used two-photon technology to measure Ca 2+ signals in identified T lymphocytes within the intact mucosa to minimize impact on tissue integrity while preserving the cellular microenvironment. We show that Ca 2+ signals in LP T cells correlate with the hyporeactivity of T cells in the intestinal immune system and furthermore link Ca 2+ signals with inflammatory bowel disease. Our data implicate that Ca 2+ signals in LP T cells do not depend on the microenvironment of the intact mucosa, since they are very similar to Ca 2+ signals in dissociated LP T cells.

Ca 2؉ entry across the plasma membrane is necessary for the activation and proliferation of T-lymphocytes. Human intestinal lamina propria lymphocytes physiologically exhibit minimal proliferation in response to antigen receptor stimulation when compared with peripheral blood T-lymphocytes. This hyporeactivity is partially abolished in inflammatory bowel disease. We hypothesized that differences in Ca 2؉ signaling could be related to the disease. To test this possibility, we measured Ca 2؉ signals in identified lymphocytes from human blood and human intestinal mucosa. Ca 2؉ signals in lamina propria T-lymphocytes from non-inflamed tissue were drastically reduced when compared with Ca 2؉ signals of blood T-lymphocytes from the same persons. However, Ca 2؉ signals in T-lymphocytes from inflamed intestinal mucosa were much higher than the ones from non-inflamed mucosa and almost reached levels of Ca 2؉ signals in peripheral blood T-cells. Furthermore, Ca 2؉ influx was closely linked to cell proliferation in both peripheral blood T-lymphocytes and lamina propria lymphocytes cells. We conclude that differences in Ca 2؉ signaling can explain the differences of T-lymphocyte reactivity in blood versus lamina propria and, importantly, also between T-lymphocytes from inflamed and non-inflamed intestinal mucosa. Ca 2؉ channels in the plasma membrane of T-lymphocytes might thus prove an excellent target to screen for immunosuppressiva to potentially treat the symptoms of inflammatory bowel disease.

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