Impaired calcium mobilization and differential tyrosine phosphorylation in intestinal intraepithelial lymphocytes

Intestinal intraepithelial lymphocytes (iIEL) exhibit a unique activation state characterized by the expression of activation markers and effector functions, but a minimal response to mitogenic signals in vitro. To further characterize this activation status, iIEL were compared with splenic T cells for two key activation signals, calcium mobilization and tyrosine phosphorylation. Calcium mobilization was impaired in iIEL treated with the calcium ionophores ionomycin or A23187, thapsigargin, or by CD3-cross-linking. The calcium mobilization defect is shared by mature and embryonic iIEL. Anti-phosphotyrosine Western blot analysis revealed that the iIEL are able to respond to T-cell receptor (TCR)-mediated signals by tyrosine phosphorylation, although the patterns of phosphorylation differ from those seen in splenic T cells. We conclude that iIEL are unable to mobilize calcium in vitro, which may be due to modulation of TCR-mediated signal transduction pathways by the microenvironment of the intestinal epithelium and/or caused by the standard isolation procedure used to prepare iIEL, which must be considered in future in vitro studies of iIEL function.