T cell receptor-triggered activation of intraepithelial lymphocytes in vitro

Intraepithelial lymphocytes (IEL) of the mouse small intestine were examined for their potential to respond to TCR signalling in vitro. Purified IEL subsets were activated using mAbs specific for CD3, TCRαβor TCRγ&. Thy-1⁺IEL, regardless of TCR type, proliferated equally well in response to anti-TCR mAb with or without exogenous IL-2. In contrast, Thy-1^- TCRαγ, CD8α^- IEL required exogenous IL-2 for proliferation. No such requirement was observed for Thy-1^- TCRγ& IEL proliferation. IEL proliferation in the absence of added IL-2 was due to an IL-2 secretion/IL-2 receptor (IL-2R) autocrine pathway, since mAbs specific for IL-2 and IL-2R inhibited IEL proliferation. Thy-1⁺ CD8β^- CD4⁺CD8⁺ IEL were unresponsive to TCR-induced proliferation but exhibited high levels of cytolytic activity upon TCR-triggerlng. Thy-1^- non-cytolytic IEL were induced to express Thy-1 and cytolytlc activity following activation in vitro. In addition, the involvement of the co-stimulatory molecule CD28 in IEL activation was tested. CD28 was weakly expressed by fresh IEL and anti-CD28 mAb had no effect on TCR-triggered proliferation. However, anti-TCR stimulation increased CD28 expression on a subset of TCRαβ IEL and the addition of anti-CD28 mAb resulted in increased IL-2 production, but not in increased proliferation. Our results indicate that IEL, including the purported extrathymlc CD8β^- subset, can respond to TCR-driven signals via proliferation and/or cytolytlc activity.