Involvement of FK506-sensitive and insensitive granule exocytosis pathways in perforin-dependent target cell lysis mediated by a CD8 + CTL clone

Abstract

The release of granzyme A and B through granule exocytosis by CD8 + CTL clone OE4 upon T cell receptor (TCR) activation was blocked by FK506 in a dose-dependent manner (IC 50=3 nM), whereas a significant granzyme release was still detectable even in the presence of excess FK506. In contrast, the production of IFN-γ was highly sensitive to FK506 (IC 50=0.01 nM) and could be completely blocked by FK506. Both FK506-sensitive and insensitive granule exocytosis pathways were involved in the actual perforin-dependent killing toward different target cells. The combination of ionomycin and phorbol ester was able to mimic TCR stimulation to induce IFN-γ production, although the same treatment triggered granule exocytosis inefficiently. Granule exocytosis and IFN-γ production following TCR activation were profoundly prevented by calphostin C. Thus, these results demonstrate that the granule exocytosis pathway in this CD8 + CTL clone depends on the activation of protein kinase C, and requires either calcineurin-dependent or independent additional signals downstream of TCR activation.