Interaction of lymphokine-activated killer cells with susceptible targets does not induce second messenger generation and cytolytic granule exocytosis

CTL activation by specific targets leads to a rapid rise of inositol phosphate (InsPs) and of cytoplasmic-free Ca²⁺ concentration ([Ca²⁺](i)). While these events are considered necessary to trigger granule secretion, Ca²⁺-independent cytolytic mechanisms have been recently proposed in addition or as an alternative to the classical Ca²⁺-dependent exocytosis model. We observed that lymphokine-activated killer (LAK) cells, obtained after stimulation with supraoptimal concentrations of IL-2 in short- or long-term cultures, kill susceptible targets in the absence of a [Ca²⁺](i) rise and InsP₃ formation. Moreover, LAK cell-mediated lysis was not associated with an increase in cytototxic granule exocytosis, as evaluated by BLT-esterase release into the culture supernatant. Furthermore, using an antigen-specific CTL clone, which acquires LAK-like activity when cultured in medium containing high IL-2 doses, second messenger generation and cytolytic granule content secretion were not detected during lysis of unrelated target cells, while killing of specific targets triggered both these processes. These findings suggest that two lytic pathways may coexist in the same effector cells: a second messenger-dependent pathway involving degranulation, which is activated after TCR interaction with specific targets, and another pathway, independent of any known second messenger generation, responsible for unrelated target cell lysis.