Interplay of T-cell receptor and interleukin-2 signalling in Vγ2Vδ2 T-cell cytotoxicity

Human peripheral blood Vγ2Vδ2 T cells are important for host defence and tumour immunity. Their unusual T-cell receptor (TCR) recognizes small molecule phosphoantigens; stimulated cells produce inflammatory cytokines and are potently cytotoxic for a variety of tumours. However, molecular mechanisms linking phosphoantigen stimulation and cytotoxicity are incompletely understood. We know that isopentenyl pyrophosphate (IPP) activates mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/Erk) and phosphoinositide 3-kinase (PI-3K)/Akt pathways; specific inhibition of Erk or Akt significantly impairs the functional response to IPP. We now show that interleukin-2 also activates MEK/Erk and PI-3K/Akt pathways but on its own, fails to induce cytokine expression or cytotoxicity. Hence, MEK/Erk and PI-3K/Akt activation are necessary but not sufficient to induce effector responses in Vγ2Vδ2 T cells and a TCR-dependent signal is still required for tumour cell killing. Cyclosporin A, an inhibitor of calcineurin, blocked calcium-dependent nuclear translocation of nuclear factor of activated T cell (NFAT) and significantly reduced IPP-induced cytokine production, degranulation and cytotoxicity. The IPP-induced calcium mobilization and NFAT translocation were necessary to activate Vγ2Vδ2 effector functions; interleukin-2, acting on the MEK/Erk pathway, regulated the strength of these responses. The TCR has a specific role in Vγ2Vδ2 T-cell killing of tumour cells, which is distinct from its role in triggering cellular proliferation in response to phosphoantigens.