Butyrophilin 3A/2A1-independent activation of human Vγ9Vδ2 γδ T cells by bacteria

The activation of human Vδ2 γδ T cells by phosphoantigens (pAg) strictly depends on transmembrane butyrophilin (BTN) molecules, specifically BTN3A isoforms and BTN2A1. Several bacteria, including <i>M. tuberculosis</i>, produce potent pAg and thus trigger a strong activation of Vδ2 T cells. The antigen-specific activation of CD4 and regulatory (Treg) T cells can be monitored by the rapid upregulation of CD154 and CD137, respectively. We have previously established that CD137 is also rapidly upregulated on Vδ2 T cells upon stimulation with pAg. In the present study, we have used antagonistic anti-BTN3A/2A1 antibodies to dissect the pAg-dependent and pAg-independent activation of Vδ2 T cells by various microbes. While the activation of Vδ2 T cells by pAg and aminobisphosphonate zoledronate was completely blocked by anti-BTN3A/2A1 antibodies, only partial inhibition was observed for activation with <i>M. tuberculosis</i> and other bacteria as analyzed by CD137/CD154 upregulation and intracellular interferon-γ expression. Similarly, anti-TCR antibody 7A5 and Lck inhibitor emodin had only a minimal inhibitory effect on activation by bacteria but strongly reduced pAg activation of Vδ2 T cells. Further studies revealed a crucial role of IL-18 in the BTN/TCR-independent early activation of Vδ2 T cells by bacteria. Neutralizing anti-IL-18 antibodies and inflammasome inhibition did not affect pAg activation of Vδ2 T cells but strongly reduced their activation by bacteria. Our results identify a BTN/TCR-independent but IL-18 and inflammasome-dependent activation pathway of Vδ2 T cells, which might be relevant for the role of Vδ2 T cells during bacterial infections.