Essential Requirements of Zoledronate-Induced Cytokine and gamma delta T Cell Proliferative Responses

The potent nitrogen-containing bisphosphonate zoledronate inhibits farnesyl pyrophosphate synthase, a key enzyme of the mevalonate pathway that is often hyperactive in malignant cells. Zoledronate activates human V gamma 9V delta 2 T cells, which are immune sentinels of cell stress and tumors, through upstream accumulation of the cognate Ag isopentenyl pyrophosphate. IL-18 was shown to enhance zoledronate-induced gamma delta T cell activation. Although monocytes have been considered important accessory cells that provide the Ag isopentenyl pyrophosphate, CD56(bright)CD11c(+) NK cells were postulated to mediate the costimulatory effects of IL-18. We report in this article that downstream depletion of geranylgeranyl pyrophosphate (GGPP), which is required for protein prenylation, caused cell stress in monocytes, followed by caspase-1-mediated maturation and release of IL-18, which, in turn, induced gamma delta T cell CCL2. Likewise, zoledronate caused a substantial delay in gamma delta T cell expansion, which could be skipped by GGPP supplementation. Moreover, repletion of GGPP, which prevented acute zoledronate toxicity, and supplementation with IL-18, which strongly upregulated IL-2R alpha (CD25) and favored the central memory phenotype, were sufficient to enable zoledronate-induced expansion of highly purified gamma delta T cells, even when starting cell numbers were as low as 10(4) gamma delta T cells. Our study reveals essential components of gamma delta T cell activation and indicates that exogenous IL-18, which can directly costimulate gamma delta T cells, eliminates the need for any accessory cells. Our findings will facilitate the generation of robust gamma delta T cells from small blood or tissue samples for cancer immunotherapy and immune-monitoring purposes.