IL-12 is required for mTOR regulation of memory CTLs during viral infection

The induction of functional memory cytotoxic T lymphocytes (CTLs) is a major goal of vaccination against intracellular pathogens. Interleukin (IL)-12 is critical for the generation of memory CTLs, and inhibition of mammalian target of rapamycin (mTOR) by rapamycin can effectively enhance the memory CTL response. Yet, the role of IL-12 in mTOR's regulation of memory CTL is unknown. Here we hypothesized that the immunostimulatory effects of mTOR on memory CTLs requires IL-12 signaling. Our results revealed that rapamycin increased the generation of memory CTLs in vaccinia virus infection, and this enhancement was dependent upon the IL-12 signal. Furthermore, IL-12 receptor deficiency diminished the secondary expansion of rapamycin-regulated memory and resultant secondary memory CTLs were abolished. Rapamycin enhanced IL-12 signaling by upregulating IL-12 receptor β2 expression and signal transducer and activator of transcription factor 4 phosphorylation in CTLs during early infection. In addition, rapamycin continually suppressed T-bet expression in both wild-type and IL-12 receptor knockout CTLs. These results indicate an essential role for IL-12 in the regulation of memory CTLs by mTOR and highlight the importance of considering the interplay between cytokines and adjuvants during vaccine design.