Regulation of Human Cytotoxic T Lymphocyte Functions by IL-12

CD8+ T cells or cytotoxic T lymphocytes (CTLs) play a major role in our defense against intracellular pathogens by secreting effector cytokines and directly killing infected cells. Upon sensing of pathogen, antigen presenting cells secrete innate cytokines such as IL-12 and IFN-α. CTL functions are specified by both antigen recognition and innate cytokines yielding a diverse population comprised of short-term effectors and long-lived memory cells. We have demonstrated that IL-12, in particular, programs effector CTL differentiation in human. Our objective was to elucidate the key pathways programmed by IL-12 that leads to effector function. Using whole transcriptome analyses, we have identified a serine/threonine protein kinase MAP3K8 or Tpl2 to be selectively induced by IL-12. Furthermore, the functionally identified effector memory CTL population expressed higher levels of MAP3K8 mRNA ex vivo compared to the naïve/central memory CTL population. MAP3K8 or Tpl2 has been shown to play an important role in activating the MAP kinase pathways in innate immune cells such as macrophages. MAP kinase pathways are three-tiered kinase cascades that coordinate many different cellular responses by relaying the instructions from extracellular signals. Using specific small molecule inhibitors, we have demonstrated that MAP3K8/Tpl2 in human CTLs functions upstream of MEK/ERK MAP kinase pathway, leading to effector functions. An inhibitor for MAP3K8/Tpl2 blocked IFN-γ and TNF-α secretion as well as cytolysis of target cells in a dose-dependent manner. However, MAP3K8/Tpl2 deficient murine CTLs did not exhibit any functional deficiency either in vitro or in vivo. Furthermore, we have found that the Tpl2 inhibitor did not block IFN-γ and TNF-α secretion from murine effector CTLs. In summary, we found that IL-12, not IFN-α, programs effector function in human CTLs at the genetic level. Additionally, MAP3K8/Tpl2, a member of the MAP kinase pathway, is regulated by IL-12 during CTL priming in human and is a critical regulator of antigen receptor-mediated effector functions. Taken together, we have discovered a species-specific role for IL-12 regulated MAP3K8/Tpl2 in effector function of human CTLs, which plays a major role in adaptive immune responses to intracellular pathogens and tumors.