90 Caspase inhibitor A major component of IFN-α/β-driven antiviral properties is the marked induction of

genes involved in antigen processing and presentation, particularly expression of class I genes and associated endocytic proteins involved in proteolysis and peptide loading. By engaging this pathway in an in vivo model of antigen cross-priming, Tough and colleagues91,92 demonstrated that IFN-α/β enhanced CD8+ T-cell expansion as well as cytolytic activity, which may explain the strong adjuvant effect of IFN-α/β on protein vaccination strategies. While the individual roles of IL-12 and IFN-α/β can be assessed in isolation in vitro, in vivo studies have revealed unique roles for IFN-α/β and IL-12 that depend upon priming conditions and the class of pathogen. Initial studies demonstrated that

the induction of IFN-α/β by CpG stimulation led to antigen-presenting cell-dependent T-cell proliferation, which required IFN-α/β signalling within the responding T cells.93 These early studies did not directly compare IFN-α/β with the powerful inflammatory effects of IL-12. However, comparing primary CD8+ responses with various pathogens, Murali-Krishna and colleagues94 demonstrated that IFN-α/β signals were required for CD8+ expansion in response to lymphocytic choriomeningitis virus (LCMV), but less so in response to vaccinia virus or Listeria monocytogenes infections.44 Based on this observation, it was postulated that antigenic load may contribute to CD8+ dependence on IFN-α/β for full expansion, as LCMV viral titres are much GSK 3 inhibitor higher during the peak of the infection than vaccinia virus titres. Furthermore, a recent study demonstrated GNAT2 that CD8+ responses to Trypanosoma cruzi were completely independent of IFN-α/β signalling.95 This is somewhat surprising given the dependence on IFN-α/β during cross-priming reported by Tough and colleagues. Nonetheless, all of these reports highlight the potential for IL-12 and IFN-α/β to significantly regulate CD8+ effector

responses, which were originally reported to be IL-12- and STAT4-independent. Interleukin-12 and IFN-α/β may also play distinct roles in regulating CD8+ T-cell memory development. First, although IL-12 has been reported to play a positive role in generating CD8+ effector cells, it seems to have an inverse role in generating memory cells. Pearce et al.96 recently demonstrated that the kinetics and magnitude of the CD8+ memory response to L. monocytogenes were significantly enhanced in IL-12Rβ2−/− cells. This observation correlated with enhanced CD8+ memory in T-bet knockout mice, as IL-12 has been reported to positively regulate T-bet expression.97,98 Moreover, as cells expand in response to antigen stimulation, the enhanced expression of T-bet driven by IL-12 generates populations of terminally differentiated cytotoxic effector cells.