Exploiting 4-1BB Costimulation for Enhancing Antiviral Vaccination

4-1BBL enhances CD8+ T cell responses induced by vectored vaccines in mice but fails to improve immunogenicity in rhesus macaques

T cells play a central role in the immune response to many of the world's major infectious diseases. In this study we investigated the tumour necrosis factor receptor superfamily costimulatory molecule, 4-1BBL (CD137L, TNFSF9), for its ability to increase T cell immunogenicity induced by a variety of recombinant vectored vaccines. To efficiently test this hypothesis, we assessed a number of promoters and developed a stable bi-cistronic vector expressing both the antigen and adjuvant. Co-expression of 4-1BBL, together with our model antigen TIP, was shown to increase the frequency of murine antigen-specific IFN-γ secreting CD8(+) T cells in three vector platforms examined. Enhancement of the response was not limited by co-expression with the antigen, as an increase in CD8(+) immunogenicity was also observed by co-administration of two vectors each expressing only the antigen or adjuvant. However, when this regimen was tested in non-human primates using a clinical malaria vaccine candidate, no adjuvant effect of 4-1BBL was observed limiting its potential use as a single adjuvant for translation into a clinical vaccine.

Role of Direct Effects of IFN-γ on T Cells in the Regulation of CD8 T Cell Homeostasis

It is well recognized that IFN-gamma plays a critical role in the control of CD8 T cell expansion and contraction during immune responses to several intracellular pathogens. However, our understanding of the mechanisms underlying the regulation of T cell fate by IFN-gamma is sorely incomplete. Specifically, it is unclear whether regulation of CD8 T cell homeostasis occurs by a T cell intrinsic IFN-gamma pathway. In this study, we have determined the role of the direct effects of IFN-gamma on T cells in regulating the expansion, contraction, and memory phases of the polyclonal CD8 T cell response to an acute viral infection. Using two complementary approaches we demonstrate that the direct effects of IFN-gamma suppress IL-7R expression on Ag-specific effector CD8 T cells, but clonal expansion or deletion of activated CD8 T cells in vivo can occur in the apparent absence of IFN-gammaR signaling in T cells. These findings have clarified fundamental features of control of T cell homeostasis by IFN-gamma in the context of CD8 T cell memory and protective immunity.