Antiviral vaccines license T cell responses by suppressing granzyme B levels in human plasmacytoid dendritic cells

Human plasmacytoid dendritic cells (pDC) are important modulators of adaptive T cell responses during viral infections. Recently, we found that human pDC produce the serine protease granzyme B (GrB), thereby regulating T cell proliferation in a GrB-dependent manner. In this study, we demonstrate that intrinsic GrB production by pDC is significantly inhibited in vitro and in vivo by clinically used vaccines against viral infections such as tick-borne encephalitis. We show that pDC GrB levels inversely correlate with the proliferative response of coincubated T cells and that GrB suppression by a specific Ab or a GrB substrate inhibitor results in enhanced T cell proliferation, suggesting a predominant role of GrB in pDC-dependent T cell licensing. Functionally, we demonstrate that GrB(high) but not GrB(low) pDC transfer GrB to T cells and may degrade the ζ-chain of the TCR in a GrB-dependent fashion, thereby providing a possible explanation for the observed T cell suppression by GrB-expressing pDC. Modulation of pDC-derived GrB activity represents a previously unknown mechanism by which both antiviral and vaccine-induced T cell responses may be regulated in vivo. Our results provide novel insights into pDC biology during vaccinations and may contribute to an improvement of prophylactic and therapeutic vaccines.