Dendritic cell maturation and death during Salmonella infection. Role of pro-inflammatory cytokines and MyD88

Abstract

The costimulatory molecules CD80 and CD86 are required for the ability of dendritic cells (DC) to induce both tolerance and immunity. This thesis investigates the control of CD80/CD86 upregulation in vivo on DC during Salmonella infection.

After oral Salmonella infection, DC in Peyer´s patches (PP), mesenteric lymph nodes (MLN) and spleen upregulated costimulatory molecules almost simultaneously despite differential seeding of these organs with bacteria. Costimulatory molecules were also induced on TNF/iNOS-producing CD11cintCD11b+ DC that accumulated in infected organs. The CD11cintCD11b+ DC were efficient at bacterial uptake but, in contrast to conventional DC, failed to process and present Salmonella Ag on MHC-II.

Using different gene-deficient mice, the pathways controlling CD80/86 upregulation on DC during Salmonella infection were dissected. Upregulation of CD80 was strictly dependent on the Toll-like receptor adaptor MyD88, whereas upregulation of CD86 was mediated by both MyD88-dependent and -independent factors. The pro-inflammatory cytokine TNF was identified as one MyD88-dependent factor required for optimal upregulation of CD80/86 in the MLN. In the absence of MyD88, upregulation of CD86 was mediated by type I interferons. However, the contribution of type I interferons to CD86 upregulation in wild type mice is only marginal, since mice lacking the type I interferon receptor (IFN-αβR) showed no major defects in CD80/86 upregulation. Despite the abrogated upregulation of CD80/86 on DC of TNFR1-/-, MyD88-/- or MyD88-/-IFN-αβR-/- mice, DC directly associated with bacteria upregulated costimulatory molecules independently of these factors.

Pro-inflammatory signaling not only upregulated costimulatory molecules on DC during Salmonella infection, but also mediated DC death. Thus, MyD88-dependent production of TNF induced DC death in Salmonella-infected mice. CD8α+ DC were most susceptible to infection-induced cell death as assessed directly ex vivo by Annexin-V and 7AAD staining, whereas recruited CD11cintCD11b+ DC were completely resistant.

Thus, the inflammatory environment imprints a distinct pattern of costimulatory molecules on DC, with MyD88-dependent factors controlling the upregulation of CD80. However, MyD88-dependent factors also induce DC death during Salmonella infection, which is likely to have a negative impact on anti-bacterial immunity.