National Human Genome Research Institute, Doris Duke Charitable Foundation, National Health Service Blood and Transplant, National Institute for Health Research, and Wellcome Trust.

Microbial molecules or cytokines can stimulate dendritic cell (DC) maturation, which involves DC migration to lymph nodes and enhanced presentation of Ag to launch T cell responses. Microbial Toll-like receptor (TLR) agonists are the most studied inducers of DC maturation, but type I interferon (IFN-I) also promotes DC maturation. In response to TLR stimulation, DC maturation involves a burst of Ag processing with enhanced expression of peptide-MHC-II complexes and co-stimulator molecules. Subsequently, MHC-II synthesis and expression in intracellular vacuolar compartments is inhibited, decreasing Ag processing function. This limits presentation to a cohort of Ags kinetically associated with the maturation stimulus and excludes presentation of Ags subsequently experienced by the DC. In contrast, our studies show that IFN-I enhances DC expression of MHC-II and co-stimulatory molecules without a concomitant inhibition of subsequent MHC-II synthesis and Ag processing. Expression of mRNA for MHC-II and the transcription factor CIITA is inhibited in DCs treated with TLR agonists but maintained in cells treated with IFN-I. Following stimulation with IFN-I, MHC-II expression is increased on the plasma membrane but is also maintained in intracellular vacuolar compartments, consistent with sustained Ag processing function. These findings suggest that IFN-I drives a distinctive DC maturation program that enhances Ag presentation to T cells without a shutdown of Ag processing, allowing continued sampling of Ags for presentation.

Dendritic cells (DCs) cross process exogenous Ags and present them by class I MHC (MHC-I) molecules to CD8+ T cells specific for Ags from viruses and bacteria such as Mycobacterium tuberculosis. Unmethylated CpG DNA signals through TLR9 to induce type I IFN (IFN-α/β), which enhances MHC-I Ag cross processing, but lipoproteins that signal through TLR2 do not induce IFN-α/β. In these studies we observed that M. tuberculosis, which expresses agonists of both TLR9 and TLR2, did not induce production of IFN-α/β or cross processing by murine DCs. Furthermore, M. tuberculosis and TLR2 agonists inhibited induction of IFN-α/β and DC cross processing by CpG DNA. Exogenous IFN-α/β effectively enhanced cross processing of M. bovis bacillus Calmette-Guérin expressing OVA, bypassing the inhibition of induction of endogenous IFN-α/β. In addition, inhibition of TLR9-induced cross processing of M. bovis bacillus Calmette-Guérin expressing OVA could be circumvented by pretreating cells with CpG DNA to induce IFN-α/β and MHC-I cross processing before inhibitory mycobacterial TLR2 agonists were present. Inhibition of the response to one TLR by another may affect the ultimate response to pathogens like M. tuberculosis that express agonists of multiple TLRs, including TLR2 and TLR9. This mechanism may contribute to immune evasion and explain why IFN-α/β provides little contribution to host immunity to M. tuberculosis. However, downregulation of certain TLR responses may benefit the host by preventing detrimental excessive inflammation that may occur in the presence of persistent infection.

Summary Macrophages and dendritic cells are phagocytic antigen presenting cells that internalize bacteria and other particulate antigens into phagosomes. The phagosome must then balance microbicidal and proteolytic degradation functions with the generation of antigenic peptides for presentation by class I and class II MHC molecules to CD8 and CD4 T cells, respectively. Understanding the host and bacterial factors that affect phagosomal antigen processing may help facilitate new strategies to eliminate pathogens.