Plasmacytoid DCs
Plasmacytoid DCs (PDCs) are a unique subset of DCs, but what is their role and how do they differ from other DCs? From previous pages you will have learnt about properties shared by all APCs, in particular that professional APCs are continually surveying their environment for foreign antigens. They recognise foreign PAMPs via PRR and then directly or indirectly clear the pathogen.
In the case of PDCs, the specific PRRs are TLR 7 and 9. These TLRs allow PDCs to specifically recognise viruses via CpG-rich DNA motifs and double-stranded-RNA. When this happens, a signalling pathway triggers a huge release of type-1 interferons (IFN), in particular IFN-α. This causes recruitment of macrophages and promotes their phagocytosis at the site of infection helping to clear the pathogen. Other cytokines are also released such as IL-6 and IL-12 causing other immune cells to infiltrate the site of infection.
PDCs only take on dendritic cell properties after activation. Before activation, PDCs appear circular and have poor antigen presenting capabilities. But once activated, PDCs become dendritic in appearance, and MHC and costimulatory molecules are upregulated allowing them to present antigen to T cells, but mainly within the site of infection. Unlike other DCs, PDCs do not lose their ability to make new MHC class II-peptide complexes after activation and they also have less migratory capacity. Why might this be important? Perhaps to allow continual but short-term presentation of endogenous viral antigens to T cells entering the site of infection in contrast to longer, more stable antigen presentation within the lymph nodes.
The primary function of PDCs is not antigen presentation, but being able to recognise and respond to viruses by releasing cytokines to recruit innate and adaptive cells to the tissue; their secondary job is antigen presentation at the site of infection and inflammation within the tissue. This makes PDCs important mediators of both the innate and adaptive immune responses.