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Plasmacytoid dendritic cells (pDCs) are a rare type of immune cell that are known to secrete large quantities of type 1 interferon (IFNs) in response to a viral infection. They constitute 0.3-0.5% of peripheral blood mononuclear cells and are found in the blood and lymphoid organs after completing development in the bone marrow. Other than conducting antiviral mechanisms, pDCs are considered to be key in linking the innate and adaptive immune systems. However, pDCs are also responsible for participating in and exacerbating certain autoimmune diseases like lupus.

Development and Characteristics
In the bone marrow, common dendritic cell progenitors expressing Flt3 (CD135) receptors are able to give rise to pDCs. Flt3 or CD135 signaling induces differentiation and proliferation of pDCs, although their mechanisms are not entirely understood. Phosphoinositide 3-kinase (PI3K)-dependent activation of rapamycin (mTOR) is believed to regulate this signaling pathway. Transcription factor E2-2 has also been found to play a key role in influencing the lineage commitment of a common DC progenitor on its course to becoming a pDC.

Unlike conventional dendritic cells (cDCs) that leave the bone marrow as precursors, pDCs leave the bone barrow to go to the lymphoid organs and peripheral blood upon completing development. Plasmacytoid dendritic cells are also distinguished from cDCs because of their ability to produce significant amounts of type-1 interferon. pDC maturation is initiated when the cell comes in contact with a virus, prompting the upregulation of MHC class I and MHC class II, co-stimulatory molecules CD80, CD86, CD83, and c-c chemokine receptor 7 (CCR7) and interferon production gradually decreases. CCR7 expression prompts the matured pDC to migrate to a lymph node where it will be able to stimulate and interact with T cells.

Plasmacytoid dendritic cells are also characterized by their expression of intracellular Toll-like receptors, TLR7 and TLR9, which interact with viral and host nucleic acids. ILT7 and BDCA-4 are also expressed on human pDC surfaces, although their signaling pathways are still obscure. However, there are speculations that the interaction between ILT7 and BST2 may have a negative regulatory effect on the cell’s interferon production. Unlike myeloid dendritic cells, myeloid antigens like CD11b, CD11c, CD13, CD14 and CD33 are not present on pDC surfaces. Furthermore, pDCs express markers CD123, CD303 (BDCA-2) and CD304 unlike other dendritic cell types.

Role in Immunity
Upon stimulation and subsequent activation of TLR7 and TLR9, these cells produce large amounts (up to 1,000 times more than other cell type) of type I interferon (mainly IFN-α and IFN-β), which are critical anti-viral compounds mediating a wide range of effects and induce maturation of the pDC. For example, the secretion of type 1 interferon triggers natural killer cells to produce IFNγ while also activating the differentiation of B cells. In addition, they can produce cytokines  IL-12, IL-6 and TNF-α as well, helping to recruit other immune cells to the site of infection.

Because they are capable of activating other immune cells, pDCs serve as a bridge between innate and adaptive immunity. A pDC's ability to stimulate T cells is heightened following maturation. As mentioned earlier, maturation also induces the expression of both MHC Class I and Class II molecules in pDCs as well, which allows the cell to optimize its antigen-presenting abilities. MHC class I on pDC surfaces are able to activate CD8+ T cells, while MHC class II have been found to activate CD4+ T cells. pDCs are also thought to be able to promote both T cell activation and tolerance.

Psoriasis
Patients who suffer from psoriasis typically exhibit skin lesions where pDCs accumulate. Inhibiting pDCs from secreting IFN diminished the appearance of the skin lesions. When DNA is released via apoptosis of an infected host cell, antibodies are produced against the host's own DNA. (see autoantibody). These anti-host DNA antibodies are able to stimulate pDCs which proceed to secrete IFN, furthering the activity of adaptive immunity.

Lupus
Although the pDC's ability to mass produce type 1 interferon can be effective in targeting a viral infection, it can also lead to Systemic lupus erythematosus if not regulated properly. Type 1 interferon production is strongly correlated with the progression of lupus, and is thought to drive excessive maturation of pDCs and activation of B cells, among many other effects. In patients with lupus, pDC levels in the circulating blood are decreased most of the pDCs have migrated toward the inflamed and affected tissues.

HIV
The mass production of type 1 interferon may result in both positive and negative outcomes in response to HIV. Although type 1 interferon is efficient at facilitating maturation in pDCs and in killing infected T cells, excessive clearance of infected T cells may have detrimental effects and further weaken the patient's compromised immune system. In addition, pDCs themselves can be infected by HIV and can lose their interferon-producing capacities in an HIV patient. Like in lupus and psoriasis, the pDCs leave peripheral circulation to the affected areas. However, it seems that in HIV, pDCs not only lose their interferon secreting properties but also die, expediting the progression of the disease. Decreases in functional, live, and uninfected pDCs have resulted in decreases in CD4+ T cells that further compromise the patient's immune defenses against HIV. Thus, maintaining balance and regulation of pDC activity is crucial for a more positive prognosis in HIV patients.