Abstract
Plasmacytoid dendritic cells (pDCs) are an antigen-presenting inflammatory cell involved in the immune response to viral infection. When activated by viral nucleic acids they secrete type 1 interferons (IFN-?, IFN-?). pDCs can also be activated by self-nucleic acids leading to a loss of tolerance and subsequent autoimmune inflammation. They are absent in normal skin but are enriched in lupus erythematosus, dermatomyositis, psoriasis, and atopic dermatitis where their secretion of interferons is thought to drive inflammation. pDCs express CD123 (interleukin 3 receptor) and CD303 (blood-derived dendritic cell antigen-2). The role of pDCs granulomatous processes is still unclear though CD123+ pDCs have been identified in granulomatous diseases including sarcoidosis, foreign body reactions, and granuloma annulare (GA). We aim to further characterize pDCs in GA using CD123 and CD303 immunohistochemical staining. We evaluated 10 cases of GA on formalin fixed tissue with hematoxylin and eosin, and immunohistochemical stains for CD123 and CD303. The percentage of total inflammatory cells marking with each stain was determined by examining 5 high power fields (400x) in areas involved by GA. We found marked differences in staining profiles. All 10 cases had prominent CD123 staining with 50% or more of the inflammatory cells present staining for CD123. In contrast, staining for CD303 was minimal, typically between 5-10% of inflammatory cells present. We hypothesize the differences in staining are multifactorial in nature. CD123 is less specific and has been reported to stain pDC precursor cells, and vascular endothelial cells. The paucity of CD303 staining could indicate a less mature pDC population within GA that has not yet expressed CD303, or a more mature pDC population with downregulated CD303 expression. Future study will focus on further characterization of the pDC population within GA and the diagnostic utility of these immunostains in other granulomatous conditions.
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