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Factor XIII A Clone EP3372 Provides Superior Staining on FFPE Human Tissue Sections Compared to Clone AC‐1A1

Factor XIII is a plasma proenzyme essential for normal clot formation. Its 'A' subunit (Factor XIII A) exists intracellularly in cells such as dermal dendritic cells, histiocytic reticulum cells as well as megakaryocytes, macrophages and blood monocytes. Detection of Factor XIII A (FXIIIA) by Immunohistochemistry (IHC) is useful in the distinction between dermatofibroma (DF) and dermatofibrosarcoma protuberans, with DFs mostly being FXIIIA positive. However, the frequently used FXIIIA antibody, clone AC‐1A1, is associated with considerable non‐specific staining in IHC. The aim of the study is to find a good IHC assay for FXIIIA. Clone AC‐1A1 concentrate antibody was optimized with two IHC platforms on FFPE human tissue sections. However, the non‐specific staining was lingering. A Rabbit monoclonal antibody clone EP3372 was subsequently obtained and optimized. The optimized protocols for clone EP3372 and clone AC‐1A1 were compared. It was found that clone EP3372 not only produced stronger staining on tumor cells of dermatofibroma than clone AC‐1A1, but also devoid of any non‐specific staining. To further characterize clone EP3372, multi normal tissue arrays and tumor arrays were stained. In the normal tissues interstitial macrophages and stromal fibroblasts showed strong cytoplasmic staining. They were found in abundance in skin, gastrointestinal tracts, bladder, lymphoid tissues; and scattered in other tissues. Strong staining was found in dermatofibroma, capillary hemangioblastoma and fibrosarcoma. In addition, FXIIIA was also diffusely positive in 5 of 13 lymphomas. This study suggests that FXIIIA clone EP3372 performs superior to clone AC‐1A1. The finding that some lymphomas are FXIIIA positive needs further investigation.