IRF8 Demonstrates Positivity in a Significant Subset of Histiocytic and Dendritic Cell Neoplasms.

Histiocytic and dendritic cell neoplasms, especially histiocytic sarcoma, can show morphologic and phenotypic overlap with immature monocytic neoplasms. IRF8 immunohistochemical staining has been demonstrated to be useful in identifying monoblasts, but it has not been extensively studied in histiocytic and dendritic cell neoplasms. IRF8 immunohistochemistry was performed on cases of histiocytic sarcoma (HS, n=6), Langerhans cell histiocytosis (LCH, n=25), Rosai Dorfman disease (RDD, n=17), follicular dendritic cell sarcoma (FDCS, n=3), and Erdheim Chester disease (ECD, n=5), along with a control group that included a subset of myeloid neoplasms with monocytic differentiation. Of 89 total cases, IRF8 was positive in 3/6 cases of HS, 3/5 cases of ECD, 12/17 cases of RDD, 7/25 cases of LCH, and 0/3 cases of FDCS. Control cases were stained similarly to previous reports, with IRF8 expression roughly correlating to monoblast count and normal staining in other control groups. We demonstrate that IRF8 is expressed in a significant subset of tested neoplasms of histiocytic and dendritic cell lineage. While we confirmed that IRF8 is useful to identify monoblasts, these results highlight that IRF8 cannot be reliably used to distinguish histiocytic sarcomas from myeloid neoplasms of monocytic lineages, and caution is advised interpreting IRF8 staining in that setting.