Plasmacytoid dendritic cell proliferations and neoplasms involving the bone marrow : Summary of the workshop cases submitted to the 18th Meeting of the European Association for Haematopathology (EAHP) organized by the European Bone Marrow Working Group, Basel 2016.

Two distinct forms of neoplasms derived from plasmacytoid dendritic cells (PDC) exist: mature PDC proliferations associated with myeloid neoplasms and blastic PDC neoplasms (BPDCN). Ten cases of PDC proliferations and neoplasms in the bone marrow have been submitted to the bone marrow workshop held at the 18th EAHP meeting. Based on observations from the submitted cases, scattered PDC (≤1% of cells) and PDC aggregates (≤10 PDC/HPF) reflect the normal bone marrow composition, while in myelodysplastic syndromes (MDS), there is a propensity for larger/more PDC aggregates (1-5% and 35 PDC/HPF). A shared PTPN11 mutation between a mature PDC proliferation and an accompanying MDS provides evidence of clonal relationship in such instances and shows that PDC are a part of the malignant clone. CD123 and CD303 should be considered backbone markers to histopathologically establish the diagnosis of BPDCN, since they are detectable in almost all cases and properly well on biopsies subjected to different fixations. Expression of some T-cell markers (e.g., CD2 and CD7 but not CD3), B-cell markers (e.g., CD79a but not CD19 and CD20), and myeloid markers (e.g., CD33 and CD117 but not myeloperoxidase) can be observed in BPDCN. Genetical data of the summarized cases corroborate the important role of chromosomal losses in BPDCN. Together with five previously reported instances, one additional workshop case with MYC rearrangement proposes that translocations of MYC may be recurrent. The frequent nature of deleterious mutations of IKZF3 and deletions of IKZF1 suggests a role for the Ikaros family proteins in BPDCN.

Spontaneous histiocytic sarcoma with possible origin from the bone marrow and lymph node in Donryu and F-344 rats.

Ninety-five male and 96 female Donryu rats reared up to 120 wk of age and 244 male and 243 female F-344 rats used as untreated controls in 5 carcinogenicity studies were examined histopathologically to clarify the primary site of histiocytic sarcoma. Histiocytic sarcoma in Donryu rats was observed in 5 of 95 (5.3%) males and 4 of 96 (4.2%) females. In F-344 rats, 4 of 244 (1.6%) males and 3 of 243 (1.2%) females had the neoplasms. Histologically, sarcomas consisting of large pleomorphic histiocytic cells were seen in the bone marrow, liver, lymph node, spleen, and lung. Among 16 sarcomas observed, 15 had neoplastic lesions in the bone marrow, and 1 F-344 rat had the lesions only in the lymph nodes. Eleven (6 F-344 rats and 5 Donryu rats) of the 15 cases had the lesions in the liver, and 4 Donryu rats had no lesions in the liver but lesions in the lymph node and/or spleen, except for 1 case where the sarcoma occurred only in the bone marrow. Among the 11 cases with the lesions both in the liver and bone marrow, neoplastic lesions were found also in the lymph node, spleen, and/or lung, but the severity of neoplastic proliferation of these organs was not so marked as that in the bone marrow except for 2 cases. Although histiocytic sarcomas in rats are considered to originate from the liver, peritoneum, or subcutis, the present results strongly suggest that some histiocytic sarcomas in Donryu and F-344 rats may also originate from the bone marrow and lymph nodes.