Triple immunostaining demonstrates the possible existence of segregated-nucleus-containing atypical monocytes in human primary myelofibrosis bone marrow: a case report.

Background: Segregated-nucleus-containing atypical monocytes have recently been identified in mice. Segregated-nucleus-containing atypical monocytes are thought to originate from the bone marrow and induce fibrosis in the drug-injured lung. The Lyc6c^- murine monocyte subset is the counterpart to human CD14^-CD16⁺⁺ non-classical monocytes; however, the human counterpart to murine segregated-nucleus-containing atypical monocytes has not yet been identified. Primary myelofibrosis is a well-known disease of progressive marrow fibrosis, and atypical megakaryocytes are thought to be closely related to fibrosis in primary myelofibrosis bone marrow. However, recently, monocytes have been reported to play an important role in marrow fibrosis in primary myelofibrosis. We speculated that, if there is a human counterpart to murine segregated-nucleus-containing atypical monocytes, it would present the same markers as murine segregated-nucleus-containing atypical monocytes, such as CD14^-CD16⁺ macrophage-1 antigen (CD11b/CD18 complex)⁺, MSR1⁺, and CEACAM1⁺, and it might exist in the bone marrow of patients with primary myelofibrosis.

Case presentation: A 74-year-old Japanese male visited our hospital for clinical follow-up after total prostatectomy for prostatic cancer. Anemia, thrombocytosis, and elevated lactate dehydrogenase were suddenly observed in a periodic examination. CALR mutation type 2 (p.K385fs*47) was observed. The histological features of the patient's bone marrow were consistent with fibrotic primary myelofibrosis. We immunohistochemically studied the bone marrow in an attempt to identify a human counterpart to murine segregated-nucleus-containing atypical monocytes. We detected a few CD16⁺MSR1⁺CEACAM1⁺ cells, but not CD14⁺MSR1⁺CEACAM1⁺ cells, by triple immunostaining. The patient is in a good condition and does not require treatment for primary myelofibrosis.

Conclusion: There is a possibility that human segregated-nucleus-containing atypical monocytes exist in the bone marrow of primary myelofibrosis patients and might be related to marrow fibrosis.