Background: The bone marrow biopsy sections of acute leukemia patients occasionally reveal a proliferation of large mononuclear cells that accompany the leukemic blasts, and this proliferation shows a starry sky pattern. We characterized these large mononuclear cells by performing immunohistochemistry with 12 different antibodies. The clinical characteristics were examined and then we determined their difference from hemophagocytic lymphohistiocytosis (HLH) and malignant histiocytic disorders.
Methods: Of the 200 acute leukemic bone marrow biopsy samples, 11 ALL and 10 AML cases showed large mononuclear cell proliferations. The panel of antibodies used for immunohistochemistry included those against the mononuclear phagocyte system, and immunohistochemistry was performed on the patients' initial specimens and the complete remission specimens. 10 normal specimens, 4 initial CML specimens and their complete hematologic response specimens were included as controls.
Results: The large mononuclear cells showed immunohistochemical results consistent with histiocytes. They were negative for the markers of dendritic cells the histiocytes and cytokines that are involved in the pathogenesis of HLH and vascular proliferation. Histiocyte proliferation was not observed in the complete remission specimens and in the initial and complete hematological response specimens of the CML patients and the normal bone marrow specimens. None of the cases fulfilled the criteria of HLH, and all 5 ALL cases, for which the immunophenotype results were available, showed a B cell phenotype.
Conclusion: We characterized the large mononuclear cell proliferations as reactive histiocyte proliferations and we differentiated these from those of secondary HLH and malignant histiocytic disorders. A proportion of the large mononuclear cells showed negative results for all 12 antibodies and they showed characteristics that were suggestive of small fat cells. The pathophysiology and the prognostic effect of the reactive histiocyte proliferation accompanying acute leukemia require further study. (Korean J Hematol 2009;44:8-13.)

Keywords: Histiocyte, Immunohistochemistry, Acute leukemia