#00063625

26-year-old female with no significant past medical history presenting with 1 day of bleeding gums, scleral hemorrhage and about 1 day of cough with non-bloody phlegm production.  She did not have fever, chills, night sweats, fatigue, SOB, CP, N/V, diarrhea, hematochezia, dysuria, extremity weakness. Her initial work up revealed peripheral blood cytopenia with circulating blasts. Subsequent work up including a bone marrow biopsy was diagnostic of acute promyelocytic leukemia with t (15:17) translocation. She underwent treatment with 4 doses of Idarubicin and a course of ATRA and ATO for 36 days. Her day 33 bone marrow demonstrated no morphological evidence of residual disease with scattered phagocytic histiocytes containing Auer rods, granules, and globules identified.

ATRA induces the terminal differentiation of APL cells that are phagocytosed by histiocytes in the bone marrow then go through cell lysis. The engulfed APL cells undergo fragmentation in the cytoplasm of histiocytes. (Shunichi Humakura, 2011). Auer rods are the crystallized form of myeloperoxidase which resist digestion by enzymes in the histiocytes. Their presence post induction phase of treatment signifies persistent APL though their persistence in histiocytes doesn’t indicate residual disease. (Christina Brown, 2006). The phenomenon contributes to the potential mechanism of clearance of neoplastic cells by histiocytes. (Kristian T. Schafernak, 2014).