A Rare Case of Mixed Phenotype Acute Leukemia- B/t Cell Type.

Mixed phenotype acute leukemia (MPAL) with B/T cell type is very rare. A 24-year-old male presented with fever, generalized body ache, cough and breathlessness, bleeding per rectum, epistaxis, and hepatosplenomegaly. His hemoglobin, total leukocyte count, and platelet count were 6.2 g/ dL, 29.06 × 103/μL, and 20 × 103/μL respectively. Peripheral blood smear & bone marrow revealed > 80% blasts that were morphologically lymphoid. The blasts were negative for all cytochemical staining. Flow cytometry immunophenotyping was performed on peripheral blood using a 6color flow cytometer Navios (Beckman Coulter) using CD1a, CD2, CD3, CD4, CD5, CD7, CD8, CD10, CD13, CD14, CD15, CD19, CD20, CD22, CD33, CD34, CD38, HLA-DR, CD45, CD56, CD61, CD64, Glycophorin A, CD117, cytoplasmicCD3, cytoplasmic CD79a, myeloperoxidase (MPO), and terminal deoxynucleotidyl transferase (TdT). The blast cells on CD45 vs SSC exhibited two distinct populations, one (45.9%) with weak and another (23.7%) with moderate expression of CD45 and low side scatter. The population of blasts with weak CD45 expression was positive for CD19 (strong), CD10 (strong), cCD79a (moderate), HLA-DR (moderate), CD38 (weak) and TdT. The population of blasts moderately positive for CD45 was positive for cytoplasmic CD3 (moderate), cCD79a, CD7 (strong), CD1a (moderate), CD4 (strong), TdT, CD5 (moderate), and CD38 (strong), and negative for surface CD3, HLA-DR, CD8, and CD2. The final diagnosis of MPAL -B/T cell type was established. The diagnostic criteria of MPAL are based on expression of strictly T-lymphoid (cytoplasmic CD3)and myeloid (MPO)-specific antigens, the latter demonstrated by either flow cytometry (FCM) or cytochemistry, and/or evidence of monocytic differentiation. B-cell lineage assignment in MPAL relies on strong expression of CD19 together with another B-cell-associated marker or, in cases with weak CD19, on the strong expression of at least 2 B-lineage markers.1 MPAL is very rare and constitutes <4% of all cases of acute leukemia. The most common type is MPALMyeloid/ B cell type. The leukemic blasts very rarely exhibit clear evidence of both T and B lineage commitment. However, as CD79a and CD10 are frequently reported in T-ALL, B/T MPAL should be differentially diagnosed.2 The precursor status of T lymphoblasts can be established by TdT, CD99, CD34, and/or CD1a positivity.3 The panel of antibodies used is important in the identification of the “mixed” phenotype. TdT and CD1a should be included in the panel when immaturity markers like CD34 are negative.