Mixed Phenotype Acute Leukemia (MPAL) comprises either blasts with myeloid and lymphoid expression or leukemia with a mixture of myeloblasts and lymphoblasts.
BIPHENOTYPIC – single population of blasts that express key markers of different lineages (e.g. T-ALL, B-ALL, and/or AML).
BILINEAL – two cell populations each diagnostic of separate acute leukemia lineages (combined populations ≥20% blasts to met threshold for acute leukemia).
CD68 (gp110) is a glycosylated lysosomal transmembrane glycoprotein, which is normally expressed on monocytes, macrophages, dendritic cells, neutrophils, basophils, myeloid progenitor cells, mast cells, activated T-cells, and a subset of blasts. The staining pattern is generally cytoplasmic. Almost all MPO+ bone marrow cells will express CD68.
In the setting of leukemia cutis, CD68 was expressed in >95% of cases (Benet, et. al). Typically, CD68 is used to identify histiocytes. Unfortunately, CD68 lacks specificity and the morphologic differential diagnosis overlaps in many situations, and use as part of a larger panel yields more specific results.
CD68R (PGM-1) is one of the epitopes of CD68, and appears to be the most monocyte specific marker of the CD68 family. Expression in monocytic differentiated AMLs is reported to be 92-94% (Rollins-Raval, et. al)
Photomicrographs
H&E section of ovary showing histiocytes.CD68 staining histiocytes in ovarian parenchyma with physiologic type changes.High power view of ovary parenchyma with histiocytes (physiologic changes).CD68 highlighting scattered macrophages in benign tonsil.
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Cronin DMP, George TI, Sundram UN. An updated approach to the diagnosis of myeloid leukemia cutis. Am J Clin Pathol. 2009;132: 101–110. doi:10.1309/AJCP6GR8BDEXPKHR
Rollins-Raval MA, Roth CG. The value of immunohistochemistry for CD14, CD123, CD33, myeloperoxidase and CD68R in the diagnosis of acute and chronic myelomonocytic leukaemias. Histopathology. 2012;60: 933–942. doi:10.1111/j.1365-2559.2012.04175.x