Table 1. Histochemical Staining Patternsa continued...
The use of monoclonal antibodies to determine cell-surface antigens of AML cells is helpful to reinforce the histologic diagnosis. Various lineage-specific monoclonal antibodies that detect antigens on AML cells should be used at the time of initial diagnostic workup, along with a battery of lineage-specific T-lymphocyte and B-lymphocyte markers to help distinguish AML from ALL and bilineal (as defined above) or biphenotypic leukemias. The expression of various cluster determinant (CD) proteins that are relatively lineage-specific for AML include CD33, CD13, CD14, CDw41 (or platelet antiglycoprotein IIb/IIIa), CD15, CD11B, CD36, and antiglycophorin A. Lineage-associated B-lymphocytic antigens CD10, CD19, CD20, CD22, and CD24 may be present in 10% to 20% of AMLs, but monoclonal surface immunoglobulin and cytoplasmic immunoglobulin heavy chains are usually absent; similarly, CD2, CD3, CD5, and CD7 lineage-associated T-lymphocytic antigens are present in 20% to 40% of AMLs.[13,14,15] The aberrant expression of lymphoid-associated antigens by AML cells is relatively common but generally has no prognostic significance.[13,14]
Immunophenotyping can also be helpful in distinguishing some FAB subtypes of AML. Testing for the presence of HLA-DR can be helpful in identifying APL. Overall, HLA-DR is expressed on 75% to 80% of AMLs but rarely expressed on APL. In addition, APL cases with PML/RARA were noted to express CD34/CD15 and demonstrate a heterogenous pattern of CD13 expression. Testing for the presence of glycoprotein Ib, glycoprotein IIb/IIIa, or Factor VIII antigen expression is helpful in making the diagnosis of M7 (megakaryocytic leukemia). Glycophorin expression is helpful in making the diagnosis of M6 (erythroid leukemia).
Less than 5% of cases of acute leukemia in children are of ambiguous lineage, expressing features of both myeloid and lymphoid lineage.[18,19,20] These cases are distinct from ALL with myeloid coexpression in that the predominant lineage cannot be determined by immunophenotypic and histochemical studies. The definition of leukemia of ambiguous lineage varies among studies, although most investigators now use criteria established by the European Group for the Immunological Characterization of Leukemias (EGIL) or the more stringent WHO criteria.[21,22,23] In the WHO classification, the presence of MPO is required to establish myeloid lineage. This is not the case for the EGIL classification.
The WHO classification system is summarized in Table 2.[23,24]
Table 2. Acute Leukemias of Ambiguous Lineage According to the WHO Classification of Tumors of Hematopoietic and Lymphoid Tissuesa
|NOS = not otherwise specified; WHO = World Health Organization.|
|a Béné MC: Biphenotypic, bilineal, ambiguous or mixed lineage: strange leukemias! Haematologica 94 (7): 891-3, 2009.Obtained from Haematologica/the Hematology Journal websitehttp://www.haematologica.org.|
|Acute undifferentiated leukemia||Acute leukemia that does not express any marker considered specific for either lymphoid or myeloid lineage|
|Mixed phenotype acute leukemia with t(9;22)(q34;q11.2);BCR-ABL1||Acute leukemia meeting the diagnostic criteria for mixed phenotype acute leukemia in which the blasts also have the (9;22) translocation or theBCR-ABL1rearrangement|
|Mixed phenotype acute leukemia with t(v;11q23);MLLrearranged||Acute leukemia meeting the diagnostic criteria for mixed phenotype acute leukemia in which the blasts also have a translocation involving theMLLgene|
|Mixed phenotype acute leukemia, B/myeloid, NOS||Acute leukemia meeting the diagnostic criteria for assignment to both B and myeloid lineage, in which the blasts lack genetic abnormalities involvingBCR-ABL1orMLL|
|Mixed phenotype acute leukemia, T/myeloid, NOS||Acute leukemia meeting the diagnostic criteria for assignment to both T and myeloid lineage, in which the blasts lack genetic abnormalities involvingBCR-ABL1orMLL|
|Mixed phenotype acute leukemia, B/myeloid, NOS—rare types||Acute leukemia meeting the diagnostic criteria for assignment to both B- and T-lineage|
|Other ambiguous lineage leukemias||Natural killer cell lymphoblastic leukemia/lymphoma|