Review of the incidence and clinical relevance of myeloid antigen-positive acute lymphoblastic leukemia.

An increasing number of papers document cases of acute leukemia in which individual blast cells co-express markers normally restricted to a single cell lineage. Numerous terms are used to refer to cases with unscheduled expression of lineage-foreign proteins; the best defined categories were hybrid acute leukemia and acute mixed-lineage leukemia. The incidence of phenotypically variant acute leukemia varies with the quality and quantity of parameters used and the stringency of the criteria employed for its definition. Considerable interest has focused on acute lymphoblastic leukemia (ALL) cells expressing one or several myeloid lineage-associated antigens (My+ ALL), CD13, CD14, CD15, CD33, and CDw65. Owing to legitimate and cryptic expression on lymphoid cells, CD11b and CD15 reagents may not be considered as specific indicators of myeloid differentiation. The reported incidence ranged from 5 to 46% in 14 studies on My+ ALL, totalling 3817 patients. Several detailed reports documented a higher incidence of My+ ALL in adults (realistically in the range 10-20%) than in children (5-10%) and in B-lineage ALL as opposed to T-lineage ALL. My+ ALL cases are more likely to display unique cytogenetic [t(9;22), 11q23, 14q32] features than My-neg ALL. There appears to be no predominant expression of a single myeloid-associated antigen among those analyzed. As the morphological diagnosis of a leukemia subtype is often imprecise, some T-neg B-neg My+ ALL cases might actually contain FAB AML-M0 populations. While the expression of myeloid-associated antigens has no apparent prognostic significance in the majority of childhood ALL subtypes, in adults myeloid antigens seem to identify a high risk group of ALL patients with a poorer response to standard ALL therapy.