CD117positive Acute Lymphocytic Leukemia

Although CD117 has a higher specificity in the recognition of AML than have CD33 and CD13, its expression is not exclusively seen on myeloid leukemic cells. Approximately 2 percent of adult ALL expressed CD117 on 100 percent of lymphoblasts in ECOG study E2993 (Paietta E, unpublished observation). These ALLs were of very immature T-cell phenotype, expressing intracytoplasmic, but not membrane CD3, TdT, CD2, and CD7 in the absence of CD5 and usually without CD4 or CD8. Other recent, acceptably large studies have reported an incidence of 3 to 10 percent, predominantly in immature T-cell phenotypes and rarely in B-lineage ALL,37,168,169,351 with variable levels of CD117 expression. High levels of CD117 are found in CD3-CD4-CD8 triple-negative thymocytes352,353 that have not yet rearranged their TCR genes,353 which is reminiscent of the predominant antigen profile of CD117+ ALL. Among myeloid antigens tested, CD13 is correlated with CD117 expression in ALL354 in the absence of MPO, CD33, CD65s, CD15, and

CD11b. Coexpression of CD34 and CD117 in ALL is not universal, occurring only in approximately half of cases.169 Given its low frequency, the prognostic significance of CD117+ ALL is not yet established.

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