CD117 (c-kit) is a restricted antigen of acute myeloid leukemia and characterizes early differentiative levels of M5 FAB subtype.

The CD117 molecule is an antigen more frequently found on early normal and leukemic hematopoietic cells, but its correlation with the FAB subtypes and with other lineage and stage associated antigens is still not well established. In this study we investigated the surface expression of CD117 antigen in 135 patients with acute leukemia in relationship to de novo or secondary origin of AML, subtypes of FAB classification, expression of other antigens such as CD34, HLA-DR, CD15, CD14, CD45RA, CD45RO, CD11b, CD11c, CD4, CD7, mixed antigen co-expression (LyAg + AML and MyAg + ALL) and features of leukemic mass.The CD117 antigen expression (clone 95C3) was determined by flow cytometry in a series of 135 patients with acute leukemia at diagnosis consecutively observed during the years 1995-1997: 82 AML (including 51 cases of de novo AML, 22 cases of AML following myelodysplastic syndromes (MDS), 9 cases of myeloid blastic crisis of chronic myeloid leukemia (BC-CML) and 53 ALL. All cases were stratified in CD117+ and CD117- groups and the differences were analyzed by using appropriate statistical analyses.CD117 antigen was found over 10% in 74% of AML without significant differences of positivity between AML after MDS or BC-CML and de novo AML. We did not note a significant correlation between FAB classification and CD117 which was expressed in 100% of M1 and M7 cases, in 80% of M0 cases, in 75% of M2 cases, in 70% of M3 cases and in 82% of M4 cases. Instead, in M5 subtype CD117 was strictly restricted to earlier stages: ten of the eleven M5b (91%) cases completely lacked CD117 antigen expression, whereas 100% of M5a cases were positive. The results of Pearson's coefficient showed: 1) a significant inverse relationship between CD117 and CD15, CD4 and CD14 (only in M5 subtypes) and CD11b, CD11c and CD45RO (in all cases); 2) a significant direct correlation between CD117 and CD34 and CD45RA (in all cases); and 3) an independent expression between CD117 and CD15 associated with a low correlation between CD117 and HLA-DR antigen (only in non-monocytic cases). In ALL, whether of B or T lineages, surface expression of CD117 was never observed.We conclude that the CD117 antigen shows a high specificity for AML, independently upon FAB classification, and represents a reliable marker in characterizing the differentiative degree of the monocytic blasts.