Biological characteristics of CD7 positive acute myelogenous leukaemia

Summary. We studied the biological characteristics of CD7⁺ acute myelogenous leukaemia (AML). We diagnosed nine out of 88 consecutive AML cases as CD7⁺ AML based on myeloperoxidase positivity and surface antigen expression. In eight of these nine cases more than 20% of leukaemic blasts were found to coexpress both CD7 and a myeloid‐associated antigen, CD33, by a two‐colour flowcytometric assay, while in the remaining case more than 90% of blasts were positive for CD7 and myeloperoxidase. CD7⁺ AML was most frequently observed in M1 among AML subtypes according to the FAB classification. An early stage‐specific antigen, CD34 was also expressed on leukaemic blasts from eight of these nine cases. Neither the T‐cell receptor (TcR)‐β nor the TcR‐γ gene was clonally rearranged in any of the cases. We then studied the proliferative responses to stimulation by various growth factors. Among interleukin‐3 (IL‐3), granulocyte macrophage colony‐stimulating factor (GM‐CSF), and granu‐locyte‐CSF (G‐CSF), IL‐3 showed the strongest stimulatory effect on DNA synthesis and leukaemic blast colony formation in 8/9 and 6/8 CD7⁺ AML cases examined, respectively. On the other hand, the strongest stimulatory effect exerted by IL‐ 3 on blast colony formation was observed in only six out of the 33 CD7⁻ AML cases examined. Furthermore, CD7⁺ AML blasts could proliferate in response to stem cell factor (SCF); SCF alone showed stimulatory effects on blast colony formation (7/8 cases), and in 5/7 SCF‐responding cases, stimulatory effects of SCF were more potent than those of IL‐3. In addition, SCF enhanced blast colony formation synergistically with IL‐ 3 in four of these seven cases. These data suggest that progenitor cells of CD7⁺ AML may possess the biological properties characteristic of immature haematopoietic stem cells.