Expression of immune inhibitory receptor ILT3 in acute myeloid leukemia with monocytic differentiation

Background: The diagnosis of AML with monocytic differentiation is limited by the lack of highly sensitive and specific monocytic markers. Immunoglobulin-like transcript 3 (ILT3) is an immune inhibitory receptor expressed by myelomonocytic cells and at high levels by tolerogenic dendritic cells. Methods: Using flow cytometry, we analyzed the expression of ILT3 in 37 patients with AML and 20 patients with no detectable disease. Results: We showed that ILT3 was expressed in all cases of AML displaying monocytic differentiation (FAB M4/M5; N = 18), but not in AML M1/M2 and M3 (N = 19; P < 0.0001). Co-expression of ILT3 and immature cell markers, such as CD34 and CD117, was observed in monoblastic leukemia. ILT3 expression was preserved after treatment in M4/M5 patients with refractory or relapsed disease. ILT3 expression was associated with the presence of cytogenetic abnormalities linked to an intermediate prognosis (P = 0.001). Rare CD45dimCD34+CD117+ILT3+ cells were identified in noninvolved bone marrow, suggesting that ILT3 expression is acquired at an early stage by normal myelomonocytic precursors. Conclusions: ILT3 is a highly sensitive and specific marker which distinguishes AML with monocytic differentiation from other types of AML. Testing of ILT3 expression should be incorporated into the initial diagnostic work-up and monitoring of patients with AML. (c) 2012 International Clinical Cytometry Society