CD11b Is a Therapy-Induced Marker for Monitoring Minimal Residual Disease in Childhood Acute Lymphoblastic Leukemia.

Assessment of minimal residual disease (MRD) has become central to the clinical management of patients with acute lymphoblastic leukemia (ALL). Among the methods available for MRD monitoring, flow cytometry (FCM), which relies on the presence of leukemia-associated aberrant immunophenotype, holds great promise for clinical application. However, the major antigens used for FCM-MRD identification (CD10, CD34, CD45, CD20, CD19, TdT) undergo expression changes during therapy. Moreover, the presence of normal hematopoietic progenitors, in particular in the regenerating bone marrow after treatment, negatively impacts the sensitivity and specificity of MRD detection. Recently, we analysed genome-wide gene expression in blasts isolated from peripheral blood of pediatric precursor B-cell (PBC)-ALL patients after one week of therapy (day 8 cells). Expression changes observed in the day 8 cells pointed to several cell surface molecules, whose expression has not been characteristic for B-lineage hematopoiesis. In particular CD11b surface antigen has been frequently up-regulated in the day 8 cells. In the present study, we addressed expression dynamics of CD11b in PBC-ALL at clinically significant MRD timepoints during induction therapy (days 15, 33 and 78; ALL-BFM protocol). To this end, a CD11b specific antibody has been included into a nine-color, single-tube panel (antibodies to CD19, CD20, CD10, CD34, CD45, CD58, CD3, and a nuclear stain Syto16), which has been applied in order to detect residual blasts among 10⁶ cells in bone marrow specimens from patients with PBC-ALL. At day 15, mean expression of CD11b (in MESF units) has been significantly increased if compared with leukemic cells at diagnosis (9600+/−2800 vs 850+/−140; p=0.005). The up-regulation by more than 10-fold has been found in 8 of 24 cases (33%), and has reached, in part, very high levels (eg, 450 MESF vs 49500 MESF at diagnosis and day 15, respectively). This indicates that CD11b expression changes are due to a therapy-induced gene up-regulation rather than to a clonal selection during clinical treatment. At the later timepoints of induction therapy, 7 of 22 patients (day 33) and 2 of 18 patients (day 78) were MRD positive. CD11b expression, if increased at day 15, retained its high values on day 33 (7900+/− 3200 MESF, 6 patients) and on day 78 (15100 MESF, 1 patient). Importantly, in contrast to leukemic cells, their normal CD19+CD10+ counterparts in both, non-leukemic and ALL bone marrow samples, remained CD11b negative. This difference has facilitated a reliable discrimination of normal and leukemic blasts in the MRD positive cases with regenerating bone marrow at day 78. In conclusion, treatment-induced up-regulation of CD11b in PBC-ALL has a promising potential as a novel marker, which may considerably improve specificity of FCM-MRD detection in bone marrow samples with a complex hematopoietic background.