Page 241 - 2021_07-Haematologica-web

Letters to the Editor
cases they were CD19-negative. Three children (2 with KMT2A gene rearrangements and 1 with germline KMT2A) developed relapses through “lineage switch” to CD19-negative acute myeloid leukemia (n=1), mixed- phenotype acute leukemia (n=1) and acute unclassifiable leukemia (n=1). Eight patients were resistant to blinatu- momab therapy, and the expression of CD19 on blast cells was retained in the bone marrow of seven of them while one patient had CD19-negative leukemic blasts after immunotherapy. Blast cells of patients who had leukemic cells in bone marrow only at the level of MFC- MRD (<5% by MFC) (n=10), were either CD19-positive (n=7) or CD19-negative (n=3).
The immunophenotypic changes of leukemic cells in 27 relapsed patients (3 cases of lineage-switch excluded) are presented in Figure 2A. For all antigens applicable for MFC-MRD assessment, except CD58, changes in expres- sion, either up- or down-modulation, were demonstrat- ed in substantial proportions of cases. We found different frequencies of changes in the expression of two markers among CD19-negative and CD19-positive relapses: CD45 and CD38 were less stable on CD19-negative blasts (Figure 2A). Expression of CD22 and CD24, which are suggested as candidates to replace CD19,10 was stud- ied at relapse in 24 and 19 patients, respectively. Total positivity (≥90%) for these antigens was found in 20 and 17 cases, respectively. We analyzed ten patients, who did not relapse, but had leukemic cells at a MRD level by MFC in bone marrow at least once during the follow-up period (Online Supplementary Table S2). Besides the under- standable downmodulation of CD19 in three out of ten patients, expression of CD10, CD20 and CD34 had changed in five, four and three cases, respectively (Figure 2B). Leukemic cells in eight resistant patients had rather
stable immunophenotypic profiles with only very rare changes in antigen expression (Figure 2C).
Since cytometric residual leukemia detection is based on investigation of the B-cell compartment, CD19 is a vital antigen for conventional MFC-MRD monitoring.9 Possible loss of this marker during CD19-negative relapse could break the well-established algorithm of MFC-MRD gating.10 If modulation in the expression of other antigens also occurs, cytometric MRD studies could become very tricky. The implication is that assessment of immunophe- notypic changes is crucial for improving antibody panels and gating algorithms in patients who undergo CD19- directed treatment.
Overall, leukemic blasts were CD19-negative in 27.1% of patients with relapses, progression or MFC-MRD-pos- itivity after blinatumomab. It was shown previously that CD19 is completely lost in 10-25% of relapses.3,4 In our series lack of the targeted antigen was noted in nine out of a total of 30 relapses (30.0%) including three “switch- es” to acute myeloid leukemia. Contrary to data published by Jabbour et al.,11 not all patients resistant to blinatu- momab preserved high CD19 expression: in one of eight cases (12.5%) leukemic cells became completely CD19- negative. Although CD19-positive relapses were mainly preceded by CD19-positive MFC-MRD and CD19-nega- tive relapses were mainly preceded by CD19-negative MFC-MRD, we observed some exceptions, demonstrat- ing that this is not a strict pattern (Online Supplementary Figure S1).
Studying the expression of other antigens commonly used for MFC-MRD evaluation (CD10, CD20, CD34, CD45, CD58, CD38), we also observed frequent changes both in the percentage of positive cells and the distribu- tion of the level of positivity. Phenotypic shifts between
Figure 3. Possible algorithm for searching for residual leukemic cells in patients with B-cell precursor acute lymphoblastic leukemia after CD19-targeted ther- apy. Various ways of B-lineage gating are shown with their limitations indicated. Consecutive investigation of all these B-cell compartments together with the CD45-defined blast region, precursor regions and aberrantly expressed antigens can help to overcome changes in expression of CD19 and other antigens, appli- cable for minimal residual disease monitoring.
haematologica | 2021; 106(7)
2011