CD20 up-regulation in pediatric B-cell precursor acute lymphoblastic leukemia during induction treatment: setting the stage for anti-CD20 directed immunotherapy

Abstract

CD20 is expressed in approximately one- half of pediatric acute lymphoblastic leukemia (ALL) cases with B-cell precursor (BCP) origin. We observed that it is occasionally up-regulated during treatment. To understand the impact of this on the potential effectiveness of anti-CD20 immunotherapy, we studied 237 CD10(+) pediatric BCP-ALL patients with Berlin-Frankfurt-Munster (BFM)-type therapy. We analyzed CD20 expression changes from diagnosis to end-induction, focusing on sample pairs with more than or equal to 0.1% residual leukemic blasts, and assessed complement-induced cytotoxicity by CD20-targeting with rituximab in vitro. CD20-positivity significantly increased from 45% in initial samples to 81% at end-induction (day 15, 71%). The levels of expression also increased; 52% of cases at end-induction had at least 90% CD20(pos) leukemic cells, as opposed to 5% at diagnosis (day 15, 20%). CD20 up-regulation was frequent in high-risk patients, patients with high minimal residual disease at end-induction, and patients who suffered later from relapse, but not in TEL/AML1 cases. Notably, up-regulation occurred in viable cells sustaining chemotherapy. In vitro, CD20 up-regulation significantly enhanced rituximab cytotoxicity and could be elicited on prednisolone incubation. In conclusion, CD20 up-regulation is frequently induced in BCP-ALL during induction, and this translates into an acquired state of higher sensitivity to rituximab. This study was registered at http://www.clinicaltrials.gov as #NCT00430118.

Figure 1

Proportions of CD20⁺ blasts in paired samples from diagnosis and follow-up. Comparisons of PB samples from day 0 versus day 8 (left plot), as well as of BM samples from diagnosis versus day 15 (center) and day 33 (right), are shown. Each data point represents one BCP-ALL case. Thresholds for determining CD20 expression on leukemic cells were set by using the background fluorescence of residual non-B lymphocytes within the same acquisition. Follow-up samples only with more than or equal to 0.1% residual leukemic cells were analyzed. Dashed lines mark the cutoff of 20% used to describe a sample as positive or negative. The numbers of samples located within each of the 4 quadrants, which are built by the cutoff lines, are shown. Identical results in paired samples would fit closely to the dotted diagonal line. In all comparative plots, it can be seen that follow-up samples have higher proportions of CD20⁺ leukemic cells than the paired samples at diagnosis.

Figure 2

CD20 up-regulation in follow-up translates into efficient rituximab-induced complement-lysis. Dot plots are derived from analyses of paired BM samples of a high-risk BCP-ALL patient at diagnosis and at end-induction. Phenotypic comparisons and cell recovery analyses after in vitro incubations with complement alone, with complement plus rituximab (0.2 mg/mL), and with complement plus rituximab plus micro-antibodies neutralizing CD55 and CD59, are shown. The dashed arrow points at the cluster of Trucount beads that were used as internal standards for absolute cell recovery assessment. Up-regulation of CD20 expression can be seen on the CD10⁺ leukemic cells (black), which remained after therapy. An almost complete reduction of intact (4′6-diamidino-2-phenylindole, dihydrochloride-negative) CD10⁺ leukemic cells can be seen on rituximab incubations, with a small remnant fraction of viable leukemic cells marked with an arrow. Of note, cells lysed by complement mostly disappear from dot plots on severe cellular disruption.

Figure 3

Dependency of rituximab-induced complement-lysis. Efficacy increases with higher intensity of CD20 expression (A) and with inhibition of complement-regulatory antigens (B). In panel A, 18 samples (9 pairs) of BCP-ALL at diagnosis (○) and from follow-up (•) were analyzed for CD20 expression levels (MFI values) and blast cell recovery on in vitro rituximab/complement incubations. The (continuous) regression line and the MFI channel = 50 (dotted line) are shown. Note that all but one follow-up sample show higher CD20 expression than initial samples along with more efficacious rituximab-lysis (best separator apparently at MFI = 50). In panel B, comparisons of complement-lysis efficacy with rituximab alone vs rituximab plus additional mini-antibodies against CD55 and CD59, denoted “augmented” lysis, are shown. Twelve samples (from 6 of the 9 pairs, as earlier in this figure legend) were tested. Differences between incubations were minor but statistically significant.

Figure 4

Prednisolone increases CD20 expression in vitro. CD20 expression levels (MFI values) of viable leukemic cells of 10 ALL samples after incubation for 3 days with various concentrations of prednisolone (none; 0.05-5 μg/mL) are shown (individual patients are characterized by specific symbols). In vivo expression changes in samples taken at diagnosis and day 8 (after the prednisolone prephase) among the same patients are shown for comparison. Note patient-specific modulation patterns.