Stability of Leukemia-Associated Immunophenotypes in Precursor B-ALL
Essentially all cases of precursor B-lymphoblastic leukemia/lymphoma (B-ALL) demonstrate multiple immunophenotypic aberrancies relative to normal maturing B-cell precursors (hematogones). The stability of these aberrancies has relevance to follow-up minimal residual disease analysis. We compared the immunophenotypes at diagnosis and relapse in 51 childhood and adult B-ALLs with flow cytometry (FC) using broad antibody panels. A total of 446 aberrancies were present at diagnosis (median, 9 per case; range, 2-14). All cases retained multiple aberrancies at relapse (median, 8 per case; range, 2-14). Antibody panels at relapse allowed assessment of 383 (85.9%) of the initial 446 aberrancies. Of these, 299 (78.1%) were persistent and 84 (21.9%) were lost at relapse. Overall, 73% of cases showed a loss of at least 1 aberrancy at relapse. However, new aberrancies were detected in 60% of cases. These findings suggest that FC is suitable for the detection of residual B-ALL, provided that follow-up studies are not too narrowly targeted.

Essentially all cases of precursor B-lymphoblastic leukemia/lymphoblastic lymphoma (B-ALL) demonstrate multiple immunophenotypic aberrancies relative to their normal counterpart, maturing B-cell precursors (hematogones). Hematogone populations demonstrate a characteristic and highly reproducible maturation pattern, whereas neoplastic lymphoblasts exhibit maturation arrest (eg, uniform expression of terminal deoxynucleotidyl transferase [TdT] and/or CD34), immunophenotypic asynchrony (coexpression of early and late antigens that are mutually exclusive in hematogones), overexpression or underexpression of normally expressed antigens, and frequent expression of cross-lineage antigens. Thus, neoplastic lymphoblasts can be consistently distinguished from hematogones by multicolor flow cytometry (FC) using appropriately selected antibody panels.

The use of FC to monitor minimal residual disease (MRD) in patients with B-ALL assumes that leukemic cells retain immunophenotypic aberrancy during and after therapy. The stability of these leukemia-associated immunophenotypes is, thus, of major interest for purposes of targeted follow-up analyses. This has been addressed in several studies. The reported immunophenotypic changes include loss of CD10, HLA-DR, TdT, and CD20 and gain or loss of myeloid antigens. However, none of these studies has comprehensively assessed the patterns of immunophenotypic aberrancy of B-ALL relative to the normal immunophenotype of hematogones. We believe a systematic comparison of the immunophenotypes of lymphoblasts and hematogones permits detection of a wide spectrum of immunophenotypic aberrancies at diagnosis, which would be expected to enhance the ability to detect MRD. Detailed knowledge of the stability of leukemia-associated immunophenotypes between diagnosis and relapse will facilitate the rational design of MRD antibody panels. To achieve this goal, we report our single institutional experience comparing the immunophenotypes of relapsed B-ALL with those of diagnostic specimens.