CD52 and CD20 as Potential Targets for Anti-Leukemic Immunotherapy in Adult Acute Lymphoblastic Leukemia.

CD52 and CD20 are potential targets for immunotherapy of adult acute lymphoblastic leukemia (ALL) with the use of commercially available humanized monoclonal antibodies (MoAbs) (alemtuzumab, rituksimab, etc.). Altough 20% antigen expression is considered diagnostic, it is thought that only patients with >50% and preferably those with >80% leukemic blasts expressing particular antigen are candidates for the treatment with the use of MoAbs. The efficiacy of immunotherapy depends, however, not only on the proportion of lymphoblasts bearing particular marker but also on the cellular antigen density. Therefore, in this study we analyzed both parameters for 29 B-lineage (common/preB n=23, prepreB n=4, B n=2) and six T-lineage (early T n=4, Thymic n=1, matureT n=1) adult ALL patients in order to determine the potential applicability of anti-CD52 and anti-CD20 therapy.

METHODS: Samples included only bone marrow aspirates. MoAbs anti CD19 (Immunotech) or anti CD7 (BD) were used for gating lymphoblasts on SSC/FL dotplot. Both tested MoAbs: anti-CD52- R-PE (Caltag, clone CF1D12) and anti-CD20-PE (BD, clone L27) were analyzed in lymphoblasts. The same method (direct staining - three-color fluorescence, flow cytometry - Epics-XL-MCL with System II software, and WinMdi 2.8) was used to compare both antigens. The expression of CD52 and CD20 antigens on blast cells was analyzed as a percentage of positivity (%), and a ratio of the mean intensity fluorescence of lymphoblasts and that of isotypic controls (MIF bl/ctrl).

RESULTS: For B-lineage ALL the proportion of patients with CD52 and CD20 positivity was as follows: > 20% positive blasts: 86% vs. 55% (p=0.02), >50% positive blasts: 79% vs. 38% (p=0.003), >80% positive blasts: 48% vs. 24% (p=0.1), respectively. For T-lineage, 2/6 (33%) patients had >50% CD52-positive blasts and none was CD20-positive.

Including patients with >50% expression, the ‘relative’ cellular antigen density did not differ for CD52 and CD20: 3.0 (2.2–5.6) vs. 3.9 (1.9–9.5) MIF bl/ctrl (p= NS). In this subgroup the MIF bl/ctr correlated positively with the proportion of blasts expressing CD52 (R Spearman 0.52, p<0.05) whereas the parallel correlation could not be found for CD20 (R Spearman 0.25, p=NS).

CONCLUSIONS: The vast majority of adult B-lineage ALL patients may potentailly be treated with anti-CD52 MoAb and less than 50% with anti-CD20 MoAb. However, if CD52- or CD20-positive, the cellular density of the antigen is similar so that the the cytostatic potential of both kinds of immunotherapy could be considered comparable as well. The cellular antigen density of both CD52 and CD20 varies between individual patients, however, in case of CD52 it may be predicted to some extent by the proportion of lymphoblasts bearing the antigen. The anti-leukemic immunotherapy could be considered for a second-line traetment of ALL as well as for in vivo purging before transplantation.