Quantification of CD52 Antigen Expression by Normal and Leukemic Lymphoid Cells: Indications for Anti-CD52 Antibody Therapy?.

CD52 is a glycosyl-phosphatidylinositol-anchored protein expressed on the surface of lymphoid cells. Polyclonal and monoclonal anti-CD52 antibodies have been utilized to purge T-lymphocyte populations from the peripheral blood and bone marrow in allogeneic transplantation procedures to prevent graft-versus-host disease. Nowadays, the anti-CD52 monoclonal antibody represents an established tool for the treatment of acute and chronic lymphoproliferative disorders. In this study, we have quantified the levels of expression of the CD52 antigen on the surface of normal B- and T-cell populations, as well as on different acute and chronic leukemic cells of B- and T-cell origin. The quantification of the CD52 antigen was performed by flow cytometry utilizing the QuantiBRITE system and the QuantiCALC software (all instruments, reagents and software came from Becton Dickinson, Immunocytometry System, San Jose, CA). The leukemic cell populations analyzed were obtained from primary B-cell chronic lymphocytic leukemia (CLL) and B- and T-lineage acute lymphoid leukemia (ALL) patients. All analyses were conducted on freshly drawn samples from untreated patients.

In a population of 20 normal adult donors, the CD20+ and CD19+ peripheral B lymphocytes showed 9,581 (median value, range 6,234 – 26,078) and 9,495 (7,259 – 26,196) molecules/cell of the CD52 antigen, respectively. T-cell populations had significantly (p = 0.0001) lower levels of expression of the CD52 antigen: 5,803 molecules/cell on CD3+/CD7+ cells (range 2,917 – 11,056 ). Interestingly, the CD4+ population expressed a significantly higher (p<0.0001) number of CD52 molecules/cell than CD8+ lymphocytes (7,531 median value, range 4,340–12,092 vs 3,178, 2,204–7,591). In 98 CLL samples, the CD19+ leukemic cells showed a median number of CD52 molecules comparable to normal values: 10,205 (range 3,154–36,801). On the contrary, in ALL samples the levels of expression of the CD52 antigen by the leukemic cells was significantly (p = 0.01) lower: in the 8 T-ALL cases analyzed the median value was 1,939 molecules/cell (range 1,110–34,509) and in the 27 B-lineage ALL 1,678 molecules/cell (range 164–7,319). These data can explain the efficacy of the anti-CD52 antibody in the treatment of CLL patients and underline that a close monitoring of the CD4+ population should be performed, particularly in patients who have also received fludarabine. In ALL patients, the clinical results obtained following the administration of the anti-CD52 antibody need to be correlated with the levels of expression of the CD52 antigen on the leukemic cells.