CD52 Expression in Waldenström's Macroglobulinemia: Implications for Alemtuzumab Therapy and Response Assessment

The efficacy of monoclonal antibody therapy is determined, at least in part, by the extent to which the target antigen is expressed. This is a complex issue in Waldenström's macroglobulinemia (WM) as it is a disorder characterized by plasma cell differentiation and therefore target antigen expression may differ between the B-cell and plasma cell compartments of the disease. In order to assess this in the context of alemtuzumab therapy, the authors used multiparameter flow cytometry to determine CD52 expression in the B-cells and plasma cells of patients with WM. CD52 expression was demonstrable in the B-cells of all cases, with a median of 99% of cells (range, 81%- 100%) expressing the antigen compared with the isotype control (n = 47). Antigen density was very similar to that seen in chronic lymphocytic leukemia (median mean fluorescence intensity [MFI], 1249; range, 175-3170). Antigen expression was, however, significantly lower in the plasma cells (median MFI, 235; range, 31-1814) in all but 1 of the cases assessed (n = 21). The clinical significance of this was assessed by examining serial bone marrow samples from patients receiving alemtuzumab as part of an ongoing clinical trial. In 4 of 5 patients, alemtuzumab therapy successfully eradicated clonal B-cells from the bone marrow, but residual plasma cells remained evident in 2 of these patients. The implications of these findings for monoclonal antibody therapy in WM are discussed.