Levels of expression of CD52 in normal and leukemic B and T cells: correlation with in vivo therapeutic responses to Campath-1H.

The CD52 antigen is expressed on most normal and neoplastic lymphoid cells. The reshaped humanized IgG1 anti-CD52 monoclonal antibody (Campath-1H) has been used in the treatment of hemopoietic and non-hemopoietic diseases for its ability to induce lymphocyte depletion both in vitro and in vivo. Good activity has been shown in patients with chronic T and B cell leukemias, in particular T-prolymphocytic leukemia (T-PLL). However, the response to treatment is not uniform and this variability may depend on differences in the level of antigen expression on the leukemic cells. To test this hypothesis, we used quantitative flow cytometry to investigate the intensity of the expression of CD52 in 45 cases of lymphoid leukemia, 24 with B-cell chronic lymphocytic leukemia (CLL), 21 with T-PLL and 12 normal controls. Normal T lymphocytes expressed higher CD52 antigen than B lymphocytes (p < 0.005) and the antigen was also significantly higher in T-PLL compared to CLL (p < 0.001). Moreover, the differences in CD52 expression were somewhat higher in Campath-1H treated patients who responded than in non responders. Although other factors may play a role in the response to Campath-1H in vivo, the quantitative estimation of CD52 expression may provide a rationale for the greater response in T-PLL and help select those patients with a higher probability of responding to this therapy.