Immunotherapy utilizing CAMPATH-1H for patients with chemotherapy-refractory 
chronic lymphocytic leukemia has yielded encouraging results with many reports 
of complete remission. Here we report the outcome of two patients with 
CD4-positive T cell prolymphocytic leukemia treated with CAMPATH-1H. Both 
patients responded rapidly to treatment and subsequently developed CD4 
lymphopenia. One patient remained in complete remission after 14 weeks of 
treatment. Serial peripheral blood flow cytometry revealed that the CD52 antigen 
was present throughout treatment. The other patient who was initially 
CD52-positive, became CD52-negative after 6 weeks of treatment, and developed 
progressive symptoms of T cell prolymphocytic leukemia. Immunotherapy was 
stopped, chemotherapy proved futile, and the patient died. This change in 
phenotype from CD52-positive to -negative during CAMPATH-1H therapy points out a 
need to develop strategies for maintaining antigenic expression during 
monoclonal antibody therapy.