Definition of the Variables Affecting Efficacy of Immunodepletion Ex Vivo of Peripheral Blood Progenitor Cell Grafts by Alemtuzumab (Campath in the Bag)

The immunodepleting effects of alemtuzumab on peripheral blood progenitor cell (PBPC) grafts for stem cell transplantation need to be better defined. The optimal graft cell concentration, antibody dose, need for complement, and whether alemtuzumab is infused with the graft during transplantation remain unclear. PBPC from 6 normal allogeneic stem cell donors harvested by apheresis were first quantitated and the cellular content defined by flow cytometry. Mononuclear cells were then incubated with incremental concentrations of alemtuzumab (.00001, .0001, .001, and .01 mg/mL) for 30 minutes at 20°C or in cell dose responses with 1, 5, and 10 × 10⁶ mononuclear cells/mL added to a fixed dose of .001 mg/mL of alemtuzumab with or without a source of complement. Cells were enumerated and analyzed by flow cytometry before and after exposure to alemtuzumab. To determine the presence of unbound anti-CD52, the supernatant of the cell dose responses were tested using the ELISA assay. Selected CD34+ lineage-negative cells were incubated with antibody at the same working concentrations and conditions and cultured in granulocyte-macrophage colony-forming unit assay. The colony numbers were compared with control cultures devoid of the antibody. Incremental concentrations of alemtuzumab led to a significant (2 log) reduction in CD3, CD4, and CD8 populations, which plateaued at .001 mg/mL. Addition of complement led to a further significant reduction in the CD4 and CD8 cells. The maximum CD4 (3 log) and CD8 (2 log) cell death was obtained at 10 × 10⁶ cells/mL. Analysis of supernatants for soluble alemtuzumab by ELISA showed a significant reduction in the free antibody concentration when the cell number was increased from 1 to 10 × 10⁶ cells/mL implying utilization/binding of the antibody by target cells. Incremental concentrations of alemtuzumab did not affect the number of granulocyte-macrophage colony-forming units. Alemtuzumab depletes all cells expressing the CD52 antigen and has higher activity on CD3, CD8, and particularly on CD4 cells, which are depleted in excess of 2 logs. From this study, we were able to derive that the optimal cell kill in the graft without detectable free alemtuzumab in the supernatant can be achieved with 1 mg of antibody per 100 mL containing 10 × 10⁹ cells and active complement (AB serum).