The depletion of T cells from haematopoietic stem cell transplants

Objective. In our laboratory, we have developed an immunorosette technique for the depletion of T cells from bone marrow transplants. Tetrameric complexes of monoclonal antibodies are able to form very stable immunorosettes, which are efficiently depleted with the aid of a blood cell separator. Major improvements over the original sheep red blood cell depletion are the use of human (patient or donor derived) erythrocytes instead of sheep-derived cells, and the possibility of using a closed system for separation in a cell separator. In contrast to bone marrow, mobilized haematopoietic stem cell transplants obtained after leucocytapheresis contain higher numbers of T cells. Therefore, a different approach is necessary. Method. We have used two CD34 selection systems (Isolex(TM) 300SA and the Clinimacs(TM)) to perform T-cell depletions from peripheral blood stem cell (PBSC) transplants. Results. Immunorosette T-cell depletion, with CD2/CD3 tetrameric complexes, of bone marrow transplants resulted in a mean 2.5 log depletion of T cells with a yield of 50% of the CD34⁺ cell population. Stem cell selection of PBSC transplants using one of the CD34 selection procedures resulted in a 4.5 log depletion of T cells for both systems, but with different results for the recovery of CD34⁺ cells. An increased yield of CD34⁺ cells was obtained with the Clinimacs(TM) procedure (57.9 ± 9.0%) in comparison to the Isolex(TM) procedure (40.1 ± 12.5%). Conclusion. Our own immunorosette depletion technique and the two tested CD34 selection methods for stem cell transplants both resulted in a very efficient T-cell depletion with the recovery of 40-60% of the CD34⁺ haematopoietic stem cells present in the transplant.