Purging
Purging is a procedure to remove unwanted cells from the peripheral blood transplant product. For autologous transplants this procedure can reduce the risk of relapse from malignant cells that still may be present in the patient after radiation therapy and/or chemotherapy. For allogeneic transplants, purging is performed to reduce the number of T cells in the transplant and minimize graft versus host disease. Removal of T cells will also affect the graft versus tumor effect which requires active T cells. Patients receiving T cell depleted transplants are more likely to have a relapse than patients receiving unpurged cell products.
The absolute number of T cells in a patient's peripheral blood sample can be measured by flow cytometry. The formula to calculate the absolute T cell count is:
Absolute T lymphocyte count = Total WBC (x109/L) x %lymphocytes from differential/100 x %T cells from flow cytometry/100
A typical reference range for adult T cells (CD3+) is 1.0-3.6 K/µL (1.0 -3.6 x 109/L)
A number of different methods have been developed to target cells for purging. They often use monoclonal antibodies to specific targets that are conjugated with magnetic particles or are attached to a solid phase. After the antibody binds to the target cells, the remaining cells can be separated by centrifugation or other physical methods of separation.
Testing for Chimerism
Complete chimerism occurs when a patient’s HSC cells have been replaced by donor cells. Partial or mixed chimerism occurs when there is a mix of patient and donor hematopoietic cells. Partial chimerism may indicate a relapse and reemergence of malignant cells in the patient. These patients may benefit from chemotherapy and a second transplant of additional HSCs from the donor. Chimerism is usually determined by molecular testing using fluorescent in situ hybridization (FISH) or polymerase chain reaction (PCR) for specific target sequences of DNA. For example, if a female patient receives a transplant from a male, FISH can be used to identify and enumerate Y chromosomes in the hematopoietic cells of the patient.
