Mixed hematopoietic chimerism after allogeneic transplantation with lymphocyte-depleted bone marrow is not associated with a higher incidence of relapse.

Using red cell phenotyping, cytogenetic analysis of blood lymphocytes, chromosome studies of bone marrow cells, and restriction fragment length polymorphism (RFLP) studies of peripheral blood cells, we demonstrated a high number of mixed chimeras after allogeneic bone marrow transplantation (BMT). Donor marrow from HLA-A, -B, and -DR identical, mixed lymphocyte culture (MLC) nonreactive siblings was depleted of 98% of lymphocytes using counterflow centrifugation. Thirty-two of 48 recipients (67%) appeared to be mixed chimeras at 6 months after transplantation. The high number of mixed chimeras is probably a result of lymphocyte depletion of the marrow graft and the high sensitivity of red cell phenotyping for the demonstration of minor cell populations (at levels as low as 0.01%). The probability of relapse-free survival from 6 months to 4 years after BMT was 85% for the mixed chimeras and 65% for the complete donor chimeras. We conclude that in this study, mixed chimerism is not associated with a higher incidence of relapse.