Mixed chimerism is frequent after allogeneic peripheral blood stem cell transplantation with positive CD34 selection, and is not reverted by low doses of donor T‐cells add‐back

The incidence of full donor chimerism (full DC) after CD34 + -selected peripheral blood stem cell transplantation (CD34 + -PBSCT) is controversial. Whereas the initial reports suggested a high incidence of full DC (hypothetically because of the high number of CD34 + cells infused) more recent works describe a high incidence of mixed lymphoid chimerism. There are no data concerning the ability of low-dose donor T-lymphocyte add-back on conversion to full DC. Methods: We prospectively monitored the chimerism status of 25 patients undergoing CD34 + -PBSCT and the effect on chimerism of delayed low doses of donor T-cell add-back (TCAB). One, two or three doses of TCAB were administered on days + 28 (2 x 10 5 CD3 + /kg), + 60 (2 x 10 5 CD3 + /kg) and +90(2 x 10 6 CD3 + /kg), respectively, when on cyclosporine A prophylaxis. Results: Incidence of full DC on day + 20 was 56%. However, all but two patients progressed to MC. Fifteen patients were scheduled to TCAB. Six patients with initial MC did not convert to full DC after TCAB. Moreover, seven patients with full DC status progressed to mixed chimerism. Conclusions: Our results indicate that low doses of TCAB administered under cyclosporine A prophylaxis have no effect on the eradication of the recipient cells. We believe that a high dose of CD34 + cells in the grafts of CD34 + -PBSCT is not enough to achieve stable full DC unless a minimum number of CD3 + cells are infused, or more intensified transplant conditioning regimens are employed.