Human G-CSF-mobilized CD34-positive peripheral blood progenitor cells callstimulate allogeneic T-cell responses: implications for graft rejection inmismatched transplantation

To investigate mechanisms of stem cell graft rejection we studied the allo- stimulatory potential of G-CSF mobilized peripheral blood progenitor cells (PBPC). CD34(+) cells were purified (>95%) in a two-step procedure using im munoaffinity columns for CD34 selection and T-depletion. The capacity of CD 34(+) cells to stimulate allogeneic T-cell responses was compared with Othe r cells from the same individual. CD34(+) cells induced potent proliferativ e responses at stimulator:responder ratios of 1:20, but were approximately 50-fold less efficient compared to dendritic cells. Furthermore, CD34(+) ce lls primed responses from partially matched allogeneic T cells in bulk cult ures. Dual-colour flow cytometry showed that the co-stimulatory molecules B 7.1. CD40 and ICAM-1 were absent on resting CD34-positive progenitor cells, but were induced during incubation with allogeneic lymphocytes due to a cy tokine-mediated effect. Up-regulation of accessory molecules on CD34(+) cel ls was reproduced by incubation with interferon-gamma or GM-CSF which enhan ced the allo-stimulatory activity of CD34(+) cells. Blocking studies with i nhibitory antibodies suggested cu-stimulatory functions for B7.2, ICAM-3, C D40 and LFA-3. CD34(+) cells were more efficient in inducing allogeneic T-c ell responses when compared to the unprocessed leukapheresis products, The reduced allo-stimulatory ability of G-CSF mobilized PBPC could be explained by the presence of CD3(+)4(+) and CD3(+)8(+) lymphocytes with suppressor a ctivity, We conclude that current methods of stem cell selection for transp lantation do not avoid allosensitization of the recipient and that further graft manipulation with add-back of lymphocytes or selection of subsets of CD34(+) cells with reduced allo-stimulatory ability may reduce graft reject ion.