Abstract
A population of Hoechst 33342-stained cells, termed side population (SP) cells, can reconstitute the hematopoietic system of syngeneic mice. This study examined whether limiting numbers of SP cells can repopulate mice across a xenogeneic MHC class I barrier. SP cells were isolated from HLA.B7 and HLA.A2.1 transgenic mice by FACS and placed in colony assays or transplanted into irradiated C57BL/6 (B/6) recipients. SP cells contained few colony-forming cells when placed directly in culture. The number of GM-CFC and HPP-CFC increased up to 3000- and 300-fold, respectively, after 7 days in IL-3- and SCF-stimulated liquid culture. BMC-derived GM-CFC increased up to only 12-fold and HPP-CFC decreased after 7 days in culture. HLA-B7 SP cells (2500–5000) were transplanted into lethal-irradiated B/6 mice. Two-color flow analysis, 4–6 weeks after transplantation, showed that HLA-B7 expression in granulocyte-, macrophage-, and lymphocyte-specific lineages from reconstituted mice was similar to that in B7 transgenic mice. Secondary transplanted B/6 mice also showed a pattern of HLA-B7 expression similar to that in transgenic mice and were followed for longer than 16 weeks with stable chimerism. When HLA-A2.1 SP cells were transplanted into sublethally irradiated mice, 50% of the mice expressed HLA-A2 by PCR analysis in short-term repopulation studies. These data confirm that limiting numbers of SP cells can repopulate the major hematopoietic lineages in lethal and sublethally irradiated mice across a human MHC class I barrier. Therefore, SP cells may be useful for establishing mixed chimerism, which may induce immunologic nonresponsiveness to donor antigens in solid organ transplantation.