FIGURE 3.

Hematopoietic progenitor cells derived from HOXB4-transduced ES cells engraft across major histocompatibility complex barriers. (a) HPCs were derived from HOXB4-transduced ES cells and infused in Rag2^−/−γ_c^−/−, the synge-neic 129SvJ or the allogenic MRL mice, and mixed chimerism was studied. Each group comprised six animals. All three mouse strains became chimeric, with the Rag2^−/−γ_c^−/− mice showing the highest degree in engraftment. The MRL showed higher engraftment than the 129SvJ, a factor we attribute possibly to different requirements for sublethal irradiation. However, it was the 129SvJ that maintained permanent engraftment over 100 days. The donor cells in the MRLs significantly decreased after 28 days and were nondetectable after 100 days in peripheral blood. (b) To determine whether blocking of costimulation protected transplanted HPCs from rejection, one group of MRL mice was left untreated. A second group was treated with an anti-CD40L antibody. Peripheral blood was analyzed after staining for GFP⁺CD45⁺, GFP⁺Gr-1⁺, and GFP⁺Ib⁺ cells 28 days posttransplantation. The anti-CD40L antibody did not improve significantly the degree of engraftment of the HPCs. (c) Chimeric mice show no altered responses to alloantigen. To determine whether alloresponses of chimeric mice to donor alloantigen were changed, splenocytes from donor-derived 129SvJ or third-party Balb/c mice were used as stimulator cells in a 4-day mixed lymphocyte reaction. The splenocytes from chimeric animals were used as responders. There was no difference of these mice's responses compared with that of control mice. These results showed no evidence of clonal deletion of alloreactive T cells. As expected, third-party responses were also the same in both controls and chimeric animals. This is a representative experiment of three different animals tested in triplicates.