Targeted T-cell depletion or CD154 blockade generates mixed hemopoietic chimerism and donor-specific tolerance in mice treated with sirolimus and donor bone marrow

Background. The administration of donor specific bone marrow (DSBM) to mice conditioned with antilymphocyte serum (ALS) and sirolimus can result in stable multilineage mixed chimerism and long-term graft survival. This study seeks to determine if either the targeted depletion of CD4 and/or CD8 pos T cells or costimulation blockade can substitute for ALS and preserve the efficacy of this regimen. Methods. C57BL/6 recipients of BALB/c skin allografts were treated with DSBM (150 X 10(6) cells), sirolimus (24 mg/kg intraperitonealy), and either ALS or various monoclonal antibodies (alphaCD4, alphaCD8, alphaCD154 alone or in combination). Recipient peripheral blood mononuclear cell (PBMC) depletion, donor chimerism, and deletion of donor reactive T cells were assessed using flow cytometry. The specificity of immunologic nonreactivity and the presence of immunoregulatory activity were assessed through a mixed lymphocyte reaction assay. Results. The administration of ALS, sirolimus, and DSBM resulted in sustained recipient PBMC depletion, transient chimerism, and prolonged graft survival. The substitution of an equivalent degree and duration of targeted depletion of either CD4 or CD8 pos T cells alone for ALS failed to produce chimerism or prolonged graft survival. In contrast, depletion of both CD4 and CD8 pos T cells resulted in durable multilineage chimerism, indefinite allograft acceptance (>350 days), and donor-specific tolerance to secondary skin grafts. Substitution of alphaCD 154 monoclonal antibody for ALS also resulted in a state of mixed chimerism and donor specific tolerance. This tolerant state appears to be maintained at least partially through clonal deletion and suppression. Conclusion. Either combined CD4 and CD8 T-cell depletion or alphaCD154 blockade can effectively substitute for ALS in producing chimerism and tolerance in this model.