Background. Administration of donor bone marrow (BM)-derived dendritic cell (DC) progenitors (DCp) that are major histocompatibility complex (MI- IC) class H+ but costimulatory molecule (CD40, CD80, CD86)-deficient can prolong mouse heart allograft survival. This is associated with microchimerism and inhibition of antidonor cytotoxic T lymphocyte (CTL) activity. Genetic modification of these donor antigen-presenting cells to express an immunosuppressive molecule(s) may enhance their in vivo survival and potential tolerogenicity. Methods. The surface phenotype of B10 (H-2b) DCp before and after gene transfer using replication-deficient adenoviral (Ad) vectors was determined by monoclonal antibody (mAb) staining and flow cytometry. Transforming growth factor-β (TGF-β) production was quantitated by enzyme-linked immunosorbent assay. Allostimulatory activity of the gene- transduced DCp was ascertained by mixed leukocyte reaction (MLR) and CTL induction. To assess their in vivo migratory activity and survival, the transduced cells were injected subcutaneously into one hind footpad of C3H (H-2(k)) mice. Tissues (draining popliteal lymph nodes [LN], spleens, and thymi) were removed 1, 2, 7, and 14 days later and stained for donor MHC class H using anti-IAb mAb in an immunohistochemical procedure. The mean number of IAb+ cells per unit area was determined. Results. Transduction with a control Ad vector (Ad-LacZ) at 50 multiplicity of infection slightly increased CD40 and CD86 expression and up-regulated the poor allostimulatory activity of the DCp assessed by MLR and CTL responses. These effects on function were negated in Ad-TGF-β1-transduced cells. After their injection into mouse footpads, the gene-transduced IAb+ cells were observed in maximal numbers in the popliteal LN at day 1 and in marginal zones and T- dependent areas of spleens (peak at day 7) but were rare in thymi. Transduction with Ad-LacZ reduced the numbers of IAb+ cells identified in both LN and spleens at all time points postinjection, suggesting that the vector alone affected DC life span in allogeneic recipients. TGF-β1 transgene expression not only fully prevented the reduction in DC induced by Ad transduction alone, but also increased numbers and prolonged the survival of donor cells in the spleen, as shown by a two-to fivefold increase in IAb+ cells at days 2-14 compared with control (Ad-LacZ-transduced) DC. Conclusion. BM-derived DCp can be transduced efficiently to express TGF-β1 using an Ad vector. They exhibit very poor allostimulatory activity and similar migration characteristics in vivo to unmodified DCp. Survival of TGF- β gene-transduced DC, however, is enhanced significantly compared with unmodified and (especially) control Ad-LacZ gene-transduced DC. Genetic engineering of donor DC to express the immunosuppressive molecule TGF-β promotes their survival in allogeneic hosts and may potentiate their previously reported tolerogenicity.