Immature syngeneic dendritic cells potentiate tolerance to pancreatic islet allografts depleted of donor dendritic cells in microgravity culture condition

BACKGROUND. Previously we showed that pancreatic islets cultured for seven days in rotating bioreactors survived for >100 days in allogeneic recipients without immunosuppression. This survival coincided with almost complete elimination of "passenger" donor dendritic cells (DCs). Herein, we examined the necessity of DCs in the generation of CD4CD25 T regulatory (Treg) cells. METHODS. Allogeneic fresh islets or islets cultured for three days in bioreactors were transplanted to streptozotocin-induced diabetic Balb/c as well as signal transducers and activators of transcription (Stat)4-deficient Balb/c or Balb/c mice. Some Balb/c recipients of fresh islet allografts were also treated with a tolerogenic protocol of anti-CD40 Ligand MR1 mAb and CTLA4Ig. RESULTS. Islet allografts cultured for three days in bioreactors survived >100 days in all Balb/c recipients and in 56% of Balb/c recipients, but in none of the Balb/c recipients; the same recipients rejected fresh islet allografts. Purified T cells from long-term surviving Balb/c recipients failed to transfer tolerance to SCID recipients of donor-type fresh islet allografts. In contrast, MR1/CTLA4Ig therapy induced tolerance to fresh islet allografts and their T cells adoptively transferred tolerance. When Balb/c or Balb/c recipients of bioreactor-cultured islets were injected intravenously with immature syngeneic DCs, they became tolerant and developed potent alloantigen-specific CD4CD25 Treg cells expressing Foxp3. CONCLUSION. Allogeneic islets depleted of donor DCs by culture in bioreactors have almost twofold better acceptance in Balb/c than in Balb/c mice, but lack Treg cells. Additional injection of host immature DCs improves tolerance in Balb/c and Balb/c recipients by inducing potent CD4CD25 Treg cells.