Orally delivered all-trans-retinoic acid- and transforming growth factor-β-loaded microparticles ameliorate type 1 diabetes in mice

Type 1 diabetes (T1D) is a multifactorial autoimmune disease that develops as a consequence of macrophage- and T cell-dependent pancreatic β-cell death. Multiple approaches for induction of anti-inflammatory/regulatory mechanisms that would attenuate T1D have been utilized, with little or no beneficial effects. To achieve prolonged stimulation of regulatory immune cells, we orally introduced microparticles (MPs) loaded with all-trans retinoic acid (ATRA) and transforming growth factor-β (TGF-β) to C57BL/6 mice treated with multiple low doses of streptozotocin (MLDS) for T1D induction. Disease incidence was significantly lower in ATRA/TGF-β MPs-treated mice, as was the degree of immune cell infiltration into the pancreatic islets. In Peyer's patches (PP), ATRA/TGF-β MPs up-regulated tolerogenic dendritic cells (tolDCs) (CD11c+CD11b-CD103+), while the proportion of mature dendritic cells was not altered. This was accompanied by reduced Th1 and Th17 proportions and up-regulation of regulatory T cells (Tregs - CD4+CD25highFoxP3+). The immune cell composition in the pancreatic lymph nodes was similar to PP. Further, the proportion of effector Tbet+CD25med cells was decreased, while the proportion of Tbet+ Treg cells that specifically inhibit Th1 response was increased. Moreover, ATRA/TGF-β MPs treatment resulted in increased Treg proliferation and frequency of CTLA-4+PD1+ and CD39+IL-10+ Tregs, suggestive of their higher suppressive capacity. Reduced pancreatic infiltration may have been a consequence of lower cell capacity for matrix degradation. In conclusion, oral application of ATRA/TGF-β MPs ameliorated T1D through potentiation of tolDCs and Tregs, inhibition of Th1 response and prevention of the immune cell entrance into the islets.