Critical role of allograft infiltrating ROR gamma t positive innate immune cells and hematopoietic progenitor cells

Zhigang Fan^1,2, Keehoon Jung^1,2, Seok Yun^1,2, Terry Strom^1,2, Maria Koulmanda^1,2

¹Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, United States; ²Wellman Center for Photomedicine, Massachusetts General Hospital, Boston, MA, United States

Using a minimally invasive in vivo imaging system to serially monitor fluorescence emitting T cell subtypes infiltrating islet allografts, we have now probed for the presence of graft infiltrating CD4⁺ Th17 cells, characterized by the expression of the nuclear transcription factor RORγt. Since Th17 cells are critical to the pathogenesis of various autoimmune disorders, we suspected that they might also play an essential role in allograft rejection. However, rather than around the time of allograft rejection (Mean Survival Time, MST=12 days), peak infiltration by RORγt⁺ cells unexpectedly occurs within the first 3 days post allogeneic islet or heart transplantation. Additionally, the phenotype of the predominant graft infiltrating RORγt⁺ cells are neither CD4⁺ Th17 cells, nor TCR γδ T cells, iNKT cells, nor recently identified innate lymphoid cells (ILCs) subtypes, but a previously uncharacterized subtype of hematopoietic progenitor cells that are lineage negative (Lin^-) and RORγt⁺. We subsequently demonstrate using both genetic and pharmacological manipulations that that early infiltrating unconventional Lin^-RORγt⁺ cells may powerfully promote allograft rejection. This effect is likely due to the ability of RORγt⁺ cells to govern local inflammation. Strikingly, the phenotype of graft infiltrating RORγt⁺ cells is amenable to pharmacological manipulation with alpha 1 anti-trypsin (AAT). Either treatment with AAT or use of RORγt deficient mice prolongs engraftment of islet allografts. Our data indicates an important extra-medullary role for Lin^-RORγt⁺hematopoietic progenitor cells in response tolocal injury induced by inflammation and an important role for RORγt gene expression in the response to allografts. Lin^-RORγt⁺hematopoietic progenitor cells may provide a novel, unrecognized target for treatment of allograft rejection.