Donor MHC class I gene transfer to recipient liver: mechanistic studies of a novel strategy for transplant tolerance induction

Abstract

While we have made progress in the treatment of acute rejection, the long-term benefits of transplantation are often truncated by chronic rejection due to activation of the recipient’s immune system to alloantigens, primarily derived from donor MHC molecules. Current immunosuppressive therapy has significant side effects including infection and malignancy, as well as toxicity to end organs. Interestingly, the liver has a unique propensity to confer immunological tolerance; liver transplants have been accepted across complete MHC mismatches. Achieving this state of operational tolerance remains the goal for allotransplantation. We confirm our previous finding that transduction of recipient liver with donor MHC class I molecules using an optimised recombinant adeno-associated viral vector (rAAVKb) leads to acceptance of skin transplants from donor 178.3 strain mice (Kk,b) to B10.BR recipients (Kk) through ‘functional silencing’ of an expanded population of alloreactive CD8 T cells (grafts viable at 250 days). Naïve B10.BR mice primed by rejecting a 178.3 skin graft and then inoculated with rAAVKb a week after rejection displayed increased numbers of peri-portal FoxP3+ cells, presumably a population of induced Tregs. To better understand the immune mechanisms underlying tolerance induction, we engineered an MHC class I molecule (rAAVD227K) with a single point mutation which abrogates CD8 co-receptor binding and therefore direct antigen recognition. 178.3 skin grafts to B10.BR mice one week after inoculation with rAAVD227K were rejected failed to demonstrate tolerance (MST 27 days). Examination of isolated hepatocytes revealed transient upregulation of the programmed cell death ligand 1 at day 7 on animals given rAAVKb but not on those given rAAVD227K. We found that blockade of the PD-L1 ligand in Kb-transduced animals resulted in liver inflammation, but did not break tolerance to subsequent 178.3 allografts. Taken together, these findings show that direct antigen recognition plays a pivotal role in harnessing the immunomodulatory effects of the liver, while PD11:PD-L1 appears to be involved in, but is not a pre-requisite for, subsequent allograft acceptance.