Introduction Graft versus host disease (GVHD) after liver transplantation (LTx) is
a rare complication yet associated with a high mortality. Donor T cells, sited in
high numbers in the liver graft are mediators of allo-reactivity. Recent studies suggest
that allo-reactive T cells are regulated by CD4/CD25/FoxP3-positive regulatory T cells
(Tregs). Here, we provide evidence of an alternative regulatory pathway involving
the membrane receptor Programmed Death–1 (PD1). Methods and Results At postoperative
day 35 (POD 35) after liver transplantation for alcoholic liver cirrhosis, our patient
presented with acute GVHD of the skin associated with high numbers of skin infiltrating
CD8-positive T cells. In addition, he developed an aplastic anemia with small lymphocytic
infiltrates in the bone marrow. Lineage-specific chimerism analysis showed that 80%
of CD8-positive cells in the peripheral blood on POD 35 and even 100% on POD 50 were
of donor origin. B-cells and CD4-positive T cells at the same time were between 50
to 60% donor, whereas the granulocytes remained of host origin. The numbers of Tregs
in the peripheral blood were subnormal with a ratio of 0.9% to 3.7% CD25FoxP3-positive
cells and did not increase during the ongoing GVHD. T cells in the peripheral blood
as well as in the bone marrow showed high expression levels of PD1. By co-staining
of HLA-class I alleles, we were able to demonstrate that in cells isolated from the
bone marrow, PD1 was positive in donor rather than host T cells, whereas the activation
marker CD69 was only expressed in T cells of the host. We isolated PD1-positive and
PD1-negative fractions of CD8-positive T cells from the peripheral blood by FACS-sorting
and used both in mixed lymphocyte reactions (MLR) with host dendritic cells as stimulators.
After one week of stimulation, the T cells were tested for allo-reactivity in an IFN-gamma
ELISPOT assay and allo-reactivity was solely present in the PD1-positive fraction.
The addition of a blocking PD1-ligand-specific antibody increased allo-reactivity
in this assay, demonstrating an ongoing and active suppression through this pathway.
Similar results were obtained in a second patient with GVHD after LTx. Conclusion
Here, we provide evidence for a PD1-dependent regulation of allo-reactivitiv T cells
in GVHD after liver transplantation. PD1 might be an interesting candidate to target
allo-reactive T cells after LTx.
liver transplantation alloreactivity tolerance
