Both Infiltrating Regulatory T Cells and Insufficient Antigen Presentation Are Involved in Long-Term Cardiac Xenograft Survival

Abstract

We have previously shown that pretransplant donor lymphocyte infusion (DLI) together with transient depletion of CD4+ T cells could induce permanent rat-to-mouse heart graft survival, whereas depleting CD4+ T cells alone failed to do so. In this study, we investigated the mechanism leading to long-term xenograft survival. We found that peripheral CD4+ T cells from DLI/anti-CD4-treated mice could mount rat heart graft rejection after adoptive transfer into B6 CD4−/− mice. Infusing donor-Ag-loaded mature dendritic cells (DCs) could break long-term cardiac xenograft survival in DLI/anti-CD4-treated mice. Interestingly, when the number and phenotype of graft-infiltrating cells were compared between anti-CD4- and DLI/anti-CD4-treated groups, we observed a significant increase in both the number and suppressive activity of αβ-TCR+CD3+CD4−CD8− double negative regulatory T cells and decrease in the numbers of CD4+ and CD8+ T cells in the xenografts of DLI/anti-CD4-treated mice. Moreover, there was a significant reduction in MHC class II-high DCs within the xenografts of DLI/anti-CD4-treated recipients. DCs isolated from the xenografts of anti-CD4- but not DLI/anti-CD4-treated recipients could stimulate CD4+ T cell proliferation. Our data indicate that functional anti-donor T cells are present in the secondary lymphoid organs of the mice that permanently accepted cardiac xenografts. Their failure to reject xenografts is associated with an increase in double negative regulatory T cells as well as a reduction in Ag stimulation by DCs found within grafts. These findings suggest that local regulatory mechanisms need to be taken into account to control anti-xenograft T cell responses.

Regulation of antigen‐expressing dendritic cells by double negative regulatory T cells

“…Furthermore, mice that were rendered tolerant to cardiac xenografts had increased numbers of DN Tregs in accepted cardiac xenografts [13]. Interestingly, DCs isolated from tolerant cardiac xenografts remained in an immature state and failed to stimulate T-cell proliferation in vitro, compared to DCs obtained from rejecting grafts [13].…”

“…Previous studies have shown that both murine and human DN Tregs can suppress immune responses through direct cytolysis of activated Ag-specific T effector cells [3,7,8]. We recently found that the number of DN Tregs was increased in the spleen and in accepted cardiac xenografts after pretransplant donor-specific lymphocyte infusion plus a short course of anti-CD4 mAb treatment in mice [13]. DCs from tolerant cardiac xenografts remained in an immature state and failed to stimulate T-cell proliferation in vitro compared to DCs from rejecting grafts [13].…”

“…We recently found that the number of DN Tregs was increased in the spleen and in accepted cardiac xenografts after pretransplant donor-specific lymphocyte infusion plus a short course of anti-CD4 mAb treatment in mice [13]. DCs from tolerant cardiac xenografts remained in an immature state and failed to stimulate T-cell proliferation in vitro compared to DCs from rejecting grafts [13]. These findings suggest that in addition to direct cytolysis of activated Ag-specific T cells, DN Tregs may inhibit graft rejection by suppressing DCs.…”

“…A single transfer of DN Tregs could prevent the onset of type I diabetes in mice [5]. Furthermore, the number of DN Tregs is increased in accepted allo-and xeno-grafts [12,13]. Adoptive transfer of Ag-activated DN Tregs can prolong donor-specific skin, heart and islet allo-or xeno-graft survival [3,8,14].…”

Consequences of double negative regulatory T cell and antigen presenting cell interaction on immune response suppression

“…4). Further studies are needed in order to determine whether alloantigen-activated DN Tregs can directly regulate costimulatory molecule expression on DCs, as has been shown using xeno-antigen-activated DN Tregs [39] as well as using CD4 + Foxp3 + Tregs [40]. Recently, Zhang et al showed that xenoantigen activated DN Tregs are able to lyse autologous B cells via the perforin pathway [41,42].…”