Antigen Exposure during Enhanced CTLA-4 Expression Promotes Allograft Tolerance In Vivo

Abstract

The role of CTLA-4 in tolerance is primarily inferred from knockout and blocking studies. Anti-CD45RB mediates allograft tolerance in mice by inducing CTLA-4 expression on CD4 cells, providing a novel opportunity to determine how therapeutic enhancement of CTLA-4 promotes tolerance. We now show that induced CTLA-4 expression normally resolves by day 17. Although thymectomy prolongs enhanced CTLA-4 expression, long-term engraftment is unaffected. To address the temporal relationship between increased CTLA-4 expression and engraftment, transplantation was delayed for various times after anti-CD45RB treatment. Delaying transplantation for 7 days (when CTLA-4 expression had peaked but treatment mAb was no longer detectable), resulted in long-term engraftment comparable to transplantation with no delay (day 0). Delaying transplantation from 10 to 18 days led to a progressively poorer outcome as CTLA-4 expression returned to baseline. This suggested that Ag exposure while CTLA-4 expression is enhanced is sufficient to induce long-term engraftment. To substantiate this, on day 0, anti-CD45RB-treated mice received BALB/c vs unrelated alloantigen, followed by transplantation of BALB/c islets 10 days later. Whereas recipients exposed to unrelated Ag experienced acute rejection, recipients exposed to donor Ag achieved long-term engraftment. Anti-CD45RB-treated mice exposed to alloantigen exhibited anergic CD4+CD25− effector cells and regulatory CD4+CD25+ cells. Moreover, CD25 depletion in the peritransplant period prevented anti-CD45RB-mediated engraftment. Thus, exposure of CD4 cells expressing CTLA-4 to donor Ag is necessary and sufficient to induce long-term engraftment which appears to be mediated by both regulation and anergy.

CD45 ligation expands Tregs by promoting interactions with DCs

“…To address this, mice treated with CsA and anti-CD45RB also received daily recombinant IL-2 (rIL-2) at a dose that augments Treg number and HP in the absence of CsA (data not gen. In agreement, transplant studies suggest that anti-CD45RB promotes antigen-specific Tregs (25,29,54). For example, anti-CD45RB generates donor-specific tolerance that is abrogated with depletion of Tregs by anti-CD25.…”

“…To rule out an effect on thymic output, we showed that anti-CD45RB increased Treg numbers in euthymic and thymectomized mice equally (Figure 2A). Concordantly, thymectomy did not prevent anti-CD45RB-induced prolongation of islet allograft survival by anti-CD45RB (29). This suggests that anti-CD45RB acts on Tregs in the periphery.…”

“…We showed previously that anti-CD45RB treatment induces donor-specific tolerance in stringent murine allograft models, and this is dependent on Tregs and induction of CTLA-4 on CD4 + cells (25)(26)(27)(28)(29). This raised the possibility that anti-CD45RB acts through generation of Tregs.…”

Cutting Edge: Transplant Tolerance Induced by Anti-CD45RB Requires B Lymphocytes

“…Recent studies correlated the effect of anti-CD45RB with up-regulation of CTLA-4 on T cells (6,7). Because CTLA-4 can deliver negative signals in T cell activation and may be expressed by regulatory T cells (Tregs), 6 anti-CD45RB-induced up-regulation of CTLA-4 may play an important role in deactivation of graft-reactive lymphocytes (8,9). In addition, induction of T cell anergy and formation of Th2 or Treg cells have been suggested in previous studies (4,10).…”

CD45RB Ligation Inhibits Allergic Pulmonary Inflammation by Inducing CTLA4 Transcription

“…In addition, we have previously shown that aCD45 treatment increases CD25 expression on most of the CTLA4-positive T cells. These CD25-positive cells from aCD45-treated mice exhibit Treg activity, suggesting that aCD45 induces Tregs (44). Approximately 70% of CTLA4-positive cells from naive mice treated with aCD45 also coexpressed Foxp3, suggesting that aCD45 may induce Tregs in vivo (D. Rothstein, unpublished data).…”