Mechanism of Action of Donor-Specific Transfusion in Inducing Tolerance

Abstract

Abstract. Donor-specific transfusion (DST) can synergize with T cell co-stimulatory blockade in inducing tolerance in several transplant models, but the mechanism of action of DST is poorly characterized. This study used genetically altered mice in an established model of cardiac transplantation to study the role of MHC and co-stimulatory molecule expression on DST cells in mediating the immunomodulatory effects of DST. In addition, to examine the role of indirect antigen presentation in the effect of DST, experiments used recipient mice that do not express MHC class II molecules on peripheral antigen-presenting cells, but do have functional CD4 ϩ T cells (II Ϫ 4 ϩ ). As previously reported, treatment with DST from wild-type donors in combination with CD154 blockade induced tolerance in wild-type recipients of cardiac allografts. Tolerance in this model is also induced despite the absence of MHC class I and II, CD40, or B7 molecules on transfused cells. In contrast, eliminating the indirect pathway using II Ϫ 4 ϩ recipients blocked the induction of long-term cardiac allograft survival by DST. These results indicate that the indirect antigen recognition pathway mediates the immunomodulatory effect of DST in inducing transplantation tolerance in vivo.Numerous studies have shown that transfusion of allogeneic donor cells (donor-specific transfusion [DST]) has the potential to induce tolerance to a specific allograft. In particular, transfusion of donor splenic leukocytes at the time of transplantation synergizes with B7/CD154 co-stimulatory signal blockade in promoting long-term allograft survival, inducing tolerance, and preventing chronic allograft rejection in some models (1-4). Postulated mechanisms of the immunomodulatory functions of DST include clonal deletion (5,6), induction of anergy (7), generation of regulatory cells (8,9), regulation of cytokine production (5,7,10), promotion of microchimerism (10), provision of soluble MHC antigens (11), or a combination of these mechanisms (5,7,10,12). However, the proximal mechanisms involving alloreactive T cell interactions with DST remain unknown. The aim of this study was to investigate the role of MHC and co-stimulatory molecule expression on the immunomodulatory functions of DST in a fully allogeneic cardiac transplant model using specific gene knockout mice as sources of DST. In addition, to investigate the role of indirect recognition of antigen provided by DST, we used as recipient transgenic mice that express MHC class II only on thymic epithelium and develop normal numbers of CD4 ϩ T cells but express no MHC class II on peripheral antigen-presenting cells (APC) (13).

Endogenous dendritic cells mediate the effects of intravenously injected therapeutic immunosuppressive dendritic cells in transplantation

“…[10][11][12]37,38 Our results indicate that even the more recently developed therapies using recipient-derived immunosuppressive DCs pulsed with donor-Ag 22-24 could also function via endogenous DCs because host (B6) MR-DCs pulsed with the BALB/c IE␣ 52-68 peptide also failed to interact directly with T cells in vivo ( Figure 3B). Our results also suggest that DST and DC-based therapies may function through a shared mechanism of reprocessing/presentation by recipient quiescent DCs in secondary lymphoid organs.…”

“…Finally, we compared the effect on allograft survival of donor-derived MR-DCs with that of DST splenocytes, the latter previously shown to be a much simpler method of regulating the antidonor response via the indirect pathway. [10][11][12]37,38 Surprisingly, DSTs with alive or apoptotic splenocytes prolonged cardiac allograft survival in mice comparable with therapy with donorderived MR-DCs (56.0 Ϯ 32.1 days, 45.9 Ϯ 26.5 days, and 52.2 Ϯ 33.4 days, respectively, Figure 7C). …”

“…B6 MHC I/II double KO cells express little, if any, MHC I/II molecules on the surface but are able to produce MHC I and MHC II IA␣ and IE␤ chains, which are retained intracellularly and constitute the source of donor-Ag in our model. 12 Thus, therapeutic immunosuppressive DCs unable to function as APCs, but capable of delivering donor-Ag to recipient APCs, significantly prolong cardiac allograft survival in our system. Finally, we compared the effect on allograft survival of donor-derived MR-DCs with that of DST splenocytes, the latter previously shown to be a much simpler method of regulating the antidonor response via the indirect pathway.…”

“…Our findings indicate that systemically injected MR-DCs do not directly present donor-Ag but rather serve as a source of donor-Ag for recipient DCs for presentation to indirect pathway T cells, down-regulation of the antidonor response, and prolongation of allograft survival, similar to DST. 11,12 Our findings suggest a shared mechanism of action between these therapies and call into question the potential clinical superiority of current DC-based therapies in transplantation.…”

“…7 However, it was later demonstrated that presentation of donor-Ag in the context of self-major histocompatibility complex (MHC) molecules by recipient Ag-presenting cells (APCs), through the indirect pathway of allorecognition, is critical for the DST effect. [10][11][12] The finding that DST sensitizes a percentage of recipients and the introduction of new immunosuppressive agents discontinued the clinical use of DST in the 1980s. 7 During the past 15 years, a new generation of cell therapies based on intravenous administration of donor-or recipient-derived dendritic cells (DCs) expanded in vitro and rendered immunosuppressive by pharmacologic or genetic methods has been used to down-regulate the host-versus-graft [13][14][15][16][17][18][19][20][21][22][23][24][25] and graft-versus-host 26 responses.…”