Rapid rejection of HLA-A2 transgenic skin graft due to indirect allorecognition

Relevance of regulatory T cell promotion of donor-specific tolerance in solid organ transplantation

Current clinical strategies to control the alloimmune response after transplantation do not fully prevent induction of the immunological processes which lead to acute and chronic immune-mediated graft rejection, and as such the survival of a solid organ allograft is limited. Experimental research on naturally occurring CD4⁺CD25^highFoxP3⁺ Regulatory T cells (Tregs) has indicated their potential to establish stable long-term graft acceptance, with the promise of providing a more effective therapy for transplant recipients. Current approaches for clinical use are based on the infusion of freshly isolated or ex vivo polyclonally expanded Tregs into graft recipients with an aim to redress the in vivo balance of T effector cells to Tregs. However mounting evidence suggests that regulation of donor-specific immunity may be central to achieving immunological tolerance. Therefore, the next stages in optimizing translation of Tregs to organ transplantation will be through the refinement and development of donor alloantigen-specific Treg therapy. The altering kinetics and intensity of alloantigen presentation pathways and alloimmune priming following transplantation may indeed influence the specificity of the Treg required and the timing or frequency at which it needs to be administered. Here we review and discuss the relevance of antigen-specific regulation of alloreactivity by Tregs in experimental and clinical studies of tolerance and explore the concept of delivering an optimal Treg for the induction and maintenance phases of achieving transplantation tolerance.

Relative contribution of direct and indirect allorecognition in developing tolerance after liver transplantation

The interaction of donor passenger leukocytes and host leukocytes in recipient secondary lymphoid tissues during the early posttransplantation period is crucial in directing host immune reactions toward allograft rejection or acceptance. Responsible T cell clones could be activated through the direct and indirect pathways of allorecognition. We examined the role of the indirect pathway in liver transplantation (LT) tolerance by depleting host antigen-presenting cells (APC) with phagocytic activity [e.g., cluster domain (CD)68+/CD163+ macrophages, CD11c+ dendritic cells (DC)] using liposome-encapsulating clodronate (LP-CL). After Lewis rat cell or liver graft transplantation, Brown Norway (BN) rat recipients pretreated with LP-CL showed a significantly reduced type 1 helper T cell cytokine up-regulation than control-LP-treated recipients. In the LT model, LP-CL treatment and host APC depletion abrogated hepatic tolerance; Lewis liver grafts in LP-CL-treated-BN recipients developed mild allograft rejection, failed to maintain donor major histocompatibility complex (MHC) class II+ leukocytes, and developed chronic rejection in challenged donor heart allografts, while control-LP-treated BN recipients maintained tolerance status and donor MHC class II+ hepatic leukocytes. Furthermore, in the BN to Lewis LT model, LP-CL recipient treatment abrogated spontaneous hepatic allograft acceptance, and graft survival rate was reduced to 43% from 100% in the control-LP group. In conclusion, the study suggests that host cells with phagocytic activity could play significant roles in developing LT tolerance.

HLA-A2 class I antigens in couples with recurrent spontaneous abortions

Class I human leukocyte antigen (HLA) antigens, locus A and B, were typed in fertile and infertile couples in cases where one of the spouses carried the HLA-A2 antigen. HLA-class I typing data were obtained from 282 participants, 63 fertile couples and 78 infertile couples with recurrent spontaneous abortions (RSA). Locus A antigens were grouped into eight broad specificities (A1, A2, A3, A9, A10, A11, A19, A28) and locus B antigens were grouped, according to HLA epitopes, in two classes (BW4 and BW6). Although the number of cases is small, significant differences in the distribution of locus A antigens were found between HLA-A2-positive (A2+) women from fertile and infertile couples. HLA-A3, A11 and A28 cross-reacting antigens were absent in women from fertile couples and present in women from infertile couples. HLA-A19, which is associated with amino acid triplets of low immunogenicity, was significantly more often observed in A2+ fertile than in infertile women. An excess of the BW4 epitope was found in A2(-) husbands from infertile couples compared to fertile ones. The results of this study support the idea that in the presence of the HLA-A2 molecule the distribution of HLA-A and B loci antigens may be different in fertile couples compared to couples with recurrent spontaneous abortions. It can be suggested that the HLA-A2 molecule, in context with specific genotypes, may contribute to the overall maternal immune response in normal and disturbed pregnancy.

Simultaneous deletion of MyD88 and Trif delays major histocompatibility and minor antigen mismatch allograft rejection

This study investigated whether ablation of all Toll-like receptor (TLR) signaling influenced skin allograft rejection across a complete donor/recipient mismatch of major histocompatibility and minor antigens. We predicted that defective TLR signaling would interfere with the activation of donor dendritic cells (DC) in vivo, by preventing DC activation in response to local environmental ("danger") signals. The ablation of TLR signals would therefore be associated with decreased activation of host T cells and decreased graft rejection. Using mice with deletions of the proximal TLR adapter proteins MyD88 or Trif, and those with simultaneous deletions of both MyD88 and Trif, we demonstrated that absence of both MyD88 and Trif adapter proteins prolonged skin graft survival, notably across a complete MHC and minor antigen barrier. Absence of MyD88 or Trif alone only had a modest effect on graft survival across even a minor MHC antigen difference. Prolonged survival of skin grafts from mice deficient in both MyD88 and Trif was associated with diminished migration of donor cells to draining lymph nodes and, subsequently, with delayed infiltration of host T cells into the grafted tissue.

Attenuated alloreactivity of dendritic cells engineered with surface-modified microspheres carrying a plasmid encoding interleukin-10

In the present study, we investigated MS(O10H6) as a carrier system to introduce a plasmid encoding murine interleukin-10 (pIL-10) to modulate alloreactivity of dendritic cells (DC). Results indicate that MS(O10H6) formed stable and protective nano-sized particles with pIL-10. Gene-modified DC elicited weak proliferation of allogeneic CD4 and CD8 T cells in vitro. Using cell-embedded Matrigel as a surrogate graft, we also showed that DC transfected with MS(O10H6) complexed with pIL-10 suppressed host cell infiltration in vivo. These data demonstrate that the self-assembled system of MS(O10H6) is an effectual delivery vehicle for plasmid-based modulation of DC-dependent allogeneic T cell responses.

Exhaustive Depletion of Graft Resident Dendritic Cells: Marginally Delayed Rejection but Strong Alteration of Graft Infiltration

BACKGROUND

Donor dendritic cells (DDC) are believed to sustain direct recognition leading to acute allograft rejection. However, DDC are also required for tolerance induction in various models.

METHODS

We studied the effect of DDC depletion on major histocompatibility complex (MHC) mismatched rat heart allografts in a strain combination characterized by a DDC-dependant tolerance induction. Grafts were depleted of DDC either by pretreating donors with cyclophosphamide (CyP) or by being parked in an intermediate recipient treated with cyclosporine A (CsA).

RESULTS

CyP depleted 95% of resident DC and no specific donor MHC class II staining was observed in parked grafts. Parked grafts survived significantly but only moderately longer than untreated grafts (10.8+/-1.9 days vs. 6.5+/-0.5 days; P<0.05). Compared to unmodified grafts, on day 5 after transplantation, the magnitude of the graft infiltrate was dramatically decreased in DDC-depleted grafts, with IgG deposition within the grafts at the time of rejection. In parallel, the cytokine transcript levels were also lower in these grafts on day 5, but reached levels similar to those of unmodified grafts by day 7, indicating a delayed pattern of rejection.

CONCLUSIONS

Taken collectively, these data suggest that DDC depletion has a greater effect on the capacity of tolerance induction than the rejection process.

In Vivo Recognition by the Host Adaptive Immune System of Microencapsulated Xenogeneic Cells

BACKGROUND

Microencapsulation is under consideration as a means of enabling pancreatic islet transplantation. To understand better the ongoing destructive host response, we examined whether the adaptive immune system of the recipient recognized polymer-encapsulated xenogeneic cells implanted intraperitoneally.

METHODS

Balb/c mice were implanted with xenogeneic Chinese hamster ovary cells, inside and outside poly(hydroxyethyl methacrylate-methyl methacrylate) microcapsules, and responses were compared with xenografted Chinese hamster skin (positive control). Capsules were localized within an agarose rod. Splenocyte proliferation upon rechallenge in vitro, antibody titer in serum, and Th1/2 polarization (assessed by interleukin-4 and interferon-gamma in supernatants of antigen-challenged splenocytes and immunoglobulin [Ig]G1 and IgG2a antibody isotypes in serum) were measured.

RESULTS

Encapsulation did not prevent a strong recipient antibody response. Splenocyte proliferation in vitro did not differ after priming by implanted cells, inside or outside capsules. Thus, the capsule membrane did not prevent indirect recognition of shed antigens. However, after 10 days of implantation, proliferation was lower than that induced by skin grafts, although this difference disappeared by 2 months. This transient T-cell suppression was unexpected because encapsulated cell viability was already compromised by 10 days. The influence of Th1/2 bias did not explain the observed suppression. Cells inside capsules elicited a consistent Th2 response, whereas cells outside capsules elicited a mixed response, and skin xenografts showed an initial Th2 response that became mixed by 2 months.

CONCLUSIONS

Encapsulation does not prevent host immune responses, but the inflammatory response to the implanted biomaterials or xenogeneic cells may be responsible both for encapsulated cell death and transient T-cell suppression.

Early passenger leukocyte migration and acute immune reactions in the rat recipient spleen during liver engraftment: with particular emphasis on donor major histocompatibility complex class II+ cells

After a short course of tacrolimus, Lewis rat liver allografts induce donor-specific nonreactivity in Brown Norway recipients that is immunosuppression-independent after 28 days. To clarify the role of donor major histocompatibility complex (MHC) class II+ cells, we investigated the migration to the recipient splenic T- and B-cell compartments of different subsets of Lewis MHC class II+ passenger leukocytes. The rise and decline of immune activation were monitored in the hepatic allograft and in the host spleen by analyses of BrdU+ (proliferating) leukocytes, TUNEL+ (apoptotic) cells, apoptosis-associated molecules, TH1/TH2 cytokine profiles, and histoimmunocytochemical examination of graft and splenic tissues. Serial flow cytometry studies during the 28-day period of drug-assisted "hepatic tolerogenesis" showed that migratory MHC class II+ cells accounted for less than half of the donor cells in the host spleen. The class II+ cells consisted mostly of B cells that homed to splenic B-cell follicles with only a sparse representation of dendritic cells that were exclusively found in the splenic periarteriolar lymphoid sheath. In parallel studies, transplantation of the less tolerogenic heart produced a diminutive version of the same events, but with far fewer donor cells in the host spleen, evidence of sustained immune activation, and the development of chronic rejection by 100 days. The data are consistent with the paradigm that migration of donor leukocytes is the prime determinant of variable tolerance induction induced by transplantation of the liver and other organs, but without regard for donor MHC class II+ expression.