Split tolerance to the MHC class I molecule H-2Dd in animals transgenic for its soluble analog

In Utero Exposure to Exosomal and B-Cell Alloantigens Lessens Alloreactivity of Recipients' Lymphocytes Rather than Confers Allograft Tolerance

According to actively acquired tolerance, antigen exposure before full immune development in fetal or early neonatal life will cause tolerance to this specific antigen. In this study, we aimed to examine whether allogeneic tolerance could be elicited by in utero exposure to surface MHC antigens of allogenic cells or soluble form of MHC exosomes. Gestational day 14 FVB/N fetuses were subjected to intraperitoneal injection of allogeneic major histocompatibility complex (MHC) exosomes or highly enriched B-cells. Postnatally, the recipients were examined for the immune responses to donor alloantigens by lymphocyte proliferative reactions and skin transplantation. In utero exposure to allogeneic MHC exosomes abolished the alloreactivity of recipients’ lymphocytes to the alloantigens, but could not confer skin allograft tolerance. In utero transplantation of highly enriched allogeneic B-cells generated low-level B-cell chimerism in the recipients. However, it only extended the survivals of skin allograft by a few days despite the lack of donor-specific alloreactivity of recipients’ lymphocyte. Thus, an early in utero contact with exosomal or B-cell alloantigens did not lead to full skin tolerance but rather, at best, only to delayed skin rejection in the presence of microchimerism made by B-cell inocula. These results argued against the theory of actively acquired tolerance, and implicated that in utero exposure to marrow cells in previous studies was a unique model of allo-tolerance induction that involved the establishment of significant hematopoietic chimerism. Taken together with the discovery of in utero sensitization to ovalbumin in our previous studies, the immunological consequences of fetal exposure to foreign antigens might vary according to the type or nature of antigens introduced.

Maternal immune responses to trophoblast: the contribution of the horse to pregnancy immunology

The horse has proven to be a distinctively informative species in the study of pregnancy immunology for several reasons. First, unique aspects of the anatomy and physiology of the equine conceptus facilitate approaches that are not possible in other model organisms, such as non-surgical recovery of early stage embryos and conceptuses and isolation of pure trophoblast cell populations. Second, pregnant mares make strong cytotoxic antibody responses to paternal major histocompatibility complex class I antigens expressed by the chorionic girdle cells, permitting detailed evaluation of the antigenicity of these invasive trophoblasts and how they affect the maternal immune system. Third, there is abundant evidence for local maternal cellular immune responses to the invading trophoblasts in the pregnant mare. The survival of the equine fetus in the face of strong maternal immune responses highlights the complex immunoregulatory mechanisms that result in materno-fetal tolerance. Finally, the parallels between human and horse trophoblast cell types, their gene expression, and function make the study of equine pregnancy highly relevant to human health. Here, we review the most pertinent aspects of equine reproductive immunology and how studies of the pregnant mare have contributed to our understanding of maternal acceptance of the allogeneic fetus.

Split immunological tolerance to trophoblast

Split immunological tolerance refers to states in which an individual is capable of mounting certain types of immune responses to a particular antigenic challenge, but is tolerant of the same antigen in other compartments of the immune system. This concept is applicable to the immunological relationship between mother and fetus, and particularly relevant in equine pregnancy. In pregnant mares, antibody responses to paternal foreign Major Histocompatibility Complex class I antigens are robust, while anti-paternal cytotoxic T cell responses are diminished compared to those mounted by non-pregnant mares. Here, we compared the distribution of the major lymphocyte subsets, the percentage of lymphocytes expressing Interferon Gamma (IFNG) and Interleukin 4 (IL4) and the level of expression of the immunoregulatory transcription factor FOXP3 between pregnant and non-pregnant mares, and between peripheral blood and the endometrium during pregnancy. In a cohort of mares in which peripheral blood lymphocytes were tested during early pregnancy and in the non-pregnant state, there were only slight changes observed during pregnancy. In contrast, comparison of peripheral blood lymphocytes with lymphocytes isolated from the endometrial cups of pregnant mares revealed striking differences in lymphocyte sub-populations. The endometrial cups contained higher numbers of IFNG+ lymphocytes, and lower numbers of lymphocytes expressing IL4. The endometrial cup lymphocytes also had higher numbers of FOXP3+ cells compared to peripheral blood lymphocytes. Taken together, these results strengthen the evidence for a state of split tolerance to trophoblast, and furthermore define sharp differences in immune reactivity during equine pregnancy between peripheral blood lymphocytes and lymphocytes at the maternal-fetal interface.

Circulating, soluble forms of major histocompatability complex antigens are not exosome-associated

In vitro studies have shown that soluble MHC (sMHC) released by cell lines is bound to nano-vesicles termed exosomes. It is thought that exosomes may represent the major reservoir of sMHC class I and II molecules in biological fluids. However, most studies have been confined to in vitro assays performed with cell lines. We show here that sMHC in the serum or plasma differs from exosome-bound sMHC in five ways: In contrast to exosome-associated sMHC, circulating sMHC is of low density, has a low apparent molecular mass (40-300 kDa) and is not detergent-labile. Moreover, the majority of MHC class II isoforms and MHC class I in blood are not physically linked and circulating HLA-DR is accessible to an antibody specific for the HLA-DR alpha-chain intracellular epitope, which is masked by its association with cellular or exosomal membranes. Finally, utilizing transcriptional activator of murine MHC class II (C2ta) promoter-mutant mice, we showed that the release of sMHC class II into the circulation is dependent on the C2ta pI promoter, but not pIII or pIV. This suggests that myeloid dendritic cells and/or macrophages, which preferentially use promoter pI of the C2ta gene, produce most of the sMHC class II found in the circulation.

Donor-derived soluble MHC antigens plus low-dose cyclosporine induce transplantation unresponsiveness independent of the thymus by down-regulating T cell-mediated alloresponses in a rat transplantation model

BACKGROUND

In vitro, soluble MHC (sMHC) antigens modulate and induce apoptosis in alloreactive and antigen-specific T cells, demonstrating their potency to regulate T cell-mediated immune responses. However, their efficacy to regulate immunological responses in vivo remains unclear. Here, we report that repetitive intraperitoneal injection of recombinant Lewis rat-derived MHC class I antigens in Dark Agouti (DA) rats modulates alloreactivity.

METHODS

RT1.A1 (Lewis derived) genes were cloned into mammalian expression vectors, and RT1.Aa (DA derived) genes were used to transfect a rat myeloma cell line. RT1.A1 molecules were injected intraperitoneally in DA recipients that subsequently underwent transplantation with Lewis-derived cardiac allografts.

RESULTS

Soluble class I antigens were secreted by the transfected cells and were shown to be heterodimeric, peptide-loaded, and conformationally folded. Injection of donor-derived soluble MHC significantly reduced the ability of recipient animals to mount a cytotoxic T-cell response to donor-derived tissue. More interestingly, this treatment significantly prolonged donor-graft survival and allowed 60% of treated animals to develop graft tolerance (>120 days), when donor sMHC were combined with a single subtherapeutic dosage of cyclosporine. Thymectomy of recipient animals before transplantation did not interfere with induction of peripheral tolerance.

CONCLUSIONS

Donor-derived sMHC are potential tolerogens for down-regulating the cytotoxic T-cell response of animals that undergo transplantation. Thus, these data provide for the first time a rationale for the application of directly injected sMHC in vivo to down-regulate immunological responses and aid the induction of graft tolerance.

Antibodies directed against the MHC-I molecule H-2Dd complexed with an antigenic peptide: similarities to a T cell receptor with the same specificity

alphabeta TCRs, which use an Ab-like structure to form a combining site, recognize molecular complexes consisting of peptides bound to MHC class I (MHC-I) or class II (MHC-II) molecules. To explore the similarities and differences between Ab and T cell recognition of similar structures, we have isolated two mAbs, KP14 and KP15, that specifically bind H-2D(d) complexed with an HIV envelope gp160-derived peptide, P18-I10. These Abs are MHC and peptide specific. Fine specificity of mAb binding was analyzed using a panel of synthetic peptides, revealing similarities between the mAb and a cloned TCR with the same specificity. These two mAbs used the same V(H) and J(H) gene segments, but different D, Vkappa, and Jkappa genes. Administered in vivo, mAb KP15 blocked the induction of CTL specific for recombinant vaccinia virus-encoded gp160, indicating its ability to bind endogenously generated MHC/peptide complexes. Analysis of the fine specificity of these mAbs in the context of their encoded amino acid sequences and the known three-dimensional structure of the H-2D(d)/P18-I10 complex suggests that they bind in an orientation similar to that of the TCR. Thus, the plasticity of the B cell receptor repertoire and the structural similarities among BCR and TCR allow Abs to effectively mimic alphabeta TCRs. Such mAbs may be useful in the therapeutic modulation of immune responses against infectious agents or harmful self Ags as well as in tracing steps in Ag processing.

Co-transplantation of donor-derived hepatocytes induces long-term tolerance to cardiac allografts in a rat model

BACKGROUND

Liver allografts transplanted between MHC-disparate mice, rats, and swine are spontaneously accepted in most strain combinations without requirement for immunosuppression. The underlying mechanism has, however, remained elusive. Here, we demonstrate that co-transplantation of donor-derived hepatocytes protect Lewis (RT1.A1) cardiac allografts from acute and chronic rejection in DA (RT1.Aa) recipients indefinitely.

METHODS

Livers of donor Lewis rats were harvested and the hepatocytes separated from hepatic leukocytes by collagenase digestion and gradient separation. DA recipient animals were transplanted Lewis cardiac allografts and simultaneously intraportally infused either Lewis-derived hepatocytes or hepatic leukocytes. Recipient animals were either not further treated or received a single dose of 15 mg/kg cyclosporine.

RESULTS

Donor hepatocytes alone significantly protected syngeneic cardiac allografts from rejection, whereas hepatic leukocytes failed to influence graft survival. In combination with cyclosporine, recipient cardiac allografts were indefinitely protected from rejection. Graft-infiltrating cells in tolerant animals presented as clusters of CD4+ T cells and stained mostly positive for interleukin-4, whereas graft-infiltrating cells in rejected allografts were predominantly positive for interferon-gamma. Adoptive transfer of splenocytes derived from tolerant animals protected Lewis cardiac allografts from rejection in DA recipients without immunosuppression. In contrast, hepatic leukocytes protected only 50% of the allografts from rejection.

CONCLUSION

We propose that donor hepatocytes induce permanent engraftment of syngeneic allografts by establishing a Th2 type alloresponse that is transferable to new graft recipients. The results of this study demonstrate that liver parenchymal cells significantly mediate spontaneously liver-induced tolerance.

HLA (A*0201) Mimicry by Anti-Idiotypic Monoclonal Antibodies

Soluble MHC Ags and anti-Id (anti-anti-MHC) Abs have both been shown to inhibit MHC alloantigen-specific B cell responses in vivo. We hypothesized that some anti-idiotypic Abs function as divalent molecular mimics of soluble HLA alloantigen. To test this idea, we studied two well-defined anti-idiotypic mAbs, T10-505 and T10-938, elicited in syngeneic BALB/c mice by immunization with CRll-351, an HLA-A2,24,28-specific mAb. Each anti-Id induced “Ab-3” Abs in rabbits that cross-reacted with HLA-A2 but not with HLA-B Ags. Furthermore, each anti-Id could bind to and block Ag recognition by Ha5C2.A2, a human homologue of mAb CRll-351. Both anti-Id mAb displayed weak reactivity with the human mAb SN66E3, which recognized an overlapping but distinct determinant of HLA-A2 Ags; neither reacted with human mAb MBW1, which recognized a nonoverlapping HLA-A2 determinant. Amino acid sequence comparison of mAb CRll-351 heavy and light chain variable region complementarity-determining regions (CDRs) with those of mAb Ha5C2.A2 and SN66E3 revealed short regions of homology with both human mAb; a large insert in the light chain CDR1 of mAb SN66E3 distinguished it from both CRll-351 and Ha5C2.A2. The amino acid sequences of mAb T10-505 and T10-938, which differed markedly from each other, revealed no homology to the α2 domain sequence of HLA-A*0201 that contains the CRll-351 mAb-defined epitope. We conclude that structurally different anti-Id Abs can mimic a polymorphic conformational epitope of an HLA Ag. In the case of T10-505 and T10-938 mimicry was not based on exact replication of the epitope by the hypervariable loops of the anti-Id mAb.

The metalloproteinase-mediated pathway is essential for generation of soluble HLA class I proteins by activated cells in vitro: proposed mechanism for soluble HLA release in transplant rejection

We and others have found donor-derived soluble beta2m-associated HLA class I proteins (sHLA/beta2m) in the serum of allograft recipients with acute and chronic rejection. Whether appearance of sHLA/beta2m and upregulated expression of donor cell-bound HLA/beta2m during allograft rejection are related events is unknown. Activation-induced upregulation of in vitro HLA/beta2m expression correlates with the surface expression of another form of HLA class I, namely beta2m-free HLA heavy chains (beta2m-free HC). We have shown that beta2m-free HC, but not beta2m-associated HC, are then cleaved by a specific membrane-bound metalloproteinase and released into supernatants as soluble 36 kDa proteins. We show now that activated peripheral blood lymphocytes produce predominantly the 36 kDa form of sHLA proteins which is present in supernatants as both beta2m-free HC and sHLA/beta2m. Importantly, the metalloprotease inhibitor BB-94 blocked not only the release of soluble beta2m-free HC, but also the appearance of sHLA/beta2m in cell supernatants. Low levels of 36 kDa beta2m-free HC were also present in human plasma of healthy donors. These data suggest an important role for the HLA class I-specific metalloproteinase in vivo in healthy individuals and during allograft rejection in the generation of soluble beta2m-free and beta2m-associated HLA proteins.

Soluble HLA class I molecules: biological significance and clinical implications

Soluble class I human leukocyte antigens (sHLAs) have been detected in serum, sweat, lymphatic fluid, urine and cerebrospinal fluid. Their biological function has, however, remained a puzzle. The physiological concentration of sHLA varies more than tenfold depending on the phenotype of the individual, and is significantly upregulated in various diseases and during inflammation. This suggests that sHLAs might serve as a marker of pathological changes. Recent experiments have shown that, in vitro, sHLAs can modulate T-cell reactivity and induce cell-activated apoptosis, implicating sHLAs in the induction and maintenance of peripheral tolerance. Therefore, sHLAs have the therapeutic potential to induce tolerance to transplants.

Soluble HLA class I in epithelial lining fluid of lung transplants: associations with graft outcome

We hypothesized that the small amounts of donor HLA-A and HLA-B proteins detected in the serum during organ allograft rejection are indicative of higher local releases within the graft itself. We determined the concentrations of total HLA class I (HLA-I) and, in selected cases, specific donor and host HLA-A and HLA-B proteins, in the epithelial lining fluid (ELF) sampled by bronchoalveolar lavage (BAL) of lung transplant recipients (n = 37) and of normal controls (n = 25). We found that 1) HLA-I proteins were enriched in the lung ELF relative to other proteins; 2) the concentration of HLA-I in the ELF of well-functioning transplants was similar to that in normal lungs; 3) HLA-I proteins and total proteins were elevated in the ELF of patients who developed chronic rejection or refractory acute rejection; 4) the concentration of HLA-I was correlated with the percentage of neutrophils but not with the percentage of lymphocytes in the ELF of transplanted lungs; and 5) only the percentage of lymphocytes was elevated in the ELF of transplant patients with active CMV infections. Total HLA-I from the ELF was found to contain a mixture of both donor- and recipient-type HLA-A and HLA-B proteins and the donor-type HLA-A2 was found to be highly enriched in the ELF relative to serum.