The mechanism of graft rejection and the concept of antigenic strength

Complementary DNA sequences encoding the rat MHC class II RT1-Bu and RT1-Du alpha and beta chains

The rat major histocompatibility complex loci RT1-B and RT1-D are equivalent to the human leucocyte antigens HLA-DQ and HLA-DR respectively. Here we describe the complementary DNA (cDNA) sequence encoding the alpha and beta chains of both the RT1-B and RT1-D locus genes of the rat RT1u haplotype. We have found entire sequence identity between five different inbred rat strains of the RT1u haplotype, which differs from previously published, incomplete sequences. This information is of considerable value for experimental studies of transplantation immunity and autoimmune disease.

T cell recognition of donor major histocompatibility complex class I peptides during allograft rejection

LEW (RTI1) recipients of DA (RTIav1) skin and kidney allografts were tested for the capacity of their T lymphocytes to proliferate to three 22-24-amino acid peptides from the hypervariable regions of the RTI-Aav1 classical MHC class I molecule. Ten days after rejecting second-set DA kidney allografts, spleen cells (but interestingly not lymph node cells) from LEW recipients showed strong, LEW antigen-presenting cell-dependent, CD4+ T cell proliferation to peptide 1 (from the alpha helical region of the alpha 1 domain). CD8+ T cells showed no response to peptide 1. There was no response by the spleen cells to peptide 2 (from the beta sheet of the alpha 2 domain) or peptide 3 (from the alpha helical region of the alpha 2 domain). Immunization of LEW rats with pure RTI-Aav1 class I H chain in Freund's adjuvant gave responses identical to that seen after grafting, i.e. good CD4+ T cell proliferation to peptide 1, but none to peptides 2 and 3. However, immunization of LEW rats with peptides 1, 2 and 3 in Freund's adjuvant resulted in good CD4+ T cell proliferation responses to each of the peptides. These data demonstrate that indirect allorecognition can be stimulated by allograft rejection, and emphasize that the physiological processing of donor antigens will influence which peptides will be important in indirect allorecognition in transplantation.

Rejection of skin allografts by indirect allorecognition of donor class I major histocompatibility complex peptides

LEW (RT1l) rats were immunized with peptides corresponding to the alpha helical region of the alpha 1 domain (peptide 1), the beta sheet of the alpha 2 domain (peptide 2), and the alpha helical region of the alpha 2 domain (peptide 3) of the RT1-Aav1 classical class I molecule of the DA (RT1av1) strain. The immunizations were without carriers, and the objective was to prime to indirect allorecognition without influencing direct recognition of the RT1-Aav1 molecule. The LEW rats mounted strong primary and secondary antibody responses to peptides 1 and 3, but only weak secondary responses to peptide 2. None of the antipeptide antibodies crossreacted with intact RT1-Aav1 class I molecules. The immunization also resulted in LEW antigen-presenting cell-dependent, CD4+ T cell proliferative responses, which were very strong against peptide 1 and weakest against peptide 2. LEW rats immunized with peptides 1 or 3, but most effectively with both peptides 1 and 3 together, showed accelerated rejection of DA skin allografts. This effect was not observed in LEW rats immunized with peptide 2. In response to the DA skin allograft, the peptide-immunized LEW rats showed markedly accelerated kinetics of antibody production to the intact RT1-Aav1 molecule. These data demonstrate that indirect allorecognition can play an important role in allograft rejection and have important implications for understanding allograft rejection and its regulation.

Allorecognition of isolated, denatured chains of class I and class II major histocompatibility complex molecules. Evidence for an important role for indirect allorecognition in transplantation

Classical RT1-A class I and RT1-B class II major histocompatibility complex (MHC) molecules were purified from DA (RT1avl) spleens, and the individual chains separated and purified by preparative polyacrylamide gel electrophoresis in sodium dodecyl sulfate. LEW (RT1l) rats were immunized with the pure class I heavy chain, the RT1-B alpha chain and the RT1-B beta chain with the aim of priming to indirect allorecognition (i.e. after processing and presentation of DA MHC chains on LEW antigen-presenting cells) in the absence of any priming to direct allorecognition (i.e. to whole, undenatured, dimeric DA MHC molecules). LEW rats immunized with each of the three DA MHC chains produced alloantibodies to these chains, suggesting that indirect allorecognition did occur, because of the requirement for cognate recognition of B cells by T helper cells. This also demonstrated polymorphism of all three chains between the DA and LEW strains. The antibodies to the isolated, denatured MHC chains did not react to the whole MHC molecules on DA cells, with the possible exception of very weak reactions in some class I heavy chain-immunized rats. DA skin grafts placed on LEW recipients immunized with each of the DA MHC chains were rejected in an accelerated fashion. Following DA skin grafting, there was an accelerated production of antibodies to whole, undenatured class I MHC molecules, even in the LEW rats preimmunized with RT1-B alpha and RT1-B beta chains. These data suggest that indirect allorecognition can play an important role in the effector mechanisms of allograft rejection, and demonstrate T helper priming as one possible mechanism whereby this might be effective.

Processed MHC class I alloantigen as the stimulus for CD4+ T-cell dependent antibody-mediated graft rejection

The traditional view of graft rejection is one of direct recognition of allogeneic MHC molecules by effector T cells, the phenotype of which may be predicted by the nature of the MHC disparity. In this article, Andrew Bradley and colleagues discuss recent evidence that suggests this view may be an oversimplification, and argue that additional effector mechanisms, such as alloantibody, need to be reconsidered.

Corneal transplantation--immunologically privileged status

Despite the relatively high success rate and the word 'immunologically privileged site', it has been known that a corneal graft can induce an allograft rejection reaction. This is especially true in the rat where orthotopic penetrating corneal grafts in certain strain combinations are rejected even when transplanted in avascular bed. Reliable microsurgical techniques, together with the availability of inbred or congenic strains and a rapidly developing knowledge of its major histocompatibility complex (MHC) and immune system in general, have made the rat a prime species in which to study the immunological events after corneal grafting. This review describes recent progress in understanding the immunological mechanisms behind corneal graft rejection. The topics discussed include the rat MHC (RT1) antigens and their distribution in the cornea; different responder status in fully allogeneic strain combinations, including the importance of multiple non-MHC antigens; and the role of antigen-presenting cells (APCs) in corneal graft rejection.

T cell requirements for the rejection of renal allografts bearing an isolated class I MHC disparity

This study has examined the cellular and humoral responses underlying the rejection of rat renal allografts bearing an isolated RT1Aa class I MHC disparity. RT1Aa disparate kidneys were rejected promptly by high responder RT1u but not by low responder RT1c recipients (median survival time 10 d and greater than 100 d, respectively). The magnitude and phenotype of the cellular infiltrate were similar in rejecting and nonrejecting RT1Aa disparate kidneys. Paradoxically, graft infiltrating cells and spleen cells from RT1u recipients showed minimal ability to lyse donor strain lymphoblasts in vitro, whereas effector cells from RT1c recipients showed modest levels of cytotoxicity. Injection of RT1u rats with MRC OX8 mAb was highly effective at selectively depleting CD8+ cells from graft recipients but had no effect in prolonging the survival of RT1Aa disparate grafts despite the complete absence of CD8+ cells from the graft infiltrate, which included numerous CD4+ T cells and macrophages. RT1u, but not RT1c, recipients mounted a strong alloantibody response against RT1Aa disparate kidneys. Immune serum obtained from RT1u recipients that had rejected a RT1Aa disparate graft was able, when injected into cyclosporin-treated RT1u recipients, to restore their ability to reject a RT1Aa, but not a third-party RT1c, kidney. These results suggest that CD8+ cells in general and CD8+ cytotoxic effector cells in particular are unnecessary for the rapid rejection of RT1Aa class I disparate kidney grafts by high responder RT1u recipients. By implication, CD4+ T cells alone are sufficient to cause prompt rejection of such grafts and they may do so by providing T cell help for the generation of alloantibody.

Effect of class II antigen matching on renal allograft survival in miniature swine

The benefit of class II major histocompatibility complex (MHC) antigen matching to renal allograft survival, in the absence of immunosuppression, has been studied in partially inbred miniature swine. Permanent (greater than 6 mo) renal allograft survival was found in 30% of recipients of either class II only or fully matched grafts. Analysis of the survival of the class II-only matched grafts by specific recipient/donor haplotype combinations indicated that survival was regulated by at least three genetic factors, including antigen gene dose, a class I MHC allele-dependent effect, and non-MHC-linked immune response phenomenon. Animals accepting class II-matched kidneys developed spontaneous tolerance to the graft, despite mounting an initial immune response marked by renal damage and the development of serum cytotoxic antibodies directed at the donor MHC antigens. The antibodies were only of the IgM class, suggesting that conversion of the humoral response to IgG was blocked. After acceptance of the kidney, three out of five animals showed specific prolongation of donor skin grafts. At the time of rejection of these skin grafts, no decrease in renal function nor reappearance of anti-donor antibodies was observed.

Relationship of tumor leucocytic infiltration to host defense mechanisms and prognosis

The interface between the tumor and the host is often the site of leucocytic infiltration. We will examine the idea that the infiltrating leucocytes of human and experimental tumors are components of the host immunological defense against the tumor, and that the presence of the infiltrate is a marker of favorable prognosis. Leucocytes could infiltrate tumors because of an active immune response, either nonspecific or specifically directed to tumor-associated antigens. Leucocyte influx may also occur because of chemotactic factors secreted by the tumor cells. Some tumors release factors which enhance vascular permeability and permit improved access by leucocytes to the tumor focus. The consequences of leucocytic infiltration include tumor cell cytolysis, cytostasis, or stimulation of proliferation. The present state of our knowledge of the interactions between tumor cells and infiltrating leucocytes precludes broad generalization of mechanisms. Further study will probably reveal that the mechanisms are diverse, and that there are some systems in which immune interactions occur at this interface and others in which they do not.

A reexamination of the role of LYT-2-positive T cells in murine skin graft rejection

We have investigated which T cell subclass defined by cytolysis with monoclonal anti-Lyt-1.2 and anti-Lyt-2.2 antibodies is required to adoptively transfer the ability to reject skin grafts. B6.Thy-1.1 spleen cells immune to graft antigens were fractionated with antibody plus C' and transferred to adult thymectomized, irradiated, bone marrow-reconstituted (ATXBM) B6.Thy-1.2 hosts that were simultaneously grafted with BALB.B skin. We found that when the ATXBM hosts were used 6 wk after irradiation and marrow reconstitution, both Lyt-1-depleted and Lyt-2-depleted immune spleen cells could transfer the ability to promptly reject skin grafts. However, such ATXBM recipients of Lyt-2-depleted cells that had rejected skin grafts were found to contain graft-specific CTL that were largely of host (B6.Thy-1.2) origin. When ATXBM hosts were used for the experiment 1 wk after irradiation and marrow reconstitution, no host-derived graft-specific CTL could be detected. However, graft rejection occurred in recipients of anti-Lyt-1- or anti-Lyt-2 plus C'-treated immune cells and specific CTL were generated from spleen cells of both groups. Thus, in the absence of a host-derived response, adoptively transferred immune Lyt-2+ cells, either resistant to, or that escaped from, antibody plus C' treatment, are able to expand in response to the antigenic stimulus provided by the graft. A more complete elimination of specific T cell subclasses is therefore needed to assess the relative contribution of a particular subset to the graft rejection process.

The major histocompatibility complex of the rat: a partial review

The major histocompatibility complex of the rat is now called RT1, and this name is becoming widely accepted. In the past five years many recombinants have been reported within RT1 that enable distinct functional regions to be identified and located relative to each other. RT1 does not at present look particularly like its closest known relative, H-2. No doubt the genetic relationship will become apparent at the DNA level. The spontaneous diabetes mellitus of the BB rat line is associated with RT1. The data so far suggest that RT1u supplies a dominant susceptibility that becomes apparent only if protection conferred by a dominant gene mapping outside the MHC is withdrawn. It seems likely that the BB rat carries a recessive mutation at this "protective" locus.