THE ROLE OF LYMPHOCYTES IN THE SENSITIZATION OF RATS TO RENAL HOMOGRAFTS

Pathways of Antigen Recognition by T Cells in Allograft Rejection

The adaptive immune response leading to the rejection of allogeneic transplants is initiated and orchestrated by recipient T cells recognizing donor antigens. T-cell allorecognition is mediated via 3 distinct mechanisms: the direct pathway in which T cells recognize allogeneic major histocompatibility complex (MHC) molecules on donor cells, the indirect pathway through which T cells interact with donor peptides bound with self-MHC molecules on recipient antigen-presenting cells, and the recently described semidirect pathway whereby T cells recognize donor MHC proteins on recipient antigen-presenting cells. In this article, we present a description of each of these allorecognition pathways and discuss their role in acute and chronic rejection of allogeneic transplants.

Path to clinical transplantation tolerance and prevention of graft-versus-host disease

Although organ and bone marrow transplantations are life-saving procedures for patients with terminal diseases, the requirement for the lifelong use of immunosuppressive drugs to prevent organ graft rejection and the development of graft versus host disease (GVHD) remain important problems. Experimental approaches to solve these problems, first in preclinical models and then in clinical studies, developed at Stanford during the past 40 years are summarized in this article. The approaches use fractionated radiation of the lymphoid tissues, a procedure initially developed to treat Hodgkin's disease, to alter the immune system such that tolerance to organ transplants can be achieved and GVHD can be prevented after the establishment of chimerism. In both instances, the desired goal was achieved when the balance of immune cells was changed to favor regulatory innate and adaptive immune cells that suppress the conventional immune cells that ordinarily promote inflammation and tissue injury.

Role of secondary lymphoid tissues in primary and memory T-cell responses to a transplanted organ

Secondary lymphoid tissues are the hub of adaptive immune responses wherein rare cognate lymphocytes encounter dendritic cells bearing antigen from peripheral tissues and differentiate into effector and memory cells that eliminate antigen. It is accepted that immune responses against microbial and tumor antigens are initiated within secondary lymphoid tissues. There is less agreement on whether the same principle applies to immune responses to a transplanted organ because an allograft expresses foreign major histocompatibility complex and contains donor antigen presenting cells that could activate T cells directly in situ leading to rejection. Recent studies confirm that although naïve T cells can be primed within the allograft, their differentiation to effect rejection is dependent on secondary lymphoid tissues. Antigen-experienced memory T cells, unlike Naïve T cells, function largely independent of secondary lymphoid tissues to cause allograft rejection. In an alloimmune response, secondary lymphoid tissues support not only immune activation but also immune regulation essential for allograft survival. Here, we will review recent findings and discuss the role of secondary lymphoid tissues in primary and memory alloimmune responses.

Non-hematopoietic allograft cells directly activate CD8+ T cells and trigger acute rejection: an alternative mechanism of allorecognition

Despite evidence that human non-hematopoietic cells, such as vascular endothelium, can activate allogeneic T lymphocytes in vitro, the prevailing view has been that hematopoietic antigen-presenting cells are required to trigger alloimmune responses in vivo. Here we report that mouse non-hematopoietic cells activate alloreactive CD8+ T lymphocytes in vitro and in vivo. We also show that vascularized cardiac allografts are acutely rejected via CD8+ direct allorecognition even if the alloantigen is not presented by hematopoietic professional antigen-presenting cells. Because activation of alloreactive CD8+ T cells by donor-type non-hematopoietic cells can continue for the life of the allograft, these findings present a new clinically relevant mechanism of allorecognition and should be taken into consideration when developing strategies to prevent allograft vasculopathy or to induce tolerance.

Immunologic 'ignorance' of vascularized organ transplants in the absence of secondary lymphoid tissue

Secondary lymphoid organs (the spleen, lymph nodes and mucosal lymphoid tissues) provide the proper environment for antigen-presenting cells to interact with and activate naive T and B lymphocytes. Although it is generally accepted that secondary lymphoid organs are essential for initiating immune responses to microbial antigens and to skin allografts, the prevailing view has been that the immune response to primarily vascularized organ transplants such as hearts and kidneys does not require the presence of secondary lymphoid tissue. The assumption has been that the immune response to such organs is initiated in the graft itself when recipient lymphocytes encounter foreign histocompatibility antigens presented by the graft's endothelial cells. In contrast to this view, we show here that cardiac allografts are accepted indefinitely in recipient mice that lack secondary lymphoid tissue, indicating that the alloimmune response to a vascularized organ transplant cannot be initiated in the graft itself. Moreover, we demonstrate that the permanent acceptance of these grafts is not due to tolerance but is because of immunologic 'ignorance'.

Sensitization of allo-specific T lymphocytes in vivo: role of antigen-presenting cells

The migratory behavior of antigen-presenting cells was investigated in vivo. Purified murine splenic dendritic cells and splenic and peritoneal macrophages were labelled and injected subcutaneously in the hind foot-pads of mice and monitored for seven days. In the first 24 h, a small quantity of label was recovered from popliteal but not inguinal lymph nodes with radioactive (111In-oxine and 3H-uridine) but not fluorescent (1,1'-dioctadecyl 3,3,3'3'-tetramethylindocarbocyanine perchlorate and fluorescein isothiocyanate) labelling of the antigen-presenting cells. Chemical fixation of the injected antigen-presenting cells had no effect on the detection of label in the popliteal lymph nodes, suggesting that it was unlikely to be due to active cellular migration. Label recovery from hind feet declined with time over the seven day period and was independent of the label type. Essentially the same observations were made whether the antigen-presenting cells were syngeneic or allogeneic to the injected mice and irrespective of the type of antigen-presenting cell used. However, allogeneic antigen-presenting cells, which did not migrate to the draining lymph nodes, successfully primed T lymphocytes in these lymph nodes as shown by a secondary in vitro mixed leukocyte reaction. Again, chemical fixation of the injected antigen-presenting cells had no effect on their ability to prime allogeneic T lymphocytes in the draining lymph nodes. These experiments suggest that, during experimental allo-sensitization via the subcutaneous route, indirect priming of allogeneic T lymphocytes may be a dominant pathway.

The immune response to intracerebral neural grafts

Neural transplantation offers a potential therapeutic approach to a variety of neurological disorders, most notably those of a degenerative nature. However, the degree of immunological privilege (i.e. isolation from an immune response) in the brain, which is not absolute, may be a significant impediment to the survival of histoincompatible grafts. The nature of this privilege, together with the specific immune events leading to neural graft rejection, are discussed. As a consequence of this immune-mediated rejection, immunosuppression in some form might be necessary to guarantee long-term graft survival. Various strategies are being explored to suppress the immune response to neural grafts, not only for future use in clinical therapies, but also to bring intracerebral allo- and xenotransplantation to the attention of the general neurobiologist.

Migration of dendritic leukocytes from cardiac allografts into host spleens. A novel pathway for initiation of rejection

It has been a long-standing dogma that host sensitization against fully-vascularized organ allografts occurs peripherally within the graft itself. In this report we show that donor-derived MHC class II-positive (Ia+) DL migrate rapidly out of mouse cardiac allografts into the recipients' spleens where they home to the peripheral white pulp and associate predominantly with CD4+ T lymphocytes. This provides a novel route for central sensitization against fully vascularized allografts, and most likely represents a pathway by which immune responses are generated against antigens on blood-borne DL emigrating from peripheral tissues.

The binding of antigen presenting cells to T lymphocytes

In the introduction, we asked how MHC molecules on the surfaces of APC make contact with antigen-specific receptors on the surfaces of T cells. We have reviewed two models in which antigen-specific contacts occur with primary leukocyte populations in vitro. One system involves resting T cells; the other sensitized T lymphoblasts. At the onset of a primary immune response, dendritic cells seem critical for binding and activating both CD4+ and CD8+ subsets. Given the current evidence, we suggest that dendritic cells literally find the right T cell clone, and not vice versa, and that dendritic cells do so by first binding and surveying T cells by an antigen-independent mechanism. In the efferent limb of immunity, other types of APC including B cells and macrophages bind and stimulate freshly sensitized T lymphoblasts. Freshly isolated epidermal Langerhans cells do not cluster T cells by an antigen-independent mechanism but acquire this capacity during epidermal suspension culture. Under the control of GM-CSF, the Langerhans cell becomes a powerful accessory cell for the primary or sensitization limb of T-dependent immune responses like the MLR and primary antibody response. Isolated lymphoid dendritic cells have many features in common with interdigitating cells in lymphoid T areas, and may be related to some of the irregularly-shaped Ia+ cells in certain epithelia and interstitial regions. Contact with dendritic cells may be important in both central and peripheral pathways of T cell sensitization in situ.

Quantification of lymphocyte activation by measurement of DNA polymerase alpha activity

Measurement of the activity of the enzyme DNA polymerase alpha has been investigated with regard to its potential usefulness as a method for the detection and quantification of lymphocyte activation in vivo. A modified enzyme assay was developed in order to optimise measurement of activity in crude homogenates of cells or tissues, thus allowing the convenient handling of multiple samples. Specificity of the assay for polymerase alpha was ensured by the inclusion in the assay mixture of dideoxythymidine triphosphate, an inhibitor of the other eukaryotic DNA polymerases. The activity of DNA polymerase alpha was found to be closely correlated with [3H]thymidine incorporation in a mitogen-stimulated in vitro system. The usefulness of the polymerase alpha method for the quantification of lymphocyte activation was validated in 3 different in vivo systems of either immune-mediated or drug-induced lymphoid cell response.

Effect of irradiation on rat renal transplant rejection

Leucocytes were selectively eliminated either from a DA renal allograft or from a WF host by irradiation of either the host or the graft on different days after the transplantation. The recovery of inflammatory leucocytes and the generation of lymphoid killer cells--that is, the natural killer (NK) cells and the cytotoxic T lymphocytes (CTL)--were analysed separately in the two compartments. Early irradiation of the graft did not affect the recovery of leucocytes in either compartment. The NK activity was only slightly reduced in the graft but was distinctly reduced in the spleen. A delay in the generation of the CTL activity was observed in the spleen. Late irradiation of the graft reduced the recovery of leucocytes in both compartments. The disappearance of the NK activity increased in the graft but not in the spleen. The CTL activity in the spleen developed normally up to day 6, whereafter it declined. After selective irradiation of the host a fair number of leucocytes remained in the graft, compared with a nearly complete disappearance of leucocytes from the graft and blood. The NK and CTL activity declined rapidly in both compartments. The data demonstrate a bidirectional interdependence between the graft and the host during the rejection.

Prolongation of canine allograft survival with donor pretreatment

Donor pretreatment of 100 mg/kg each of cyclophosphamide (CY) and methylprednisolone (P) infused 5 hours before nephrectomy invariably prolongs the survival of DLA mismatched, MLC incompatible nonlittermate Beagle renal allografts as well as the survival of mongrel renal allografts. The effect of donor pretreatment appears to be mediated by cyclophosphamide and its metabolites because methylprednisolone pretreatment does not significantly prolong survival. Methylprednisolone is needed, however, because it abolishes cyclophosphamide pretreatment mediated early but transient postoperative renal (allograft) insufficiency. The effect of donor pretreatment appears to be mediated by drugs residing in the graft; mannitol infusions given 1 hour prior to donor nephrectomy or peroperatively into the recipient decrease the renal cortical content of carbon 14 cyclophosphamide and its metabolites and abolishes the prolonged survival. Because donor pretreated kidneys contain less than 0.5% of the infused dose of carbon 14 cyclophosphamide, the drugs appear to exert their effect locally in the transplanted kidney. Donor pretreatment mediating prolonged canine renal allograft survival appears to be an example of influencing a biological process by a localized drug delivery by virtue of unique properties of the drug and because early postoperatively host sensitization occurs mainly at the site of the graft.

Studies of the immediately vascularized skin allograft

In order to evaluate the effect of the nature of revascularization on survival rates of skin allografts, a rat model was devised which provided simultaneous and immediate establishment of arterial and venous blood supply to a composite skin-renal allograft. For 5 days after transplantation the skin portion of the composite grafts appeared normal, with minimal inflammation in 11 recipients. From 6 to 11 days, in 6 rats, active rejection developed with a marked inflammatory reaction of the skin-kidney interface. In 10 rats in which the composite grafts remained in the secondary hosts for 12--21 days rejection of the skin was complete. The renal portion of all composite grafts appeared normal in all three panels and all recipients of composite grafts rejected subsequent orthotopic skin grafts in an accelerated manner, with median survival time of 8.2 +/- 0.3 days compared with 11.5 +/- 0.7 days in untreated Fisher leads to Lewis controls. These results demonstrate that the skin on the composite graft retained its immunogenicity, that immediately vascularized skin allografts between AgB compatible rats will be rejected in a normal time and that anatomical factors are not sufficient to account for the difference in survival times between skin and solid organs.

Immunogenetic aspects of intracerebral skin transplantation in inbred rats

This study was undertaken to evaluate the ability of intracerebral skin grafts transplanted across different genetic disparities in the major histocompatibility complex (RT1) to elicit an immune response in inbred rats, as determined by histologic examination and by the ability of the grafts to sensitize the recipients to subsequent orthotopic skin grafts. The ability of intracerebral skin allografts to sensitize rats to transplantation antigens is related to the specific genetic disparity between the graft and the host: sensitization appears to occur more consistently across an A region barrier than across a B region barrier. Histologic changes of intracerebral graft rejection are more severe in rats with two intracerebral grafts than in those with one. The degree of histologic change attributable to intracerebral allograft rejection correlates with the ability of these grafts to sensitize the recipient. In certain strains intracerebral sensitization is accomplished with two grafts but not with one, indicating an antigenic dose requirement for intracerebral sensitization.

The pathology of renal homograft rejection. A review

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Recruitment of effector lymphocytes by initiator lymphocytes. In vivo migration of in vitro sensitized initiator T lymphocytes

We previously found that mouse T lymphocytes sensitized in vitro against allo- or syngeneic fibroblasts, upon injection into syngeneic recipients, do not themselves differentiate into effector cells, but recruit effector T lymphocytes within the draining lymph nodes. As a result of sensitization, these initiator lymphocytes acquire a trypsin-sensitive membrane property which is necessary for recruitment. We now report studies on the in vivo migratory behavior of initiator lymphocytes following sensitization. We injected 51Cr-labeled initiator lymphocytes into recipient footpads and found significantly increased migration of sensitized cells to the draining popliteal lymph node (PLN) during the first day. By amputation of the foot at various times, we showed that migration during the first 12-24 hours was critical for subsequent recruitment. Trypsin treatment of initiator lymphocytes abolished this accelerated migration. Lymphocytes triggered nonspecifically by Con A migrated to the PLN like antigen-sensitized cells. We also compared the migration of injected lymphocytes from the footpad to the PLN in graft-versus-host and host-versus-graft reactions, and found these reactions to differ both from each other and from recruitment in terms of lymphocyte migration. These findings are discussed in terms of the physiology of the cell-mediated immune response and the notion of peripheral sensitization.

Specific suppression of immune responses

The models we have discussed in detail demonstrate specific suppression of immune reactivity produced in normal adult animals by antibody and antigen. The mechanism of homeostasis of suppression in these models depends on continued exposure to antigen and on an active response by the host. The active response may include production of antibody directed against specific receptors as well as antibody directed against antigen. Thus, specific regulation of both antibody and cell mediated immunity to an antigen might be achieved by the use of only the biological agents of the response: antigen, antibody, and possibly antibody to receptors. The general implication is that these same biological agents are responsible for autoregulation of immune reactions occurring in nature. Presumably, these agents may be used to suppress or reverse immune responses for appropriate clinical objectives.

Skeletal muscle as a privileged site for orthotopic skin allografts

A semi-privileged status for rat skin allografts may be achieved by placing them on extensive open beds formed by panniculus carnosus muscle which prevents contact of the transplant with host skin. Such allografts enjoy approximately a twofold increase in their life expectancy, even if transplanted across a strong histocompatibility barrier. Experiments are described which rule out stress or a "central" weakening of response, such as enhancement, as explanations of this phenomenon. Intact skin "islands" separated from surrounding host skin on all sides by a broad border of bared panniculus were also found to serve as privileged sites. Dye injected into these islands failed to reach the regional nodes until about the 15th day after their preparation. These studies indicate that a lymphatic deficit is responsible for the observed privileged status of the allografts.

The host cell response in the local graft-versus-host reaction induced in the kidneys of F 1 rats by parental thoracic duct lymphocytes

Radioautographs of infiltrative cells in the kidneys of (Lewis x BN)F(1) rats labeled with tritiated thymidine (TdR(3)H) before the subcapsular injection of parental (Lewis) thoracic duct lymphocytes (TDL) showed a predominantly host-proliferative response by 4 days after grafting. The immediate renal incorporation of TdR(3)H was used to measure the local graft-vs.-host (GVH) reactions. Substantial reactions could still be induced in the face of the considerable degree of leukopenia after 400 R whole body gamma-irradiation. These results suggest that radioresistant cells are capable of carrying on the appropriate host activities and that the weakness of GVH reactions induced after higher doses of irradiation may be due to impairment of the mitotic mechanism of host cells. The importance of circulating leukocytes as a source of immunogenic stimulation was nevertheless substantiated by inducing local GVH reactions with Lew TDL in chimeric parental-type rats that had been repopulated with F(1) bone marrow. This result also emphasizes the nonspecific nature of tissue destruction in the renal GVH reaction in confirmation of Elkins. In this and other situations in which B cells were the predominant F(1) type elements available for interaction with parental-type TDL the reactions were nearly equivalent or equivalent to those in the appropriate controls. Typical local GVH reactions could be induced in heavily irradiated hosts by an inoculum of combined parental and F(1)-type TDL in the apparent absence of mononuclear phagocytes. The possible relationship between the activation of host lymphocytes, the involvement of B cells, and the nonspecific nature of tissue damage in the renal GVH is discussed.