Lymphokine-activated killer cells in rats: analysis of progenitor and effector cell phenotype and relationship to natural killer cells.

The progenitor and effector cell phenotype of lymphokine-activated killer (LAK) cells generated in F344 rats by recombinant human interleukin 2 (IL-2) (rIL-2) were analyzed. Highly purified populations of peripheral blood large granular lymphocytes (LGL) exhaustively depleted of T-cells were fully capable of generating high levels of LAK activity by 3 to 5 days in culture while purified populations of resting T-cells devoid of LGL could not generate LAK activity. This pure population of LGL expressed surface markers characteristic of rat natural killer (NK) cells [i.e., OX8+, asialomonoganglioside (asialo-GM1+), laminin+, OX19-, R1-3B3-, W3/25-, Ia-, surface immunoglobulin negative (SIg-)]. Further evidence that NK cells were the progenitors of cells with LAK activity was obtained by treatment of spleen or peripheral blood lymphocytes with anti-laminin or anti-asialo-GM1 antibodies plus complement or with the lysosomotropic agent L-leucine methyl ester. These treatments effectively depleted LGL/NK cell activity and the subsequent generation of rIL-2-induced LAK activity. Analysis of the LAK effector phenotype by cell sorting demonstrated that the majority of cells with LAK activity were OX8+, asialo-GM1+, laminin+, OX6+, OX19-, R1-3B3-, W3/25-, and SIg-. Furthermore, treatment of LAK cells with L-leucine methyl ester also significantly reduced their cytolytic activity. Thus, the LAK effector cells were also LGL and expressed surface marker characteristic of activated NK cells and not those of mature T- or B-cells. The proliferative response of rat spleen or blood lymphocytes to rIL-2 appeared to be primarily associated with LGL/NK cells since depletion of NK cells by anti-asialo-GM1 or anti-laminin antibody plus complement or by L-leucine methyl ester significantly (P less than 0.001) reduced the incorporation of [3H]thymidine into DNA. In contrast, depletion of T-cells (by anti-T-cell antibody plus complement) did not significantly affect rIL-2-induced proliferation. Similarly, T-cell-depleted, highly purified populations of LGL gave substantial proliferative responses to rIL-2. These studies clearly indicate that in the rat, the major cell population activated by rIL-2 is the LGL/NK cell and these cells appear to represent the major population of cells in blood or spleen which generate broad antitumor (LAK) cytotoxicity.

Cardiac transplantation: immune mechanisms and alloantigens involved in graft rejection.

Recent advances in surgical and chemotherapeutic approaches to cardiac transplantation have resulted in substantial improvements in patient survival. Rejection of the cardiac allograft by the recipient's immune system and complications associated with immunosuppressive therapy remain, however, important clinical problems. The rejection of the transplanted heart is a complex set of immunological reactions. Previous experimental work has indicated that a variety of mechanisms may be involved in the pathogenesis of heart rejection; these include alloantibodies, specifically sensitized cytotoxic T-lymphocytes, lymphokine-dependent T-lymphocyte delayed-type hypersensitivity reactions, and natural killer cells. The current knowledge of the role that genetic incompatibilities play in stimulating the cardiac allograft reaction and the effector mechanisms implicated in the pathogenesis of the rejection process will be discussed.

Chronic graft-versus-host disease in rats after syngeneic bone marrow transplantation.

A disease similar to the chronic graft-versus-host disease (cGVHD) seen following transplantation of human bone marrow was observed after syngeneic and allogeneic bone marrow transplantation in rats. Bone marrow grafts were exchanged between donors and recipients that were syngeneic or genetically different for the RT2 erythrocyte antigen locus by the use of AUG/AUG.2B and PVG/PVG.2A congenic pair donor/recipient strain combinations. After an initial period of well-being (120-180 days posttransplantation), several AUG and AUG.2B recipients of syngeneic or RT2-mismatched bone marrow developed clinical signs compatible with cGVHD. The clinical signs of the disease included: erythema, diffuse alopecia, thickened skin folds, and conjunctivitis. Laboratory findings included peripheral blood eosinophilia and impaired in-vitro proliferative responses to third-party spleen cells in the mixed lymphocyte reaction. Histological examination of the tissues of a limited number of rats with cGVHD showed subepidermal mononuclear inflammation with atrophy of the epidermis and adnexa of the skin, as well as plasmacytic hyperplasia of the lymphoid tissues. None of the PVG or PVG.2A recipients of syngeneic or RT2-mismatched marrow developed cGVHD. The development of cGVHD in AUG.2B recipients of syngeneic marrow and the absence of the disease in reciprocal marrow grafts between the PVG/PVG.2A rat strains suggests that the development of the disease in the AUG and AUG.2B recipients of RT2-mismatched bone marrow is not due to the RT2 disparity, but may be due to an autologous immune reaction. Furthermore, the finding that the cGVHD is only observed when the AUG and AUG.2B strains are used as recipients--not when the PVG or PVG.2A strains are used as recipients--suggests that the development of the disease is associated with the genetic background of the host and is independent of the background of the donor. It is possible that the use of high-dose cyclophosphamide treatment is involved in the pathogenesis of cGVHD, because the disease is observed only when the recipients are conditioned for transplantation with this immunosuppressive agent.

The genetics of bone marrow transplantation in the rat.

We have used a variety of standard inbred, recombinant, and congenic rat strains to establish the effect of MHC and non-MHC genetic incompatibilities on bone marrow transplantation. The role of these loci in the successful establishment of bone marrow engraftment was first determined by examining the potential of donor marrow to protect recipient rats from a lethal dose of the myeloablative drug busulfan. Similar donor/recipient combinations were then used to examine the effects of the same incompatibilities on the induction of fatal graft-versus-host disease (GVHD) in recipients conditioned with a combination of busulfan and cyclophosphamide. The data obtained indicate that: (1) a difference for the entire MHC of the rat is sufficient to prevent marrow engraftment and to produce fatal GVHD; (2) the class I RT1.A locus of the rat MHC has a differential effect on bone marrow transplantation, since disparity for this locus prevents successful marrow engraftment, while this gene has little effect on the development of fatal GVHD; (3) disparity for the class I RT1.E locus has no effect on bone marrow engraftment and does not stimulate GVHD; (4) disparity for the class II locus, RT1.D, can prevent marrow engraftment and elicit fatal GVHD; and (5) incompatibility for non-MHC genes can prevent the establishment of bone marrow engraftment and elicit fatal GVHD.

Differential usage of the T cell receptor repertoire for allorecognition of heart, liver, and kidney grafts.

We have previously demonstrated that the immune response to cardiac allografts in the ACI-to-LEW rat strain combination involves a limited use of the TCR V beta gene repertoire. In the present study we analyzed the expression of V beta genes by T cells infiltrating kidney and liver allografts to test whether a limited use of the T cell receptor (TCR) repertoire is a common denominator for immune responses to allografts. Graft-infiltrating lymphocytes (GIL) were isolated from allografts on different days after transplantation and analyzed for the expression of V beta genes using a semi-quantitative polymerase chain reaction (PCR) without manipulations in tissue culture. We detected a limited expression of the V beta gene repertoire in fresh GIL harvested from both kidney and liver allografts early in graft rejection. The level of TCR repertoire usage, however, was influenced by the type of graft. The rejection of heart and kidney allografts was associated with more limited use of the V beta gene repertoire when compared with that seen for the rejection of liver allografts. The limited use of the V beta gene repertoire was only apparent when analyzed early in graft rejection; as the rejection reaction progressed T cells using a more diverse V beta repertoire infiltrated the graft. The limited use of TCR repertoire of the early T cell response to allografts may provide the opportunity to therapeutically disrupt the rejection reaction by targeting selected T cell populations for elimination at the time of organ transplantation.

Induction of allograft nonresponsiveness after intrathymic inoculation with donor class I allopeptides. I. Correlation of graft survival with antidonor IgG antibody subclasses.

We have recently demonstrated that cardiac allograft rejection in the PVG.R8-to-PVG.1U rat strain combination involves the recognition of a isolated class I (RT1.Aa) molecules as peptides in the context of the recipient MHC molecules. Three synthetic peptides (P1, P2, and P3) corresponding to the alpha-helices of the RT1.Aa molecule served as T-cell epitopes for graft rejection. In this study, we demonstrate that two of these peptides (P2 and P3) are sufficient to induce immune nonresponsiveness (median survival time >237 days) to cardiac allografts when presented to the recipient immune system in the thymus 7 days before transplantation. This effect was time dependent, as intrathymic inoculation 60 days before transplantation did not prolong graft survival (median survival time=12 days). Previous studies have demonstrated a critical role for alloantibody responses in mediating graft rejection in this rat strain combination. We, therefore, studied the role alloantibody responses may play in the observed immune nonresponsiveness. The titers of alloantibody in serum samples harvested from graft recipients at different times after transplantation were measured. We used recipient primary aortic endothelial cells genetically manipulated to express the donor RT1.Aa molecule as targets in an enzyme-linked immunosorbent assay. High titers of anti-RT1.Aa IgM antibody were detected in unmanipulated controls at the time of graft rejection. The IgM antibody switched to high IgG titers in intrathymically inoculated rats with accelerated or delayed rejection. Graft rejection in intrathymically manipulated recipients that had achieved a transient state of immunological nonresponsiveness correlated with higher titers of the IgG2b alloantibody. In marked contrast, the long-term graft survivors expressed undetectable or low levels of the IgG2b antibody and moderate to high levels of the IgG1 and IgG2a subclasses. These data suggest that the IgG2b alloantibody may contribute to the rejection reaction, whereas IgG1 and IgG2a may be involved in active enhancement of graft survival.

Removal of natural human xenoantibodies to pig vascular endothelium by perfusion of blood through pig kidneys and livers.

We have examined the nature of the binding of human xenoantibodies to pig liver and kidney vascular endothelium. Our results demonstrate that human serum contains IgM and IgG xenoantibodies that bind to pig vascular endothelium, and that the pattern of antibody binding is similar for both livers and kidneys. Immunohistochemical analysis of pig kidneys after perfusion with human blood demonstrated the binding of both IgM and IgG xenoantibodies, complement (C3), and fibrinogen to the vascular and glomerular endothelium. An ELISA assay of the perfusate after perfusion of 500 ml of human blood through a single pig kidney for 60 min demonstrated a significant reduction in the amount of human IgM (67%) and IgG (55%) binding to pig aortic endothelium. Similar perfusion experiments conducted with pig livers were associated with minimal immunohistochemical evidence of the binding of human xenoantibodies to liver vascular endothelium. Immunofluorescence staining for IgM, IgA, C3, and C1q was negative or minimally positive in the liver vascular endothelium. Sinusoidal endothelium were weakly positive for IgG and fibrinogen. The perfusion of the pig liver with human blood was, however, associated with a significant reduction in the subsequent binding of IgM and IgG to pig kidney vascular endothelium. Pig liver perfusion was also responsible for the removal of both IgM and IgG xenoantibodies capable of reacting with pig aortic endothelium, as measured by an ELISA assay of the perfusate. These results suggest that both pig kidney and livers are capable of absorbing the xenoantibodies that may be responsible for mediating a hyperacute rejection of pig xenografts and that the distribution of the target antigens for these antibodies is similar in the two organs.

Peptides derived from alpha-helices of allogeneic class I major histocompatibility complex antigens are potent inducers of CD4+ and CD8+ T cell and B cell responses after cardiac allograft rejection.

We studied the rejection of cardiac allografts in a rat strain combination (PVG.R8 to PVG.1U) disparate for a single class I MHC antigen (RT1.Aa) to test the extent by which this molecule is recognized as peptides in association with recipient MHC molecules during graft rejection and the contribution of this recognition process to the rejection reaction. Three synthetic peptides that correspond to the portions of alpha-helices of the alpha 1 (P1, P2) and alpha 2 (P3) domains of the donor RT1.Aa molecule were used in this study. Splenocytes from heart allograft recipients at rejection responded in a proliferation assay to all 3 peptides and in a cytotoxic assay to peptides P1 and P2. The peptide-mediated proliferation and cytolytic reactions were blocked by antibodies against CD4/class II MHC and CD8 molecules. Serum from graft recipients at rejection contained significant titers of antibodies to peptides. Presensitization of graft recipients with the peptides resulted in a marked increase in peptide-mediated T cell and antibody responses. Although all 3 peptides were effective in eliciting active immune responses, the P3-mediated response was minimal when compared with those mediated by P1 and P2. Recipients presensitized with the peptides rejected their grafts in 5 days compared with 6 days for unsensitized animals. Recipients presensitized with donor-irradiated splenocytes and aortic endothelial cells, on the other hand, rejected their grafts in 1 and 3 days, respectively, which suggests that immunization with the whole RT1.Aa molecule is required to stimulate accelerated rejection of the graft. This rejection was associated with high titers of donor cell-specific antibodies that exhibited moderate cross-reactivity with the peptides. Our results clearly demonstrate that (1) the donor RT1.Aa molecule is recognized as peptides in the context of recipient class I and class II MHC molecules during the rejection of heart allografts, and (2) peptides derived from this molecule are highly immunogenic in that they contain epitopes recognized by CD4+ and CD8+ T cells and alloantibodies. Immune responses elicited by these peptides, however, did not significantly affect the rate of rejection. These results suggest that acute rejection of allografts may be mediated primarily by the direct recognition of intact MHC molecules.

Pretransplant injection of allograft recipients with donor blood or lymphocytes permits allograft tolerance without the presence of persistent donor microchimerism.

Donor-recipient microchimerism has recently been suggested to play a critical role in the induction and maintenance of allograft tolerance. In this study we sought evidence for this hypothesis using the LEW-to-ACI cardiac allograft as a model system. Donor-specific tolerance to cardiac allografts was induced by intravenous or intraportal injection of graft recipients with donor peripheral blood, T cells, or B cells 7 days before transplantation. All the graft recipients injected with donor antigens accepted donor heart grafts indefinitely when compared with control recipients that rejected donor allografts in 12 days. Long-term graft survivors rejected third-party BN heart allografts in 14 days without an adverse effect on the survival of the first LEW heart allografts, demonstrating the specificity of the tolerance. Tissue lysates prepared from heart, kidney, liver, bone marrow, thymus, lymph nodes, and spleen of tolerant (>120 days) graft recipients were analyzed for the presence of donor DNA using LEW T cell receptor C beta gene-specific primers for polymerase chain reaction that detects donor DNA at > or = 1:10,000 dilution. Donor DNA was detected in 77% of tolerant graft recipients. Chimeric recipients showed variations in the levels and presence of donor DNA in different tissues. The status of donor microchimerism, with respect to its presence and tissue distribution, was dependent upon the donor cell type and route of injection used for the induction of tolerance. Intraportal injection of the graft recipients with donor peripheral blood resulted in the highest degree of chimerism, whereas intravenous injection with donor B cells did not induce detectable microchimerism in this group of recipients. These data clearly demonstrate that the presence of microchimerism is common following administration of donor cells, but that its presence is not an absolute requirement for the long-term survival of allografts.

The synergism of brequinar sodium and cyclosporine used in combination to prevent cardiac allograft rejection in the rat.

Brequinar sodium (BQR) is a novel immunosuppressive drug that inhibits cell proliferation by virtue of its disruption of the de novo pyrimidine biosynthesis. The basis of the immunosuppressive activity of BQR is distinctively different from that of cyclosporine (CsA), and we have recently evaluated in vivo and in vitro the efficacy of the two drugs when used in combination. Subtherapeutic doses of BQR and CsA were tested for their ability to prolong heterotopic cardiac allograft survival in the MHC- and non-MHC-mismatched ACI-->LEW rat strain combination. The graft survival data derived from these experiments were analyzed using the median-effect analysis to establish the immunosuppressive interaction of both drugs. The administration of BQR 3 mg/kg three times weekly or CsA 2.5 mg/kg daily moderately prolonged cardiac allograft survival, with a mean survival of 10 +/- 0.5 and 16 +/- 5.3 days, respectively. The use of the two drugs in combination with the same dose schedule exerted a synergistic effect on graft survival, prolonging the graft function to a mean of 31 +/- 5.7 days. Sera from animals treated with the two drugs displayed, when compared with single treatment groups (BQR 3 mg/kg and CsA 2.5 mg/kg), significantly (P < 0.01) increased in vitro inhibition of lymphocyte proliferation following stimulation with PHA. Finally, a clear correlation between the mean survival time and BQR plasma levels of animals treated with BQR alone or in combination with CsA was seen. Those treatment groups with BQR levels below 2 micrograms/ml (1.5 and 3.0 mg/kg/3x/week) had a mean graft survival of less than 10 days). In contrast, recipients treated with a combination of low doses of BQR and CsA displayed higher drug plasma levels (> 2 micrograms/ml) and longer mean graft survival times. These observations suggest that BQR and CsA may be highly effective when used in combination to prevent organ allograft rejection for clinical transplantation.

Lymphokine-activated killer cell purging of leukemia cells from bone marrow prior to syngeneic transplantation.

We have studied the effect of using lymphokine-activated killer (LAK) cells as an in vitro means for eliminating leukemic cells from normal bone marrow prior to transplantation of experimental animals. Rat LAK cells exhibit broad cytolytic activity against a variety of hematopoietic neoplasms, but do not kill normal bone marrow cells or lectin-stimulated blasts. Bone marrow was harvested from normal Fischer 344 rats, combined with increasing numbers of CRNK-16 tumor cells, and then incubated with LAK cells. The BM/tumor/LAK mixture was then administered to untreated Fischer rats, and the ability of the LAK cells to purge the bone marrow of neoplastic cells and prevent the transmission of the leukemia to recipient animals monitored. Our results demonstrate that LAK cells are capable of efficiently purging the bone marrow of neoplastic cells. Treatment of the BM/tumor mixtures with LAK cells is associated with significant prolongation of survival in the higher tumor doses (10(5) tumor cells/recipient) and complete elimination of the tumor in a high percentage of recipients at lower tumor levels (10(3)-10(4) tumor cells/recipient). At levels of BM transfer comparable to that used in humans, there was no evidence of a failure of LAK-treated bone marrow to reconstitute lethally conditioned recipient animals. However, with lower numbers of BM cells, there was an increased mortality in animals receiving LAK-treated BM, suggesting a minimal inhibition of pluripotent hematopoietic stem cell function when suboptimal numbers of BM cells are used for reconstitution. These experiments demonstrate that LAk cells are capable of eliminating neoplastic cells in bone marrow without significant destruction of immature syngeneic stem cells. LAK cells display a broad range of cytolytic activity against hematopoietic and solid tissue tumors, and are therefore capable of eliminating small numbers of tumor cells from a wide variety of neoplastic diseases of the marrow. The ability to detect and eliminate malignant cells, without interfering with reconstitution with donor marrow, suggests that immune therapy with LAK cells can be a relatively simple and efficient method to purge bone marrow prior to autologous transplantation in patients following high-dose chemotherapy for neoplastic diseases.

Human serum reactivity to porcine endothelial cells after antisense-mediated down-regulation of GpIIIa expression.

The hyperacute rejection of vascularized grafts exchanged between discordant species is a result of the binding of preformed natural antibodies to the endothelium of the donor organ, and the subsequent activation of the complement system. Human natural antibodies to pig endothelial cell antigens appear to be predominantly directed at carbohydrate epitopes expressed by a variety of porcine integrins, including GpIIIa. The identification of porcine xenoantigens whose recognition by human natural antibodies results in hyperacute rejection would allow for the development of strategies to genetically modify the xenograft reaction. We have used antisense technology to down-regulate the expression of one of seven recently identified xenoantigens from the surface of pig aortic endothelial cells. Down-regulation of GpIIIa on endothelial cells resulted in a 20.8% decrease in the mean channel shift (MCS) of IgM natural antibody binding from pooled human sera, and a 28-35% decrease in the MCS of IgM binding from two high-titer individuals. The MCS for human IgG natural antibody binding to the surface of pig cells decreased by 27%. Natural antibody-mediated cytotoxicity to pig endothelial cells was not significantly altered, as indicated by a 2.5-6% decline in complement-mediated cytotoxicity. These results indicate that down-regulation of GpIIIa alone may not be sufficient to significantly alter xenograft rejection. Our results also suggest, however, that antisense-mediated regulation of a functionally important target antigen is technically feasible and may represent a strategy to prevent the xenograft reaction.

Genetic control of the humoral immune response to xenografts. I. Functional characterization of rat monoclonal antibodies to hamster heart xenografts.

The rejection of cardiac xenografts in the hamster-to-rat combination is characterized by the production of IgM antibodies that result in the rapid loss of the graft. We have recently produced rat monoclonal antibodies (mAb) to hamster heart xenografts in an attempt to develop reagents for use in identifying the target antigens for this reaction and to study the nature of the genetic control of the humoral response. The monoclonals were created by the fusion of myeloma cells with splenic lymphocytes from LEW rat recipients of hamster cardiac xenografts. The hybridomas were screened for antibody production, reactivity to hamster cell surface antigens, and the ability to mediate hyperacute rejection of hamster heart xenografts. A panel of monoclonal antibodies has been identified that are capable of inducing hyperacute rejection. All of these mAbs are IgM and bind strongly to hamster vascular endothelium. None of the mAbs were lymphocytotoxic or bound to hamster lymphocytes or erythrocytes. Immunopathologic studies demonstrated that these mAbs react specifically with hamster vascular endothelium and mediate a complement-dependent humoral reaction leading to the destruction of the cardiac xenografts. One of the mAbs (designated as HAR-1) has been characterized in detail. HAR-1 detects antigens distributed in the vascular endothelium, epithelium of bronchi in the lung, small intestine, tubules of kidney, and selective components of lymphoid organs--e.g., the stromal cells of the spleen and thymic medullary epithelium. Western blot analysis of hamster heart proteins with HAR-1 showed multiple bands with two major bands migrating at 80 kDa and 48 kDa. Absorption of the HAR-1 antibody with 48 individual carbohydrate molecules demonstrated that the strongest reactivity of the antibody is with a sialyl-Lea carbohydrate antigen.

A Class II monoclonal antibody specific for the RT1.B, rather than the RT1.D, product of the rat major histocompatibility complex.

An allospecific monoclonal antibody, 79.7.5., has been shown to be specific for a Class II histocompatibility product of the ACI (RT1.AaBaDa) rat. To further specify the reaction of this antibody to the B or D locus Class II products of RT1, we examined the binding of this antibody to peripheral blood lymphocytes (PBLs) of the WRC rat (haplotype RT1.AnBnDa). Radiolabelled monoclonal antibody 79.7.5. did not bind to PBLs from the WRC rat, but it did bind to PBLs from the WRA rat (RT1.AdBaDa) and the DA (RT1.AaBaDa) rat. These results were confirmed using radiolabelled Staphylococcus protein A in an indirect binding assay. In addition, binding of 79.7.5 could be inhibited by alloantiserum BN anti-BN.1A (DA) (directed against AaBaDa) but not by BN anti WRC (directed against Da). These data demonstrate that monoclonal antibody 79.7.5. reacts with the a allele product of RT1.B rather than RT1.D. This antibody can be used to probe the structures and functional roles of different Class II products of the rat major histocompatibility complex.

Genetic control of the humoral responses to xenografts. III. Identification of the immunoglobulin V(H) genes responsible for encoding rat immunoglobin G xenoantibodies to hamster heart grafts.

BACKGROUND: We have previously reported that the early phases of the immune response of rats to hamster xenografts are characterized by the production of IgM xenoantibodies encoded by a restricted group of Ig germline V(H) genes (V(H)HAR family). In the later phases of the reaction, an IgM to IgG isotype switch occurs and our study examines the structure of the rearranged V(H)HAR genes used to encode IgG antibodies after this isotype switch. METHODS: A quantitative polymerase chain reaction was used to investigate the changes in the levels of V(H)HAR+ IgG mRNA seen after xenotransplantation. cDNA libraries specific for V(H)HAR+ Iggamma chain were established from total RNA extracted from splenocytes of naive rats and xenograft recipients of hamster hearts at days 4, 8, 21, and 28 posttransplantation. Colony filter hybridization was used to estimate the relative frequency of the use of individual V(H)HAR+ IgG subclasses. Selected IgG clones from day 21 cDNA libraries were sequenced and analyzed for VH-D-J(H) gene usage and antibody combining site structure. RESULTS: The level of mRNA for V(H)HAR+ IgG increased 6-fold in xenograft recipients at day 21 post-transplantation when compared with naive animals. The relative frequency of isotype usage for V(H)HAR+ IgG1 antibodies alone increased from 22.3% at day 0 to 37.4% at day 21 PTx. Ten IgG clones from the day 21 cDNA libraries have been sequenced for the rearranged V(H)-D-J(H) genes. Thirty percent (3/10) of these IgG clones used V(H)HAR genes for the coding of heavy chain variable region with limited numbers of nucleic acid substitutions (>98% identity with their germline progenitors) although others demonstrated increased variation in nucleotide sequences (95-97% identity) when compared with germline V(H) genes. Analysis of the canonical binding site structure from the predicted amino acid sequences demonstrated that the majority of IgG clones (9/10) displayed a similar pattern of conserved configurations for their combining sites. CONCLUSIONS: The change in IgM to IgG antibody production in the later stages of the humoral immune response of rats to hamster xenografts is associated with an IgM to IgG isotype switch and an increased production of antibodies of the IgG1 isotype. Rat anti-hamster IgG xenoantibodies continue to express the V(H)HAR family of V(H) genes, many in their original germline configuration, to encode antibody recognition of the hamster target antigens. There are, however, a majority of antibodies for which the V(H) genes express evidence of increased nucleic acid sequence variation when compared to currently available germline sequences. The source of this variation is not known but may represent the expression of as yet unidentified germline genes and/or the introduction of T cell-driven somatic mutations. Despite the appearance of this variation, the unusual level of conservation in key antigen binding sites within the V(H) region suggests the variation, independent of its origin, may have a limited influence on the restricted nature of the host antibody response to xenografts.

Cardiac allograft rejection and enhancement in natural recombinant rat strains.

Cardiac allografts were carried out using rat strain combinations with recombinant haplotypes. Rejection times were determined in unsensitized recipients, recipients that had been immunized to produce hyperacute or accelerated rejection, and recipients that had been i.v. immunized with 10(7) bone marrow cells to produce active enhancement. The graft survival results obtained suggest that there is a major influence of the A region or classical type I transplantation antigens in first set and hyperacute rejection whereas immunization to both A or B region (Ia) antigens may produce markedly prolonged survival. These data suggest major differential influences of major histocompatibility complex (MHC) subregions depending on the specific donor-recipient incompatibility as well as suggesting major influences in these three allograft models depending on the background of the strain used.

Platelet-activating factor and hyperacute rejection. The effect of a platelet-activating factor antagonist, SRI 63-441, on rejection of xenografts and allografts in sensitized hosts.

The pathogenesis of hyperacute transplantation reactions includes the activation of a cascade of nonspecific inflammatory reactions that precipitates the destruction of the target organ. Platelet-activating factor (PAF) represents an important component of these inflammatory cascades, and we have examined the influence of a specific PAF receptor antagonist (SRI 63-441) on the inhibition of hyperacute rejection in two experimental models, the rejection of rat cardiac allografts by presensitized recipients and guinea pig-to-rat and mouse-to-rat cardiac xenografts. Our results demonstrate that inhibition of PAF function by SRI 63-441 has a variable effect on the survival of cardiac allografts in presensitized rat recipients. In the ACI to sensitized BN cardiac allograft model, the use of SRI 63-441 alone, or in combination with CsA, FK506, or prostaglandin E2 (PGE2), does not prolong graft survival. As we have previously reported, SRI 63-441 does act as a single agent to prolong the survival of ACI to sensitized LEW grafts, and this survival effect is synergistic when combined with CsA. Here we extend these results to demonstrate that this survival is also extended when FK506 is used in the ACI-to-LEW model. Concordant mouse-to-rat cardiac xenografts are also relatively resistant to prolongation of graft survival following treatment with SRI 63-441 alone or in combination with CsA or FK506. Discordant xenografts appear to be more susceptible to inhibition of the rejection reaction with SRI 63-441. When either donor or recipient animals were treated with SRI 63-441 alone, or in combination with CsA or FK506, there was significant prolongation of guinea pig-to-rat cardiac xenograft survival. These results are consistent with our earlier description of the effectiveness of SRI 63-441 in preventing the rejection of cat-to-rabbit kidney xenografts. We believe that these results demonstrate that the use of the SRI 63-441 to specifically interfere with the function of PAF has the effect of prolonging graft survival in those systems in which performed antibody and/or complement activation are important components of the hyperacute reaction. This synthetic drug is representative of a family of compounds whose structure can be modified to balance their therapeutic and toxicity activities, and may prove to be important components of a polytherapeutic approach to the control of graft rejection in sensitized patients or following discordant xenografting.

The graft-versus-host reactivity in AG-B/MLR disparate strains of rats.

Inbred strains of rats can currently be classified into eight Ag-B groups. Within an Ag-B group, individual strains generally share identity both the Ag-B histocompatibility antigens and mixed lymphocyte responses. In this report we present data from three strains which are Ag-B and mixed lymphocyte reaction (MLR) disparate: KGH (Ag-B7, MLR-1), MNR (Ag-B4, MLR-5), and B3 (Ag-B3, MLR-4). Popliteal lymph node assays involving these three strains and standard inbred strains demonstrate that the graft-versus-host reaction and MLR reactions in the rat are closely related. Positive graft-versus-host reactions were observed only in strain combinations incompatible for the MLR and were unaffected by differences in their Ag-B histocompatibility antigens. The close association of the MLR and graft-versus-host reaction provides additional evidence that the Ag-B/MLR disparity in these strains is the result of natural genetic recombinations within the major histocompatibility complex.

Suppression of allograft rejection with FK506. I. Prolonged cardiac and liver survival in rats following short-course therapy.

Heterotopic heart and orthotopic liver grafts from ACI donors were transplanted to Lewis rat recipients that were treated with a 3 (or 4) day course of FK506 IM that was started on postoperative day 0, 2, 3, 4, 5, or 6. Hearts, which rejected after a median of 6 days in untreated controls, always had prolonged survival (median 91 days) when treatment was started on postoperative day 4. The results were inferior when treatment was started earlier or later than this, but even when the first dose of FK506 was on postoperative day 5, one day before rejection was imminent in controls, the median survival was 50 days. The poorest results with a median graft survival of only 36 days were obtained when injections were on days 0-3. Results were similar with liver grafts that rejected after a median time of 10 days in nontreated controls but that usually survived permanently after a 3 (or 4) day FK506 course starting on day 0, 2, 3, or 4. Therapy started on day 6 was too late.

FK506 suppression of heart and liver allograft rejection. II: The induction of graft acceptance in rats.

Lewis recipients of orthotopic ACI livers had permanent graft acceptance induced with 3 doses of i.m. FK506 in the early postoperative period. They were studied 100 and 300 days posttransplantation. The recipients rejected ACI as well as Brown Norway (BN) (third-party) skin grafts, and had lymphocytes with substantial reactivity by mixed lymphocyte culture testing against ACI and third-party (BN) alloantigens. Lymphocyte subset redistribution had not occurred in the peripheral blood or spleens of these animals, and there was no evidence of suppressor cell activation by in vitro and in vivo tests. Graft-versus-host reactivity in splenic lymphoid tissues of these recipients was demonstrated with the popliteal lymph node assay. Attempts at adaptive transfer with recipient lymphocytes were unsuccessful. Heart graft acceptance was far more difficult to accomplish than liver graft acceptance, and probably was never permanent. ACI heart graft prolongation in LEW recipients after a brief induction with FK506 lasted for no more than 3 months in most animals. The temporary heart graft acceptance was specific for hearts of the original ACI donor strain but not for ACI skin. Results of studies of lymphocyte subsets and suppressor cell activity were similar to those in the liver recipients. These studies illustrate how poorly graft acceptance is understood and how badly further work is needed to clarify its mechanism.