Use of C6-deficient rats to evaluate the mechanism of hyperacute rejection of discordant cardiac xenografts

C plays a critical role in the hyperacute rejection (HAR) of discordant xenografts (Xg), but the relative contribution of early vs late C components is unknown. In this study, genetic differences in C6 activity were correlated with HAR of guinea pig cardiac Xg by the rat. Seven rat strains were tested for C activity. Six strains (PVG.R1 (R1), PVG.1A (1A), DA, W/F, F344, LEW) had readily detectable C activity in the total and alternative pathways. Some PVG rats also had adequate C activity [PVG (C+)] but others [PVG (C-)] had a profound C6 deficiency. All rats with adequate C activity (n = 35) rejected cardiac Xg between 15 and 80 min. PVG (C+) (n = 6) rats also rejected cardiac Xg hyperacutely (26 +/- 12 min), whereas PVG (C-) (n = 16) rats, which had high preformed IgM natural antibody titers, rejected cardiac Xg in 1 to 2 days (2678 +/- 542 min). Transfer of serum from R1 rats to PVG (C-) recipients with vigorously beating Xg caused HAR of cardiac Xg within 116 +/- 75 min. Transfer of fresh PVG (C-) serum or heat-inactivated R1 serum did not induce HAR. HAR was characterized by intravascular platelet aggregation and interstitial hemorrhage, whereas Xg transplanted to PVG (C-) recipients had patent vessels at 30 min but were heavily infiltrated by granulocytes and monocytes at 2 days. These findings indicate that a deficiency in C6 prevents HAR but allows an accelerated acute rejection that may be mediated by the generation of vasoactive and chemotactic C3a and C5a.

Use of C6-deficient rats to evaluate the mechanism of hyperacute rejection of discordant cardiac xenografts

C plays a critical role in the hyperacute rejection (HAR) of discordant xenografts (Xg), but the relative contribution of early vs late C components is unknown. In this study, genetic differences in C6 activity were correlated with HAR of guinea pig cardiac Xg by the rat. Seven rat strains were tested for C activity. Six strains (PVG.R1 (R1), PVG.1A (1A), DA, W/F, F344, LEW) had readily detectable C activity in the total and alternative pathways. Some PVG rats also had adequate C activity [PVG (C+)] but others [PVG (C-)] had a profound C6 deficiency. All rats with adequate C activity (n = 35) rejected cardiac Xg between 15 and 80 min. PVG (C+) (n = 6) rats also rejected cardiac Xg hyperacutely (26 +/- 12 min), whereas PVG (C-) (n = 16) rats, which had high preformed IgM natural antibody titers, rejected cardiac Xg in 1 to 2 days (2678 +/- 542 min). Transfer of serum from R1 rats to PVG (C-) recipients with vigorously beating Xg caused HAR of cardiac Xg within 116 +/- 75 min. Transfer of fresh PVG (C-) serum or heat-inactivated R1 serum did not induce HAR. HAR was characterized by intravascular platelet aggregation and interstitial hemorrhage, whereas Xg transplanted to PVG (C-) recipients had patent vessels at 30 min but were heavily infiltrated by granulocytes and monocytes at 2 days. These findings indicate that a deficiency in C6 prevents HAR but allows an accelerated acute rejection that may be mediated by the generation of vasoactive and chemotactic C3a and C5a.

The effect of xenoreactive antibody and B cell depletion on hyperacute rejection of guinea pig-to-rat cardiac xenografts

Xenotransplantation between phylogenetically distant species is prevented by hyperacute rejection (HAR), a process that is thought to be initiated by the binding of naturally occurring xenoreactive antibodies (NAb) to the endothelium of the xenograft (Xg) with subsequent activation of the classical pathway of C. The relative role of direct alternative pathway C activation in HAR is controversial. To evaluate the role of NAb in HAR of discordant rodent Xg, LEW rats were treated from the day of birth with i.p. injections of rabbit anti-rat IgM antiserum (RARM), or with mAb specific for rat kappa-light chain (OX12) or rat class II MHC (14-4-4S, Y-3P, or 10-2.16), in an effort to deplete B cells and NAb. These rats then underwent xenotransplantation with discordant guinea pig hearts. RARM was effective in depleting rats (n = 5) of B cells, serum IgM, and rat NAb directed against guinea pig cells, but guinea pig cardiac Xg survival was not prolonged compared with PBS-treated controls (n = 5), possibly due to the rabbit NAb specific for guinea pig cardiac tissue that were passively transferred in the RARM preparation. Of the anti-B cell mAb used to avoid this passive transfer of NAb, mAb 14-4-4S was highly effective (n = 9) in depleting the peripheral blood and spleen of B cells and the serum of IgM and NAb. Guinea pig cardiac Xg survival, however, was again not prolonged (n = 5), and rejected Xg from the B cell- and NAb-depleted recipients demonstrated rat C3 deposition in the absence of rat IgM and IgG. This study demonstrates that while neonatal anti-B cell antibody treatment can effectively deplete B cells and NAb in the rat, such treatment does not significantly prolong cardiac Xg survival in this well-established guinea pig to rat xenotransplantation model. These findings suggest that in addition to NAb depletion, inhibition of alternative C pathway activation and other humoral mechanisms may be necessary to prevent HAR and allow successful xenotransplantation.

CD4 mAb therapy modulates alloantibody production and intracardiac graft deposition in association with selective inhibition of Th1 lymphokines

The accelerated (24 h) rejection of (LEWxBN)F1 cardiac allografts (Tx) in LEW rats sensitized with BN skin grafts, is abrogated with CD4 mAb (BWH-4) administration between skin (day -7) and heart (day 0) transplantation (Tx survival ca. 11 days, p < 0.0001). This study analyzed the effects of CD4-targeted therapy upon host IgG and IgM alloantibody (allo-Ab) within the serum by two-color flow cytometry, and within the Tx, by immunohistology. These data were correlated with mRNA and protein production profiles of Th1 (IL-2, IFN-gamma) vs Th2 (IL-4) specific cytokines (polymerase chain reaction and/or immunohistology). Skin grafts elicited a strong systemic IgM allo-Ab response, which peaked at the time of cardiac Tx rejection at 24 h. It was associated with extensive deposits of IgM on Tx endothelium. Treatment with BWH-4 mAb diminished circulating IgM allo-Ab levels, and only low levels of IgM could be detected at the Tx site. Conversely, the low circulating IgG allo-Ab levels during rejection at 24 h in untreated recipients were accompanied by a strong labeling for intra-Tx IgG. BWH-4 mAb therapy did not prevent totally the switch of the IgM to IgG, but the IgG allo-Ab response was earlier, less intense and more transient than in untreated recipients. Accelerated rejection triggered sequential lymphokine mRNA expression in cardiac Tx, with the peak of transcription for IL-2 (6-12 h) preceding that for IL-4 (24 h). Interestingly, although CD4 targeted therapy virtually ablated the induction of IL-2 mRNA, it preserved transcription of the IL-4 gene. BWH-4 mAb therapy decreased otherwise abundant intra-Tx IL-2 and IFN-gamma, but allowed a vigorous elaboration of IL-4, confirming the translation of mRNA to the protein in vivo. Thus, CD4 mAb-mediated abrogation of accelerated cardiac Tx injury correlates with suppression of Th1 responses (depressed IL-2 and IFN-gamma production), but sparing of the Th2 function (enhanced IL-4 elaboration). Indeed, CD4 mAb-induced allo-Ab depression and immunosuppressive effects may reflect selective targeting of proinflammatory Th1-like cells and the multifaceted effects of IL-4 produced by unopposed Th2-like cells.

CD4 mAb therapy modulates alloantibody production and intracardiac graft deposition in association with selective inhibition of Th1 lymphokines

The accelerated (24 h) rejection of (LEWxBN)F1 cardiac allografts (Tx) in LEW rats sensitized with BN skin grafts, is abrogated with CD4 mAb (BWH-4) administration between skin (day -7) and heart (day 0) transplantation (Tx survival ca. 11 days, p &lt; 0.0001). This study analyzed the effects of CD4-targeted therapy upon host IgG and IgM alloantibody (allo-Ab) within the serum by two-color flow cytometry, and within the Tx, by immunohistology. These data were correlated with mRNA and protein production profiles of Th1 (IL-2, IFN-gamma) vs Th2 (IL-4) specific cytokines (polymerase chain reaction and/or immunohistology). Skin grafts elicited a strong systemic IgM allo-Ab response, which peaked at the time of cardiac Tx rejection at 24 h. It was associated with extensive deposits of IgM on Tx endothelium. Treatment with BWH-4 mAb diminished circulating IgM allo-Ab levels, and only low levels of IgM could be detected at the Tx site. Conversely, the low circulating IgG allo-Ab levels during rejection at 24 h in untreated recipients were accompanied by a strong labeling for intra-Tx IgG. BWH-4 mAb therapy did not prevent totally the switch of the IgM to IgG, but the IgG allo-Ab response was earlier, less intense and more transient than in untreated recipients. Accelerated rejection triggered sequential lymphokine mRNA expression in cardiac Tx, with the peak of transcription for IL-2 (6-12 h) preceding that for IL-4 (24 h). Interestingly, although CD4 targeted therapy virtually ablated the induction of IL-2 mRNA, it preserved transcription of the IL-4 gene. BWH-4 mAb therapy decreased otherwise abundant intra-Tx IL-2 and IFN-gamma, but allowed a vigorous elaboration of IL-4, confirming the translation of mRNA to the protein in vivo. Thus, CD4 mAb-mediated abrogation of accelerated cardiac Tx injury correlates with suppression of Th1 responses (depressed IL-2 and IFN-gamma production), but sparing of the Th2 function (enhanced IL-4 elaboration). Indeed, CD4 mAb-induced allo-Ab depression and immunosuppressive effects may reflect selective targeting of proinflammatory Th1-like cells and the multifaceted effects of IL-4 produced by unopposed Th2-like cells.

Cytokines, adhesion molecules, and the pathogenesis of chronic rejection of rat renal allografts

Little is known of the host immune mechanisms responsible for initiation and progression of chronic rejection. We describe immunopathologic features associated with progressively deteriorating function of kidney allografts in the F344-to-Lewis rat strain combination, which differ at MHC and non-MHC loci. Initial rejection in untreated recipients was controlled by a brief course of CsA (5 mg/kg/day, for 10 days), resulting in > 80% of recipients surviving up to a year despite declining renal function. In contrast to controls (isografts placed in untreated or CsA-treated Lewis rats), allografts from 12-16 weeks post-Tx showed segmental or global glomerulosclerosis, increasing tubular atrophy, interstitial fibrosis, and intimal proliferation leading ultimately to vascular occlusion. By flow cytometry, IgM and IgG alloantibodies peaked at 2-4 weeks, with a gradual decline to baseline thereafter. Immunohistology showed early and progressive deposition of IgM, IgG, C3, and fibrin in vessel walls and glomeruli. In addition, by 12 weeks, extensive infiltration by activated (IL-2R+) macrophages and CD4+ T cells were noted in glomeruli and blood vessels, in conjunction with staining for the cytokines TNF-alpha, IL-1, and IL-6. The persistent and dense intraglomerular expression of IL-6 was of particular interest, given its potent mitogenic effects for mesangial cells in vitro, and suggests a role for this cytokine as a mediator of mesangial expansion, advanced glomerular injury, and glomerulosclerosis in chronic rejection. Parallel timing of IL-6 and TNF-alpha expression was shown in serum samples by ELISA and bioassays. In vitro binding studies showed increased binding of naive host lymphocytes to allograft versus isografts, correlating with upregulation (peaking at week 16) of intercellular adhesion molecule-1 expression by graft endothelium. We conclude that cytokine production and upregulation of adhesion molecules occurring as part of a cellular immune response may be as important to the etiology of chronic rejection as the hitherto widely emphasized antibody-mediated host responses.

The association of enhancement of renal allograft survival by donor-specific blood transfusion with host MHC-linked inhibition of IgG anti-donor class I alloantibody responses

Donor-specific blood transfusion (DSBT) in animals and humans can either promote subsequent renal graft survival or lead to sensitization and graft rejection. Using a rat renal allograft model, we have examined whether the effects of DSBT on renal allograft outcome and IgG alloantibody responses are linked to the host MHC. In F1 rats produced by mating PVG (RT1c), a low IgG alloantibody responder to transfused ACI (RT1a) blood, with 3 different high-IgG responders [W/F (RT1u), LOU (RT1u), and LEW (RT1l)], high IgG alloantibody production was found to be inherited as a dominant trait and associated with acute rejection of ACI renal allografts. DSBT given to offspring of (PVG x W/F)F1 rats backcrossed to W/F with either RT1u/c (u/c) or RT1u/u) (u/u) phenotype induced high-IgG-alloantibody responses that were associated with acute renal allograft rejection. Likewise, offspring of (PVG x W/F)F1 rats backcrossed to PVG expressing the u/c phenotype had high IgG responses to ACI DSBT associated with acute renal allograft rejection. In contrast, DSBT given to backcrossed recipients expressing the RT1c/c (c/c) phenotype elicited a transient IgM response that switched to a very low IgG response and was associated with renal allograft acceptance. Analysis of IgG isotypes demonstrated that DSBT prevented production of IgG1 and IgG2a, and to a lesser extent IgG2b and IgG2c alloantibodies in c/c but not u/c renal allograft recipients. The differences in the level and isotype of IgG alloantibody responses found in sera of DSBT-pretreated backcross rats of u/c and c/c phenotypes were also present in allograft eluates and splenocyte cultures. Likewise, DSBT-pretreated renal allograft recipients of the c/c phenotype produced lower levels of alloantibodies directed to class I RT1.Aa antigens compared with their u/c counterparts; in contrast, no difference was found in alloantibody responses to class II RT1.Ba antigens. These findings demonstrate that the variable ability of DSBT to enhance renal allograft survival correlates with the inhibition of antidonor class I alloantibody responses of all IgG subclasses by a mechanism that is linked to host MHC.

Characterization of human anti-porcine "natural antibodies" recovered from ex vivo perfused hearts--predominance of IgM and IgG2

Hyperacute rejection is a major obstacle to successful transplantation of vascularized xenogeneic organs and is believed to be mediated at least in part by performed xenoreactive "natural antibodies" (NAb). In this study, human NAb that could be involved in hyperacute rejection of pig heart xenografts were identified and characterized using an ex vivo model in which pig hearts were perfused with whole blood from individual human or pig donors. This ex vivo perfusion model allows for the continuous monitoring of physiologic parameters of cardiac function as well as sequential sampling of tissue and blood. Pig hearts perfused with allogeneic pig blood maintained normal function for at least 4 hr, whereas those perfused with xenogeneic AB+ human blood never achieved normal function and rejected completely after 30 min. In three separate experiments involving different human blood donors and pig hearts, sequential samples of perfused blood revealed a progressive depletion of anti-porcine NAb. Samples of all three rejected cardiac xenografts were homogenized, and the specifically bound human anti-porcine antibodies were eluted with citric acid. The eluted antibodies were enriched approximately 50-120-fold for anti-porcine reactivity compared with serum from the corresponding donor. Eluates contained NAb of predominantly IgM and IgG2 isotypes. Immunofluorescence histology confirmed the deposition of IgM and IgG2 but not other IgG subclasses in the rejected pig hearts. Since IgG2 utilized predominantly in response to bacterial polysaccharide antigens, our findings are consistent with the possibility that some NAb arise via crossreactivity with microbial antigens and are predominantly directed against carbohydrate rather than protein antigens.

Different patterns of sensitization following renal allograft rejection in an inbred rat strain combination

Exposure to MHC antigens by kidney transplantation can result in long-term sensitization or tolerance. In order to characterize immune responses to an acutely rejected renal allograft, ACI (RT1a) kidneys were transplanted into untreated male PVG (RT1c) recipients and allowed to reject while one native kidney remained in situ for host survival. Two distinct patterns of recipient sensitization were found based on early IgM responses to RT1.Aa following rejection, with "weakly sensitized" and "strongly sensitized" groups comprising approximately 40% and 60%, respectively, of the recipients. Serum taken from strongly versus weakly sensitized animals at the time of peak IgM responses (7 days post-transplantation) showed greater ability to block binding of anti-RT1.Aa mAb (R2/15S, R2/10P, or YR1/100) to PVG.1A (RT1a class I and II on a PVG background) lymph node target cells. Sera obtained from strongly sensitized recipients during peak IgG responses (4 weeks after kidney transplantation) demonstrated significantly higher IgG2a and IgG2b alloantibody levels than weakly sensitized rats at all serum dilutions (1:4-1:1024). Allografts harvested 10 days after transplantation from strongly sensitized recipients (strong R2/10P-blocking serum, n = 9) had a vascular pattern of rejection characterized primarily by extensive vascular endothelial damage, glomerular and cortical necrosis, and gross infarction of the graft. In contrast, grafts harvested from weakly sensitized recipients at 10 days, 21 days, or > 6 months (n = 6, 10, and 6, respectively) posttransplantation showed a significantly different pattern of rejection, with moderate interstitial lymphocytic infiltrates but substantial preservation of the general kidney architecture. When challenged 13 weeks posttransplantation, strongly sensitized animals rejected RT1.Aa class I-disparate PVG.R1 skin grafts in an accelerated fashion, whereas such grafts survived indefinitely on weakly sensitized recipients. These findings indicate that two patterns of renal allograft rejection can occur between a fixed strain disparity, one of which results in long-term sensitization and the other with partial tolerance to donor class I MHC antigens, as evidenced by (1) decreased production of IgM, IgG2a and IgG2b isotype alloantibody to donor class I MHC antigen epitopes after kidney rejection and (2) acceptance of donor class I-disparate skin grafts.

The effect of bursectomy on natural xenoreactive antibodies and vascularized rat cardiac xenograft rejection in the chicken

The clinical application of xenotransplantation between distantly related species is currently prevented by the occurrence of hyperacute rejection (HAR). Controversy exists over the importance of natural xenoreactive antibody (NAb)-mediated activation of the classical complement pathway vs. direct activation of the alternative C pathway in this process. In order to evaluate HAR of xenografts (Xgs) in the absence of NAb, this study utilized K strain leghorn chickens that were bursectomized (Bx) on day 17 in ovo (n = 18) to prevent B cell development and production of NAb. Aged-matched untreated siblings served as controls (n = 13). Based on pretransplant antibody levels, the Bx chickens were divided into two groups: totally Bx (Total Bx, n = 9) and partially Bx (Part Bx, n = 9). Chickens then underwent heterotopic cardiac xenotransplantation using PVG rats as donors, where the Xg was connected to the circulation of the chicken recipient utilizing cannulae. For the control group, Xg survival was 28 +/- 3 min (mean +/- SEM), while Part Bx prolonged survival to 80 +/- 15 min. Total Bx extended rat Xg survival to 102 +/- 11 min, with 5 of 9 Xgs functioning well at the time of termination of the study (90-120 min). Three chickens in the Total Bx group with rat cardiac Xgs that were functioning at 120 min were given a 1 ml i.v. injection of heat inactivated control chicken serum. This led to loss of Xg function within 10 min, confirming the important role for NAb in HAR in this species combination. Histologic examination of the Xgs following perfusion revealed significant arterial endothelial injury in the control and Part Bx groups but not in the Total Bx group. Conversely, Xgs from the Total Bx group showed marked venous congestion, which was not seen in the other two groups. This study demonstrates that: (1) Bx effectively eliminates NAb; (2) Xg survival is significantly prolonged in the absence of NAb in this rat-to-chicken xenogeneic combination; (3) the presence of NAb is associated with arterial endothelial injury; and (4) in the absence of NAb, marked venous congestion and injury occurs, which is possibly mediated by alternative C pathway activation or other humoral mechanisms.

The effects of donor-specific blood transfusion enhancement of rat renal allografts on host NK cell responses

Donor-specific blood transfusion (DSBT) given 1-2 weeks prior to transplantation prolongs the survival of fully allogeneic ACI (RT1a) renal allografts in PVG (RT1c) recipients from 6-8 days to greater than 100 days. We have previously demonstrated that ACI kidneys transplanted to autologous blood transfusion (ABT)- or DSBT-pretreated PVG recipients stimulated an increase in CD8+ (OX8+) cells in the peripheral blood by 6 days after transplantation. To determine whether this increase represents a general expansion of the entire CD8+ population or only a subpopulation of CD8+ cells, subset analysis was performed on peripheral blood lymphocytes depleted of cells reactive with monoclonal antibodies against rat alpha beta T cell receptor (TCR), CD8, or NK cells (R7.3, OX8, or 3.2.3, respectively). Phenotypic studies of PBL depleted of CD8+ cells demonstrated that all 3.2.3+ NK cells coexpressed CD8; depletion of 3.2.3+ PBL revealed a second subpopulation of CD8+3.2.3- cells comprised predominantly of alpha beta TCR+ T cells. In naive PVG rats the prevalence of these two CD8+ subpopulations was approximately equal. Both ABT- and DSBT-pretreated renal allograft recipients demonstrated a significant and equivalent expansion of the CD8+ cell subpopulation that coexpresses the 3.2.3 NK marker. In contrast, the second subpopulation of CD8+3.2.3- cells did not change significantly after allografting. There were also no differences between DSBT and ABT pretreated rats in activity of PBL against the NK targets YAC-1 and Doxie at 6 days after renal transplantation, though the level of activity was modestly increased compared with naive controls. These findings indicate that renal transplantation in the rat is associated with a significant increase in PBL with the NK phenotype (CD8+3.2.3+) and a modest increase of NK activity, but that DSBT enhancement does not affect this NK cell response.

IgG ALLOANTIBODY RESPONSES TO DONOR-SPECIFIC BLOOD TRANSFUSION IN DIFFERENT RAT STRAIN COMBINATIONS AS A PREDICTOR OF RENAL ALLOGRAFT SURVIVAL1,2

Donor-specific blood transfusion prolongs the survival of fully allogeneic ACI (RT1a) renal allografts in PVG (RT1c) recipients from 7-10 days to greater than 100 days. We have observed significant differences in the alloantibody (Ab1) responses to ACI renal allografts in control and DSBT-treated PVG recipients: DSBT is associated with decreased IgG and IgM alloantibody circulating in serum, deposited in the allograft, and produced in culture by splenocytes. In the present studies the effects of DSBT on alloantibody production and renal allograft survival were extended to examine other recipient strains: F344 (RT1lv1), BN (RT1n), W/F (RT1u) and LEW (RT1l). Animals of each recipient strain were injected i.v. with 0.5 ml of ACI blood alone or followed by a renal allograft. Studies on the kinetics of IgM and IgG alloantibody responses were performed by flow cytometry on lymphocytes from donor ACI, PVG, and PVG.R1 (RT1.Aa class I MHC antigen on PVG background) rats. In F344 and PVG rats, DSBT from ACI rats elicited a transient IgM response that peaked at day 7 and was not followed by a switch to IgG. In control PBS transfused F344 recipients, an ACI renal allograft stimulated both IgM and IgG alloantibody production. DSBT pretreatment significantly decreased circulating IgG alloantibody following ACI renal transplantation and prolonged graft survival in F344 recipients. In DSBT-treated F344 recipients that rejected ACI renal allografts acutely, small amounts of IgG (5-12 mode channel shift) were detected in sera harvested 7 days after transplantation, whereas almost no IgG was detected in the sera from DSBT treated F344 rats that accepted their renal allografts indefinitely. In contrast, DSBT alone from ACI to BN, W/F, or LEW strains elicited a transient IgM response that peaked at day 7 and was followed by a strong IgG response that peaked on days 10-14 and remained high through day 21. DSBT failed to prolong ACI renal allograft survival in any of these strains (survival less than 11 days in control and DSBT rats). The alloantibody response to DSBT in all five recipient strains examined was directed primarily to RT1.Aa class I MHC antigens, as determined by binding studies on lymphocytes from ACI, PVG and PVG.R1 rats and alloantibody blocking studies using biotinylated rat monoclonal antibodies to distinct epitopes of the RT1.Aa antigen. The relative magnitude of blocking of R2/10P and R2/15S binding by sera from BN, W/F, and LEW rats was: control allograft recipients greater than DSBT pretreated allograft recipients greater than DSBT alone.(ABSTRACT TRUNCATED AT 400 WORDS)

IgM AND IgG ALLOANTIBODY RESPONSES TO MHC CLASS I AND II FOLLOWING RAT RENAL ALLOGRAFT REJECTION

Renal allograft rejection in rats and humans is a potent inducer of alloantibody to donor major histocompatibility complex antigens. Alloantibody in such presensitized recipients can cause hyperacute rejection of subsequent renal allografts. In order to characterize alloantibody production in rats presensitized by renal graft rejection, ACI (RT1a) kidneys were transplanted into untreated fully allogeneic PVG (RT1c) recipients and allowed to reject while one native kidney remained in situ for host survival. Serum samples collected at weekly intervals were analyzed by flow cytometry for IgM and IgG antibody binding to ACI lymphoid target cells. The specificity of alloantibody responses was assessed by (1) differential binding to congenic rat strain target cells expressing only donor class I (PVG.R1) versus both donor class I and II (PVG.1A) antigens, (2) differential binding to unseparated donor lymphoid target cells versus lymphoid target cells depleted of class II MHC antigen-expressing cells, and (3) specific blocking of monoclonal antibodies to donor class I (R2/10P, R2/15S) or class II (F17.23.2) epitopes. Alloantibody responses to both donor class I and II MHC antigens were detected. The initial IgM response to donor class I MHC antigens peaked at the time of rejection, followed by a steady decline to relatively low levels by 4 weeks posttransplantation. The IgM response to donor class II MHC antigens was found to be cyclical with apparent peaks at day 7 and 5-6 weeks. The IgG response to donor class I and class II MHC antigens reached maximum by 5-6 weeks before slowly decreasing. IgM and IgG alloantibody specific for class I and class II MHC antigens could be detected through 19 weeks posttransplantation. The effects on circulating alloantibody of two manipulations, posttransplantation donor specific blood transfusion and allograft removal, were examined in this model. The alloantibody responses to class I MHC antigens were not affected by giving DSBT weekly beginning at day 14 after transplantation. However, posttransplantation DSBT eliminated the second peak of IgM alloantibody to class II MHC antigens seen approximately 5-6 weeks posttransplantation and also decreased circulating IgG specific for class II antigens. Transplantectomy at day 5-7 days after transplantation had no apparent effect on circulating IgM or IgG alloantibody through 7 weeks posttransplantation. These data indicate that in a fully allogeneic rat renal allograft model alloantibody responses are elicited to both class I and II MHC donor antigens, but that the kinetics and regulation of the responses to class I differ from those to class II alloantigens.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of soluble complement receptor type 1 on hyperacute xenograft rejection

In the guinea pig-to-rat model of hyperacute xenograft (Xg) rejection, the effect of complement inhibition using systemically administered soluble complement receptor type 1 (sCR1) on discordant cardiac Xg survival was investigated. In PBS-treated control Xg recipients (n = 13), hyperacute rejection was rapid, with a mean Xg survival of 17 +/- 4 min. Therapy with sCR1 prolonged survival of cardiac Xgs in a dose-dependent manner. A 3 mg/kg bolus of sCR1 (n = 4) prolonged Xg survival to 64 +/- 29 min (not significant). Increasing the sCR1 dose to 5.9 mg/kg (n = 4) significantly delayed Xg rejection to 71 +/- 17 min (P-0.026, log-rank test vs. control). In 10 recipients treated with 15 mg/kg sCR1, mean Xg survival was further prolonged to 189 +/- 36 min (P-0.0004) with no adverse effects. While 2 of 8 recipients receiving 60 mg/kg sCR1 died with functioning Xgs at 30 and 300 min due to anastomotic bleeding, Xg survival averaged over 12 hr (747 +/- 100 min, P-0.0004) in the remaining 6 recipients. sCR1 administration significantly inhibited serum complement activity in a parallel dose-dependent fashion, with the 60 mg/kg dose reducing complement activity by 95 +/- 1 and 96 +/- 1% five and 30 min following Xg reperfusion, respectively. Immunofluorescence microscopy revealed rat IgM bound to all cardiac Xgs in control as well as sCR1-treated recipients. In addition, serial histologic examination of cardiac Xgs harvested within 21 min of graft reperfusion revealed occlusive platelet aggregates within the coronary vessels as well as interstitial hemorrhage and myocardial necrosis in Xgs from control recipients, all of which were only minimally present in Xgs from recipients treated with sCR1. These studies show that complement inhibition with sCR1 significantly delays hyperacute cardiac Xg rejection in this discordant model and may be an important component in a therapeutic protocol for xenotransplantation.