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Biomedical subjects

W M Baldwin

Publications and source records attributed to W M Baldwin.

At least 37 records · Page 2Linked to original sources
PubMed1996

Inhibition of complement, evoked antibody, and cellular response prevents rejection of pig-to-primate cardiac xenografts.

Complement (C) inhibition alone using a recombinant soluble form of complement receptor type 1 (sCR1) prevents hyperacute rejection but not subsequent irreversible accelerated acute rejection of discordant pig-to-cynomolgus monkey cardiac xenografts, which occurs within 1 week. To inhibit accelerated acute rejection, which is associated with a rise in serum xenoreactive antibody (Ab) and a cellular infiltrate, triple therapy with standard immunosuppressive agents (cyclosporine, cyclophosphamide, and steroids [CCS]) was combined with continuous C inhibition using sCR1. Each of two monkeys that received sCR1 + CCS showed minimal evidence of rejection when killed on days 21 and 32 in comparison to a monkey that received sCR1 + subtherapeutic CCS (rejected at 11 days) and a control that received CCS alone (rejected at 38 min). Prolonged xenograft survival was associated with low Ab levels and a minimal cellular infiltrate, suggesting that combined inhibition of C, xenoreactive Ab responses, and cellular immunity may be a useful approach in overcoming the immune barriers to discordant xenotransplantation.

Animals↗
PubMed1996

Functional activity of anti-C6 antibodies elicited in C6-deficient rats reconstituted by liver allografts. Ability to inhibit hyperacute rejection of discordant cardiac xenografts.

A critical role of the complement membrane attack complex (C5b-9) in mediating hyperacute rejection has been demonstrated previously in fully C6-deficient PVG (C-)(RT1c) rats that reject guinea pig cardiac xenografts at a delayed tempo (45 +/- 9 hr; n=16) compared with C6-sufficient PVG (C+)(RT1c) hosts (0.5 +/- 2 hr; n=6). We have investigated whether selective depletion of C6 from Lewis rats by antibody therapy prevents hyperacute rejection. A polyclonal rat-antirat C6 antibody was induced in PVG (C-) recipients by orthotopic liver transplants from congenic PVG (C+) donors. These liver grafts produced high levels of C6 that reconstituted the complement function of PVG (C-) hosts by 7 days, but the recipients responded within 28 days with the synthesis of an IgG1 antibody to rat C6. The antiserum inhibited hemolytic complement activity of Lewis (RT1(1)) rats in vivo and in vitro. The effect of C6 depletion on Xg survival was investigated by injecting Lewis rats with 2 ml of rat-antirat C6 antiserum before and 1 ml after reperfusion of the guinea pig cardiac xenograft. Lewis rats rejected guinea pig cardiac xenografts after treatment with this rat-antirat C6 antiserum in 38 +/ -11 hr (n=3). Treatment with normal control sera from PVG (C-) rats did not prolong guinea pig cardiac xenograft survival in the Lewis rats (1 +/- 0.7 hr; n=3)(P<0.0043). Injection of 3 ml of the IgG fraction purified from the rat-antirat C6 antibody (33 mg/ml) prolonged xenograft survival to 13.6 +/- 4 hr (n=4) compared with the survival of 0.61 +/- 0.3 hr (n=4) after injection of control rat IgG (33 mg/ml) (P<0.005). These results demonstrate that specific depletion of C6 by antibody therapy has a significant effect on guinea pig cardiac xenograft survival in the Lewis rat. These findings further suggest that C6 depletion may be beneficial to patients undergoing hyperacute rejection of xenografts or allografts.

Animals↗
PubMed1996

Induction of heat shock protein in cardiac allograft rejection--a cyclosporine-suppressible response.

The cellular response to a wide variety of stresses results in the synthesis of a family of proteins termed heat shock proteins (HSPs). To determine if acute allograft rejection could induce these proteins in a transplanted graft, we examined the HSP response to acute cardiac allograft rejection and analyzed the effect of immunosuppression upon this response. Donor hearts obtained from either Lewis (LEW) or ACI rats were heterotopically transplanted in recipient LEW rats. There were 4 experimental groups: untreated isografted (LEW to LEW) animals (n = 14), untreated allografted (ACI to LEW) animals (n = 14), cyclosporine-treated (10 mg/kg SQ/day) isografted animals (n = 12), and cyclosporine-treated allografted animals (n = 12). Animals were sacrificed on posttransplantation day 2, 4, or 6 (time of rejection for untreated allografts); n = 4-5 for each time point per group. At these times tissue obtained from the transplanted heart was examined histologically and analyzed for HSP72 by quantitative Northern and Western blots. The level of HSP72 in the untreated allografts progressively increased between 2, 4, and 6 days posttransplantation and was significantly greater than that of the untreated isografts at all time points. The HSP72 response in cyclosporine-treated allografts was significantly reduced at 4 and 6 days posttransplantation compared with the untreated allografts. In contrast, there was no difference in the HSP response in treated versus untreated isografts. Additionally, there was no difference in HSP levels in cyclosporine-treated isografts and allografts. These findings demonstrate that HSP expression in the transplanted heart correlates directly with the evolution of acute allograft rejection, and that immunosuppressive therapy inhibits the HSP response. These studies also raise the possibility of a functional role for HSPs in the allogeneic immune response.

Animals↗
PubMed1995

Extrahepatic synthesis of C6 in the rat is sufficient for complement-mediated hyperacute rejection of a guinea pig cardiac xenograft.

The liver is the major source of complement (C) components, but extrahepatic sources of C, such as macrophages and endothelial cells, have been hypothesized to contribute to inflammation. Our experiments demonstrate that extrahepatically produced C6 can contribute to hyperacute rejection. PVG (RT1c) rats with normal C activity (PVG (C+)) reject guinea pig cardiac xenografts in 0.5 +/- 0.2 hr, but fully C6-deficient PVG (RT1c) rats (PVG (C-)) reject guinea pig cardiac xenografts in 45 +/- 9 hr. PVG (C+) rats, which received liver transplants from PVG (C-) rats and retained all extrahepatic sources of C6, rejected guinea pig cardiac xenografts in 0.6 +/- 0.03 hr (n = 3). PVG (C-) rats, which received bone marrow transplants from PVG (C+) rats, had C6 levels restored to 10% of that of the donor and rejected guinea pig cardiac xenografts in 9 +/- 3.2 hr (n = 5). Thus, extrahepatic sources of C6 can contribute to xenograft rejection.

Animals↗
PubMed1995

The contribution of terminal complement components to acute and hyperacute allograft rejection in the rat.

Acute rejection and antibody-mediated hyperacute allograft rejection are affected by activation of the complement cascade. Split products of early complement components influence the localization, activation, and effector functions of platelets, granulocytes, monocytes, and lymphocytes, while the formation of membrane attack complex (C5b-C9) can lead to rapid cell destruction. Therefore, we compared acute and Ab-mediated hyperacute allograft rejection in a recently described model of C6 deficient PVG (C-) (RT1c) rats and their normal counterpart PVG (C+) (RT1c) rats. Cardiac allografts from fully MHC disparate ACI donors were heterotopically grafted into naive and skin graft sensitized PVG (C-) and PVG (C+) rats. ACI cardiac allografts were rejected acutely (8.3 +/- 2 days; n = 7) by naive PVG (C+) recipients, but survived significantly longer in PVG (C-) recipients (22 +/- 10 days; n = 10). Presensitized PVG (C+) rats rejected ACI cardiac allografts hyperacutely in 6.1 +/- 2.4 hr (n = 5). In contrast, ACI cardiac allografts transplanted into presensitized (PVG (C-) rats had markedly longer survival of 91 +/- 14 hr (n = 5). The alloantibody responses of naive PVG (C+) and PVG (C-) recipients 7 days after cardiac allografting, and of presensitized PVG (C+) and PVG (C-) recipients at time of cardiac allografting were not significantly different as measured by flow cytometry against ACI lymphocytes. Immunofluorescence demonstrated deposition of IgM, IgG and C3 in ACI allografts in PVG (C-) as well as in PVG (C+) recipients. Deposition of C6 was only found in grafts rejected by PVG (C+). The significantly longer survival of ACI cardiac allografts in C6-deficient PVG (C-) rats indicates that the membrane attack complex contributes to acute as well as antibody-mediated hyperacute allograft rejection.

Acute Disease↗
PubMed1994

Hepatic and extrahepatic biosynthesis of complement factor C6 in the rat.

The relative contributions of liver and bone marrow (BM) constituents to systemic C6 production were compared in a rat model. Liver grafts were transplanted from C6-sufficient PVG (RT1c) rats (PVG (C+)) to profoundly C6-deficient PVG rats (PVG (C-)). C6 levels were restored to 32% within 24 h and reached more than 80% of that of the PVG (C+) donor within 7 days post-grafting, which indicates that the liver is a primary source of systemic C6 production. When livers were transplanted from PVG (C-) to PVG (C+) rats (n = 3), levels of C6 dropped to 42% of pretransplant levels within 24 h and remained between 30 and 40% for more than 100 days after grafting. To determine the source of extrahepatic C6 production, BM was transplanted from PVG (C+) to PVG (C-) rats after total body irradiation. Levels of C6 increased from undetectable levels to 5% of C6 levels of the donor PVG (C+) rat within 60 days. Replenishing PVG (C-) recipients with BM after treatment of recipients with busulfan, which preferentially allows reconstitution with donor myelomonocyte stem cells, resulted in restoration of C activity. Treatment with cyclophosphamide before BM transplantation, which preferentially allows reconstitution of lymphoid stem cells, did not restore hemolytic C activity in PVG (C-) rats. These results were confirmed directly by the successful restoration of C activity with BM depleted of lymphocytes by counterflow centrifugal elutriation from PVG (C+) rats. These in vivo experiments demonstrate that the liver is a primary, but not the sole, source of C6 in the rat and that extrahepatic sources, such as myelomonocytes, and not lymphoid cells in the BM produce a significant amount of systemic C6 in the rat.

Animals↗
PubMed1993

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.

Animals↗
PubMed1993

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.

Animals↗
PubMed1993

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.

Animals↗
PubMed1993

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.

Animals↗
PubMed1993

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.

Animals↗
PubMed1993

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.

Animals↗
PubMed1993

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.

Animals↗
PubMed1993

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.

Acute Disease↗