Search PubMed⌕ Search

Biomedical subjects

J L Platt

Publications and source records attributed to J L Platt.

At least 91 records · Page 5Linked to original sources
PubMed1998

High dose intravenous immunoglobulin does not affect complement-bacteria interactions.

Pooled IgG preparations for i.v. use (IVIg) have been shown to possess anticomplementary activity in autoimmune and systemic inflammatory diseases. Both in vitro and in vivo, IVIg is a preferential acceptor of activated C4 and C3, thus diverting complement activation from the target surface. We explored the effect of IVIg on complement-bacteria interactions in an attempt both to determine the safety of IVIg preparations in relation to natural immunity to bacteria and to extend our knowledge of the physiologic mechanism of action of IVIg. Using both complement-sensitive and complement-resistant bacterial strains, we investigated the effect of IVIg on C3 binding to bacterial surfaces. In all cases, whether complement could be directly activated by bacteria through the classical or the alternative pathway, IVIg had no effect on the amount of C3 bound to bacteria. In addition, IVIg did not inhibit complement-dependent bacterial lysis. Interestingly, increasing concentrations of IVIg induced an increase in C1q binding, suggesting the presence of low affinity complement-fixing antibacterial Abs in certain preparations. Using serum samples from patients treated with IVIg, complement binding to and lysis of complement-sensitive bacterial strains were not modified as compared with normal controls and pretreatment samples, although a decrease in C3 binding to sensitized human erythrocytes was observed. Our data suggest that IVIg does not affect direct complement-bacteria interactions, although it is a potent agent to use for diversion of complement activation on sensitized target surfaces.

Animals↗
PubMed1998

Regulation of platelet heparanase during inflammation: role of pH and proteinases.

Heparan sulfate is rapidly degraded by an endoglycosidase (heparanase) secreted by activated platelets. Since the cleavage and release of heparan sulfate would profoundly alter the local physiology of the endothelium, platelet heparanase activity should be tightly regulated. Consistent with this hypothesis, platelet heparanase was found to degrade endothelial cell heparan sulfate at pH 6.0 but not at pH 7.4, even though 25% of maximum activity was detected at pH 7.4. Loss of heparanase activity occurred rapidly (t1/2 is approximately equal to 20 min) and reversibly at physiologic pH but did not occur at acidic pH (<7.0). Inactivation of heparanase at pH 7.4 did not affect heparin binding and was reversed by 0.5 M NaCl or by heparan sulfate but not by chondroitin sulfate, suggesting inactive heparanase could be tethered on cell surfaces and the function regulated by heparan sulfate. Heparanase was gradually inactivated by trypsin and urokinase (t1/2 = 5 h) but resisted cleavage by leukocyte cathepsin G, leukocyte elastase, plasmin, and thrombin. These findings are consistent with a model in which platelet heparanase is active at the low pH of inflammation but inactive under physiologic conditions preventing inadvertent cleavage of heparan sulfate and loss of physiologic functions of endothelial cells.

Animals↗
PubMed1998

Total respiratory support from swine lungs in primate recipients.

UNLABELLED: The use of nonhuman lung donors, such as swine, has the potential to provide an unlimited supply of organs. However, hyperacute rejection has prevented pulmonary xenotransplantation. OBJECTIVE: Our aim was to test the hypothesis that immunodepletion by pretransplantation swine lung perfusion will prevent hyperacute swine-to-primate pulmonary xenograft rejection and allow for a functional swine pulmonary xenograft. METHODS: Seven baboons underwent left pneumonectomy followed by orthotopic transplantation of the swine left lung. Four baboons received immunodepletion by perfusion with swine lungs before transplantation, and three received no treatment before transplantation. RESULTS: After transplantation, pulmonary xenografts from immunodepleted baboons had a low pulmonary vascular resistance and a high pulmonary blood flow compared with control animals, which had a high pulmonary vascular resistance and a low pulmonary blood flow. After 60 minutes of reperfusion, three of four immunodepleted animals also tolerated complete occlusion of the right pulmonary artery, with the baboon relying completely on the swine pulmonary xenograft for respiratory function for 11 hours. Pathologic analysis of peripheral lung biopsy specimens taken from control lungs displayed alveolar disruption and hemorrhage within small vessels, whereas swine lungs transplanted into immunodepleted baboons displayed little histologic evidence of injury. Furthermore, pulmonary xenografts transplanted into immunodepleted baboons demonstrated excellent respiratory function and adequate hemodynamics during occlusion of the right pulmonary artery. CONCLUSION: Hyperacute pulmonary xenograft rejection can be prevented by pretransplantation swine lung perfusion. Swine pulmonary xenografts can provide complete respiratory support in primates when rejection is prevented.

Animals↗
PubMed1998

Immunology of xenotransplantation.

The transplantation of tissues and organs between individuals of different species, that is, xenotransplantation, engenders a variety of immune responses. Xenogeneic immune responses mediated by naturally-occurring antibodies and complement lead to hyperacute and acute vascular rejection of vascularized organ grafts and may also cause vascular rejection of cell and tissue grafts. Under some circumstances, however, a vascularized organ graft may evade humoral rejection despite the presence of anti-donor antibodies in the circulation of the recipient; this condition is called accommodation. Xenogeneic immune responses mediated by T lymphocytes and natural killer cells may cause acute cellular rejection. The extent to which cellular rejection of xenografts resembles cellular rejection of allografts remains to be determined. New insights into the molecular mechanisms underlying the immune responses to xenotransplantation has shed light on the pathogenesis of immunological disease and has allowed the development of specific immunomodulatory strategies that may facilitate clinical application of xenotransplantation.

Animals↗
PubMed1998

The promise of xenografting.

Advances in understanding the rejection of foreign tissues has renewed enthusiasm about the possibility of overcoming the present organ shortage by transplanting porcine kidneys into humans. Currently, three known forms of organ rejection stand as obstacles to xenotransplantation as a clinical reality--hyperacute rejection, acute vascular rejection, and cellular rejection. Progress in the knowledge of xenoreactive antibodies and of complement, along with new transgenic technologies, have enabled researchers to overcome hyperacute rejection in xenografts. The advances have brought into focus such issues as the ability of the porcine kidney to replace the physiologic functions of the human kidney and the risk associated with the potential transmission of infectious agents from animals to humans. Despite the remaining hurdles to clinical application of xenotransplantation, the rapid pace of research and emerging technologies would seem to make xenotransplantation a renal replacement therapy of great promise.

Animals↗
PubMed1998

Differential recognition by proteins of alpha-galactosyl residues on endothelial cell surfaces.

The binding of proteins to cell surface carbohydrates contributes to cell-cell interactions in development, immunity, and various physiologic processes. Such interactions are presumably dictated not only by the chemical structure of the carbohydrate but also reflect the properties of the protein and the microenvironment in which the protein-carbohydrate interaction occurs. To explore the factors influencing the recognition of cell surface carbohydrates by proteins, the extent to which three classes of proteins--anti-Gal alpha 1-3Gal IgM found in higher primates, Griffonia simplicifolia type I lectin, isolectin B4 (GS-IB4), and alpha-galactosidase--interact with Gal alpha 1-3Gal on porcine cell surfaces was tested. Although the Gal alpha 1-3Gal residues expressed on porcine endothelial cells and recognized by anti-Gal alpha 1-3Gal IgM and GS-IB4 were both sensitive to cleavage by alpha-galactosidase, the sites recognized by GS-IB4 were more sensitive to cleavage than sites recognized by anti-Gal alpha 1-3Gal IgM. Cross-blocking studies on porcine cell surfaces revealed that a significant proportion of anti-Gal alpha 1-3Gal IgM bound to sites not recognized by GS-IB4; however, anti-Gal alpha 1-3Gal IgM and GS-IB4 recognized the same sites on solubilized membrane proteins and model compounds. These results suggest that features of the cell surface such as the three-dimensional arrangement of Gal alpha 1-3Gal and characteristics of the protein such as size and valency play critical roles in specific interactions on cell surfaces.

Animals↗
PubMed1998

Fate of antigen in xenotransplantation: implications for acute vascular rejection and accommodation.

Antigen down-modulation plays a critical role in xenotransplants involving humoral responses against the Forssman antigen and may play a role in the long-term survival of ABO-incompatible allografts. The present study investigates the fate of porcine antigens in pig-to-primate xenotransplantation. Human antibodies bound to the glycocalyx of cultured porcine aortic endothelial cells as judged by electron microscopy and were shed from the cell surface in a complex with fibronectin, a glycoprotein that is found in the apical membrane glycocalyx of cultured cells. Antibody was shed in a metabolically dependent process with a t(1/2) of 2 to 3 hours. However, the amount of antigen on the cell surface did not change appreciably within 24 hours, suggesting that antigen modulation did not occur. Over the ensuing days, antigen expression decreased, although the change was always less than 50% of baseline. Changes in antigen expression were due for the most part to changes in expression of alpha-galactosyl residues. Consistent with results obtained in vitro, antigen expression in porcine organ transplants remained at approximately the baseline level as determined by immunofluorescence analysis of IgM binding to graft endothelium. If, as these results suggest, antigen is not down-modulated in pig-to-primate xenotransplantation, then therapies aimed at prolonged xenograft survival must focus on antibody or genetic manipulation of antigen expression.

Animals↗
PubMed1998

Decreased fibrinolytic activity in porcine-to-primate cardiac xenotransplantation.

BACKGROUND: One major barrier to successful xenotransplantation is acute vascular rejection, a process pathologically characterized by microvascular thrombosis and diffuse fibrin deposition in transplant blood vessels. This pathologic picture may result from a disturbance in the coagulant or fibrinolytic pathways that regulate normal vascular patency. This study evaluated the regulation of fibrinolytic activity defined by tissue plasminogen activator and plasminogen activator inhibitor-1 as it may exist in the setting of acute vascular rejection. MATERIALS AND METHODS, RESULTS: Serial biopsies from cardiac xenotransplants evaluated by immunofluorescence microscopy demonstrated progressive decreases in tissue plasminogen activator and increases in plasminogen activator inhibitor-1. In vitro studies measuring fibrinolytic activity of cell culture medium from porcine aortic endothelial cells stimulated with human serum or autologous porcine serum revealed that human serum triggered as much as 93% increase in antifibrinolytic activity. CONCLUSIONS: These findings demonstrate that porcine vascular endothelial cells change toward an antifibrinolytic state following stimulation with human xenoreactive antibodies and complement. The shift is at least partly explained by an increased ratio of plasminogen activator inhibitor-1 to tissue plasminogen activator, and is at least in part mediated by the activation of complement. This increased antifibrinolytic activity may contribute to the thrombotic diathesis seen in acute vascular rejection in pig-to-primate xenografts.

Animals↗
PubMed1997

Induction of procoagulant function in porcine endothelial cells by human natural killer cells.

NK cells may mediate effector functions other than target cell cytotoxicity. To explore such noncytotoxic effector mechanisms, we tested whether human PBL and purified NK (CD56+) cells might induce expression of tissue factor by cultured porcine aortic endothelial cells. Tissue factor is the major coagulation factor that binds to factor VIIa and initiates coagulation. The addition of freshly isolated NK cells but not T cells to endothelial cells resulted in the induction of tissue factor activity. NK-depleted (CD56-) effector cells did not induce tissue factor activity; however, the combination of CD56+ cells and NK-depleted cells induced tissue factor activity to the same extent as unseparated cells. PBL induced tissue factor mRNA in porcine endothelial cells and NK depletion resulted in a significant decrease of the induction. Induction of tissue factor activity in porcine endothelial cells by human NK cells required direct cell-to-cell contact, as transfer of supernatants from NK-endothelial cell cultures to secondary cultures did not induce tissue factor activity, and anti-LFA-1alpha Abs inhibited the induction of tissue factor activity. Induction of tissue factor activity in endothelial cells by NK cells may represent one of a variety of ways in which NK cells mediate noncytotoxic effects.

Animals↗
PubMed1997

Modulation of eicosanoid metabolism in endothelial cells in a xenograft model. Role of cyclooxygenase-2.

Lipid inflammatory mediators are thought to play a critical role in the pathogenesis of vascular injury. Among the events which might cause the synthesis of eicosanoids in blood vessels is activation of the complement. To evaluate how complement might influence eicosanoid metabolism, we investigated endothelial cells exposed to xenoreactive antibodies and complement, as might occur in rejecting xenografts where severe vascular injury is a typical feature. While resting porcine aortic endothelial cells released only prostaglandin (PG) I2, endothelial cells stimulated with xenoreactive antibodies and complement released PGE2 and thromboxane A2 (TXA2), in addition to increased amounts of PGI2. This alteration in eicosanoid metabolism was associated with induction of cyclooxygenase (Cox)-2 and thromboxane synthase, but not Cox-1. Unlike results seen in other systems, the upregulation of Cox-2 and the subsequent release of eicosanoids by endothelial cells was not directly induced by complement but rather required production of IL-1alpha, which acted on endothelial cells as an autocrine factor. Since eicosanoids have a potent effect on inflammation, vascular tone and platelet aggregation, we postulated that the abnormalities in eicosanoid release induced by xenoreactive antibodies and complement might provide one explanation for the vascular injury, focal ischemia, and thrombosis observed in acute vascular rejection and other vasculitides mediated by complement.

Animals↗
PubMed1997

A molecular epidemiological probe for pig microchimerism.

We have cloned and characterized a single-copy DNA sequence from the porcine alpha-1,3-galactosyltransferase gene that corresponds to a 547-base pair intron separating exons 3 and 4 of the protein coding domain. Polymerase chain reaction amplification of this sequence from flanking oligonucleotides generates a species-specific DNA probe (pgt34) capable of recognizing 50 pg chimeric template DNA at a pig to human cellular ratio of 1/10,000. Homologous DNA sequence is not identified in the macaque, baboon, or human genome by Southern hybridization. Analysis of a discordant model of pig to baboon xenotransplantation demonstrates peripheral blood microchimerism in the presence of a functioning pig kidney xenograft and persistence of microchimerism in lymphatic tissue after graft removal. This probe should be useful for tracking the fate of porcine cells in patients undergoing xenotransplantation of whole organs or free tissues such as pancreatic islet cells and should facilitate studies of microchimerism in experimental models of pig to monkey xenotransplantation.

Animals↗
PubMed1997

Transgenic pigs expressing human CD59 and decay-accelerating factor produce an intrinsic barrier to complement-mediated damage.

We characterize a line of transgenic pigs that express the human complement-regulatory proteins human CD59 and human decay-accelerating factor. These genes, under the control of heterologous promoters, are expressed in a variety of organs, including the vasculature of the heart, kidney, and liver. We demonstrate that moderate levels of these gene products are sufficient to protect peripheral blood cells from human or baboon complement. Using pig to baboon heterotopic heart transplants, we show that expression of these proteins is sufficient to block the complement-mediated damage that is the hallmark of such xenografts, when nontransgenic organs are used. These results indicate that there is significant species specificity of intrinsic complement regulatory protein function. This specificity is evident in transgenic organs in which low levels of human CD59 and human decay-accelerating factor expression significantly effect the humoral immune response that causes xenograft rejection. This result suggests that transgenic organs with high levels of human complement-regulatory protein expression will be sufficient to alleviate the humoral immunological barriers that currently block the use of xenogeneic organs for human transplantation.

Animals↗