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Biomedical subjects

S C Robson

Publications and source records attributed to S C Robson.

At least 109 records · Page 6Linked to original sources

Activation of human platelets by the membrane-expressed A1 domain of von Willebrand factor.

Platelet activation and microthrombus formation are invariable features of xenograft rejection and the vascular injury observed when porcine organs are transplanted into primates. This pathological process could be mediated, at least in part, by aberrant interactions of von Willebrand Factor (vWF) associated with the donor vasculature with host platelets. Unlike human vWF, native porcine vWF (pvWF) interacts with human GPIb independently of shear stress or nonphysiological stimuli, eg, ristocetin. We therefore contrasted the potential of isolated human and porcine vWF-A1-domains to interact with human platelets in vitro. Both human and porcine vWF-A1-domains expressed as glycosyl phosphatidylinositol-linked FLAG fusion proteins on COS-7 cells induced GPIb-dependent aggregation and intracellular Ca++ uptake of platelets, independent of both the remainder of the vWF protein and additional modifying factors. Porcine A1-domains were more potent than human homologues, and in addition ristocetin could boost platelet aggregation only with the human A1-domain. Putative conformational changes in the porcine A1-domain could result in the heightened, ristocetin-independent interactions observed with human platelets and may be of importance for xenograft survival.

Amino Acid Sequence↗

Thrombin activates nuclear factor-kappaB and potentiates endothelial cell activation by TNF.

Thrombin is the central bioregulatory enzyme in hemostasis and is generated in vascular beds in which inflammatory responses are ongoing. In this study, we examined the effect of thrombin, both alone and in combination with TNF, on gene expression in porcine aortic endothelial cells (EC). Thrombin (1-10 U/ml) induced increased mRNA levels of E-selectin, monocyte chemoattractant protein-1, IL-8, plasminogen activator inhibitor-1, and IkappaB-alpha. These effects were mimicked by a thrombin receptor-activating peptide; preincubation of thrombin with hirudin blocked the induction of mRNA, suggesting that the increased gene expression was due to thrombin-specific activity. Because these genes are known to contain nuclear-factor-kappaB (NF-kappaB)-binding elements in their promoter region, we next examined the ability of thrombin to activate this transcription factor. As detected by electrophoretic mobility shift assay, thrombin (10 U/ml) or thrombin receptor-activating peptide (100 microM) stimulated increased NF-kappaB-binding activity. Supershift analysis revealed that these complexes were comprised principally of the RelA (p65) and NF-kappaB1 (p50) Rel family members. Thrombin alone did not substantively increase protein levels of E-selectin despite the increase in E-selectin mRNA levels. However, thrombin (3-10 U/ml) stimulated a 10-fold enhancement in the ability of TNF (0.3-1.0 ng/ml) to induce E-selectin surface expression. Similar potentiation of TNF-induced NF-kappaB activity and E-selectin transcription by thrombin was observed in experiments utilizing luciferase reporter constructs expressed in bovine aortic EC. The ability of thrombin to potentiate TNF-induced EC activation thus provides an important mechanism by which products of the coagulation cascade may enhance cytokine-mediated inflammatory responses.

Animals↗

Xenogeneic endothelial cells activate human prothrombin.

BACKGROUND: Delayed xenograft rejection is characterized by platelet activation and fibrin deposition and is thought to occur independently of complement activation. We have therefore investigated the potential for xenogeneic endothelial cells (EC) to regulate the conversion of prothrombin to thrombin, a central component of the final common pathway of coagulation and an important platelet agonist. METHODS AND RESULTS: Quiescent porcine aortic EC (PAEC) were found to convert high levels of human prothrombin to thrombin (0.234+/-0.019 IU/ml) when compared with human aortic EC (0.017+/-0 IU/ml, 30-min time point, chromogenic assay; P<0.001). PAEC activation by human complement resulted in comparable levels of thrombin generation. Prothrombin conversion by PAEC as determined by generation of F1+2 (1.909+/-0.119 nmol/L) and formation of thrombin-antithrombin III complexes (125.611+/-6.373 microg/L) was significantly greater than the matched human aortic EC values (F1+2: 1.539+/-0.03 nmol/L, P<0.001; thrombin-antithrombin III: 1.833+/-0.104 microg/L, P<0.001). Sequential analysis of prothrombin activation by PAEC indicated generation of the intermediate meizothrombin followed by autolytically accelerated thrombin formation. Subsequent experiments established important cross-species' incompatibilities with respect to porcine thrombomodulin interaction with human thrombin and protein C in that PAEC had a reduced capacity to generate activated human protein C in vitro. CONCLUSION: These observations indicate a potentially important molecular barrier involving blood coagulation that may impact on the planned clinical application of porcine transgenic organs.

Animals↗

Effect of porcine endothelial tissue factor pathway inhibitor on human coagulation factors.

BACKGROUND: Delayed xenograft rejection (DXR) is characterized by inflammation and vascular thrombosis. Activation of coagulation may occur as a result of tissue factor (TF) expression on both activated donor endothelial cells (EC) and recipient infiltrating monocytes (Mo). In addition, natural anticoagulants associated with porcine endothelial cells may not function adequately across species. METHODS: In the present study, we examined the interaction of the TF pathway of coagulation with the natural anticoagulant TF pathway inhibitor, in xenogeneic leukocyte-EC cultures in vitro, and during rejection of discordant xenografts in vivo. RESULTS: Coculture of human Mo with pig aortic EC (PAEC) resulted in 1.7-fold and 2-fold higher induction of Mo TF and Mo intercellular adhesion molecule-1, respectively, when compared with coculture with human aortic endothelial cells (HAEC). In addition, TF-dependent and -independent activation of coagulation factor X was higher on PAEC than on HAEC. Low levels of mRNA for tissue factor pathway inhibitor (TFPI) and its variant, TFPI-2, in resting PAEC were up-regulated by stimulation with tumor necrosis factor alpha. Procoagulant activity of recombinant human TF complexed to activated factor VII was inhibited by PAEC and HAEC-associated TFPI by 22% and 56%, respectively. In contrast, human activated factor X (factor Xa) activity was inhibited by human, but not porcine, EC-associated TFPI, suggesting functional incompatibility of PAEC for human factor Xa. Endothelial TFPI was detected in pig control organs and after hyperacute rejection, but was lost from the vasculature during DXR. CONCLUSIONS: Lack of appropriate human factor Xa inhibition by porcine EC during hyperacute rejection and loss of porcine EC TFPI during DXR could promote the development of a procoagulant environment leading to xenograft rejection.

Amino Acid Sequence↗

Loss of ATP diphosphohydrolase activity with endothelial cell activation.

Quiescent endothelial cells (EC) regulate blood flow and prevent intravascular thrombosis. This latter effect is mediated in a number of ways, including expression by EC of thrombomodulin and heparan sulfate, both of which are lost from the EC surface as part of the activation response to proinflammatory cytokines. Loss of these anticoagulant molecules potentiates the procoagulant properties of the injured vasculature. An additional thromboregulatory factor, ATP diphosphohydrolase (ATPDase; designated as EC 3.6.1.5) is also expressed by quiescent EC, and has the capacity to degrade the extracellular inflammatory mediators ATP and ADP to AMP, thereby inhibiting platelet activation and modulating vascular thrombosis. We describe here that the antithrombotic effects of the ATPDase, like heparan sulfate and thrombomodulin, are lost after EC activation, both in vitro and in vivo. Because platelet activation and aggregation are important components of the hemostatic changes that accompany inflammatory diseases, we suggest that the loss of vascular ATPDase may be crucial for the progression of vascular injury.

Adenosine Diphosphate↗

Renal abnormalities on obstetric ultrasound as a presentation of DiGeorge syndrome.

We describe three pregnancies that presented with renal anomalies on obstetric ultrasound as the main abnormality and were subsequently found to have interstitial deletions within chromosome 22q11. A cardiac defect, double-outlet right ventricle, was also seen in the first case. Amnio infusion was refused in the second pregnancy and the perimembranous ventricular septal defect was not identified prior to termination. In the third case, there was no cardiac defect. The genitourinary abnormalities were a right hydroureter and hydronephrosis with a ureterocele bulging into the bladder lumen, bilateral multicystic kidneys with associated oligohydramnios, and a left multicystic kidney with right renal agenesis and associated oligohydramnios. Absence of thymus at autopsy in all three cases led to fluorescent in situ hybridization studies looking for the submicroscopic deletion of chromosome 22q11 associated with DiGeorge syndrome.

Adolescent↗

Autonomic neuropathy in extra-hepatic portal vein thrombosis: evidence for impaired autonomic reflex arc.

BACKGROUND/AIMS: Autonomic dysfunction is common in cirrhosis, and may be associated with increased mortality and hyperdynamic circulatory changes. Our aim was to investigate whether autonomic disturbances occur in extrahepatic portal vein thrombosis and their correlation with hemodynamic abnormalities. PATIENTS AND METHODS: Heart rate variation in response to standing, deep breathing, and Valsalva maneuver, and blood pressure response to sustained handgrip and to standing, were studied in 16 subjects with portal vein thrombosis (10 males, 30.8+/-2.8 years: mean+/-SE), 12 with cirrhosis (7 males, 52+/-2.3 years), and 10 healthy controls (7 males, 30.8+/-3.0 years). Supine resting, and 10- and 30-min standing epinephrine and norepinephrine levels were measured and results correlated with cardiac output. RESULTS: Autonomic dysfunction occurred in 62% of portal vein thrombosis and 75% of cirrhosis subjects, but in no controls (p<0.02). Similarly, postural hypotension occurred in portal vein thrombosis (-10.25+/-0.65 mmHg, p=0.003) and cirrhosis (-7.42+/-0.82 mmHg, p=0.007) but not in controls. All groups had similar baseline epinephrine and norepinephrine concentrations. Epinephrine increased significantly in controls (45%, p<0.01 and 49%, p<0.02) after 10 of 30 min standing but not in the portal vein thrombosis or the cirrhotic group, and norepinephrine increased after 10 and 30 min standing in cirrhotics (128%, p<0.004 and 130%, p<0.008) and controls (129%, p<0.002 and 116%, p<0.004), but not portal vein thrombosis (34.5% and 39%, NS vs baseline). Portal vein thrombosis and cirrhosis groups had increased cardiac output (4441+/-509 and 3262+/-292) vs controls (1763+/-212 ml/min/m2, p<0.002), but there was no correlation with autonomic neuropathy or with catecholamine levels. CONCLUSIONS: Autonomic dysfunction and impaired catecholamine response to orthostatic stress occur commonly in portal vein thrombosis and suggest an impairment of the autonomic reflex arc, but changes do not correlate with hemodynamic abnormalities.

Adult↗

Holt-Oram syndrome is caused by mutations in TBX5, a member of the Brachyury (T) gene family.

Holt-Oram syndrome is a developmental disorder affecting the heart and upper limb, the gene for which was mapped to chromosome 12 two years ago. We have now identified a gene for this disorder (HOS1). The gene (TBX5) is a member of the Brachyury (T) family corresponding to the mouse Tbx5 gene. We have identified six mutations, three in HOS families and three in sporadic HOS cases. Each of the mutations introduces a premature stop codon in the TBX5 gene product. Tissue in situ hybridization studies on human embryos from days 26 to 52 of gestation reveal expression of TBX5 in heart and limb, consistent with a role in human embryonic development.

Abnormalities, Multiple↗

Modulation of vascular ATP diphosphohydrolase by fatty acids.

Endothelial cells (EC) possess several protective thromboregulatory mechanisms that may be perturbed by cell activation or injury. Vascular ATP-diphosphohydrolase (ATPDase) has been demonstrated on both aortic EC and smooth muscle cells and may play a key regulatory role in hemostasis and platelet reactivity by converting extracellular ATP and ADP to AMP. We have examined the role of exogenous saturated or unsaturated fatty acids in the modulation of EC associated ATPDase activity in vitro. EC growth was not dramatically influenced by supplementation with fatty acids whereas viability was enhanced by oleic, butyrate and eicosapentaenoic acid. EC cultures supplemented with saturated or a monounsaturated (oleic acid) fatty acid(s) had markedly increased ATPDase activity, whereas those exposed to polyunsaturated fatty acids showed substantive decreases. Exogenous oleic acid could also protect against the significant loss of ATPDase activity, following exposure to reactive oxygen intermediates in vitro. We conclude that endothelial ATPDase activity may be regulated by exogenous fatty acids and that underlying mechanisms include alterations in the nature of the phospholipid composition of EC membranes that influence responses to oxidative stress reactions.

Animals↗

Third trimester fetal growth and measures of carbohydrate and lipid metabolism in umbilical venous blood at term.

AIM: To compare measures of carbohydrate and lipid metabolism in umbilical venous blood after birth at term in pregnancies with normal and retarded fetal growth during the third trimester. METHODS: Three groups of pregnancies reaching term, in which fetal growth had been prospectively monitored by repeated ultrasound measurements during the third trimester, were studied. Sequential fetal abdominal circumference measurements remained above the 10th centile in 42 (normal size, normal growth group), below the 10th centile but did not depart further than 1.5 SD (small, normal growth group), or below the 10th centile and subsequently fell away by more than 1.5 SD before delivery (small, growth retarded group). Birthweight, neonatal morphometric measures (ponderal index, mid arm:head circumference ratio, subscapular and triceps skinfold thickness), umbilical venous blood concentrations of glucose, insulin, pro-insulin, des 31,32 proinsulin, total cholesterol, free cholesterol, cholesterol ester, triglycerides, lipoprotein (a), apolipoprotein A-1 and apolipoprotein B were measured. RESULTS: The median birthweight of the three groups was significantly different (3570, 2569, and 2277 g, respectively). Median values of ponderal index and mid arm:head circumference ratio were significantly lower in the small, growth retarded group and did not differ between the small and normal size groups with normal growth. Both groups with small fetuses had significantly lower mean glucose and cholesterol ester concentrations, and higher mean free cholesterol:cholesterol ester ratios, compared with the normal size, normal growth group. The group showing fetal growth retardation had mean total cholesterol and mean cholesterol ester concentrations that were significantly lower than those of both the other two groups. Mean des 31,32 proinsulin concentrations were low in both groups of small fetuses, but only significantly so in the group without fetal growth retardation. Mean insulin, proinsulin, free cholesterol, triglycerides, lipoprotein(a), apolipoprotein A-1, apolipoprotein B concentrations and the ratio of A-1:B were similar in all three groups. CONCLUSION: The similarity in the umbilical venous blood carbohydrate and lipid profile at term between pregnancies with documented third trimester fetal growth retardation and those with "genetically" small babies argues against a major role for intrauterine nutritional deprivation as a cause for the association between birth-weight and subsequent adult disease.

Adolescent↗

High prevalence of GBV-C hepatitis G virus infection in a rural South African population.

A novel virus, GBV-C/hepatitis G virus (GBV-C/HGV), has been cloned and characterised recently. GBV-C/HGV global epidemiology and risk factors for acquisition are currently unclear. We aimed to establish the determinants of this infection in a rural South African (SA) population. The study population included two samples, namely a community-based sample, and consenting persons from a nonspecialist outpatient department in the same district. A questionnaire regarding demographic details and putative risk factors was administered; blood samples were taken on which a polymerase chain reaction (PCR) was performed for both 5'NCR and NS5a regions of GBV-C/HGV using commercially available primers and probes. Two hundred and forty-nine people were studied with a mean GBV-C/HGV prevalence of 10.4%. Outpatient department and community prevalences differed significantly (18.0% and 6.3%, respectively, P = 0.004). GBV-C/HGV infection was associated with excessive alcohol consumption (P = 0.02; OR, 4.18) and a lack of waterborne sewerage (P = 0.04). PCR amplification of the NS5a region of all but two South African GBV-C/HGV positive samples showed poor reactivity. The prevalence of GBV-C/HGV in rural SA appears to be higher than that reported from Europe and North America. Infection appeared to be associated with excess alcohol intake and a history of previous blood transfusion. The discrepant NS5a and 5'NCR PCR sensitivity in this study raises the possibility of genetic differences in southern African GBV-C/HGV.

Adolescent↗

Apyrase administration prolongs discordant xenograft survival.

Platelet thrombi and vascular inflammation are prominent features of discordant xenograft rejection. The purinergic nucleotides ATP and ADP, which are secreted from platelets and released by injured endothelial cells (EC), are important mediators of these reactions. Quiescent EC express the ectoenzyme ATP-diphosphohydrolase (ATPDase; an apyrase), which exerts an important thromboregulatory function by hydrolyzing both ATP and ADP. We have shown that ATPDase activity is rapidly lost from the surface of the EC following ischemia-reperfusion injury and during xenograft rejection. The aim of this study was to supplement ATPDase activity within xenografts by infusion of soluble apyrases, and thereby validate the importance of local ATPDase activity in the modulation of xenograft rejection. Lewis rats underwent heterotopic cardiac xenografting from guinea pigs and apyrase was administered intravenously (200 U/kg) as a single dose to evaluate effects on hyperacute rejection (HAR). This initial dose was followed by a continuous apyrase infusion (8.0 U/kg/hr) directly into the graft aorta in combination with systemic cobra venom factor (CVF) administration to deplete complement when delayed xenograft rejection (DXR) was studied. Functional apyrase levels in vivo were assessed by the capacity of blood samples taken at the time of surgery and rejection to inhibit platelet aggregation in vitro. Apyrase administration significantly prolonged graft survival in HAR and DXR. Functional assays showed inhibition of platelet aggregation suggesting effective systemic antiaggregatory effects of the administered apyrases. Histologic studies showed that apyrase administration abrogated local platelet aggregation and activation in HAR and DXR. Our data demonstrate that local administration of apyrase prolonged discordant xenograft survival. These observations emphasize the potential importance of purinergic mediators in platelet activation during xenograft rejection.

Adenosine Diphosphate↗

Identification and characterization of CD39/vascular ATP diphosphohydrolase.

Vascular ATP diphosphohydrolase (ATPDase) is a plasma membrane-bound enzyme that hydrolyses extracellular ATP and ADP to AMP. Analysis of amino acid sequences available from various mammalian and avian ATPDases revealed their close homology with CD39, a putative B-cell activation marker. We, therefore, isolated CD39 cDNA from human endothelial cells and expressed this in COS-7 cells. CD39 was found to have both immunological identity to, and functional characteristics of, the vascular ATPDase. We also demonstrated that ATPDase could inhibit platelet aggregation in response to ADP, collagen, and thrombin, and that this activity in transfected COS-7 cells was lost following exposure to oxidative stress. ATPDase mRNA was present in human placenta, lung, skeletal muscle, kidney, and heart and was not detected in brain. Multiple RNA bands were detected with the CD39 cDNA probe that most probably represent different splicing products. Finally, we identified an unique conserved motif, DLGGASTQ, that could be crucial for nucleotide binding, activity, and/or structure of ATPDase. Because ATPDase activity is lost with endothelial cell activation, overexpression of the functional enzyme, or a truncated mutant thereof, may prevent platelet activation associated with vascular inflammation.

Adenosine Triphosphatases↗

Effect of repetitive high-dose treatment with soluble complement receptor type 1 and cobra venom factor on discordant xenograft survival.

Hyperacute xenograft rejection may be modified by the activation and depletion of complement (C) using cobra venom factor (CVF). This method of prolonging xenograft survival is toxic and associated with systemic inflammation, which may potentially contribute to the pathologic features of delayed xenograft rejection. Soluble complement receptor type 1 (sCR1) inhibits both the classical and alternative C pathways and thus limits the production of proinflammatory products such as the anaphylatoxins. Hence, we investigated the effects of various sCR1 and CVF regimens, and combinations thereof, in the discordant guinea pig-to-Lewis rat cardiac xenograft model. Mean graft survival time (MST) was significantly prolonged with repetitive dosing (MST=22 hr) or continuous infusion of sCR1 (MST=32 hr) as compared with unmodified controls (MST=15 min). However, sCR1 did not prevent intragraft deposition of C3 or neutrophil infiltration and resulted in only partial inhibition of C-mediated hemolytic activity in vitro. Grafts in rats treated with a single dose of CVF (MST=67 hr) or repetitive doses of CVF (MST=69 hr) survived significantly longer than those treated with sCR1 alone, and lacked C3 deposition or neutrophil accumulation. Sera from these animals were completely depleted of C-mediated hemolytic activity. Animals treated with a single dose of CVF, or sCRI plus a single dose of CVF (MST=64 hr), had similar xenograft survival times. However, immunohistologic studies showed that addition of sCR1 to a single dose of CVF resulted in decreased macrophage activation and reduced levels of cytokines (tumor necrosis factor-alpha and interleukin-1beta) within xenografts as compared with that in recipients treated with CVF alone. Such decreased macrophage activation may result from the binding of C4b by sCR1, since combination therapy was associated with decreased intragraft C4b as compared with either therapy alone. High doses of sCR1 were well tolerated by rats and significantly prolonged discordant xenograft survival (MST=32 hr), although not to the same extent as CVF. The modification of the intragraft immune responses seen with CVF/sCR1 combination therapy may augment further therapeutic manipulations to achieve discordant xenograft survival without the attendant toxicity associated with repeated CVF administration.

Animals↗

Inhibition of platelet integrin GPIIbIIIa prolongs survival of discordant cardiac xenografts.

The integrin GPIIbIIIa is known to be crucial to the formation of platelet aggregates and potentiates adhesion to subendothelial matrices via fibrin(ogen), von Willebrand factor, and vitronectin. Given the demonstration by us and others of widespread platelet aggregation during xenograft rejection, we hypothesized that platelet thrombi might contribute to graft dysfunction during development of hyperacute rejection (HAR), as well as during what we have termed delayed xenograft rejection (DXR), e.g., as seen in complement-depleted rat recipients of guinea pig cardiac xenografts. We therefore tested the effects of a specific GPIIbIIIa antagonist (SDZ GPI 562) during xenograft rejection. Lewis rats received heterotopic guinea pig cardiac xenografts and were treated with GPI 562 alone (HAR model) or in combination with cobra venom factor (CVF) (DXR model). A high (0.5 mg/kg) or a low dose (0.1 mg/kg) of GPI 562 was administered perioperatively and then given twice daily in the same dose until rejection. CVF was given daily until rejection. Plasma drawn after the first dose of GPI 562 and at the time of rejection was tested for the ability to inhibit ADP-stimulated platelet aggregation in vitro. Rejected grafts were analyzed by immunohistology. Plasma from animals in the high-dose group completely inhibited platelet aggregation in vitro, whereas plasma from the low-dose group resulted in only partial inhibition. Similarly, whereas low-dose GPI 562 failed to prolong graft survival, high-dose GPI 562 showed a statistically significant increase in graft survival in both HAR and DXR groups. Immunohistologic studies of HAR showed little effect of GPI 562 on platelet aggregation or activation and no effect on fibrin deposition. However, the combination of high-dose GPI 562 and CVF resulted in a significant decrease in intragraft platelet aggregation, P-selectin expression, and leukocyte infiltration compared with CVF alone. In conclusion, GPIIbIIIa antagonist therapy can inhibit platelet aggregation in vitro and prolong xenograft survival. The diminution of intragraft platelet microthrombi formation and leukocyte infiltration suggests an important role for platelet-dependent mechanisms in leukocyte recruitment during DXR.

Animals↗

Thrombin inhibition in an ex vivo model of porcine heart xenograft hyperacute rejection.

Prominent components of vascularized xenograft rejection such as platelet activation and microvascular thrombosis may be dependent upon thrombin generation in vivo. To study potential therapeutic benefits of a synthetic low-molecular-weight thrombin inhibitor, SDZ MTH 958, in hyperacute porcine heart rejection by human blood ex vivo, a working model of hyperacute rejection of porcine by fresh, heparinized (6 microM/ml) human blood with or without 1 microM SDZ MTH 958 was used. Thrombin-antithrombin complexes (TAT) and prothrombin fragment F1.2 levels as markers of thrombin activation were determined, and biopsies from rejected hearts were analyzed by immunohistopathology. Control porcine hearts (n=8) underwent a rapid and consistent decline in cardiac output, ceasing function by 60 min. Experimental cardiac output values of 14 ml/g (SEM 1.2) were significantly higher than seen in controls (5 ml/g SEM 0.6) after 5 min of cardiac work, and prolonged survival times up to 120 min were noted (P<0.05). Activity of SDZ MTH 958 was confirmed by functional assays throughout perfusion. Levels of TAT and F1.2 increased consistently in control samples when compared with plasma samples containing SDZ MTH 958. Immunohistopathological examination confirmed diminished fibrin deposition, reduced leukocyte adherence to endothelium, impaired diapedesis and less tissue necrosis in the hearts perfused with SDZ MTH 958. SDZ MTH 958, in this xenoperfusion model, prolonged survival, enhanced function of the explanted organ, and improved histological features at the time of rejection. Effective and specific antagonism of thrombin may be useful as an adjunct therapy to complement inhibition for xenograft rejection

Acute Disease↗