Bochdalek hernia in the adult: demographics, presentation, and surgical management

BACKGROUND

Bochdalek hernias are a very rare form of diaphragmatic hernias. There are no robust studies that reveal the true natural history of this disease process. The aim of this study was to summarize clinically relevant data for the purpose of assisting surgeons with the work-up, diagnosis, and treatment of adult patients with Bochdalek hernia.

METHODS

A literature search was performed using PubMed, Google scholar, EMBASE and the following keywords: Bochdalek hernia, congenital diaphragmatic hernia, and posterolateral hernia. All case reports and series after 1955 that pertained to adults were included in the review. The following data points were queried: age, sex, presentation, studies utilized during work-up, laterality, surgical approach, hernia sac management, specific minimally invasive surgical techniques, and follow-up.

RESULTS

A total of 124 articles comprising 173 patients met the inclusion criteria. Based on the data provided, several conclusions regarding this disease process can be made. Most patients present with symptoms related to their hernia (86%). Pain is the most common complaint (69%). While laparotomy is the most widely used surgical approach (38%), minimally invasive surgical techniques have gained popularity since their first report in 1995. Laparoscopic repair can be performed with a low complication rate (7%) and short hospital stay (4 days).

CONCLUSIONS

Using modern surgical techniques to include laparoscopy, repair can be performed safely, with a short hospital stay, and with minimal morbidity or mortality.

Sterol regulatory element-binding proteins: transcriptional activators of lipid synthesis

Sterol regulatory element-binding proteins (SREBPs) are a family of transcription factors that regulate lipid homoeostasis. Three SREBP isoforms control the expression of more than 30 genes required for the biosynthesis of cholesterol, fatty acids, triacylglycerols and phospholipids. The unique regulation and activation properties of each SREBP isoform facilitates the co-ordinate regulation of all essential lipid building blocks required for cell membranes as well as for very-low-density lipoprotein formation in hepatocytes.

Identification of a mammalian long chain fatty acyl elongase regulated by sterol regulatory element-binding proteins

Fatty acids are synthesized de novo from acetyl-CoA and malonyl-CoA through a series of reactions mediated by acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS). In rodents, the principal fatty acid produced by FAS is palmitic acid (16:0). Sterol regulatory element-binding proteins (SREBPs) enhance the transcription of many genes responsible for fatty acid synthesis. In transgenic mice that overexpress SREBPs in liver, the rate of fatty acid synthesis is markedly increased, owing to the activation of these biosynthetic genes, which include ATP citrate lyase, ACC, FAS, and stearoyl-CoA desaturase. The fatty acids that accumulate in livers of SREBP transgenic mice are 18 carbons rather than 16 carbons in length, suggesting that the enzymes required for the elongation of palmitic to stearic acid may be induced. Here, we report the cDNA cloning of a murine long chain fatty acyl elongase (LCE) that was identified initially by oligonucleotide array analysis of mRNA from SREBP transgenic mouse livers. LCE mRNA is highly expressed in liver and adipose tissue. The cDNA encodes a protein of 267 amino acids that shares sequence identity with previously identified very long chain fatty acid elongases. Cells that overexpress LCE show enhanced addition of 2-carbon units to C12-C16 fatty acids. We provide evidence that LCE catalyzes the rate-limiting condensing step in this reaction. The current studies suggest that mouse LCE expression is increased by SREBPs and that the enzyme is a component of the elusive mammalian elongation system that converts palmitic to stearic acid.

Decreased lipid synthesis in livers of mice with disrupted Site-1 protease gene

Site-1 protease (S1P) cleaves membrane-bound sterol regulatory element-binding proteins (SREBPs), allowing their transcription-stimulating domains to translocate to the nucleus where they activate genes governing lipid synthesis. S1P is a potential target for lipid-lowering drugs, but the effect of S1P blockade in animals is unknown. Here, we disrupt the S1P gene in mice. Homozygous germ-line disruptions of S1P were embryonically lethal. To disrupt the gene inducibly in liver, we generated mice homozygous for a floxed S1P allele and heterozygous for a transgene encoding Cre recombinase under control of the IFN-inducible MX1 promoter. When IFN was produced, 70-90% of S1P alleles in liver were inactivated, and S1P mRNA and protein were reduced. Nuclear SREBPs declined, as did mRNAs for SREBP target genes. Cholesterol and fatty acid biosynthesis in hepatocytes declined by 75%. Low density lipoprotein (LDL) receptor mRNA declined by 50%, as did the clearance of (125)I-labeled LDL from plasma, but plasma cholesterol fell, suggesting that LDL production was reduced. These data raise the possibility that S1P inhibitors may be effective lipid-lowering agents, but they suggest that nearly complete inhibition will be required.

SREBP cleavage-activating protein (SCAP) is required for increased lipid synthesis in liver induced by cholesterol deprivation and insulin elevation

In liver, the synthesis of cholesterol and fatty acids increases in response to cholesterol deprivation and insulin elevation, respectively. This regulatory mechanism underlies the adaptation to cholesterol synthesis inhibitors (statins) and high calorie diets (insulin). In nonhepatic cells, lipid synthesis is controlled by sterol regulatory element-binding proteins (SREBPs), membrane-bound transcription factors whose active domains are released proteolytically to enter the nucleus and activate genes involved in the synthesis and uptake of cholesterol and fatty acids. SCAP (SREBP cleavage-activating protein) is a sterol-regulated escort protein that transports SREBPs from their site of synthesis in the endoplasmic reticulum to their site of cleavage in the Golgi. Here, we produced a conditional deficiency of SCAP in mouse liver by genomic recombination mediated by inducible Cre recombinase. SCAP-deficient mice showed an 80% reduction in basal rates of cholesterol and fatty acid synthesis in liver, owing to decreases in mRNAs encoding multiple biosynthetic enzymes. Moreover, these mRNAs failed to increase normally in response to cholesterol deprivation produced by a cholesterol synthesis inhibitor and to insulin elevation produced by a fasting-refeeding protocol. These data provide in vivo evidence that SCAP and the SREBPs are required for hepatic lipid synthesis under basal and adaptive conditions.

Insulin inhibits transcription of IRS-2 gene in rat liver through an insulin response element (IRE) that resembles IREs of other insulin-repressed genes

Recent data indicate that sustained elevations in plasma insulin suppress the mRNA for IRS-2, a component of the insulin signaling pathway in liver, and that this deficiency contributes to hepatic insulin resistance and inappropriate gluconeogenesis. Here, we use nuclear run-on assays to show that insulin inhibits transcription of the IRS-2 gene in the livers of intact rats. Insulin also inhibited transcription of a reporter gene driven by the human IRS-2 promoter that was transfected into freshly isolated rat hepatocytes. The human promoter contains a heptanucleotide sequence, TGTTTTG, that is identical to the insulin response element (IRE) identified previously in the promoters of insulin-repressed genes. Single base pair substitutions in this IRE decreased transcription of the IRS-2-driven reporter in the absence of insulin and abolished insulin-mediated repression. We conclude that insulin represses transcription of the IRS-2 gene by blocking the action of a positive factor that binds to the IRE. Sustained repression of IRS-2, as occurs in chronic hyperinsulinemia, contributes to hepatic insulin resistance and accelerates the development of the diabetic state.

Disruption of the sterol 27-hydroxylase gene in mice results in hepatomegaly and hypertriglyceridemia. Reversal by cholic acid feeding

Sterol 27-hydroxylase (CYP27) participates in the conversion of cholesterol to bile acids. We examined lipid metabolism in mice lacking the Cyp27 gene. On normal rodent chow, Cyp27(-/-) mice have 40% larger livers, 45% larger adrenals, 2-fold higher hepatic and plasma triacylglycerol concentrations, a 70% higher rate of hepatic fatty acid synthesis, and a 70% increase in the ratio of oleic to stearic acid in the liver versus Cyp27(+/+) controls. In Cyp27(-/-) mice, cholesterol 7alpha-hydroxylase activity is increased 5-fold, but bile acid synthesis and pool size are 47 and 27%, respectively, of those in Cyp27(+/+) mice. Intestinal cholesterol absorption decreases from 54 to 4% in knockout mice, while fecal neutral sterol excretion increases 2.5-fold. A compensatory 2.5-fold increase in whole body cholesterol synthesis occurs in Cyp27(-/-) mice, principally in liver, adrenal, small intestine, lung, and spleen. The mRNA for the cholesterogenic transcription factor sterol regulatory element-binding protein-2 (SREBP-2) and mRNAs for SREBP-2-regulated cholesterol biosynthetic genes are elevated in livers of mutant mice. In addition, the mRNAs encoding the lipogenic transcription factor SREBP-1 and SREBP-1-regulated monounsaturated fatty acid biosynthetic enzymes are also increased. Hepatic synthesis of fatty acids and accumulation of triacylglycerols increases in Cyp27(-/-) mice and is associated with hypertriglyceridemia. Cholic acid feeding reverses hepatomegaly and hypertriglyceridemia but not adrenomegaly in Cyp27(-/-) mice. These studies confirm the importance of CYP27 in bile acid synthesis and they reveal an unexpected function of the enzyme in triacylglycerol metabolism.

Anabolic function of the type II isozyme of hexokinase in hepatic lipid synthesis

The mRNA encoding the Type II isozyme of hexokinase was markedly elevated in livers of transgenic mice overexpressing the transcriptionally active nuclear form of sterol regulatory element binding protein-1a (nSREBP-1a), but not in transgenic mice overexpressing the nSREBP-1c or nSREBP-2 isoforms. Cellulose acetate electrophoresis and immunoblotting results confirmed selective increase of the Type II isozyme in livers of transgenic mice expressing nSREBP-1a. SREBP-1a has previously been shown to activate transcription of genes encoding enzymes involved in biosynthesis of fatty acids and glycerolipids and to a lesser extent the enzymes of cholesterol biosynthesis. Thus, these results are consistent with the view that the Type II isozyme serves an anabolic function, providing precursors and reducing equivalents required for increased rates of hepatic lipid synthesis.

Xenopus NK cells identified by novel monoclonal antibodies

Early-thymectomized (Tx) Xenopus frogs, which are permanently deficient in T cells, are used as a model sytem for the characterization of novel monoclonal antibodies (mAb) which identify candidate NK cells at the amphibian level of evolution. Hybridomas, generated from mice immunized with splenocytes from Tx Xenopus following B cell and thrombocyte depletion, were screened by flow cytometry. Three mAb (1F8, 4D4 and 1G5) were identified that stained increased proportions of splenocytes from Tx compared with control frogs. These mAb identified lymphoid populations from Xenopus spleen, liver and gut which, after 48 h culture in growth factor-rich medium, exhibited spontanous killing of MHC-deficient allotumor targets. mAb-defined splenocytes also rapidly induced apoptosis of such tumor targets. Dual color analysis confirmed that NK cells are neither T nor B cells. Cytospins of splenocytes isolated with anti-NK mAb revealed large lymphoid cells with distinct pseudopodia. Immunohistology indicated each anti-NK mAb routinely labeled cells within the gut epithelium but NK cells were difficult to visualize in spleen sections. Western blotting of spleen, liver and intestinal lysates subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that 1G5 reacted strongly with protein bands of approximately 70 - 85 kDa, whereas mAb 1F8 and 4D4 stained less intensely, but identified similar protein bands.

Insulin selectively increases SREBP-1c mRNA in the livers of rats with streptozotocin-induced diabetes

Sterol regulatory element binding proteins (SREBPs) enhance transcription of genes encoding enzymes of cholesterol and fatty acid biosynthesis and uptake. In the current experiments, we observed a decline in the mRNA encoding one SREBP isoform, SREBP-1c, in the livers of rats that were rendered diabetic by treatment with streptozotocin. There was no change in the mRNA encoding SREBP-1a, which is derived from the same gene as SREBP-1c but uses a different promoter. The ratio of SREBP-1c:1a transcripts fell 25-fold from 5:1 in control rats to 0.2:1 in the diabetic animals. The SREBP-1c mRNA rose nearly to normal, and the 1c:1a ratio increased 17-fold when the diabetic rats were treated for 6 h with insulin. These treatments produced no change in the mRNA for SREBP-2, which is encoded by a separate gene. The SREBP-1c mRNA also fell selectively in freshly isolated rat hepatocytes and rose when the cells were treated with insulin. Considered together with recent data on hepatocytes [Foretz, M., Pacot, C., Dugal, I., et al. (1999) Mol. Cell. Biol. 19, 3760-3768], the current in vivo studies suggest that insulin may stimulate lipid synthesis in the liver by selectively inducing transcription of the SREBP-1c gene.

Increased levels of nuclear SREBP-1c associated with fatty livers in two mouse models of diabetes mellitus

Hepatic steatosis is common in non-insulin-dependent diabetes and can be associated with fibrosis and cirrhosis in a subset of individuals. Increased rates of fatty acid synthesis have been reported in livers from rodent models of diabetes and may contribute to the development of steatosis. Sterol regulatory element-binding proteins (SREBPs) are a family of regulated transcription factors that stimulate lipid synthesis in liver. In the current studies, we measured the content of SREBPs in livers from two mouse models of diabetes, obese ob/ob mice and transgenic aP2-SREBP-1c436 (aP2-SREBP-1c) mice that overexpress nuclear SREBP-1c only in adipose tissue. The aP2-SREBP-1c mice exhibit a syndrome that resembles congenital generalized lipodystrophy in humans. Both lines of mice develop hyperinsulinemia, hyperglycemia, and hepatic steatosis. Nuclear SREBP-1c protein levels were significantly elevated in livers from ob/ob and aP2-SREBP-1c mice compared with wild-type mice. Increased nuclear SREBP-1c protein was associated with elevated mRNA levels for known SREBP target genes involved in fatty acid biosynthesis, which led to significantly higher rates of hepatic fatty acid synthesis in vivo. These studies suggest that increased levels of nuclear SREBP-1c contribute to the elevated rates of hepatic fatty acid synthesis that leads to steatosis in diabetic mice.

Disruption of LDL receptor gene in transgenic SREBP-1a mice unmasks hyperlipidemia resulting from production of lipid-rich VLDL

Transgenic mice that overexpress the nuclear form of sterol regulatory element binding protein-1a (SREBP-1a) in liver (TgBP-1a mice) were shown previously to overproduce cholesterol and fatty acids and to accumulate massive amounts of cholesterol and triglycerides in hepatocytes. Despite the hepatic overproduction of lipids, the plasma levels of cholesterol ( approximately 45 mg/dl) and triglycerides ( approximately 55 mg/dl) were not elevated, perhaps owing to degradation of lipid-enriched particles by low-density lipoprotein (LDL) receptors. To test this hypothesis, in the current studies we bred TgBP-1a mice with LDL receptor knockout mice. As reported previously, LDLR-/- mice manifested a moderate elevation in plasma cholesterol ( approximately 215 mg/dl) and triglycerides ( approximately 155 mg/dl). In contrast, the doubly mutant TgBP-1a;LDLR-/- mice exhibited marked increases in plasma cholesterol ( approximately 1,050 mg/dl) and triglycerides ( approximately 900 mg/dl). These lipids were contained predominantly within large very-low-density lipoprotein (VLDL) particles that were relatively enriched in cholesterol and apolipoprotein E. Freshly isolated hepatocytes from TgBP-1a and TgBP-1a;LDLR-/- mice overproduced cholesterol and fatty acids and secreted increased amounts of these lipids into the medium. Electron micrographs of livers from TgBP-1a mice showed large amounts of enlarged lipoproteins within the secretory pathway. We conclude that the TgBP-1a mice produce large lipid-rich lipoproteins, but these particles do not accumulate in plasma because they are degraded through the action of LDL receptors.

Sterol regulatory element-binding proteins: activators of cholesterol and fatty acid biosynthesis

A family of transcription factors designated sterol regulatory element-binding proteins (SREBPs) mediates the previously described end-product feedback regulation of cholesterol biosynthesis. In addition, SREBPs are emerging as important regulators of fatty acid synthesis. The current review focuses on the in-vivo regulation of SREBPs in liver and the coordinate regulation of SREBP-activated target genes.

Warfarin therapy: evolving strategies in anticoagulation

Warfarin is the oral anticoagulant most frequently used to control and prevent thromboembolic disorders. Prescribing the dose that both avoids hemorrhagic complications and achieves sufficient suppression of thrombosis requires a thorough understanding of the drug's unique pharmacology. Warfarin has a complex dose-response relationship that makes safe and effective use a challenge. For most indications, the dose is adjusted to maintain the patient's International Normalized Ratio (INR) at 2 to 3. Because of the delay in factor II (prothrombin) suppression, heparin is administered concurrently for four to five days to prevent thrombus propagation. Loading doses of warfarin are not warranted and may result in bleeding complications. Interactions with other drugs must be considered, and therapy in elderly patients requires careful management. Current dosing recommendations are reviewed, and practical guidelines for the optimal use of warfarin are provided.

Nuclear sterol regulatory element-binding proteins activate genes responsible for the entire program of unsaturated fatty acid biosynthesis in transgenic mouse liver

Previous studies have shown that the rate of fatty acid synthesis is elevated by more than 20-fold in livers of transgenic mice that express truncated nuclear forms of sterol regulatory element-binding proteins (SREBPs). This was explained in part by an increase in the levels of mRNA for the two major enzymes of fatty acid synthesis, acetyl-CoA carboxylase and fatty acid synthase, whose transcription is stimulated by SREBPs. Fatty acid synthesis also requires a source of acetyl-CoA and NADPH. In the current studies we show that the levels of mRNA for ATP citrate lyase, the enzyme that produces acetyl-CoA, are also elevated in the transgenic livers. In addition, we found marked elevations in the mRNAs for malic enzyme, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase, all of which produce NADPH. Finally, we found that overexpressing two of the SREBPs (1a and 2) led to elevated mRNAs for stearoyl-CoA desaturase 1 (SCD1), an isoform that is detectable in nontransgenic livers, and SCD2, an isoform that is not detected in nontransgenic livers. This stimulation led to an increase in total SCD activity in liver microsomes. Together, all of these changes would be expected to lead to a marked increase in the concentration of monounsaturated fatty acids in the transgenic livers, and this was confirmed chromatographically. We conclude that expression of nuclear SREBPs is capable of activating the entire coordinated program of unsaturated fatty acid biosynthesis in mouse liver.

Blunted feedback suppression of SREBP processing by dietary cholesterol in transgenic mice expressing sterol-resistant SCAP(D443N)

Feedback regulation of cholesterol biosynthesis is mediated by membrane-bound transcription factors designated sterol regulatory element-binding proteins (SREBP)-1 and -2. In sterol-deprived cultured cells, SREBPs are released from membranes by a proteolytic process that is stimulated by SREBP cleavage-activating protein (SCAP), a membrane protein containing a sterol-sensing domain. Sterols suppress SREBP cleavage by blocking the action of SCAP, thereby decreasing cholesterol synthesis. A point mutation in SCAP(D443N) causes resistance to sterol suppression. In this article, we produced transgenic mice that express mutant SCAP(D443N) in liver. In these livers the nuclear content of SREBP-1 and -2 was increased, mRNAs encoding proteins involved in uptake and synthesis of cholesterol and fatty acids were elevated, and the livers were engorged with cholesteryl esters and triglycerides enriched in monounsaturated fatty acids. When the mice were challenged with a high cholesterol diet, cleavage of SREBP-1 and -2 was reduced in wild-type livers and less so in transgenic livers. We conclude that SCAP(D443N) stimulates proteolytic processing of native SREBPs in liver and decreases the normal sterol-mediated feedback regulation of SREBP cleavage, suggesting a central role for SCAP as a sterol sensor in liver.

T-cell and natural killer cell development in thymectomized Xenopus

The Xenopus early-thymectomy model system is used to investigate the extent to which the thymus controls T-cell development and to probe the evolution of natural killer (NK) cells. Loss of T-cell function following thymectomy, together with the paucity of cells expressing monoclonal antibody-defined T-cell surface markers, and greatly reduced expression of T-cell receptor beta transcripts in spleen, liver and intestine, indicate that T-cell development in minimal in the absence of the thymus. Our findings therefore mitigate against the idea that a substantial extrathymic pathway of T-cell development exists in early vertebrate evolution. Rather, they suggest that in this amphibian representative T cells are predominately thymus dependent. In vitro studies with control and thymectomized Xenopus splenocytes reveal that a non-T/non-B population and also two T-cell subsets all display natural cytotoxicity towards allogeneic thymus lymphoid tumour cells (which are deficient in MHC antigen expression). Since Xenopus thymectomized early in larval development are permanently deficient in T cells, they may provide a useful phylogenetic model for the study of NK cells.

Activation of cholesterol synthesis in preference to fatty acid synthesis in liver and adipose tissue of transgenic mice overproducing sterol regulatory element-binding protein-2

We produced transgenic mice that express a dominant-positive truncated form of sterol regulatory element-binding protein-2 (SREBP-2) in liver and adipose tissue. The encoded protein lacks the membrane-binding and COOH-terminal regulatory domains, and it is therefore not susceptible to negative regulation by cholesterol. Livers from the transgenic mice showed increases in mRNAs encoding multiple enzymes of cholesterol biosynthesis, the LDL receptor, and fatty acid biosynthesis. The elevations in mRNA for 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase and HMG CoA reductase were especially marked (13-fold and 75-fold, respectively). As a result, the transgenic livers showed a 28-fold increase in the rate of cholesterol synthesis and a lesser fourfold increase in fatty acid synthesis, as measured by intraperitoneal injection of [3H]water. These results contrast with previously reported effects of dominant-positive SREBP-1a, which activated fatty acid synthesis more than cholesterol synthesis. In adipose tissue of the SREBP-2 transgenics, the mRNAs for cholesterol biosynthetic enzymes were elevated, but the mRNAs for fatty acid biosynthetic enzymes were not. We conclude that SREBP-2 is a relatively selective activator of cholesterol synthesis, as opposed to fatty acid synthesis, in liver and adipose tissue of mice.

Regulation of sterol regulatory element binding proteins in livers of fasted and refed mice

Hepatic lipid synthesis is known to be regulated by food consumption. In rodents fasting decreases the synthesis of cholesterol as well as fatty acids. Refeeding a high carbohydrate/low fat diet enhances fatty acid synthesis by 5- to 20-fold above the fed state, whereas cholesterol synthesis returns only to the prefasted level. Sterol regulatory element binding proteins (SREBPs) are transcription factors that regulate genes involved in cholesterol and fatty acid synthesis. Here, we show that fasting markedly reduces the amounts of SREBP-1 and -2 in mouse liver nuclei, with corresponding decreases in the mRNAs for SREBP-activated target genes. Refeeding a high carbohydrate/low fat diet resulted in a 4- to 5-fold increase of nuclear SREBP-1 above nonfasted levels, whereas nuclear SREBP-2 protein returned only to the nonfasted level. The hepatic mRNAs for fatty acid biosynthetic enzymes increased 5- to 10-fold above nonfasted levels, a pattern that paralleled the changes in nuclear SREBP-1. The hepatic mRNAs for enzymes involved in cholesterol synthesis returned to the nonfasted level, closely following the pattern of nuclear SREBP-2 regulation. Transgenic mice that overproduce nuclear SREBP-1c failed to show the normal decrease in hepatic mRNA levels for cholesterol and fatty acid synthetic enzymes upon fasting. We conclude that SREBPs are regulated by food consumption in the mouse liver and that the decline in nuclear SREBP-1c upon fasting may explain in part the decrease in mRNAs encoding enzymes of the fatty acid biosynthetic pathway.

Natural cytotoxicity towards allogeneic tumour targets in Xenopus mediated by diverse splenocyte populations

We have recently demonstrated NK-like activity in the spleen of the clawed frog, Xenopus laevis. This paper investigates the cellular basis of this natural cytotoxicity. Significant levels of cytotoxicity towards B3B7 allogeneic thymus tumour targets, that express neither class Ia nor class II MHC proteins, occurred after splenocytes from either control or early-thymectomized (Tx) year-old Xenopus were cultured for 48 hours. Killing by Tx cells required their culture in growth factor-rich medium (GFM) obtained from concanavalin A-stimulated cells. Immunomagnetic cell sorting revealed that cytotoxic effectors in both control and Tx frogs were found in the B cell-depleted population, but never in the B cell-enriched fraction. Splenocytes from control Xenopus, depleted of T cells by magnetic sorting and following culture in GFM, also developed natural cytotoxicity towards allotumour cells. Magnetic cell sorting also revealed that purified (CD5+) T cells cultured for 48 hours in GFM also became able to lyse the allogeneic tumour targets. Cytotoxicity mediated by T cells resided not only in the CD5+, CD8+ population, but also in the CD5+, CD8- (putative CD4+) T cell subset. Ontogenetic studies revealed that splenocytes from 6-7 week-old (stage 56-57) control larvae, even after 48 hr culture in GFM, were unable to spontaneously lyse the allotumour targets, whereas cultured splenocytes from 6 month old froglets were effective killers. Thymocytes from larvae or adults routinely failed to kill tumour cells. The work highlights the need to use Tx Xenopus to further explore non-T-cell-mediated, NK-like cytotoxicity at the amphibian level of evolution.

Cholesterol feeding reduces nuclear forms of sterol regulatory element binding proteins in hamster liver

Cholesterol feeding reduces the mRNAs encoding multiple enzymes in the cholesterol biosynthetic pathway and the low density lipoprotein receptor in livers of hamsters. Here we show that cholesterol feeding also reduces the levels of the nuclear NH2-terminal domains of sterol regulatory element binding proteins (SREBPs), which activate transcription of sterol-regulated genes. We show that livers of hamsters, like those of mice and humans, predominantly produce SREBP-2 and the 1c isoform of SREBP-1. Both are produced as membrane-bound precursors that must be proteolyzed to release the transcriptionally active NH2-terminal domains. Diets containing 0.1% to 1.0% cholesterol decreased the amount of nuclear SREBP-1c without affecting the amount of the membrane precursor or its mRNA, suggesting that cholesterol inhibits the proteolytic processing of SREBP-1 in liver as it does in cultured cells. Cholesterol also appeared to reduce the proteolytic processing of SREBP-2. In addition, at high levels of dietary cholesterol the mRNA encoding SREBP-2 declined and the amount of the precursor also fell, suggesting that cholesterol accumulation also may inhibit transcription of the SREBP-2 gene. The high-cholesterol diets reduced the amount of low density lipoprotein receptor mRNA by 30% and produced a more profound 70-90% reduction in mRNAs encoding 3-hydroxy-3-methylglutaryl CoA synthase and reductase. Treatment with lovastatin and Colestipol, which increases hepatic demands for cholesterol, increased the amount of SREBP-2 mRNA as well as the precursor and nuclear forms of the protein. This treatment caused a reciprocal decline in SREBP-1c mRNA and protein. Considered together, these data suggest that SREBPs play important roles in controlling transcription of sterol-regulated genes in liver, as they do in cultured cells.

Elevated levels of SREBP-2 and cholesterol synthesis in livers of mice homozygous for a targeted disruption of the SREBP-1 gene

The synthesis of cholesterol and its uptake from plasma LDL are regulated by two membrane-bound transcription factors, designated sterol regulatory element binding protein-1 and -2 (SREBP-1 and SREBP-2). Here, we used the technique of homologous recombination to generate mice with disruptions in the gene encoding the two isoforms of SREBP-1, termed SREBP-1a and SREBP-1c. Heterozygous gene-disrupted mice were phenotypically normal, but 50- 85% of the homozygous (-/-) mice died in utero at embryonic day 11. The surviving -/- mice appeared normal at birth and throughout life. Their livers expressed no functional SREBP-1. There was a 1.5-fold upregulation of SREBP-2 at the level of mRNA and a two- to threefold increase in the amount of mature SREBP-2 in liver nuclei. Previous studies showed that SREBP-2 is much more potent than SREBP-1c, the predominant hepatic isoform of SREBP-1, in activating transcription of genes encoding enzymes of cholesterol synthesis. Consistent with this observation, the SREBP-1 -/- animals manifested elevated levels of mRNAs for 3-hydroxy-3-methylglutaryl coenzyme A synthase and reductase, farnesyl diphosphate synthase, and squalene synthase. Cholesterol synthesis, as measured by the incorporation of [3H]water, was elevated threefold in livers of the -/- mice, and hepatic cholesterol content was increased by 50%. Fatty acid synthesis was decreased in livers of the -/- mice. The amount of white adipose tissue was not significantly decreased, and the levels of mRNAs for lipogenic enzymes, adipocyte lipid binding protein, lipoprotein lipase, and leptin were normal in the -/- mice. We conclude from these studies that SREBP-2 can replace SREBP-1 in regulating cholesterol synthesis in livers of mice and that the higher potency of SREBP-2 relative to SREBP-1c leads to excessive hepatic cholesterol synthesis in these animals.

Differential expression of exons 1a and 1c in mRNAs for sterol regulatory element binding protein-1 in human and mouse organs and cultured cells

The 5' end of the mRNA-encoding sterol regulatory element binding protein-1 (SREBP-1) exists in two forms, designated 1a and 1c. The divergence results from the use of two transcription start sites that produce two separate 5' exons, each of which is spliced to a common exon 2. Here we show that the ratio of SREBP-1c to 1a transcripts varies markedly among organs of the adult mouse. At one extreme is the liver, in which the 1c transcript predominates by a 9:1 ratio. High 1c:1a ratios are also found in mouse adrenal gland and adipose tissue and in human liver and adrenal gland. At the other extreme is the spleen, which shows a reversed 1c:1a ratio (1:10). In five different lines of cultured cells, including the HepG2 line derived from human hepatocytes, the 1a transcript predominated (1c:1a ratio < 1:2). In mouse 3T3-L1 preadipocytes, the 1a transcript was present, but the 1c transcript was not detectable. When these cells were differentiated into adipocytes by hormone treatment in culture, the amount of 1a transcript rose markedly (8.2-fold), and the 1c transcript remained virtually undetectable. We conclude that the SREBP-1a and 1c transcripts are controlled independently by regulatory regions that respond differentially to organ-specific and metabolic factors.

Isoform 1c of sterol regulatory element binding protein is less active than isoform 1a in livers of transgenic mice and in cultured cells

We have produced transgenic mice whose livers express a dominant positive NH2-terminal fragment of sterol regulatory element binding protein-1c (SREBP-1c). Unlike full-length SREBP-1c, the NH2-terminal fragment enters the nucleus without a requirement for proteolytic release from cell membranes, and hence it is immune to downregulation by sterols. We compared SREBP-1c transgenic mice with a line of transgenic mice that produces an equal amount of the NH2-terminal fragment of SREBP-1a. SREBP-1a and -1c are alternate transcripts from a single gene that differ in the first exon, which encodes part of an acidic activation domain. The 1a protein contains a long activation domain with 12 negatively charged amino acids, whereas the 1c protein contains a short activation domain with only 6 such amino acids. As previously reported, livers of the SREBP-1a transgenic mice were massively enlarged, owing to accumulation of triglycerides and cholesterol. SREBP-1c transgenic livers were only slightly enlarged with only a moderate increase in triglycerides, but not cholesterol. The mRNAs for the LDL receptor and several cholesterol biosynthetic enzymes were elevated in SREBP-la transgenic mice, but not in 1c transgenic mice. The mRNAs for fatty acid synthase and acetyl CoA carboxylase were elevated 9- and 16-fold in la animals, but only 2- and 4-fold in 1c animals. Experiments with transfected cells confirmed that SREBP-1c is a much weaker activator of transcription than SREBP-1a when both are expressed at levels approximating those found in nontransfected cells. SREBP-1c became a strong activator only when expressed at supraphysiologic levels. We conclude that SREBP-1a is the most active form of SREBP-1 and that SREBP-1c may be produced when cells require a lower rate of transcription of genes regulating cholesterol and fatty acid metabolism.

Overproduction of cholesterol and fatty acids causes massive liver enlargement in transgenic mice expressing truncated SREBP-1a

The NH2-terminal domain of sterol-regulatory element binding protein-1a (SREBP-1a) activates transcription of genes encoding enzymes of cholesterol and fatty acid biosynthesis in cultured cells. This domain is synthesized as part of a membrane-bound precursor that is attached to the nuclear envelope and endoplasmic reticulum. In sterol-depleted cells a two-step proteolytic process releases this NH2-terminal domain, which enters the nucleus and activates transcription. Proteolysis is suppressed by sterols, thereby suppressing transcription. In the current experiments we produce transgenic mice that overexpress a truncated version of human SREBP-1a that includes the NH2-terminal domain but lacks the membrane attachment site. This protein enters the nucleus without a requirement for proteolysis, and therefore it cannot be down-regulated. Expression was driven by the phosphoenolpyruvate carboxykinase (PEPCK) promoter, which gives high level expression in liver. When placed on a low carbohydrate/high protein diet to induce the PEPCK promoter, the transgenic mice developed progressive and massive enlargement of the liver, owing to the engorgement of hepatocytes with cholesterol and triglycerides. The mRNAs encoding 3-hydroxy-3-methylglutaryl CoA (HMG CoA) synthase, HMG CoA reductase, squalene synthase, acetyl-CoA carboxylase, fatty acid synthase, and stearoyl-CoA desaturase-1 were all elevated markedly, as was the LDL receptor mRNA. The rates of cholesterol and fatty acid synthesis in liver were elevated 5- and 25-fold, respectively. Remarkably, plasma lipid levels were not elevated. The amount of white adipose tissue decreased progressively as the liver enlarged. These studies indicate that the NH2-terminal domain of SREBP-1a can produce major effects on lipid synthesis and storage in the liver.

NK-like activity against allogeneic tumour cells demonstrated in the spleen of control and thymectomized Xenopus

This paper addresses the issue of natural killer (NK) cell evolution by searching for NK-like activity in an amphibian representative, the immunologically well-characterized clawed frog, Xenopus laevis. Using in vitro 6 h 51chromium release assays, we have shown that splenocyte effectors from early thymectomized (Tx) year-old frogs, but not from control siblings, are able to spontaneously lyse allogeneic thymus tumour cell lines that lack MHC antigen expression. Such lytic capacity can be readily induced in control Xenopus and elevated in Tx frogs by a single injection of tumour cells, with maximal splenocyte cytotoxicity occurring 3 days postinjection, the amount of 51Cr-release correlating directly with effector: target ratios. Splenocytes, even those from tumour-injected frogs, are unable to lyse allogeneic splenic lymphoblasts or erythrocyte targets, even when the latter are coated with IgY (the Xenopus IgG equivalent); moreover, we were unable to demonstrate any splenocyte-induced lysis of the human NK cell target K562. Lymphokine-activated killing (LAK) in Xenopus is suggested, since Tx splenocytes cultured in cytokine-rich medium (from concanavalin A-stimulated control splenocytes) display significantly elevated killing of allogeneic tumour targets. Flow cytometric analysis highlights the loss of T cell markers from the spleen of Tx frogs and reveals a variable staining pattern of both control and Tx splenocytes when treated with a mAb that binds to both fish non-specific cytotoxic cells and human NK cells. Prospects for identifying the cellular basis of NK-like activity in Xenopus are discussed in the light of these experiments.

Identification of a candidate CD5 homologue in the amphibian Xenopus laevis

We identified a novel T cell Ag in the South African clawed toad (Xenopus laevis) by a mAb designated 2B1. This Ag is present in relatively high levels on most thymocytes, approximately 65% of splenocytes, 55% of PBL, and 65% of intestinal lymphocytes, but is rarely seen on IgM+ B cells in any of these tissues. Lymphocytes bearing the 2B1 Ag proliferate in response to stimulation with Con A or PHA, whereas the 2B1- lymphocytes are reactive to LPS. Biochemical analysis indicates that this Ag is a differentially phosphorylated glycoprotein of 71 to 82 kDa. The protein core of 64 kDa bears both N- and O-linked carbohydrate side chains. The amino-terminal protein sequence of the 2B1 Ag shares significant homology with both the macrophage scavenger receptor type 1 motif and the mammalian CD5/CD6 family. The biochemical characteristics and cellular distribution of the 2B1 Ag suggest that it represents the CD5 homologue in X. laevis. While T cells constitutively express this highly conserved molecule, Xenopus B cells acquire the CD5 homologue only when they are stimulated in the presence of T cells.

Ontogeny and thymus-dependence of T cell surface antigens in Xenopus: flow cytometric studies on monoclonal antibody-stained thymus and spleen

Recently generated anti-Xenopus T cell monoclonal antibodies (mAbs) to the 120 kDa XTLA-1 determinant and against the putative CD5 and CD8 homologues, together with anti-IgM and anti-MHC class II mAbs, are used in dual colour flow cytometric experiments to characterize cell surface antigenic expression on lymphocytes in thymus and spleen of Xenopus laevis during larval and early adult life and also in metamorphosis-inhibited animals. Histological confirmation of T cell emergence early in larval ontogeny is supplied by cryostat sections stained for CD8. Five-day thymectomy, i.e. prior to T-lineage cell differentiation in the thymus, abolishes T cell marker expression in the spleen for up to 1 year. Moreover, late larval (20 days) or early adult (3 months) thymectomy (i.e. removal after peripheralization of T cells has occurred) also leads to severe depletion of mAb-defined T cells in the spleen.

Identification of a candidate CD5 homologue in the amphibian Xenopus laevis

We identified a novel T cell Ag in the South African clawed toad (Xenopus laevis) by a mAb designated 2B1. This Ag is present in relatively high levels on most thymocytes, approximately 65% of splenocytes, 55% of PBL, and 65% of intestinal lymphocytes, but is rarely seen on IgM+ B cells in any of these tissues. Lymphocytes bearing the 2B1 Ag proliferate in response to stimulation with Con A or PHA, whereas the 2B1- lymphocytes are reactive to LPS. Biochemical analysis indicates that this Ag is a differentially phosphorylated glycoprotein of 71 to 82 kDa. The protein core of 64 kDa bears both N- and O-linked carbohydrate side chains. The amino-terminal protein sequence of the 2B1 Ag shares significant homology with both the macrophage scavenger receptor type 1 motif and the mammalian CD5/CD6 family. The biochemical characteristics and cellular distribution of the 2B1 Ag suggest that it represents the CD5 homologue in X. laevis. While T cells constitutively express this highly conserved molecule, Xenopus B cells acquire the CD5 homologue only when they are stimulated in the presence of T cells.

Regulatory effects of n-3 polyunsaturated fatty acids on hepatic LDL uptake in the hamster and rat

These studies were undertaken to investigate the mechanisms involved in the regulation of hepatic low density lipoprotein (LDL) transport by n-3 fatty acids in the hamster and rat. Animals were fed n-3 or n-6 fatty acids with a cholesterol-free, very-low-fat semisynthetic diet, or with a diet enriched with cholesterol and saturated fat. Although the enrichment of liver lipids with dietary n-3 fatty acids was similar in hamsters and rats, the effect of n-3 fatty acids on hepatic LDL transport differed in the two species. In the hamster, n-3 fatty acids had no effect on hepatic receptor-dependent LDL uptake in animals fed a cholesterol-free, very-low-fat diet and suppressed receptor-dependent transport in animals fed a diet enriched with cholesterol and saturated triglyceride. In hamsters fed n-3 fatty acids, changes in receptor-dependent LDL transport were accompanied by parallel changes in LDL receptor mRNA, indicating regulation of the receptor at the pretranslational level. In the rat, on the other hand, dietary n-3 fatty acids enhanced hepatic receptor-dependent LDL uptake by nearly twofold regardless of the background diet; however, hepatic LDL receptor protein and mRNA were unchanged. Dietary n-3 fatty acids did not enhance hepatic chylomicron remnant clearance in the rat. These studies confirm marked species differences in response to n-3 fatty acids and suggest that n-3 fatty acids accelerate hepatic receptor-dependent LDL transport in the rat by altering the distribution or recycling of LDL receptors or via effects on a different receptor pathway.

Regulation of hepatic 7 alpha-hydroxylase expression and response to dietary cholesterol in the rat and hamster

Although dietary cholesterol raises plasma total and low density lipoprotein (LDL) cholesterol concentrations, the response to a given intake of cholesterol varies enormously among different species and even among individuals of the same species. The mechanisms responsible for differing sensitivity to dietary cholesterol were examined by comparing the rat, which is able to adapt to large fluctuations in sterol intake or loss with little change in plasma LDL levels, with the hamster, where changes in sterol balance strongly influence plasma LDL concentrations. When fed the same cholesterol-free diet, hepatic 7 alpha-hydroxylase activity was 16-fold higher in the rat than in the hamster. As a consequence, rates of hepatic cholesterol synthesis were 20-fold higher in the rat than in the hamster. In both species, hepatic cholesterol synthesis was suppressed > 90% in response to increasing loads of dietary cholesterol. However, the quantitative importance of this adaptive mechanism was much greater in the rat since the absolute reduction in hepatic cholesterol synthesis in the rat (2,110 nmol/h/g) was much larger than in the hamster (103 nmol/h/g). In the rat, the high basal level of 7 alpha-hydroxylase expression was further induced by substrate (cholesterol) allowing these animals to convert excess dietary cholesterol to bile acids efficiently. In contrast, the low basal level of enzyme expression in the hamster was not induced by dietary cholesterol. Thus, the low basal rates of bile acid and cholesterol synthesis coupled with a lack of 7 alpha-hydroxylase induction by cholesterol render the hamster much more sensitive than the rat to the cholesterolemic effects of dietary cholesterol.

Regulation of hepatic 7 alpha-hydroxylase expression by dietary psyllium in the hamster

Soluble fiber consistently lowers plasma total and low density lipoprotein (LDL)-cholesterol concentrations in humans and various animal models including the hamster; however, the mechanism of this effect remains incompletely defined. We performed studies to determine the activity of dietary psyllium on hepatic 7 alpha-hydroxylase, 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase and LDL receptor expression in the hamster. In animals fed a cholesterol-free semisynthetic diet containing 7.5% cellulose (avicel) as a fiber source, substitution of psyllium for avicel increased hepatic 7 alpha-hydroxylase activity and mRNA levels by 3-4-fold. Comparable effects on 7 alpha-hydroxylase expression were observed with 1% cholestyramine. Psyllium also increased hepatic 7 alpha-hydroxylase activity and mRNA in animals fed a diet enriched with cholesterol and triglyceride. Activation of 7 alpha-hydroxylase was associated with an increase in hepatic cholesterol synthesis that was apparently not fully compensatory since the cholesterol content of the liver declined. Although dietary psyllium did not increase hepatic LDL receptor expression in animals fed the cholesterol-free, very-low-fat diet, it did increase (or at least restore) receptor expression that had been downregulated by dietary cholesterol and triglyceride. Thus, 7.5% dietary psyllium produced effects on hepatic 7 alpha-hydroxylase and LDL metabolism that were similar to those of 1% cholestyramine. Induction of hepatic 7 alpha-hydroxylase activity by dietary psyllium may account, in large part, for the hypocholesterolemic effect of this soluble fiber.

Dietary fatty acids regulate hepatic low density lipoprotein (LDL) transport by altering LDL receptor protein and mRNA levels

The concentration of LDL in plasma is strongly influenced by the amount and the type of lipid in the diet. Recent studies in the hamster have shown that dietary fatty acids differentially affect circulating LDL levels primarily by altering receptor-dependent LDL uptake in the liver. To investigate the mechanistic basis of this effect, rates of receptor-dependent LDL transport in the liver were correlated with LDL receptor protein and mRNA levels in hamsters fed safflower oil or coconut oil and varying amounts of cholesterol. Hepatic LDL receptor activity was significantly lower in animals fed coconut oil than in animals fed safflower oil at all levels of cholesterol intake (26, 53, and 61% lower at cholesterol intakes of 0, 0.06, and 0.12%, respectively). These fatty acid-induced changes in hepatic LDL receptor activity were accompanied by parallel changes in hepatic LDL receptor protein and mRNA levels, suggesting that dietary fatty acids regulate the LDL receptor pathway largely at the mRNA level.

Incomplete tolerance induced in Xenopus by larval tissue allografting: evidence from immunohistology and mixed leucocyte culture

Application of adult skin allografts to Xenopus larvae has been a favoured protocol for probing the development of self-tolerance. A more physiologic approach is presented here that examines the immunologic outcome of grafting semi- or fully allogeneic larval skin or spleen to age-matched, larval Xenopus (X. laevis/X. gilli clonal hybrids). Following such grafting at 2 or 4 weeks-of-age, young froglets (4-5-months-old) are generally unable to reject second-set skin transplants, but destroy third-party skin vigorously, the MHC class II-rich spleen proving especially effective at inducing this tolerance. In contrast, following larval grafting of semiallogeneic tissues, mixed leucocyte culture performed at the end of metamorphosis (6 weeks) and again at 6 months reveals splenocyte reactivity toward donor-strain stimulators. Immunohistological findings extend this observation of anti-donor reactivity (suggesting incomplete tolerance) to the graft site. Thus despite excellent health when viewed externally, apparently tolerated second-set skin transplants display localised infiltration (especially into the epidermis) by CD8+ T cells and increased numbers of MHC class I and II-expressing cells by 3 weeks post-grafting. The immunologic implications of these findings are discussed.

Skin xenograft rejection in Xenopus--immunohistology and effect of thymectomy

Immunohistology, using the T-lineage-specific monoclonal antibody XT-1 and an anti-IgM mAb, illustrates differences in the cellular basis of skin allograft and xenograft destruction displayed by control froglets (X. laevis). Thus T cells predominate within allografts, whereas B-lineage cells accumulate under xenografts (from X. tropicalis). The possibility that T cells do not play a central role in mediating xenograft rejection is consistent with the finding that early thymectomy (at 7 days) has minimal effect on rejection end points of X. tropicalis transplants. However, rejection of skin from a "phylogenetically less distant" xenogeneic species (X. borealis) is shown here to be impaired in early thymectomized X. laevis. Differences in the extent to which the thymus has been shown to influence skin xenograft rejection in Xenopus are discussed.

In vitro cytotoxicity in adult Xenopus generated against larval targets and minor histocompatibility antigens

Our experiments reveal that application of several minor H antigen-disparate skin grafts to adult Xenopus over an 18-month period can lead to in vitro generation of CML reactivity toward these minor antigens. Furthermore, we demonstrate that, following MHC-disparate skin graft rejection, adult effectors can efficiently kill both adult and larval donor-strain targets; this killing is MHC-specific and requires MLC restimulation with cells syngeneic to the skin graft donor. The ability to kill larval lymphoblasts, which have been shown elsewhere to be MHC class I-negative but class II-positive, suggests the probable importance of class II-restricted killing in this species.

Attempts to break perimetamorphically induced skin graft tolerance by treatment of Xenopus with cyclophosphamide and interleukin-2

The maintenance of skin allotolerance induced by perimetamorphic application of MHC-disparate skin to isogeneic Xenopus is investigated. Removal of the perimetamorphically applied first-set graft after 4 weeks did not, in general, completely break allotolerance; however, many second-set semi-allogeneic grafts, applied up to 14 weeks after first-set removal, were no longer maintained in perfect condition. Skin allografts tolerated for up to 42 weeks continued to express donor histocompatibility antigens, as indicated by their survival times when transplanted back to the original donor or recipient strain. Treatment with human recombinant IL-2 (rIL-2), shown elsewhere to be an effective immunoregulatory lymphokine for Xenopus in vivo, failed to cause long-term-tolerated 1st-set allografts, or newly-applied 2nd-set grafts, to be rejected. In contrast, cyclophosphamide (CyP) treatment led to acute (less than 4 weeks) destruction of both 1st- and 2nd-set allografts; breaking of tolerance was regularly seen when donor and host differed by two MHC haplotypes, but occurred infrequently in semiallogeneic combinations. The experiments suggest that skin-induced allotolerance is maintained by an immunosuppressive mechanism, that is CyP-sensitive but resistant to rIL-2 treatment.

Amphibian metamorphosis: an immunologic opportunity!

Anuran amphibian metamorphosis is an immunologically interesting period. For the investigator, it provides an unusual opportunity for analyzing both humoral regulation of the immune response and the development and maintenance of self-tolerance. Some of the questions one can ask are: Why don't immunocompetent larvae destroy antigenically disparate adult cells as they differentiate within them during metamorphosis? Do the dramatic hormonal changes occurring during this period regulate immunological function? How do animals in metamorphosis protect themselves from their immunologically hostile environment?

In vivo studies on allotolerance perimetamorphically induced in control and thymectomized Xenopus

Implantation of either major histocompatibility complex (MHC)-disparate thymus to 7-day thymectomized (Tx) Xenopus in late larval life, or allogeneic skin to perimetamorphic controls, routinely induces tolerance towards implant-strain skin grafts applied in adult life. To characterize this allotolerance further, additional in vivo approaches were attempted. Injection of gamma-irradiated (5000 rads) implant-strain splenocytes into frogs bearing tolerant skin grafts revealed (within 3 days) significantly elevated tritiated thymidine uptake by host spleen cells, compared to siblings injected with isogeneic cells. Although this in vivo 'mixed leucocyte reaction' proved to be thymus dependent, the identity of the cells involved awaits clarification. When non-restored Tx Xenopus are injected with live MHC-disparate splenocytes, graft-versus-host (GVH)-induced mortality ensued within 2 weeks. Such GVH disease also occurred (albeit more chronically) when Tx allothymus-implanted animals were given MHC-incompatible splenocytes, but only when these came from the thymus donor strain. Splenocytes from thymus-implanted animals failed to achieve GVH-induced splenomegaly when transferred to appropriate hosts (bearing MHC antigens of the thymus donor strain). Overall, the experiments indicate that alloreactivity against donor cells is impaired but not completely inhibited in Xenopus following perimetamorphic implantation.

Cytoarchitecture of the Xenopus thymus following gamma-irradiation

This paper describes in vitro and in vivo attempts to deplete the 4- to 8-month-old Xenopus laevis (J strain) thymus of its lymphocyte compartment. Gamma irradiation (2-3000 rad) of the excised thymus, followed by two weeks in organ culture, is effective in removing lymphocytes, but causes drastic reduction in size and loss of normal architecture. In contrast, in vivo whole-body irradiation (3000 rad) and subsequent in situ residence for 8-14 days proves successful in providing a lymphocyte-depleted froglet thymus without loss of cortical and medullary zones. In vivo-irradiated thymuses are about half normal size, lack cortical lymphocytes, but still retain some medullary thymocytes; they show no signs of lymphocyte regeneration when subsequently organ cultured for 2 weeks. Light microscopy of 1 micron, plastic-embedded sections and electron microscopy reveal that a range of thymic stromal cell types are retained and that increased numbers of cysts, mucous and myoid cells are found in the thymus following whole-body irradiation. In vivo-irradiated thymuses are therefore suitable for implantation studies exploring the role of thymic stromal cells in tolerance induction of differentiating T lymphocytes.

Thymocyte/stromal cell chimaerism in allothymus-grafted Xenopus: developmental studies using the X. borealis fluorescence marker

These experiments employ the X. borealis (quinacrine-fluorescence) cell marker to illustrate that froglet (normal or in vivo-irradiated) thymuses, alloimplanted to 4- to 6-week-old, 7-day-thymectomized hosts, become filled with host lymphoid cells, while a range of thymic stromal cell types (e.g. epithelial derivatives and reticuloendothelial cells) remain donor derived. A time-course study of 4 micron historesin-embedded sections reveals that for normal thymus implants, host cells begin to immigrate in good number only after metamorphosis. In contrast, 3000 rad-irradiated thymus implants begin to be repopulated with host lymphocytes within 2 weeks postimplantation, when hosts are still at a late larval stage of development. Despite rapid colonization by host lymphoid cells, irradiated thymuses remain small and often disappear in early adult life. Donor-derived lymphocytes frequent the blood and both the red pulp and perifollicular regions of the spleen following normal thymus implantation, whereas such thymic emigrants were not seen in the periphery of thymectomized hosts grafted with irradiated thymus glands.

Impaired rejection of minor-histocompatibility-antigen-disparate skin grafts and acquisition of tolerance to thymus donor antigens in allothymus-implanted, thymectomized Xenopus

We examine whether J strain Xenopus laevis and X laevis/gilli (LG) isogeneic hybrids, thymectomized (Tx) at 7 days and implanted with larval or postmetamorphic thymus (either normal or irradiated) in late larval life, can be used to explore the role of the developing thymus in the education of minor histocompatibility (H) antigen-reactive T cells and in the acquisition of tolerance to MHC (major histocompatibility complex) antigens. Tx LG Xenopus implanted with isogeneic thymus display fully restored ability to reject both MHC-incompatible and minor H-disparate grafts. Allothymuses (both MHC- and minor-H-disparate) restore normal rejection of skin grafts with MHC incompatibility to thymus donor and Tx (J or LG) host, but reactivity to grafts with putative minor H disparity is impaired. Defective minor-H-disparate skin graft rejection occurs even when the Tx host has received a thymus of the "correct" MHC haplotype (i.e., the same MHC as the assay skin graft). This defect may be attributable to inadequate establishment of minor-H-reactive T lymphocytes, to sharing of minor antigens, or to "nonspecificity" of tolerance induction to minor antigens, since such tolerance is demonstrated here following perimetamorphic grafting of skin to control LG clones. Skin grafts from Xenopus isogeneic to the donors of the MHC-incompatible larval and adult thymus implants are always tolerated by Tx hosts. In contrast, in vitro one-way MLC (mixed leukocyte culture) reactivity of splenocytes to thymus donor MHC antigens occurs when certain donor/host combinations are used but not when others are tested. Postmetamorphic thymus implants appear to promote in vitro MLC tolerance more effectively than larval thymus implants.

Elevated CK-MB and CK-BB in serum and tumor homogenate of a patient with lung cancer

Elevated serum levels of creatine kinase (CK) were found in a patient with small-(oat) cell carcinoma of the lung. Fractionation of the enzyme showed markedly elevated CK-MB and CK-BB isoenzymes. Clinical and subsequent pathological examination showed no evidence of infarction, inflammation, or tumor involvement of the heart; however, analysis of tumor tissue for CK showed predominance of CK-MB and CK-BB isoenzymes, thus implicating tumor as the source of the circulating levels of CK-MB and CK-BB. Our case is the first to document CK-MB from neoplastic tissue homogenates, and illustrates that markedly elevated circulating levels of CK-MB, or increased levels of CK-MB in combination with CK-BB may point away from a myocardial source, and toward the existence of a malignancy.

Changes in antibody and complement production in Xenopus laevis during postmetamorphic development revealed in a primary in vivo or in vitro antibody response

Xenopus laevis (G-line) mounts a primary plaque forming cell (PFC) response either in vivo or in vitro following challenge with foreign erythrocytes. Methods are described for generating and assaying the response, which specify criteria such as antigen dose, antigen choice, response kinetics, and complement source. The results suggest that at the peak of the primary response (approximately day 6), animals of different ages produce predominantly different 'classes' of antibody which display markedly different complement-fixing characteristics. Antibodies produced by larvae and 4-month-old postmetamorphic animals appear here to be unable to fix either guinea pig complement (GPC') or adult Xenopus complement, but can readily fix complement from 6-month-old Xenopus. The proportion of spleen PFC's producing antibody capable of fixing GPC' progressively increases from about six months to 18 months of age. Possible explanations for such ontogenetic changes are discussed.

Restoration of the antibody response to sheep erythrocytes in thymectomized Xenopus implanted with MHC-compatible or MHC-incompatible thymus

These experiments make use of an amphibian model system for investigating the role of the thymus in T helper cell education. Clawed toads (Xenopus laevis), thymectomized at 7 days, are unable to mount an antibody response to thymus-dependent antigens, such as sheep red blood cells (SRBC). When thymectomized larvae are implanted with larval thymuses (either irradiated or non-irradiated), incompatible at the major histocompatibility complex (MHC) or with MHC-compatible or -incompatible ‘adult’ thymuses, their splenic plaque-forming cell response and serum haemolytic antibody production to SRBC are both restored, to some extent. However, levels of mercaptoethanol-resistant antibody were extremely poor in those animals implanted with MHC-incompatible ‘adult’ thymus. Larval thymus implants were shown, by ploidy-labelling studies, to become repopulated with host-derived lymphocytes. Whether or not these lymphocytes acquire their MHC restriction specificities in the thymus awaits clarification.