Evaluation of FluSight influenza forecasting in the 2021-22 and 2022-23 seasons with a new target laboratory-confirmed influenza hospitalizations

Accurate forecasts can enable more effective public health responses during seasonal influenza epidemics. Forecasting teams were asked to provide national and jurisdiction-specific probabilistic predictions of weekly confirmed influenza hospital admissions for one through four weeks ahead for the 2021-22 and 2022-23 influenza seasons. Across both seasons, 26 teams submitted forecasts, with the submitting teams varying between seasons. Forecast skill was evaluated using the Weighted Interval Score (WIS), relative WIS, and coverage. Six out of 23 models outperformed the baseline model across forecast weeks and locations in 2021-22 and 12 out of 18 models in 2022-23. Averaging across all forecast targets, the FluSight ensemble was the 2nd most accurate model measured by WIS in 2021-22 and the 5th most accurate in the 2022-23 season. Forecast skill and 95% coverage for the FluSight ensemble and most component models degraded over longer forecast horizons and during periods of rapid change. Current influenza forecasting efforts help inform situational awareness, but research is needed to address limitations, including decreased performance during periods of changing epidemic dynamics.

Intra-articular therapy with recombinant human GDF5 arrests disease progression and stimulates cartilage repair in the rat medial meniscus transection (MMT) model of osteoarthritis

OBJECTIVE

Investigation of osteoarthritis (OA) risk alleles suggests that reduced levels of growth and differentiation factor-5 (GDF5) may be a precipitating factor in OA. We hypothesized that intra-articular recombinant human GDF5 (rhGDF5) supplementation to the OA joint may alter disease progression.

METHODS

A rat medial meniscus transection (MMT) joint instability OA model was used. Animals received either one intra-articular injection, or two or three bi-weekly intra-articular injections of either 30 μg or 100 μg of rhGDF5 beginning on day 21 post surgery after structural pathology had been established. Nine weeks after MMT surgery, joints were processed for histological analysis following staining with toluidine blue. Control groups received intra-articular vehicle injections, comprising a glycine-buffered trehalose solution. OA changes in the joint were evaluated using histopathological end points that were collected by a pathologist who was blinded to treatment.

RESULTS

Intra-articular rhGDF5 supplementation reduced cartilage lesions on the medial tibial plateau in a dose-dependent manner when administered therapeutically to intercept OA disease progression. A single 100 μg rhGDF5 injection on day 21 slowed disease progression at day 63. A similar effect was achieved with two bi-weekly injections of 30 μg. Two bi-weekly injections of 100 μg or three bi-weekly injections of 30 μg stopped progression of cartilage lesions. Importantly, three biweekly injections of 100 μg rhGDF5 stimulated significant cartilage repair.

CONCLUSIONS

Intra-articular rhGDF5 supplementation can prevent and even reverse OA disease progression in the rat MMT OA model. Collectively, these results support rhGDF5 supplementation as an intra-articular disease modifying OA therapy.

Disruption of host-seeking behaviour by the salmon louse, Lepeophtheirus salmonis, using botanically derived repellents

The potential for developing botanically derived natural products as novel feed-through repellents for disrupting settlement of the salmon louse, Lepeophtheirus salmonis (Caligidae) upon farmed Atlantic salmon, Salmo salar, was investigated using an established laboratory vertical Y-tube behavioural bioassay for assessing copepodid behaviour. Responses to artificial sea water conditioned with the odour of salmon, or to the known salmon-derived kairomone component, α-isophorone, in admixture with selected botanical materials previously known to interfere with invertebrate arthropod host location were recorded. Materials included oils extracted from garlic, Allium sativum (Amaryllidaceae), rosemary, Rosmarinus officinalis (Lamiaceae), lavender, Lavandula angustifolia (Lamiaceae), and bog myrtle, Myrica gale (Myricaceae), and individual components (diallyl sulphide and diallyl disulphide from garlic; allyl, propyl, butyl, 4-pentenyl and 2-phenylethyl isothiocyanate from plants in the Brassica genus). Removal of attraction to salmon-conditioned water (SCW) or α-isophorone was observed when listed materials were presented at extremely low parts per trillion (ppt), that is picograms per litre or 10^-12 level. Significant masking of attraction to SCW was observed at a level of 10 ppt for diallyl disulphide and diallyl sulphide, and allyl isothiocyanate and butyl isothiocyanate. The potential of very low concentrations of masking compounds to disrupt Le. salmonis copepodid settlement on a host fish has been demonstrated in vitro.

Contrasting expression of immune genes in scaled and scaleless skin of Atlantic salmon infected with young stages of Lepeophtheirus salmonis

Atlantic salmon skin tissues with and without scales were taken from two preferred sites of salmon louse (Lepeophtheirus salmonis) attachment, behind the dorsal fin (scaled) and from the top of the head (scaleless), respectively. Tissues were profiled by qPCR of 32 genes to study responses to copepodids, 4 days post infection (dpi), and during the moult of copepodids to the chalimus stage, at 8 dpi. Basal/constitutive differences were found for many immune-related genes between the two skin sites; e.g., mannose binding protein C was over 100 fold higher expressed in the scaled skin from the back in comparison to the skin without scales from the head. With lice-infection, at 4 dpi most genes in both tissues showed lower values than in the non-infected control. By 8 dpi, the majority of responses increased towards the control levels, including cytokines of Th1, Th17 and Th2 pathways. Immunohistochemistry of three immune factors revealed an even distribution of MHC class II positive cells throughout epidermis, including the top layer of keratinocytes, marked compartmentalization of Mx⁺ and CD8α⁺ cells close to stratum basale, and an increase in numbers of CD8α⁺ cells in response to infection. In conclusion, suppression of immune genes during the copepodid stage likely sets off a beneficial situation for the parasite. At the moult to chalimus stage 8 dpi, only few genes surpassed the non-infected control levels, including CD8α. The gene expression pattern was reflected in the increased number of CD8α expressing cells, thus revealing a relatively minor activation of skin T-cell defenses in Atlantic salmon in response to L. salmonis infection.

Cytokine expression in the intestine of rainbow trout (Oncorhynchus mykiss) during infection with Aeromonas salmonicida

Gene expression of a number of cytokines in the intestine of rainbow trout (Oncorhynchus mykiss) was investigated after challenge with a pathogenic strain of Aeromonas salmonicida. Fish were exposed to A. salmonicida by immersion in a bacterial suspension (bath challenge) and tissue samples of the distal and proximal intestine were collected at days 0, 2, 4, 6 and 8 post-exposure. Head kidney tissue was also collected to assess the effect in a systemic immune tissue. A classic profile of pro-inflammatory cytokine upregulation was observed in the proximal intestine of fish infected by bath challenge, as determined by semi-quantitative RT-PCR. Expression of IL-1beta, IL-8, TNF-alpha and IFN-gamma was increased in the proximal intestine. TGF-beta was significantly decreased in the distal intestine. In the head kidney, infection with A. salmonicida by bath challenge caused decreased expression levels of IL-1beta, IL-8, TNF-alpha and TGF-beta. The results are discussed in the context of potential immune mechanisms in the gut to prevent infection.

Effects of a p38 MAP kinase inhibitor on bone ingrowth and tissue differentiation in rabbit chambers

The effects of an oral p38 mitogen-activated protein kinase (MAPK) inhibitor and polyethylene particles separately and together on tissue differentiation in the bone harvest chamber (BHC) in rabbits over a 3-week treatment period were investigated. The harvested tissue was analyzed histomorphometrically for markers of bone formation (percentage of bone area), osteoblasts (alkaline phosphatase staining), and osteoclasts (CD51, the alpha chain of the vitronectin receptor). Polyethylene particles decreased the percentage of bone ingrowth and staining for alkaline phosphatase. The p38 MAPK inhibitor alone decreased alkaline phosphatase staining. When the oral p38 MAPK inhibitor was given and the chamber contained polyethylene particles, there was a suppression of bone ingrowth and alkaline phosphatase staining. In contrast to oral non-steroidal anti-inflammatory drugs (NSAIDs) and local Interleukin-1 receptor antagonist (IL-1ra) administration, the oral p38 MAPK inhibitor alone did not suppress bone formation when given during the initial phase of tissue differentiation. Particle-induced inflammation and the foreign body reaction were not curtailed when the p38 MAPK inhibitor was given simultaneously with particles. Additional experiments are needed to establish the efficacy of p38 MAPK inhibitor administration on mitigating an established inflammatory and foreign body reaction that parallels the clinical situation more closely.

A role for schwann cells in the neuroregenerative effects of a non-immunosuppressive fk506 derivative, jnj460

FK506 and its non-immunosuppressive derivatives represent a class of pharmacological agents referred to as immunophilin ligands that have been reported to promote neuroregeneration and survival in several experimental models; however their cellular and molecular mechanisms of action have not been well established. Here we characterize a new immunophilin ligand that interacts with both FK506 binding protein 12 (FKBP12) and FKBP52, and demonstrate that JNJ460 induces neurite outgrowth from freshly explanted dorsal root ganglia (DRG) in a Schwann cell-dependent manner. Purified cultures of neurons fail to respond to these drugs, but cultures containing Schwann cells and neurons respond with neurite outgrowth, as do neurons grown in conditioned medium from JNJ460-treated Schwann cells. Using microarray analysis and a transcription reporter assay, we show that JNJ460 induces a series of transcriptional changes that occur in a temporal cascade. Among the Schwann cell-expressed genes upregulated following JNJ460 treatment is the POU transcription factor SCIP, which has been shown to regulate Schwann cell gene transcription and differentiation. JNJ460 potentiated transforming growth factor beta (TGF-beta)-induced transcriptional activation and SCIP induction in Schwann cells, by altering the interaction between FKBP12 and the TGF-beta type I receptor, TbetaR1. Finally, to test whether JNJ460 enhances neurite regeneration in vivo, we treated animals with JNJ460 for 30 days following mechanical transection of the sciatic nerve and demonstrated myelin and axonal hypertrophy at the ultrastructural level. Collectively, these data suggest that Schwann cells play an important role in the biological effects of immunophilin ligands by affecting neuron-glial signaling during regeneration.

SUMMARY

The cellular and molecular mechanisms responsible for the regenerative effects of immunophilin ligands are not well understood. Here we show that the neuritogenic effects of JNJ460 in a DRG model depend on interactions between neurons and Schwann cells. Treatment of purified Schwann cells with JNJ460 alters Schwann cell gene expression, and promotes the generation of factors that act on neurons. These data indicate that Schwann cells play an important role in the actions of immunophilin ligands.

Radiological review of accident and emergency radiographs: a 1-year audit

AIM

To assess the impact and cost effectiveness of a system of radiological review of accident and emergency (A&E) plain films.

MATERIALS AND METHODS

Review documentation was studied retrospectively over a 1-year period. Six hundred and eighty-four actual or suspected errors in the initial radiological interpretation by A&E staff were highlighted by radiologists in training. These selected 'red reports' were then further reviewed by a musculoskeletal radiologist and a more senior member of the A&E team.

RESULTS

Three hundred and fifty-one missed or strongly suspected fractures were detected, with ankle, finger and elbow lesions predominating. Other errors included 11 missed chest radiograph abnormalities and 24 A&E false-positives. Radiologists in training tended to over-report abnormalities with an 18% false-positive rate when compared to the subsequent musculoskeletal radiology opinion. Following review, further action was taken by A&E staff in 286 (42.6%) of cases. No operative intervention was required in those patients with a delayed or missed A&E diagnosis. Consideration is given to the cost of providing this form of review and the impact of medico-legal factors.

CONCLUSION

Compared with the large numbers of patients seen and radiographed in a busy A&E department, the number of radiological errors was small. There were even fewer changes in management. Despite this, concern over litigation, clinical governance and future work patterns in A&E make this form of review a useful means of risk reduction in a teaching hospital.Williams, S. M. (2000). Clinical Radiology55, 861-865

Coordinate packaging of newly synthesized phosphatidylcholine and phosphatidylglycerol in lamellar bodies in alveolar type II cells

Methylamine, a weak base, inhibits packaging of newly synthesized phosphatidylcholine (PC) in lamellar bodies in 20-22 h cultured alveolar type II cells, suggesting a role for acidic pH of lamellar bodies. In this study, we tested if (i) the packaging of PC is similarly regulated in freshly isolated type II cells and (ii) methylamine also inhibits the packaging of other surfactant phospholipids, particularly, phosphatidylglycerol (PG). The latter would suggest coordinated packaging so as to maintain the phospholipid composition of lung surfactant. During the short-term metabolic labeling experiments in freshly isolated type II cells, methylamine treatment decreased the incorporation of radioactive precursors into PC, disaturated PC (DSPC), and PG of lamellar bodies but not of the microsomes, when compared with controls. The calculated packaging (the percentage of microsomal lipid packaged in lamellar bodies) of each phospholipid was similarly decreased (approximately 50%) in methylamine-treated cells, suggesting coordinated packaging of surfactant phospholipids in lamellar bodies. Equilibrium-labeling studies with freshly isolated type II cells (as is routinely done for studies on surfactant secretion) +/- methylamine showed that in methylamine-treated cells, the secretion of PC and PG was decreased (possibly due to decreased packaging), but the phospholipid composition of released surfactant (measured by radioactivity distribution) was unchanged; and the PC content (measured by mass or radioactivity) of lamellar bodies was lower, but the PC composition (as percentage of total phospholipids) was unchanged when compared with control cells. We speculate that the newly synthesized surfactant phospholipids, PC, DSPC, and PG, are coordinately transported into lamellar bodies by a mechanism requiring the acidic pH, presumably, of lamellar bodies.

Administration of helper-dependent adenoviral vectors and sequential delivery of different vector serotype for long-term liver-directed gene transfer in baboons

The efficiency of first-generation adenoviral vectors as gene delivery tools is often limited by the short duration of transgene expression, which can be related to immune responses and to toxic effects of viral proteins. In addition, readministration is usually ineffective unless the animals are immunocompromised or a different adenovirus serotype is used. Recently, adenoviral vectors devoid of all viral coding sequences (helper-dependent or gutless vectors) have been developed to avoid expression of viral proteins. In mice, liver-directed gene transfer with AdSTK109, a helper-dependent adenoviral (Ad) vector containing the human alpha(1)-antitrypsin (hAAT) gene, resulted in sustained expression for longer than 10 months with negligible toxicity to the liver. In the present report, we have examined the duration of expression of AdSTK109 in the liver of baboons and compared it to first-generation vectors expressing hAAT. Transgene expression was limited to approximately 3-5 months with the first-generation vectors. In contrast, administration of AdSTK109 resulted in transgene expression for longer than a year in two of three baboons. We have also investigated the feasibility of circumventing the humoral response to the virus by sequential administration of vectors of different serotypes. We found that the ineffectiveness of readministration due to the humoral response to an Ad5 first-generation vector was overcome by use of an Ad2-based vector expressing hAAT. These data suggest that long-term expression of transgenes should be possible by combining the reduced immunogenicity and toxicity of helper-dependent vectors with sequential delivery of vectors of different serotypes.

Giant coronary artery aneurysm presenting as a mediastinal mass

An asymptomatic mediastinal mass in a 72-year-old man was imaged by computer tomography, magnetic resonance imaging, and coronary arteriography, revealing a giant atheromatous aneurysm of the right coronary artery.

Adeno-associated virus 2-mediated gene transfer in vivo: organ-tropism and expression of transduced sequences in mice

Adeno-associated virus 2 (AAV), a non-pathogenic human parvovirus, is gaining attention as a vector for its potential use in human gene therapy. However, few studies have examined the safety and the efficacy of this vector system in vivo. We report here that recombinant AAV vectors, when directly injected intravenously in mice, accumulated predominantly in liver cells, suggesting that AAV may possess in vivo organ-tropism for liver. The transduced lacZ reporter gene was expressed in hepatocytes in the liver and, at the level examined, did not appear to induce any detectable cytotoxic T lymphocyte response against beta Gal. AAV-mediated transduction of murine hematopoietic progenitor cells ex vivo followed by transplantation into lethally irradiated syngeneic mice also revealed high-efficiency gene transfer into progeny cells without any observable cytotoxicity or deleterious effect. The transduced reporter gene sequences were also expressed in mice in vivo. The AAV-based vectors may thus prove useful as a potentially safe alternative to the more commonly used retrovirus- and adenovirus-based vector systems.

Detection of telomerase activity in exfoliated cancer cells in colonic luminal washings and its related clinical implications

Telomerase is a ribonucleoprotein capable of replacing telomeric DNA sequences that are lost at each cell division. Under normal circumstances, it is active in rapidly dividing embryonic cells and in stem cell populations but not in terminally differentiated somatic cells. Much attention has recently focused on the hypothesis that activity of this enzyme is necessary for cells to become immortal. This predicts that telomerase activity should be detectable in malignant cells and tissues but not in their normal counterparts, which slowly senesce and die. In accordance with this notion, telomerase activity has been reported in a wide range of malignancies, including those of the gastrointestinal tract, breast and lung. In the present study, we used a polymerase chain reaction (PCR)-based assay for telomerase activity, designated the "telomeric repeat amplification protocol (TRAP)', to examine initially 35 colonic carcinomas, their corresponding normal tissues and 12 inflammatory bowel disease (IBD) lesions. We detected strong enzyme activity in 32 (92%) of the 35 colon carcinomas while there was no activity in 30 (86%) of 35 matched normal colonic tissue specimens and only very weak activity in the remainder. Four of seven specimens of ulcerative colitis and two of five Crohn's disease lesions were negative, and the rest were only weakly positive. These results led us to examine whether telomerase could be detected in carcinoma cells exfoliated into the colonic lumen. We assayed lysates of exfoliated cells in luminal washings from colectomy specimens of 15 patients with colon carcinoma and nine with IBD. Telomerase activity was detected in washings from 9 (60%) of the 15 colon carcinoma cases but not in any from cases with IBD, suggesting that it can be a good marker for the detection of colon carcinoma, possibly even in non-invasively obtained samples.

Methylamine decreases trafficking and packaging of newly synthesized phosphatidylcholine in lamellar bodies in alveolar type II cells

Lung lamellar bodies, the storage organelles for lung surfactant phosphatidylcholine (PC), maintain an acidic pH that can be increased with weak bases. This study investigates the effect of a weak base, methylamine, on the pH in lamellar bodies and on the trafficking and packaging of newly synthesized PC in lamellar bodies. Methylamine increased the pH of isolated lung lamellar bodies and of lamellar bodies in intact cells. Metabolic labelling of isolated type II cells with [methyl-3H]choline showed that although methylamine (2.5-10 mM) did not alter the labelling of cellular or microsomal PC and disaturated PC, it decreased the labelling of the PC and disaturated PC in lamellar bodies. The packaging of PC in lamellar bodies (the specific activities ratio between the PC in lamellar bodies and the microsomal PC) also decreased in a time- and concentration-dependent manner. The cellular synthesis of PC or its packaging into lamellar bodies was unaltered by brefeldin A, suggesting that the Golgi was not involved in PC packaging. Although methylamine also increased surfactant secretion, the inhibition of PC packaging in lamellar bodies seems unrelated to the secretagogue effect, (1) on the basis of metabolic consequences of increased secretion and (2) because ATP, another secretagogue, did not inhibit PC packaging. Methylamine seems to inhibit PC packaging by inhibiting trafficking of PC to lipid-rich light subcellular fractions. Together our results suggest that the trafficking of surfactant PC into lamellar bodies might be sensitive to changes in the pH of lamellar bodies.

Protein kinase C in intracellular pH regulation in alveolar type II cells

The Na+/H+ exchanger and Na(+)-HCO3- cotransporter have been implicated in regulation of intracellular pH (pHi) in alveolar type II cells. This study demonstrates that activation of protein kinase C (PKC) stimulates both of these ion transporters in type II cells. Treatment of type II cells with 80 nM phorbol 12-myristate 13-acetate (PMA) increased the resting pHi in a time-dependent manner. Compared with control cells, the rates of recovery from an acid load increased with PMA treatment, reaching a maximum at 15 min, and returned to control levels by 3 h. The PMA-stimulated changes in recovery rate were sensitive to H-7, a PKC inhibitor. For PMA treatment up to 2 h, these recoveries were also sensitive to dimethylamiloride (DMA), an inhibitor of Na+/H+ exchanger activity, and to HCO3-, suggesting activation of both the Na+/H+ exchanger and the Na(+)-HCO3- cotransporter. After prolonged (3 h) treatment with PMA, however, the recovery was insensitive to DMA but was sensitive to HCO3-, suggesting that the Na+/H+ exchanger was no longer active and that most of the recovery was mediated by the Na(+)-HCO3- cotransporter. PMA treatment also altered the Na+ kinetics of the recovery from an acid load with respect to the Michaelis constant (Km) and maximal ion flux (Vmax), suggesting protein modifications of each transporter. We suggest that PKC activation in type II cells results in acute and long-term changes in pHi regulatory mechanisms mediated by the Na+/H+ exchanger and by the Na(+)-HCO3- cotransporter.

The Immunophilin Ligand Rapamycin: A Probe for the Analysis of the Relationship of Apoptosis to the Cell Cycle

A common consequence of growth factor deprivation in actively proliferating cells is arrest in the G1 phase of the cell cycle and subsequent apoptosis (programmed cell death). We have utilized the potent cell cycle inhibitor rapamycin (Rapa) to define more precisely the point within G1 where apoptosis is initiated during IL-2 deprivation in murine CTLL cells. Our results indicate that apoptosis is not initiated at a single point within G1, but rather can be initiated in at least two distinct points of the cell cycle, early and late G1. In nonproliferating CTLL cells arrested with Rapa in late G1, IL-2 prevents the initiation of an apoptotic response. This suggests that an IL-2-mediated signal is necessary to prevent apoptosis when proliferation is blocked, whether the block to proliferation arrests cells in early or late G1.

Developmental and tissue-specific expression of human CD4 in transgenic rabbits

A major obstacle to understanding AIDS is the lack of a suitable small animal model for studying HIV-1 infection and the subsequent development of AIDS, and for testing diagnostic, therapeutic, and preventive modalities. Our goal is to produce a rabbit model for the study of AIDS. Here we report on the generation of transgenic rabbits that express the human CD4 (hCD4) gene. The transgene, which contains the coding region for hCD4 and approximately 23 kb of sequence upstream of the translation start site, was used previously to direct hcD4 expression on the surface of CD4+ T cells of transgenic mice (Gillespie et al., 1993: Mol Cell Biol 13:2952-2958). The hCD4 transgene was detected in five males and two females derived from the microinjection in five males and two females derived from the microinjection of 271 rabbit embryos. Both hCD4 RNA and protein were expressed in peripheral blood lymphocytes (PBLs) from all five males but neither of the females. Human CD4 was expressed on PBLs from F1 offspring of all founder males. T-cell subset analysis revealed that hCD4 expression was restricted to rabbit CD4 (rCD4) expressing lymphocytes; mature rCD4- rCD8+ lymphocytes did not express hCD4. In preliminary studies, PBLs from hCD4 transgenic rabbits produced greater amounts of HIV-1 p24 core protein following HIV-1 infection in vitro than HIV-1 p24 antigen in nontransgenic rabbit infected cultures. These results extend to rabbits our previous observation that this transgene contains the sequence elements required for high-level expression in the appropriate cells of transgenic mice. Furthermore, these and previous studies demonstrating that expression of hCD4 protein enhances HIV-1 infection of rabbit T cells in vitro, coupled with reports that normal, nontransgenic rabbits are susceptible to HIV-1 infection, suggests that the hCD4 transgenic rabbits described herein will have an increased susceptibility to HIV-1 infection. In vivo HIV-1 infection studies with these rabbits are under way.

Alpha 3 beta 1 and alpha 6 beta 1 integrins mediate laminin/merosin binding and function as costimulatory molecules for human thymocyte proliferation

Integrins comprise a superfamily of alpha beta heterodimers that serve as cell signaling as well as adhesion molecules. We demonstrate that the alpha 3 beta 1 and alpha 6 beta 1 integrins are laminin/merosin receptors expressed in human thymocytes. By reverse transcriptase-PCR analysis, we determined that the alpha 3A beta 1, but not the alpha 3B beta 1, cytoplasmic structural variant of alpha 3 beta 1 is expressed in thymocytes. In contrast, both alpha 6A beta 1 and alpha 6B beta 1 cytoplasmic structural variants of alpha 6 beta 1 are expressed. A small percentage (10 to 15%) of human thymocytes bind to immobilized laminin, and even fewer (3 to 5%) bind to merosin, the laminin isoform normally present in the thymus. This binding, however, can be increased to 39 to 41% after activation of thymocytes with Mn2+ (or PMA). Binding to either laminin or merosin is completely inhibited by anti-beta 1 mAb or by a mixture of anti-alpha 3 and anti-alpha 6 mAbs, indicating that both alpha 3 beta 1 and alpha 6 beta 1 participate in thymocyte adhesion to the laminin family of extracellular matrix proteins. The protein kinase C inhibitors, calphostin C and staurosporine, inhibit Mn(2+)-enhanced thymocyte binding, suggesting that protein kinase C activity is crucial for the binding. Furthermore, the data indicate that at least two divalent cation binding sites serve to regulate integrin binding activity. Finally, we show that both immobilized laminin and merosin have costimulatory function for anti-CD3-induced thymocyte proliferation, and both anti-alpha 3 and anti-alpha 6 mAbs can block this proliferative response. The cooperative function of alpha 3 beta 1 and alpha 6 beta 1 evidenced in the laminin/merosin binding and proliferation assays suggests that thymocyte-merosin interactions may play an important role in thymic T cell development.

Alpha 3 beta 1 and alpha 6 beta 1 integrins mediate laminin/merosin binding and function as costimulatory molecules for human thymocyte proliferation

Integrins comprise a superfamily of alpha beta heterodimers that serve as cell signaling as well as adhesion molecules. We demonstrate that the alpha 3 beta 1 and alpha 6 beta 1 integrins are laminin/merosin receptors expressed in human thymocytes. By reverse transcriptase-PCR analysis, we determined that the alpha 3A beta 1, but not the alpha 3B beta 1, cytoplasmic structural variant of alpha 3 beta 1 is expressed in thymocytes. In contrast, both alpha 6A beta 1 and alpha 6B beta 1 cytoplasmic structural variants of alpha 6 beta 1 are expressed. A small percentage (10 to 15%) of human thymocytes bind to immobilized laminin, and even fewer (3 to 5%) bind to merosin, the laminin isoform normally present in the thymus. This binding, however, can be increased to 39 to 41% after activation of thymocytes with Mn2+ (or PMA). Binding to either laminin or merosin is completely inhibited by anti-beta 1 mAb or by a mixture of anti-alpha 3 and anti-alpha 6 mAbs, indicating that both alpha 3 beta 1 and alpha 6 beta 1 participate in thymocyte adhesion to the laminin family of extracellular matrix proteins. The protein kinase C inhibitors, calphostin C and staurosporine, inhibit Mn(2+)-enhanced thymocyte binding, suggesting that protein kinase C activity is crucial for the binding. Furthermore, the data indicate that at least two divalent cation binding sites serve to regulate integrin binding activity. Finally, we show that both immobilized laminin and merosin have costimulatory function for anti-CD3-induced thymocyte proliferation, and both anti-alpha 3 and anti-alpha 6 mAbs can block this proliferative response. The cooperative function of alpha 3 beta 1 and alpha 6 beta 1 evidenced in the laminin/merosin binding and proliferation assays suggests that thymocyte-merosin interactions may play an important role in thymic T cell development.

Constitutive activation of integrin alpha 4 beta 1 defines a unique stage of human thymocyte development

Our understanding of thymocyte development and of the positive and negative selection events involved in shaping the repertoire of mature T lymphocytes has been greatly facilitated by the use of transgenic and gene knockout animals. Much less is known about the factors that control the homing and population of the thymus by T cell precursors and the subsequent migration of developing thymocytes through the thymic architecture. As the integrins represent a candidate group of cell surface receptors that may regulate thymocyte development, we have analyzed the expression and function of alpha 4 beta 1 and alpha 5 beta 1 on human thymocytes. A major portion of double positive (CD4+ CD8+) human thymocytes express alpha 4 beta 1 in a constitutively active form and adhere to fibronectin and vascular cell adhesion molecule 1. alpha 4 beta 1 expression is similar on adherent and nonadherent populations, thus, activity reflects the receptor state and not simple expression. The adherent cells are immature, expressing high levels of CD4/CD8 and low levels of CD3 and CD69. In contrast, nonadherent cells possess the phenotype of thymocytes after positive selection, expressing intermediate levels of CD4 and/or CD8 and high levels of CD3 and CD69. The adherent population fails to respond to activation with anti-CD3 and fibronectin, whereas nonadherents exhibit an alpha 5 beta 1-dependent proliferation. Differential regulation of alpha 4 beta 1 and alpha 5 beta 1 receptors may provide a mechanism controlling cellular traffic, differentiation, and positive selection of thymocytes.

Expression of merosin in the thymus and its interaction with thymocytes

Merosin is a 700 kDa multichain protein that has several properties in common with laminin, one of the major extracellular matrix proteins present in basement membranes. Both contain B1 and B2 light chains, but merosin has a distinct M-chain compared with the laminin A-chain. Merosin is present in the basement membrane of placenta, striated muscle, and peripheral nerve. Using the RT-PCR assay, we demonstrate that merosin, but not laminin, is also expressed in the thymus. A partial cDNA sequence of the mouse merosin M-chain was 88% identical to the human M-chain, and the deduced amino acid sequences were 95% identical. 18-24% of adult mouse thymocytes bound to human merosin in vitro. Mg2+, but not Ca2+, was essential for binding. Binding was inhibited by antibodies recognizing VLA alpha 6 or beta 1, suggesting that the VLA-6 integrin is a merosin receptor. An anti-beta 4 integrin subunit mAb failed to inhibit binding, suggesting that the alpha 6 beta 4 integrin was not involved. Thymocytes were fractionated into immature and mature populations based on their expression of the heat stable antigen, recognized by the J11d mAb. Virtually all thymocytes expressed VLA-6, but only immature thymocytes (J11d+) bound to meroson. PMA treatment did not significantly increase the binding of J11d+ thymocytes nor did induce binding in the mature J11d- population. In constrast, both splenic T cells and unseparated lymph node cells showed enhanced binding to merosin after PMA stimulation. The expression of merosin in the thymus and its selective interaction with immature thymocytes suggest that thymocyte-merosin interactions may play a role in T cell development.

Expression of merosin in the thymus and its interaction with thymocytes

Merosin is a 700 kDa multichain protein that has several properties in common with laminin, one of the major extracellular matrix proteins present in basement membranes. Both contain B1 and B2 light chains, but merosin has a distinct M-chain compared with the laminin A-chain. Merosin is present in the basement membrane of placenta, striated muscle, and peripheral nerve. Using the RT-PCR assay, we demonstrate that merosin, but not laminin, is also expressed in the thymus. A partial cDNA sequence of the mouse merosin M-chain was 88% identical to the human M-chain, and the deduced amino acid sequences were 95% identical. 18-24% of adult mouse thymocytes bound to human merosin in vitro. Mg2+, but not Ca2+, was essential for binding. Binding was inhibited by antibodies recognizing VLA alpha 6 or beta 1, suggesting that the VLA-6 integrin is a merosin receptor. An anti-beta 4 integrin subunit mAb failed to inhibit binding, suggesting that the alpha 6 beta 4 integrin was not involved. Thymocytes were fractionated into immature and mature populations based on their expression of the heat stable antigen, recognized by the J11d mAb. Virtually all thymocytes expressed VLA-6, but only immature thymocytes (J11d+) bound to meroson. PMA treatment did not significantly increase the binding of J11d+ thymocytes nor did induce binding in the mature J11d- population. In constrast, both splenic T cells and unseparated lymph node cells showed enhanced binding to merosin after PMA stimulation. The expression of merosin in the thymus and its selective interaction with immature thymocytes suggest that thymocyte-merosin interactions may play a role in T cell development.

Multiple changes in VLA protein glycosylation, expression, and function occur during mouse T cell ontogeny

VLA molecules (beta 1 integrins) are cell-surface alpha beta heterodimers that bind to extracellular matrix (ECM) proteins such as fibronectin, laminin, and collagen. We analyzed the expression, structure, and function of VLAs on mouse thymocytes, as a first step toward understanding their role in T cell development. Two major forms of beta 1 were detected, which differed in their extent of N-glycosylation and sialylation. No evidence for alternative splicing of beta 1 mRNA was obtained by PCR analyses. The larger (135 kDa, nonreduced) more basic beta 1 was the only form on nonmature (J11d+) adult thymocytes, with 135 kDa and larger beta 1-chains expressed on fetal thymocytes from day 14 of gestation through birth. The smaller (120 kDa, nonreduced), more acidic beta-chain was found exclusively on mature (J11d-) thymocytes and peripheral lymphocytes. VLA alpha-chain expression was also analyzed. Virtually all thymocytes were VLA-alpha 4+, -alpha 6+. VLA-alpha 5 was detected in both J11d+ and J11d- thymocytes by immunoprecipitation, but could not be analyzed in detail because of the lack of an appropriate mAb for flow cytometry. VLA-alpha 1 and -alpha 2 were immunoprecipitated only from cells within the J11d- population. Only J11d+ thymocytes (12 to 15%) bound to fibronectin (via VLA-4 and VLA-5) and binding to laminin (via VLA-6) was two to fourfold higher in J11d+ compared with J11d- thymocytes (60 to 80% vs 20 to 30%). The high percentage of J11d+ thymocytes adhering to laminin suggests that VLA-6 exists in an activated state on most thymocytes, in contrast to resting peripheral T cells. These data show that major changes in VLA glycosylation, expression, and ECM-binding capacity occur as thymocytes mature, supporting the hypothesis that VLA-ECM interactions play a role in T cell ontogeny.

Multiple changes in VLA protein glycosylation, expression, and function occur during mouse T cell ontogeny

VLA molecules (beta 1 integrins) are cell-surface alpha beta heterodimers that bind to extracellular matrix (ECM) proteins such as fibronectin, laminin, and collagen. We analyzed the expression, structure, and function of VLAs on mouse thymocytes, as a first step toward understanding their role in T cell development. Two major forms of beta 1 were detected, which differed in their extent of N-glycosylation and sialylation. No evidence for alternative splicing of beta 1 mRNA was obtained by PCR analyses. The larger (135 kDa, nonreduced) more basic beta 1 was the only form on nonmature (J11d+) adult thymocytes, with 135 kDa and larger beta 1-chains expressed on fetal thymocytes from day 14 of gestation through birth. The smaller (120 kDa, nonreduced), more acidic beta-chain was found exclusively on mature (J11d-) thymocytes and peripheral lymphocytes. VLA alpha-chain expression was also analyzed. Virtually all thymocytes were VLA-alpha 4+, -alpha 6+. VLA-alpha 5 was detected in both J11d+ and J11d- thymocytes by immunoprecipitation, but could not be analyzed in detail because of the lack of an appropriate mAb for flow cytometry. VLA-alpha 1 and -alpha 2 were immunoprecipitated only from cells within the J11d- population. Only J11d+ thymocytes (12 to 15%) bound to fibronectin (via VLA-4 and VLA-5) and binding to laminin (via VLA-6) was two to fourfold higher in J11d+ compared with J11d- thymocytes (60 to 80% vs 20 to 30%). The high percentage of J11d+ thymocytes adhering to laminin suggests that VLA-6 exists in an activated state on most thymocytes, in contrast to resting peripheral T cells. These data show that major changes in VLA glycosylation, expression, and ECM-binding capacity occur as thymocytes mature, supporting the hypothesis that VLA-ECM interactions play a role in T cell ontogeny.

Developmentally regulated expression of the beta 4 integrin on immature mouse thymocytes

Integrins are a superfamily of alpha beta heterodimers, most of which serve as cell surface receptors for extracellular matrix proteins. In this report, we demonstrate that the recently described alpha 6 beta 4 integrin, previously thought to be limited to epithelial cells and Schwann cells, is expressed on immature mouse thymocytes. The presence of alpha 6 beta 4 is controlled by regulation of beta 4 expression, because alpha 6 was expressed by virtually all cells examined, paired with the beta 1 integrin chain to form VLA-6. During fetal ontogeny, beta 4 was highly expressed by 35% of day-13 thymocytes, 75% of day-14 to -15 thymocytes, then rapidly declined to low levels by birth. In neonates and adults, beta 4 expression was highest on CD4- CD8- CD3- and TCR(+)-gamma delta subsets. Correlation of IL-2R, CD44 and beta 4 on CD4- CD8- thymocytes revealed maximal levels on the intermediate CD44- IL-2R+ subset. Most CD4- CD8+ TCR- thymocytes and a significant fraction of CD4+ CD8+ thymocytes were beta 4lo, whereas the most mature J11d- single positive thymocytes were beta-4. Overall, down-regulation of beta 4 was associated with up-regulation of CD4, CD8, and CD3 in the thymus. alpha 6 beta 4 was undetectable on fetal liver or bone marrow cells, lymphocytes from lymph node, spleen, or blood, and mitogen-activated splenic T cells cultured up to 10 wk with IL-2. The data suggest that alpha 6 beta 4 is up-regulated after pro-T cells enter the thymus and may have a thymus-specific function for T cells. The developmentally regulated pattern of expression and the prominence of alpha 6 beta 4 on day-13 to -16 fetal and adult CD4- CD8- CD3- thymocytes further suggest this unusual integrin may play a role in early T cell development, including stages before acquisition of the TCR.

Developmentally regulated expression of the beta 4 integrin on immature mouse thymocytes

Integrins are a superfamily of alpha beta heterodimers, most of which serve as cell surface receptors for extracellular matrix proteins. In this report, we demonstrate that the recently described alpha 6 beta 4 integrin, previously thought to be limited to epithelial cells and Schwann cells, is expressed on immature mouse thymocytes. The presence of alpha 6 beta 4 is controlled by regulation of beta 4 expression, because alpha 6 was expressed by virtually all cells examined, paired with the beta 1 integrin chain to form VLA-6. During fetal ontogeny, beta 4 was highly expressed by 35% of day-13 thymocytes, 75% of day-14 to -15 thymocytes, then rapidly declined to low levels by birth. In neonates and adults, beta 4 expression was highest on CD4- CD8- CD3- and TCR(+)-gamma delta subsets. Correlation of IL-2R, CD44 and beta 4 on CD4- CD8- thymocytes revealed maximal levels on the intermediate CD44- IL-2R+ subset. Most CD4- CD8+ TCR- thymocytes and a significant fraction of CD4+ CD8+ thymocytes were beta 4lo, whereas the most mature J11d- single positive thymocytes were beta-4. Overall, down-regulation of beta 4 was associated with up-regulation of CD4, CD8, and CD3 in the thymus. alpha 6 beta 4 was undetectable on fetal liver or bone marrow cells, lymphocytes from lymph node, spleen, or blood, and mitogen-activated splenic T cells cultured up to 10 wk with IL-2. The data suggest that alpha 6 beta 4 is up-regulated after pro-T cells enter the thymus and may have a thymus-specific function for T cells. The developmentally regulated pattern of expression and the prominence of alpha 6 beta 4 on day-13 to -16 fetal and adult CD4- CD8- CD3- thymocytes further suggest this unusual integrin may play a role in early T cell development, including stages before acquisition of the TCR.

Mechanisms of autoimmunity in the context of T-cell tolerance: insights from natural and transgenic animal model systems

There are a number of mechanisms which cooperate to produce and maintain T-cell tolerance. First, and perhaps most important, is the clonal deletion in the thymus of T cells with high affinity for self antigens. However, to ensure that a wide repertoire of T cells is available in the periphery to combat foreign antigens, the threshold of clonal deletion may be set low enough so that T cells whose TCR's have sub-threshold affinity for self antigens mature and migrate to the periphery. T cells which recognize self antigen-derived peptides not expressed or presented in the thymus will also fail to be deleted. For those self-reactive T cells which are not deleted in the thymus, other mechanisms may produce tolerance, including an undefined alteration of signalling pathways which produces clonal anergy, and lowering the avidity of the TCR for its ligand by downregulating coreceptor and accessory molecules. Active suppression of T-cell responses in another well-described phenomenon whose mechanism is undefined. From our observations with the model systems discussed here, we have observed three distinct mechanisms by which T-cell tolerance can be circumvented, allowing autoimmune phenomena to occur. These mechanisms may have relevance for different types of autoimmune diseases seen in humans. In gld mice, the autoimmune disease seems to be related to a global defect in T-cell differentiation and function, which allows for the expansion of autoimmune B cells. While we showed that clonal deletion of V beta-bearing T cells is appropriate in certain cases, aberrant lymphokine secretion by the abnormal T cells or disruption of immune system regulation are most probably responsible for allowing autoantibody production. While human lupus erythematosis shares much of the pathology of lpr and gld mice, there is no expansion of T cells with a similar phenotype in human lupus. There are environmental factors which must play a role in the development of human lupus, since the incidence of the disease does not follow an absolute genetic pattern. The escape from clonal deletion and subsequent reactivation of autoimmune T cells which we observed in V beta 8.1 TCR-transgenic mice can be a model for human autoimmune diseases such as multiple sclerosis and type I diabetes, in which T cells are directed against a specific autoantigen. According to this model, susceptibility loci for autoimmune disease such as the MHC would function by producing different repertoires of T cells which in some cases could gain autoreactivity following activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Origin and selection of peripheral CD4-CD8- T cells bearing alpha/beta T cell antigen receptors in autoimmune gld mice

We have analyzed the origin and development of unusual CD4-CD8- alpha/beta T cell receptor-positive peripheral T cells produced in large numbers by mice homozygous for the gld mutation (C3H-gld/gld). These mice may be an important model for investigating processes controlling T cell development. Bone marrow transfers demonstrated that the gld defect was intrinsic to bone marrow-derived cells. Clonal deletion of potentially autoreactive cells was observed in peripheral gld CD4-CD8-, CD4+CD8-, and CD4-CD8+ T cells, as well as mature thymocytes. This suggests that gld CD4-CD8- T cells have passed through the thymus in ontogeny and that gld autoimmunity does not result from a general defect in elimination of self-reactive thymocytes. These observations, combined with demethylation of the CD8 gene in the CD4-CD8- population, support prior expression of CD4 and/or CD8 in gld CD4-CD8- T cell ontogeny, perhaps at a CD4+CD8+ stage. Steroid sensitivity of gld thymocytes and CD4-CD8- T cells was normal. Therefore, we found no gross abnormalities in two major mechanisms of inducible cell death in the gld thymus, the clonal deletion process associated with tolerance and the steroid-inducible endogenous endonuclease thought to be involved in apoptosis of unselected thymocytes. The data suggest that if gld CD4-CD8- T cells arise via escape from normal elimination in the thymus, they must do so by a novel defect in thymic selection (perhaps related to aberrant positive signals) and/or are expanded by an extrathymic process which allows clonal deletion to occur.

Immune response to a molecularly defined internal image idiotope

A monoclonal anti-idiotope termed 87.92.6 mimics the neutralization/cell-attachment site of the reovirus type 3 hemagglutinin (HA3). The second complementarity determining regions of the VH and VL of 87.92.6 share sequence similarity with a determinant on the HA3. We have used synthetic peptides (termed VH, VL, and Reo peptides, respectively) to probe the immunologic significance of this sequence similarity. Antibodies specific for Reo peptide or VL peptide neutralized reovirus type 3 infectivity. Although Reo peptide was an effective immunogen by itself, free VL peptide or VH peptide were unable to elicit antibodies unless they were linked to each other (VH-VL peptide). Immunization with reo peptide, 87.92.6, or the HA3 elicited a specific lymphocyte proliferative response to VH peptide, indicating that VH peptide may bear an important TH determinant. As found previously for 87.92.6, VL peptide elicited a delayed-type hypersensitivity response specific for reovirus type 3. Reovirus type 3 specific cytolytic lymphocytes specifically lysed targets coated with VH-VL peptide, but not VH or VL peptide alone. These results suggest that immune cross-reactivity between an external Ag and an internal image antibody can be understood at the primary structural level. These observations may have important implications for understanding the development of autoantibodies, network interactions, and the regulation of immune responses.

Immune response to a molecularly defined internal image idiotope

A monoclonal anti-idiotope termed 87.92.6 mimics the neutralization/cell-attachment site of the reovirus type 3 hemagglutinin (HA3). The second complementarity determining regions of the VH and VL of 87.92.6 share sequence similarity with a determinant on the HA3. We have used synthetic peptides (termed VH, VL, and Reo peptides, respectively) to probe the immunologic significance of this sequence similarity. Antibodies specific for Reo peptide or VL peptide neutralized reovirus type 3 infectivity. Although Reo peptide was an effective immunogen by itself, free VL peptide or VH peptide were unable to elicit antibodies unless they were linked to each other (VH-VL peptide). Immunization with reo peptide, 87.92.6, or the HA3 elicited a specific lymphocyte proliferative response to VH peptide, indicating that VH peptide may bear an important TH determinant. As found previously for 87.92.6, VL peptide elicited a delayed-type hypersensitivity response specific for reovirus type 3. Reovirus type 3 specific cytolytic lymphocytes specifically lysed targets coated with VH-VL peptide, but not VH or VL peptide alone. These results suggest that immune cross-reactivity between an external Ag and an internal image antibody can be understood at the primary structural level. These observations may have important implications for understanding the development of autoantibodies, network interactions, and the regulation of immune responses.

Major histocompatibility complex class I-specific cytolytic T cells, derived from gld mice, lacking Thy-1, CD4, and CD8

Thy-1 is a major cell surface molecule expressed on murine thymocytes and peripheral T cells. Its physiological function is unknown, but in vitro studies suggest that Thy-1 may transmit activation signals to T cells and may play a role in the growth and/or differentiation of thymocytes [Kroczek, R. A., Gunter, K. C., Seligmann, B. & Shevach, E. M. (1986) J. Immunol. 136, 4379-4384; Kroczek, R. A., Gunter, K. C., Germain, R. N. & Shevach, E. M. (1986) Nature (London) 322, 181-184]. However, not all mouse thymocytes are Thy-1+ [Scollay, R., Wilson, A., D'Amico, A., Kelly, K., Egerton, M., Pearse, M., Wu, L. & Shortman, K. (1988) Immunol. Rev. 104, 81-120]. In addition, C3H-gld/gld mice accumulate large numbers of Thy-1- (and Thy-1+) T-cell antigen receptor-positive CD8- CD4- (double negative) T cells in peripheral lymphoid organs. Our previous studies of these Thy-1- and Thy-1+ double negatives suggested that lack of Thy-1 expression correlated with diminished capacity to respond to T-cell stimuli. In this report, we describe a Thy-1- alpha/beta T-cell receptor-positive major histocompatibility complex-specific cytotoxic T-cell clone derived from C3H-gld/gld lymph node-residing cells. The data show that, at least in this system, Thy-1 (and CD8/CD4) expression is not required for growth, cytolytic activity, or expression of functional T-cell receptor complexes in vitro and raise the possibility that Thy-1 expression may not be obligatory in vivo for development of cytotoxic T-lymphocyte precursors in gld mice.

The antigen-major histocompatibility complex-T cell receptor interaction. A structural analysis

In summary, we wish to propose that regions on MHC molecules interact with complementary regions on processed peptide antigens, and that the resultant Ag-MHC complex forms a conformation with separate functional regions that are able to interact with similarly complementary areas on T cell receptors. It is the product of these interactions that determines whether a given peptide Ag is capable of binding the MHC molecule, and whether a given Ag-MHC complex is capable of stimulating a particular T cell. As more becomes known about the molecular aspects of MHC-restricted, Ag-specific T cell activation, it will become clear which amino acid residues on the Ag, MHC, and TCR are involved in these interactions.

Characterization of Lyt-2-, L3T4- class I-specific cytolytic clones in C3H-gld/gld mice. Implications for functions of accessory molecules and programmed development

We report the first demonstration of Thy-1+, Lyt-2-, L3T4- MHC-specific CTL clones derived from the Lyt-2-, L3T4- subset of lymph node cells of C3H-gld/gld mice. These clones express alpha/beta heterodimeric TCRs on the cell surface and specifically recognize class I molecules on target cells. Lyt-2 and L3T4 molecules are therefore not essential for the induction, recognition, and killing of antigen-specific CTL. In addition, these studies suggest that antigen specificity development for class I structures may occur before Lyt-2 gene activation in the differentiation of T cells.

Anatomy of herpes simplex virus DNA. V. Terminally repetitive sequences

Native DNA from four strains of herpes simplex virus 1 (HSV-1) circularized after digestion with the lambda exonuclease, indicating that the molecules were terminally repetitious. In two strains, the terminal repetition was evident in nearly 50% of the DNA molecules. Maximal circularization was observed when only 0.25 to 0.5% of the DNA was depolymerized by the exonuclease, suggesting that the minimal size of the terminally repetitious regions is in the range of 400 to 800 bases pairs. More extensive exonuclease treatment resulted in a reduction in the frequency of circularization. To determine whether the terminally repetitive regions themselves contained self-annealing sequences that were precluding circularization of more extensively digested DNA, the terminal fragments from HinIII restriction endonuclease digests were isolated, denatured, and tested for their ability to self-anneal. The results of hydroxyapatite column chromatography and electron microscope examination of the terminal regions are consistent with this hypothesis.

Anatomy of herpes simplex virus DNA. II. Size, composition, and arrangement of inverted terminal repetitions

Electron microscope studies on self-annealed intact single strands and on partially denatured molecules show that herpes simplex virus 1 DNA consists of two unequal regions, each bounded by inverted redundant sequences. Thus the region L (70 percent of the contour length of the DNA) separates the left terminal region a1b from its inverted repeat b'a'1, each of which comprises 6 percent of the DNA. The region S (9.4 percent of DNA) separates the right terminal region cas (4.3 percent of the DNA) from its inverted repeat a'sc'. The regions of the two termini which are inverted and repeated itnernally differ in topology. Thus, cas is guanine plus cytosine rich, whereas only the terminal 1 percent of the a1b region, designated as subregion a1, is guanine plus cytosine rich.