Apoptosis-mediated ADAM10 activation removes a mucin barrier promoting T cell efferocytosis

Efferocytic clearance of apoptotic cells in general, and T cells in particular, is required for tissue and immune homeostasis. Transmembrane mucins are extended glycoproteins highly expressed in the cell glycocalyx that function as a barrier to phagocytosis. Whether and how mucins may be regulated during cell death to facilitate efferocytic corpse clearance is not well understood. Here we show that normal and transformed human T cells express a subset of mucins which are rapidly and selectively removed from the cell surface during apoptosis. This process is mediated by the ADAM10 sheddase, the activity of which is associated with XKR8-catalyzed flipping of phosphatidylserine to the outer leaflet of the plasma membrane. Mucin clearance enhances uptake of apoptotic T cells by macrophages, confirming mucins as an enzymatically-modulatable barrier to efferocytosis. Together these findings demonstrate a glycocalyx regulatory pathway with implications for therapeutic intervention in the clearance of normal and transformed apoptotic T cells.

Profound structural conservation of chemically cross-linked HIV-1 envelope glycoprotein experimental vaccine antigens

Chemical cross-linking is used to stabilize protein structures with additional benefits of pathogen and toxin inactivation for vaccine use, but its use has been restricted by the potential for local or global structural distortion. This is of particular importance when the protein in question requires a high degree of structural conservation for inducing a biological outcome such as the elicitation of antibodies to conformationally sensitive epitopes. The HIV-1 envelope glycoprotein (Env) trimer is metastable and shifts between different conformational states, complicating its use as a vaccine antigen. Here we have used the hetero-bifunctional zero-length reagent 1-Ethyl-3-(3-Dimethylaminopropyl)-Carbodiimide (EDC) to cross-link two soluble Env trimers, selected well-folded trimer species using antibody affinity, and transferred this process to good manufacturing practice (GMP) for experimental medicine use. Cross-linking enhanced trimer stability to biophysical and enzyme attack. Cryo-EM analysis revealed that cross-linking retained the overall structure with root-mean-square deviations (RMSDs) between unmodified and cross-linked Env trimers of 0.4-0.5 Å. Despite this negligible distortion of global trimer structure, we identified individual inter-subunit, intra-subunit, and intra-protomer cross-links. Antigenicity and immunogenicity of the trimers were selectively modified by cross-linking, with cross-linked ConS retaining bnAb binding more consistently than ConM. Thus, the EDC cross-linking process improves trimer stability whilst maintaining protein folding, and is readily transferred to GMP, consistent with the more general use of this approach in protein-based vaccine design.

Modular capsid decoration boosts adenovirus vaccine-induced humoral immunity against SARS-CoV-2

Adenovirus vector vaccines have been widely and successfully deployed in response to coronavirus disease 2019 (COVID-19). However, despite inducing potent T cell immunity, improvement of vaccine-specific antibody responses upon homologous boosting is modest compared with other technologies. Here, we describe a system enabling modular decoration of adenovirus capsid surfaces with antigens and demonstrate potent induction of humoral immunity against these displayed antigens. Ligand attachment via a covalent bond was achieved using a protein superglue, DogTag/DogCatcher (similar to SpyTag/SpyCatcher), in a rapid and spontaneous reaction requiring only co-incubation of ligand and vector components. DogTag was inserted into surface-exposed loops in the adenovirus hexon protein to allow attachment of DogCatcher-fused ligands on virus particles. Efficient coverage of the capsid surface was achieved using various ligands, with vector infectivity retained in each case. Capsid decoration shielded particles from vector neutralizing antibodies. In prime-boost regimens, adenovirus vectors decorated with the receptor-binding domain of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) spike induced >10-fold higher SARS-CoV-2 neutralization titers compared with an undecorated vector encoding spike. Importantly, decorated vectors achieved equivalent or superior T cell immunogenicity against encoded antigens compared with undecorated vectors. We propose capsid decoration using protein superglues as a novel strategy to improve efficacy and boostability of adenovirus-based vaccines and therapeutics.

An immunodominant NP105-113-B*07:02 cytotoxic T cell response controls viral replication and is associated with less severe COVID-19 disease

NP105-113-B*07:02-specific CD8+ T cell responses are considered among the most dominant in SARS-CoV-2-infected individuals. We found strong association of this response with mild disease. Analysis of NP105-113-B*07:02-specific T cell clones and single-cell sequencing were performed concurrently, with functional avidity and antiviral efficacy assessed using an in vitro SARS-CoV-2 infection system, and were correlated with T cell receptor usage, transcriptome signature and disease severity (acute n = 77, convalescent n = 52). We demonstrated a beneficial association of NP105-113-B*07:02-specific T cells in COVID-19 disease progression, linked with expansion of T cell precursors, high functional avidity and antiviral effector function. Broad immune memory pools were narrowed postinfection but NP105-113-B*07:02-specific T cells were maintained 6 months after infection with preserved antiviral efficacy to the SARS-CoV-2 Victoria strain, as well as Alpha, Beta, Gamma and Delta variants. Our data show that NP105-113-B*07:02-specific T cell responses associate with mild disease and high antiviral efficacy, pointing to inclusion for future vaccine design.

Human MAIT cells respond to and suppress HIV-1

Human MAIT cells sit at the interface between innate and adaptive immunity, are polyfunctional and are capable of killing pathogen infected cells via recognition of the Class IB molecule MR1. MAIT cells have recently been shown to possess an antiviral protective role in vivo and we therefore sought to explore this in relation to HIV-1 infection. There was marked activation of MAIT cells in vivo in HIV-1-infected individuals, which decreased following ART. Stimulation of THP1 monocytes with R5 tropic HIVBAL potently activated MAIT cells in vitro. This activation was dependent on IL-12 and IL-18 but was independent of the TCR. Upon activation, MAIT cells were able to upregulate granzyme B, IFNγ and HIV-1 restriction factors CCL3, 4, and 5. Restriction factors produced by MAIT cells inhibited HIV-1 infection of primary PBMCs and immortalized target cells in vitro. These data reveal MAIT cells to be an additional T cell population responding to HIV-1, with a potentially important role in controlling viral replication at mucosal sites.

Immunological and pathological outcomes of SARS-CoV-2 challenge following formalin-inactivated vaccine in ferrets and rhesus macaques

There is an urgent requirement for safe and effective vaccines to prevent COVID-19. A concern for the development of new viral vaccines is the potential to induce vaccine-enhanced disease (VED). This was reported in several preclinical studies with both SARS-CoV-1 and MERS vaccines but has not been reported with SARS-CoV-2 vaccines. We have used ferrets and rhesus macaques challenged with SARS-CoV-2 to assess the potential for VED in animals vaccinated with formaldehyde-inactivated SARS-CoV-2 (FIV) formulated with Alhydrogel, compared to a negative control vaccine. We showed no evidence of enhanced disease in ferrets or rhesus macaques given FIV except for mild transient enhanced disease seen 7 days after infection in ferrets. This increased lung pathology was observed at day 7 but was resolved by day 15. We also demonstrate that formaldehyde treatment of SARS-CoV-2 reduces exposure of the spike receptor binding domain providing a mechanistic explanation for suboptimal immunity.

Inclusion of cGAMP within virus-like particle vaccines enhances their immunogenicity

Cyclic GMP-AMP (cGAMP) is an immunostimulatory molecule produced by cGAS that activates STING. cGAMP is an adjuvant when administered alongside antigens. cGAMP is also incorporated into enveloped virus particles during budding. Here, we investigate whether inclusion of cGAMP within viral vaccine vectors enhances their immunogenicity. We immunise mice with virus-like particles (VLPs) containing HIV-1 Gag and the vesicular stomatitis virus envelope glycoprotein G (VSV-G). cGAMP loading of VLPs augments CD4 and CD8 T-cell responses. It also increases VLP- and VSV-G-specific antibody titres in a STING-dependent manner and enhances virus neutralisation, accompanied by increased numbers of T follicular helper cells. Vaccination with cGAMP-loaded VLPs containing haemagglutinin induces high titres of influenza A virus neutralising antibodies and confers protection upon virus challenge. This requires cGAMP inclusion within VLPs and is achieved at markedly reduced cGAMP doses. Similarly, cGAMP loading of VLPs containing the SARS-CoV-2 Spike protein enhances Spike-specific antibody titres. cGAMP-loaded VLPs are thus an attractive platform for vaccination.

The RNA sensor MDA5 detects SARS-CoV-2 infection

Human cells respond to infection by SARS-CoV-2, the virus that causes COVID-19, by producing cytokines including type I and III interferons (IFNs) and proinflammatory factors such as IL6 and TNF. IFNs can limit SARS-CoV-2 replication but cytokine imbalance contributes to severe COVID-19. We studied how cells detect SARS-CoV-2 infection. We report that the cytosolic RNA sensor MDA5 was required for type I and III IFN induction in the lung cancer cell line Calu-3 upon SARS-CoV-2 infection. Type I and III IFN induction further required MAVS and IRF3. In contrast, induction of IL6 and TNF was independent of the MDA5-MAVS-IRF3 axis in this setting. We further found that SARS-CoV-2 infection inhibited the ability of cells to respond to IFNs. In sum, we identified MDA5 as a cellular sensor for SARS-CoV-2 infection that induced type I and III IFNs.

SARS-CoV-2 RNA detected in blood products from patients with COVID-19 is not associated with infectious virus

Background: Laboratory diagnosis of SARS-CoV-2 infection (the cause of COVID-19) uses PCR to detect viral RNA (vRNA) in respiratory samples. SARS-CoV-2 RNA has also been detected in other sample types, but there is limited understanding of the clinical or laboratory significance of its detection in blood. Methods: We undertook a systematic literature review to assimilate the evidence for the frequency of vRNA in blood, and to identify associated clinical characteristics. We performed RT-PCR in serum samples from a UK clinical cohort of acute and convalescent COVID-19 cases (n=212), together with convalescent plasma samples collected by NHS Blood and Transplant (NHSBT) (n=462 additional samples). To determine whether PCR-positive blood samples could pose an infection risk, we attempted virus isolation from a subset of RNA-positive samples. Results: We identified 28 relevant studies, reporting SARS-CoV-2 RNA in 0-76% of blood samples; pooled estimate 10% (95%CI 5-18%). Among serum samples from our clinical cohort, 27/212 (12.7%) had SARS-CoV-2 RNA detected by RT-PCR. RNA detection occurred in samples up to day 20 post symptom onset, and was associated with more severe disease (multivariable odds ratio 7.5). Across all samples collected ≥28 days post symptom onset, 0/494 (0%, 95%CI 0-0.7%) had vRNA detected. Among our PCR-positive samples, cycle threshold (ct) values were high (range 33.5-44.8), suggesting low vRNA copy numbers. PCR-positive sera inoculated into cell culture did not produce any cytopathic effect or yield an increase in detectable SARS-CoV-2 RNA. There was a relationship between RT-PCR negativity and the presence of total SARS-CoV-2 antibody (p=0.02). Conclusions: vRNA was detectable at low viral loads in a minority of serum samples collected in acute infection, but was not associated with infectious SARS-CoV-2 (within the limitations of the assays used). This work helps to inform biosafety precautions for handling blood products from patients with current or previous COVID-19.

SARS-CoV-2 RNA detected in blood products from patients with COVID-19 is not associated with infectious virus

Background: Laboratory diagnosis of SARS-CoV-2 infection (the cause of COVID-19) uses PCR to detect viral RNA (vRNA) in respiratory samples. SARS-CoV-2 RNA has also been detected in other sample types, but there is limited understanding of the clinical or laboratory significance of its detection in blood. Methods: We undertook a systematic literature review to assimilate the evidence for the frequency of vRNA in blood, and to identify associated clinical characteristics. We performed RT-PCR in serum samples from a UK clinical cohort of acute and convalescent COVID-19 cases (n=212), together with convalescent plasma samples collected by NHS Blood and Transplant (NHSBT) (n=462 additional samples). To determine whether PCR-positive blood samples could pose an infection risk, we attempted virus isolation from a subset of RNA-positive samples. Results: We identified 28 relevant studies, reporting SARS-CoV-2 RNA in 0-76% of blood samples; pooled estimate 10% (95%CI 5-18%). Among serum samples from our clinical cohort, 27/212 (12.7%) had SARS-CoV-2 RNA detected by RT-PCR. RNA detection occurred in samples up to day 20 post symptom onset, and was associated with more severe disease (multivariable odds ratio 7.5). Across all samples collected ≥28 days post symptom onset, 0/494 (0%, 95%CI 0-0.7%) had vRNA detected. Among our PCR-positive samples, cycle threshold (ct) values were high (range 33.5-44.8), suggesting low vRNA copy numbers. PCR-positive sera inoculated into cell culture did not produce any cytopathic effect or yield an increase in detectable SARS-CoV-2 RNA. Conclusions: vRNA was detectable at low viral loads in a minority of serum samples collected in acute infection, but was not associated with infectious SARS-CoV-2 (within the limitations of the assays used). This work helps to inform biosafety precautions for handling blood products from patients with current or previous COVID-19.

Structural and immunologic correlates of chemically stabilized HIV-1 envelope glycoproteins

Inducing broad spectrum neutralizing antibodies against challenging pathogens such as HIV-1 is a major vaccine design goal, but may be hindered by conformational instability within viral envelope glycoproteins (Env). Chemical cross-linking is widely used for vaccine antigen stabilization, but how this process affects structure, antigenicity and immunogenicity is poorly understood and its use remains entirely empirical. We have solved the first cryo-EM structure of a cross-linked vaccine antigen. The 4.2 Å structure of HIV-1 BG505 SOSIP soluble recombinant Env in complex with a CD4 binding site-specific broadly neutralizing antibody (bNAb) Fab fragment reveals how cross-linking affects key properties of the trimer. We observed density corresponding to highly specific glutaraldehyde (GLA) cross-links between gp120 monomers at the trimer apex and between gp120 and gp41 at the trimer interface that had strikingly little impact on overall trimer conformation, but critically enhanced trimer stability and improved Env antigenicity. Cross-links were also observed within gp120 at sites associated with the N241/N289 glycan hole that locally modified trimer antigenicity. In immunogenicity studies, the neutralizing antibody response to cross-linked trimers showed modest but significantly greater breadth against a global panel of difficult-to-neutralize Tier-2 heterologous viruses. Moreover, the specificity of autologous Tier-2 neutralization was modified away from the N241/N289 glycan hole, implying a novel specificity. Finally, we have investigated for the first time T helper cell responses to next-generation soluble trimers, and report on vaccine-relevant immunodominant responses to epitopes within BG505 that are modified by cross-linking. Elucidation of the structural correlates of a cross-linked viral glycoprotein will allow more rational use of this methodology for vaccine design, and reveals a strategy with promise for eliciting neutralizing antibodies needed for an effective HIV-1 vaccine.

Astrocytes Resist HIV-1 Fusion but Engulf Infected Macrophage Material

HIV-1 disseminates to diverse tissues and establishes long-lived viral reservoirs. These reservoirs include the CNS, in which macrophage-lineage cells, and as suggested by many studies, astrocytes, may be infected. Here, we have investigated astrocyte infection by HIV-1. We confirm that astrocytes trap and internalize HIV-1 particles for subsequent release but find no evidence that these particles infect the cell. Astrocyte infection was not observed by cell-free or cell-to-cell routes using diverse approaches, including luciferase and GFP reporter viruses, fixed and live-cell fusion assays, multispectral flow cytometry, and super-resolution imaging. By contrast, we observed intimate interactions between HIV-1-infected macrophages and astrocytes leading to signals that might be mistaken for astrocyte infection using less stringent approaches. These results have implications for HIV-1 infection of the CNS, viral reservoir formation, and antiretroviral therapy.

Performance of Pediatric Risk of Mortality Score Among Critically Ill Children With Heart Disease

OBJECTIVE

To evaluate the performance of the Pediatric Risk of Mortality 3 (PRISM-3) score in critically ill children with heart disease.

METHODS

Patients <18 years of age admitted with cardiac diagnoses (cardiac medical and cardiac surgical) to one of the participating pediatric intensive care units in the Virtual Pediatric Systems, LLC, database were included. Performance of PRISM-3 was evaluated with discrimination and calibration measures among both cardiac surgical and cardiac medical patients.

RESULTS

The study population consisted of 87,993 patients, of which 49% were cardiac medical patients (n = 43,545) and 51% were cardiac surgical patients (n = 44,448). The ability of PRISM-3 to distinguish survivors from nonsurvivors was acceptable for the entire cohort (c-statistic 0.86). However, PRISM-3 did not perform as well when stratified by varied severity of illness categories. Pediatric Risk of Mortality 3 underpredicted mortality among patients with lower severity of illness categories (quintiles 1-4) whereas it overpredicted mortality among patients with greatest severity of illness category (fifth quintile). When stratified by Society of Thoracic Surgeons-European Association for Cardiothoracic Surgery (STS-EACTS) categories, PRISM-3 overpredicted mortality among the STS-EACTS mortality categories 1, 2, and 3 and underpredicted mortality among the STS-EACTS mortality categories 4 and 5. Pediatric Risk of Mortality 3 overpredicted mortality among centers with high cardiac surgery volume whereas it underpredicted mortality among centers with low cardiac surgery volume.

CONCLUSION

Data from this large multicenter study do not support the use of PRISM-3 in cardiac surgical or cardiac medical patients. In this study, the ability of PRISM-3 to distinguish survivors from nonsurvivors was fair at best, and the accuracy with which it predicted death was poor.

Macrophage infection via selective capture of HIV-1-infected CD4+ T cells

Macrophages contribute to HIV-1 pathogenesis by forming a viral reservoir and mediating neurological disorders. Cell-free HIV-1 infection of macrophages is inefficient, in part due to low plasma membrane expression of viral entry receptors. We find that macrophages selectively capture and engulf HIV-1-infected CD4⁺ T cells leading to efficient macrophage infection. Infected T cells, both healthy and dead or dying, were taken up through viral envelope glycoprotein-receptor-independent interactions, implying a mechanism distinct from conventional virological synapse formation. Macrophages infected by this cell-to-cell route were highly permissive for both CCR5-using macrophage-tropic and otherwise weakly macrophage-tropic transmitted/founder viruses but restrictive for nonmacrophage-tropic CXCR4-using virus. These results have implications for establishment of the macrophage reservoir and HIV-1 dissemination in vivo.

•

Macrophages selectively capture and engulf HIV-1-infected T cells

•

Uptake of HIV-1-infected T cells drives efficient macrophage infection

•

T cell capture is viral Env independent; macrophage infection is Env-receptor dependent

•

This represents a route for macrophage infection by transmitted/founder viruses

Relationship between risk-adjustment tools and the pediatric logistic organ dysfunction score

BACKGROUND

The Risk-Adjusted Classification for Congenital Heart Surgery (RACHS-1) method and Aristotle Basic Complexity (ABC) scores correlate with mortality. However, low mortality rates in congenital heart disease (CHD) make use of mortality as the primary outcome measure insufficient. Demonstrating correlation between risk-adjustment tools and the Pediatric Logistic Organ Dysfunction (PELOD) score might allow for risk-adjusted comparison of an outcome measure other than mortality.

METHODS

Data were obtained from the Virtual PICU Systems database. Patients with postoperative CHD between 2009 and 2010 were included. Correlation between RACHS-1 category and PELOD score and between ABC level and PELOD score was examined using Spearman rank correlation. Consistency of PELOD scores across institutions for given levels of case complexity was examined using Kruskal-Wallis nonparametric analysis of variance.

RESULTS

A total of 1,981 patient visits among 12 institutions met inclusion criteria. Positive correlations between PELOD score and RACHS-1 category (r s = .353, P < .0001) as well as between PELOD score and ABC level (r s = .328, P < .0001) were demonstrated. Variability in PELOD scores across individual centers for given levels of case complexity was observed (P < .04).

CONCLUSIONS

Risk-Adjusted Classification for Congenital Heart Surgery categories and ABC levels correlate with postoperative organ dysfunction as measured by PELOD. However, the correlation was weak, potentially due to limitations of the PELOD score itself. Identification of a more accurate metric of morbidity for the congenital heart disease population is needed.

High multiplicity HIV-1 cell-to-cell transmission from macrophages to CD4+ T cells limits antiretroviral efficacy

OBJECTIVE

Few studies have examined the efficacy of antiretroviral therapy (ART) in the context of cell-to-cell transmission. We aimed to determine whether the activity of ART is limited by the mode of HIV-1 spread between cells and the type of immune cell implicated in transmission, or is independent of these variables.

DESIGN

ART activity was evaluated in primary cells using in-vitro cell-free and cell to-cell HIV-1 infection systems.

METHODS

HIV-1 cell-free or cell-to-cell transmission between infected monocyte-derived macrophages (MDMs) and autologous target CD4+ T cells was measured in the presence or absence of reverse transcriptase and integrase inhibitors. Viral infection was evaluated using luciferase-reporter infectious molecular HIV-1 clones carrying macrophage-tropic envelope glycoproteins (Envs). Cell-free HIV-1 was titrated to yield different multiplicities of CD4+ T-cell infection.

RESULTS

Whereas cell-free infection of CD4+ T cells was substantially reduced by all inhibitors, cell-to-cell spread from macrophages to CD4+ T cells was largely resistant to inhibition. However, when multiplicity of infection was controlled for, we observed no difference in antiretroviral inhibition of cell-to-cell or cell-free infection.

CONCLUSION

Cell-to-cell spread of HIV-1 reduces the probability of antiretroviral inhibition, but it is the number of infectious viruses transferred between cells rather than the specific mode of viral spread or transmitting cell type that governs antiretroviral activity. High multiplicity infection in vivo is more likely to occur by cell-to-cell transmission, and these data will inform use of ART against viral reservoirs.

Bilateral pulmonary artery banding as rescue intervention in high-risk neonates

BACKGROUND

Presentation in shock and preoperative infection remain risk factors for neonatal cardiac surgery. This report describes bilateral pulmonary artery banding (bPAB) in ductal-dependent lesions with systemic outflow obstruction as rescue intervention before surgery with cardiopulmonary bypass in these high-risk neonates.

METHODS

A retrospective chart review was conducted for 10 patients who underwent bPAB before conventional surgery with cardiopulmonary bypass. Patient characteristics including birth weight, gestational age, cardiac and noncardiac diagnoses, preoperative and postoperative markers of organ function, and outcome measures were examined.

RESULTS

The majority of patients (8 of 10) were considered high-risk owing to multiorgan dysfunction syndrome. The median age at bPAB was 12 days (range, 5 to 26 days), and the median interval between bPAB and second surgery was 10.5 days (range, 5 to 79 days). Organ function improved after admission and continued to improve after bPAB in 9 of 10 patients. No patient experienced new complications between bPAB and subsequent operation. Of 8 patients who had stage I palliation, 5 have undergone or are awaiting completion Fontan, 1 underwent Kawashima procedure, 1 underwent orthotopic heart transplant, and 1 with hypoplastic left heart syndrome and intact atrial septum died at 44 days old. Both patients who underwent biventricular repair are alive and well. Median follow-up for survivors was 2.9 years (range, 0.25 to 6.25 years).

CONCLUSIONS

Bilateral pulmonary artery banding is safe in ductal-dependent lesions with systemic outflow obstruction. High-risk patients with preoperative organ dysfunction or infection can recover within a short period and become lower risk candidates for complex congenital heart surgery using cardiopulmonary bypass.

Discrepancies between medication orders and infusion pump programming in a paediatric intensive care unit

BACKGROUND

Errors and the incorrect use of medications are significant sources of risk and harm to children in US hospitals. The risk associated with medication infusions has led to recommendations for the adoption of technologies including computer order physician entry (CPOE) and 'smart' infusion pumps despite a paucity of evidence demonstrating the ability of these technologies to reduce harm to paediatric inpatients.

OBJECTIVE

To measure discrepancies between medication orders for infusions entered into a CPOE system and the medication being infused as measured by the programmed settings of the smart infusion pump within a paediatric intensive care unit.

METHODS

This study used a prospective, observational design in a 30-bed paediatric intensive care unit. Data were simultaneously collected from the medication orders in the CPOE system and the bedside smart infusion pumps by trained observers. Analysis consisted of a line-by-line comparison of order observation data with the pump observation data.

CONCLUSIONS

Of 296 observations of medication infusions and 231 observations of intravenous fluid infusions, the frequency of discrepancies between orders entered and pumps programming ranged from 24.3% for observed medications to 42.4% for observed fluids. Anti-infectives (100%), concentrated electrolytes (46.7%) and anticoagulants (46.2%) were associated with greatest discrepancy between orders and programmed doses.

Likely role of APOBEC3G-mediated G-to-A mutations in HIV-1 evolution and drug resistance

The role of APOBEC3 (A3) protein family members in inhibiting retrovirus infection and mobile element retrotransposition is well established. However, the evolutionary effects these restriction factors may have had on active retroviruses such as HIV-1 are less well understood. An HIV-1 variant that has been highly G-to-A mutated is unlikely to be transmitted due to accumulation of deleterious mutations. However, G-to-A mutated hA3G target sequences within which the mutations are the least deleterious are more likely to survive selection pressure. Thus, among hA3G targets in HIV-1, the ratio of nonsynonymous to synonymous changes will increase with virus generations, leaving a footprint of past activity. To study such footprints in HIV-1 evolution, we developed an in silico model based on calculated hA3G target probabilities derived from G-to-A mutation sequence contexts in the literature. We simulated G-to-A changes iteratively in independent sequential HIV-1 infections until a stop codon was introduced into any gene. In addition to our simulation results, we observed higher ratios of nonsynonymous to synonymous mutation at hA3G targets in extant HIV-1 genomes than in their putative ancestral genomes, compared to random controls, implying that moderate levels of A3G-mediated G-to-A mutation have been a factor in HIV-1 evolution. Results from in vitro passaging experiments of HIV-1 modified to be highly susceptible to hA3G mutagenesis verified our simulation accuracy. We also used our simulation to examine the possible role of A3G-induced mutations in the origin of drug resistance. We found that hA3G activity could have been responsible for only a small increase in mutations at known drug resistance sites and propose that concerns for increased resistance to other antiviral drugs should not prevent Vif from being considered a suitable target for development of new drugs.

The search for new drugs to battle HIV-1 infections is a continuing struggle. APOBEC3G proteins have been shown to deaminate C-residues in HIV-1 minus strand DNA during its synthesis, resulting in G-to-A mutations in the RNA genome. The HIV-1 Vif protein has evolved to counteract APOBEC3G and thereby escape these frequently deleterious mutations, making Vif an attractive target for new drugs. However, a partial block of Vif could result in an increased although low-level HIV-1 G-to-A mutation rate. Here we investigated APOBEC3G mutation footprints in HIV-1 evolution and the potential risk for known drug resistance from sublethal G-to-A mutations. Using computer simulations, the accuracies of which were verified by infection experiments, we detected evolutionary APOBEC3G mutation footprints in the HIV-1 genome. We predict that the risk that APOBEC3G-induced G-to-A mutations will cause drug resistance is very low. We therefore propose that concerns for increased resistance to other antiviral drugs should not prevent Vif from being considered a suitable target for development of new drugs.

APOBEC3G induces a hypermutation gradient: purifying selection at multiple steps during HIV-1 replication results in levels of G-to-A mutations that are high in DNA, intermediate in cellular viral RNA, and low in virion RNA

Background

Naturally occurring Vif variants that are unable to inhibit the host restriction factor APOBEC3G (A3G) have been isolated from infected individuals. A3G can potentially induce G-to-A hypermutation in these viruses, and hypermutation could contribute to genetic variation in HIV-1 populations through recombination between hypermutant and wild-type genomes. Thus, hypermutation could contribute to the generation of immune escape and drug resistant variants, but the genetic contribution of hypermutation to the viral evolutionary potential is poorly understood. In addition, the mechanisms by which these viruses persist in the host despite the presence of A3G remain unknown.

Results

To address these questions, we generated a replication-competent HIV-1 Vif mutant in which the A3G-binding residues of Vif, Y⁴⁰RHHY⁴⁴, were substituted with five alanines. As expected, the mutant was severely defective in an A3G-expressing T cell line and exhibited a significant delay in replication kinetics. Analysis of viral DNA showed the expected high level of G-to-A hypermutation; however, we found substantially reduced levels of G-to-A hypermutation in intracellular viral RNA (cRNA), and the levels of G-to-A mutations in virion RNA (vRNA) were even further reduced. The frequencies of hypermutation in DNA, cRNA, and vRNA were 0.73%, 0.12%, and 0.05% of the nucleotides sequenced, indicating a gradient of hypermutation. Additionally, genomes containing start codon mutations and early termination codons within gag were isolated from the vRNA.

Conclusion

These results suggest that sublethal levels of hypermutation coupled with purifying selection at multiple steps during the early phase of viral replication lead to the packaging of largely unmutated genomes, providing a mechanism by which mutant Vif variants can persist in infected individuals. The persistence of genomes containing mutated gag genes despite this selection pressure indicates that dual infection and complementation can result in the packaging of hypermutated genomes which, through recombination with wild-type genomes, could increase viral genetic variation and contribute to evolution.

Species-specific inhibition of APOBEC3C by the prototype foamy virus protein bet

The APOBEC3 cytidine deaminases are part of the intrinsic defense of cells against retroviruses. Lentiviruses and spumaviruses have evolved essential accessory proteins, Vif and Bet, respectively, which counteract the APOBEC3 proteins. We show here that Bet of the Prototype foamy virus inhibits the antiviral APOBEC3C activity by a mechanism distinct to Vif: Bet forms a complex with APOBEC3C without inducing its degradation. Bet abolished APOBEC3C dimerization as shown by coimmunoprecipitation and cross-linking experiments. These findings implicate a physical interaction between Bet and the APOBEC3C. Subsequently, we identified the Bet interaction domain in human APOBEC3C in the predicted APOBEC3C dimerization site. Taken together, these data support the hypothesis that Bet inhibits incorporation of APOBEC3Cs into retroviral particles. Bet likely achieves this by trapping APOBEC3C protein in complexes rendering them unavailable for newly generated viruses due to direct immobilization.

Identification of two distinct human immunodeficiency virus type 1 Vif determinants critical for interactions with human APOBEC3G and APOBEC3F

Human cytidine deaminases APOBEC3G (A3G) and APOBEC3F (A3F) inhibit replication of Vif-deficient human immunodeficiency virus type 1 (HIV-1). HIV-1 Vif overcomes these host restriction factors by binding to them and inducing their proteasomal degradation. The Vif-A3G and Vif-A3F interactions are attractive targets for antiviral drug development because inhibiting the interactions could allow the host defense mechanism to control HIV-1 replication. It was recently reported that the Vif amino acids D(14)RMR(17) are important for functional interaction and degradation of the previously identified Vif-resistant mutant of A3G (D128K-A3G). However, the Vif determinants important for functional interaction with A3G and A3F have not been fully characterized. To identify these determinants, we performed an extensive mutational analysis of HIV-1 Vif. Our analysis revealed two distinct Vif determinants, amino acids Y(40)RHHY(44) and D(14)RMR(17), which are essential for binding to A3G and A3F, respectively. Interestingly, mutation of the A3G-binding region increased Vif's ability to suppress A3F. Vif binding to D128K-A3G was also dependent on the Y(40)RHHY(44) region but not the D(14)RMR(17) region. Consistent with previous observations, subsequent neutralization of the D128K-A3G antiviral activity required substitution of Vif determinant D(14)RMR(17) with SEMQ, similar to the SERQ amino acids in simian immunodeficiency virus SIV(AGM) Vif, which is capable of neutralizing D128K-A3G. These studies are the first to clearly identify two distinct regions of Vif that are critical for independent interactions with A3G and A3F. Pharmacological interference with the Vif-A3G or Vif-A3F interactions could result in potent inhibition of HIV-1 replication by the APOBEC3 proteins.

The Akoya pearl oyster shell as an archival monitor of lead exposure

The Akoya pearl oyster (Pinctada imbricata) was experimentally exposed to (a) constant levels of lead (Pb) at 180 microg L(-1) for nine weeks, or (b) two short term (pulse) exposures of Pb at 180 microg L(-1) (three weeks each) with an intervening depuration period (three weeks), to assess its utility as an (i) accumulative monitor of Pb contamination and an (ii) archival monitor for discriminating constant versus pulsed Pb exposure events. P. imbricata showed similar reductions in growth (based on shell morphology and wet weight) and Pb accumulation patterns for whole tissue and shell in response to both Pb exposure regimes. Thus the whole oyster was deemed an inappropriate accumulative monitor for assessing short-term temporal variation of Pb exposure and effect. However, using secondary ion mass spectrometry, Pb was shown to accumulate in the successively deposited nacreous layers of the shell of P. imbricata, documenting the exposure history of constant versus pulsed Pb events. Patterns of Pb deposition not only reflected the frequency of Pb exposure events but also their relative durations. Thus, the shell of P. imbricata may be employed as a suitable biological archive of Pb exposure.

Stable inhibition of hepatitis B virus proteins by small interfering RNA expressed from viral vectors

BACKGROUND

There has been much research into the use of RNA interference (RNAi) for the treatment of human diseases. Many viruses, including hepatitis B virus (HBV), are susceptible to inhibition by this mechanism. However, for RNAi to be effective therapeutically, a suitable delivery system is required.

METHODS

Here we identify an RNAi sequence active against the HBV surface antigen (HBsAg), and demonstrate its expression from a polymerase III expression cassette. The expression cassette was inserted into two different vector systems, based on either prototype foamy virus (PFV) or adeno-associated virus (AAV), both of which are non-pathogenic and capable of integration into cellular DNA. The vectors containing the HBV-targeted RNAi molecule were introduced into 293T.HBs cells, a cell line stably expressing HBsAg. The vectors were also assessed in HepG2.2.15 cells, which secrete infectious HBV virions.

RESULTS

Seven days post-transduction, a knockdown of HBsAg by approximately 90%, compared with controls, was detected in 293T.HBs cells transduced by shRNA encoding PFV and AAV vectors. This reduction has been observed up to 5 months post-transduction in single cell clones. Both vectors successfully inhibited HBsAg expression from HepG2.2.15 cells even in the presence of HBV replication. RT-PCR of RNA extracted from these cells showed a reduction in the level of HBV pre-genomic RNA, an essential replication intermediate and messenger RNA for HBV core and polymerase proteins, as well as the HBsAg messenger RNA.

CONCLUSIONS

This work is the first to demonstrate that delivery of RNAi by viral vectors has therapeutic potential for chronic HBV infection and establishes the ground work for the use of such vectors in vivo.

Foamy virus Bet proteins function as novel inhibitors of the APOBEC3 family of innate antiretroviral defense factors

Foamy viruses are a family of complex retroviruses that establish common, productive infections in a wide range of nonhuman primates. In contrast, humans appear nonpermissive for foamy virus replication, although zoonotic infections do occur. Here we have analyzed the ability of primate and mouse APOBEC3G proteins to inhibit the infectivity of primate foamy virus (PFV) virions produced in their presence. We demonstrate that several APOBEC3 proteins can potently inhibit the infectivity of a PFV-based viral vector. This inhibition correlated with the packaging of inhibitory APOBEC3 proteins into PFV virions, due to a specific PFV Gag/APOBEC3 interaction, and resulted in the G to A hypermutation of PFV reverse transcripts. While inhibition of PFV virion infectivity by primate APOBEC3 proteins was largely relieved by coexpression of the PFV Bet protein, a cytoplasmic auxiliary protein of previously uncertain function, Bet failed to relieve inhibition caused by murine APOBEC3. PFV Bet bound to human, but not mouse, APOBEC3 proteins in coexpressing cells, and this binding correlated with the specific inhibition of their incorporation into PFV virions. Of note, both PFV Bet and a second Bet protein, derived from an African green monkey foamy virus, rescued the infectivity of Vif-deficient human immunodeficiency virus type 1 (HIV-1) virions produced in the presence of African green monkey APOBEC3G and blocked the incorporation of this host factor into HIV-1 virion particles. However, neither foamy virus Bet protein reduced APOBEC3 protein expression levels in virion producer cells. While these data identify the foamy virus Bet protein as a functional ortholog of the HIV-1 Vif auxiliary protein, they also indicate that Vif and Bet block APOBEC3 protein function by distinct mechanisms.

Transient foamy virus vector production by adenovirus vectors

The genome of the prototype foamy virus (PFV) has been introduced into an adenoviral/PFV hybrid vector and tested for stable in vitro gene transfer. Three different adenoviruses are used to encode: (i) the PFV structural genes gag and pol (Ad-GagPolDeltaPacI); (ii) the PFV structural gene env (Ad-Env); and (iii) the PFV vector genome (Ad-MD9) encoding the transgene (the enhanced green fluorescent protein (eGFP) gene). Following cotransduction by the three adenoviruses, the target cells become transient PFV vector-producing cells, resulting in the in situ release of recombinant PFV at a titre of up to 10(3) vector particles/ml, which can then infect surrounding cells, leading to stable integration of the expression cassette. Stable eGFP expression, observed for up to 60 days (11 passages) in cells transduced with all three adenoviral vectors, was shown by PCR to be the result of PFV integration. In contrast, cells transduced with only the adenovirus encoding the PFV vector genome showed a marked decrease in eGFP expression by passage 2 (16 days post-transduction) and did not contain integrated PFV vector. In short, this paper describes the production of a hybrid vector capable of high in vitro transduction and stable transgene expression using adenovirus and PFV vectors.

Human foamy virus integrase fails to catalyse the integration of a circular DNA molecule containing an LTR junction sequence

The presence of closed circular forms of the linear DNA genome of human foamy virus (HFV) has not been established. The ability of the HFV integrase (IN) to catalyse the integration of these circular forms (termed 2 long terminal repeat (LTR) circles) was investigated, with a view to producing a novel hybrid vector. To this end, a construct was made containing, in addition to the enhanced green fluorescent protein (eGFP) marker gene, the last 27 bp of the 3' U5 LTR region of HFV fused to the first 28 bp of the 5' U3 LTR, the latter representing a 2LTR circle. Marker gene expression following transfection of both 293 and 293T cells indicated that the level of integration was not significantly increased by the HFV IN. Moreover, correctly integrated provirus-like forms of the input plasmid could not be detected by PCR. Taken together, these results show that the HFV IN is not able to integrate a circular molecule containing an LTR junction and, hence, the technique is not exploitable as a tool to produce hybrid vectors for gene therapy.

Incorporation of a molecular hinge into molecular tweezers by using tandem cycloadditions onto 5,6-dimethylenenorbornene

Site-selective 1,3-dipolar coupling at the norbornene pi-bond of 5,6-dimethylenenorbornene 1 yields cycloadducts with an end-fused 1,3-diene system which have been reacted with N=N (or C=C) dienophiles to produce ribbon molecules, in which the internal diazacyclohexene (or cyclohexene) subunits are capable of acting as conformational hinges. Direct coupling of 5,6-dimethylenenorbornene with 1,3,4-oxadiazoles or dual coupling with bis(cyclobutene epoxides) afforded bis(1,3-dienes) that diastereoselectively react with dienophiles to produce new, conformationally mobile, molecular tweezers.

The R region found in the human foamy virus long terminal repeat is critical for both Gag and Pol protein expression

It has been suggested that sequences located within the 5' noncoding region of human foamy virus (HFV) are critical for expression of the viral Gag and Pol structural proteins. Here, we identify a discrete approximately 151-nucleotide sequence, located within the R region of the HFV long terminal repeat, that activates HFV Gag and Pol expression when present in the 5' noncoding region but that is inactive when inverted or when placed in the 3' noncoding region. Sequences that are critical for the expression of both Gag and Pol include not only the 5' splice site positioned at +51 in the R region, which is used to generate the spliced pol mRNA, but also intronic R sequences located well 3' to this splice site. Analysis of total cellular gag and pol mRNA expression demonstrates that deletion of the R region has little effect on gag mRNA levels but that R deletions that would be predicted to leave the pol 5' splice site intact nevertheless inhibit the production of the spliced pol mRNA. Gag expression can be largely rescued by the introduction of an intron into the 5' noncoding sequence in place of the R region but not by an intron or any one of several distinct retroviral nuclear RNA export sequences inserted into the mRNA 3' noncoding sequence. Neither the R element nor the introduced 5' intron markedly affects the cytoplasmic level of HFV gag mRNA. The poor translational utilization of these cytoplasmic mRNAs when the R region is not present in cis also extended to a cat indicator gene linked to an internal ribosome entry site introduced into the 3' noncoding region. Together these data imply that the HFV R region acts in the nucleus to modify the cytoplasmic fate of target HFV mRNA. The close similarity between the role of the HFV R region revealed in this study and previous data (M. Butsch, S. Hull, Y. Wang, T. M. Roberts, and K. Boris-Lawrie, J. Virol. 73:4847--4855, 1999) demonstrating a critical role for the R region in activating gene expression in the unrelated retrovirus spleen necrosis virus suggests that several distinct retrovirus families may utilize a common yet novel mechanism for the posttranscriptional activation of viral structural protein expression.

Palindromic sequence plays a critical role in human foamy virus dimerization

The retroviral RNA genome is dimeric, consisting of two identical strands of RNA linked near their 5' ends by a dimer linkage structure. Previously it was shown that human foamy virus (HFV) RNA transcribed in vitro contained three sites, designated SI, SII, and SIII, which contributed to the dimerization process (O. Erlwein, D. Cain, N. Fischer, A. Rethwilm, and M. O. McClure, Virology 229:251-258, 1997). To characterize these sites further, a series of mutants were designed and tested for their ability to dimerize in vitro. The primer binding site and a G tetrad in SI were dispensable for dimerization. However, a mutant that changed the 3' end of SI migrated slower on nondenaturing gels than wild-type RNA dimers. The sequence composition of the SII palindrome, consisting of 10 nucleotides, proved to be critical for in vitro dimerization, since mutations within this sequence or replacement of the sequence with a different palindrome of equal length impaired in vitro dimerization. The length of the palindrome also seems to play an important role. A moderate extension to 12 nucleotides was tolerated, whereas an extension to 16 nucleotides or more impaired dimerization. When nucleotides flanking the palindrome were mutated in a random fashion, dimerization was unaffected. Changing the SIII sequence also led to decreased dimer formation, confirming its contribution to the dimerization process. Interesting mutants were cloned into the infectious molecular clone of HFV, HSRV-2, and were transfected into BHK-21 cells. Mutations in SII that reduced dimerization in vitro also abolished virus replication. In contrast, constructs containing mutations in SI and SIII replicated to some extent in cell culture after an initial drop in viral replication. Analysis of the SIM1 mutant revealed reversion to the wild type but with the insertion of an additional two nucleotides. Analysis of cell-free virions demonstrated that both replication-competent and replication-defective mutants packaged nucleic acid. Thus, efficient dimerization is a critical step for HFV to generate infectious virus, but HFV RNA dimerization is not a prerequisite for packaging.

Sudden death due to metronidazole/ethanol interaction

Metronidazole (Flagyl), a commonly prescribed antimicrobial agent, can produce a reaction similar to that of disulfiram (Antabuse) when administered to patients drinking ethanol. This drug/chemical interaction results in accumulation of acetaldehyde in the blood. Acetaldehyde is hepatotoxic, cardiotoxic, and arrythmogenic; no lethal serum acetaldehyde level has been established. Sudden death has been reported in patients taking disulfiram while using ethanol; no fatalities have been reported due to ethanol/ metronidazole interactions. Described is a case of a 31-year-old woman who died moments after an assault by a male companion, during which he inflicted minor physical trauma to her upper arm. Toxicologic analysis yielded elevated concentrations of serum ethanol (162 mg/d), acetaldehyde (4.6 mg/d), and metronidazole (0.42 mg/L). The cause of death was reported to be cardiac dysrhythmia due to acetaldehyde toxicity due to an ethanol/ metronidazole interaction. Autonomic stress associated with the assault is likely to have contributed to this woman's death. The mechanism of death is examined.

Maleimide-Fused Cyclobutadienes and Dewar Furans: Trapping With Dienes as a Route to Propellane Photosubstrates With Potential for Oxiren Production

N-Methylmaleimide-fused cyclobutadiene (15) and its 1,2-dimethyl derivative (41) have been generated by zinc/titanium tetrachloride treatment of the vicinal dibromide (39) and (24) respectively. Prior epoxidations of (24) and (39) produce mixtures of dibromo epoxides which yield the related N-methylmaleimide-fused Dewar furans (16) and (17) upon similar debromination (Zn/TiCl4). None of these reactive intermediates is sufficiently stable for direct observation, but each was trapped in adduct form by Diels-Alder reaction with furan. Attempts to form fused Dewar furan (17) by flash vacuum pyrolysis of the furan adduct (34) led to the production of the isomeric furo [3,4-c] pyrrole (43), the structure of which was confirmed by separate synthesis. Stereochemical assignments have been made to most adducts, and AM1 calculations have been used to predict the stereoselectivities and facial selectivities in these cycloaddition reactions. Reaction of these Dewar furans with cyclopentadienones produced propellanes which were decarbonylated to their respective cyclohexadienes and used as photosubstrates for the generation of oxiren or dimethyloxiren . Attempts to trap oxiren, generated by photolysis of propellane (12), with isobenzofuran were unsuccessful.

A 19F NMR study of the interaction of 3-fluoro-4-demethoxydaunomycin with the hexanucleotide d(TCCGGA)2

19F NMR spectroscopy has been used to study the binding of the fluorinated anthracycline 3-fluoro-4-demethoxydaunomycin with the hexanucleotide d(TCCGGA)2. In the spectrum of the 1:1 anthracycline-d(TCCGGA)2 complex four resonances of approximately equal intensity were observed. This indicated that 4-demethoxydaunomycin intercalated at all possible sites with similar affinity. This suggests that the specific high affinity binding sites that are observed in anthracycline-DNA footprinting experiments are strongly regulated by the local DNA conformation.

The Photochemical Generation of Isoindene and Its Dimerization to Diastereomeric 1-(Indian-1-yl)indenes

Isoindene (1), generated by ultraviolet irradiation ( Vycor filter, acetone solution) of 1,4- dihydro-1,4-methanonaphthalene-2,3-dione (14), is shown to undergo homodimerization at -60°. This cycloaddition involves the intermolecular transfer of a hydrogen atom from one isoindene molecule to a second isoindene molecule in an [(8π+2σ)+ Sπ ] pericyclic process. The gross structure of the resulting diastereomeric dimers was confirmed by synthesis from 1-bromoindan (25) and indenyl anion (26). The stereochemistry of the dimers was determined by comparing their dihydro derivatives (33) and (34) with authentic samples. The authentic samples were themselves prepared by cis hydrogenation (Pd/C) of (E)-2,2′,3,3′-tetrahydro-1,1′-bi-1H-indenylidene (30) which yielded (34), and trans hydrogenation (Na/NH3) of (30) which formed (33) together with (34) (ratio 7:3). The (E) stereochemistry of (30), which was a critical feature in the stereochemical assignments to (33) and (34), was confirmed by n.O.e. studies on it and its (D4)derivative (32). Complete analysis of the proton, proton coupling was conducted on (32). This identified a 5J coupling between protons C 3-H and C 7-H which contributed a 6% scalar coupling to the observed n.O.e . of 39% at C 7-H. This information allowed unambiguous stereochemical assignments to be made to (33). The chirality of 1,1′-bi-1H-indenyls (43) and (44), prepared by the radical coupling of 1H-inden-1-ylmagnesium bromide, was also determined by a similar hydrogenation process.

The Photochemistry of Isobenzofuran. I. Structure of the Dimers Resulting From Ultraviolet Irradiation of Isobenzofuran in Acetone and Ether Solution

Irradiation of isobenzofuran (1) in acetone solution yields a symmetrical [π8s+π8s] dimer (24) involving bonding at the peri -position of the furan moiety in each molecule. The anti-stereochemistry of this dimer was established by a novel application of lanthanide induced shift spectroscopy. In contrast, irradiation of isobenzofuran (1) in ether solution yielded the unsymmetrical dimer (34) as the major product, together with lesser amounts of the symmetrical dimer (24), and small amounts of a new dimer (50) resulting from [π8s+π4s] cycloaddition , where the 8π system of the isobenzofuran of one molecule reacts with the carbocyclic 4π diene of the other. The structure of the unsymmetrical dimer (34) was confirmed by synthesis. No evidence for the Dewar form (7e) of isobenzofuran could be obtained in these reactions conducted at -60° and monitored by 1H n.m.r. spectroscopy. However, the small, but persistent, production of o-phthalaldehyde may implicate an intermediate derived from (7e).

Reaction of Dienophiles With a 5,6-Bismethylenenorborn-2-en-7-one: the Synthesis of a Stable Norborna-2,5-dien-7-one

Reaction of hemicyclone (10) with (E)-1,4-dibromobut-2-ene (9) under forcing conditions yielded a 1:1 adduct (11) which served as the precursor for the 5,6-bismethylenenorborn-2-en-7-one (12) through fluoride-promoted dehydrobromination. The conjugated diene group in (12) enters into Diels-Alder reactions with reactive dienophiles to produce intermediate norbornadienones en route to aromatic products. It has been possible to identify the norborna-2,5-dien-7-one (16) resulting from the reaction of (12) with N- tolyltriazoline dione (15), thereby offering authentic evidence for the rare norborna-2,5-dien-7-one ring system. In addition, (12) has been used as a reagent to evaluate facial selectivity in its reaction with the transient Dewar benzene (24). The cyclobutamaleimide moiety in (24) is screened on each face, yet facial discrimination in its reaction with (12) is high (7: 1). Thermolysis of the 5,6-bismethylenenorborn-2-en-7-one (12) yields the substituted benzocyclobutene (14), a reaction which may involve the o-xylylene (13) as intermediate.

The Adducts of Isobenzofuran and cis-3,4-Dichlorocyclobutene, and Their Conversion Into Formal Dewar Furan Adducts

cis-3,4-Dichlorocyclobutene (19) reacts with isobenzofuran (4) to produce three of four possible [ π;45+π 28] cycloaddition products. The isomer (27) is the dominant cycloadduct , which indicates that significant stabilization by the chlorine atoms with isobenzofuran occurs in the transition state leading to this product. Both adducts (26) and (27) are dechlorinated to form the exo cyclobutene (31), and the other adduct, the isomer (28), yields the endo cyclobutene (33); these products correspond to the formal adducts of cyclobutadiene with isobenzofuran . Separate reactions of cyclobutenes (31) and (33) with 3-chloroperbenzoic acid produce a single anti epoxide in each case, (32) and (34), respectively; these products are identical with the pair of isobenzofuran adducts derived from Dewar furan itself, as reported elsewhere.

Acetylene Stacking: a New Concept for the Synthesis of Cyclic Polyenes. Application to the Preparation of a 1,6-Bridged Bicyclo[4.2.0]octa-2,4,7-triene and Some Novel Oxepins

A new protocol for a general synthesis of cyclic polyenes is presented; in this protocol two new concepts are introduced. The first, acetylene stacking, is the generic term covering both the synthesis of a target molecule and the specific retrosynthesis upon which it is based. This retrosynthetic analysis involves conversion of the target molecule, through a series of transannular [ π2s+π2s] valence isomerizations , into a key polycyclobutanoid intermediate (24). This intermediate is cleaved, in turn, through a similar number of [ σ2s+ σ2s] electrocyclic fragmentations, to yield a set of acetylene synthons . The synthetic phase is based on the recombination of the corresponding acetylene synthetic equivalents in a stacking mode which leads to the target molecule by way of the same polycyclobutanoid intermediate (24). To achieve the ordered assembly of the acteylene synthons, in the required stacking mode, it has been necessary to develop the second concept, namely transfer technology. This involves the use of special transfer reagents, to act as synthetic equivalents and provide the molecular equivalent of acetylenes without ever producing acetylenes per se. Similar transfer reagents are described for cyclobutadienes which serves to demonstrate the wider implications of this new concept in organic synthesis. The bridged bicyclic polyene 8-oxatricyclo [4.3.2.01,6]undeca-2,4,10-triene (73), and the heterocycle 2,4,5,7,8-pentamethyl-1H-oxepino[4,5-c]pyrrole-1,3(2H)- dione (91) are used as the target molecules to illustrate this new synthesis protocol involving sets of stacked acetylenes (or cyclobutadienes ) as the synthetic base. The conversion of the oxepin (91) into isomeric oxepins, and the role of their tautomeric benzene oxides, present as equilibrium partners, is discussed.

The Preparation of Dimethyl (1-α,2-β,5-β,6-α)-9-Oxatricyclo[4.2.1.02,5]nona-3,7-diene -3,4-dicarboxylate: a New Transfer Reagent for Dimethyl Cyclobutadiene-1,2-dicarboxylate

A synthesis, based on transfer technology, of the title compound is reported and its advantages over previously described reagents are discussed.

New Thermal and Photochemical Routes to Dewar Furan and Other Isomers on the C4H4O Energy Surface: the Role of Isobenzofuran as a Trapping Agent

Photolysis of the specially synthesized substrate (39) leads to quantitative fragmentation into the phthalimide (56) and Dewar furan (4a). Dewar furan has only transient existence even at -65°, yet can be trapped effectively with isobenzofuran but not furan. Rapid isomerization to cycloprop-2-enecarbaldehyde (57) occurs at the photolysis temperatures and this product is also trapped by the isobenzofuran . In the absence of trapping agent, photolysis of (39) produces some furan but no 1H n.m.r . evidence can be obtained for (4a) or (57), even at low temperatures (-85°). Separate irradiation of (57) causes extensive polymerization, without yielding other recognizable products. Furan is concluded, therefore, to arise from photoisomerization of (4a) rather than photochemical or thermal isomerization of (57). Separate thermal study of (57) shows that isomerization to furan only occurs above 420°. Flash vacuum pyrolysis of the polycyclic epoxide (72) provides a new retro- Diels-Alder route to (57) which likely proceeds via (4a) as an intermediate. At high temperatures (57) is isomerized to furan. A new Dewar benzene oxide (41) and Dewar benzene (45) are reported en route to the photosubstrates (39) and (50) respectively. Photolysis of (50) provides a high-yielding source of cyclobutadiene , which in the absence of trapping agent yields the syn-dimer (59).

The Synthesis of Bisanthraquinones Joined by Polyether Linkages

Derivatives of quinizarin linked by polyether chains have been prepared by the annelation of bis (quinone monoacetals) with the anions of substituted 3-cyanophthalides.

Towards the Synthesis of a Dewar Furan Stabilized by a Pinching Group: the Role of Dewar o-Xylylenes as Dienes and Dienophiles

The synthesis of a photosubstrate (42) suitable for the production of a Dewar furan (46) containing a four-carbon bridge at the 1,4-position is described. The route to this product illustrates an example of transfer technology in synthesis, by using the oxanorbornadiene (29). Other active species such as Dewar benzene (32) and Dewar o-xylylene (50) play key roles as intermediates. Diels-Alder cycloadditions are also central to the overall strategy, and some observations are made regarding the facial specificities operating, especially in regard to Dewar o-xylylene (50) and its dimerization. Attempts to isolate the photoproducts, presumably involving the Dewar furan (46) as intermediate, were unsuccessful.

Anodic-Oxidation Products of 2',5'-Dimethoxyacetanilide Derivatives, and Their Application to the Synthesis of Aminoanthraquinones

Anodic oxidation of simple 2′,5′-dimethoxyacetanilides proceeds under mildly basic conditions to afford either monomeric or dimeric quinone bisacetals, depending on the structure of the substrate. Similarly, the oxidation of linked derivatives of 2′,5′-dimethoxyacetanilides is shown to exhibit a comparable sensitivity, with non-alkylated amides being efficiently converted into bis(quinone bisacetals). The regiospecificity of acetal hydrolysis for both simple and linked quinone bisacetals is shown to be dependent upon the nature of the N-acyl group. The annelation of these hydrolysis products with the anion of 3-cyanophthalide to yield (acylamino)anthraquinones is also reported.

A 19F-NMR study of 3-fluoro-4-demethoxydaunomycin intercalation complexes with the hexanucleotide d(CTGCAG)2

Equilibrium systems containing intercalation complexes formed between the novel anthracycline drug, 3-fluoro-4-demethoxydaunomycin (3FD), and the hexanucleotide duplex d(CTGCAG)2 have been studied by 19F-NMR spectroscopy. Solutions containing a 1:1 molar ratio of 3FD/d(CTGCAG)2 gave four 19F signals which have been assigned to each of four possible intercalation isomers for the 1:1 3FD.d(CTGCAG)2 complex, which we denote by [d(CTGCAG)2][3FD]; these were where 3FD bound between the 5'-CT-3', 5'-TG-3', 5'-GC-3' or 5'-CA-3' base sequences, with the drug sugar moiety lying in the minor groove and pointed in the 3' direction in each case. Changes in temperature and NaCl concentration affecting the equilibrium distribution of these isomers were studied and indicated that no overriding binding site preference prevailed under standard biochemical conditions. Formation of some of the 2:1 3FD.d(CTGCAG)2 complex occurred when a solution of [d(CTGCAG)2][3FD] was exposed to excess 3FD; however, this complex was unstable to gel filtration and no co-operative binding of the second 3FD molecule was observed.

Effects of solute conditions on the relative affinities of the oligonucleotides d(G-C)5 and d(A-T)5 for the anthracycline drug 2-fluoro-4-demethoxydaunomycin

19F NMR has been used to show that changes in NaCl concentration, as well as the presence of lysine or arginine, affect the equilibrium distribution of the synthetic anthracycline 2-fluoro-4-demethoxydaunomycin (2FD) between binding sites on d(G-C)5 and d(A-T)5 in a 1:1:1 molar aqueous system: 2FD/d(G-C)5/d(A-T)5. Varying the pH between 6.2 and 7.7 had no effect. NaCl concentrations below 0.1 M led to a d(G-C)5 preference while above 0.1 M a preference for d(A-T)5 was observed. At comparable solute concentrations, use of either lysine or arginine resulted in a significant drug preference for d(G-C)5 compared to systems containing only NaCl.

A 19F-NMR study of 2-fluoro-4-demethoxydaunomycin intercalation complexes with the decanucleotides d(G-C)5 and d(A-T)5

Binding configurations and equilibria of intercalation complexes formed by the novel anthracycline drug, 2-fluoro-4-demethoxydaunomycin (2FD), with the decanucleotides d(G-C)5 and d(A-T)5 have been studied by 19F-NMR spectroscopy. The 19F chemical shift of 2FD bound to d(A-T)5 was approximately 1.5 ppm downfield of that observed for 2FD bound to d(G-C)5. By mixing equimolar amounts of aqueous d(G-C)5, d(A-T)5 and 2FD, the distribution of drug between the nucleotides was followed by observing relative peak intensities and showed no G-C or A-T binding preference at room temperature. It was shown that each decanucleotide duplex bound three 2FD molecules, giving a neighbour exclusion parameter, n, of n = 3 for this drug. The stoichiometric complexes, which we denote by [d(A-T)5][2FD]3 and [d(G-C)5][2FD]3, were also purified and isolated in this study.

Anthracyclines. XVI. Further Comments Concerning the Phthalide Anion Annelation of Quinone Monoacetals

.Yields of tricyclic and tetracyclic quinones prepared by annelation of quinone monoacetals with the anion of 3-phenylsulfonylphthalides are shown to be sensitive to the substituent on both the phthalide and the quinone monoacetal partner. In contrast, reactions with the anions of 3-cyanophthalides afford high yields of condensed products in all cases examined.