Mucosal correlates of protection after influenza viral challenge of vaccinated and unvaccinated healthy volunteers

The induction of systemic antibody titers against hemagglutinin has long been the main focus of influenza vaccination strategies, but mucosal immunity has also been shown to play a key role in the protection against respiratory viruses. By vaccinating and challenging healthy volunteers, we demonstrated that inactivated influenza vaccine (IIV) modestly reduced the rate of influenza while predominantly boosting serum antibody titers against hemagglutinin (HA) and HA stalk, a consequence of the low neuraminidase (NA) content of IIV and the intramuscular route of administration. The viral challenge induced nasal and serum responses against both HA and NA. Correlations between mucosal IgA and serum IgG against specific antigens were low, whether before or after challenge, suggesting a compartmentalization of immune responses. Even so, volunteers who developed viral shedding for multiple days had lower baseline titers across both systemic and mucosal compartments as compared to those with no shedding or a single day of shedding. Regression analysis showed that pre-challenge HA inhibition titers were the most consistent correlate of protection across clinical outcomes combining shedding and symptoms, with NA inhibition titers and HA IgG levels only predicting the duration of shedding. Despite the inclusion of data from multiple binding and functional antibody assays against HA and NA performed on both serum and nasal samples, multivariate models were unable to account for the variability in outcomes, emphasizing our imperfect understanding of immune correlates in influenza and the importance of refining models with assessments of innate and cellular immune responses.IMPORTANCEThe devastating potential of influenza has been well known for over 100 years. Despite the development of vaccines since the middle of the 20th century, influenza continues to be responsible for substantial global morbidity and mortality. To develop next-generation vaccines with enhanced effectiveness, we must synthesize our understanding of the complex immune mechanisms culminating in protection. Our study outlines the differences in immune responses to influenza vaccine and influenza infection, identifying potential gaps in vaccine-induced immunity, particularly at the level of the nasal mucosa. Furthermore, this research underscores the need to refine our imperfect models while recognizing potential pitfalls in past and future attempts to identify and measure correlates of protection.

An inactivated multivalent influenza A virus vaccine is broadly protective in mice and ferrets

Influenza A viruses (IAVs) present major public health threats from annual seasonal epidemics and pandemics and from viruses adapted to a variety of animals including poultry, pigs, and horses. Vaccines that broadly protect against all such IAVs, so-called "universal" influenza vaccines, do not currently exist but are urgently needed. Here, we demonstrated that an inactivated, multivalent whole-virus vaccine, delivered intramuscularly or intranasally, was broadly protective against challenges with multiple IAV hemagglutinin and neuraminidase subtypes in both mice and ferrets. The vaccine is composed of four β-propiolactone-inactivated low-pathogenicity avian IAV subtypes of H1N9, H3N8, H5N1, and H7N3. Vaccinated mice and ferrets demonstrated substantial protection against a variety of IAVs, including the 1918 H1N1 strain, the highly pathogenic avian H5N8 strain, and H7N9. We also observed protection against challenge with antigenically variable and heterosubtypic avian, swine, and human viruses. Compared to control animals, vaccinated mice and ferrets demonstrated marked reductions in viral titers, lung pathology, and host inflammatory responses. This vaccine approach indicates the feasibility of eliciting broad, heterosubtypic IAV protection and identifies a promising candidate for influenza vaccine clinical development.

Lung epithelial and endothelial damage, loss of tissue repair, inhibition of fibrinolysis, and cellular senescence in fatal COVID-19

Coronavirus disease 2019 (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is characterized by respiratory distress, multiorgan dysfunction, and, in some cases, death. The pathological mechanisms underlying COVID-19 respiratory distress and the interplay with aggravating risk factors have not been fully defined. Lung autopsy samples from 18 patients with fatal COVID-19, with symptom onset-to-death times ranging from 3 to 47 days, and antemortem plasma samples from 6 of these cases were evaluated using deep sequencing of SARS-CoV-2 RNA, multiplex plasma protein measurements, and pulmonary gene expression and imaging analyses. Prominent histopathological features in this case series included progressive diffuse alveolar damage with excessive thrombosis and late-onset pulmonary tissue and vascular remodeling. Acute damage at the alveolar-capillary barrier was characterized by the loss of surfactant protein expression with injury to alveolar epithelial cells, endothelial cells, respiratory epithelial basal cells, and defective tissue repair processes. Other key findings included impaired clot fibrinolysis with increased concentrations of plasma and lung plasminogen activator inhibitor-1 and modulation of cellular senescence markers, including p21 and sirtuin-1, in both lung epithelial and endothelial cells. Together, these findings further define the molecular pathological features underlying the pulmonary response to SARS-CoV-2 infection and provide important insights into signaling pathways that may be amenable to therapeutic intervention.

Sex Differences in Influenza: The Challenge Study Experience

BACKGROUND

Preclinical animal studies and retrospective human studies suggest that adult females have worse outcomes from influenza than males. Prospective studies in humans are missing.

METHODS

Data from 164 healthy volunteers who underwent Influenza A/California/04/2009/H1N1 challenge were compiled to compare differences between sexes. Baseline characteristics, including hormone levels, hemagglutination-inhibition (HAI) titers, neuraminidase-inhibition titers (NAI), and outcomes after challenge were compared. Linear and logistic regression models were built to determine significant predictor variables with respect to outcomes of interest.

RESULTS

Hemagglutination-inhibition (HAI) titers were similar between the sexes, but neuraminidase-inhibition titers (NAI) were higher in males than females at 4-weeks and 8-weeks post-challenge. Females were more likely to have symptoms (mean 0.96 vs 0.80, p=.003) and to have a higher number of symptoms (median 3 vs 4, p=.011) than males. Linear and logistic regression models showed that pre-challenge NAI titers, but not HAI titers or sex hormone levels, were predictive of all shedding and symptom outcomes of interest.

CONCLUSIONS

Females in our cohorts were more likely to be symptomatic and to have a higher number of symptoms than males. NAI titers predicted all outcomes of interest and may explain differential outcomes between the sexes.

The effect of calcium and magnesium on activity, immunogenicity, and efficacy of a recombinant N1/N2 neuraminidase vaccine

Despite the importance of immunity against neuraminidase (NA), NA content and immunogenicity are neglected in current influenza vaccines. To address this, a recombinant N1/N2 NA vaccine (NAV) was developed. Stability assays were used to determine optimal temperature and buffer conditions for vaccine storage. The effect of divalent cation-related enhancement of NA stability and activity on N1 and N2 immunogenicity and efficacy against viral challenge was assessed. Differences in activity between N1 and N2 and cation-related activity enhancement did not translate into differences in immunogenicity or efficacy. NAV-vaccinated mice showed robust antibody titers against N1 and N2, and after challenge with influenza A (H1N1) virus, decreased viral titers and decreased antiviral and inflammatory responses by transcriptomic analysis. These findings provide guidance for optimal storage and assessment of NA-based vaccines and confirm the importance of NA in influenza vaccination strategies in attenuating viral replication and limiting inflammatory responses necessary to clear infection.

Virus infection of the CNS disrupts the immune-neural-synaptic axis via induction of pleiotropic gene regulation of host responses

Treatment for many viral infections of the central nervous system (CNS) remains only supportive. Here we address a remaining gap in our knowledge regarding how the CNS and immune systems interact during viral infection. By examining the regulation of the immune and nervous system processes in a nonhuman primate model of West Nile virus neurological disease, we show that virus infection disrupts the homeostasis of the immune-neural-synaptic axis via induction of pleiotropic genes with distinct functions in each component of the axis. This pleiotropic gene regulation suggests an unintended off-target negative impact of virus-induced host immune responses on the neurotransmission, which may be a common feature of various viral infections of the CNS.

Influenza A Reinfection in Sequential Human Challenge: Implications for Protective Immunity and "Universal" Vaccine Development

BACKGROUND

Identification of correlates of protection against human influenza A virus infection is important in development of broadly protective ("universal") influenza vaccines. Certain assumptions underlie current vaccine developmental strategies, including that infection with a particular influenza A virus should offer long-term or lifelong protection against that strain, preventing reinfection. In this study we report observations made when 7 volunteers participated in sequential influenza challenge studies where they were challenged intranasally using the identical influenza A(H1N1)pdm09 virus approximately 1 year apart. We evaluate and describe the outcomes of these 7 rechallenge participants and discuss what these results may suggest about correlates of protection and development of more broadly protective influenza vaccines.

METHODS

Seven participants were enrolled in 2 viral challenge studies at 7.5- to 18.5-month intervals. Both challenge studies used the identical lot of influenza A (H1N1)pdm09 virus administered intranasally. We evaluated pre- and postchallenge hemagglutination inhibition, neuraminidase inhibition, and stalk antibody titers; peripheral blood leukocyte host gene expression response profiles; daily viral detection via nasal wash; and clinical signs and symptoms.

RESULTS

At least 3 of 7 participants demonstrated confirmed laboratory evidence of sequential infection, with 5 of 7 demonstrating clinical evidence.

CONCLUSIONS

The data presented in this report demonstrate that sequential infection with the identical influenza A virus can occur and suggest it may not be rare. These data raise questions about immune memory responses in an acute superficial respiratory mucosal infection and their implications in development of broadly protective influenza vaccines. Further investigation of these observations is warranted.

CLINICAL TRIALS REGISTRATION

NCT01646138; NCT01971255.

Establishment of a Pig Influenza Challenge Model for Evaluation of Monoclonal Antibody Delivery Platforms

mAbs are a possible adjunct to vaccination and drugs in treatment of influenza virus infection. However, questions remain whether small animal models accurately predict efficacy in humans. We have established the pig, a large natural host animal for influenza, with many physiological similarities to humans, as a robust model for testing mAbs. We show that a strongly neutralizing mAb (2-12C) against the hemagglutinin head administered prophylactically at 15 mg/kg reduced viral load and lung pathology after pandemic H1N1 influenza challenge. A lower dose of 1 mg/kg of 2-12C or a DNA plasmid-encoded version of 2-12C reduced pathology and viral load in the lungs but not viral shedding in nasal swabs. We propose that the pig influenza model will be useful for testing candidate mAbs and emerging delivery platforms prior to human trials.

Pre-existing immunity to influenza virus hemagglutinin stalk might drive selection for antibody-escape mutant viruses in a human challenge model

The conserved region of influenza hemagglutinin (HA) stalk (or stem) has gained attention as a potent target for universal influenza vaccines^1-5. Although the HA stalk region is relatively well conserved, the evolutionarily dynamic nature of influenza viruses⁶ raises concerns about the possible emergence of viruses carrying stalk escape mutation(s) under sufficient immune pressure. Here we show that immune pressure on the HA stalk can lead to expansion of escape mutant viruses in study participants challenged with a 2009 H1N1 pandemic influenza virus inoculum containing an A388V polymorphism in the HA stalk (45% wild type and 55% mutant). High level of stalk antibody titers was associated with the selection of the mutant virus both in humans and in vitro. Although the mutant virus showed slightly decreased replication in mice, it was not observed in cell culture, ferrets or human challenge participants. The A388V mutation conferred resistance to some of the potent HA stalk broadly neutralizing monoclonal antibodies (bNAbs). Co-culture of wild-type and mutant viruses in the presence of either a bNAb or human serum resulted in rapid expansion of the mutant. These data shed light on a potential obstacle for the success of HA-stalk-targeting universal influenza vaccines-viral escape from vaccine-induced stalk immunity.

The 1918 influenza pandemic: 100 years of questions answered and unanswered

The 2018-2019 period marks the centennial of the "Spanish" influenza pandemic, which caused at least 50 million deaths worldwide. The unprecedented nature of the pandemic's sudden appearance and high fatality rate serve as a stark reminder of the threat influenza poses. Unusual features of the 1918-1919 pandemic, including age-specific mortality and the high frequency of severe pneumonias, are still not fully understood. Sequencing and reconstruction of the 1918 virus has allowed scientists to answer many questions about its origin and pathogenicity, although many questions remain. This Review summarizes key findings and still-to-be answered questions about this deadliest of human events.

Deep sequencing of 2009 influenza A/H1N1 virus isolated from volunteer human challenge study participants and natural infections

Nasal wash samples from 15 human volunteers challenged with GMP manufactured influenza A/California/04/2009(H1N1) and from 5 naturally infected influenza patients of the 2009 pandemic were deep sequenced using viral targeted hybridization enrichment. Ten single nucleotide polymorphism (SNP) positions were found in the challenge virus. Some of the nonsynonymous changes in the inoculant virus were maintained in some challenge participants, but not in others, indicating that virus is evolving away from the Vero cell adapted inoculant, for example SNPs in the neuraminidase. Many SNP sites in challenge patients and naturally infected patients were found, many not identified previously. The SNPs identified, and phylogenetic analyses, showed that intrahost evolution of the virus are different in challenge participants and naturally infected patients. This study, using hybridization enrichment without PCR, provided an accurate and unbiased assessment of differential intrahost viral evolution from a uniform influenza inoculant in humans and comparison to naturally infected patients.

Deep Sequencing of H7N9 Influenza A Viruses from 16 Infected Patients from 2013 to 2015 in Shanghai Reveals Genetic Diversity and Antigenic Drift

Influenza A virus (IAV) infections are a major public health concern, including annual epidemics, epizootic outbreaks, and pandemics. A significant IAV epizootic outbreak was the H7N9 avian influenza A outbreak in China, which was first detected in 2013 and which has spread over 5 waves from 2013 to 2017, causing human infections in many different Chinese provinces. Here, RNA from primary clinical throat swab samples from 20 H7N9-infected local patients with different clinical outcomes, who were admitted and treated at one hospital in Shanghai, China, from April 2013 to April 2015, was analyzed. Whole-transcriptome amplification, with positive enrichment of IAV RNA, was performed, all 20 samples were subjected to deep sequencing, and data from 16 samples were analyzed in detail. Many single-nucleotide polymorphisms, including ones not previously reported, and many nonsynonymous changes that could affect hemagglutinin head and stalk antibody binding epitopes were observed. Minor populations representing viral quasispecies, including nonsynonymous hemagglutinin changes shared by antigenically variant H7N9 clades identified in the most recent wave of H7N9 infections in 2016 to 2017, were also identified.IMPORTANCE H7N9 subtype avian influenza viruses caused infections in over 1,400 humans from 2013 to 2017 and resulted in almost 600 deaths. It is important to understand how avian influenza viruses infect and cause disease in humans and to assess their potential for efficient person-to-person transmission. In this study, we used deep sequencing of primary clinical material to assess the evolution and potential for human adaptation of H7N9 influenza viruses.

Longitudinal peripheral blood transcriptional analysis of a patient with severe Ebola virus disease

The 2013-2015 outbreak of Ebola virus disease in Guinea, Liberia, and Sierra Leone was unprecedented in the number of documented cases, but there have been few published reports on immune responses in clinical cases and their relationships with the course of illness and severity of Ebola virus disease. Symptoms of Ebola virus disease can include severe headache, myalgia, asthenia, fever, fatigue, diarrhea, vomiting, abdominal pain, and hemorrhage. Although experimental treatments are in development, there are no current U.S. Food and Drug Administration-approved vaccines or therapies. We report a detailed study of host gene expression as measured by microarray in daily peripheral blood samples collected from a patient with severe Ebola virus disease. This individual was provided with supportive care without experimental therapies at the National Institutes of Health Clinical Center from before onset of critical illness to recovery. Pearson analysis of daily gene expression signatures revealed marked gene expression changes in peripheral blood leukocytes that correlated with changes in serum and peripheral blood leukocytes, viral load, antibody responses, coagulopathy, multiple organ dysfunction, and then recovery. This study revealed marked shifts in immune and antiviral responses that preceded changes in medical condition, indicating that clearance of replicating Ebola virus from peripheral blood leukocytes is likely important for systemic viral clearance.

1918 pandemic influenza virus and Streptococcus pneumoniae co-infection results in activation of coagulation and widespread pulmonary thrombosis in mice and humans

To study bacterial co-infection following 1918 H1N1 influenza virus infection, mice were inoculated with the 1918 influenza virus, followed by Streptococcus pneumoniae (SP) 72 h later. Co-infected mice exhibited markedly more severe disease, shortened survival time and more severe lung pathology, including widespread thrombi. Transcriptional profiling revealed activation of coagulation only in co-infected mice, consistent with the extensive thrombogenesis observed. Immunohistochemistry showed extensive expression of tissue factor (F3) and prominent deposition of neutrophil elastase on endothelial and epithelial cells in co-infected mice. Lung sections of SP-positive 1918 autopsy cases showed extensive thrombi and prominent staining for F3 in alveolar macrophages, monocytes, neutrophils, endothelial and epithelial cells, in contrast to co-infection-positive 2009 pandemic H1N1 autopsy cases. This study reveals that a distinctive feature of 1918 influenza virus and SP co-infection in mice and humans is extensive expression of tissue factor and activation of the extrinsic coagulation pathway leading to widespread pulmonary thrombosis.

The role of viral, host, and secondary bacterial factors in influenza pathogenesis

Influenza A virus infections in humans generally cause self-limited infections, but can result in severe disease, secondary bacterial pneumonias, and death. Influenza viruses can replicate in epithelial cells throughout the respiratory tree and can cause tracheitis, bronchitis, bronchiolitis, diffuse alveolar damage with pulmonary edema and hemorrhage, and interstitial and airspace inflammation. The mechanisms by which influenza infections result in enhanced disease, including development of pneumonia and acute respiratory distress, are multifactorial, involving host, viral, and bacterial factors. Host factors that enhance risk of severe influenza disease include underlying comorbidities, such as cardiac and respiratory disease, immunosuppression, and pregnancy. Viral parameters enhancing disease risk include polymerase mutations associated with host switch and adaptation, viral proteins that modulate immune and antiviral responses, and virulence factors that increase disease severity, which can be especially prominent in pandemic viruses and some zoonotic influenza viruses causing human infections. Influenza viral infections result in damage to the respiratory epithelium that facilitates secondary infection with common bacterial pneumopathogens and can lead to secondary bacterial pneumonias that greatly contribute to respiratory distress, enhanced morbidity, and death. Understanding the molecular mechanisms by which influenza and secondary bacterial infections, coupled with the role of host risk factors, contribute to enhanced morbidity and mortality is essential to develop better therapeutic strategies to treat severe influenza.

Treatment with the reactive oxygen species scavenger EUK-207 reduces lung damage and increases survival during 1918 influenza virus infection in mice

The 1918 influenza pandemic caused over 40 million deaths worldwide, with 675,000 deaths in the United States alone. Studies in several experimental animal models showed that 1918 influenza virus infection resulted in severe lung pathology associated with dysregulated immune and cell death responses. To determine if reactive oxygen species produced by host inflammatory responses play a central role in promoting severity of lung pathology, we treated 1918 influenza virus-infected mice with the catalytic catalase/superoxide dismutase mimetic, salen-manganese complex EUK-207 beginning 3 days postinfection. Postexposure treatment of mice infected with a lethal dose of the 1918 influenza virus with EUK-207 resulted in significantly increased survival and reduced lung pathology without a reduction in viral titers. In vitro studies also showed that EUK-207 treatment did not affect 1918 influenza viral replication. Immunohistochemical analysis showed a reduction in the detection of the apoptosis marker cleaved caspase-3 and the oxidative stress marker 8-oxo-2'-deoxyguanosine in lungs of EUK-207-treated animals compared to vehicle controls. High-throughput sequencing and RNA expression microarray analysis revealed that treatment resulted in decreased expression of inflammatory response genes and increased lung metabolic and repair responses. These results directly demonstrate that 1918 influenza virus infection leads to an immunopathogenic immune response with excessive inflammatory and cell death responses that can be limited by treatment with the catalytic antioxidant EUK-207.

Seasonal H3N2 influenza A virus fails to enhance Staphylococcus aureus co-infection in a non-human primate respiratory tract infection model

Staphylococcus aureus community-acquired pneumonia is often associated with influenza or an influenza-like syndrome. Morbidity and mortality due to methicillin-resistant S. aureus (MRSA) or influenza and pneumonia, which includes bacterial co-infection, are among the top causes of death by infectious diseases in the United States. We developed a non-lethal influenza A virus (IAV) (H3N2)/S. aureus co-infection model in cynomolgus macaques (Macaca fascicularis) to test the hypothesis that seasonal IAV infection predisposes non-human primates to severe S. aureus pneumonia. Infection and disease progression were monitored by clinical assessment of animal health; analysis of blood chemistry, nasal swabs, and X-rays; and gross pathology and histopathology of lungs from infected animals. Seasonal IAV infection in healthy cynomolgus macaques caused mild pneumonia, but unexpectedly, did not predispose these animals to subsequent severe infection with the community-associated MRSA clone USA300. We conclude that in our co-infection model, seasonal IAV infection alone is not sufficient to promote severe S. aureus pneumonia in otherwise healthy non-human primates. The implication of these findings is that comorbidity factors in addition to IAV infection are required to predispose individuals to secondary S. aureus pneumonia.

High-throughput RNA sequencing of a formalin-fixed, paraffin-embedded autopsy lung tissue sample from the 1918 influenza pandemic

Most biopsy and autopsy tissues are formalin-fixed and paraffin-embedded (FFPE), but this process leads to RNA degradation that limits gene expression analysis. The RNA genome of the 1918 pandemic influenza virus was previously determined in a 9-year effort by overlapping RT-PCR from post-mortem samples. Here, the full genome of the 1918 virus at 3000× coverage was determined in one high-throughput sequencing run of a library derived from total RNA of a 1918 FFPE sample after duplex-specific nuclease treatments. Bacterial sequences associated with secondary bacterial pneumonias were also detected. Host transcripts were well represented in the library. Compared to a 2009 pandemic influenza virus FFPE post-mortem library, the 1918 sample showed significant enrichment for host defence and cell death response genes, concordant with prior animal studies. This methodological approach should assist in the analysis of FFPE tissue samples isolated over the past century from a variety of diseases.

Identification of a novel splice variant form of the influenza A virus M2 ion channel with an antigenically distinct ectodomain

Segment 7 of influenza A virus produces up to four mRNAs. Unspliced transcripts encode M1, spliced mRNA2 encodes the M2 ion channel, while protein products from spliced mRNAs 3 and 4 have not previously been identified. The M2 protein plays important roles in virus entry and assembly, and is a target for antiviral drugs and vaccination. Surprisingly, M2 is not essential for virus replication in a laboratory setting, although its loss attenuates the virus. To better understand how IAV might replicate without M2, we studied the reversion mechanism of an M2-null virus. Serial passage of a virus lacking the mRNA2 splice donor site identified a single nucleotide pseudoreverting mutation, which restored growth in cell culture and virulence in mice by upregulating mRNA4 synthesis rather than by reinstating mRNA2 production. We show that mRNA4 encodes a novel M2-related protein (designated M42) with an antigenically distinct ectodomain that can functionally replace M2 despite showing clear differences in intracellular localisation, being largely retained in the Golgi compartment. We also show that the expression of two distinct ion channel proteins is not unique to laboratory-adapted viruses but, most notably, was also a feature of the 1983 North American outbreak of H5N2 highly pathogenic avian influenza virus. In identifying a 14th influenza A polypeptide, our data reinforce the unexpectedly high coding capacity of the viral genome and have implications for virus evolution, as well as for understanding the role of M2 in the virus life cycle.

Influenza A virus is a pathogen capable of infecting a wide range of avian and mammalian hosts, causing seasonal epidemics and pandemics in humans. In recent years, the unexpected coding capacity of the virus has begun to be unravelled, with the identification of three more protein products (PB1-F2, PB1-N40 and PA-X) on top of the 10 viral proteins originally identified 30 years ago. Here, we identify a 14th primary translation product, made from segment 7. Previously established protein products from segment 7 include the matrix (M1) and ion channel (M2) proteins. M2, made from a spliced transcript, has multiple roles in the virus lifecycle including in entry and budding. In a laboratory setting, it is possible to generate M2 deficient viruses, but these are highly attenuated. However, upon serial passage a virus lacking the M2 splice donor site quickly recovered wild type growth properties, without reverting the original mutation. Instead we found a compensatory single nucleotide mutation had upregulated another segment 7 mRNA. This mRNA encoded a novel M2-like protein with a variant extracellular domain, which we called M42. M42 compensated for loss of M2 in tissue culture cells and animals, although it displayed some differences in subcellular localisation. Our study therefore identifies a further novel influenza protein and gives insights into the evolution of the virus.

Protection against a lethal H5N1 influenza challenge by intranasal immunization with virus-like particles containing 2009 pandemic H1N1 neuraminidase in mice

Highly pathogenic H5N1 influenza shares the same neuraminidase (NA) subtype with the 2009 pandemic (H1N1pdm09), and cross-reactive NA immunity might protect against or mitigate lethal H5N1 infection. In this study, mice were either infected with a sublethal dose of H1N1pdm09 or were vaccinated and boosted with virus-like particles (VLP) consisting of the NA and matrix proteins, standardized by NA activity and administered intranasally, and were then challenged with a lethal dose of HPAI H5N1 virus. Mice previously infected with H1N1pdm09 survived H5N1 challenge with no detectable virus or respiratory tract pathology on day 4. Mice immunized with H5N1 or H1N1pdm09 NA VLPs were also fully protected from death, with a 100-fold and 10-fold reduction in infectious virus, respectively, and reduced pathology in the lungs. Human influenza vaccines that elicit not only HA, but also NA immunity may provide enhanced protection against the emergence of seasonal and pandemic viruses.

An overlapping protein-coding region in influenza A virus segment 3 modulates the host response

Influenza A virus (IAV) infection leads to variable and imperfectly understood pathogenicity. We report that segment 3 of the virus contains a second open reading frame ("X-ORF"), accessed via ribosomal frameshifting. The frameshift product, termed PA-X, comprises the endonuclease domain of the viral PA protein with a C-terminal domain encoded by the X-ORF and functions to repress cellular gene expression. PA-X also modulates IAV virulence in a mouse infection model, acting to decrease pathogenicity. Loss of PA-X expression leads to changes in the kinetics of the global host response, which notably includes increases in inflammatory, apoptotic, and T lymphocyte-signaling pathways. Thus, we have identified a previously unknown IAV protein that modulates the host response to infection, a finding with important implications for understanding IAV pathogenesis.

Insights on influenza pathogenesis from the grave

The 1918-1919 'Spanish' influenza virus caused the worst pandemic in recorded history and resulted in approximately 50 million deaths worldwide. Efforts to understand what happened and to use these insights to prevent a future similar pandemic have been ongoing since 1918. In 2005 the genome of the 1918 influenza virus was completely determined by sequencing fragments of viral RNA preserved in autopsy tissues of 1918 victims, and using reverse genetics, infectious viruses bearing some or all the 1918 virus gene segments were reconstructed. These studies have yielded much information about the origin and pathogenicity of the 1918 virus, but many questions still remain.

Obese mice have increased morbidity and mortality compared to non-obese mice during infection with the 2009 pandemic H1N1 influenza virus

Please cite this paper as: Easterbrook et al. (2011) Obese mice have increased morbidity and mortality compared to non‐obese mice during infection with the 2009 pandemic H1N1 influenza virus. Influenza and Other Respiratory Viruses 5(6), 418–425.

Background  Obesity has been identified as an independent risk factor for severe or fatal infection with 2009 pandemic H1N1 influenza (2009 pH1N1), but was not previously recognized for previous pandemic or seasonal influenza infections.

Objectives  Our aim was to evaluate the role of obesity as an independent risk factor for severity of infection with 2009 pH1N1, seasonal H1N1, or a pathogenic H1N1 influenza virus.

Methods  Diet‐induced obese (DIO) and their non‐obese, age‐matched control counterparts were inoculated with a 2009 pH1N1, A/California/04/2009 (CA/09), current seasonal H1N1, A/NY/312/2001 (NY312), or highly pathogenic 1918‐like H1N1, A/Iowa/Swine/1931 (Sw31), virus.

Results  Following inoculation with CA/09, DIO mice had higher mortality (80%) than control mice (0%) and lost more weight during infection. No effect of obesity on morbidity and mortality was observed during NY312 or Sw31 infection. Influenza antigen distribution in the alveolar regions of the lungs was more pronounced in DIO than control mice during CA/09 infection at 3 days post‐inoculation (dpi), despite similar virus titers. During CA/09 infection, localized interferon‐β and proinflammatory cytokine protein responses in the lungs were significantly lower in DIO than control mice. Conversely, serum cytokine concentrations were elevated in DIO, but not control mice following infection with CA/09. The effect of obesity on differential immune responses was abrogated during NY312 or Sw31 infection.

Conclusions  Together, these data support epidemiologic reports that obesity may be a risk factor for severe 2009 pandemic H1N1 influenza infection, but the role of obesity in seasonal or highly virulent pandemic influenza infection remains unclear.

The ability of pandemic influenza virus hemagglutinins to induce lower respiratory pathology is associated with decreased surfactant protein D binding

Pandemic influenza viral infections have been associated with viral pneumonia. Chimeric influenza viruses with the hemagglutinin segment of the 1918, 1957, 1968, or 2009 pandemic influenza viruses in the context of a seasonal H1N1 influenza genome were constructed to analyze the role of hemagglutinin (HA) in pathogenesis and cell tropism in a mouse model. We also explored whether there was an association between the ability of lung surfactant protein D (SP-D) to bind to the HA and the ability of the corresponding chimeric virus to infect bronchiolar and alveolar epithelial cells of the lower respiratory tract. Viruses expressing the hemagglutinin of pandemic viruses were associated with significant pathology in the lower respiratory tract, including acute inflammation, and showed low binding activity for SP-D. In contrast, the virus expressing the HA of a seasonal influenza strain induced only mild disease with little lung pathology in infected mice and exhibited strong in vitro binding to SP-D.

Immunization with 1976 swine H1N1- or 2009 pandemic H1N1-inactivated vaccines protects mice from a lethal 1918 influenza infection

Please cite this paper as: Easterbrook et al. (2011) Immunization with 1976 swine H1N1‐ or 2009 pandemic H1N1‐inactivated vaccines protects mice from a lethal 1918 influenza infection. Influenza and Other Respiratory Viruses DOI: 10.1111/j.1750‐2659.2010.00191.x.

Background  Zoonotic infections with H1N1 influenza viruses that evolved initially from the 1918 virus (1918) and adapted to swine threatened a pandemic in 1976 (1976 swH1N1) and a novel reassortant H1N1 virus caused a pandemic in 2009–2010 (2009 pH1N1). Epidemiological and laboratory animal studies show that protection from severe 2009 pH1N1 infection is conferred by vaccination or prior infection with 1976 swH1N1 or 1918.

Objectives  Our aim was to demonstrate cross‐protection by immunization with 2009 pH1N1 or 1976 swH1N1 vaccines following a lethal challenge with 1918. Further, the mechanisms of cross‐protective antibody responses were evaluated.

Methods  Mice were immunized with 1976 swH1N1, 2009 pH1N1, 2009 seasonal trivalent, or 1918 vaccines and challenged with 1918. Cross‐reactive antibody responses were assessed and protection monitored by survival, weight loss, and pathology in mice.

Results and Conclusions  Vaccination with the 1976 swH1N1 or 2009 pH1N1 vaccines protected mice from a lethal challenge with 1918, and these mice lost no weight and had significantly reduced viral load and pathology in the lungs. Protection was likely due to cross‐reactive antibodies detected by microneutralization assay. Our data suggest that the general population may be protected from a future 1918‐like pandemic because of prior infection or immunization with 1976 swH1N1 or 2009 pH1N1. Also, influenza protection studies generally focus on cross‐reactive hemagglutination‐inhibiting antibodies; while hemagglutinin is the primary surface antigen, this fails to account for other influenza viral antigens. Neutralizing antibody may be a better correlate of human protection against pathogenic influenza strains and should be considered for vaccine efficacy.

Influenza virus evolution, host adaptation, and pandemic formation

Newly emerging or "re-emerging" viral diseases continue to pose significant global public health threats. Prototypic are influenza viruses that are major causes of human respiratory infections and mortality. Influenza viruses can cause zoonotic infections and adapt to humans, leading to sustained transmission and emergence of novel viruses. Mechanisms by which viruses evolve in one host, cause zoonotic infection, and adapt to a new host species remain unelucidated. Here, we review the evolution of influenza A viruses in their reservoir hosts and discuss genetic changes associated with introduction of novel viruses into humans, leading to pandemics and the establishment of seasonal viruses.

Prior infection with classical swine H1N1 influenza viruses is associated with protective immunity to the 2009 pandemic H1N1 virus

BACKGROUND

The 2009 H1N1 pandemic emerged even though seasonal H1N1 viruses have circulated for decades. Epidemiological evidence suggested that the current seasonal vaccine did not offer significant protection from the novel pandemic, and that people over the age of 50 were less susceptible to infection.

OBJECTIVES

In a mouse challenge study with the 2009 pandemic H1N1 virus, we evaluated protective immune responses elicited by prior infection with human and swine influenza A viruses.

RESULTS

Mice infected with A/Mexico/4108/2009 (Mex09) showed significant weight loss and 40% mortality. Prior infection with a 1976 classical swine H1N1 virus resulted in complete protection from Mex09 challenge. Prior infection with either a 2009 or a 1940 seasonal H1N1 influenza virus provided partial protection and a >100-fold reduction in viral lung titers at day 4 post-infection.

CONCLUSIONS

These findings indicate that in experimental animals recently induced immunity to 1918-derived H1N1 seasonal influenza viruses, and to a 1976 swine influenza virus, afford a degree of protection against the 2009 pandemic virus. Implications of these findings are discussed in the context of accumulating data suggesting partial protection of older persons during the 2009 pandemic.

An early 'classical' swine H1N1 influenza virus shows similar pathogenicity to the 1918 pandemic virus in ferrets and mice

The 1918 pandemic influenza virus has demonstrated significant pathogenicity in animal models and is the progenitor of 'classical' swine and modern seasonal human H1N1 lineages. Here we characterize the pathogenicity of an early 'classical' swine H1N1 influenza A virus isolated in 1931 compared to the pathogenicity of the 1918 pandemic virus and a seasonal H1N1 virus in mice and ferrets. A/Swine/Iowa/31 (Sw31) and the 1918 influenza viruses were uniformly lethal in mice at low doses and produced severe lung pathology. In ferrets, Sw31 and 1918 influenza viruses caused severe clinical disease and lung pathology with necrotizing bronchiolitis and alveolitis. The modern H1N1 virus caused little disease in either animal model. These findings revealed that in these models the virulence factors of the 1918 influenza virus are likely preserved in the Sw31 virus and suggest that early swine viruses may be a good surrogate model to study 1918 virulence and pathogenesis.

Applications of high-throughput genomics to antiviral research: evasion of antiviral responses and activation of inflammation during fulminant RNA virus infection

Host responses can contribute to the severity of viral infection, through the failure of innate antiviral mechanisms to recognize and restrict the pathogen, the development of intense systemic inflammation leading to circulatory failure or through tissue injury resulting from overly exuberant cell-mediated immune responses. High-throughput genomics methods are now being used to identify the biochemical pathways underlying ineffective or damaging host responses in a number of acute and chronic viral infections. This article reviews recent gene expression studies of 1918 H1N1 influenza and Ebola hemorrhagic fever in cell culture and animal models, focusing on how genomics experiments can be used to increase our understanding of the mechanisms that permit those viruses to cause rapidly overwhelming infection. Particular attention is paid to how evasion of type I IFN responses in infected cells might contribute to over-activation of inflammatory responses. Reviewing recent research and describing how future studies might be tailored to understand the relationship between the infected cell and its environment, this article discusses how the rapidly growing field of high-throughput genomics can contribute to a more complete understanding of severe, acute viral infections and identify novel targets for therapeutic intervention.

Aberrant innate immune response in lethal infection of macaques with the 1918 influenza virus

The 1918 influenza pandemic was unusually severe, resulting in about 50 million deaths worldwide. The 1918 virus is also highly pathogenic in mice, and studies have identified a multigenic origin of this virulent phenotype in mice. However, these initial characterizations of the 1918 virus did not address the question of its pathogenic potential in primates. Here we demonstrate that the 1918 virus caused a highly pathogenic respiratory infection in a cynomolgus macaque model that culminated in acute respiratory distress and a fatal outcome. Furthermore, infected animals mounted an immune response, characterized by dysregulation of the antiviral response, that was insufficient for protection, indicating that atypical host innate immune responses may contribute to lethality. The ability of influenza viruses to modulate host immune responses, such as that demonstrated for the avian H5N1 influenza viruses, may be a feature shared by the virulent influenza viruses.

Suppression of proinflammatory signal transduction and gene expression by the dual nucleic acid binding domains of the vaccinia virus E3L proteins

Cells have evolved elaborate mechanisms to counteract the onslaught of viral infections. To activate these defenses, the viral threat must be recognized. Danger signals, or pathogen-associated molecular patterns, that are induced by pathogens include double-stranded RNA (dsRNA), viral single-stranded RNA, glycolipids, and CpG DNA. Understanding the signal transduction pathways activated and host gene expression induced by these danger signals is vital to understanding virus-host interactions. The vaccinia virus E3L protein is involved in blocking the host antiviral response and increasing pathogenesis, functions that map to separate C-terminal dsRNA- and N-terminal Z-DNA-binding domains. Viruses containing mutations in these domains allow modeling of the role of dsRNA and Z-form nucleic acid in the host response to virus infection. Deletions in the Z-DNA- or dsRNA-binding domains led to activation of signal transduction cascades and up-regulation of host gene expression, with many genes involved in the inflammatory response. These data suggest that poxviruses actively inhibit cellular recognition of viral danger signals and the subsequent cellular response to the viral threat.

Genomic analysis of increased host immune and cell death responses induced by 1918 influenza virus

The influenza pandemic of 1918-19 was responsible for about 50 million deaths worldwide. Modern histopathological analysis of autopsy samples from human influenza cases from 1918 revealed significant damage to the lungs with acute, focal bronchitis and alveolitis associated with massive pulmonary oedema, haemorrhage and rapid destruction of the respiratory epithelium. The contribution of the host immune response leading to this severe pathology remains largely unknown. Here we show, in a comprehensive analysis of the global host response induced by the 1918 influenza virus, that mice infected with the reconstructed 1918 influenza virus displayed an increased and accelerated activation of host immune response genes associated with severe pulmonary pathology. We found that mice infected with a virus containing all eight genes from the pandemic virus showed marked activation of pro-inflammatory and cell-death pathways by 24 h after infection that remained unabated until death on day 5. This was in contrast with smaller host immune responses as measured at the genomic level, accompanied by less severe disease pathology and delays in death in mice infected with influenza viruses containing only subsets of 1918 genes. The results indicate a cooperative interaction between the 1918 influenza genes and show that study of the virulence of the 1918 influenza virus requires the use of the fully reconstructed virus. With recent concerns about the introduction of highly pathogenic avian influenza viruses into humans and their potential to cause a worldwide pandemic with disastrous health and economic consequences, a comprehensive understanding of the global host response to the 1918 virus is crucial. Moreover, understanding the contribution of host immune responses to virulent influenza virus infections is an important starting point for the identification of prognostic indicators and the development of novel antiviral therapies.

Hijacking of the host-cell response and translational control during influenza virus infection

Influenza virus is a major public health problem with annual deaths in the US of 36,000 with pandemic outbreaks, such as in 1918, resulting in deaths exceeding 20 million worldwide. Recently, there is much concern over the introduction of highly pathogenic avian influenza H5N1 viruses into the human population. Influenza virus has evolved complex translational control strategies that utilize cap-dependent translation initiation mechanisms and involve the recruitment of both viral and host-cell proteins to preferentially synthesize viral proteins and prevent activation of antiviral responses. Influenza virus is a member of the Orthomyxoviridae family of negative-stranded, segmented RNA viruses and represents a particularly attractive model system as viral replication strategies are closely intertwined with normal cellular processes including the host defense and stress pathways. In this chapter, we review the parallels between translational control in influenza virus infected cells and in stressed cells with a focus on selective translation of viral mRNAs and the antagonism of the dsRNA and host antiviral responses. Moreover, we will discuss how the use of genomic technologies such as DNA microarrays and high through-put proteomics can be used to gain new insights into the control of protein synthesis during viral infection and provide a near comprehensive view of virus-host interactions.

Global suppression of the host antiviral response by Ebola- and Marburgviruses: increased antagonism of the type I interferon response is associated with enhanced virulence

We studied the effect of filovirus infection on host cell gene expression by characterizing the regulation of gene expression responses in human liver cells infected with Zaire Ebolavirus (ZEBOV), Reston Ebolavirus (REBOV), and Marburgvirus (MARV), using transcriptional profiling and bioinformatics. Expression microarray analysis demonstrated that filovirus infection resulted in the up-regulation of immune-related genes and the down-regulation of many coagulation and acute-phase proteins. These studies further revealed that a common feature of filovirus virulence is suppression of key cellular antiviral responses, including TLR-, interferon (IFN) regulatory factor 3-, and PKR-related pathways. We further showed that ZEBOV and MARV were more potent antagonists of the IFN response and inhibited the expression of most of the IFN-stimulated genes (ISGs) observed in mock-infected IFN-alpha-2b treated cells, compared to REBOV infection, which activated more than 20% of these ISGs. Finally, we examined IFN-related gene expression in filovirus-infected cells treated with IFN-alpha-2b. These experiments revealed that a majority of genes induced in mock-infected cells treated with type I IFN were antagonized in treated ZEBOV- and MARV-infected cells, while in contrast, REBOV infection resulted in a significant increase in ISG expression. Analysis of STAT1 and -2 phosphorylation following IFN treatment showed a significant reduction of STAT phosphorylation for MARV but not for ZEBOV and REBOV, indicating that different mechanisms might be involved in antagonizing IFN signaling pathways by the different filovirus species. Taken together, these studies showed a correlation between antagonism of type I IFN responses and filovirus virulence.

Virus infection and the interferon response: a global view through functional genomics

The primary focus of this chapter is on providing an overview of how we use the tools of functional genomics to study virus infection, the interferon response, and the mechanisms by which viruses attenuate or evade this response to ensure successful replication. We provide examples of the types of analyses we perform, experimental design considerations, and the tools and techniques we use for data processing and mining. We have not attempted to provide detailed protocols for performing microarray or proteomics experiments because even individual components of such analyses, for example, techniques for the isolation and amplification of ribonucleic acid, could easily be the subject of an entire chapter. Rather, our goal is to show how data obtained from global gene expression and protein profiling can be used to gain new insights into virus-host interactions, with particular emphasis on the interferon response and its modulation by virus infection.

Global host immune response: pathogenesis and transcriptional profiling of type A influenza viruses expressing the hemagglutinin and neuraminidase genes from the 1918 pandemic virus

To understand more fully the molecular events associated with highly virulent or attenuated influenza virus infections, we have studied the effects of expression of the 1918 hemagglutinin (HA) and neuraminidase (NA) genes during viral infection in mice under biosafety level 3 (agricultural) conditions. Using histopathology and cDNA microarrays, we examined the consequences of expression of the HA and NA genes of the 1918 pandemic virus in a recombinant influenza A/WSN/33 virus compared to parental A/WSN/33 virus and to an attenuated virus expressing the HA and NA genes from A/New Caledonia/20/99. The 1918 HA/NA:WSN and WSN recombinant viruses were highly lethal for mice and displayed severe lung pathology in comparison to the nonlethal New Caledonia HA/NA:WSN recombinant virus. Expression microarray analysis performed on lung tissues isolated from the infected animals showed activation of many genes involved in the inflammatory response, including cytokine, apoptosis, and lymphocyte genes that were common to all three infection groups. However, consistent with the histopathology studies, the WSN and 1918 HA/NA:WSN recombinant viruses showed increased up-regulation of genes associated with activated T cells and macrophages, as well as genes involved in apoptosis, tissue injury, and oxidative damage that were not observed in the New Caledonia HA/NA:WSN recombinant virus-infected mice. These studies document clear differences in gene expression profiles that were correlated with pulmonary disease pathology induced by virulent and attenuated influenza virus infections.

Selective translation of eukaryotic mRNAs: functional molecular analysis of GRSF-1, a positive regulator of influenza virus protein synthesis

To understand the regulation of cap-dependent translation initiation mediated by specific 5' untranslated region (UTR) RNA-protein interactions in mammalian cells, we have studied the selective translation of influenza virus mRNAs. Previous work has shown that the host cell mRNA binding protein guanine-rich sequence factor 1 (GRSF-1) bound specifically to conserved viral 5' UTR sequences and stimulated translation of viral 5' UTR-driven mRNAs in vitro. In the present study, we have characterized the functional domains of GRSF-1 and mapped the RNA binding activity of GRSF-1 to RRM 2 (amino acids 194 to 275) with amino-terminal deletion glutathione S-transferase (GST)-GRSF-1 proteins. When these mutants were assayed for functional activity in vitro, deletion of an Ala-rich region (Delta[2-94]) appeared to diminish translational stimulation, while deletion of the Ala-rich region in addition to RRM 1 (Delta[2-194]) resulted in a 4-fold increase in translational activation over wild-type GRSF-1 (an overall 20-fold increase in activity). We have also mapped the GRSF-1 RNA binding site on influenza virus NP and NS1 5' UTRs, which was determined to be the sequence AGGGU. With polysome fractionation and cDNA microarray analysis, we have identified cellular and viral mRNAs containing putative GRSF-1 binding sites that were transcriptionally up-regulated and selectively recruited to polyribosomes following influenza virus infection. Taken together, these studies demonstrate that RRM 2 is critical for GRSF-1 RNA binding and translational activity. Further, our data suggest GRSF-1 functions by selectively recruiting cellular and viral mRNAs containing 5' UTR GRSF-1 binding sites to polyribosomes, which is mediated through interactions with cellular proteins.

Post-transcriptional regulation of luteinizing hormone receptor mRNA in the ovary by a novel mRNA-binding protein

Luteinizing hormone (LH) receptor mRNA is post-transcriptionally regulated. An ovarian cytosolic LH receptor mRNA-binding protein (LRBP) identified in our laboratory binds to a polypyrimidine-rich bipartite sequence in the coding region of LH receptor mRNA. The present studies show a role for LRBP in the regulation of LH receptor mRNA. We demonstrated that increased LH receptor mRNA degradation occurs during hormone-induced LH receptor down-regulation. Furthermore, increased degradation of LH receptor mRNA was seen when partially purified LRBP was included in an in vitro mRNA decay reaction. The LH receptor mRNA binding activity of LRBP measured by RNA electrophoretic mobility shift analysis showed an inverse relationship to LH receptor mRNA levels during different physiological states. These results suggest that LRBP is a physiological regulator of LHR mRNA expression in the ovary and provides a novel mechanism for the regulation of LH receptor expression in the ovary.

Sequence-specific binding of a hormonally regulated mRNA binding protein to cytidine-rich sequences in the lutropin receptor open reading frame

In previous studies, a lutropin receptor mRNA binding protein implicated in the hormonal regulation of lutropin receptor mRNA stability was identified. This protein, termed LRBP-1, was shown by RNA gel electrophoretic mobility shift assay to specifically interact with lutropin receptor RNA sequences. The present studies have examined the specificity of lutropin receptor mRNA recognition by LRBP-1 and mapped the contact site by RNA footprinting and by site-directed mutagenesis. LRBP-1 was partially purified by cation-exchange chromatography, and the mRNA binding properties of the partially purified LRBP-1 were examined by RNA gel electrophoretic mobility shift assay and hydroxyl-radical RNA footprinting. These data showed that the LRBP-1 binding site is located between nucleotides 203 and 220 of the receptor open reading frame, and consists of the bipartite polypyrimidine sequence 5'-UCUC-X(7)-UCUCCCU-3'. Competition RNA gel electrophoretic mobility shift assays demonstrated that homoribopolymers of poly(rC) were effective RNA binding competitors, while poly(rA), poly(rG), and poly(rU) showed no effect. Mutagenesis of the cytidine residues contained within the LRBP-1 binding site demonstrated that all the cytidines in the bipartite sequence contribute to LRBP-1 binding specificity. Additionally, RNA gel electrophoretic mobility supershift analysis showed that LRBP-1 was not recognized by antibodies against two well-characterized poly(rC) RNA binding proteins, alphaCP-1 and alphaCP-2, implicated in the regulation of RNA stability of alpha-globin and tyrosine hydroxylase mRNAs. In summary, we show that partially purified LRBP-1 binds to a polypyrimidine sequence within nucleotides 203 and 220 of lutropin receptor mRNA with a high degree of specificity which is indicative of its role in posttranscriptional control of lutropin receptor expression.

Identification of a hormonally regulated luteinizing hormone/human chorionic gonadotropin receptor mRNA binding protein. Increased mrna binding during receptor down-regulation

To elucidate the molecular events associated with the regulation of luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptor mRNA stability during hCG-induced receptor down-regulation, we have identified an LH/hCG receptor-specific mRNA binding protein. Proteins were isolated from control and down-regulated rat ovary and were incubated with in vitro transcribed RNAs corresponding to the full-length LH/hCG receptor, as well as 5'- and 3'-truncated receptor forms. Resultant ribonucleoprotein complexes were analyzed by RNA gel mobility shift. A prominent Mr 50,000 ribonucleoprotein complex was identified with the following characteristics: 1) specificity for LH/hCG receptor open reading frame sequences located between nucleotides 102 and 282; 2) lack of competition by nonspecific RNAs; 3) a 3-fold increase in RNA binding activity during hCG-induced receptor down-regulation; and 4) limited tissue expression. This report describes the first evidence of an LH/hCG receptor mRNA binding protein, which we term LRBP-1, for luteinizing hormone receptor RNA binding protein-1. This protein is a candidate for a trans-acting factor involved in the hormonal regulation of LH/hCG receptor mRNA stability in rat ovary.