Behavioral thermoregulation in mice inoculated with influenza virus

Evidence for SARS-CoV-2 infected Golden Syrian hamsters (Mesocricetus auratus) reducing daily energy expenditure and body core temperature

Golden Syrian hamsters (Mesocricetus auratus) are a well-established animal model for human infections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) due to their susceptibility to SARS-CoV-2 infection, robust virus replication and pathological manifestations similar to human COVID-19 pneumonia. To investigate the physiological changes upon infection in this animal model, we explored the alterations in daily energy expenditure (DEE), water turnover, body mass, body temperature, and locomotor activity in non-infected and SARS-CoV-2 infected Golden Syrian hamsters for four days post SARS-CoV-2. DEE was measured using the doubly labelled water method, which allows for the accurate estimation of carbon dioxide production and, consequently, energy expenditure in animals. Additionally, we investigated total water intake (TWI), which comprises drinking water, preformed water in food, and metabolic water. Using intraperitoneally implanted data loggers, we also monitored body core temperature and locomotor activity in some of the animals. Here we provide evidence for infected hamsters exhibiting significantly lower DEE and TWI compared to non-infected animals. We also observed an increase in body weight in the non-infected animals, while infected animals experienced weight loss. Further, infected animals showed a significantly decreased body temperature, indicating a generally lowered metabolic rate.

Tumor Temperature: Friend or Foe of Virus-Based Cancer Immunotherapy

The temperature of a solid tumor is often dissimilar to baseline body temperature and, compared to healthy tissues, may be elevated, reduced, or a mix of both. The temperature of a tumor is dependent on metabolic activity and vascularization and can change due to tumor progression, treatment, or cancer type. Despite the need to function optimally within temperature-variable tumors, oncolytic viruses (OVs) are primarily tested at 37 °C in vitro. Furthermore, animal species utilized to test oncolytic viruses, such as mice, dogs, cats, and non-human primates, poorly recapitulate the temperature profile of humans. In this review, we discuss the importance of temperature as a variable for OV immunotherapy of solid tumors. Accumulating evidence supports that the temperature sensitivity of OVs lies on a spectrum, with some OVs likely hindered but others enhanced by elevated temperatures. We suggest that in vitro temperature sensitivity screening be performed for all OVs destined for the clinic to identify potential hinderances or benefits with regard to elevated temperature. Furthermore, we provide recommendations for the clinical use of temperature and OVs.

Circadian rhythmicity of body temperature and metabolism

This article reviews the literature on the circadian rhythms of body temperature and whole-organism metabolism. The two rhythms are first described separately, each description preceded by a review of research methods. Both rhythms are generated endogenously but can be affected by exogenous factors. The relationship between the two rhythms is discussed next. In endothermic animals, modulation of metabolic activity can affect body temperature, but the rhythm of body temperature is not a mere side effect of the rhythm of metabolic thermogenesis associated with general activity. The circadian system modulates metabolic heat production to generate the body temperature rhythm, which challenges homeothermy but does not abolish it. Individual cells do not regulate their own temperature, but the relationship between circadian rhythms and metabolism at the cellular level is also discussed. Metabolism is both an output of and an input to the circadian clock, meaning that circadian rhythmicity and metabolism are intertwined in the cell.

Immunological Responses to the Relapsing Fever Spirochete Borrelia turicatae in Infected Rhesus Macaques: Implications for Pathogenesis and Diagnosis

The global public health impact of relapsing fever (RF) spirochetosis is significant, since the pathogens exist on five of seven continents. The hallmark sign of infection is episodic fever and the greatest threat is to the unborn. With the goal of better understanding the specificity of B-cell responses and the role of immune responses in pathogenicity, we infected rhesus macaques with Borrelia turicatae (a new world RF spirochete species) by tick bite and monitored the immune responses generated in response to the pathogen. Specifically, we evaluated inflammatory mediator induction by the pathogen, host antibody responses to specific antigens, and peripheral lymphocyte population dynamics. Our results indicate that B. turicatae elicits from peripheral blood cells key inflammatory response mediators (interleukin-1β and tumor necrosis factor alpha), which are associated with preterm abortion. Moreover, a global decline in peripheral B-cell populations was observed in all animals at 14 days postinfection. Serological responses were also evaluated to assess the antigenicity of three surface proteins: BipA, BrpA, and Bta112. Interestingly, a distinction was observed between antibodies generated in nonhuman primates and mice. Our results provide support for the nonhuman primate model not only in studies of prenatal pathogenesis but also for diagnostic and vaccine antigen identification and testing.

Increased mortality from influenza infection in long-chain acyl-CoA dehydrogenase knockout mice

We previously showed that the mitochondrial fatty acid oxidation enzyme long-chain acyl-CoA dehydrogenase (LCAD) is expressed in alveolar type II pneumocytes and that LCAD-/- mice have altered breathing mechanics and surfactant defects. Here, we hypothesized that LCAD-/- mice would be susceptible to influenza infection. Indeed, LCAD-/- mice demonstrated increased mortality following infection with 2009 pandemic influenza (A/CA/07/09). However, the mortality was not due to increased lung injury, as inflammatory cell counts, viral titers, and histology scores all showed non-significant trends toward milder injury in LCAD-/- mice. To confirm this, LCAD-/- were infected with a second, mouse-adapted H1N1 virus (A/PR/8/34), to which they responded with significantly less lung injury. While both strains become increasingly hypoglycemic over the first week post-infection, LCAD-/- mice lose body weight more rapidly than wild-type mice. Surprisingly, while acutely fasted LCAD-/- mice develop hepatic steatosis, influenza-infected LCAD-/- mice do not. They do, however, become more hypothermic than wild-type mice and demonstrate increased blood lactate values. We conclude that LCAD-/- mice succumb to influenza from bioenergetic starvation, likely due to increased reliance upon glucose for energy.

An efficient and inexpensive method for measuring long-term thermoregulatory behavior

Thermoregulatory ability and behavior influence organismal responses to their environment. By measuring thermal preferences, researchers can better understand the effects that temperature tolerances have on ecological and physiological responses to both biotic and abiotic stressors. However, because of funding limitations and confounders, measuring thermoregulation can often be difficult. Here, we provide an effective, affordable (~$50 USD per unit), easy to construct, and validated apparatus for measuring the long-term thermal preferences of animals. In tests, the apparatus spanned temperatures from 9.29 to 33.94°C, and we provide methods to further increase this range. Additionally, we provide simple methods to non-invasively measure animal and substrate temperatures and to prevent temperature preferences of the focal organisms from being confounded with temperature preferences of its prey and its humidity preferences. To validate the apparatus, we show that it was capable of detecting individual-level consistency and among individual-level variation in the preferred body temperatures of Southern toads (Anaxyrus terrestris) and Cuban tree frogs (Osteopilus septentrionalis) over three-weeks. Nearly every aspect of our design is adaptable to meet the needs of a multitude of study systems, including various terrestrial amphibious, and aquatic organisms. The apparatus and methods described here can be used to quantify behavioral thermal preferences, which can be critical for determining temperature tolerances across species and thus the resiliency of species to current and impending climate change.

Temperature-Sensitive Mutants in the Influenza A Virus RNA Polymerase: Alterations in the PA Linker Reduce Nuclear Targeting of the PB1-PA Dimer and Result in Viral Attenuation

The influenza virus RNA-dependent RNA polymerase catalyzes genome replication and transcription within the cell nucleus. Efficient nuclear import and assembly of the polymerase subunits PB1, PB2, and PA are critical steps in the virus life cycle. We investigated the structure and function of the PA linker (residues 197 to 256), located between its N-terminal endonuclease domain and its C-terminal structured domain that binds PB1, the polymerase core. Circular dichroism experiments revealed that the PA linker by itself is structurally disordered. A large series of PA linker mutants exhibited a temperature-sensitive (ts) phenotype (reduced viral growth at 39.5°C versus 37°C/33°C), suggesting an alteration of folding kinetic parameters. The ts phenotype was associated with a reduced efficiency of replication/transcription of a pseudoviral reporter RNA in a minireplicon assay. Using a fluorescent-tagged PB1, we observed that ts and lethal PA mutants did not efficiently recruit PB1 to reach the nucleus at 39.5°C. A protein complementation assay using PA mutants, PB1, and β-importin IPO5 tagged with fragments of the Gaussia princeps luciferase showed that increasing the temperature negatively modulated the PA-PB1 and the PA-PB1-IPO5 interactions or complex stability. The selection of revertant viruses allowed the identification of different types of compensatory mutations located in one or the other of the three polymerase subunits. Two ts mutants were shown to be attenuated and able to induce antibodies in mice. Taken together, our results identify a PA domain critical for PB1-PA nuclear import and that is a "hot spot" to engineer ts mutants that could be used to design novel attenuated vaccines.

IMPORTANCE

By targeting a discrete domain of the PA polymerase subunit of influenza virus, we were able to identify a series of 9 amino acid positions that are appropriate to engineer temperature-sensitive (ts) mutants. This is the first time that a large number of ts mutations were engineered in such a short domain, demonstrating that rational design of ts mutants can be achieved. We were able to associate this phenotype with a defect of transport of the PA-PB1 complex into the nucleus. Reversion substitutions restored the ability of the complex to move to the nucleus. Two of these ts mutants were shown to be attenuated and able to produce antibodies in mice. These results are of high interest for the design of novel attenuated vaccines and to develop new antiviral drugs.

Development of a mouse-adapted live attenuated influenza virus that permits in vivo analysis of enhancements to the safety of live attenuated influenza virus vaccine

The live attenuated influenza virus vaccine (LAIV) is preferentially recommended for use in persons 2 through 49 years of age but has not been approved for children under 2 or asthmatics due to safety concerns. Therefore, increasing safety is desirable. Here we describe a murine LAIV with reduced pathogenicity that retains lethality at high doses and further demonstrate that we can enhance safety in vivo through mutations within NS1. This model may permit preliminary safety analysis of improved LAIVs.

Real-time monitoring of disease progression in rhesus macaques infected with Borrelia turicatae by tick bite

The hallmark of disease caused by tick- and louse-borne relapsing fever due to Borrelia infection is cyclic febrile episodes, which in humans results in severe malaise and may lead to death. To evaluate the pathogenesis of relapsing fever due to spirochetes in an animal model closely related to humans, disease caused by Borrelia turicatae after tick bite was compared in 2 rhesus macaques in which radiotelemetry devices that recorded body temperatures in 24-hour increments were implanted. The radiotelemetry devices enabled real-time acquisition of core body temperatures and changes in heart rates and electrocardiogram intervals for 28 consecutive days without the need to constantly manipulate the animals. Blood specimens were also collected from all animals for 14 days after tick bite, and spirochete densities were assessed by quantitative polymerase chain reaction. The complexity of disease caused by relapsing-fever spirochetes was demonstrated in the nonhuman primates monitored in real time. The animals experienced prolonged episodes of hyperthermia and hypothermia; disruptions in their diurnal patterns and repolarization of the heart were also observed. This is the first report of the characterizing disease progression with continuous monitoring in an animal model of relapsing fever due to Borrelia infection.

Feeling too hot or cold after breast cancer: is it just a nuisance or a potentially important prognostic factor?

There is widespread recognition among both patients and caregivers that breast cancer patients often experience debilitating deficiencies in their ability to achieve thermal comfort, feeling excessively hot or cold under circumstances when others are comfortable. However, this symptom receives little clinical or scientific attention beyond identification and testing of drugs that minimise menopausal-like symptoms. Could some of these symptoms represent an important prognostic signal? Could thermal discomfort be among other cytokine-driven sickness behaviour symptoms seen in many breast cancer patients? While the literature reveals a strong link between treatment for breast cancer and some menopausal vasomotor symptoms (e.g. hot flashes also known as "hot flushes"), there is little data on quantitative assessment of severity of different types of symptoms and their possible prognostic potential. However, recent, intriguing studies indicating a correlation between the presence of hot flashes and reduced development of breast cancer recurrence strongly suggests that more study on this topic is needed. In comparison to reports on the phenomenon of breast cancer-associated hot flashes, there is essentially no scientific study on the large number of women who report feeling excessively cold after breast cancer treatment. Since similar acquired thermal discomfort symptoms can occur in patients with cancers other than breast cancer, there may be as yet unidentified cancer- or treatment-driven factor related to temperature dysregulation. In general, there is surprisingly little information on the physiological relationship between body temperature regulation, vasomotor symptoms, and cancer growth and progression. The goal of this article is twofold: (1) to review the scientific literature regarding acquired deficits in thermoregulation among breast cancer survivors and (2) to propose some speculative ideas regarding the possible basis for thermal discomfort among some of these women. Specifically, we suggest a potential association with excessive pro-inflammatory cytokine activity, similar to other cytokine-driven symptoms experienced after breast cancer, including fatigue and depression. We highlight the similarity of some breast cancer-associated thermal discomfort symptoms to those which occur during fever, suggesting the possibility that there may be common underlying changes in pro-inflammatory cytokine activity in both conditions. We anticipate that this contribution will stimulate additional scientific interest among researchers in identifying potential mechanisms and prognostic significance of this under-studied aspect of breast cancer biology and survivorship.

The effect on mortality of antipyretics in the treatment of influenza infection: systematic review and meta-analysis

OBJECTIVE

To determine whether antipyretic treatment for influenza infection influences the risk of mortality in animal models and humans.

DESIGN

A systematic search of Medline, Embase and the Cochrane Register of Controlled Trials was undertaken to identify randomized placebo-controlled trials of antipyretic use in influenza infection in animal models or humans that reported mortality. A quantitative meta-analysis of the risk of death using Peto's one step odds ratio with calculation of the pooled risk of death and standard evaluation of heterogeneity was undertaken.

SETTING

Not applicable.

PARTICIPANTS

Not applicable.

MAIN OUTCOME MEASURES

Risk of mortality associated with antipyretic use in influenza infection.

RESULTS

Eight studies from three publications met the inclusion criteria. No human studies were identified. The risk of mortality was increased by antipyretic use in influenza-infected animals with a fixed effects pooled odds ratio of 1.34 (95% CI 1.04-1.73). An increased risk was observed with aspirin, paracetamol and diclofenac.

CONCLUSION

In animal models, treatment with antipyretics for influenza infection increases the risk of mortality. There are no randomized placebo-controlled trials of antipyretic use in influenza infection in humans that reported data on mortality and a paucity of clinical data by which to assess their efficacy. We suggest that randomized placebo-controlled trials of antipyretic use in human influenza infection are urgently required, and that these are sufficiently powered to investigate a potential effect on mortality.

Thermoregulatory, behavioral, and metabolic responses to heatstroke in a conscious mouse model

The typical core temperature (Tc) profile displayed during heatstroke (HS) recovery consists of initial hypothermia followed by delayed hyperthermia. Anecdotal observations led to the conclusion that these Tc responses represent thermoregulatory dysfunction as a result of brain damage. We hypothesized that these Tc responses are mediated by a change in the temperature setpoint. Tc (± 0.1°C; radiotelemetry) of male C57BL/6J mice was monitored while they were housed in a temperature gradient with ambient temperature (Ta) range of 20–39°C to monitor behaviorally selected Ta (Ts) or an indirect calorimeter (Ta = 25°C) to monitor metabolism (V̇o2) and calculate respiratory exchange ratio (RER). Responses to mild and severe HS (thermal area 249.6 ± 18.9 vs. 299.4 ± 19.3°C·min, respectively) were examined through 48 h of recovery. An initial hypothermia following mild HS was associated with warm Ts (∼32°C), ∼35% V̇o2 decrease, and RER ∼0.71 that indicated reliance on fatty acid oxidation. After 24 h, mild HS mice developed hyperthermia associated with warm Ts (∼32°C), ∼20% V̇o2 increase, and RER ∼0.85. Severe HS mice appeared poikilothermic-like in the temperature gradient with Tc similar to Ts (∼20°C), and these mice failed to recover from hypothermia and develop delayed hyperthermia. Cellular damage (hematoxylin and eosin staining) was undetectable in the hypothalamus or other brain regions in severe HS mice. Overall, decreases and increases in Tc were associated with behavioral and autonomic thermoeffectors that suggest HS elicits anapyrexia and fever, respectively. Taken together, Tc responses of mild and severe HS mice suggest a need for reinterpretation of the mechanisms of thermoregulatory control during recovery.

The olfactory nerve has a role in the body temperature and brain cytokine responses to influenza virus

Mouse-adapted human influenza virus is detectable in the olfactory bulbs of mice within hours after intranasal challenge and is associated with enhanced local cytokine mRNA and protein levels. To determine whether signals from the olfactory nerve influence the unfolding of the acute phase response (APR), we surgically transected the olfactory nerve in mice prior to influenza infection. We then compared the responses of olfactory-nerve-transected (ONT) mice to those recorded in sham-operated control mice using measurements of body temperature, food intake, body weight, locomotor activity and immunohistochemistry for cytokines and the viral antigen, H1N1. ONT did not change baseline body temperature (Tb); however, the onset of virus-induced hypothermia was delayed for about 13 h in the ONT mice. Locomotor activity, food intake and body weights of the two groups were similar. At 15 h post-challenge fewer viral antigen-immunoreactive (IR) cells were observed in the olfactory bulb (OB) of ONT mice compared to sham controls. The number of tumor necrosis factor alpha (TNFalpha)- and interleukin 1beta (IL1beta)-IR cells in ONT mice was also reduced in the OB and other interconnected regions in the brain compared to sham controls. These results suggest that the olfactory nerve pathway is important for the initial pathogenesis of the influenza-induced APR.

Attenuation of the influenza virus sickness behavior in mice deficient in Toll-like receptor 3

Certain sickness behaviors occur consistently in influenza-infected humans and mice. These include body temperature changes, somnolence, and anorexia. Several cytokines serve as mediators of the influenza acute phase response (APR), including these sickness behaviors, and one likely inducer of these cytokines is dsRNA produced during viral replication. TLR3 is known to be one of the host cellular components capable of recognizing dsRNA and activating cytokine synthesis. To determine the role of TLR3-detected viral dsRNA in the causation of viral symptoms, TLR3-deficient mice (TLR3 knockouts, or KOs) were infected with a marginally-lethal dose of mouse-adapted X-31 influenza virus. TLR3 KOs and their wild-type (WT) controls were monitored for baseline body temperature, locomotor activity, and sleep profiles prior to infection. Both mouse strains were then infected and monitored for changes in these sickness behaviors plus body weight changes and mortality for up to 14days post-infection. Consistent with the observations that influenza pathology is reduced in TLR3 KOs, we showed that hypothermia after post-infection day 5 and the total loss of body weight were attenuated in the TLR3 KOs. Sleep changes characteristic of this infection model [particularly increased non-rapid-eye-movement sleep (NREMS)] were also attenuated in TLR3 KOs and returned to baseline values more rapidly. Locomotor activity suppression was similar in both strains. Therefore virus-associated dsRNA detected by TLR3 appears to play a substantial role in mediating several aspects of the influenza syndrome in mice.

Influenza virus- and cytokine-immunoreactive cells in the murine olfactory and central autonomic nervous systems before and after illness onset

Influenza virus invades the olfactory bulb (OB) and enhances cytokine mRNAs therein at the time of illness onset. Here we show that viral antigen immunoreactivity co-localized with glial markers in the OB but could not be detected in other brain areas. Interleukin 1beta- and tumor necrosis factor alpha-immunoreactivity co-localized with neuronal markers in olfactory and central autonomic systems, and the number of cytokine-immunoreactive neurons increased at the time of illness onset [15 h post-inoculation (PI)] but not before (10 h PI). These results suggest that the OB virus influences the brain cytokines and therefore the onset of illness.

Universal influenza A M2e-HBc vaccine protects against disease even in the presence of pre-existing anti-HBc antibodies

The extracellular domain of influenza A virus matrix protein 2 (M2e) is strongly conserved. Therefore, vaccines based on M2e can induce broad-spectrum immunity against influenza. We have mainly used recombinant virus-like particles derived from Hepatitis B virus core (HBc) as carrier for efficacious presentation of the M2e antigen. Here, we address whether pre-existing HBc-specific immunity interferes with the protective immune response obtained by M2e-HBc vaccination. Anti-HBc antibodies were induced by immunizing mice with unsubstituted HBc virus-like particles in the presence of two different adjuvants. We demonstrate that pre-existing HBc-specific antibodies affect neither the induction of M2e-specific antibody responses to vaccination with M2e-HBc particles, nor the protective efficacy of the resulting response. These results suggest that vaccination with M2e-HBc can induce protective anti-M2e antibodies even in anti-HBc positive individuals. The implications of these findings are discussed in the context of the clinical development of an M2e-based universal influenza vaccine, which recently successfully completed a Phase I trial.

Effect of environmental temperature on sleep, locomotor activity, core body temperature and immune responses of C57BL/6J mice

Ambient temperature exerts a prominent influence on sleep. In rats and humans, low ambient temperatures generally impair sleep, whereas higher temperatures tend to promote sleep. The purpose of the current study was to evaluate sleep patterns and core body temperatures of C57BL/6J mice at ambient temperatures of 22, 26 and 30 degrees C under baseline conditions, after sleep deprivation (SD), and after infection with influenza virus. C57BL/6J mice were surgically implanted with electrodes for recording electroencephalogram (EEG) and electromyogram (EMG) and with intraperitoneal transmitters for recording core body temperature (T(c)) and locomotor activity. The data indicate that higher ambient temperatures (26 and 30 degrees C) promote spontaneous slow wave sleep (SWS) in association with reduced delta wave amplitude during SWS in C57BL/6J mice. Furthermore, higher ambient temperatures also promote recuperative sleep after SD. Thus, in mice, higher ambient temperatures reduced sleep depth under normal conditions, but augmented the recuperative response to sleep loss. Mice infected with influenza virus while maintained at 22 or 26 degrees C developed more SWS, less rapid eye movement sleep, lower locomotor activity and greater hypothermia than did mice maintained at 30 degrees C during infection. In addition, despite equivalent viral titers, mice infected with influenza virus at 30 degrees C showed less leucopenia and lower cytokine induction as compared with 22 and 26 degrees C, respectively, suggesting that less inflammation develops at the higher ambient temperature.

Interferon type I receptor-deficient mice have altered disease symptoms in response to influenza virus

The role of type I interferons (IFNs) in mediation of acute viral symptoms (fever, somnolence, anorexia, etc.) is unknown. To determine the role of type I IFN in selected symptom development, body temperature and sleep responses to a marginally lethal dose of X-31 influenza virus were examined in mice with a targeted mutation of the IFN receptor type I (IFN-RI knockouts) and compared to wild-type 129 SvEv control mice. Mice were monitored for 48 h to determine baseline temperature and sleep profiles prior to infection, and then for 9 days following infection. Hypothermic responses to virus were perceptible beginning at 64 h post-infection (PI) and were more marked in KO mice until 108 h, when hypothermia became more exaggerated in wild-type controls. Temperatures of wild-type mice continued to decline through day 9 while temperatures in IFN-RI KO mice stabilized. Time spent in non-rapid eye movement sleep (NREMS) increased in KO mice when hypothermia was marked and then returned to baseline levels, while NREMS continued to increase in wild-type mice through day 9. Other sleep parameters [time spent in rapid eye movement sleep (REMS), relative NREMS EEG slow wave activity, NREMS EEG power density] were all reduced in wild-type mice compared to KOs from days 3 to 8 while REMS low frequency EEG power density increased in wild-type relative to KOs. In conclusion, our results indicate that the presence of functional type I IFN slightly ameliorates disease symptoms early in the X-31 infection while exacerbating disease symptoms later in the infection.

Macrophage participation in influenza-induced sleep enhancement in C57BL/6J mice

Mice develop changes in sleep during the nonspecific immune response that occurs during the initial few days after inoculation with influenza virus. T lymphocytes, neutrophils, macrophages, and natural killer (NK) cells all participate in the early host response to influenza infection. All of these cell types are potential sources of endogenous substances that modulate sleep, but the contributory role of each cell type to the alteration of somnolence during infection has not been determined. To investigate which cell types contribute to the sleep enhancement that develops during influenza infection in mice, the sleep patterns of C57BL/6J mice with perturbations of particular facets of host immune response capabilities were assessed before and after influenza infection. Targeted mutation of the gene Ccl3 (macrophage inflammatory protein 1 alpha) prevented development of the dark phase sleep enhancement that is characteristic of C57BL/6J mice after influenza infection. Other experimental treatments that impair macrophage or monocyte function also produced significant (administration of pentoxifylline or CNI-1493) or marginally significant (deletion of the interferon-gamma gene or intranasal administration of carrageenan) changes in influenza-induced sleep enhancement in C57BL/6J mice. In contrast, functional impairments of NK cells, neutrophils, and T lymphocytes did not significantly influence sleep responses. These data therefore support a contributory role for macrophages, but not for NK cells, neutrophils, and T lymphocytes, in eliciting the sleep response typical of influenza-infected C57BL/6J mice.

The hypothalamic-pituitary-adrenal axis and viral infection

The hypothalamic-pituitary-adrenal (HPA) axis plays an important immunomodulatory role during viral infection. Activation of the HPA axis ultimately leads to elevated plasma levels of glucocorticoid (GC) hormones with the ability to mediate adaptive behavioral, metabolic, cardiovascular and immune system effects. In this review, we focus on the modulation of anti-viral immunity and viral pathogenesis by the HPA axis.

Alterations in EEG activity and sleep after influenza viral infection in GHRH receptor-deficient mice

Viral infections induce excess non-rapid eye movement sleep (NREMS) in mice. Growth hormone-releasing hormone receptor (GHRH receptor) was previously identified as a candidate gene responsible for NREMS responses to influenza challenge in mice. The dwarf lit/lit mouse with a nonfunctional GHRH receptor was used to assess the role of the GHRH receptor in viral-induced NREMS. After influenza A virus infection the duration and intensity [electroencephalogram (EEG) delta power] of NREMS increased in heterozygous mice with the normal phenotype, whereas NREMS and EEG delta power decreased in homozygous lit/lit mice. Lit/lit mice developed a pathological state with EEG slow waves and enhanced muscle tone. Other influenza-induced responses (decreases in rapid eye movement sleep, changes in the EEG high-frequency bands during the various stages of vigilance, hypothermia, and decreased motor activity) did not differ between the heterozygous and lit/lit mice. GH replacement failed to normalize the NREMS responses in the lit/lit mice after influenza inoculation. Decreases in NREMS paralleled hypothermia in the lit/lit mice. Lung virus levels were similar in the two mouse strains. Lit/lit mice had a higher death rate after influenza challenge than the heterozygotes. In conclusion, GHRH signaling is involved in the NREMS response to influenza infection.

Influenza infection promotes macrophage traffic into arteries of mice that is prevented by D-4F, an apolipoprotein A-I mimetic peptide

BACKGROUND

We reported that HDL loses its antiinflammatory properties during acute influenza A infection in mice, and we hypothesized that these changes might be associated with increased trafficking of macrophages into the artery wall. The present study tested this hypothesis.

METHODS AND RESULTS

D-4F, an apolipoprotein A-I mimetic peptide, or vehicle in which it was dissolved (PBS) was administered daily to LDL receptor-null mice after a Western diet and after influenza infection. D-4F treatment increased plasma HDL cholesterol and paraoxonase activity compared with PBS and inhibited increases in LDL cholesterol and peak levels of interleukin-6 after infection. Lung viral titers were reduced by 50% in mice receiving D-4F. Injection of female mice with male macrophages, which were detected with real-time polymerase chain reaction to measure the male Sry gene, revealed a marked increase in macrophage traffic into the aortic arch and innominate arteries after infection that was prevented by administration of D-4F.

CONCLUSIONS

We conclude that loss of antiinflammatory properties of HDL after influenza infection in mice is associated with increased arterial macrophage traffic that can be prevented by administration of D-4F.

Tumor necrosis factor (TNF) ligand and TNF receptor deficiency affects sleep and the sleep EEG

Tumor necrosis factor (TNF) and lymphotoxin-alpha (LT-alpha) are proinflammatory cytokines involved in host defense and pathogenesis of various diseases. In addition, there is evidence that TNF is involved in sleep. TNF and LT-alpha both bind to the tumor necrosis factor receptors (TNFR). Recently, it was shown that TNF receptor 1 (TNFR1) knockout mice (R1KO) sleep less during the light period than controls. We investigated the effect of a TNF and LT-alpha double deficiency on sleep in mice (Ligand KO) and compared their sleep with that of R1KO, TNFR2 knockout (R2KO) mice, and wild-type (WT) controls. All mice were adapted to a 12:12 h light:dark cycle and their electroencephalographs (EEG) and electromyographs (EMG) were continuously recorded during a baseline day, 6-h sleep deprivation (SD), and 18-h recovery. Ligand KO and R2KO had 15% less rapid eye movement (REM) sleep during the baseline light period due to a reduction in REM sleep episode frequency. After SD, all genotypes showed an initial increase in slow-wave activity (SWA) (EEG power density between 0.75 and 4.0 Hz) in non-REM sleep, which gradually declined in the following hours. In Ligand KO the increase was mainly caused by an increase in fast SWA (2.75-4.0 Hz), which was also increased in R2KO. In contrast, in R1KO mice the increase was limited to the slow portion of SWA (0.75-2.5 Hz). R2KO and WT mice showed increases in both frequency ranges. The sub-division into fast and slow SWA frequencies corresponds to previous electrophysiological data where the two types of slow-waves were induced by either excitatory or inhibitory stimuli. Our data suggest that in Ligand KO the SWA increase is caused by an increase in excitatory input to the cortex, whereas in R1KO this input seems to be almost absent.

Feeding behavior during sialodacryoadenitis viral infection in rats

Sialodacryoadenitis (SDA) is a highly contagious common viral infection in rats, akin to mumps in humans. Anorexia occurs during such viral infection. But the pattern of the decrease in food intake (a decrease in either meal size and meal number or both) during spontaneous viral infection has not been previously characterized. We observed the onset of anorexia and an abnormal feeding pattern during an opportunistic SDA viral infection in our rat colony. We thus studied seven male rats. Before the viral infection there was a positive association between food intake and meal number (P<.05). After infection food intake decreased by 68%. This occurred via a significant decrease in meal size (by 69%) (P<.05); and a nonsignificant decrease in meal number (P=.71). This pattern of decreased food intake is similar to that occurring during indomethacin-induced ulcerative ileitis, where we previously measured an increase in plasma tumor-necrosis factor (TNF)-alpha. Anorexia in response to bacterial lipopolysaccharide administration, which is also linked to plasma TNF-alpha, is however, caused only via a decrease in meal number. The differences in the decrease in the feeding pattern between the SDA viral and a bacterial infection suggest that factors other than TNF-alpha alone play a significant role in the mechanism of anorexia during a viral infection.

Animal models of pulmonary infection in the compromised host: potential usefulness for studying health effects of inhaled particles

Pulmonary infection leading to pneumonia is a significant cause of morbidity and mortality worldwide. Airborne particles have been associated with pneumonia through epidemiological research, but the mechanisms by which particles affect the incidence of pneumonia are not well established. The purpose of this review is to examine the potential of animal models to improve our understanding of the mechanisms by which inhaled particles might affect the incidence and resolution of pulmonary infection. The pathogenesis of pneumonia in most animal models differs from that in humans because humans frequently have underlying diseases that predispose them to infection with relatively low doses of pathogens. Normal, healthy animals lack the underlying pathology often found in humans and clear bacteria and viruses rapidly from their lungs. To overcome this, animals are administered large inocula of pathogens, are treated with agents that cause mucosal lesions, or are treated with immunosuppressive drugs. Alternatively, pathogenic bacteria are protected from phagocytosis by encasing them in agar. No one animal model will replicate a human disease in its entirety, and the choice of model depends upon how well the animal infection mimics the particular human response being examined. The advantages and disadvantages of animal models in current use for bacterial and viral infections important in the etiology of human pneumonia are reviewed in detail. Considerable data indicate that prior exposure to particles compromises the ability of experimental animals to resolve a subsequent infection. In addition, information is available on the effects of particle exposure on various portions of respiratory defense including phagocytic function, ciliary movement, inflammation, and antibody response in the absence of infection. In contrast, little research to date has examined the consequences of particle exposure on the host defense mechanisms of animals already infected or on their ability to resolve their infection.

Distinct pathogenesis of hong kong-origin H5N1 viruses in mice compared to that of other highly pathogenic H5 avian influenza viruses

In 1997, an outbreak of virulent H5N1 avian influenza virus occurred in poultry in Hong Kong (HK) and was linked to a direct transmission to humans. The factors associated with transmission of avian influenza virus to mammals are not fully understood, and the potential risk of other highly virulent avian influenza A viruses infecting and causing disease in mammals is not known. In this study, two avian and one human HK-origin H5N1 virus along with four additional highly pathogenic H5 avian influenza viruses were analyzed for their pathogenicity in 6- to 8-week-old BALB/c mice. Both the avian and human HK H5 influenza virus isolates caused severe disease in mice, characterized by induced hypothermia, clinical signs, rapid weight loss, and 75 to 100% mortality by 6 to 8 days postinfection. Three of the non-HK-origin isolates caused no detectable clinical signs. One isolate, A/tk/England/91 (H5N1), induced measurable disease, and all but one of the animals recovered. Infections resulted in mild to severe lesions in both the upper and lower respiratory tracts. Most consistently, the viruses caused necrosis in respiratory epithelium of the nasal cavity, trachea, bronchi, and bronchioles with accompanying inflammation. The most severe and widespread lesions were observed in the lungs of HK avian influenza virus-infected mice, while no lesions or only mild lesions were evident with A/ck/Scotland/59 (H5N1) and A/ck/Queretaro/95 (H5N2). The A/ck/Italy/97 (H5N2) and the A/tk/England/91 (H5N1) viruses exhibited intermediate pathogenicity, producing mild to moderate respiratory tract lesions. In addition, infection by the different isolates could be further distinguished by the mouse immune response. The non-HK-origin isolates all induced production of increased levels of active transforming growth factor beta following infection, while the HK-origin isolates did not.

Role of fever in disease

Infection, trauma, and injury result in a stereotypical response that includes loss of food appetite, increased sleepiness, muscle aches, and fever. For thousands of years fever was considered a protective response, and fevers were induced by physicians to combat certain infections. But with the advent of antipyretic drugs, physicians started to reduce fevers, and fever therapy was virtually abandoned. As a result of (1) studies on the evolution of fever, (2) further understanding of just how tightly the process of fever is regulated, and (3) detailed studies on how fever affects host morbidity and mortality, the view of fever as a host defense response has reemerged. However, data indicate that not all fevers are protective and that high fevers are maladaptive. These issues are discussed in the context of the evolution of host defense responses versus modern medical technology. In short, we speculate that patients who would not have survived severe sepsis in the past are now being kept alive and that the occasionally high fevers seen in these patients may be maladaptive.

The use of knockout mice to understand the role of cytokines in fever

1. In most instances, data obtained using knockout mice to dissect the role of cytokines in fever are similar to data obtained by other, more traditional experimental techniques. 2. Interleukin (IL)-1beta appears to be critically involved in fever caused by some routes of infection/inflammation (e.g. localized inflammation with turpentine). This cytokine has only a small role in fevers caused by i.p. injection of lipopolysaccharide (LPS). These IL-1beta-induced fevers in knockout mice appear to be via the induction of IL-6, similar to LPS-induced fevers in rats. Interleukin-6 also appears to be critically involved in turpentine-induced fever. 3. The precise role of tumour necrosis factor (TNF) in fever is controversial. Data obtained from knockout mice lacking both TNF receptors do not support a pyrogenic role for TNF in fever either to i.p. injection of LPS, s.c. injection of turpentine or following caecal ligation and puncture. 4. The roles of these cytokines in fevers induced by injection of LPS, IL-1beta, turpentine and caecal ligation and puncture are summarized. The data show the complexity of the febrile response. Depending on the types of inflammatory/infectious stimuli, different cytokines play important roles. Because other cytokines are thought to be involved in fever (e.g. macrophage inflammatory protein, interferons), considerable work is still needed to dissect the precise roles of cytokines in fever.

Strain differences in the somnogenic effects of interferon inducers in mice

Increased slow-wave sleep accompanies influenza infection in C57BL/6 mice but not BALB/c mice. These strains of mice possess different alleles of the genetic lucus If-1, which codes for high (If-1h; C57BL/6) and low (If-1(1); BALB/c) production of interferon (IFN), a putative sleep-inducing cytokine. To evaluate the contribution of the If-1 gene to differences in murine sleep propensity, sleep patterns were evaluated in mice treated with the IFN inducers polyinosinic:polycytidilic acid (pIC) or Newcastle disease virus (NDV), with influenza virus, or with murine interferon (IFN-alpha) or IFN-alpha/beta. As compared with baseline values, C57BL/6 mice exhibited increased slow-wave sleep after all three challenges, but BALB/c mice did not. Congenic B6.C-H28c mice, which bear the BALB/c allele for low IFN production on the C57BL/6 genetic background, showed enhanced slow-wave sleep after influenza infection but not after NDV. Exogenous IFN did not enhance slow-wave sleep in either C57BL/6 or BALB/c mice. These data suggest that the If-1 allele may influence the somnogenic responsiveness of mice under some conditions but that additional mechanisms may contribute to sleep enhancement during infectious disease.

A mechanism for the inhibition of fever by a virus

Poxviruses encode proteins that block the activity of cytokines. Here we show that the study of such virulence factors can contribute to our understanding of not only virus pathogenesis but also the physiological role of cytokines. Fever is a nonspecific response to infection that contributes to host defense. Several cytokines induce an elevation of body temperature when injected into animals, but in naturally occurring fever it has been difficult to show that any cytokine has a critical role. We describe the first example of the suppression of fever by a virus and the molecular mechanism leading to it. Several vaccinia virus strains including smallpox vaccines express soluble interleukin 1 (IL-1) receptors, which bind IL-1 beta but not IL-1 alpha. These viruses prevent the febrile response in infected mice, whereas strains that naturally or through genetic engineering lack the receptor induce fever. Repair of the defective IL-1 beta inhibitor in the smallpox vaccine Copenhagen, a more virulent virus than the widely used vaccine strains Wyeth and Lister, suppresses fever and attenuates the disease. The vaccinia-induced fever was inhibited with antibodies to IL-1 beta. These findings provide strong evidence that IL-1 beta, and not other cytokines, is the major endogenous pyrogen in a poxvirus infection.

The adaptive value of fever

There is overwhelming evidence in favor of fever being an adaptive host response to infection that has persisted throughout the animal kingdom for hundreds of millions of years. As such, it is probable that the use of antipyretic/anti-inflammatory/analgesic drugs, when they lead to suppression of fever, results in increased morbidity and mortality during most infections; this morbidity and mortality may not be apparent to most health care workers because fever is only one of dozens of host defense responses. Furthermore, most infections are not life-threatening and subtle changes in morbidity are not easily detected.