Hydrogen sulphide reduces renal ischemia-reperfusion injury by enhancing autophagy and reducing oxidative stress.

AIM: Renal ischemia-reperfusion injury (IRI) is a major cause of acute kidney injury. Hydrogen sulphide (H2S) exerts a protective effect in renal IRI. The present study was carried out to investigate the effects of exogenous H2S on renal IRI by regulating autophagy in mice. METHODS: Mice were randomly assigned to control, IRI and NaHS (an H2S donor, 28, 56 and 100 µmol/kg) groups. Renal IRI was induced by clamping the bilateral renal pedicles with non-traumatic arterial clamp for 45 min and then reperfused for 24 h. Mice were administered intraperitoneally with NaHS 20 min prior to renal ischemia. Sham group mice underwent the same procedures without clamping. Serum and kidney tissues were harvested 24 h after reperfusion for functional, histological, oxidative stress, and autophagic determination. RESULTS: Compared with the control group, the concentrations of serum creatinine (Scr), blood urea nitrogen (BUN), and malondialdehyde (MDA), the protein levels of LC3II/I, Beclin-1 and P62, as well as the number of autophagosomes were significantly increased, but the activity of superoxide dismutase (SOD) was decreased after renal IRI. NaHS pre-treatment dramatically attenuated renal IRI-induced renal dysfunction, histological changes, MDA concentration and p62 expression in a dose-dependent manner. However, NaHS increased the SOD activity and the protein levels of LC3II/I and Beclin-1. CONCLUSION: These results indicate that exogenous H2S protects the kidney from IRI through enhancement of autophagy and reduction of oxidative stress. Novel H2S donors could be developed in the treatment of renal IRI.

Brain endothelial CXCL12 attracts protective natural killer cells during ischemic stroke.

BACKGROUND: The innate lymphoid cell (ILC) family consists of NK cells, ILC type 1, 2, 3 and lymphoid tissue inducer cells. They have been shown to play important roles in homeostasis and immune responses and are generally considered tissue resident. Not much is known about the presence of ILC members within the central nervous system and whether they are tissue resident in this organ too. Therefore, we studied the presence of all ILC members within the central nervous system and after ischemic brain insult. METHODS: We used the photothrombotic ischemic lesion method to induce ischemic lesions within the mouse brain. Using whole-mount immunofluorescence imaging, we established that the ILCs were present at the rim of the lesion. We quantified the increase of all ILC members at different time-points after the ischemic lesion induction by flow cytometry. Their migration route via chemokine CXCL12 was studied by using different genetic mouse models, in which we induced deletion of Cxcl12 within the blood-brain barrier endothelium, or its receptor, Cxcr4, in the ILCs. The functional role of the ILCs was subsequently established using the beam-walk sensorimotor test. RESULTS: Here, we report that ILCs are not resident within the mouse brain parenchyma during steady-state conditions, but are attracted towards the ischemic stroke. Specifically, we identify NK cells, ILC1s, ILC2s and ILC3s within the lesion, the highest influx being observed for NK cells and ILC1s. We further show that CXCL12 expressed at the blood-brain barrier is essential for NK cells and NKp46+ ILC3s to migrate toward the lesion. Complementary, Cxcr4-deficiency in NK cells prevents NK cells from entering the infarct area. Lack of NK cell migration results in a higher neurological deficit in the beam-walk sensorimotor test. CONCLUSIONS: This study establishes the lack of ILCs in the mouse central nervous system at steady-state and their migration towards an ischemic brain lesion. Our data show a role for blood-brain barrier-derived CXCL12 in attracting protective NK cells to ischemic brain lesions and identifies a new CXCL12/CXCR4-mediated component of the innate immune response to stroke.

Comprehensive analysis of lipid metabolism in influenza virus infection.

Influenza A virus (IAV) exploits host metabolic pathways to support its replication. To improve the understanding of lipid metabolic changes that could occur upon IAV infection, a comprehensive analysis of lipid metabolites in A549 cells infected with the avian H9N2 virus at the different time points was performed. It was found that H9N2 infection could largely promote the level of lipid metabolites. Further, these metabolites were mainly included in glycerophospholipids (GPs), sphingolipids (SPs), glycerolipids (GLs), fatty acids (FAs), sterollipids (STs), triglycerides (TGs), and prenol lipids (PRs). Specifically, Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that these metabolites were mainly associated with the glycerphospholipid metabolism, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, and autophagy. Furthermore, it is interesting to note that these metabolites, including FFA(19:1), PE(P-17:0_20:3), PE(P-18:1_20:2), LPC(14:0/0:0), PE(O-18:0_20:3), and MGDG(16:0_18:1), are upregulated and shared in the top 10 at 12 h, 24 h, 36 h, and 48 h after H9N2 infection, indicative of the possibility of acting as biomarkers for the diagnosis in the lung infected with influenza virus. These pathways and altered metabolites could provide new understandings about biological characteristics and pathogenicity of influenza virus and have the potential to serve as biomarkers for influenza.

Comparative analysis of changes in diarrhea and gut microbiota in Beigang pigs.

Increasing evidence indicated that the gut microbiota is a large and complex organic combination, which is closely related to the host health. Diarrhea is a disease with devastating effects on livestock that has been demonstrated to be associated with gut microbiota. Currently, studies on gut microbiota and diarrhea have involved multiple species, but changes in gut microbiota of Beigang pigs during diarrhea have not been characterized. Here, we described gut microbial changes of Beigang pigs during diarrhea. Results indicated that a total of 4423 OTUs were recognized in diarrheic and healthy Beigang pigs, and Firmicutes and Bacteroidota were the most dominant phyla regardless of health status. However, the major components of the gut microbiota changed between diarrheic and healthy Beigang pigs. Bacterial taxonomic analysis revealed that the relative abundances of 3 phyla (Synergistota, Actinobacteriota and Spirochaetota) and 30 genera increased significantly during diarrhea, whereas the relative abundances of 3 phyla (Patescibacteria, Bacteroidota and Fibrobacterota) and 41 genera decreased significantly. In conclusion, this study found significant changes in the gut microbiota of Beigang pigs during diarrhea. Meanwhile, this also lays the foundation for the prevention and treatment of diarrhea in Beigang pigs and the further discovery of more anti-diarrhea probiotics.

Fracture strength and failure mechanism of graphene-containing grain boundaries and pores.

Grain boundaries and pores commonly manifest in graphene sheets during experimental preparation. Additionally, pores have been intentionally incorporated into graphene to fulfill specific functions for various applications. However, how does the simultaneous presence of pores and grain boundaries impact the mechanical properties of graphene? This paper establishes uniaxial tension models of single-layer graphene-containing pores and three types of experimentally observed. The effect of interaction between pores and grain boundaries on the fracture strength of graphene was studied respectively for three types of grain boundaries by employing molecular dynamics simulations and considering factors such as pore size, the distance between pores and grain boundaries, and loading angle. A competitive mechanism between the intrinsic strength of pristine graphene with grain boundaries (referred to as pristine GGBs), which varies with the loading angle and the fracture strength of graphene sheets with pores that changes with the size of the pores, governs the fracture strength and failure modes of GGBs with pores. When the former exceeds the latter, the fracture strength of GGBs with pores primarily depends on the size of the pores, and fractures occur at the edges of the pores. Conversely, when the former is lower, the fracture strength of GGBs with pores relies on the loading angle and the distance between pores and grain boundaries, leading to grain boundary rupture. If the two strengths are comparable, the failure modes are influenced by the distance between pores and grain boundaries as well as the loading angle. The findings further elucidate the impact of coexisting grain boundaries and pores on the fracture behavior of graphene, providing valuable guidance for the precise design of graphene-based devices in the future.

Auxetic ographene: a new 2D Dirac nodal-ring semimetal carbon-based material with a high negative Poisson's ratio.

Auxetic and semimetallic materials possess many advanced applications due to the negative Poisson's ratio (NPR) effect and unique electronic properties. However, candidates with the above properties are rather scarce, especially in the 2D carbon materials. Here, a new 2D NPR material with a Dirac nodal ring, named ographene, is identified using first-principles calculations. Ographene possesses anisotropic Young's modulus and unusual in-plane NPR (-0.11), which mainly originated from its puckered tetrahedron structure. In addition, the electronic band structure calculations show that ographene is a topological node-ring semimetal with high Fermi velocity. Moreover, the electronic band structure is robust against external strain. The intrinsic NPR coupled with robust electronic properties renders auxetic ographene promising for applications in electronics and mechanics areas.

Reconfiguring graphene to achieve intrinsic negative Poisson's ratio and strain-tunable bandgap.

A new two-dimensional carbon-based material consisting of pentagonal and hexagonal elements is identified by numerical experiments, which is called phgraphene and possesses not only a tunable semimetallic feature but also a direction-dependent even sign-changed Poisson's ratio. The structural stability of such a new material is first checked systematically. It is found that phgraphene has a similar energy as theγ-graphyne, a thermally stable structure from the room temperature to 1500 K, and elastic constants satisfying the Born-Huang criterion. Both the band structure and density of states are further verified with different techniques, which demonstrate a Dirac semimetallic characteristic of phgraphene. A more interesting finding is that the band structure can be easily tuned by an external loading, resulting in the transition from semimetal to semiconductor or from type I to type III. As a new material that may be applied in the future, the mechanical property of phgraphene is further evaluated. It shows that phgraphene is a typically anisotropic material, which has not only direction-dependent Young's moduli but also direction-dependent even sign-changed Poisson's ratios. The microscopic mechanisms of both the electrical and mechanical properties are revealed. Such a versatile material with tunable band structure and auxetic effect should have promising applications in the advanced nano-electronic field in the future.

A Dirac nodal surface semi-metallic carbon-based structure as a universal anode material for metal-ion batteries with high performance.

The rapid development of electronic devices requires high power storage batteries. However, reported 3D carbon-based materials are semiconductors or metals and are used in Li- or Na-ion batteries with low capacities. Thus, it is of interest to discover whether there is a universal semi-metallic material for use in high performance Li-, Na-, and K-ion batteries. Inspired by the recent synthesis of 3D carbon-based materials, in the research reported here, a 3D regular porous structure (bct-C56) is designed using graphene sheets. The porous carbon-based material has mechanical, dynamic, thermal, and mechanical stabilities. Interestingly, bct-C56 exhibits semi-metallic features with two Dirac nodal surfaces with mirror symmetry, as well as high Fermi velocities, indicating high electron-transport abilities. More excitingly, its theoretical capacities are 743.8, 478.2, and 425.0 mA h g-1, with diffusion barriers of 0.05-0.12, 0.07-0.12, and 0.03-0.05 eV, average OCVs of 0.31, 0.45, and 0.59 V, and volume expansion levels of 1.2%, 0.02%, and 3.1%, in Li-, Na-, and K-ion batteries, respectively. All these excellent characteristics suggest that semi-metallic bct-C56 is a universal anode material for use in metal-ion batteries with a fast charge-discharge rate. In this research, not only was a new material with a Dirac nodal surface feature designed, but it also offers an approach for the creation of high performance and universal metal-ion battery anodes with 3D porous carbon materials.

A modified slow sand filtration system of epikarst spring water in karst mountainous areas, China.

Epikarst springs are commonly used for drinking water in karst mountainous areas, but they tend to bring health risks to residents because of their vulnerability. In this work, a modified slow sand filtration system (M-SSF) was established as a case study to purify and conserve the epikarst spring water. The outcomes indicate that the purification of M-SSF relies mainly on the adsorption and ion exchange of the filter medium (mixtures of heat-treated red clay and crushed limestone, MHRCCL) during the schmutzdecke juvenility, and on the schmutzdecke-formed food chain of pollutants → bacteria → protozoa after the schmutzdecke maturity. The closed water cellar lined with ceramic tiles could reduce the deterioration of epikarst spring water during storage. Via 16S rRNA sequencing, it was found that the high abundance of TM6_Dependentiae in purified epikarst spring water (PESW) suggested that the M-SSF system relies on the formation of a closed food chain to achieve effective water purification. The decrease of Pseudarcicella abundance in PESW indicated that M-SSF could effectively prevent the water quality from external influences represented by leeches. Besides, the 16S function prediction was used to qualitatively characterize microbial nitrogen metabolism, as well as organic matter degradation in water purification.

Contribution of Connexin Hemichannels to the Pathogenesis of Acute Lung Injury.

Connexin (Cx) family members form hemichannels (HCs) and gap junctions (GJs). Biological functions of Cx HCs have not been adequately characterized due to the inability to selectively target HCs or GJs. Recently, we developed a 6-mer peptide mimetic (P5) of the first extracellular loop of Cx43 and showed that it can block the permeability of HCs but not GJs formed by Cx43. In this study, we further characterized the HC blocking property of P5 and investigated the role of Cx HCs in acute lung injury (ALI). We found that P5 administration decreased HC permeability, in pulmonary microvascular endothelial cells, HepG2 cells, and even Cx43-deficient astrocytes, which express different sets of Cxs, suggesting that P5 is a broad spectrum Cx HC blocker. In addition, P5 reduced HC permeability of alveolar cells in vivo. Moreover, P5 decreased endotoxin-induced release, by vascular endothelial cells in vitro, of high mobility group box protein 1 (HMGB1), a critical mediator of acute lung injury (ALI), and reduced HMGB1 accumulation in bronchoalveolar lavage fluid (BALF) of mice subjected to intratracheal endotoxin instillation. Furthermore, P5 administration resulted in a significant decrease in the concentrations of ALT, AST, and LDH in the BALF, the accumulation of leukocytes in alveoli, and the mortality rate of mice subjected to ALI. Wright-Giemsa staining showed that P5 caused similar reductions of both neutrophils and monocytes in BALF of ALI mice. Together, these results suggest that Cx HCs mediate HMGB1 release, augment leukocyte recruitment, and contribute to ALI pathology.

Stabilization of two-dimensional penta-silicene for flexible lithium-ion battery anodes via surface chemistry reconfiguration.

Silicon-based two-dimensional (2D) materials have unique properties and extraordinary engineering applications. However, penta-silicene is unstable. Herein, by employing first-principles calculations, we provide a facile surface chemistry method, i.e. functionalization, to acquire and reconfigure stable penta-silicene for use in flexible lithium-ion batteries. Our results of density functional theory calculations showed that the reconfigured penta-silicene nanosheets possess a broad range of properties, including semiconductors with an indirect bandgap, semiconductors with a direct bandgap, semimetals and metals. For fluorinated penta-silicene, a fluorine-concentration-induced transition from a semiconductor to a metal is found. For fully fluorinated penta-silicene, a mechanically induced transition from a semiconductor with an indirect bandgap to a semiconductor with a direct bandgap is obtained. Our calculation results showed the reconfigured penta-silicene is a high-performance anode for use in flexible lithium (Li)-ion batteries. A transition from a semiconductor to a metal with adsorption of Li atoms indicates a high electrical conductivity. It possesses low Li diffusion barriers (0.08-0.28 eV), demonstrating a high mobility of Li ions. The metallic feature and low Li diffusion barriers reveal that it has an ultrafast charge/discharge rate. This work suggests that surface chemistry reconfiguration provides new stable materials with excellent mechanical properties and tunable electronic properties for their promising applications in flexible metal-ion batteries and solar batteries as well as nanoelectronics devices.

Highly negative Poisson's ratio in a flexible two-dimensional tungsten carbide monolayer.

Auxetic materials have numerous promising engineering applications such as fracture resistance and energy storage due to their negative Poisson's ratios (NPRs). However, compared to materials possessing positive Poisson's ratios (PPRs), auxetic materials are rare. In this paper, by employing first principles calculations, we found a high NPR two-dimensional (2D) material, tungsten carbide (W2C), in the transition metal carbides (MXenes). Our results of the relatively moderate Young's modulus and fracture strength as well as the critical strain showed that the 2D monolayer W2C is an extraordinary flexible material. Our DFT results also demonstrated that W2C possesses high NPRs while Hf2C and Ta2C have PPRs. Furthermore, the mechanically induced deformation mechanism and the NPR formation mechanism of W2C have been proposed. Such an intrinsic NPR in W2C is attributed to the strong coupling between the C-p and W-d orbitals in the pyramid structural unit. The mechanically induced deformation mechanism and the PPR formation mechanism of Hf2C have also been determined. The intrinsic NPR for W2C transforms to PPR upon the surface functionalization induced. The behavior occurs due to the W-C interaction weakening. The excellent NPR in the 2D MXene material combined with other outstanding properties such as the metallic state would bring about its promising engineering prospects, ranging from the metal-ion battery, to automobiles and aircraft.

Modulation of the electronic and mechanical properties of phagraphene via hydrogenation and fluorination.

Recently, a new carbon sheet, phagraphene, was proposed by theoretical calculations [Nano Lett. 2015, 15, 6182]. In this paper, hydrogenated and fluorinated phagraphene (denoted as H-PHA and F-PHA) sheets have been systematically studied using first-principles calculations. The results of formation energy, ab initio molecular dynamics, phonon dispersion and elastic constants confirm that the modified phagraphene sheets are thermodynamically and dynamically as well as mechanically stable. We find that hydrogenation or fluorination is an effective way to modulate the bandgap, and we also find that adsorption-induced semimetal-semiconductor transition and adsorption-induced semimetal-insulator transition occur. Configuration-dependent bandgaps for partially H-PHA and configuration-independent bandgaps for fully H-PHA are determined. Adsorption-ratio-dependent bandgaps of H-PHA and F-PHA are also identified. Bandgaps calculated from HSE06 and PBE functionals of fully H-PHA are larger than those of F-PHA, and they are comparable to hydrogenated/fluorinated penta-graphene while they are larger than their corresponding graphene. Dependence of bandgaps of fully H-PHA and F-PHA on the tensile strain is investigated, and our calculations show that an insulator-semiconductor transition occurs upon increasing the tensile strain. Our results also show that the mechanical properties can be controlled using hydrogenation and fluorination. The calculations of Young's modulus and Poisson's ratio reveal that functionalized phagraphene sheets possess suitable stiffness and resistance to volume deformation, and both are smaller than those of the pristine phagraphene.

Anti-inflammatory and retinal protective effects of capsaicin on ischaemia-induced injuries through the release of endogenous somatostatin.

The mechanisms regarding the retinal protective and anti-inflammatory effects of capsaicin (CAP) remain unclear. Somatostatin is contained in CAP-sensitive sensory neurons, including nerve terminals, from which it can be released by capsaicin. The present study provides a novel neurohumoral regulatory mechanism for CAP-induced-endogenous somatostatin in a retinal ischaemia-reperfusion (I/R) mouse model. CAP (0.5 mg/kg) was injected subcutaneously 5 minutes after I/R. A selective somatostatin-depleting agent, cysteamine, was applied subcutaneously 4 hours before the experiment to examine the effects of endogenous somatostatin. Ischaemia and oxidative stress-induced inflammatory factors (CXCL10, CXCR3 and NF-κB p65) were also examined in the present study. The morphometric evaluation showed that the retinal thickness was increased 24 hours after I/R injury and attenuated 7 days after I/R injury. The number of ganglion cells was reduced 7 days after I/R injury. The application of CAP significantly prevented retinal I/R damage. Cysteamine pretreatment reversed the effects of CAP. Inhibition of CXCL10/CXCR3 and NF-κB (especially in astrocytes and microglia/macrophage) was involved in capsaicin-induced retinal protection through endogenous somatostatin. CAP has anti-inflammatory and neuroprotective effects in ischaemia-induced retinal injuries through endogenous somatostatin. Novel therapeutic remedies for inflammation or neuronal injuries were developed based on the systemic humoral effects related to CAP.

Poly(ADP-ribosyl)ation of FOXP3 Protein Mediated by PARP-1 Protein Regulates the Function of Regulatory T Cells.

Poly(ADP-ribose) polymerase 1 (PARP-1) is an ADP-ribosylating enzyme participating in diverse cellular functions. The roles of PARP-1 in the immune system, however, have not been well understood. Here we find that PARP-1 interacts with FOXP3 and induces its poly(ADP-ribosyl)ation. By using PARP-1 inhibitors, we show that reduced poly(ADP-ribosyl)ation of FOXP3 results in not only FOXP3 stabilization and increased FOXP3 downstream genes but also enhanced suppressive function of regulatory T cells. Our results suggest that PARP-1 negatively regulates the suppressive function of Treg cells at the posttranslational level via FOXP3 poly(ADP-ribosyl)ation. This finding has implications for developing PARP-1 inhibitors as potential agents for the prevention and treatment of autoimmune diseases.

Tunable thermal transport and mechanical properties of graphyne heterojunctions.

By employing molecular dynamics simulations, a family of graphyne heterojunctions (GYHJs) made by two different graphynes (GYs) have been designed and prepared. The dependence of tunable properties of GYHJs, such as thermal conductivity, mechanical properties, interfacial thermal resistance and rectification, on the composition and type of GYHJs is determined. Upon changing the composition of a GYHJ, one can keep a constant value of its fracture strength (and/or Young's modulus), while tuning its thermal conductivity. The thermal conductivities of GYHJs in the zigzag direction are larger than those in the armchair direction, indicating GYHJs are anisotropic. By decreasing the percentage of γ-GY, the thermal conductivities of GYHJs γ-GY/6,6,12-GY/γ-GY and γ-GY/14-GY/γ-GY decrease linearly in the armchair direction, whereas they undergo three stages (first decrease, then keep a constant value, and finally increase) in the zigzag direction. Regarding the mechanical response, by increasing the percentage of the graphyne in the GYHJ which possesses smaller Young's modulus, the Young's modulus of the GYHJ decreases. These findings would provide significant insights into the potential applications of graphyne-family materials in nanodevices.

Mechanical properties and failure mechanisms of graphene under a central load.

By employing molecular dynamics simulations, the evolution of deformation of a monolayer graphene sheet under a central transverse loading are investigated. Dependence of mechanical responses on the symmetry (shape) of the loading domain, on the size of the graphene sheet, and on temperature, is determined. It is found that the symmetry of the loading domain plays a central role in fracture strength and strain. By increasing the size of the graphene sheet or increasing temperature, the tensile strength and fracture strain decrease. The results have demonstrated that the breaking force and breaking displacement are sensitive to both temperature and the symmetry of the loading domain. In addition, we find that the intrinsic strength of graphene under a central load is much smaller than that of graphene under a uniaxial load. By examining the deformation processes, two failure mechanisms are identified namely, brittle bond breaking and plastic relaxation. In the second mechanism, the Stone-Wales transformation occurs.

A transferable force field for CdS-CdSe-PbS-PbSe solid systems.

A transferable force field for the PbSe-CdSe solid system using the partially charged rigid ion model has been successfully developed and was used to study the cation exchange in PbSe-CdSe heteronanocrystals [A. O. Yalcin et al., "Atomic resolution monitoring of cation exchange in CdSe-PbSe heteronanocrystals during epitaxial solid-solid-vapor growth," Nano Lett. 14, 3661-3667 (2014)]. In this work, we extend this force field by including another two important binary semiconductors, PbS and CdS, and provide detailed information on the validation of this force field. The parameterization combines Bader charge analysis, empirical fitting, and ab initio energy surface fitting. When compared with experimental data and density functional theory calculations, it is shown that a wide range of physical properties of bulk PbS, PbSe, CdS, CdSe, and their mixed phases can be accurately reproduced using this force field. The choice of functional forms and parameterization strategy is demonstrated to be rational and effective. This transferable force field can be used in various studies on II-VI and IV-VI semiconductor materials consisting of CdS, CdSe, PbS, and PbSe. Here, we demonstrate the applicability of the force field model by molecular dynamics simulations whereby transformations are initiated by cation exchange.

The global landscapes of lysine crotonylation in pseudorabies virus infection.

Lysine crotonylation is a common occurrence in eukaryotic cells, regulating various physiological functions, including chromatin remodeling, cellular growth, and development. However, its involvement in viral infections has rarely been documented. In this study, we reveal that pseudorabies virus (PRV) infection significantly alters the global lysine crotonylation levels in porcine kidney PK-15 cells. Specifically, we identified a few viral proteins, including UL54, gM, gD, UL19, UL37, and UL46, which undergo crotonylation modification. Our observations indicate that at 20 h post-infection (hpi), 551 crotonylation sites were reduced across 345 proteins, while 47 new sites emerged in 37 proteins compared to the control group. By 40 hpi, 263 sites had decreased in 190 proteins, while 389 new sites appeared in 240 proteins. Deeper analysis revealed that the proteins with altered crotonylation levels were primarily involved in binding, catalytic activity, biosynthetic processes, ribosome activity, and metabolic processes. Additionally, our findings underscored the significance of ribosomes and the endoplasmic reticulum (ER), which were enriched with proteins exhibiting altered crotonylation. Overall, our study for the first time offers new insights into the relationship between crotonylation and herpes virus infection, paving the way for future investigations into the role of crotonylation in viral infections.