ABSTRACT
Metabolic syndrome increases the risk of severe disease due to viral infection. Yet few studies have assessed the pathogenesis of respiratory viruses in high-risk populations. Here, we summarize how metabolic dysregulation impairs immune responses, and we define the role of metabolism during influenza virus and coronavirus infections. We also discuss the use of various in vitro, in vivo, and ex vivo models to elucidate the contributions of host factors to viral susceptibility, immunity, and disease severity.
INTRODUCTION
Metabolic syndrome (MetS) is a cluster of metabolic disorders that can lead to serious health conditions (1). Established features of MetS are visceral adiposity, insulin resistance, glucose intolerance, endothelial dysfunction, hypertension, and dyslipidemia. MetS is clinically diagnosed as the cooccurrence of three or more of these pathologies, which are interrelated because of overlapping metabolic pathways and pathophysiologic mechanisms (1). Obesity is one of the principal causes of MetS and can lead to the development of type 2 diabetes mellitus (T2DM), stroke, cardiovascular disease, and hypertension. This is problematic given that obesity is occurring at epidemic rates worldwide. According to the World Health Organization, obesity rates have nearly tripled since 1975, resulting in more than 650 million adults being obese in 2016 (2). Strikingly, global obesity rates are expected to exceed 50% of the adult population by 2050 (2).
In addition to the aforementioned noncommunicable diseases, recent work has begun to shed light on the influence of MetS on viral infection and host susceptibility. Viruses can metabolically engineer host cells by manipulating gene expression and lipid metabolism to enhance viral replication and progeny release while enabling the virus to evade host immune responses. Because metabolic disorders impair immune responses at homeostasis, viral infection further compromises these responses and potentiates metabolic disease severity (3). Although MetS-associated comorbidities are considered high-risk factors for increased severity of viral infection, few studies have attempted to define how underlying metabolic diseases influence viral pathogenesis. Here, we will review the current state of knowledge regarding the impact of MetS on influenza virus and coronavirus pathogenesis. We will also discuss the development of experimental systems to model viral disease progression in individuals with MetS.
OBESITY
Obesity can develop when excess caloric intake and insufficient physical activity lead to an energy imbalance and the accumulation of adipose tissue. Chronic low-grade inflammation is a well-established feature of obesity that is maintained by the secretion of proinflammatory cytokines and reactive oxygen species (ROS) from excess adipose tissue (3). Obesity is also linked to dysregulated lipid synthesis, which can aggravate pulmonary inflammation and contribute to increased disease severity in obese hosts during respiratory viral infections (4). Even in the absence of infection, systemic inflammation has detrimental effects on host innate and adaptive immune responses (3).
Influenza virus.During the 2009 H1N1 influenza pandemic, epidemiologic studies first recognized obesity as a high-risk factor for developing severe influenza virus infection and increasing influenza-related deaths (3). Among hospitalized patients with confirmed 2009 H1N1 infection, nearly one-third were morbidly obese (3). Obesity increases both the duration of stay in the intensive care unit (ICU) and the need for invasive mechanical ventilation during influenza virus infection (3). However, the factors contributing to the increased disease severity in obese hosts are only now becoming better understood.
Animal studies suggest that prolonged inflammation and delayed wound repair in the lungs of obese hosts lead to pulmonary edema and enhanced viral pathogenicity (5). Infection with the 2009 H1N1 influenza virus significantly diminished the extent of epithelial regeneration, increased pulmonary barrier permeability, and elevated mortality in genetically obese and diet-induced obese (DIO) mice (5). Even after 14 days postinfection, monocyte infiltration and alveolar exudates were prominent in the lungs of DIO mice, which could be attributed to the increased secretion of monocyte chemotactic protein 1 (MCP-1) in obese microenvironments. The proinflammatory state also upregulates the expression of the suppressor of cytokine signaling (SOCS) protein, which negatively regulates JAK–STAT signaling and suppresses interferon (IFN) induction and cytokine production (3). Indeed, the chronic state of metainflammation dampens antiviral immune responses to influenza virus infection and results in severe pathogenesis.
Obesity can also have an impact on influenza virus transmission. In household transmission studies in Nicaragua, the association between obesity and the duration of influenza virus shedding was examined over three seasons (3). Reverse transcription-PCR (RT-PCR) analysis indicated that influenza A virus (IAV) shedding lasted 42% longer in symptomatic obese adults than in symptomatic nonobese adults. Even obese adults who were asymptomatic or mildly ill shed IAV more than twice as long as asymptomatic or mildly ill nonobese adults (3.21 versus 1.57 days) (3). Interestingly, this association was specific to IAV; no associations were identified between obesity and increased influenza B virus shedding. A second study demonstrated that obese college students had higher viral loads in exhaled droplets than lean students (3). Among symptomatic students, infectious influenza virus was detected in 39% of fine aerosol samples collected from exhaled breath and spontaneous coughs. Increasing body mass index (BMI) correlated positively with viral aerosol shedding. These findings suggest that the prolonged viral shedding associated with obesity increases the release of infectious aerosols and facilitates airborne transmission of influenza viruses.
Considering the highly mutable nature of RNA viruses, the influence of obesity on the influenza virus population has also been investigated. A recent study found that obesity-induced impairment of interferons promoted the emergence of more-pathogenic viral variants (6). Serial passaging of H1N1 virus through genetically obese and DIO mice generated more-virulent IAV populations because of the increased induction of single-nucleotide variants in influenza virus-specific polymerase proteins (6). As a result, viruses passaged in obese hosts replicated to higher titers than viruses passaged in nonobese hosts and led to higher mortality rates in lean mice. The viral evolution phenotype was also corroborated in normal human bronchial epithelial (NHBE) cells: virus derived from obese-donor NHBE cells exhibited enhanced replication kinetics and blunted host interferon responses (6). In summary, these studies have demonstrated that obesity is associated with more-severe disease, higher viral titers in exhaled breath, and prolonged transmission and that changes in the viral population may facilitate the emergence of more-pathogenic influenza virus variants (3).
Arguably, vaccination is the most effective strategy for protection against influenza virus strains. However, studies have suggested that obesity impairs vaccination-induced responses to influenza virus infection. Although obese adults who received seasonal influenza vaccines generated robust antibody titers, obesity doubled the risk of developing influenza virus infection and influenza-like illness despite the vaccination (7). Even when adjuvants were used, vaccinated genetically obese and DIO mice were not protected against influenza virus challenge (8). Although influenza virus-infected obese mice generated neutralizing antibody responses, both the breadth and the magnitude of their serologic responses were significantly reduced, and this contributed to substantially increased morbidity and mortality rates in obese mice (8). While the mechanism by which influenza severity is increased in obese hosts is not clearly understood, suppressed antiviral responses and impaired wound repair in these hosts may limit protection against respiratory infections. These findings suggest that using antibody titers to define immune correlates of protection in high-risk groups may be misleading. Hence, more-consistent methods are needed to assess protective serologic responses and vaccine efficacy in all populations.
Coronavirus.Severe viral pathogenesis in obese populations is not only evident during influenza virus infection. The Centers for Disease Control and Prevention (CDC) recently recognized obesity as a high-risk factor for severe illness caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which emerged in December 2019 (9). In the United States, widespread SARS-CoV-2 infection has resulted in more than 2.7 million confirmed cases of coronavirus disease 2019 (COVID-19) and more than 129,000 deaths at the time of writing in June 2020 (10). A retrospective cohort study investigated the association between BMI and the severity of COVID-19. Among hospitalized patients with confirmed SARS-CoV-2 infection, almost 50% were considered obese and were admitted into intensive care requiring mechanical ventilation (11). This is not surprising, because excess body weight and fat deposition apply pressure to the diaphragm, which further increases the difficulty of breathing during a viral infection (4). What additional physiological factors are contributing to the increased risk of COVID-19 complications associated with obesity?
SARS-CoV-2 and SARS-CoV both use the angiotensin-converting enzyme 2 (ACE2) as their receptor (4). Studies in DIO mice demonstrated that high-fat diets dysregulated ACE2 expression and significantly increased the levels in adipose tissue (12). Excess adiposity is a physiologic feature commonly associated with obesity, which suggests that obesity results in the increased availability of high-affinity binding sites for SARS-CoV-2. Additionally, chronic inflammation within the obesogenic milieu may limit the detection and clearance of SARS-CoV-2 by immune cells, but the question remains as to whether the proinflammatory state restricts immune surveillance and contributes to increased severity of viral disease in obese hosts.
Obesity is also associated with leptin resistance and lipotoxicity. The accumulation of lipids can be exploited by viruses for lipid raft formation to enhance viral entry, replication, and progeny release (Fig. 1). A recent study analyzed the role of SARS-CoV-2 in dysregulating lipid metabolism and also investigated the influence of lipogenesis on ACE2 expression (4). Transcriptome analysis revealed that SARS-CoV-2 infection significantly upregulated lipid-specific genes and increased levels of proinflammatory cytokines in infected NHBE cells (4). Upregulated lipid metabolism may provide more opportunities for the virus to hijack host cells to complete its replication cycle. The combined effects of chronic systemic inflammation and the induction of a cytokine storm by SARS-CoV-2 could predispose obese populations to more-severe infection and unfavorable disease outcomes (4). In addition, obesity-associated lipogenesis antagonized the sterol-response element binding protein-1 (SREBP-1), a transcription factor that prevents lipotoxicity and controls Ace2 expression (4). The downregulation of SREBP-1 expression led to the upregulation of Ace2 in type 2 pneumocytes, which may contribute to severe SARS-CoV-2 infection in the lower respiratory tracts of obese hosts. Delineating how obesity contributes to disease severity following SARS-CoV-2 infection and identifying potential therapeutic targets are critical priorities, especially since few treatments are currently available. Although the antiviral drug remdesivir was recently approved by the U.S. Food and Drug Administration, the therapeutic dosing and adverse effects of this drug in obese patients remain unclear (13).
Obesity and T2DM impair host immunity and enhance the severity of influenza virus infection. During IAV infection in healthy hosts, surfactant protein D (SP-D) binds to IAV surface glycoproteins to inhibit IAV binding to the sialic acid receptors on the surfaces of the lung epithelia. Also, IAV particles that arrive at the bronchial epithelial cell surface are trapped in the mucus and transported out of the respiratory tract via the rhythmic movement of cilia. Circulating CD4+ and CD8+ T cells can migrate from the blood vessels to the lung tissue to target IAV and prevent damage to the lung epithelial-endothelial barrier. In contrast, excess glucose in the obese and T2DM microenvironments binds to SP-D and restricts the neutralization of IAV. IAV infection also leads to the loss of cilia on the bronchial epithelial cells of obese individuals and hence promotes IAV binding to the sialic acid receptors. Additionally, glycolytic efflux leads to glucose intolerance and insulin resistance, which alter the cytokine milieu and contribute to a chronic state of metainflammation. As IAV infection progresses, this significantly upregulates proinflammatory cytokines, such as IL-6 and IFN-γ, that potentiate inflammation, further damaging the epithelial layer. Obesity is also associated with upregulated neutrophil extravasation during IAV infection, whereby neutrophils attach to blood vessels, migrate to the epithelial cell layer, and release reactive oxygen species (ROS) that induce alveolar damage. Damage to type 2 pneumocytes results in severe influenza virus infection, increased barrier permeability, and the development of acute lung injury, which can result in viral pneumonia. Furthermore, obesity can result in upregulated lipid synthesis and increased production of lipid rafts, which can be exploited by IAV for viral entry, replication, and the release of progeny virions. Accelerated glycolysis increases ATP sources that are used for assembling V-ATPase and for transporting protons into the endosome. This results in the acidification of endosomes and the upregulation of IAV M2 activity, which collectively promote the uncoating and release of viral genomic material to facilitate viral replication.
TYPE 2 DIABETES MELLITUS
Type 2 diabetes mellitus develops when the body is unable to respond to insulin (14). The etiology of T2DM is often associated with obesity because chronic inflammation and hyperglycemia are induced by excess adipose tissue, but this condition can also occur independently of obesity. Insulin resistance develops in pancreatic β cells and contributes to the increase in proinflammatory cytokines that results in systemic inflammation. Additionally, infiltrating macrophages have been implicated in the late stage of T2DM because interleukin 1β (IL-1β) expression in islets facilitates the accumulation of proinflammatory macrophages in β cells (14). This leads to β-cell failure, which is the triggering factor for the transition from an insulin-resistant state to the development of T2DM (14). Few studies have investigated whether the T2DM milieu has an impact on viral infection.
Influenza virus.Epidemiologic studies suggest that patients with T2DM are more likely than those without T2DM to suffer influenza-related death (15). During the 2009 H1N1 pandemic, having T2DM tripled the risk of influenza-related hospitalization and quadrupled the likelihood of admission to the ICU (15). In agreement with these studies, diabetic mice demonstrated increased susceptibility and elevated lung viral titers when challenged with IAV (16). Excess blood glucose in transgenic diabetic mice that overexpressed an insulin promoter in their islet β cells promoted binding interactions with the surfactant protein D (SP-D) at the lung interface. At homeostasis, SP-D serves as an innate host defense because of its ability to bind to glycans on the influenza virus glycoproteins and neutralize infectious virus particles (Fig. 1). However, glucose preferentially binds SP-D and, when present in excess, abolishes the protective properties of SP-D. Consequently, elevated glucose levels enhanced viral entry and replication in the lungs of diabetic mice (16). Ketoacidosis provided a favorable environment for the release of virus from endosomes, further promoting viral replication and the release of viral progeny, and enhancing viral pathogenesis (Fig. 1) (16).
Subsequent studies confirmed the glycolytic regulation of influenza virus in vitro. In addition to dysregulating SP-D binding interactions, influenza viruses can also manipulate V-ATPase proton pumps to lower the pH within endosomes so as to facilitate viral progeny release (17). Because V-ATPase is ATP dependent, upregulated glycolysis promotes V-ATPase assembly, increases proton transport, and significantly enhances viral replication (Fig. 1) (17). A recent study demonstrated that variable glycemic conditions in the T2DM milieu also upregulated viral replication ex vivo (18). Cocultures of primary human epithelial and endothelial cells were treated with glucose and infected with IAV. Compared to untreated cocultures, glucose-treated cocultures had significantly more viral mRNA and increased ROS levels, which induced epithelial cell death (Fig. 1). This was recapitulated in vivo, where glucose variability in DIO mice also increased the severity of primary and secondary IAV infections (18). At 1 week postinfection, IAV-infected obese prediabetic mice exhibited reduced blood oxygen saturation, significant upregulation of gamma interferon (IFN-γ), and increased levels of oxidative stress, all of which impaired their lung function (18). Indeed, T2DM confers an increased risk of acute influenza virus infection and severe disease pathogenesis.
Coronavirus.Type 2 diabetes mellitus is also regarded as a high-risk factor for severe SARS-CoV-2 infection. During previous outbreaks of infections with coronaviruses, such as Middle East respiratory syndrome coronavirus (MERS-CoV) and SARS-CoV, T2DM was the most common comorbidity in severe cases. In mouse models of MERS-CoV infection, transgenic diabetic mice that expressed human dipeptidyl peptidase 4 (DPP4) in their lung epithelial cells experienced more weight loss and a longer duration of disease, as a result of dysregulated immune responses, than nondiabetic mice (19). Histopathologic and flow cytometric analyses of infected lungs revealed delayed recruitment of inflammatory macrophages and CD4+ T-cell lymphocytopenia, which resulted in aggravated lung pathology. The impact of T2DM on disease severity has also been demonstrated in human cohorts. A recent study of 174 patients with diabetes who had confirmed COVID-19 found that these patients were at a significantly higher risk for severe pneumonia than nondiabetic patients who were affected by COVID-19 (20). Chest computed tomography (CT) scans revealed more-severe lung abnormalities in patients with diabetes, which aligned with the increased neutrophil infiltration and lymphocytopenia seen in these patients (20). There was also a profound increase in serum IL-6 levels, which is a predictive biomarker for disease severity (Fig. 1). These data imply that SARS-CoV-2 causes severe disease in obese patients and in those with T2DM by inducing bilateral pneumonia and a cytokine storm that damages the lung epithelial-endothelial barrier.
Upregulated cytokine production in the T2DM pulmonary microenvironment leads to endothelial dysfunction and pulmonary damage (18). ACE2 is expressed primarily in the heart and kidneys and regulates heart function and blood pressure (21). Consequently, these ACE2-expressing tissues are vulnerable to SARS-CoV-2. Recent CDC data revealed that 32% of ICU patients with COVID-19 in the United States were diabetic (9). Another study reported cardiac injury in nearly 20% of 416 hospitalized patients with COVID-19 in Wuhan, China (22). Strikingly, more than 50% of the patients with documented heart damage had a history of hypertension. These clinical reports suggest that because blood vessels are damaged by SARS-CoV-2 infection, patients with T2DM who have preexisting vasculature damage are at a higher risk of serious SARS-CoV-2 disease. However, the mechanism by which SARS-CoV-2 attacks the heart and blood vessels has not yet been elucidated.
Patients with T2DM who have hypertension or heart disease are commonly treated with angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs). ACEIs and ARBs significantly increase ACE2 expression to maintain pulmonary homeostasis (21). Clinicians and researchers were initially concerned that angiotensin inhibitor therapies could facilitate SARS-CoV-2 adhesion and entry and thereby significantly increase the risk of severe COVID-19. Multiple studies now suggest that ACEIs and ARBs do not lead to poorer outcomes in COVID-19 and that people taking ACEIs or ARBs should not change their therapy unless advised to do so by their physicians (23, 24). This is a rapidly changing area of investigation, with many ongoing preclinical and clinical studies investigating the impacts of hypertension and of ACEIs and ARBs on COVID-19 susceptibility and disease severity.
EXPERIMENTAL MODELS OF VIRAL PATHOGENESIS IN METABOLIC SYNDROMES
To understand the impact of MetS on viral pathogenesis and disease severity, reliable and reproducible experimental systems are necessary. Various approaches have been developed to model metabolic disease. However, the available models do not accurately recapitulate the metabolic reprogramming of human hosts because of the lack of system complexity and the metabolic differences between species. Selecting appropriate models for investigating viral pathogenesis is also challenging because susceptibility to infection, disease phenotypes, and the need for virus adaptation differ between model species. These limitations necessitate the development of more biologically relevant models to better define how diseased metabolic states potentiate viral pathogenesis and to provide opportunities for preclinical testing in high-risk populations.
Exogenous administration of glucose, insulin, and metabolic intermediates to immortalized mammalian cell lines has been used to model leptin resistance and insulin resistance in vitro (Table 1). In such studies, it is important to use cell lines that are susceptible and permissive to viral growth and replication. For example, specific cell lines, such as Vero cells, that express ACE2 will be appropriate for examining the viral kinetics of SARS-CoV-2 and for exploring potential therapeutic options. However, cell cultures consist of a monolayer of nonpolarized cells that is not representative of the apical-basal polarity that is present in vivo, which is necessary for regulating ion transport, lipid metabolism, and barrier integrity. Cell cultures neither reproduce the proximity of tissues to pulmonary endothelia nor account for the signaling pathways and associated metabolic intermediates that could influence disease outcomes during viral infection (17).
Experimental systems for studying influenza virus and coronavirus pathogenesis in human metabolic syndromes
Because of the limited host diversity offered by immortalized cell lines, primary human cell cultures are increasingly being used as ex vivo models to enable understanding of the impact of viral infections on biological processes and host cellular responses. More recently, cocultures of human primary epithelial and endothelial cells have been used to model cell-to-cell and immune cell interactions that occur in vivo (Table 1). This approach is commonly used to model how viral infection disrupts the epithelial-endothelial barrier in humans to increase the severity of viral disease (18). Mice are also invaluable models for studying metabolic disease and viral pathogenesis because of their genetic tractability and the ease with which experiments can be performed. Obese mouse models can be generated by genetic manipulation or by a high-fat diet and are used for understanding how metabolically diseased microenvironments influence susceptibility and responses to primary and secondary viral infections. Since most human hosts with diabetes are obese, obese mouse models are often used to reflect insulin resistance and glycemic variability (18).
Previous studies have attempted to model glycemic oscillations via short-term glucose injections in vivo. In reality, obesity and T2DM induce years of metabolic reprogramming and are also influenced by diet, physical activity, stress levels, and sleep duration, which cannot be readily modeled in the laboratory. Additionally, murine strains are resistant to disease following infection with human influenza virus and hence require virus adaptation (19). MERS-CoV and SARS-CoV-2 are unable to replicate in wild-type mice because of differences in the binding domains of the DPP4 and ACE2 receptors, respectively (19). Hence, transgenic models are used to express human DPP4 in appropriate lung epithelial cells so that the replication of MERS-CoV in human hosts and their susceptibility can be modeled in vivo (19).
Although murine models have greatly advanced research on the severity of viral disease, mice are not the ideal model for understanding the pathogenicity and virulence of many respiratory viruses, because of differences in the distribution of receptors along the respiratory tract, the need for virus adaptation, and the inability of the animals to transmit virus or display symptoms of infection. Ferrets are most widely accepted for modeling human influenza and many other acute respiratory viral infections (25). Ferrets can be infected with human influenza virus without prior adaptation, and they display disease signs like those seen in humans because of similarities in the distribution of sialic acid residues in the respiratory tracts of ferrets and humans (25). Most importantly, ferrets are considered the best model for airborne and contact transmission of infectious respiratory viruses (Table 1). A recent study confirmed that ferrets are highly susceptible to SARS-CoV-2 infection based on the observation of heightened body temperatures, increased viral titers in the upper respiratory tract, and the development of acute bronchiolitis in infected animals (25). SARS-CoV-2 was transmitted via direct contact and by aerosols between infected and naïve ferrets, suggesting that ferrets may be an appropriate animal model for the study of SARS-CoV-2. Therefore, DIO ferrets should be considered for investigating how obesity and associated comorbidities affect the transmission and disease severity of SARS-CoV-2.
While experimental models have been developed to reflect metabolic syndromes and to study viral pathogenesis, few studies have attempted to elucidate the pathogenesis of viral infection and the severity of disease in humans. Taking into account the issues arising from interspecies differences, the complexity of metabolic pathways, and tissue connectivity, generating biologically relevant models remains a difficult task. The continual emergence of viral pathogens, along with the increasing prevalence of MetS, highlights the importance of defining how metabolic reprogramming influences vulnerability to infection and the efficacy of therapeutic approaches. Ideally, animal species, sex, and environmental differences should be considered when one is using animal models, because those factors can have an impact on susceptibility to metabolic disease, infection pathogenesis, and response to treatment.
CONCLUSION AND FUTURE PERSPECTIVES
The increasing prevalence of metabolic syndrome is a major public health issue. Over the years, humans have adopted sedentary lifestyles, and dietary patterns have shifted to excessive food consumption and poor nutrition. Overnutrition has led to the constellation of metabolic abnormalities that not only contributes to metabolic reprogramming but also limits host innate and adaptive immunity. Impaired immune responses and chronic inflammation in metabolically diseased microenvironments provide the ideal conditions for viral exploitation of host cells and enhanced viral pathogenesis. Consequently, obesity, T2DM, and associated comorbidities have been established as risk factors for severe influenza virus and coronavirus infections. Delayed defense mechanisms favor viral spread, enhanced replication, and persistent infections in these high-risk populations. Defining the role of MetS-associated immune dysfunction in disease severity is fundamental for combating these emerging viruses. Modeling MetS-induced physiology with biologically relevant systems will bridge the gaps between metabolic inflexibility, impaired immune responses, and increased disease severity in humans.
In conjunction with adult obesity, childhood obesity is also increasing at an alarming rate. In 2018, more than 40 million children below the age of 5 years were overweight or obese (2). Although obese children are more likely than nonobese children to become obese adults with associated comorbidities, the impact of prolonged metabolic dysregulation on host immunity is not well understood. Also, patients with T2DM have an increased risk of developing nonalcoholic steatohepatitis and nonalcoholic fatty liver diseases, which lead to excessive fat deposition in the liver and long-term metabolic dysfunction. Future studies should assess the long-term effects of aberrant cellular metabolism on host immune responses in order to understand the implications for infection burden, morbidity, and mortality. Furthermore, seasonal influenza vaccines do not stimulate protective antibody responses in obese recipients, and no specific treatments for SARS-CoV-2 in obese populations are currently available. Future research should seek to delineate how metabolic abnormalities increase viral pathogenesis, since this information will play an essential role in global preparedness against emerging seasonal and pandemic virus strains.
Finally, while sex differences are not the focus of this review, it should be noted that more work is needed to understand how biological sex and MetS will impact influenza virus/SARS-CoV-2 susceptibility, disease severity, and vaccine efficacy in order to properly protect all populations.
ACKNOWLEDGMENTS
We thank Keith A. Laycock for scientific editing of this Gem.
M.S. performed the literature search, wrote and edited manuscript drafts, designed the figure, constructed the table, and submitted the manuscript. R.H. edited manuscript drafts and the table, created the figure, and finalized manuscript submission. S.S.-C. secured grant funding and contributed to manuscript revision and submission.
Funding was provided by ALSAC, by the St. Jude Graduate School of Biomedical Sciences, and by NIAID grants R01 AI140766-01, HHSN272201400006C, and 75N93019C00052 to S.S.-C. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
FOOTNOTES
- Received 26 May 2020.
- Accepted 9 July 2020.
- Accepted manuscript posted online 13 July 2020.
- Copyright © 2020 American Society for Microbiology.
