Determining Infection Outcome Research Articles

Age-dependent differences in efferocytosis determine the outcome of opsonophagocytic protection from invasive pathogens

In early life, susceptibility to invasive infection skews toward a small subset of microbes, whereas other pathogens associated with diseases later in life, including Streptococcus pneumoniae (Spn), are uncommon among neonates. To delineate mechanisms behind age-dependent susceptibility, we compared age-specific mouse models of invasive Spn infection. We show enhanced CD11b-dependent opsonophagocytosis by neonatal neutrophils improved protection against Spn during early life. The augmented function of neonatal neutrophils was mediated by higher CD11b surface expression at the population level due to dampened efferocytosis, which also resulted in more CD11bhi "aged" neutrophils in peripheral blood. Dampened efferocytosis during early life could be attributed to the lack of CD169+ macrophages in neonates and reduced systemic expressions of multiple efferocytic mediators, including MerTK. On experimentally impairing efferocytosis later in life, CD11bhi neutrophils increased and protection against Spn improved. Our findings reveal how age-dependent differences in efferocytosis determine infection outcome through the modulation of CD11b-driven opsonophagocytosis and immunity.

Untangling Cellular Host-Pathogen Encounters at Infection Bottlenecks.

Bacterial pathogens can invade the tissue and establish a protected intracellular niche at the site of invasion that can spread locally (e.g., microcolonies) or to systemic sites (e.g., granulomas). Invasion of the tissue and establishment of intracellular infection are rare events that are difficult to study in the in vivo setting but have critical clinical consequences, such as long-term carriage, reinfections, and emergence of antibiotic resistance. Here, I discuss Salmonella interactions with its host macrophage during early stages of infection and their critical role in determining infection outcome. The dynamics of host-pathogen interactions entail highly heterogenous host immunity, bacterial virulence, and metabolic cross talk, requiring in vivo analysis at single-cell resolution. I discuss models and single-cell approaches that provide a global understanding of the establishment of a protected intracellular niche within the tissue and the host-pathogen landscape at infection bottlenecks during early stages of infection. Studying cellular host-pathogen interactions in vivo can improve our knowledge of the trajectory of infection between the initial inoculation with a dose of pathogens and the appearance of symptoms of disease.

Host- and Age-Dependent Transcriptional Changes in Mycobacterium tuberculosis Cell Envelope Biosynthesis Genes after Exposure to Human Alveolar Lining Fluid.

Tuberculosis (TB) infection, caused by the airborne pathogen Mycobacterium tuberculosis (M.tb), resulted in almost 1.4 million deaths in 2019, and the number of deaths is predicted to increase by 20% over the next 5 years due to the COVID-19 pandemic. Upon reaching the alveolar space, M.tb comes into close contact with the lung mucosa before and after its encounter with host alveolar compartment cells. Our previous studies show that homeostatic, innate soluble components of the alveolar lining fluid (ALF) can quickly alter the cell envelope surface of M.tb upon contact, defining subsequent M.tb–host cell interactions and infection outcomes in vitro and in vivo. We also demonstrated that ALF from 60+ year old elders (E-ALF) vs. healthy 18- to 45-year-old adults (A-ALF) is dysfunctional, with loss of homeostatic capacity and impaired innate soluble responses linked to high local oxidative stress. In this study, a targeted transcriptional assay shows that M.tb exposure to human ALF alters the expression of its cell envelope genes. Specifically, our results indicate that A-ALF-exposed M.tb upregulates cell envelope genes associated with lipid, carbohydrate, and amino acid metabolism, as well as genes associated with redox homeostasis and transcriptional regulators. Conversely, M.tb exposure to E-ALF shows a lesser transcriptional response, with most of the M.tb genes unchanged or downregulated. Overall, this study indicates that M.tb responds and adapts to the lung alveolar environment upon contact, and that the host ALF status, determined by factors such as age, might play an important role in determining infection outcome.

Interactions of the Skin Pathogen Haemophilus ducreyi With the Human Host

The obligate human pathogen Haemophilus ducreyi causes both cutaneous ulcers in children and sexually transmitted genital ulcers (chancroid) in adults. Pathogenesis is dependent on avoiding phagocytosis and exploiting the suppurative granuloma-like niche, which contains a myriad of innate immune cells and memory T cells. Despite this immune infiltrate, long-lived immune protection does not develop against repeated H. ducreyi infections—even with the same strain. Most of what we know about infectious skin diseases comes from naturally occurring infections and/or animal models; however, for H. ducreyi, this information comes from an experimental model of infection in human volunteers that was developed nearly three decades ago. The model mirrors the progression of natural disease and serves as a valuable tool to determine the composition of the immune cell infiltrate early in disease and to identify host and bacterial factors that are required for the establishment of infection and disease progression. Most recently, holistic investigation of the experimentally infected skin microenvironment using multiple “omics” techniques has revealed that non-canonical bacterial virulence factors, such as genes involved in central metabolism, may be relevant to disease progression. Thus, the immune system not only defends the host against H. ducreyi, but also dictates the nutrient availability for the invading bacteria, which must adapt their gene expression to exploit the inflammatory metabolic niche. These findings have broadened our view of the host-pathogen interaction network from considering only classical, effector-based virulence paradigms to include adaptations to the metabolic environment. How both host and bacterial factors interact to determine infection outcome is a current focus in the field. Here, we review what we have learned from experimental H. ducreyi infection about host-pathogen interactions, make comparisons to what is known for other skin pathogens, and discuss how novel technologies will deepen our understanding of this infection.

The Role of Macrophages in Staphylococcus aureus Infection.

Staphylococcus aureus is a member of the human commensal microflora that exists, apparently benignly, at multiple sites on the host. However, as an opportunist pathogen it can also cause a range of serious diseases. This requires an ability to circumvent the innate immune system to establish an infection. Professional phagocytes, primarily macrophages and neutrophils, are key innate immune cells which interact with S. aureus, acting as gatekeepers to contain and resolve infection. Recent studies have highlighted the important roles of macrophages during S. aureus infections, using a wide array of killing mechanisms. In defense, S. aureus has evolved multiple strategies to survive within, manipulate and escape from macrophages, allowing them to not only subvert but also exploit this key element of our immune system. Macrophage-S. aureus interactions are multifaceted and have direct roles in infection outcome. In depth understanding of these host-pathogen interactions may be useful for future therapeutic developments. This review examines macrophage interactions with S. aureus throughout all stages of infection, with special emphasis on mechanisms that determine infection outcome.

The Role of the Gut Microbiome in Resisting Norovirus Infection as Revealed by a Human Challenge Study.

Norovirus infections take a heavy toll on worldwide public health. While progress has been made toward understanding host responses to infection, the role of the gut microbiome in determining infection outcome is unknown. Moreover, data are lacking on the nature and duration of the microbiome response to norovirus infection, which has important implications for diagnostics and host recovery. Here, we characterized the gut microbiomes of subjects enrolled in a norovirus challenge study. We analyzed microbiome features of asymptomatic and symptomatic individuals at the genome (population) and gene levels and assessed their response over time in symptomatic individuals. We show that the preinfection microbiomes of subjects with asymptomatic infections were enriched in Bacteroidetes and depleted in Clostridia relative to the microbiomes of symptomatic subjects. These compositional differences were accompanied by differences in genes involved in the metabolism of glycans and sphingolipids that may aid in host resilience to infection. We further show that microbiomes shifted in composition following infection and that recovery times were variable among human hosts. In particular, Firmicutes increased immediately following the challenge, while Bacteroidetes and Proteobacteria decreased over the same time. Genes enriched in the microbiomes of symptomatic subjects, including the adenylyltransferase glgC, were linked to glycan metabolism and cell-cell signaling, suggesting as-yet unknown roles for these processes in determining infection outcome. These results provide important context for understanding the gut microbiome role in host susceptibility to symptomatic norovirus infection and long-term health outcomes.IMPORTANCE The role of the human gut microbiome in determining whether an individual infected with norovirus will be symptomatic is poorly understood. This study provides important data on microbes that distinguish asymptomatic from symptomatic microbiomes and links these features to infection responses in a human challenge study. The results have implications for understanding resistance to and treatment of norovirus infections.

Seronegative hepatitis C virus infection in Polish blood donors-Virological characteristics of index donations and follow-up observations.

Nucleic acid testing (NAT) was implemented in Poland in 1999 for screening of plasma for fractionation and for all blood donors in 2002. To analyze seronegative NAT‐positive samples representing hepatitis C virus (HCV) window‐period (WP) in the years 2000 to 2016 and to determine infection outcome. We analyzed results of 17 502 739 donations screened in minipools (6‐48) or individually. Index samples underwent viral load (VL) quantification, genotyping and Ag, and anti‐HCV re‐testing using chemiluminescence (CMIA), electrochemiluminescence (ECLIA), and fourth‐generation enzyme‐linked immunosorbent assay (IV EIA) assays. HCV‐seronegative infections were identified in 126 donations (7.2/mln donations; 95% confidential intervals, 6.0‐8.6). Frequency of NAT yields was decreasing over time. Of the initial 126 seronegative index cases 106 were retested: 32.1% were reactive in IV EIA, 11.3% in ECLIA, and 1.9% in CMIA. The lowest VL correlated with absent anti‐HCV and HCV Ag, while VL was highest when the antigen was detectable and then it decreased when anti‐HCV appeared at a level detectable by sensitive third generation tests while retesting. The proportion of genotype 1 was 38.9% in samples positive only for HCV RNA and 71.4% in samples that were anti‐HCV reactive in re‐testing. In parallel, genotype 3 frequency was 50% in the former group and 21% in the latter. NAT is an effective measure to limit HCV transmission by transfusion and IV EIA seems to have higher clinical sensitivity than ECLIA. Samples representing likely successive phases of early HCV infection were characterized by different genotype distribution probably due to very early elimination of genotype 3.

Defective Viral Genomes Alter How Sendai Virus Interacts with Cellular Trafficking Machinery, Leading to Heterogeneity in the Production of Viral Particles among Infected Cells.

Defective viral genomes (DVGs) generated during RNA virus replication determine infection outcome by triggering innate immunity, diminishing virulence, and, in many cases, facilitating the establishment of persistent infections. Despite their critical role during virus-host interactions, the mechanisms regulating the production and propagation of DVGs are poorly understood. Visualization of viral genomes using RNA fluorescent in situ hybridization revealed a striking difference in the intracellular localization of DVGs and full-length viral genomes during infections with the paramyxovirus Sendai virus. In cells enriched in full-length virus, viral genomes clustered in a perinuclear region and associated with cellular trafficking machinery, including microtubules and the GTPase Rab11a. However, in cells enriched in DVGs, defective genomes distributed diffusely throughout the cytoplasm and failed to interact with this cellular machinery. Consequently, cells enriched in full-length genomes produced both DVG- and full-length-genome-containing viral particles, while DVG-high cells poorly produced viral particles yet strongly stimulated antiviral immunity. These findings reveal the selective production of both standard and DVG-containing particles by a subpopulation of infected cells that can be differentiated by the intracellular localization of DVGs. This study highlights the importance of considering this functional heterogeneity in analyses of virus-host interactions during infection.IMPORTANCE Defective viral genomes (DVGs) generated during Sendai virus infections accumulate in the cytoplasm of some infected cells and stimulate antiviral immunity and cell survival. DVGs are packaged and released as defective particles and have a significant impact on infection outcome. We show that the subpopulation of DVG-high cells poorly engages the virus packaging and budding machinery and do not effectively produce viral particles. In contrast, cells enriched in full-length genomes are the primary producers of both standard and defective viral particles during infection. This study demonstrates heterogeneity in the molecular interactions occurring within infected cells and highlights distinct functional roles for cells as either initiators of immunity or producers and perpetuators of viral particles depending on their content of viral genomes and their intracellular localization.

Impact of Engineered Expression of Mitochondrial Association Factor 1b on Toxoplasma gondii Infection and the Host Response in a Mouse Model.

The opportunistic intracellular parasite Toxoplasma gondii causes a lifelong chronic infection capable of reactivating in immunocompromised individuals, which can lead to life-threatening complications. Following invasion of the host cell, host mitochondria associate with the parasitophorous vacuole membrane. This phenotype is T. gondii strain specific and is mediated by expression of a host mitochondrial association-competent (HMA+) paralog of the parasite protein mitochondrial association factor 1 (MAF1b). Previous work demonstrated that expression of MAF1b in strains that do not normally associate with host mitochondria increases their fitness during acute infection in vivo However, the impact of MAF1b expression during chronic T. gondii infection is unclear. In this study, we assess the impact of MAF1b expression on cyst formation and cytokine production in mice. Despite generally low numbers of cysts generated by the in vitro culture-adapted strains used in this study, we find that parasites expressing MAF1b have higher numbers of cysts in the brains of chronically infected mice and that MAF1b+ cyst burden significantly increases during the course of chronic infection. Consistent with this, mice infected with MAF1b+ parasites have higher levels of the serum cytokines RANTES and VEGF (vascular endothelial growth factor) at day 57 postinfection, although this could be due to higher parasite burden at this time point rather than direct manipulation of these cytokines by MAF1b. Overall these data indicate that MAF1b expression may also be important in determining infection outcome during the chronic phase, either by directly altering the cytokine/signaling environment or by increasing proliferation during the acute and/or chronic phase.IMPORTANCE The parasite Toxoplasma gondii currently infects approximately one-third of the world's population and causes life-threatening toxoplasmosis in individuals with undeveloped or weakened immune systems. Current treatments are unable to cure T. gondii infection, leaving infected individuals with slow-growing tissue cysts for the remainder of their lives. Previous work has shown that expression of the parasite protein mitochondrial association factor 1 (MAF1b) is responsible for the association of T. gondii parasites with host mitochondria and provides a selective advantage during acute infection. Here we examine the impact of MAF1b expression during chronic T. gondii infection. We find that mice infected with MAF1b-expressing parasites have higher cyst burden and cytokine levels than their wild-type counterparts. A better understanding of the genes involved in establishing and maintaining chronic infection will aid in discovering effective therapeutics for chronically infected individuals.

Studies in the mouse model identify strain variability as a major determinant of disease outcome in Leishmania infantum infection.

BackgroundVisceral leishmaniasis is a severe and potentially fatal disease caused by protozoa of the genus Leishmania, transmitted by phlebotomine sandflies. In Europe and the Mediterranean region, L. infantum is the commonest agent of visceral leishmaniasis, causing a wide spectrum of clinical manifestations, including asymptomatic carriage, cutaneous lesions and severe visceral disease. Visceral leishmaniasis is more frequent in immunocompromised individuals and data obtained in experimental models of infection have highlighted the importance of the host immune response, namely the efficient activation of host’s macrophages, in determining infection outcome. Conversely, few studies have addressed a possible contribution of parasite variability to this outcome.MethodsIn this study, we compared three isolates of L. infantum regarding their capacity to grow in the organs of mice, the way they activate the host’s macrophages and other components of the immune response and also their capacity to cope with host’s antimicrobial mechanisms, namely reactive oxygen and nitrogen species.ResultsWe found that the three parasite strains significantly differed regarding the degree to which they induced nitric oxide synthase (NOS2) and arginase expression in infected macrophages and the pattern of cytokine production they induced in the host, resulting in different degrees of inflammatory response in infected livers. Additionally, the three strains also significantly differed in their in vitro susceptibility to reactive oxygen and nitrogen species. This variability was reflected in the capacity of each strain to persist and proliferate in the organs of wild-type as well as NOS2- and phagocyte oxidase- deficient mice.ConclusionsThe results obtained in this study show that parasite strain variability is an important determinant of disease outcome in L. infantum visceral leishmaniasis, with relevant implications for studies on host-pathogen interaction and also for leishmanicidal drug development.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-015-1259-6) contains supplementary material, which is available to authorized users.

Adaptive response in hepatitis B virus infection

Hepatitis B virus (HBV) is a major cause of acute and chronic liver inflammation worldwide. The immune response against the virus represents a key factor in determining infection outcome, in terms of both viral clearance and the perpetuation of liver damage. Significant advances have recently been achieved regarding the functions of antiviral CD8+ T cells, leading to a better understanding of their abnormalities during chronic infection as well as the pathways to be manipulated to reverse the immune impairment of chronic infection. In this review, we aimed to analyse the patterns of adaptive immunity that develop during acute infection and the profiles in chronic infection. In addition to CD8+ T cells, which are the best-described subset to date, we reviewed and commented on the direct and indirect roles of CD4+ T cells and B cells.

Evidence that hepatitis C virus genome partly controls infection outcome.

Infection by hepatitis C virus (HCV) leads to one of two outcomes; either the infection resolves within approximately 6 months or the virus can persist indefinitely. Host genetics are known to affect the likelihood of clearance or persistence. By contrast, the importance of the virus genotype in determining infection outcome is unknown, as quantifying this effect traditionally requires well-characterized transmission networks, which are rare. Extending phylogenetic approaches previously developed to estimate the virus control over set-point viral load in HIV-1 infections, we simulate inheritance of a binary trait along a phylogenetic tree, use this data to quantify how infection outcomes cluster and ascertain the effect of virus genotype on these. We apply our method to the Hepatitis C Incidence and Transmission Study in prisons (HITS-p) data set from Australia, as this cohort prospectively identified incident cases including viraemic subjects who ultimately clear the virus, thus providing us with a unique collection of sequences from clearing infections. We detect significant correlations between infection outcome and virus distance in the phylogeny for viruses of Genotype 1, with estimates lying at around 67%. No statistically significant estimates were obtained for viruses of Genotype 3a.

Multi-Scale Modeling Predicts a Balance of Tumor Necrosis Factor-α and Interleukin-10 Controls the Granuloma Environment during Mycobacterium tuberculosis Infection

Interleukin-10 (IL-10) and tumor necrosis factor-α (TNF-α) are key anti- and pro-inflammatory mediators elicited during the host immune response to Mycobacterium tuberculosis (Mtb). Understanding the opposing effects of these mediators is difficult due to the complexity of processes acting across different spatial (molecular, cellular, and tissue) and temporal (seconds to years) scales. We take an in silico approach and use multi-scale agent based modeling of the immune response to Mtb, including molecular scale details for both TNF-α and IL-10. Our model predicts that IL-10 is necessary to modulate macrophage activation levels and to prevent host-induced tissue damage in a granuloma, an aggregate of cells that forms in response to Mtb. We show that TNF-α and IL-10 parameters related to synthesis, signaling, and spatial distribution processes control concentrations of TNF-α and IL-10 in a granuloma and determine infection outcome in the long-term. We devise an overall measure of granuloma function based on three metrics – total bacterial load, macrophage activation levels, and apoptosis of resting macrophages – and use this metric to demonstrate a balance of TNF-α and IL-10 concentrations is essential to Mtb infection control, within a single granuloma, with minimal host-induced tissue damage. Our findings suggest that a balance of TNF-α and IL-10 defines a granuloma environment that may be beneficial for both host and pathogen, but perturbing the balance could be used as a novel therapeutic strategy to modulate infection outcomes.

Is There a Role for HTLV-1-Specific CTL in Adult T-Cell Leukemia/Lymphoma?

ATLL is an aggressive malignancy of T cells that affects about 5% of individuals infected with HTLV-1. The precise mechanism of oncogenesis is not known, but there is evidence that two regulatory viral proteins, Tax and HBZ, are involved. A high set point proviral load is associated with development of ATLL or a chronic inflammatory condition, HAM/TSP. Several lines of evidence, including HLA class 1 association studies and in vitro killing assays, indicate that cytotoxic T lymphocytes are instrumental in determining this proviral load set point. Prior studies have focused chiefly on the CTL response to the immunodominant Tax protein: efficient lysis of Tax-expressing cells inversely correlates with proviral load in nonmalignant infection. However, a recent study showed that strong binding of peptides from HBZ, but not Tax, to HLA class 1 molecules was associated with a low proviral load and a reduced risk of developing HAM/TSP, indicating an important role for HBZ-specific CTL in determining infection outcome. In comparison with nonmalignant infection, HTLV-1-specific CTLs in ATLL patients are reduced in frequency and functionally deficient. Here we discuss the nature of protective CTL responses in nonmalignant HTLV-1 infection and explore the potential of CTLs to protect against ATLL.

The Human Gastric PathogenHelicobacter pyloriand Its Association with Gastric Cancer and Ulcer Disease

With the momentous discovery in the 1980's that a bacterium,Helicobacter pylori, can cause peptic ulcer disease and gastric cancer, antibiotic therapies and prophylactic measures have been successful, only in part, in reducing the global burden of these diseases. To date, ~700,000 deaths worldwide are still attributable annually to gastric cancer alone. Here, we reviewH. pylori's contribution to the epidemiology and histopathology of both gastric cancer and peptic ulcer disease. Furthermore, we examine the host-pathogen relationship andH. pyloribiology in context of these diseases, focusing on strain differences, virulence factors (CagA and VacA), immune activation and the challenges posed by resistance to existing therapies. We consider also the important role of host-genetic variants, for example, in inflammatory response genes, in determining infection outcome and the role ofH. pyloriin other pathologies—some accepted, for example, MALT lymphoma, and others more controversial, for example, idiopathic thrombocytic purpura. More recently, intriguing suggestions thatH. pylorihas protective effects in GERD and autoimmune diseases, such as asthma, have gained momentum. Therefore, we consider the basis for these suggestions and discuss the potential impact for future therapeutic rationales.

Host and Pathogen Glycosaminoglycan-Binding Proteins Modulate Antimicrobial Peptide Responses inDrosophila melanogaster

During group B streptococcal infection, the alpha C protein (ACP) on the bacterial surface binds to host cell surface heparan sulfate proteoglycans (HSPGs) and facilitates entry of bacteria into human epithelial cells. Previous studies in a Drosophila melanogaster model showed that binding of ACP to the sulfated polysaccharide chains (glycosaminoglycans) of HSPGs promotes host death and is associated with higher bacterial burdens. We hypothesized that ACP-glycosaminoglycan binding might determine infection outcome by altering host responses to infection, such as expression of antimicrobial peptides. As glycosaminoglycans/HSPGs also interact with a number of endogenous secreted signaling molecules in Drosophila, we examined the effects of host and pathogen glycosaminoglycan/HSPG-binding structures in host survival of infection and antimicrobial peptide expression. Strikingly, host survival after infection with wild-type streptococci was enhanced among flies overexpressing the endogenous glycosaminoglycan/HSPG-binding morphogen Decapentaplegic-a transforming growth factor β-like Drosophila homolog of mammalian bone morphogenetic proteins-but not by flies overexpressing a mutant, non-glycosaminoglycan-binding Decapentaplegic, or the other endogenous glycosaminoglycan/HSPG-binding morphogens, Hedgehog and Wingless. While ACP-glycosaminoglycan binding was associated with enhanced transcription of peptidoglycan recognition proteins and antimicrobial peptides, Decapentaplegic overexpression suppressed transcription of these genes during streptococcal infection. Further, the glycosaminoglycan-binding domain of ACP competed with Decapentaplegic for binding to the soluble glycosaminoglycan heparin in an in vitro assay. These data suggest that, in addition to promoting bacterial entry into host cells, ACP competes with Decapentaplegic for binding to glycosaminoglycans/HSPGs during infection and that these bacterial and endogenous glycosaminoglycan-binding structures determine host survival and regulate antimicrobial peptide transcription.

Leishmania amazonensis: Participation of regulatory T and B cells in the in vitro priming (PIV) of CBA/J spleen cells susceptible response

CBA/J mice are resistant to Leishmania major and susceptible to Leishmania amazonensis. Early events determine infection outcome. Until now, PIV (in vitro priming) immune response to L. amazonensis has not been assessed. Herein, we have shown that compared to L. major, L. amazonensis induced higher parasite burden associated to similar IL-4, IFN-γ, and TNF-α mRNA expressions and IFN-γ and IL-10 levels. Although similar amounts of IL-10 were detected, the frequency of intracellular IL-10 positive B cells was enhanced in spleen cells stimulated with anti-CD3/anti-CD28, or anti-CD3/anti-CD28 and L. amazonensis, compared to L. major-stimulation. Interestingly, IL-10- producing B cells were reduced in response to anti-CD3/anti-CD28 stimulation combined with L. major compared to the other groups. L. amazonensis may favor T regulatory cell development, since 40% of all the CD4 +CD25 + were CD25 high cells. These data suggest that in PIV, susceptibility to L. amazonensis is not related to Th cell polarization, but to the presence and activity of regulatory T and B cells.