The blood-brain barrier internalises Cryptococcus neoformans via the EphA2-tyrosine kinase receptor

Preclinical Models for Cryptococcosis of the CNS and Their Characterization Using In Vivo Imaging Techniques

Infections caused by Cryptococcus neoformans and Cryptococcus gattii remain a challenge to our healthcare systems as they are still difficult to treat. In order to improve treatment success, in particular for infections that have disseminated to the central nervous system, a better understanding of the disease is needed, addressing questions like how it evolves from a pulmonary to a brain disease and how novel treatment approaches can be developed and validated. This requires not only clinical research and research on the microorganisms in a laboratory environment but also preclinical models in order to study cryptococci in the host. We provide an overview of available preclinical models, with particular emphasis on models of cryptococcosis in rodents. In order to further improve the characterization of rodent models, in particular the dynamic aspects of disease manifestation, development, and ultimate treatment, preclinical in vivo imaging methods are increasingly used, mainly in research for oncological, neurological, and cardiac diseases. In vivo imaging applications for fungal infections are rather sparse. A second aspect of this review is how research on models of cryptococcosis can benefit from in vivo imaging methods that not only provide information on morphology and tissue structure but also on function, metabolism, and cellular properties in a non-invasive way.

Eph receptors and ephrins in cancer progression

Evidence implicating Eph receptor tyrosine kinases and their ephrin ligands (that together make up the 'Eph system') in cancer development and progression has been accumulating since the discovery of the first Eph receptor approximately 35 years ago. Advances in the past decade and a half have considerably increased the understanding of Eph receptor-ephrin signalling mechanisms in cancer and have uncovered intriguing new roles in cancer progression and drug resistance. This Review focuses mainly on these more recent developments. I provide an update on the different mechanisms of Eph receptor-ephrin-mediated cell-cell communication and cell autonomous signalling, as well as on the interplay of the Eph system with other signalling systems. I further discuss recent advances in elucidating how the Eph system controls tumour expansion, invasiveness and metastasis, supports cancer stem cells, and drives therapy resistance. In addition to functioning within cancer cells, the Eph system also mediates the reciprocal communication between cancer cells and cells of the tumour microenvironment. The involvement of the Eph system in tumour angiogenesis is well established, but recent findings also demonstrate roles in immune cells, cancer-associated fibroblasts and the extracellular matrix. Lastly, I discuss strategies under evaluation for therapeutic targeting of Eph receptors-ephrins in cancer and conclude with an outlook on promising future research directions.

Macropinocytosis as a potential mechanism driving neurotropism of Cryptococcus neoformans

Cryptococcus neoformans can invade the central nervous system by crossing the blood-brain barrier via a transcellular mechanism that relies on multiple host factors. In this narrative, we review the evidence that a direct interplay between C. neoformans and brain endothelial cells forms the basis for invasion and transmigration across the brain endothelium. Adherence and internalization of C. neoformans is dependent on transmembrane proteins, including a hyaluronic acid receptor and an ephrin receptor tyrosine kinase. We consider the role of EphA2 in facilitating the invasion of the central nervous system by C. neoformans and highlight experimental evidence supporting macropinocytosis as a potential mechanism of internalization and transcytosis. How macropinocytosis might be conclusively demonstrated in the context of C. neoformans is also discussed.

A multi-arm, parallel, preclinical study investigating the potential benefits of acetazolamide, candesartan, and triciribine in combination with fluconazole for the treatment of cryptococcal meningoencephalitis

Cryptococcus neoformans, an opportunistic fungal pathogen, primarily infects immunodeficient patients frequently causing cryptococcal meningoencephalitis (CM). Increased intracranial pressure (ICP) is a serious complication responsible for increased morbidity and mortality in CM patients. Non-invasive pharmacological agents that mitigate ICP could be beneficial in treating CM patients. The objective of the study was to investigate the efficacy of acetazolamide (AZA), candesartan (CAN), and triciribine (TCBN), in combination with the antifungal fluconazole, on C. neoformans-induced endothelial, brain, and lung injury in an experimental mouse model of CM. Our study shows that C. neoformans increases the expression of brain endothelial cell (BEC) junction proteins Claudin-5 (Cldn5) and VE-Cadherin to induce pathological cell-barrier remodeling and gap formation associated with increased Akt and p38 MAPK activation. All three agents inhibited C. neoformans-induced endothelial gap formation, only CAN and TCBN significantly reduced C. neoformans-induced Cldn5 expression, and only TCBN was effective in inhibiting Akt and p38MAPK. Interestingly, although C. neoformans did not cause brain or lung edema in mice, it induced lung and brain injuries, which were significantly reversed by AZA, CAN, or TCBN. Our study provides novel insights into the direct effects of C. neoformans on BECs in vitro, and the potential benefits of using AZA, CAN, or TCBN in the management of CM patients.

Glioblastoma-targeted, local and sustained drug delivery system based on an unconventional lipid nanocapsule hydrogel

The objective of this work was to develop an implantable therapeutic hydrogel that will ensure continuity in treatment between surgery and radiochemotherapy for patients with glioblastoma (GBM). A hydrogel of self-associated gemcitabine-loaded lipid nanocapsules (LNC) has shown therapeutic efficacy in vivo in murine GBM resection models. To improve the targeting of GBM cells, the NFL-TBS.40-63 peptide (NFL), was associated with LNC. The LNC-based hydrogels were formulated with the NFL. The peptide was totally and instantaneously adsorbed at the LNC surface, without modifying the hydrogel mechanical properties, and remained adsorbed to the LNC surface after the hydrogel dissolution. In vitro studies on GBM cell lines showed a faster internalization of the LNC and enhanced cytotoxicity, in the presence of NFL. Finally, in vivo studies in the murine GBM resection model proved that the gemcitabine-loaded LNC with adsorbed NFL could target the non-resected GBM cells and significantly delay or even inhibit the apparition of recurrences.

Cryptococcus neoformans Infection in the Central Nervous System: The Battle between Host and Pathogen

Cryptococcus neoformans (C. neoformans) is a pathogenic fungus with a global distribution. Humans become infected by inhaling the fungus from the environment, and the fungus initially colonizes the lungs. If the immune system fails to contain C. neoformans in the lungs, the fungus can disseminate to the blood and invade the central nervous system, resulting in fatal meningoencephalitis particularly in immunocompromised individuals including HIV/AIDS patients. Following brain invasion, C. neoformans will encounter host defenses involving resident as well as recruited immune cells in the brain. To overcome host defenses, C. neoformans possesses multiple virulence factors capable of modulating immune responses. The outcome of the interactions between the host and C. neoformans will determine the disease progression. In this review, we describe the current understanding of how C. neoformans migrates to the brain across the blood–brain barrier, and how the host immune system responds to the invading organism in the brain. We will also discuss the virulence factors that C. neoformans uses to modulate host immune responses.

IL-23 signaling prevents ferroptosis-driven renal immunopathology during candidiasis

During infection the host relies on pattern-recognition receptors to sense invading fungal pathogens to launch immune defense mechanisms. While fungal recognition and immune effector responses are organ and cell type specific, during disseminated candidiasis myeloid cells exacerbate collateral tissue damage. The β-glucan receptor ephrin type-A 2 receptor (EphA2) is required to initiate mucosal inflammatory responses during oral Candida infection. Here we report that EphA2 promotes renal immunopathology during disseminated candidiasis. EphA2 deficiency leads to reduced renal inflammation and injury. Comprehensive analyses reveal that EphA2 restrains IL-23 secretion from and migration of dendritic cells. IL-23 signaling prevents ferroptotic host cell death during infection to limit inflammation and immunopathology. Further, host cell ferroptosis limits antifungal effector functions via releasing the lipid peroxidation product 4-hydroxynonenal to induce various forms of cell death. Thus, we identify ferroptotic cell death as a critical pathway of Candida-mediated renal immunopathology that opens a new avenue to tackle Candida infection and inflammation.

Elevated Intracranial Pressure in Cryptococcal Meningoencephalitis: Examining Old, New, and Promising Drug Therapies

Despite the availability of effective antifungal therapy, cryptococcal meningoencephalitis (CM) remains associated with elevated mortality. The spectrum of symptoms associated with the central nervous system (CNS) cryptococcosis is directly caused by the high fungal burden in the subarachnoid space and the peri-endothelial space of the CNS vasculature, which results in intracranial hypertension (ICH). Management of intracranial pressure (ICP) through aggressive drainage of cerebrospinal fluid by lumbar puncture is associated with increased survival. Unfortunately, these procedures are invasive and require specialized skills and supplies that are not readily available in resource-limited settings that carry the highest burden of CM. The institution of pharmacologic therapies to reduce the production or increase the resorption of cerebrospinal fluid would likely improve clinical outcomes associated with ICH in patients with CM. Here, we discuss the potential role of multiple pharmacologic drug classes such as diuretics, corticosteroids, and antiepileptic agents used to decrease ICP in various neurological conditions as potential future therapies for CM.

Experimental models for pharmacokinetic and pharmacodynamic studies of antifungals used in cryptococcosis treatment

Studies on cryptococcosis in the mammal animal model have demonstrated the occurrence of central nervous system infection and similarities in fungal pathogenicity with clinical and immunological features of the human infection. Although there is still a lack of studies involving pharmacokinetics (PK) and pharmacodynamics (PD) in animal models of cryptococcosis in the literature, these experimental models are useful for understanding this mycosis and antifungal effectiveness in improving the therapeutic schemes. The scope of this review is to describe and discuss the main mammal animal models for PK and PD studies of antifungals used in cryptococcosis treatment. Alternative models and computational methods are also addressed. All approaches for PK/PD studies are relevant to investigating drug-infection interaction and improving cryptococcosis therapy.

Identification of lncRNAs and Their Regulatory Relationships with mRNAs in Response to Cryptococcus neoformans Infection of THP-1 Cells

Aims. Cryptococcosis is an invasive fungal disease that is associated with an increasing prevalence along with a very high fatality and is primarily caused by Cryptococcus. However, its mechanism to cause pathogenicity is not yet completely understood. In this study, we aim to screen the lncRNA markers in human monocytic (THP-1) cells infected by Cryptococcus neoformans (C. neoformans) through high-throughput sequencing technology and to explore its effects on biological functions. Methods. We initially conducted an lncRNA microarray analysis of the THP-1 cells infected by C. neoformans and normal THP-1 cells. Based upon these data, RT-qPCR was used to verify the expressions of the selected lncRNAs and mRNAs. We then performed functional and pathway enrichment analyses. Lastly, target prediction was performed by using the lncRNA target tool which was based on the differentially expressed lncRNAs. Results. We determined 81 upregulated and 96 downregulated lncRNAs using microarray. In addition, the profiling data showed 42 upregulated and 57 downregulated genes and discovered that neuroactive ligand-receptor interaction, tyrosine metabolism, and phenylalanine metabolism are extremely impaired in the regulation of C. neoformans infection. GO enrichment analysis of the 99 differentially expressed mRNAs exhibited that these modules showed different signaling pathways and biological mechanisms like protein binding and metal ion binding. Moreover, lncRNAs and mRNAs were analyzed for their coexpression relations. A qRT-PCR analysis confirmed that the expression of the top 10 differently expressed mRNA and lincRNA. The expressions of the lncRNAs after C. neoformans infection in THP-1 cells were detected by RNA-sequence, suggesting that microarray analysis could reveal lncRNAs having functional significance that might be linked with the progression of patients. Conclusion. The current study analyzed the differential lncRNAs and mRNAs in C. neoformans infection and predicted the corresponding pathways and their correlations that can offer new potential insights into the mechanistic basis of this condition.

Fungal CNS Infections in Africa: The Neuroimmunology of Cryptococcal Meningitis

Cryptococcal meningitis (CM) is the leading cause of central nervous system (CNS) fungal infections in humans, with the majority of cases reported from the African continent. This is partly due to the high burden of HIV infection in the region and reduced access to standard-of-care including optimal sterilising antifungal drug treatments. As such, CM is responsible for 10-15% of all HIV-related mortality, with a large proportion being preventable. Immunity to the causative agent of CM, Cryptococcus neoformans, is only partially understood. IFNγ producing CD4⁺ T-cells are required for the activation of myeloid cells, especially macrophages, to enable fungal killing and clearance. However, macrophages may also act as a reservoir of the fungal yeast cells, shielding them from host immune detection thus promoting latent infection or persistent chronic inflammation. In this chapter, we review the epidemiology and pathogenesis of CNS fungal infections in Africa, with a major focus on CM, and the antifungal immune pathways operating to protect against C. neoformans infection. We also highlight the areas of research and policy that require prioritisation to help reduce the burden of CNS fungal diseases in Africa.

Expression of IL-20 Receptor Subunit β Is Linked to EAE Neuropathology and CNS Neuroinflammation

Considerable clinical evidence supports that increased blood-brain barrier (BBB) permeability is linked to immune extravasation of CNS parenchyma during neuroinflammation. Although BBB permeability and immune extravasation are known to be provoked by vascular endothelial growth factor-A (i.e., VEGF-A) and C-X-C motif chemokine ligand 12 (CXCL12), respectively, the mechanisms that link both processes are still elusive. The interleukin-20 (i.e., IL-20) cytokine signaling pathway was previously implicated in VEGF-mediated angiogenesis and is known to induce cellular response by way of signaling through IL-20 receptor subunit β (i.e., IL-20RB). Dysregulated IL-20 signaling is implicated in many inflammatory pathologies, but it's contribution to neuroinflammation has yet to be reported. We hypothesize that the IL-20 cytokine, and the IL cytokine subfamily more broadly, play a key role in CNS neuroinflammation by signaling through IL-20RB, induce VEGF activity, and enhance both BBB-permeability and CXCL12-mediated immune extravasation. To address this hypothesis, we actively immunized IL-20RB^-/- mice and wild-type mice to induce experimental autoimmune encephalomyelitis (EAE) and found that IL-20RB^-/- mice showed amelioration of disease progression compared to wild-type mice. Similarly, we passively immunized IL-20RB^-/- mice and wild-type mice with myelin-reactive Th1 cells from either IL-20RB^-/- and wild-type genotype. Host IL-20RB^-/- mice showed lesser disease progression than wild-type mice, regardless of the myelin-reactive Th1 cells genotype. Using multianalyte bead-based immunoassay and ELISA, we found distinctive changes in levels of pro-inflammatory cytokines between IL-20RB^-/- mice and wild-type mice at peak of EAE. We also found detectable levels of all cytokines of the IL-20 subfamily within CNS tissues and specific alteration to IL-20 subfamily cytokines IL-19, IL-20, and IL-24, expression levels. Immunolabeling of CNS region-specific microvessels confirmed IL-20RB protein at the spinal cord microvasculature and upregulation during EAE. Microvessels isolated from macaques CNS tissues also expressed IL-20RB. Moreover, we identified the expression of all IL-20 receptor subunits: IL-22 receptor subunit α-1 (IL-22RA1), IL-20RB, and IL-20 receptor subunit α (IL-20RA) in human CNS microvessels. Notably, human cerebral microvasculature endothelial cells (HCMEC/D3) treated with IL-1β showed augmented expression of the IL-20 receptor. Lastly, IL-20-treated HCMEC/D3 showed alterations on CXCL12 apicobasal polarity consistent with a neuroinflammatory status. This evidence suggests that IL-20 subfamily cytokines may signal at the BBB via IL-20RB, triggering neuroinflammation.

In vitro infection models to study fungal-host interactions

Fungal infections (mycoses) affect over a billion people per year. Approximately, two million of these infections are life-threatening, especially for patients with a compromised immune system. Fungi of the genera Aspergillus, Candida, Histoplasma and Cryptococcus are opportunistic pathogens that contribute to a substantial number of mycoses. To optimize the diagnosis and treatment of mycoses, we need to understand the complex fungal-host interplay during pathogenesis, the fungal attributes causing virulence and how the host resists infection via immunological defenses. In vitro models can be used to mimic fungal infections of various tissues and organs and the corresponding immune responses at near-physiological conditions. Furthermore, models can include fungal interactions with the host-microbiota to mimic the in vivo situation on skin and mucosal surfaces. This article reviews currently used in vitro models of fungal infections ranging from cell monolayers to microfluidic 3D organ-on-chip (OOC) platforms. We also discuss how OOC models can expand the toolbox for investigating interactions of fungi and their human hosts in the future.

Considerations When Developing Blood-Brain Barrier Crossing Drug Delivery Technology

Efficient therapeutic transport across the neurovasculature remains a challenge for developing medicine to treat central nervous system (CNS) disorders (Bell and Ehlers, Neuron 81:1-3, 2014). This chapter is meant to provide some insight and key considerations for developing and evaluating various technologies and approaches to CNS drug delivery. First, a brief review of various biological barriers, including the immune system, cellular and protein components of the blood-brain barrier (BBB), and clearance mechanisms in peripheral organs is provided. Next, a few examples and learnings from existing BBB-crossing modalities will be reviewed. Insight from "BBBomic" databases and thoughts on basic requirements for successful in vivo validation studies are discussed. Finally, an additional engineering barrier, namely manufacturing and product scalability, is highlighted as it relates to clinical translation and feasibility for developing BBB-crossing delivery technologies. A goal of this chapter is to provide an overview of the many barriers to the successful delivery of medicines into the brain. An emphasis will be placed on biotherapeutic and gene therapy applications for the treatment of neurological and neurodegenerative disorders.

Identifying Loci Associated With Bovine Corona Virus Infection and Bovine Respiratory Disease in Dairy and Feedlot Cattle

Bovine coronavirus (BCoV) is associated with respiratory and enteric infections in both dairy and beef cattle worldwide. It is also one of a complex of pathogens associated with bovine respiratory disease (BRD), which affects millions of cattle annually. The objectives of this study were to identify loci and heritability estimates associated with BCoV infection and BRD in dairy calves and feedlot cattle. Dairy calves from California (n = 1,938) and New Mexico (n = 647) and feedlot cattle from Colorado (n = 915) and Washington (n = 934) were tested for the presence of BCoV when classified as BRD cases or controls following the McGuirk scoring system. Two comparisons associated with BCoV were investigated: (1) cattle positive for BCoV (BCoV⁺) were compared to cattle negative for BCoV (BCoV^-) and (2) cattle positive for BCoV and affected with BRD (BCoV⁺BRD⁺) were compared to cattle negative for BCoV and BRD (BCoV^-BRD^-). The Illumina BovineHD BeadChip was used for genotyping, and genome-wide association analyses (GWAA) were performed using EMMAX (efficient mixed-model association eXpedited). The GWAA for BCoV⁺ identified 51 loci (p < 1 × 10^-5; 24 feedlot, 16 dairy, 11 combined) associated with infection with BCoV. Three loci were associated with BCoV⁺ across populations. Heritability estimates for BCoV⁺ were 0.01 for dairy, 0.11 for feedlot cattle, and 0.03 for the combined population. For BCoV⁺BRD⁺, 80 loci (p < 1 × 10^-5; 26 feedlot, 25 dairy, 29 combined) were associated including 14 loci across populations. Heritability estimates for BCoV⁺BRD⁺ were 0.003 for dairy, 0.44 for feedlot cattle, and 0.07 for the combined population. Several positional candidate genes associated with BCoV and BRD in this study have been associated with other coronaviruses and respiratory infections in humans and mice. These results suggest that selection may reduce susceptibility to BCoV infection and BRD in cattle.

Unraveling Axon Guidance during Axotomy and Regeneration

During neuronal development and regeneration axons extend a cytoskeletal-rich structure known as the growth cone, which detects and integrates signals to reach its final destination. The guidance cues “signals” bind their receptors, activating signaling cascades that result in the regulation of the growth cone cytoskeleton, defining growth cone advance, pausing, turning, or collapse. Even though much is known about guidance cues and their isolated mechanisms during nervous system development, there is still a gap in the understanding of the crosstalk between them, and about what happens after nervous system injuries. After neuronal injuries in mammals, only axons in the peripheral nervous system are able to regenerate, while the ones from the central nervous system fail to do so. Therefore, untangling the guidance cues mechanisms, as well as their behavior and characterization after axotomy and regeneration, are of special interest for understanding and treating neuronal injuries. In this review, we present findings on growth cone guidance and canonical guidance cues mechanisms, followed by a description and comparison of growth cone pathfinding mechanisms after axotomy, in regenerative and non-regenerative animal models.

Dissecting the localization of Tilapia tilapinevirus in the brain of the experimentally infected Nile tilapia, Oreochromis niloticus (L.)

Tilapia tilapinevirus or tilapia lake virus (TiLV) is an emerging virus that inflicts significant mortality on farmed tilapia globally. Previous studies reported detection of the virus in multiple organs of the infected fish; however, little is known about the in-depth localization of the virus in the central nervous system. Herein, we determined the distribution of TiLV in the entire brain of experimentally infected Nile tilapia. In situ hybridization (ISH) using TiLV-specific probes revealed that the virus was broadly distributed throughout the brain. The strongest positive signals were dominantly detected in the forebrain (responsible for learning, appetitive behaviour and attention) and the hindbrain (involved in controlling locomotion and basal physiology). The permissive cell zones for viral infection were observed mostly to be along the blood vessels and the ventricles. This indicates that the virus may productively enter into the brain through the circulatory system and widen broad regions, possibly through the cerebrospinal fluid along the ventricles, and subsequently induce the brain dysfunction. Understanding the pattern of viral localization in the brain may help elucidate the neurological disorders of the diseased fish. This study revealed the distribution of TiLV in the whole infected brain, providing new insights into fish-virus interactions and neuropathogenesis.

Mechanisms of fungal dissemination

Fungal infections are an increasing threat to global public health. There are more than six million fungal species worldwide, but less than 1% are known to infect humans. Most of these fungal infections are superficial, affecting the hair, skin and nails, but some species are capable of causing life-threatening diseases. The most common of these include Cryptococcus neoformans, Aspergillus fumigatus and Candida albicans. These fungi are typically innocuous and even constitute a part of the human microbiome, but if these pathogens disseminate throughout the body, they can cause fatal infections which account for more than one million deaths worldwide each year. Thus, systemic dissemination of fungi is a critical step in the development of these deadly infections. In this review, we discuss our current understanding of how fungi disseminate from the initial infection sites to the bloodstream, how immune cells eliminate fungi from circulation and how fungi leave the blood and enter distant organs, highlighting some recent advances and offering some perspectives on future directions.

Neonatal LPS exposure reduces ATP8A2 level in the prefrontal cortex in mice via increasing IFN-γ level

Neonatal lipopolysaccharide (LPS) exposure can cause depressive-like behaviors in rodents involving elevated interferon (IFN)-γ. Studies have linked down-regulation of prefrontal cortex (PFC) ATPase phospholipid transporting 8A2(ATP8A2) expression to depressive-like behaviors. In non-neuronal cells, IFN-γ could reduce ATP8A2 expression. Therefore, we hypothesized that neonatal LPS exposure might induce PFC ATP8A2 down-regulation by increasing the IFN-γ level. Here, C57BL6/J mice of both sexes received 3-dose-injections of LPS (50 μg/kg body weight, i.p.) on postnatal day (PND)5, PND7, and PND9. LPS-treated mice showed a transiently decreased PFC ATP8A2 expression indicated by western blot results. Moreover, a significant negative correlation of PFC ATP8A2 expression was found with the IFN-γ level. Using neutralizing mAb, IFN-γ was identified as the key mediator of LPS-induced PFC ATP8A2 decrease indicated by western blot and immunofluorescence results. In sum, neonatal LPS exposure reduced ATP8A2 level in PFC in mice via increasing IFN-γ level. This finding may help further understand the mechanism underlying LPS-induced impairments in brain development and function.

It's all in your head: antifungal immunity in the brain

As the incidence rate of invasive fungal infections has increased with the use of modern medical interventions, so too has the occurrence of fungi invading the brain. Fungi such as Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus often infect immunocompromised individuals, and can use several strategies to invade the central nervous system (CNS) by penetrating the blood-brain barrier. Once in the brain parenchyma the specialized resident immune cells need to effectively recognize the fungus and mount an appropriate immune response to clear the infection, without causing debilitating immune-mediated toxicity and neuronal damage. Here we review the current knowledge pertaining to the antifungal response of the CNS and highlight areas where future research is required.

Epstein-Barr Virus gH/gL and Kaposi's Sarcoma-Associated Herpesvirus gH/gL Bind to Different Sites on EphA2 To Trigger Fusion

Both Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are human gammaherpesviruses and are important in a variety of malignancies. Eph family receptor tyrosine kinase A2 (EphA2) is a cellular receptor for KSHV and EBV. Previous studies identified five conserved residues (ELEFN^50-54) in the N-terminal domain of KSHV gH that are critical for Eph binding and KSHV infection. However, the specific domains of EBV gH/gL important for EphA2 binding are not well described. We found that the KSHV gH (ELEFN^50-54) motif is important for higher KSHV fusion and that EBV gH/gL does not utilize a similar motif for fusion activity. We previously identified that an EBV gL N-glycosylation mutant (gL-N⁶⁹L/S⁷¹V) was hyperfusogenic in epithelial cells but not in B cells. To determine whether this glycosylation site may be the binding region for EphA2, we compared the EphA2 binding activity of EBV gH/gL and the EBV gH/gL-N⁶⁹L/S⁷¹V mutant. We found that EBV gH/gL-N⁶⁹L/S⁷¹V had higher binding affinity for EphA2, indicating that the EBV gL N-glycosylation site might be responsible for inhibiting the binding of gH/gL to EphA2. Loss of N-glycosylation at this site may remove steric hindrance that reduces EBV gH/gL binding to EphA2. In addition, the mutations located in the large groove of EBV gH/gL (R¹⁵²A and G⁴⁹C) also have decreased binding with EphA2. Taken together, our data indicate that the binding site of EphA2 on EBV gH/gL is at least in part proximal to the EBV gL glycosylation site, which in part accounts for differences in EphA2 binding affinity by KSHV.IMPORTANCE Virus entry into target cells is the first step for virus infection. Understanding the overall entry mechanism, including the binding mechanism of specific virus glycoproteins with cellular receptors, can be useful for the design of small molecule inhibitors and vaccine development. Recently, EphA2 was identified as an important entry receptor for both KSHV and EBV. In the present study, we investigated the required binding sites within EphA2 and EBV gH/gL that mediate the interaction of these two proteins allowing entry into epithelial cells and found that it differed in compared to the interaction of KSHV gH/gL with EphA2. Our discoveries may uncover new potential interventional strategies that block EBV and KSHV infection of target epithelial cells.

EphA2-Dependent Internalization of A. fumigatus Conidia in A549 Lung Cells Is Modulated by DHN-Melanin

Dectin-1 and ephrin type-A receptor 2 (EphA2) receptors recognize β-glucan present in the fungal cell wall. Inhibition of Dectin-1 with the monoclonal 2a11 antibody was shown to reduce internalization of conidia of the human pathogen Aspergillus fumigatus into epithelial cells. In this study, we investigated the role of the EphA2 receptor present on A549 epithelial type II lung cells in the interaction with A. fumigatus conidia. We assessed whether EphA2 is involved in association and internalization of conidia by receptor inhibition by an antibody or by using the kinase inhibitor dasatinib. A 50% reduction of internalization of conidia was observed when this receptor was blocked with either the EphA2-specific monoclonal antibody or dasatinib, which was similar when Dectin-1 was inhibited with the 2a11 monoclonal antibody. Inhibition of both receptors reduced the internalization to 40%. EphA2 inhibition was also assessed in a hydrophobin deletion strain (ΔrodA) that exposes more β-glucan and a dihydroxynaphthalene (DHN)-melanin deletion strain (ΔpksP) that exposes more glucosamine and glycoproteins. The ΔrodA strain behaved similar to the wild-type strain with or without EphA2 inhibition. In contrast, the ΔpksP mutant showed an increase in association to the A549 cells and a decrease in internalization. Internalization was not further decreased by EphA2 inhibition. Taken together, the presence of DHN-melanin in the spore cell wall results in an EphA2-dependent internalization of conidia of A. fumigatus into A549 cells.

Diverse species in the genus Cryptococcus: Pathogens and their non-pathogenic ancestors

The genus Cryptococcus comprises of more than 30 species. It consists of clinically significant pathogenic Cryptococcus neoformans/Cryptococcus gattii species complex comprising of a minimum of seven species. These pathogens cost more than 200,000 lives annually by causing cryptococcal meningoencephalitis. The evolution of the pathogenic species from closely related non-pathogenic species of the Cryptococcus amylolentus complex is of particular importance and several advances have been made to understand their phylogenetic and genomic relationships. The current review briefly describes the sexual reproduction process followed by an individual description of the members focusing on their key attributes and virulence mechanisms of the pathogenic species. A special section on phylogenetic studies is aimed at understanding the evolutionary divergence of pathogens from non-pathogens. Recent findings from our group pertaining to parameters affecting codon usage bias in six pathogenic and three non-pathogenic ancestral species and their corroboration with existing phylogenetic reports are also included in the current review.

Redundant Trojan horse and endothelial-circulatory mechanisms for host-mediated spread of Candida albicans yeast

The host innate immune system has developed elegant processes for the detection and clearance of invasive fungal pathogens. These strategies may also aid in the spread of pathogens in vivo, although technical limitations have previously hindered our ability to view the host innate immune and endothelial cells to probe their roles in spreading disease. Here, we have leveraged zebrafish larvae as a model to view the interactions of these host processes with the fungal pathogen Candida albicans in vivo. We examined three potential host-mediated mechanisms of fungal spread: movement inside phagocytes in a "Trojan Horse" mechanism, inflammation-assisted spread, and endothelial barrier passage. Utilizing both chemical and genetic tools, we systematically tested the loss of neutrophils and macrophages and the loss of blood flow on yeast cell spread. Both neutrophils and macrophages respond to yeast-locked and wild type C. albicans in our model and time-lapse imaging revealed that macrophages can support yeast spread in a "Trojan Horse" mechanism. Surprisingly, loss of immune cells or inflammation does not alter dissemination dynamics. On the other hand, when blood flow is blocked, yeast can cross into blood vessels but they are limited in how far they travel. Blockade of both phagocytes and circulation reduces rates of dissemination and significantly limits the distance of fungal spread from the infection site. Together, this data suggests a redundant two-step process whereby (1) yeast cross the endothelium inside phagocytes or via direct uptake, and then (2) they utilize blood flow or phagocytes to travel to distant sites.

An Antivirulence Approach for Preventing Cryptococcus neoformans from Crossing the Blood-Brain Barrier via Novel Natural Product Inhibitors of a Fungal Metalloprotease

Fungal infections like cryptococcal meningitis are difficult to resolve because of the limited therapies available. The small arsenal of antifungal drugs reflect the difficulty in finding available targets in fungi because like mammalian cells, fungi are eukaryotes. The limited efficacy, toxicity, and rising resistance of antifungals contribute to the high morbidity and mortality of fungal infections and further underscore the dire but unmet need for new antifungal drugs. The traditional approach in antifungal drug development has been to target fungal growth, but an attractive alternative is to target mechanisms of pathogenesis. An important attribute of Cryptococcus neoformans (Cn) pathogenesis is its ability to enter the central nervous system. Here, we describe a large-scale screen that identified three natural products that prevented Cn from crossing the blood-brain barrier by inhibiting the virulence factor Mpr1 without affecting the growth of Cn. We propose that compounds identified here could be further developed as antivirulence therapy that would be administered preemptively or serve as a prophylactic in patients at high risk for developing cryptococcal meningitis.

Cryptococcus neoformans (Cn) is the leading cause of fungal meningitis, a deadly disease with limited therapeutic options. Dissemination to the central nervous system hinges on the ability of Cn to breach the blood-brain barrier (BBB) and is considered an attribute of Cn virulence. Targeting virulence instead of growth for antifungal drug development has not been fully exploited despite the benefits of this approach. Mpr1 is a secreted fungal metalloprotease not required for fungal growth, but rather, it functions as a virulence factor by facilitating Cn migration across the BBB. This central role for Mpr1, its extracellular location, and lack of expression in mammalian cells make Mpr1 a high-value target for an antivirulence approach aimed at developing therapeutics for cryptococcal meningitis. To test this notion, we devised a large-scale screen to identify compounds that prohibited Cn from crossing the BBB by selectively blocking Mpr1 proteolytic activity, without inhibiting the growth of Cn. A phytochemical natural product-derived library was screened to identify new molecular scaffolds of prototypes unique to a Cn microecosystem. Of the 240 pure natural products examined, 3 lead compounds, abietic acid, diosgenin, and lupinine inhibited Mpr1 proteolytic activity with 50% inhibitory concentration (IC₅₀) values of <10 μM, displayed little to no mammalian cell toxicity, and did not affect Cn growth. Notably, the lead compounds blocked Cn from crossing the BBB, without damaging the barrier integrity, suggesting the bioactive molecules had no off-target effects. We propose that these new drug scaffolds are promising candidates for the development of antivirulence therapy against cryptococcal meningitis.

Epithelial cell infection by Epstein-Barr virus

Epstein-Barr Virus (EBV) is etiologically associated with multiple human malignancies including Burkitt lymphoma and Hodgkin disease as well as nasopharyngeal and gastric carcinoma. Entry of EBV into target cells is essential for virus to cause disease and is mediated by multiple viral envelope glycoproteins and cell surface associated receptors. The target cells of EBV include B cells and epithelial cells. The nature and mechanism of EBV entry into these cell types are different, requiring different glycoprotein complexes to bind to specific receptors on the target cells. Compared to the B cell entry mechanism, the overall mechanism of EBV entry into epithelial cells is less well known. Numerous receptors have been implicated in this process and may also be involved in additional processes of EBV entry, transport, and replication. This review summarizes EBV glycoproteins, host receptors, signal molecules and transport machinery that are being used in the epithelial cell entry process and also provides a broad view for related herpesvirus entry mechanisms.

Role of EphA2 in host defense against oro-pharyngeal candidiasis

Oral defense should be able to sense the burden of, and distinguish between fungal commensals and pathogens, so that an adequate inflammatory response can be set up. Recently, Ephrin type-A receptor 2 (EphA2) was identified on oral epithelial cells and neutrophils that recognizes Candida albicans and induces adaptive protective host responses against this organism. The studies have increased our knowledge of how epithelial cells and neutrophils contribute to host defense against oral yeast infection.

Harnessing the Activity of the Fungal Metalloprotease, Mpr1, To Promote Crossing of Nanocarriers through the Blood-Brain Barrier

Cryptococcus neoformans (Cn) is the leading cause of fungal meningitis primarily in immunosuppressed patients. Cn invades the central nervous system by overcoming the highly restricted blood-brain barrier (BBB). We previously determined that a secreted fungal metalloprotease, Mpr1, that also confers crossing ability to yeast upon CnMPR1 expression in Saccharomyces cerevisiae is central to this process. This led us to question whether Mpr1 could be engineered to function as part of a nanocarrier delivery vehicle. Here, a eukaryotic expression system produced proteolytically active Mpr1 recombinant protein that was successfully conjugated to functionalized quantum dot (QD) nanoparticles and readily internalized by brain microvascular endothelial cells. An in vitro BBB model showed QD-Mpr1 crossed the BBB significantly better than mock QD, and QD-Mpr1 did not damage BBB integrity. Internalization of QD-Mpr1 occurred by membrane invaginations and endocytic pits typical of receptor-mediated endocytosis involving clathrin-coated entry points. This study substantiates the notion that fungal mechanisms of BBB entry may be harnessed for new drug delivery platform technologies.

PAMPs of the Fungal Cell Wall and Mammalian PRRs

Fungi are opportunistic pathogens that infect immunocompromised patients and are responsible for an estimated 1.5 million deaths every year. The antifungal innate immune response is mediated through the recognition of pathogen-associated molecular patterns (PAMPs) by the host's pattern recognition receptors (PRRs). PRRs are immune receptors that ensure the internalisation and the killing of fungal pathogens. They also mount the inflammatory response, which contributes to initiate and polarise the adaptive response, controlled by lymphocytes. Both the innate and adaptive immune responses are required to control fungal infections. The immune recognition of fungal pathogen primarily occurs at the interface between the membrane of innate immune cells and the fungal cell wall, which contains a number of PAMPs. This chapter will focus on describing the main mammalian PRRs that have been shown to bind to PAMPs from the fungal cell wall of the four main fungal pathogens: Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans and Pneumocystis jirovecii. We will describe these receptors, their functions and ligands to provide the reader with an overview of how the immune system recognises fungal pathogens and responds to them.

EphA2 contributes to disruption of the blood-brain barrier in cerebral malaria

Disruption of blood-brain barrier (BBB) function is a key feature of cerebral malaria. Increased barrier permeability occurs due to disassembly of tight and adherens junctions between endothelial cells, yet the mechanisms governing junction disassembly and vascular permeability during cerebral malaria remain poorly characterized. We found that EphA2 is a principal receptor tyrosine kinase mediating BBB breakdown during Plasmodium infection. Upregulated on brain microvascular endothelial cells in response to inflammatory cytokines, EphA2 is required for the loss of junction proteins on mouse and human brain microvascular endothelial cells. Furthermore, EphA2 is necessary for CD8+ T cell brain infiltration and subsequent BBB breakdown in a mouse model of cerebral malaria. Blocking EphA2 protects against BBB breakdown highlighting EphA2 as a potential therapeutic target for cerebral malaria.

Vomocytosis: Too Much Booze, Base, or Calcium?

Macrophages are well known for their phagocytic activity and their role in innate immune responses. Macrophages eat non-self particles, via a variety of mechanisms, and typically break down internalized cargo into small macromolecules. However, some pathogenic agents have the ability to evade this endosomal degradation through a nonlytic exocytosis process termed vomocytosis.

Macrophages are well known for their phagocytic activity and their role in innate immune responses. Macrophages eat non-self particles, via a variety of mechanisms, and typically break down internalized cargo into small macromolecules. However, some pathogenic agents have the ability to evade this endosomal degradation through a nonlytic exocytosis process termed vomocytosis. This phenomenon has been most often studied for Cryptococcus neoformans, a yeast that causes roughly 180,000 deaths per year, primarily in immunocompromised (e.g., human immunodeficiency virus [HIV]) patients. Existing dogma purports that vomocytosis involves distinctive cellular pathways and intracellular physicochemical cues in the host cell during phagosomal maturation. Moreover, it has been observed that the immunological state of the individual and macrophage phenotype affect vomocytosis outcomes. Here we compile the current knowledge on the factors (with respect to the phagocytic cell) that promote vomocytosis of C. neoformans from macrophages.

Central Nervous System Cryptococcal Infections in Non-HIV Infected Patients

Central nervous system (CNS) cryptococcosis in non-HIV infected patients affects solid organ transplant (SOT) recipients, patients with malignancy, rheumatic disorders, other immunosuppressive conditions and immunocompetent hosts. More recently described risks include the use of newer biologicals and recreational intravenous drug use. Disease is caused by Cryptococcus neoformans and Cryptococcus gattii species complex; C. gattii is endemic in several geographic regions and has caused outbreaks in North America. Major virulence determinants are the polysaccharide capsule, melanin and several ‘invasins’. Cryptococcal plb1, laccase and urease are essential for dissemination from lung to CNS and crossing the blood–brain barrier. Meningo-encephalitis is common but intracerebral infection or hydrocephalus also occur, and are relatively frequent in C. gattii infection. Complications include neurologic deficits, raised intracranial pressure (ICP) and disseminated disease. Diagnosis relies on culture, phenotypic identification methods, and cryptococcal antigen detection. Molecular methods can assist. Preferred induction antifungal therapy is a lipid amphotericin B formulation (amphotericin B deoxycholate may be used in non-transplant patients) plus 5-flucytosine for 2–6 weeks depending on host type followed by consolidation/maintenance therapy with fluconazole for 12 months or longer. Control of raised ICP is essential. Clinicians should be vigilant for immune reconstitution inflammatory syndrome.

How Environmental Fungi Cause a Range of Clinical Outcomes in Susceptible Hosts

Environmental fungi are globally ubiquitous and human exposure is near universal. However, relatively few fungal species are capable of infecting humans, and among fungi, few exposure events lead to severe systemic infections. Systemic infections have mortality rates of up to 90%, cost the US healthcare system $7.2 billion annually, and are typically associated with immunocompromised patients. Despite this reputation, exposure to environmental fungi results in a range of outcomes, from asymptomatic latent infections to severe systemic infection. Here we discuss different exposure outcomes for five major fungal pathogens: Aspergillus, Blastomyces, Coccidioides, Cryptococcus, and Histoplasma species. These fungi include a mold, a budding yeast, and thermal dimorphic fungi. All of these species must adapt to dramatically changing environments over the course of disease. These dynamic environments include the human lung, which is the first exposure site for these organisms. Fungi must defend themselves against host immune cells while germinating and growing, which risks further exposing microbe-associated molecular patterns to the host. We discuss immune evasion strategies during early infection, from disruption of host immune cells to major changes in fungal cell morphology.

Emerging Roles for Eph Receptors and Ephrin Ligands in Immunity

Eph receptors are the largest family of receptor tyrosine kinases and mediate a myriad of essential processes in humans from embryonic development to adult tissue homeostasis through interactions with membrane-bound ephrin ligands. The ubiquitous expression of Eph receptors and ephrin ligands among the cellular players of the immune system underscores the importance of these molecules in orchestrating an optimal immune response. This review provides an overview of the various roles of Eph receptors and ephrin ligands in immune cell development, activation, and migration. We also discuss the role of Eph receptors in disease pathogenesis as well as the implications of Eph receptors as future immunotherapy targets. Given the diverse and critical roles of Eph receptors and ephrin ligands throughout the immune system during both resting and activated states, this review aims to highlight the critical yet underappreciated roles of this family of signaling molecules in the immune system.

Axon Guidance Molecules Guiding Neuroinflammation

Axon guidance molecules (AGMs), such as Netrins, Semaphorins, and Ephrins, have long been known to regulate axonal growth in the developing nervous system. Interestingly, the chemotactic properties of AGMs are also important in the postnatal period, such as in the regulation of immune and inflammatory responses. In particular, AGMs play pivotal roles in inflammation of the nervous system, by either stimulating or inhibiting inflammatory responses, depending on specific ligand-receptor combinations. Understanding such regulatory functions of AGMs in neuroinflammation may allow finding new molecular targets to treat neurodegenerative diseases, in which neuroinflammation underlies aetiology and progression.

Past and Current Perspectives in Modeling Bacteria and Blood-Brain Barrier Interactions

The central nervous system (CNS) barriers are highly specialized cellular barriers that promote brain homeostasis while restricting pathogen and toxin entry. The primary cellular constituent regulating pathogen entry in most of these brain barriers is the brain endothelial cell (BEC) that exhibits properties that allow for tight regulation of CNS entry. Bacterial meningoencephalitis is a serious infection of the CNS and occurs when bacteria can cross specialized brain barriers and cause inflammation. Models have been developed to understand the bacterial - BEC interaction that lead to pathogen crossing into the CNS, however, these have been met with challenges due to these highly specialized BEC phenotypes. This perspective provides a brief overview and outlook of the in vivo and in vitro models currently being used to study bacterial brain penetration, and opinion on improved models for the future.

Cryptococcal Meningitis and Anti-virulence Therapeutic Strategies

Fungal infections of the central nervous system are responsible for significant morbidity and mortality. Cryptococcus neoformans (Cn) is the primary cause of fungal meningitis. Infection begins in the lung after inhalation of fungal spores but often spreads to other organs, particularly the brain in immunosuppressed individuals. Cn’s ability to survive phagocytosis and endure the onslaught of oxidative attack imposed by the innate immune response facilitates dissemination to the central nervous system (CNS). Despite the success of Cn at bypassing innate immunity, entry into the heavily protected brain requires that Cn overwhelm the highly restricted blood-brain barrier (BBB). This is a formidable task but mounting evidence suggests that Cn expresses surface-bound and secreted virulence factors including urease, metalloprotease, and hyaluronic acid that can undermine the BBB. In addition, Cn can exploit multiple routes of entry to gain access to the CNS. In this review, we discuss the cellular and molecular interface of Cn and the BBB, and we propose that the virulence factors mediating BBB crossing could be targeted for the development of anti-virulence drugs aimed at preventing fungal colonization of the CNS.

Ephrin Receptor A4 is a New Kaposi's Sarcoma-Associated Herpesvirus Virus Entry Receptor

Kaposi's sarcoma-associated herpesvirus (KSHV) is a human gammaherpesvirus associated with the development of Kaposi's sarcoma (KS). KSHV target cells include endothelial cells, B cells, monocytes, epithelial cells, dendritic cells, macrophages, and fibroblasts. KSHV entry into target cells is a complex multistep process and is initiated by the binding and interaction of viral envelope glycoproteins with the cellular receptors. In the current studies, we have found that EphA4 promotes KSHV glycoprotein H/glycoprotein L (gH/gL)-mediated fusion and infection better than does ephrin A2 (EphA2) in HEK293T cells, indicating that EphA4 is a new KSHV entry receptor. To confirm that epithelial cells express EphA2 and EphA4, we analyzed the expression of EphA2 and EphA4 in epithelial cells, endothelial cells, B cells, monocytes, fibroblasts using RNA sequencing (RNA-seq) data analysis of existing data sets. We found that these cell types broadly express both EphA2 and EphA4, with the exception of monocytes and B cells. To confirm EphA4 is important for KSHV fusion and infection, we generated EphA2 and EphA4 single- and double-knockout cells. We found that both EphA2 and EphA4 play a role in KSHV fusion and infection, since EphA2-EphA4 double-knockout cells had the greatest decrease in fusion activity and infection compared to single-knockout cells. Fusion and infection of KSHV were rescued in the EphA2-EphA4 double-knockout cells upon overexpression of EphA2 and/or EphA4. EphA2 binds to both Epstein-Barr virus (EBV) and KSHV gH/gL; however, EphA4 binds only to KSHV gH/gL. Taken together, our results identify EphA4 as a new entry receptor for KSHV.IMPORTANCE The overall entry mechanism for herpesviruses is not completely known, including those for the human gammaherpesviruses Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein-Barr virus (EBV). To fully understand the herpesvirus entry process, functional receptors need to be identified. In the current study, we found that EphA4 can also function for a KSHV entry receptor along with EphA2. Interestingly, we found that EphA4 does not function as an entry receptor for EBV, whereas EphA2 does. The discovery of EphA4 as a KSHV entry receptor has important implications for KSHV pathogenesis in humans, may prove useful in understanding the unique pathogenesis of KSHV infection in humans, and may uncover new potential targets that can be used for the development of novel interventional strategies.

The structure-function analysis of the Mpr1 metalloprotease determinants of activity during migration of fungal cells across the blood-brain barrier

Cryptococcal meningoencephalitis, the most common form of cryptococcosis, is caused by the opportunistic fungal pathogen, Cryptococcus neoformans. Molecular strategies used by C. neoformans to invade the central nervous system (CNS) have been the focus of several studies. Recently, the role of a novel secreted metalloprotease (Mpr1) in the pathogenicity of C. neoformans was confirmed by studies demonstrating that Mpr1 mediated the migration of fungal cells into the CNS. Given this central function, the aim here was to identify the molecular determinants of Mpr1 activity and resolve their role in the migration of cryptococci across the blood-brain barrier (BBB). The Mpr1 protein belongs to an understudied group of metalloproteases of the M36 class of fungalysins unique to fungi. They are generally synthesized as propeptides with fairly long prodomains and highly conserved regions within their catalytic core. Through structure-function analysis of Mpr1, our study identified the prodomain cleavage sites of Mpr1 and demonstrated that when mutated, the prodomain appears to remain attached to the catalytic C-terminus of Mpr1 rendering a nonfunctional Mpr1 protein and an inability for cryptococci to cross the BBB. We found that proteolytic activity of Mpr1 was dependent on the coordination of zinc with two histidine residues in the active site of Mpr1, since amino acid substitutions in the HExxH motif abolished Mpr1 proteolytic activity and prevented the migration of cryptococci across the BBB. A phylogenetic analysis of Mpr1 revealed a distinct pattern likely reflecting the neurotropic nature of C. neoformans and the specific function of Mpr1 in breaching the BBB. This study contributes to a deeper understanding of the molecular regulation of Mpr1 activity and may lead to the development of specific inhibitors that could be used to restrict fungal penetration of the CNS and thus prevent cryptococcal meningoencephalitis-related deaths.

Mechanisms of Pulmonary Escape and Dissemination by Cryptococcus neoformans

Cryptococcus neoformans is a common environmental saprophyte and human fungal pathogen that primarily causes disease in immunocompromised individuals. Similar to many environmentally acquired human fungal pathogens, C. neoformans initiates infection in the lungs. However, the main driver of mortality is invasive cryptococcosis leading to fungal meningitis. After C. neoformans gains a foothold in the lungs, a critical early step in invasion is transversal of the respiratory epithelium. In this review, we summarize current knowledge relating to pulmonary escape. We focus on fungal factors that allow C. neoformans to disseminate from the lungs via intracellular and extracellular routes.

A conserved Eph family receptor-binding motif on the gH/gL complex of Kaposi's sarcoma-associated herpesvirus and rhesus monkey rhadinovirus

Kaposi’s sarcoma-associated herpesvirus (KSHV) is a human oncogenic virus associated with Kaposi’s sarcoma and two B-cell malignancies. The rhesus monkey rhadinovirus (RRV) is a virus of nonhuman primates that is closely related to KSHV. Eph family receptor tyrosine kinases (Ephs) are cellular receptors for the gH/gL glycoprotein complexes of both KSHV and RRV. Through sequence analysis and mutational screens, we identified conserved residues in the N-terminal domain of KSHV and RRV glycoprotein H that are critical for Eph-binding in vitro. Homology-based structural predictions of the KSHV and RRV gH/gL complexes based on the Epstein-Barr-Virus gH/gL crystal structure located these amino acids in a beta-hairpin on gH, which is likely stabilized by gL and is optimally positioned for protein-protein interactions. Guided by these predictions, we generated recombinant RRV and KSHV strains mutated in the conserved motif as well as an RRV gL null mutant. Inhibition experiments using these mutants confirmed that disruption of the identified Eph-interaction motif or of gL expression resulted in complete detargeting from Ephs. However, all mutants were infectious on all cell types tested, exhibiting normal attachment but a reduction in infectivity of up to one log order of magnitude. While Eph-binding-negative RRV mutants were replication-competent on fibroblasts, their infectivity was comparatively more reduced on endothelial cells with a substantial subpopulation of endothelial cells remaining resistant to infection. Together, this provides evidence for a cell type-specific use of Ephs by RRV. Furthermore, our results demonstrate that gL is dispensable for infection by RRV. Its deletion caused a reduction in infectivity similar to that observed after mutation of Eph-binding residues in gH. Our findings would be compatible with an ability of KSHV and RRV to use other, less efficient entry mediators in lieu of Ephs, although these host factors may not be uniformly expressed by all cells.

In immunocompromised individuals in general and in the context of HIV infection in particular, KSHV is a major cause of cancer and B-cell proliferative malignancies. We identified and mutated conserved residues in the N-terminal domain of the gH/gL glycoprotein complex of KSHV and the related monkey virus RRV that are critical for the interaction with cellular receptors from the Eph family. These findings provide important insight into the function of the γ-herpesviral entry machinery. Using recombinant KSHV and RRV carrying these mutations, we demonstrated that while not strictly essential, gH/gL-Eph interactions are important for efficient infection—for RRV also in a cell-specific manner—but not for attachment of KSHV and RRV. The Eph-detargeted virus mutants described in this study can be used to further dissect the requirements for KSHV and RRV entry and to identify potential alternative entry mediators. Domains and residues on the viral glycoproteins with critical roles in receptor recognition, such as the Eph-binding motif described in this paper, can be informative for the design of inhibitory monoclonal antibodies.