RIPK3 and caspase 8 collaborate to limit herpes simplex encephalitis

Caspase-8 activity mediates TNFα production and restricts Coxiella burnetii replication during murine macrophage infection

Coxiella burnetii is an obligate intracellular bacteria that causes the global zoonotic disease Q Fever. Treatment options for chronic infection are limited, and the development of novel therapeutic strategies requires a greater understanding of how C. burnetii interacts with immune signaling. Cell death responses are known to be manipulated by C. burnetii, but the role of caspase-8, a central regulator of multiple cell death pathways, has not been investigated. In this research, we studied bacterial manipulation of caspase-8 signaling and the significance of caspase-8 to C. burnetii infection, examining bacterial replication, cell death induction, and cytokine signaling. We measured caspase, RIPK, and MLKL activation in C. burnetii-infected tumor necrosis factor alpha (TNFα)/cycloheximide-treated THP-1 macrophage-like cells and TNFα/ZVAD-treated L929 cells to assess apoptosis and necroptosis signaling. Additionally, we measured C. burnetii replication, cell death, and TNFα induction over 12 days in RIPK1-kinase-dead, RIPK3-kinase-dead, or RIPK3-kinase-dead-caspase-8-/- bone marrow-derived macrophages (BMDMs) to understand the significance of caspase-8 and RIPK1/3 during infection. We found that caspase-8 is inhibited by C. burnetii, coinciding with inhibition of apoptosis and increased susceptibility to necroptosis. Furthermore, C. burnetii replication was increased in BMDMs lacking caspase-8, but not in those lacking RIPK1/3 kinase activity, corresponding with decreased TNFα production and reduced cell death. As TNFα is associated with the control of C. burnetii, this lack of a TNFα response may allow for the unchecked bacterial growth we saw in caspase-8-/- BMDMs. This research identifies and explores caspase-8 as a key regulator of C. burnetii infection, opening novel therapeutic doors.

Astrocytic RIPK3 exerts protective anti-inflammatory activity during viral encephalitis via induction of serpin protease inhibitors

Flaviviruses pose a significant threat to public health due to their ability to infect the central nervous system (CNS) and cause severe neurologic disease. Astrocytes play a crucial role in the pathogenesis of flavivirus encephalitis through their maintenance of blood-brain barrier (BBB) integrity and their modulation of immune cell recruitment and activation within the CNS. We have previously shown that receptor interacting protein kinase-3 (RIPK3) is a central coordinator of neuroinflammation during CNS viral infection, a function that occurs independently of its canonical function in inducing necroptotic cell death. To date, however, roles for necroptosis-independent RIPK3 signaling in astrocytes are poorly understood. Here, we use mouse genetic tools to induce astrocyte-specific deletion, overexpression, and chemogenetic activation of RIPK3 to demonstrate an unexpected anti-inflammatory function for astrocytic RIPK3. RIPK3 activation in astrocytes was required for host survival in multiple models of flavivirus encephalitis, where it restricted neuropathogenesis by limiting immune cell recruitment to the CNS. Transcriptomic analysis revealed that, despite inducing a traditional pro-inflammatory transcriptional program, astrocytic RIPK3 paradoxically promoted neuroprotection through the upregulation of serpins, endogenous protease inhibitors with broad immunomodulatory activity. Notably, intracerebroventricular administration of SerpinA3N in infected mice preserved BBB integrity, reduced leukocyte infiltration, and improved survival outcomes in mice lacking astrocytic RIPK3. These findings highlight a previously unappreciated role for astrocytic RIPK3 in suppressing pathologic neuroinflammation and suggests new therapeutic targets for the treatment of flavivirus encephalitis.

Neuronal DAMPs exacerbate neurodegeneration via astrocytic RIPK3 signaling

Astrocyte activation is a common feature of neurodegenerative diseases. However, the ways in which dying neurons influence the activity of astrocytes is poorly understood. Receptor interacting protein kinase-3 (RIPK3) signaling has recently been described as a key regulator of neuroinflammation, but whether this kinase mediates astrocytic responsiveness to neuronal death has not yet been studied. Here, we used the 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine model of Parkinson's disease to show that activation of astrocytic RIPK3 drives dopaminergic cell death and axon damage. Transcriptomic profiling revealed that astrocytic RIPK3 promoted gene expression associated with neuroinflammation and movement disorders, and this coincided with significant engagement of damage-associated molecular pattern signaling. In mechanistic experiments, we showed that factors released from dying neurons signaled through receptor for advanced glycation endproducts to induce astrocytic RIPK3 signaling, which conferred inflammatory and neurotoxic functional activity. These findings highlight a mechanism of neuron-glia crosstalk in which neuronal death perpetuates further neurodegeneration by engaging inflammatory astrocyte activation via RIPK3.

RIPK3 signaling and its role in regulated cell death and diseases

Receptor-interacting protein kinase 3 (RIPK3), a member of the receptor-interacting protein kinase (RIPK) family with serine/threonine protein kinase activity, interacts with RIPK1 to generate necrosomes, which trigger caspase-independent programmed necrosis. As a vital component of necrosomes, RIPK3 plays an indispensable role in necroptosis, which is crucial for human life and health. In addition, RIPK3 participates in the pathological process of several infections, aseptic inflammatory diseases, and tumors (including tumor-promoting and -suppressive activities) by regulating autophagy, cell proliferation, and the metabolism and production of chemokines/cytokines. This review summarizes the recent research progress of the regulators of the RIPK3 signaling pathway and discusses the potential role of RIPK3/necroptosis in the aetiopathogenesis of various diseases. An in-depth understanding of the mechanisms and functions of RIPK3 may facilitate the development of novel therapeutic strategies.

RIPK3 promotes neuronal survival by suppressing excitatory neurotransmission during CNS viral infection

While recent work has identified roles for immune mediators in the regulation of neural activity, the capacity for cell intrinsic innate immune signaling within neurons to influence neurotransmission remains poorly understood. However, the existing evidence linking immune signaling with neuronal function suggests that modulation of neurotransmission may serve previously undefined roles in host protection during infection of the central nervous system. Here, we identify a specialized function for RIPK3, a kinase traditionally associated with necroptotic cell death, in preserving neuronal survival during neurotropic flavivirus infection through the suppression of excitatory neurotransmission. We show that RIPK3 coordinates transcriptomic changes in neurons that suppress neuronal glutamate signaling, thereby desensitizing neurons to excitotoxic cell death. These effects occur independently of the traditional functions of RIPK3 in promoting necroptosis and inflammatory transcription. Instead, RIPK3 promotes phosphorylation of the key neuronal regulatory kinase CaMKII, which in turn activates the transcription factor CREB to drive a neuroprotective transcriptional program and suppress deleterious glutamatergic signaling. These findings identify an unexpected function for a canonical cell death protein in promoting neuronal survival during viral infection through the modulation of neuronal activity, highlighting new mechanisms of neuroimmune crosstalk.

Caspase-8 activity mediates TNFα production and restricts Coxiella burnetii replication during murine macrophage infection

Coxiella burnetii is an obligate intracellular bacteria which causes the global zoonotic disease Q Fever. Treatment options for infection are limited, and development of novel therapeutic strategies requires a greater understanding of how C. burnetii interacts with immune signaling. Cell death responses are known to be manipulated by C. burnetii, but the role of caspase-8, a central regulator of multiple cell death pathways, has not been investigated. In this research, we studied bacterial manipulation of caspase-8 signaling and the significance of caspase-8 to C. burnetii infection, examining bacterial replication, cell death induction, and cytokine signaling. We measured caspase, RIPK, and MLKL activation in C. burnetii-infected TNFα/CHX-treated THP-1 macrophage-like cells and TNFα/ZVAD-treated L929 cells to assess apoptosis and necroptosis signaling. Additionally, we measured C. burnetii replication, cell death, and TNFα induction over 12 days in RIPK1-kinase-dead, RIPK3-kinase-dead, or RIPK3-kinase-dead-caspase-8-/- BMDMs to understand the significance of caspase-8 and RIPK1/3 during infection. We found that caspase-8 is inhibited by C. burnetii, coinciding with inhibition of apoptosis and increased susceptibility to necroptosis. Furthermore, C. burnetii replication was increased in BMDMs lacking caspase-8, but not in those lacking RIPK1/3 kinase activity, corresponding with decreased TNFα production and reduced cell death. As TNFα is associated with the control of C. burnetii, this lack of a TNFα response may allow for the unchecked bacterial growth we saw in caspase-8-/- BMDMs. This research identifies and explores caspase-8 as a key regulator of C. burnetii infection, opening novel therapeutic doors.

Herpes simplex virus infection induces necroptosis of neurons and astrocytes in human fetal organotypic brain slice cultures

BACKGROUND

Herpes simplex virus (HSV) encephalitis (HSE) is a serious and potentially life-threatening disease, affecting both adults and newborns. Progress in understanding the virus and host factors involved in neonatal HSE has been hampered by the limitations of current brain models that do not fully recapitulate the tissue structure and cell composition of the developing human brain in health and disease. Here, we developed a human fetal organotypic brain slice culture (hfOBSC) model and determined its value in mimicking the HSE neuropathology in vitro.

METHODS

Cell viability and tissues integrity were determined by lactate dehydrogenase release in supernatant and immunohistological (IHC) analyses. Brain slices were infected with green fluorescent protein (GFP-) expressing HSV-1 and HSV-2. Virus replication and spread were determined by confocal microscopy, PCR and virus culture. Expression of pro-inflammatory cytokines and chemokines were detected by PCR. Cell tropism and HSV-induced neuropathology were determined by IHC analysis. Finally, the in situ data of HSV-infected hfOBSC were compared to the neuropathology detected in human HSE brain sections.

RESULTS

Slicing and serum-free culture conditions were optimized to maintain the viability and tissue architecture of ex vivo human fetal brain slices for at least 14 days at 37 °C in a CO2 incubator. The hfOBSC supported productive HSV-1 and HSV-2 infection, involving predominantly infection of neurons and astrocytes, leading to expression of pro-inflammatory cytokines and chemokines. Both viruses induced programmed cell death-especially necroptosis-in infected brain slices at later time points after infection. The virus spread, cell tropism and role of programmed cell death in HSV-induced cell death resembled the neuropathology of HSE.

CONCLUSIONS

We developed a novel human brain culture model in which the viability of the major brain-resident cells-including neurons, microglia, astrocytes and oligodendrocytes-and the tissue architecture is maintained for at least 2 weeks in vitro under serum-free culture conditions. The close resemblance of cell tropism, spread and neurovirulence of HSV-1 and HSV-2 in the hfOBSC model with the neuropathological features of human HSE cases underscores its potential to detail the pathophysiology of other neurotropic viruses and as preclinical model to test novel therapeutic interventions.

RIPK3 promotes brain region-specific interferon signaling and restriction of tick-borne flavivirus infection

Innate immune signaling in the central nervous system (CNS) exhibits many remarkable specializations that vary across cell types and CNS regions. In the setting of neuroinvasive flavivirus infection, neurons employ the immunologic kinase receptor-interacting kinase 3 (RIPK3) to promote an antiviral transcriptional program, independently of the traditional function of this enzyme in promoting necroptotic cell death. However, while recent work has established roles for neuronal RIPK3 signaling in controlling mosquito-borne flavivirus infections, including West Nile virus and Zika virus, functions for RIPK3 signaling in the CNS during tick-borne flavivirus infection have not yet been explored. Here, we use a model of Langat virus (LGTV) encephalitis to show that RIPK3 signaling is specifically required in neurons of the cerebellum to control LGTV replication and restrict disease pathogenesis. This effect did not require the necroptotic executioner molecule mixed lineage kinase domain like protein (MLKL), a finding similar to previous observations in models of mosquito-borne flavivirus infection. However, control of LGTV infection required a unique, region-specific dependence on RIPK3 to promote expression of key antiviral interferon-stimulated genes (ISG) in the cerebellum. This RIPK3-mediated potentiation of ISG expression was associated with robust cell-intrinsic restriction of LGTV replication in cerebellar granule cell neurons. These findings further illuminate the complex roles of RIPK3 signaling in the coordination of neuroimmune responses to viral infection, as well as provide new insight into the mechanisms of region-specific innate immune signaling in the CNS.

The Host-Pathogen Interplay: A Tale of Two Stories within the Cornea and Posterior Segment

Ocular infectious diseases are an important cause of potentially preventable vision loss and blindness. In the following manuscript, we will review ocular immunology and the pathogenesis of herpesviruses and Pseudomonas aeruginosa infections of the cornea and posterior segment. We will highlight areas of future research and what is currently known to promote bench-to-bedside discoveries to improve clinical outcomes of these debilitating ocular diseases.

Neuronal DAMPs exacerbate neurodegeneration via astrocytic RIPK3 signaling

Astrocyte activation is a common feature of neurodegenerative diseases. However, the ways in which dying neurons influence the activity of astrocytes is poorly understood. RIPK3 signaling has recently been described as a key regulator of neuroinflammation, but whether this kinase mediates astrocytic responsiveness to neuronal death has not yet been studied. Here, we used the MPTP model of Parkinson's disease to show that activation of astrocytic RIPK3 drives dopaminergic cell death and axon damage. Transcriptomic profiling revealed that astrocytic RIPK3 promoted gene expression associated with neuroinflammation and movement disorders, and this coincided with significant engagement of DAMP signaling. Using human cell culture systems, we show that factors released from dying neurons signal through RAGE to induce RIPK3-dependent astrocyte activation. These findings highlight a mechanism of neuron-glia crosstalk in which neuronal death perpetuates further neurodegeneration by engaging inflammatory astrocyte activation via RIPK3.

RIPK3 promotes brain region-specific interferon signaling and restriction of tick-borne flavivirus infection

Innate immune signaling in the central nervous system (CNS) exhibits many remarkable specializations that vary across cell types and CNS regions. In the setting of neuroinvasive flavivirus infection, neurons employ the immunologic kinase receptor-interacting kinase 3 (RIPK3) to promote an antiviral transcriptional program, independently of the traditional function of this enzyme in promoting necroptotic cell death. However, while recent work has established roles for neuronal RIPK3 signaling in controlling mosquito-borne flavivirus infections, including West Nile virus and Zika virus, functions for RIPK3 signaling in the CNS during tick-borne flavivirus infection have not yet been explored. Here, we use a model of Langat virus (LGTV) encephalitis to show that RIPK3 signaling is specifically required in neurons of the cerebellum to control LGTV replication and restrict disease pathogenesis. This effect did not require the necroptotic executioner molecule mixed lineage kinase domain like protein (MLKL), a finding similar to previous observations in models of mosquito-borne flavivirus infection. However, control of LGTV infection required a unique, region-specific dependence on RIPK3 to promote expression of key antiviral interferon-stimulated genes (ISG) in the cerebellum. This RIPK3-mediated potentiation of ISG expression was associated with robust cell-intrinsic restriction of LGTV replication in cerebellar granule cell neurons. These findings further illuminate the complex roles of RIPK3 signaling in the coordination of neuroimmune responses to viral infection, as well as provide new insight into the mechanisms of region-specific innate immune signaling in the CNS.

Approaches to Evaluating Necroptosis in Virus-Infected Cells

The immune system functions to protect the host from pathogens. To counter host defense mechanisms, pathogens have developed unique strategies to evade detection or restrict host immune responses. Programmed cell death is a major contributor to the multiple host responses that help to eliminate infected cells for obligate intracellular pathogens like viruses. Initiation of programmed cell death pathways during the early stages of viral infections is critical for organismal survival as it restricts the virus from replicating and serves to drive antiviral inflammation immune recruitment through the release of damage-associated molecular patterns (DAMPs) from the dying cell. Necroptosis has been implicated as a critical programmed cell death pathway in a diverse set of diseases and pathological conditions including acute viral infections. This cell death pathway occurs when certain host sensors are triggered leading to the downstream induction of mixed-lineage kinase domain-like protein (MLKL). MLKL induction leads to cytoplasmic membrane disruption and subsequent cellular destruction with the release of DAMPs. As the role of this cell death pathway in human disease becomes apparent, methods identifying necroptosis patterns and outcomes will need to be further developed. Here, we discuss advances in our understanding of how viruses counteract necroptosis, methods to quantify the pathway, its effects on viral pathogenesis, and its impact on cellular signaling.

Type I Interferon Signaling Is Critical During the Innate Immune Response to HSV-1 Retinal Infection

Purpose

Acute retinal necrosis (ARN) is a herpesvirus infection of the retina with blinding complications. In this study, we sought to create a reproducible mouse model of ARN that mimics human disease to better understand innate immunity within the retina during virus infection.

Methods

C57Bl/6J wild type (WT) and type I interferon receptor-deficient (IFNAR-/-) mice were infected with varying amounts of herpes simplex virus type 1 (HSV-1) via subretinal injection. Viral titers, optical coherence tomography (OCT) and fundus photography, the development of encephalitis, and ocular histopathology were scored and compared between groups of WT and IFNAR-/- mice.

Results

The retina of WT mice could be readily infected with HSV-1 via subretinal injection resulting in retinal whitening and full-thickness necrosis as determined by in vivo imaging and histopathology. In IFNAR-/- mice, HSV-1-induced retinal pathology was significantly worse when compared with WT mice, and viral titers were significantly elevated within two days after infection and persisted to day 5 after infection within the retina. These results were also observed in the brain where there were significantly higher viral titers and frequency of encephalitis in IFNAR-/- when compared to WT mice.

Conclusions

Collectively, these findings show that our new mouse model of ARN mimics human disease and can be used to study innate immunity within the retina. We conclude that type I interferons are critical in containing HSV-1 locally within retinal tissues and prohibiting spread into the brain.