Rousette Bat Dendritic Cells Overcome Marburg Virus-Mediated Antiviral Responses by Upregulation of Interferon-Related Genes While Downregulating Proinflammatory Disease Mediators

Abstract

Dysregulated and maladaptive immune responses are at the forefront of human diseases caused by infection with zoonotic viral hemorrhagic fever viruses. Elucidating mechanisms of how the natural animal reservoirs of these viruses coexist with these agents without overt disease, while permitting sufficient replication to allow for transmission and maintenance in a population, is important for understanding the viral ecology and spillover to humans. The Egyptian rousette bat (ERB) has been identified as a reservoir for Marburg virus (MARV), a filovirus and the etiological agent of the highly lethal Marburg virus disease. Little is known regarding how these bats immunologically respond to MARV infection. In humans, macrophages and dendritic cells (DCs) are primary targets of infection, and their dysregulation is thought to play a central role in filovirus diseases, by disturbing their normal functions as innate sensors and adaptive immune response facilitators while serving as amplification and dissemination agents for the virus. The infection status and responses to MARV in bat myeloid-lineage cells are uncharacterized and likely represent an important modulator of the bat’s immune response to MARV infection. Here, we generate DCs from the bone marrow of rousette bats. Infection with a bat isolate of MARV resulted in a low level of transcription in these cells and significantly downregulated DC maturation and adaptive immune-stimulatory pathways while simultaneously upregulating interferon-related pathogen-sensing pathways. This study provides a first insight into how the bat immune response is directed toward preventing aberrant inflammatory responses while mounting an antiviral response to defend against MARV infection. IMPORTANCE Marburg viruses (MARVs) cause severe human disease resulting from aberrant immune responses. Dendritic cells (DCs) are primary targets of infection and are dysregulated by MARV. Dysregulation of DCs facilitates MARV replication and virus dissemination and influences downstream immune responses that result in immunopathology. Egyptian rousette bats (ERBs) are natural reservoirs of MARV, and infection results in virus replication and shedding, with asymptomatic control of the virus within weeks. The mechanisms that bats employ to appropriately respond to infection while avoiding disease are unknown. Because DC infection and modulation are important early events in human disease, we measured the transcriptional responses of ERB DCs to MARV. The significance of this work is in identifying cell type-specific coevolved responses between ERBs and MARV, which gives insight into how bat reservoirs are able to harbor MARV and permit viral replication, allowing transmission and maintenance in the population while simultaneously preventing immunopathogenesis.

Asymptomatic Infection of Marburg Virus Reservoir Bats Is Explained by a Strategy of Immunoprotective Disease Tolerance

“…To further validate that in vivo CD14 + splenocyte responses to MARV reflected direct infection of and replication in monocytes/macrophages, which are primary MARV targets in primates, we infected ex vivo CD14 + splenocytes from naive colony ERBs with a recombinant version of the same MARV bat isolate used for our ERB inoculations. 7,21,25,26,36,37 This rMARV encodes a ZsGreen (ZsG) fluorescent protein able to visualize successful filovirus replication. 36,37 As expected, CD14 + populations showed robust ex vivo infection, with ZsG fluorescence increasing across the cell monolayer in both breadth and intensity out to D3 ( Figure S1B).…”

“…Thus, these two species-specific antibodies efficiently and selectively target and magnetically isolate ERB cells characteristic of monocytes/macrophages and B cells, agreeing with their cytometric analysis in our previously published BMDC ex vivo study, and ERB CD14 + monocytic cells are a direct cellular target of MARV infection, reflecting the established MARV tropism of their primate counterparts. 7,21,25 MARV-Infected ERBs Become Temporarily Viremic with Virus Dissemination to Tissues MARV RNA levels in blood were first detected on D1, peaked at D5, and were undetectable by D14 ( Figure 2A). Consistent with previous studies, similar viral RNA (vRNA) kinetics were observed in skin at the inoculation site ( Figure 2B) and in whole liver and spleen ( Figure 2C), persisting in the spleen for longer (D14) than in liver or skin (D8).…”

“…[18][19][20] Further, infecting ERB bone marrow-derived dendritic cells (BMDCs) failed to increase inflammatory gene expression, while experimental MARV infections of ERBs produced minimal, if any, gross histological signs of inflammation, even in tissues with highest viral loads. 3,5,6,21,22 The culmination of these data led us to hypothesize that ERBs have indeed developed a system of disease tolerance to MARV. However, significant testing of this hypothesis has so far been limited to immortalized cell lines, ex vivo tissue culture infections, or genomic approaches that cannot reproduce or examine the complexity and context of an immune response in a whole animal, which to our knowledge remains uncharacterized at a broad transcriptional level for any bat reservoir of a human-pathogenic virus, including ERBs infected with MARV.…”

Possibility for reverse zoonotic transmission of SARS-CoV-2 to free-ranging wildlife: A case study of bats

“…In vitro and laboratory studies demonstrate that bats can specifically regulate naïve immunity pathways to effectively cope with viral infection [114]. For example, dendritic cells generated from the bone marrow of the Egyptian rousette (Rousettus aegyptiacus) infected with Marburg virus down-regulate immune-stimulatory pathways and maturation of cells targeted by the virus while up-regulating pathogen-sensing pathways [115]. Unique bat immune regulation may occur with MERS-CoV infection, at least under experimental conditions [101].…”

Egyptian Rousette IFN-ω Subtypes Elicit Distinct Antiviral Effects and Transcriptional Responses in Conspecific Cells

“…Even if MARV infection of these cell lines does not lead to IFN-ω induction, it is still possible that various primary cell types could serve as a source of IFN-ω in vivo. It has recently been shown that Egyptian rousette dendritic cells that are infected with a bat isolate of MARV upregulate IFNs and ISGs, though the few IFNω genes included in the Nanostring-based study were not reported to be significantly upregulated (63).…”