The gastrointestinal tract, which is constantly exposed to a multitude of stimuli, is considered responsible for maintaining the homeostasis of the host. It is inhabited by billions of microorganisms, the gut microbiota, which form a mutualistic relationship with the host. Although the microbiota is generally recognized as beneficial, at the same time, together with pathogens, they are a permanent threat to the host. Various populations of epithelial cells provide the first line of chemical and physical defense against external factors acting as the interface between luminal microorganisms and immunocompetent cells in lamina propria. In this review, we focus on some essential, innate mechanisms protecting mucosal integrity, thus responsible for maintaining intestine homeostasis. The characteristics of decisive cell populations involved in maintaining the barrier arrangement, based on mucus secretion, formation of intercellular junctions as well as production of antimicrobial peptides, responsible for shaping the gut microbiota, are presented. We emphasize the importance of cross-talk between gut microbiota and epithelial cells as a factor vital for the maintenance of the homeostasis of the GI tract. Finally, we discuss how the imbalance of these regulations leads to the compromised barrier integrity and dysbiosis considered to contribute to inflammatory disorders and metabolic diseases.

Herpes simplex encephalitis (HSE) is a severe neurological disease in children and adults caused by herpes simplex virus. This review discusses recent findings on the role of Toll-like receptor 3 (TLR3) deficiencies in the HSE development. Critical checkpoints in the TLR3 signaling that contribute to innate response are discussed, including the importance of TLR3 ligand recognition site and transportation in the cell. We also indicate unresolved issues in the TLR3 functioning that might lead to thorough understanding of immunity during HSE. Such a knowledge base will lead to discovery and design of a rationale therapeutic and preventive approach against HSE.

Timely and precise delivery of the endosomal Toll-like receptors (TLRs) to the ligand recognition site is a critical event in mounting an effective antimicrobial immune response, however, the same TLRs should maintain the delicate balance of avoiding recognition of self-nucleic acids. Such sensing is widely known to start from endosomal compartments, but recently enough evidence has accumulated supporting the idea that TLR-mediated signaling pathways originating in the cell membrane may be engaged in various cells due to differential expression and distribution of the endosomal TLRs. Therefore, the presence of endosomal TLRs on the cell surface could benefit the host responses in certain cell types and/or organs. Although not fully understood why, TLR3, TLR7, and TLR9 may occur both in the cell membrane and intracellularly, and it seems that activation of the immune response can be initiated concurrently from these two sites in the cell. Furthermore, various forms of endosomal TLRs may be transported to the cell membrane, indicating that this may be a normal process orchestrated by cysteine proteases—cathepsins. Among the endosomal TLRs, TLR3 belongs to the evolutionary distinct group and engages a different protein adapter in the signaling cascade. The differently glycosylated forms of TLR3 are transported by UNC93B1 to the cell membrane, unlike TLR7, TLR8, and TLR9. The aim of this review is to reconcile various views on the cell surface positioning of endosomal TLRs and add perspective to the implication of such receptor localization on their function, with special attention to TLR3. Cell membrane-localized TLR3, TLR7, and TLR9 may contribute to endosomal TLR-mediated inflammatory signaling pathways. Dissecting this signaling axis may serve to better understand mechanisms influencing endosomal TLR-mediated inflammation, thus determine whether it is a necessity for immune response or simply a circumstantial superfluous duplication, with other consequences on immune response.

Mitochondria are multifunctional organelles that participate in numerous processes in response to viral infection, but they are also a target for viruses. The aim of this study was to define subcellular events leading to alterations in mitochondrial morphology and function during infection with ectromelia virus (ECTV). We used two different cell lines and a combination of immunofluorescence techniques, confocal and electron microscopy, and flow cytometry to address subcellular changes following infection. Early in infection of L929 fibroblasts and RAW 264.7 macrophages, mitochondria gathered around viral factories. Later, the mitochondrial network became fragmented, forming punctate mitochondria that co-localized with the progeny virions. ECTV-co-localized mitochondria associated with the cytoskeleton components. Mitochondrial membrane potential, mitochondrial fission–fusion, mitochondrial mass, and generation of reactive oxygen species (ROS) were severely altered later in ECTV infection leading to damage of mitochondria. These results suggest an important role of mitochondria in supplying energy for virus replication and morphogenesis. Presumably, mitochondria participate in transport of viral particles inside and outside of the cell and/or they are a source of membranes for viral envelope formation. We speculate that the observed changes in the mitochondrial network organization and physiology in ECTV-infected cells provide suitable conditions for viral replication and morphogenesis.

Current data strongly suggest herpes simplex virus type 1 (HSV-1) infection in the brain as a contributing factor to Alzheimer’s disease (AD). The consequences of HSV-1 brain infection are multilateral, not only are neurons and glial cells damaged, but modifications also occur in their environment, preventing the transmission of signals and fulfillment of homeostatic and immune functions, which can greatly contribute to the development of disease. In this review, we discuss the pathological alterations in the central nervous system (CNS) cells that occur, following HSV-1 infection. We describe the changes in neurons, astrocytes, microglia, and oligodendrocytes related to the production of inflammatory factors, transition of glial cells into a reactive state, oxidative damage, Aβ secretion, tau hyperphosphorylation, apoptosis, and autophagy. Further, HSV-1 infection can affect processes observed during brain aging, and advanced age favors HSV-1 reactivation as well as the entry of the virus into the brain. The host activates pattern recognition receptors (PRRs) for an effective antiviral response during HSV-1 brain infection, which primarily engages type I interferons (IFNs). Future studies regarding the influence of innate immune deficits on AD development, as well as supporting the neuroprotective properties of glial cells, would reveal valuable information on how to harness cytotoxic inflammatory milieu to counter AD initiation and progression.

Poxviruses have evolved numerous mechanisms to avoid the immune response of the infected host, and many of these mechanisms have not been fully described. Here, we studied the transcriptional response of innate immune genes in BALB/c and C57BL/6 peritoneal macrophages following infection with the Moscow strain of ectromelia virus (ECTV-Mos) with the aim of delineating innate immune genes that contribute to the difference between susceptibility and resistance to lethal infection. We show a generalized downregulation of many genes in four categories (toll-like receptor signaling, NOD-like receptor signaling, RIG-I-like receptor signaling, and type I interferon signaling) of antiviral innate immune receptors, downstream signaling pathways, and responsive components. Two important observations were made. First, 14 innate antiviral genes were differentially expressed with fold change upregulation of two and above occurring in C57BL/6 mice, known to be resistant to ECTV-Mos infection, whereas the same genes were downregulated in BALB/c mice with fold change of two and below. Second, the cathepsin group of genes was downregulated in both strains of mice but with profound fold changes of 17, 38, and 62 downregulation for CtsL, CtsB, and CtsS, respectively, in C57BL/6 mice. We show that a poxvirus profoundly downregulates both the mRNA and protein expression of these three cathepsins and this change appears to support virus replication. Based on these data we propose that the variations in gene expression observed may contribute to the difference in resistance/susceptibility between BALB/c and C57BL/6 mice to lethal infection by ECTV-Mos.

Toll-like receptors (TLRs) sense the presence of pathogen-associated molecular patterns. Nevertheless, the mechanisms modulating TLR-triggered innate immune responses are not yet fully understood. Complex regulatory systems exist to appropriately direct immune responses against foreign or self-nucleic acids, and a critical role of hepatocyte growth factor-regulated tyrosine kinase substrate (HRS), endosomal sorting complex required for transportation-0 (ESCRT-0) subunit, has recently been implicated in the endolysosomal transportation of TLR7 and TLR9. We investigated the involvement of Syk, Hrs, and STAM in the regulation of the TLR3 signaling pathway in a murine astrocyte cell line C8-D1A following cell stimulation with a viral dsRNA mimetic. Our data uncover a relationship between TLR3 and ESCRT-0, point out Syk as dsRNA-activated kinase, and suggest the role for Syk in mediating TLR3 signaling in murine astrocytes. We show molecular events that occur shortly after dsRNA stimulation of astrocytes and result in Syk Tyr-342 phosphorylation. Further, TLR3 undergoes proteolytic processing; the resulting TLR3 N-terminal form interacts with Hrs. The knockdown of Syk and Hrs enhances TLR3-mediated antiviral response in the form of IFN-β, IL-6, and CXCL8 secretion. Understanding the role of Syk and Hrs in TLR3 immune responses is of high importance since activation and precise execution of the TLR3 signaling pathway in the brain seem to be particularly significant in mounting an effective antiviral defense. Infection of the brain with herpes simplex type 1 virus may increase the secretion of amyloid-β by neurons and astrocytes and be a causal factor in degenerative diseases such as Alzheimer’s disease. Errors in TLR3 signaling, especially related to the precise regulation of the receptor transportation and degradation, need careful observation as they may disclose foundations to identify novel or sustain known therapeutic targets.

Background Cathepsins are a group of endosomal proteases present in many cells including dendritic cells (DCs). The activity of cathepsins is regulated by their endogenous inhibitors – cystatins. Cathepsins are crucial to antigen processing during viral and bacterial infections, and as such are a prerequisite to antigen presentation in the context of major histocompatibility complex class I and II molecules. Due to the involvement of DCs in both innate and adaptive immune responses, and the quest to understand the impact of poxvirus infection on host cells, we investigated the influence of ectromelia virus (ECTV) infection on cathepsin and cystatin levels in murine conventional DCs (cDCs). ECTV is a poxvirus that has evolved many mechanisms to avoid host immune response and is able to replicate productively in DCs. Results Our results showed that ECTV-infection of JAWS II DCs and primary murine GM-CSF-derived bone marrow cells down-regulated both mRNA and protein of cathepsin B, L and S, and cystatin B and C, particularly during the later stages of infection. Moreover, the activity of cathepsin B, L and S was confirmed to be diminished especially at later stages of infection in JAWS II cells. Consequently, ECTV-infected DCs had diminished ability to endocytose and process a soluble antigen. Close examination of cellular protein distribution showed that beginning from early stages of infection, the remnants of cathepsin L and cystatin B co-localized and partially co-localized with viral replication centers (viral factories), respectively. Moreover, viral yield increased in cDCs treated with siRNA against cathepsin B, L or S and subsequently infected with ECTV. Conclusions Taken together, our results indicate that infection of cDCs with ECTV suppresses cathepsins and cystatins, and alters their cellular distribution which impairs the cDC function. We propose this as an additional viral strategy to escape immune responses, enabling the virus to replicate effectively in infected cells. Electronic supplementary material The online version of this article (10.1186/s12866-019-1471-1) contains supplementary material, which is available to authorized users.