Persistent virus infection inhibits type I interferon production by plasmacytoid dendritic cells to facilitate opportunistic infections

Emerging studies indicate an association between virus-induced impairment in type I interferon (IFN-I) production and enhanced susceptibility to opportunistic infections, which represent a major health problem. Here, we provide in vivo evidence that lymphocytic choriomeningitis virus (LCMV) infection of its natural murine host dramatically diminishes the unique capacity of plasmacytoid dendritic cells (pDCs) to secrete high levels of systemic IFN-I. While both acute and persistent LCMV infections suppress pDC IFN-I response, only the persistent virus induces a long-lasting diversion of this innate immune pathway. The consequent reduction in IFN-I production serves to impair natural killer cell responses in LCMV-infected mice challenged subsequently with murine cytomegalovirus (MCMV) as an opportunistic pathogen. This innate defect also compromises the host's ability to counteract early MCMV spread. These findings provide a mechanistic explanation for the occurrence of opportunistic infections following viral insults and have important implications for treating such medical complications.     

Ross River virus envelope glycans contribute to type I interferon production in myeloid dendritic cells

Alphaviruses are mosquito-transmitted viruses that cause significant human disease, and understanding how these pathogens successfully transition from the mosquito vector to the vertebrate host is an important area of research. Previous studies demonstrated that mosquito and mammalian-cell-derived alphaviruses differentially induce type I interferons (alpha/beta interferon [IFN-alpha/beta]) in myeloid dendritic cells (mDCs), where the mosquito cell-derived virus is a poor inducer of IFN-alpha/beta compared to the mammalian-cell-derived virus. Furthermore, the reduced IFN-alpha/beta induction by the mosquito cell-derived virus is attributed to differential N-linked glycosylation. To further evaluate the role of viral envelope glycans in regulating the IFN-alpha/beta response, studies were performed to assess whether the mosquito cell-derived virus actively inhibits IFN-alpha/beta induction or is simply a poor inducer of IFN-alpha/beta. Coinfection studies using mammalian- and mosquito cell-derived Ross River virus (mam-RRV and mos-RRV, respectively) indicated that mos-RRV was unable to suppress IFN-alpha/beta induction by mam-RRV in mDC cultures. Additionally, a panel of mutant viruses lacking either individual or multiple N-linked glycosylation sites was used to demonstrate that N-linked glycans were essential for high-level IFN-alpha/beta induction by the mammalian-cell-derived virus. These results suggest that the failure of the mosquito cell-derived virus to induce IFN-alpha/beta is due to a lack of complex carbohydrates on the virion rather than the active suppression of the DC antiviral response.     

SOCS-1 inhibition of type I interferon restrains Staphylococcus aureus skin host defense

The skin innate immune response to methicillin-resistant Staphylococcus aureus (MRSA) culminates in the formation of an abscess to prevent bacterial spread and tissue damage. Pathogen recognition receptors (PRRs) dictate the balance between microbial control and injury. Therefore, intracellular brakes are of fundamental importance to tune the appropriate host defense while inducing resolution. The intracellular inhibitor suppressor of cytokine signaling 1 (SOCS-1), a known JAK/STAT inhibitor, prevents the expression and actions of PRR adaptors and downstream effectors. Whether SOCS-1 is a molecular component of skin host defense remains to be determined. We hypothesized that SOCS-1 decreases type I interferon production and IFNAR-mediated antimicrobial effector functions, limiting the inflammatory response during skin infection. Our data show that MRSA skin infection enhances SOCS-1 expression, and both SOCS-1 inhibitor peptide-treated and myeloid-specific SOCS-1 deficient mice display decreased lesion size, bacterial loads, and increased abscess thickness when compared to wild-type mice treated with the scrambled peptide control. SOCS-1 deletion/inhibition increases phagocytosis and bacterial killing, dependent on nitric oxide release. SOCS-1 inhibition also increases the levels of type I and type II interferon levels in vivo. IFNAR deletion and antibody blockage abolished the beneficial effects of SOCS-1 inhibition in vivo. Notably, we unveiled that hyperglycemia triggers aberrant SOCS-1 expression that correlates with decreased overall IFN signatures in the infected skin. SOCS-1 inhibition restores skin host defense in the highly susceptible hyperglycemic mice. Overall, these data demonstrate a role for SOCS-1-mediated type I interferon actions in host defense and inflammation during MRSA skin infection.     

IRF family proteins and type I interferon induction in dendritic cells

Dendritic cells （DC）, although a minor population in hematopoietic cells, produce type I interferons （IFN） and other cytokines and are essential for innate immunity. They are also potent antigen presenters and regulate adaptive immunity. Among DC subtypes plasmacytoid DC （pDC） produce the highest amounts of type I IFN. In addition, pro- and anti-inflammatory cytokines such as IL-12 and IL-10 are induced in DC in response to Toll like receptor （TLR） signaling and upon viral infection. Proteins in the IRF family control many aspects of DC activity. IRF-8 and IRF-4 are essential for DC development. They differentially control the development of four DC subsets. IRF-8^-/- mice are largely devoid of pDC and CD8α^＋ DC, while IRF-4^-/- mice lack CD4^＋ DC. IRF-8^-/-, IRF4^-/-, double knock-out mice have only few CD8α CD4^-DC that lack MHC Ⅱ. IRF proteins also control type Ⅰ IFN induction in DC. IRF-7, activated upon TLR signaling is required for IFN induction not only in pDC, but also in conventional DC （cDC） and non-DC cell types. IRF-3, although contributes to IFN induction in fibroblasts, is dispensable in IFN induction in DC. Our recent evidence reveals that type Ⅰ IFN induction in DC is critically dependent on IRF-8, which acts in the feedback phase of IFN gene induction in DC. Type Ⅰ IFN induction in pDC is mediated by MyD88 dependent signaling pathway, and differs from pathways employed in other cells, which mostly rely on TLR3 and RIG-Ⅰ family proteins. Other pro-inflammatory cytokines are produced in an IRF-5 dependent manner. However, IRF-5 is not required for IFN induction, suggesting the presence of separate mechanisms for induction of type Ⅰ IFN and other pro-inflammatory cytokines. IFN and other cytokines produced by activated DC in turn advance DC maturation and change the phenotype and function of DC. These processes are also likely to be governed by IRF family proteins.     

Phospholipid Scramblase 1 regulates Toll-like receptor 9-mediated type I interferon production in plasmacytoid dendritic cells

Toll-like receptor 9 (TLR9) senses microbial DNA in the endosomes of plasmacytoid dendritic cells (pDCs) and triggers MyD88-dependent type I interferon (IFN) responses. To better understand TLR9 biology in pDCs, we established a yeast two-hybrid library for the identification of TLR9-interacting proteins. Here, we report that an IFN-inducible protein, phospholipid scramblase 1 (PLSCR1), interacts with TLR9 in pDCs. Knockdown of PLSCR1 expression by siRNA in human pDC cell line led to a 60-70% reduction of IFN-α responses following CpG-ODN (oligodeoxynucleotide) stimulation. Primary pDCs from PLSCR1-deficient mice produced lower amount of type 1 IFN than pDCs from the wild-type mice in response to CpG-ODN, herpes simplex virus and influenza A virus. Following CpG-A stimulation, there were much lower amounts of TLR9 in the early endosomes together with CpG-A in pDCs from PLSCR1-deficient mice. Our study demonstrates that PLSCR1 is a TLR9-interacting protein that plays an important role in pDC''s type 1 IFN responses by regulating TLR9 trafficking to the endosomal compartment.     

Differential induction of type I interferon responses in myeloid dendritic cells by mosquito and mammalian-cell-derived alphaviruses

Dendritic cells (DCs) are an important early target cell for many mosquito-borne viruses, and in many cases mosquito-cell-derived arboviruses more efficiently infect DCs than viruses derived from mammalian cells. However, whether mosquito-cell-derived viruses differ from mammalian-cell-derived viruses in their ability to induce antiviral responses in the infected dendritic cell has not been evaluated. In this report, alphaviruses, which are mosquito-borne viruses that cause diseases ranging from encephalitis to arthritis, were used to determine whether viruses grown in mosquito cells differed from mammalian-cell-derived viruses in their ability to induce type I interferon (IFN) responses in infected primary dendritic cells. Consistent with previous results, mosquito-cell-derived Ross River virus (mos-RRV) and Venezuelan equine encephalitis virus (mos-VEE) exhibited enhanced infection of primary myeloid dendritic cells (mDCs) compared to mammalian-cell-derived virus preparations. However, unlike the mammalian-cell-derived viruses, which induced high levels of type I IFN in the infected mDC cultures, mos-RRV and mos-VEE were poor IFN inducers. Furthermore, the poor IFN induction by mos-RRV contributed to the enhanced infection of mDCs by mos-RRV. These results suggest that the viruses initially delivered by the mosquito vector differ from those generated in subsequent rounds of replication in the host, not just with respect to their ability to infect dendritic cells but also in their ability to induce or inhibit antiviral type I IFN responses. This difference may have an important impact on the mosquito-borne virus's ability to successfully make the transition from the arthropod vector to the vertebrate host.     

Protective role of beta interferon in host defense against influenza A virus

Type I interferon (IFN), which includes the IFN-alpha and -beta subtypes, plays an essential role in host defense against influenza A virus. However, the relative contribution of IFN-beta remains unresolved. In mice, type I IFN is effective against influenza viruses only if the IFN-induced resistance factor Mx1 is present, though most inbred mouse strains, including the recently developed IFN-beta-deficient mice, bear only defective Mx1 alleles. We therefore generated IFN-beta-deficient mice carrying functional Mx1 alleles (designated Mx-BKO) and compared them to either wild-type mice bearing functional copies of both IFN-beta and Mx1 (designated Mx-wt) or mice carrying functional Mx1 alleles but lacking functional type I IFN receptors (designated Mx-IFNAR). Influenza A virus strain SC35M (H7N7) grew to high titers and readily formed plaques in monolayers of Mx-BKO and Mx-IFNAR embryo fibroblasts which showed no spontaneous expression of Mx1. In contrast, Mx-wt embryo fibroblasts were found to constitutively express Mx1, most likely explaining why SC35M did not grow to high titers and formed no visible plaques in such cells. In vivo challenge experiments in which SC35M was applied via the intranasal route showed that the 50% lethal dose was about 20-fold lower in Mx-BKO mice than in Mx-wt mice and that virus titers in the lungs were increased in Mx-BKO mice. The resistance of Mx-BKO mice to influenza A virus strain PR/8/34 (H1N1) was also substantially reduced, demonstrating that IFN-beta plays an important role in the defense against influenza A virus that cannot be compensated for by IFN-alpha.     

Virus infection of dendritic cells: portal for host invasion and host defense

Dendritic cells (DCs) act as a portal for virus invasion and as the most potent antigen-presenting cells in antiviral host defense. Human immunodeficiency virus (HIV)-1 has served as the paradigm for virus interaction with DCs. HIV-1 infection of DCs via its primary CD4 receptor and secondary chemokine receptors leads to full virus replication (cis infection), whereas binding to C-type lectin receptors results both in cis replication, as well as transfer and replication of virus in CD4(pos) T cells (trans infection). DCs respond to this invasion by processing viral proteins through MHC class I and II pathways and undergoing a maturation that enhances their presentation of antigen to T cells for induction of adaptive antiviral immunity. HIV-1 and other viruses have evolved mechanisms to subvert this immune function. Engineering of DCs with various forms of viral immunogens and co-treatment with cytokines and chemokines is being used as an immunotherapy for HIV-1 and other viral infections.     

The host type I interferon response to viral and bacterial infections

Type I interferons (IFN) are well studied cytokines with anti-viral and immune-modulating functions. Type I IFNs are produced following viral infections, but until recently, the mechanisms of viral recognition leading to IFN production were largely unknown. Toll like receptors (TLRs) have emerged as key transducers of type I IFN during viral infections by recognizing various viral components. Furthermore, much progress has been made in defining the signaling pathways downstream of TLRs for type I IFN production. TLR7 and TLR9 have become apparent as universally important in inducing type I IFN during infection with most viruses, particularly by plasmacytoid dendritic cells. New intracellular viral pattern recognition receptors leading to type I IFN production have been identified. Many bacteria can also induce the up-regulation of these cytokines. Interestingly, recent studies have found a detrimental effect on host cells if type Ⅰ IFN is produced during infection with the intracellular gram-positive bacterial pathogen, Listeria monocytogenes. This review will discuss the recent advances made in defining the signaling pathways leading to type I IFN production.     

Moxibustion activates host defense against herpes simplex virus type I through augmentation of cytokine production

Moxibustion is a technique used in traditional oriental medicine, the aim of which is to cure and/or prevent illness by activating a person's ability for self‐healing. In this study, we assessed how moxibustion would affect the immune system and whether it would augment protective immunity. Mice were treated with moxibustion at Zusanli (ST36) acupoints; we analyzed mortality and cytokine activity in sera after infection with herpes simplex virus type 1 (HSV‐1), and cytokine gene expression in the skin and the spleen without a virus challenge. Our study demonstrates that pretreatment of BALB/c mice with moxibustion resulted in a marked increase in the survival rate after infection with lethal doses of HSV‐1, and elevated serum levels of IL‐1β and IFN‐γ on days 1 and 6 post‐infection with HSV‐1. Semi‐quantitative RT‐PCR assay showed that moxibustion treatment augmented the expression of IL‐1α, IL‐1β, IL‐6, universal‐IFN‐α, MIP‐1α, and TNF‐α mRNA in the skin, and IL‐1α, IL‐1β, IL‐12p40, IL‐15, u‐IFN‐α, MIP‐1α, and TNF‐α mRNA in the spleen. Moreover, moxibustion induces augmentation of natural killer cell activity. Collectively, our study demonstrates that moxibustion activates protective responses against HSV‐1 infection through the activation of cytokine production including IFN, and of NK cells.     

A nonredundant role for plasmacytoid dendritic cells in host defense against the human fungal pathogen Aspergillus fumigatus

While plasmacytoid dendritic cells (pDCs), a natural type I interferon (IFN)-producing cell type, are regarded as critical for innate immunity to viruses, their role in defense against fungal infections remains unknown. We examined the interactions of pDCs with hyphae of the invasive human fungal pathogen Aspergillus fumigatus. Human pDCs spread over hyphae and inhibited their growth. Antifungal activity was retained in pDC lysates, did not require direct fungal contact, and was partially reversed by zinc. Incubation with hyphae resulted in pDC cytotoxicity, partly due to fungal gliotoxin secretion. Following hyphal stimulation, pDCs released proinflammatory cytokines via a TLR9-independent mechanism. Pulmonary challenge of mice with A. fumigatus resulted in a substantial influx of pDCs into lungs, and pDC-depleted mice were hypersusceptible to invasive aspergillosis. These data demonstrate the antifungal activity of pDCs against A. fumigatus and establish their nonredundant role in host defenses against invasive aspergillosis in vivo.     

Differential roles of type I interferon signaling in tumor versus host cells in experimental glioma models

Despite multimodal treatment approaches including surgery, radiotherapy and chemotherapy, the median survival for patients with glioblastoma remains in the range of one year and thus poor. Type I interferons (IFN) are involved in immune responses to viral infection and exhibit anti-tumor activity in certain cancers.Here we explored the biological relevance of constitutive type I IFN signaling in murine glioma models in vitro and in vivo. CT-2A, GL-261, SMA-497, SMA-540 and SMA-560 murine glioma cells expressed IFN type I receptors IFNAR1 and IFNAR2 and were responsive to exogenous IFN stimulation. CRISPR/Cas9-mediated deletion of IFNAR1 decreased the baseline expression of type I IFN response genes in GL-261 cells, but neither in CT-2A nor in SMA-560 cells. IFNAR1 deletion slowed growth in GL-261 and SMA-560, but not in CT-2A cells. However, only the growth of IFNAR1-depleted GL-261 tumors and not that of SMA-560 tumors was delayed in vivo upon orthotopic tumor cell implantation into syngeneic mice. This survival gain was no longer detected when the IFNAR1-depleted GL-261 cells were inoculated into IFNAR1-deficient mice. Altogether these data suggest that constitutive type I IFN signaling in gliomas may be pro-tumorigenic, but only in a microenvironment that is proficient for type I IFN signaling in the host.     

The role of Paneth cells and their antimicrobial peptides in innate host defense

The intestinal epithelium is the largest surface area that is exposed to various pathogens in the environment, however, in contrast to the colon the number of bacteria that colonize the small intestine is extremely low. Paneth cells, one of four major epithelial cell lineages in the small intestine, reside at the base of the crypts and have apically oriented secretory granules. These granules contain high levels of antimicrobial peptides that belong to the alpha-defensin family. Paneth cells secrete these microbicidal granules that contain alpha-defensins when exposed ex vivo to bacteria or their antigens, and recent evidence reveals that antimicrobial peptides, particularly alpha-defensins, that are present in Paneth cells contribute to intestinal innate host defense.