Bacterial stimulation upregulates the surface expression of the stress protein gp96 on B cells in the frog Xenopus

Urotensin II upregulates migration and cytokine gene expression in leukocytes of the African clawed frog, Xenopus laevis

Urotensin II (UII) exhibits diverse physiological actions including vasoconstriction, locomotor activity, osmoregulation, and immune response via the UII receptor (UTR) in mammals. However, in amphibians the function of the UII-UTR system remains unknown. In the present study, we investigated the potential immune function of UII using leukocytes isolated from the African clawed frog, Xenopus laevis. Stimulation of male frogs with lipopolysaccharide increased mRNA expression of UII and UTR in leukocytes, suggesting that inflammatory stimuli induce activation of the UII-UTR system. Migration assays showed that both UII and UII-related peptide enhanced migration of leukocytes in a dose-dependent manner, and that UII effect was inhibited by the UTR antagonist urantide. Inhibition of Rho kinase with Y-27632 abolished UII-induced migration, suggesting that it depends on the activation of RhoA/Rho kinase. Treatment of isolated leukocytes with UII increased the expression of several cytokine genes including tumor necrosis factor-α, interleukin-1β, and macrophage migration inhibitory factor, and the effects were abolished by urantide. These results suggest that in amphibian leukocytes the UII-UTR system is involved in the activation of leukocyte migration and cytokine gene expression in response to inflammatory stimuli.

GP96 interacts with HHV-6 during viral entry and directs it for cellular degradation

CD46 and CD134 mediate attachment of Human Herpesvirus 6A (HHV-6A) and HHV-6B to host cell, respectively. But many cell types interfere with viral infection through rapid degradation of viral DNA. Hence, not all cells expressing these receptors are permissive to HHV-6 DNA replication and production of infective virions suggesting the involvement of additional factors that influence HHV-6 propagation. Here, we used a proteomics approach to identify other host cell proteins necessary for HHV-6 binding and entry. We found host cell chaperone protein GP96 to interact with HHV-6A and HHV-6B and to interfere with virus propagation within the host cell. In human peripheral blood mononuclear cells (PBMCs), GP96 is transported to the cell surface upon infection with HHV-6 and interacts with HHV-6A and -6B through its C-terminal end. Suppression of GP96 expression decreased initial viral binding but increased viral DNA replication. Transient expression of human GP96 allowed HHV-6 entry into CHO-K1 cells even in the absence of CD46. Thus, our results suggest an important role for GP96 during HHV-6 infection, which possibly supports the cellular degradation of the virus.

Inhibition of local immune responses by the frog-killing fungus Batrachochytrium dendrobatidis

Amphibians are suffering unprecedented global declines. A leading cause is the infectious disease chytridiomycosis caused by the chytrid fungus Batrachochytrium dendrobatidis. Chytridiomycosis is a skin disease which disrupts transport of essential ions leading to death. Soluble factors produced by B. dendrobatidis impair amphibian and mammalian lymphocytes in vitro, but previous studies have not shown the effects of these inhibitory factors in vivo. To demonstrate in vivo inhibition of immunity by B. dendrobatidis, a modified delayed-type-hypersensitivity (DTH) protocol was developed to induce innate and adaptive inflammatory swelling in the feet of Xenopus laevis by injection of killed bacteria or phytohemagglutinin (PHA). Compared to previous protocols for PHA injection in amphibians, this method induced up to 20-fold greater inflammatory swelling. Using this new protocol, we measured DTH responses induced by killed bacteria or PHA in the presence of B. dendrobatidis supernatants. Swelling induced by single injection of PHA or killed bacteria was not significantly affected by B. dendrobatidis supernatants. However, swelling caused by a secondary injection of PHA, was significantly reduced by B. dendrobatidis supernatants. As previously described in vitro, factors from B. dendrobatidis appear to inhibit lymphocyte-mediated inflammatory swelling but not swelling caused by an inducer of innate leukocytes. This suggests that B. dendrobatidis is capable of inhibiting lymphocytes in a localized response to prevent adaptive immune responses in the skin. The modified protocol used to induce inflammatory swelling in the present study may be more effective than previous methods to investigate amphibian immune competence, particularly in nonmodel species.

The invasive chytrid fungus of amphibians paralyzes lymphocyte responses

The chytrid fungus, Batrachochytrium dendrobatidis, causes chytridiomycosis and is a major contributor to global amphibian declines. Although amphibians have robust immune defenses, clearance of this pathogen is impaired. Because inhibition of host immunity is a common survival strategy of pathogenic fungi, we hypothesized that B. dendrobatidis evades clearance by inhibiting immune functions. We found that B. dendrobatidis cells and supernatants impaired lymphocyte proliferation and induced apoptosis; however, fungal recognition and phagocytosis by macrophages and neutrophils was not impaired. Fungal inhibitory factors were resistant to heat, acid, and protease. Their production was absent in zoospores and reduced by nikkomycin Z, suggesting that they may be components of the cell wall. Evasion of host immunity may explain why this pathogen has devastated amphibian populations worldwide.

Bap, a biofilm matrix protein of Staphylococcus aureus prevents cellular internalization through binding to GP96 host receptor

The biofilm matrix, composed of exopolysaccharides, proteins, nucleic acids and lipids, plays a well-known role as a defence structure, protecting bacteria from the host immune system and antimicrobial therapy. However, little is known about its responsibility in the interaction of biofilm cells with host tissues. Staphylococcus aureus, a leading cause of biofilm-associated chronic infections, is able to develop a biofilm built on a proteinaceous Bap-mediated matrix. Here, we used the Bap protein as a model to investigate the role that components of the biofilm matrix play in the interaction of S. aureus with host cells. The results show that Bap promotes the adhesion but prevents the entry of S. aureus into epithelial cells. A broad analysis of potential interaction partners for Bap using ligand overlayer immunoblotting, immunoprecipitation with purified Bap and pull down with intact bacteria, identified a direct binding between Bap and Gp96/GRP94/Hsp90 protein. The interaction of Bap with Gp96 provokes a significant reduction in the capacity of S. aureus to invade epithelial cells by interfering with the fibronectin binding protein invasion pathway. Consistent with these results, Bap deficient bacteria displayed an enhanced capacity to invade mammary gland epithelial cells in a lactating mice mastitis model. Our observations begin to elucidate the mechanisms by which components of the biofilm matrix can facilitate the colonization of host tissues and the establishment of persistent infections.

Sensing disease and danger: a survey of vertebrate PRRs and their origins

A key facet of the innate immune response lays in its ability to recognize and respond to invading microorganisms and cellular disturbances. Through the use of germ-line encoded PRRs, the innate immune system is capable of detecting invariant pathogen motifs termed pathogen-associated molecular patterns (PAMPS) that are distinct from host encoded proteins or products released from dying cells, which are known as damage-associated molecular patterns (DAMPs). PAMPs and DAMPs include both protein and nucleic acids for the detection and response to pathogens and metabolic "danger" signals. This is by far one of the most active areas of research as recent studies have shown retinoic acid inducible gene 1 (RIG1)-like receptors (RLRs), the nucleotide-binding domain, leucine-rich repeat containing proteins (NLRs) and Toll-like receptors (TLRs) and the recently described AIM-like receptors (ALRs) are responsible for initiating interferon production or the assembly and activation of the inflammasome, ultimately resulting in the release of bioactive IL-1 family members. Overall, the vertebrate PRR recognition machinery consists of seven domains (e.g., Death, NACHT, CARD, TIR, LRR, PYD, helicase), most of which can be traced to the very origins of the deuterostomes. This review is intended to provide an overview of the basic components that are used by vertebrates to detect and respond to pathogens, with an emphasis on these receptors in fish as well as a brief note on their likely origins.

Heat-shock proteins as powerful weapons in vaccine development

Heat-shock proteins (HSPs) have been known as multifunctional proteins. They facilitate the folding and unfolding of proteins, participate in vesicular transport processes, prevent protein aggregation in the densely packed cytosol and are involved in signaling processes. HSPs have been involved in different fields, including autoimmunity, immunity to infections and tumor immunology. Although there are many different kinds of HSPs, only some HSPs, including HSP70 and Gp96, have immunological properties. HSP molecules have been applied into DNA- or protein (peptide)-based vaccines as antigens, chaperones or adjuvants. HSP-based vaccines have been shown to immunize against cancer and infectious diseases in both prophylactic and therapeutic protocols. The immunogenicity of HSPs results from two different properties: a peptide-dependent capacity to chaperone and elicit adaptive cytotoxic T-lymphocyte responses against antigenic peptides and a peptide-independent immunomodulatory capacity. Furthermore, HSPs could be immunoregulatory agents with potent and widely applicable therapeutic uses. Accordingly, certain HSPs, such as HSP70 and Gp96, are highly effective carrier molecules for cross-presentation. Their ability in eliciting immune responses against different pathogens (parasite and virus) and their role in cancer immunity will be discussed in this review.

Molecular and Cellular Requirements for Enhanced Antigen Cross-Presentation to CD8 Cytotoxic T Lymphocytes

MHC class I-mediated cross-priming of CD8 T cells by APCs is critical for CTL-based immunity to viral infections and tumors. We have shown previously that tumor-secreted heat shock protein gp96-chaperoned peptides cross prime CD8 CTL that are specific for genuine tumor Ags and for the surrogate Ag OVA. We now show that tumor-secreted heat shock protein gp96-chaperoned peptides enhance the efficiency of Ag cross-priming of CD8 CTL by several million-fold over the cross-priming activity of unchaperoned protein alone. Gp96 also acts as adjuvant for cross-priming by unchaperoned proteins, but in this capacity gp96 is 1000-fold less active than as a peptide chaperone. Mechanistically, the in situ secretion of gp96-Ig by transfected tumor cells recruits and activates dendritic cells and NK cells to the site of gp96 release and promotes CD8 CTL expansion locally. Gp96-mediated cross-priming of CD8 T cells requires B7.1/2 costimulation but proceeds unimpeded in lymph node-deficient mice, in the absence of NKT and CD4 cells and without CD40L. Gp96-driven MHC I cross-priming of CD8 CTL in the absence of lymph nodes provides a novel mechanism for local, tissue-based CTL generation at the site of gp96 release. This pathway may constitute a critically important, early detection, and rapid response mechanism that is operative in parenchymal tissues for effective defense against tissue damaging antigenic agents.

Aminoacyl-tRNA synthetase-interacting multifunctional protein 1/p43 controls endoplasmic reticulum retention of heat shock protein gp96: its pathological implications in lupus-like autoimmune diseases

Aminoacyl-tRNA synthetase-interacting multifunctional protein 1 (AIMP1; previously known as p43) is a multifunctional protein that was initially found in multitRNA synthetase complex. In the present study, screening of the AIMP1-binding proteins revealed that AIMP1 can form a molecular complex with heat shock protein gp96. AIMP1 enhances gp96 dimerization and the interaction between gp96 and KDEL receptor-1 (KDELR-1), which mediates the retrieval of KDEL-containing proteins from Golgi to the endoplasmic reticulum (ER). The interaction between gp96 and KDELR-1 was reduced in AIMP1-deficient cells, and this disturbed ER retention of gp96 and increased its cell surface localization. Moreover, this localization of gp96 at the cell surface was suppressed by its interaction with AIMP1 and enhanced by the depletion of endogenous AIMP1. In addition, AIMP1-deficient mice showed dendritic cell activation attributable to increased gp96 surface presentation and lupus-like autoimmune phenotypes. These results suggest that AIMP1 acts as a regulator of the ER retention of gp96 and provide a new perspective of the regulatory mechanism underlying immune stimulation by gp96.

Stress cytokines: pivotal proteins in immune regulatory networks; Opinion

Stress proteins have three immunological regulatory functions: within the cell, on the cell membrane as signalling receptors, and in the extracellular environment as stress cytokines. They can activate the immune system by providing danger signals or they may downregulate immune and inflammatory responses. In addition, they can modulate immune responses by acting as chaperones for antigenic peptides while they themselves are processed and presented to T cells as self-peptides. We predict that the exploitation of the downregulatory properties of stress cytokines will have therapeutic applications in the treatment of human chronic inflammatory diseases, such as rheumatoid arthritis.

Increased splenocyte mitogenesis following sympathetic denervation in Xenopus laevis

Studies in mammals reveal that ablation of the sympathetic nervous system (SNS) can alter in vivo and in vitro parameters of immunity. To shed some light on the phylogenetic history of the interactions between the SNS and the immune system, we studied the effects of chemical sympathectomy on the proliferative response of frog splenocytes to mitogens. Adult Xenopus laevis were injected with 6-hydroxydopamine 3 days before removal of spleen cells for culture with mitogens. Splenocytes from sympathectomized frogs exhibited an increased proliferative response to the T cell mitogens PHA and ConA and the B cell mitogen, LPS. That sympathectomy appears to effect a release from tonic inhibition by the SNS in Xenopus is consistent with comparable experiments in mice. It also reveals a phylogenetically ancient origin for SNS-immune system communications.