Cathepsin S, but not cathepsin L, participates in the MHC class II-associated invariant chain processing in large yellow croaker (Larimichthys crocea)

Antigen Presentation and Autophagy in Teleost Adaptive Immunity

Infectious diseases are a burden for aquaculture. Antigen processing and presentation (APP) to the immune effector cells that fight pathogens is key in the adaptive immune response. At the core of the adaptive immunity that appeared in lower vertebrates during evolution are the variable genes encoding the major histocompatibility complex (MHC). MHC class I molecules mainly present peptides processed in the cytosol by the proteasome and transported to the cell surface of all cells through secretory compartments. Professional antigen-presenting cells (pAPC) also express MHC class II molecules, which normally present peptides processed from exogenous antigens through lysosomal pathways. Autophagy is an intracellular self-degradation process that is conserved in all eukaryotes and is induced by starvation to contribute to cellular homeostasis. Self-digestion during autophagy mainly occurs by the fusion of autophagosomes, which engulf portions of cytosol and fuse with lysosomes (macroautophagy) or assisted by chaperones (chaperone-mediated autophagy, CMA) that deliver proteins to lysosomes. Thus, during self-degradation, antigens can be processed to be presented by the MHC to immune effector cells, thus, linking autophagy to APP. This review is focused on the essential components of the APP that are conserved in teleost fish and the increasing evidence related to the modulation of APP and autophagy during pathogen infection.

PU.1 Regulates Cathepsin S Expression in Large Yellow Croaker (Larimichthys crocea) Macrophages

Different morphologies have been detected in teleost macrophages. In this study, two macrophage cell lines were sub-cloned from a large yellow croaker head kidney cell line, LYCK. One type of sub-cloned cells was fusiform but the other was round, named LYC-FM and LYC-RM cells respectively, based on their morphologies. Both types showed the characteristics of macrophages, including expression of macrophage-specific marker genes, possession of phagocytic and bactericidal activities, and production of reactive oxygen species (ROS) and nitric oxide (NO). The transcription factor PU.1, crucial for the development of macrophages in mammals, was found to exist in two transcripts, PU.1a and PU.1b, in large yellow croaker, and constitutively expressed in LYC-FM and LYC-RM cells. The expression levels of PU.1a and PU.1b could be upregulated by recombinant large yellow croaker IFN-γ protein (rLcIFN-γ). Further studies showed that both PU.1a and PU.1b increased the expression of cathepsin S (CTSS) by binding to different E26−transformation−specific (Ets) motifs of the CTSS promoter. Additionally, we demonstrated that all three domains of PU.1a and PU.1b were essential for initiating CTSS expression by truncated mutation experiments. Our results therefore provide the first evidence that teleost PU.1 has a role in regulating the expression of CTSS.

In silico and gene expression analysis of the acute inflammatory response of gilthead seabream (Sparus aurata) after subcutaneous administration of carrageenin

Inflammation is one of the main causes of loss of homeostasis at both the systemic and molecular levels. The aim of this study was to investigate in silico the conservation of inflammation-related proteins in the gilthead seabream (Sparus aurata L.). Open reading frames of the selected genes were used as input in the STRING database for protein-protein interaction network analysis, comparing them with other teleost protein sequences. Proteins of the large yellow croaker (Larimichthys crocea L.) presented the highest percentages of identity with the gilthead seabream protein sequence. The gene expression profile of these proteins was then studied in gilthead seabream specimens subcutaneously injected with carrageenin (1%) or phosphate-buffered saline (control) by analyzing skin samples from the injected zone 12 and 24 h after injection. Gene expression analysis indicated that the mechanisms necessary to terminate the inflammatory response to carrageenin and recover skin homeostasis were activated between 12 and 24 h after injection (at the tested dose). The gene analysis performed in this study could contribute to the identification of the main mechanisms of acute inflammatory response and validate the use of carrageenin as an inflammation model to elucidate these mechanisms in fish.

Genome-wide identification, expression signature and immune functional analysis of two cathepsin S (CTSS) genes in turbot (Scophthalmus maximus L.)

Cathepsins, a superfamily of hydrolytic enzymes produced and enclosed within lysosomes, play multiple roles at physiological and pathological states. Cathepsin S is a lysosomal cysteine endopeptidase of the papain family, and exerts critical roles in the regulation of MHC class II immune responses. In the present study, we captured two Cathepsin S genes in turbot (SmCTSS1 and SmCTSS2.1), characterized their expression patterns following V. anguillarum and S. iniae infections, and explored their binding ability and agglutination capability. Firstly, the SmCTSS1 contained a 990 bp ORF encoding 329 amino acids, while SmCTSS2.1 contained a 1,014 bp ORF encoding 337 amino acids. The phylogenetic analysis revealed that both genes showed the closest relationship to their counterparts of Japanese flounder (Paralichthys olivaceus). In addition, both genes were ubiquitously expressed in all examined healthy tissues, with the highest expression level observed in spleen and intestine, respectively, while the lowest expression level both observed in liver. Both SmCTSS1 and SmCTSS2.1 were significantly differentially expressed, and exhibited general down-regulations at most time points in skin and intestine after two bacterial infections. Finally, both rSmCTSS1 and rSmCTSS2.1 showed significant binding ability to three examined microbial ligands (LPS, PGN and LTA), and strong agglutination effect to different bacteria (E. tarda, S. agalactiae, S. aureus and V. anguillarum). Collectively, this study provided valuable data for understanding the roles of CTSS in the host defense against bacterial infections in turbot, and indicated the potential vital roles of CTSS in innate immune responses of teleost species.

Characterization of Matrix Metalloprotease-9 Gene from Nile tilapia (Oreochromis niloticus) and Its High-Level Expression Induced by the Streptococcus agalactiae Challenge

The bacterial diseases of tilapia caused by Streptococcus agalactiae have resulted in the high mortality and huge economic loss in the tilapia industry. Matrix metalloproteinase-9 (MMP-9) may play an important role in fighting infection. However, the role of MMP-9 in Nile tilapia against S. agalactiae is still unclear. In this work, MMP-9 cDNA of Nile tilapia (NtMMP-9) has been cloned and characterized. NtMMP-9 has 2043 bp and encodes a putative protein of 680 amino acids. NtMMP-9 contains the conserved domains interacting with decorin and inhibitors via binding forces compared to those in other teleosts. Quantitative real-time-polymerase chain reaction (qPCR) analysis reveals that NtMMP-9 distinctly upregulated following S. agalactiae infection in a tissue- and time-dependent response pattern, and the tissues, including liver, spleen, and intestines, are the major organs against a S. agalactiae infection. Besides, the proteolytic activity of NtMMP-9 is also confirmed by heterologous expression and zymography, which proves the active function of NtMMP-9 interacting with other factors. The findings indicate that NtMMP-9 was involved in immune responses against the bacterial challenge at the transcriptional level. Further work will focus on the molecular mechanisms of NtMMP-9 to respond and modulate the signaling pathways in Nile tilapia against S. agalactiae invasion and the development of NtMMP-9-related predictive biomarkers or vaccines for preventing bacterial infection in the tilapia industry.

Characterization, expression, enzymatic activity, and functional identification of cathepsin S from black rockfish Sebastes schlegelii

Cathepsin S belong to the cathepsin L-like family of cysteine cathepsins. It is well known that Cathepsin S participate in various physiological processes and host immune defense in mammals. However, in teleost fish, the function of cathepsin S is less investigated. In the present study, a cathepsin S homologue (SsCTSS) from the teleost fish black rockfish (Sebastes schlegelii) were identified and examined at expression and functional levels. In silico analysis showed that three domains, including signal peptide, cathepsin propeptide inhibitor I29 domain, and functional domain Pept_C1, were existed in the cathepsin. SsCTSS possesses a peptidase domain with three catalytically essential residues (Cys25, His162, and Asn183). Phylogenetic profiling indicated that SsCTSS are evolutionally close to the cathepsin S of other teleost fish. The expression of SsCTSS in immune-related tissues was upregulated in a time-dependent manner upon bacterial pathogen infection. Purified recombinant SsCTSS (rSsCTSS) exhibited apparent peptidase activity, which was remarkably declined in the presence of the cathepsin inhibitor E-64. rSsCTSS showed strong binding ability to LPS and PGN, the major constituents of the outer membranes of Gram-negative and Gram-positive bacteria, respectively. rSsCTSS also exhibited the capability of agglutination to different bacteria. The knockdown of SsCTSS attenuated the ability of host to eliminate pathogenic bacteria. Taken together, our results suggested that SsCTSS functions as cysteine protease which might be involved in the antibacterial immunity of black rockfish.

Cathepsin L promotes secretory IgA response by participating in antigen presentation pathways during Mycoplasma Hyopneumoniae infection

Cathepsin L (CTSL) has been proved to help contain leishmaniasis and mycoplasma infection in mice by supporting cellular immune responses, but the regulatory functions of CTSL on mucosal immune responses haven't been tested and remain undefined. Here, we investigated the effects of CTSL on SIgA responses and invariant chain (Ii) degradations in the co-cultured swine dendritic cells (DCs) and B cells system in vitro. When the cells system were transfected with vector CTSL-GFP or incubated with recombinant CTSL (rCTSL) before they were infected with Mycoplasma hyopneumoniae (M.hp), SIgA significantly increased and Ii chain was degraded into smaller intermediates, while SIgA decreased when CTSL was knockdown or inhibited with E64. To confirm the SIgA responses promoted by CTSL contribute to the resistance to mycoplasma pneumonia, pigs injected with rCTSL before they were challenged with M.hp, showed milder clinical symptoms and histopathological damage of lungs, less mycoplasma burden together with higher secretion of SIgA, percentages of CD4+ T cells and level of MHC II molecules comparing with the group without rCTSL. Collectively, these results suggested that rCTSL could provide effective protection for piglets against mycoplasma pneumonia by enhancing M.hp-specific mucosal immune responses through its role in antigen presentation by processing the invariant chain.

Peptidylarginine deiminase and deiminated proteins are detected throughout early halibut ontogeny - Complement components C3 and C4 are post-translationally deiminated in halibut (Hippoglossus hippoglossus L.)

Post-translational protein deimination is mediated by peptidylarginine deiminases (PADs), which are calcium dependent enzymes conserved throughout phylogeny with physiological and pathophysiological roles. Protein deimination occurs via the conversion of protein arginine into citrulline, leading to structural and functional changes in target proteins. In a continuous series of early halibut development from 37 to 1050° d, PAD, total deiminated proteins and deiminated histone H3 showed variation in temporal and spatial detection in various organs including yolksac, muscle, skin, liver, brain, eye, spinal cord, chondrocytes, heart, intestines, kidney and pancreas throughout early ontogeny. For the first time in any species, deimination of complement components C3 and C4 is shown in halibut serum, indicating a novel mechanism of complement regulation in immune responses and homeostasis. Proteomic analysis of deiminated target proteins in halibut serum further identified complement components C5, C7, C8 C9 and C1 inhibitor, as well as various other immunogenic, metabolic, cytoskeletal and nuclear proteins. Post-translational deimination may facilitate protein moonlighting, an evolutionary conserved phenomenon, allowing one polypeptide chain to carry out various functions to meet functional requirements for diverse roles in immune defences and tissue remodelling.

Molecular cloning and functional characterization of cathepsin B from Nile tilapia (Oreochromis niloticus)

Cathepsin B (CatB) has been widely known for its hydrolytic ability and involvement in the innate immunity. However, the mechanism of CatB from teleosts participating in immunoregulation remains poorly understood; and the sequence of CatB from Nile tilapia (NtCatB) has not been cloned and characterized. In this study, the coding sequence of NtCatB was cloned, and then characterized by bioinformatic analysis and heterologous expression. The deduced amino acid sequence (330-aa) of NtCatB contains the representative features of CatB. Quantitative real-time PCR revealed the extensive mRNA expression of NtCatB in six tissues of healthy Nile tilapia, and its transcription level was significantly up-regulated after Streptococcus agalactiae challenge. NtCatB may interact with some immunological function proteins and take part in the regulatory pathway. These results suggest that NtCatB is likely to be involved in the immune reaction. The mature region (residues 79-328, mNtCatB) of NtCatB was cloned and transferred to pET-28a for expressing the recombinant protein. The purified recombinant mNtCatB was verified with the activity of 992.34 U mg^-1 min^-1 under the optimal condition using a substrate hydrolyzing assay. The recombinant cystatin-A1-like can effectively inhibit the activity of the recombinant mNtCatB, and their binding form was predicted by molecular docking. Our results contribute to elucidating the immunological functions of NtCatB.

Functional activities of interferon gamma in large yellow croaker Larimichthys crocea

Interferon gamma (IFN-γ) is a T helper cell type 1 (Th1) cytokine that plays important roles in almost all phases of immune and inflammatory responses. Although IFN-γ gene in large yellow croaker Larimichthys crocea has been reported, little is known about its bioactivity. In this study, large yellow croaker IFN-γ (LycIFN-γ) gene was found to be constitutively expressed in all tissues tested, with the highest levels in blood and heart. Based on stimulation with polyinosinic-polycytidylic acid [poly (I:C)] or inactivated trivalent bacterial vaccine, LycIFN-γ mRNA was significantly increased in spleen and head kidney tissues. LycIFN-γ transcripts were also detected in head kidney granulocytes, primary head kidney macrophages (PKM), head kidney leukocytes, and large yellow croaker head kidney cell line (LYCK), and were significantly up-regulated by poly(I:C) or lipopolysaccharide (LPS) in head kidney leukocytes. Recombinant LycIFN-γ protein (rLycIFN-γ) produced in Escherichia coli could enhance respiratory burst responses in PKM. Furthermore, rLycIFN-γ not only induced the expression of iNOS gene and release of NO, but also up-regulated the expression of proinflammatory cytokines TNF-α and IL-1β in PKM. These findings therefore indicated that LycIFN-γ has a role in mediating inflammatory response. In addition, rLycIFN-γ could significantly up-regulate expression of IFN-γ receptor CRFB13, signal transduction factor STAT1, transcription factors IRF1 and T-bet, and Th1-related cytokines IFN-γ and IL-2 in head kidney leukocytes, suggesting that LycIFN-γ may have the potential to promote Th1 immune response in large yellow croaker. Taken together, our results show that LycIFN-γ may be involved in inflammatory response and promote Th1 immune response as its mammalian counterpart.

Molecular characterization and gene expression of cathepsin L in Nile tilapia (Oreochromis niloticus)

Cathepsin L (CatL) has been widely known for its involvement in the innate immunity. However, it still remains poorly understand how CatL modulates the immune system of teleosts. Moreover, the CatL of Nile tilapia (NtCatL) has not been cloned or characterized. In this study, the gene encoding NtCatL was cloned, and was characterized by bioinformatics analysis, heterologous expression and protease activity assay. The coding sequence of NtCatL is 1017 bp in length and encodes 338 amino acid residues with a predicted molecular weight of 38.487 kDa and a theoretical isoelectric point of 5.79. NtCatL possesses the features of a typical cathepsin L, including one signal peptide, one propeptide region, and one papain family cysteine protease domain containing four active site residues (Gln¹³⁵, Cys¹⁴¹, His²⁸¹, and Asn³⁰⁵). The prediction of protein-protein interaction shows that NtCatL may interact with some functional proteins for realizing an immune function. Real-time quantitative PCR revealed the widespread transcriptional expression of NtCatL in six tissues of healthy Nile tilapia, and the NtCatL mRNA is significantly up-regulated after Streptococcus agalactiae challenge. These results suggest that NtCatL is likely to be involved in the immune reaction of Nile tilapia. Recombinant proteins from the mature domain (residues 117-337) of NtCatL were obtained by heterologous expression using pET28a and Rosetta (DE3) competent cells. A protein product with the high purity was obtained by using TALON Superflow purification rather than adopting HisTrap HP columns. The protease activity of the recombinant protein was verified by using a substrate hydrolyzing assay. This work has cloned and characterized the CatL from Nile tilapia for the first time, and contributes to elucidating the immunological functions of CatL.

Grass carp (Ctenopharyngodon idellus) invariant chain of the MHC class II chaperone protein associates with the class I molecule

The invariant chain (Ii) is an important immune molecule, as it assists major histocompatibility complex (MHC) class II molecules to present antigenic peptides. The relationship between the Ii and MHC molecules in teleosts remains poorly understood. This study focused on the molecular structure of grass carp Ii (gIi), its organ distribution, correlations with gene transcription, and the association with MHC. gIi cDNA was cloned using designed degenerate primers and the rapid amplification of cDNA ends method (RACE). The gIi sequence was 92%-96% similar to that of other teleosts, but only 52%-67% similar to that of mammals, respectively. The gIi gene was distributed in all 12 organs examined by PCR. The gIi gene transcription levels were markedly higher in organs enriched with immune cells than in other organs (P < 0.01). Moreover, positive correlations were detected between transcription levels of the gIi and gMhcI or II genes in different organs (r = 8.415-8.523, P = 0.001). The gIi co-localized on endomembrane systems with either class I or II molecules in co-transfected cells observed by a laser confocal. Further testing confirmed that the gIi bound gMHCI and II molecules. Taken together, these results indicate that the gIi is associated with MHC class I and II molecules, suggesting homology of both MHC molecules.

Identification and activity of a paralog of cathepsin S from yellow catfish (Pelteobagrus fulvidraco) involved in immune response

Cathepsin S, a papain-like cysteine peptidase, is an important regulator and signaling molecule with diverse biological actions in addition to immune presentation. However, our understanding of its structure and properties remains limited. Herein, a full-length cathepsin Sa from yellow catfish was cloned and named PfCTSSa. It contained 1366 bp, including a 981 bp ORF flanked by a 123 bp 5'-untranslated region (UTR) and a 262 bp 3'-UTR. This ORF encoded a 36.5 kD cysteine protease with the deduced amino acid sequence having a 76% sequence identity with Ictalurus punctatus ctssa. Additionally, PfCTSSa was found to be a paralog of cathepsin S since it generated a new cluster with cathepsin Sa in the phylogenic tree. Furthermore, PfCTSSa was found to contain more N-glycosylation sites than cathepsin S. The recombinant PfCTSSa was overexpressed in E. coli BL21 (DE3) and appeared to have the strongest activity at pH 8.5 and 35 °C in a concentration-dependent manner, with activity further affected by metal ions and detergents. Moreover, PfCTSSa mRNA was highly expressed in classic and mucosal immune tissues, although constitutively distributed in all of the examined tissues. Yellow catfish were then challenged with inactivated Aeromonas hydrophila and PfCTSSa was remarkably increased in the head kidney, liver and spleen when compared to the PBS control. Collectively, these results indicate that PfCTSSa is a paralog of cathepsin S and functions in the yellow catfish immune response.

A cystatin F homologue from large yellow croaker (Larimichthys crocea) inhibits activity of multiple cysteine proteinases and Ii chain processing in vitro

Cystatin F, a member of the family II cystatins, plays important roles in immune response-related processes through inhibiting specific enzyme targets. In this study, a cystatin F homologue, LycCysF, was identified and characterized from large yellow croaker (Larimichthys crocea). The deduced LycCysF protein exhibits a typical structural feature of type II cystatins, including three evolutionally conserved motifs, Gly(35), QVVRG(79-83) and PW(130-131). Tissue expression analysis showed that LycCysF mRNA was expressed in all tissues examined, albeit at different levels. Recombinant LycCysF (rLycCysF) produced in Pichia pastoris could inhibit the activity of multiple cysteine proteases, including papain, legumain and recombinant large yellow croaker cathepsin B, L and S. Moreover, rLycCysF could inhibit the Ii chain processing by recombinant cathepsin S in vitro. These data suggest that LycCysF may participate in regulation of cathepsins and MHC-II associated Ii chain processing. In addition, mammalian cystatin F is produced as an inactive dimer, becoming activated by proteolysis in the endo/lysosome of immune cells and then exerts its function of regulating downstream proteases activity. However, the N-terminal extension and two additional cysteine residues responsible for dimer formation are absent in LycCysF and cystatin F from other fish species, reptiles and Aves, indicating that these proteins can not form dimer and may regulate the proteases activity via an alternate pathway distinct from mammalian cystatin F. To our knowledge, this is the first report on molecular characteristics of a teleost cystatin F and its role in Ii chain processing.