Tiehui Wang

Evolution of interleukin-1β

Abstract All jawed vertebrates possess a complex immune system, which is capable of anticipatory and innate immune responses. Jawless vertebrates posses an equally complex immune system but with no evidence of an anticipatory immune response. From these findings it has been speculated that the initiation and regulation of the immune system within vertebrates will be equally complex, although very little has been done to look at the evolution of cytokine genes, despite well-known biological activities within vertebrates. In recent years, cytokines, which have been well characterised within mammals, have begun to be cloned and sequenced within non-mammalian vertebrates, with the number of cytokine sequences available from primitive vertebrates growing rapidly. The identification of cytokines, which are mammalian homologues, will give a better insight into where immune system communicators arose and may also reveal molecules, which are unique to certain organisms. Work has focussed on interleukin-1 (IL-1), a major mediator of inflammation which initiates and/or increases a wide variety of non-structural, function associated genes that are characteristically expressed during inflammation. Other than mammalian IL-1β sequences there are now full cDNA sequences and genomic organisations available from bird, amphibian, bony fish and cartilaginous fish, with many of these genes having been obtained using an homology cloning approach. This review considers how the IL-1β gene has changed through vertebrate evolution and whether its role and regulation are conserved within selected non-mammalian vertebrates.

The interleukins of fish

Interleukins are a subgroup of cytokines, molecules involved in the intercellular regulation of the immune system. The term interleukin was first coined in 1979 to refer to molecules that signal between different leucocyte types, although not exclusively restricted to leucocyte communication. Whilst it is now known that interleukins are produced by a wide variety of cell types, nevertheless many are synthesised by CD4(+) T helper cells, macrophages/monocytes and endothelial cells. The nomenclature is relatively straightforward, with interleukin 1 the first discovered and interleukin 2 the second, etc. However, whilst 35 interleukins are currently described in mammals, several are in fact terms referring to subfamilies of more molecules, as with the IL-1 family where 11 members (IL-1F1-IL-1F11) are present, and the IL-17 family where 6 members (IL-17A-IL-17F) are present. So the total is much higher and splice variants and allelic variation increase this diversity further. This review will focus on what is known about interleukins in fish, and will refer to the major subfamilies rather than try to work through 35 descriptions in a row. It is clear that many direct homologues of molecules known in mammals are present in fish, but that not all are present and some novel interleukins exist that may have arisen from fish specific gene duplication events.

The cytokine networks of adaptive immunity in fish

Cytokines, produced at the site of entry of a pathogen, drive inflammatory signals that regulate the capacity of resident and newly arrived phagocytes to destroy the invading pathogen. They also regulate antigen presenting cells (APCs), and their migration to lymph nodes to initiate the adaptive immune response. When naive CD4+ T cells recognize a foreign antigen-derived peptide presented in the context of major histocompatibility complex class II on APCs, they undergo massive proliferation and differentiation into at least four different T-helper (Th) cell subsets (Th1, Th2, Th17, and induced T-regulatory (iTreg) cells in mammals. Each cell subset expresses a unique set of signature cytokines. The profile and magnitude of cytokines produced in response to invasion of a foreign organism or to other danger signals by activated CD4+ T cells themselves, and/or other cell types during the course of differentiation, define to a large extent whether subsequent immune responses will have beneficial or detrimental effects to the host. The major players of the cytokine network of adaptive immunity in fish are described in this review with a focus on the salmonid cytokine network. We highlight the molecular, and increasing cellular, evidence for the existence of T-helper cells in fish. Whether these cells will match exactly to the mammalian paradigm remains to be seen, but the early evidence suggests that there will be many similarities to known subsets. Alternative or additional Th populations may also exist in fish, perhaps influenced by the types of pathogen encountered by a particular species and/or fish group. These Th cells are crucial for eliciting disease resistance post-vaccination, and hopefully will help resolve some of the difficulties in producing efficacious vaccines to certain fish diseases.

The First Cytokine Sequence Within Cartilaginous Fish: IL-1β in the Small Spotted Catshark (Scyliorhinus canicula)

Cartilaginous fish are considered the most primitive living jawed vertebrates with a complex immune system typical of all jawed vertebrates. Cytokine homologs are found within jawless and bony fish, although no cytokine or cytokine receptor genes have been sequenced in cartilaginous fish. In this study the complete coding sequence of the small spotted catshark (Scyliorhinus canicula) IL-1β gene is presented that contains a short 5′ untranslated region (54 bp), a 903-bp open reading frame, a 379-bp 3′ untranslated region, a polyadenylation signal, and eight mRNA instability motifs. The predicted translation (301 amino acids) has highest identity to trout IL-1β (31.7%), with greatest homology within the putative 12 β-sheets. The IL-1 family signature is also present, but there is no apparent signal peptide. As with other nonmammalian IL-1β sequences, the IL-1-converting enzyme cut site is absent. Expression of the IL-1β transcript is detectable by RT-PCR in the spleen and testes, induced in vivo with LPS. Furthermore, a 7-fold increase of transcript levels in splenocytes incubated for 5 h with LPS was seen. The genomic organization comprises six exons and five introns with highest homology seen in exons encoding the largest amount of secondary structure per amino acid. Southern blot analysis suggests at least two copies of the IL-1β gene or genes related to the 3′ end of the IL-1β sequence are present in the catshark. The cloning of IL-1β in S. canicula, the first cytokine sequenced within cartilaginous fish, verifies previous bioactivity evidence for the presence of inflammatory cytokines.

Functional Characterization of a Nonmammalian IL-21: Rainbow Trout Oncorhynchus mykiss IL-21 Upregulates the Expression of the Th Cell Signature Cytokines IFN-γ, IL-10, and IL-22

In mammals, IL-21 is a common γ chain cytokine produced by activated CD4+ T cells and NKT cells that acts on multiple lineages of cells. Although IL-21 has also been discovered in birds, amphibians, and fish, to date, no functional studies have been reported for any nonmammalian IL-21 molecule. We have sequenced an IL-21 gene (tIL-21) in rainbow trout, which has a six-exon/five-intron structure, is expressed in immune tissues, and is induced by bacterial and viral infection and the T cell stimulant PHA. In contrast to mammals, calcium ionophore and PMA act synergistically to induce tIL-21. Recombinant tIL-21 (rtIL-21) induced a rapid and long-lasting (4–72 h) induction of expression of IFN-γ, IL-10, and IL-22, signature cytokines for Th1-, Th2-, and Th17-type responses, respectively, in head kidney leukocytes. However, rtIL-21 had little effects on the expression of other cytokines studied. rtIL-21 maintained the expression of CD8α, CD8β, and IgM at a late stage of stimulation when their expression was significantly decreased in controls and increased the expression of the Th cell markers CD4, T-bet, and GATA3. Intraperitoneal injection of rtIL-21 confirmed the in vitro bioactivity and increased the expression of IFN-γ, IL-10, IL-21, IL-22, CD8, and IgM. Inhibition experiments revealed that the activation of JAK/STAT3, Akt1/2, and PI3K pathways were responsible for rtIL-21 action. This study helps to clarify the role of IL-21 in lower vertebrates for the first time, to our knowledge, and suggests IL-21 is a likely key regulator of T and B cell function in fish.

Cloning and expression of the first nonmammalian interleukin-11 gene in rainbow trout Oncorhynchus mykiss

Interleukin (IL)‐11 is a multifunctional cytokine that stimulates hematopoietic progenitor cells and exerts a series of important immunomodulatory effects. It was believed to be restricted to mammals, but here we report the first nonmammalian IL‐11gene, in rainbow trout (Oncorhynchus mykiss). A trout IL‐11 cDNA clone was isolated that contains a 5′‐untranslated region (UTR) of 400 bp, an open reading frame of 612 bp and a large 3′‐UTR of 1924 bp. Analysis of a genomic DNA clone from a trout lambda library revealed that the trout IL‐11 gene has the same five exon/four intron gene organization, as well as the same intron phase, as mammalian IL‐11 genes. The 204 amino acid trout IL‐11 translation has a predicted signal peptide of 26 amino acids and mature peptide of 178 amino acids, with a calculated molecular mass of 20.5 kDa and a theoretical pI of 9.83. The mature peptide contains a cysteine residue and a potential N‐linked glycosylation site that are not present in mammals. Phylogenetic analysis clearly grouped trout IL‐11 with IL‐11 molecules from other species and separated from other members of the IL‐6 family. The IL‐11 gene is highly expressed in intestine and gills in healthy fish and its expression can also be detected in spleen, head kidney, brain, skin and muscle. Bacterial infection of rainbow trout markedly up‐regulates IL‐11 expression in liver, head kidney and spleen. IL‐11 expression is also up‐regulated in RTS‐11 cells (a trout macrophage cell line), which constitutively expressed the lowest level of IL‐11 of the four trout cell lines examined, after stimulation with bacteria, lipopolysaccharide, poly(I:C) and recombinant trout IL‐1β. Only a single transcript of 3.2 kb could be detected in lipopolysaccharide or recombinant IL‐1β‐stimulated RNA samples by northern blotting. The expression results, showing that IL‐11 is widely distributed and modulated by infection and other cytokines, suggest that fish IL‐11 is an active player in the cytokine network and the host immune response to infection.

Two Macrophage Colony-Stimulating Factor Genes Exist in Fish That Differ in Gene Organization and Are Differentially Expressed

Mammalian macrophage CSF (MCSF; CSF-1) is the primary regulator of the mononuclear phagocyte lineage. We, for the first time, report the complete sequencing of five MCSF cDNAs from three fish species, rainbow trout, zebrafish, and goldfish. Despite the difference in the lengths of the MCSF transcripts, all of the fish MCSF molecules encode a signal peptide, a CSF-1 domain, a transmembrane domain, and an intracellular region. Each fish MCSF gene has a unique exon/intron structure. The primordial MCSF gene may have had a nine exon/eight intron structure. In this model, insertion of an intron in exon 6 in primitive fish created the fish type I MCSF, while the loss of this exon or part of the original exon 6 created the fish type II MCSF. Investigation of alternative splicing variants in trout suggests that no mammalian equivalent splice variants exist. The two trout MCSF genes are differentially expressed in vivo and contributed differently to the high-level expression of MCSF in spleen and head kidney. In contrast to the up-regulation of MCSF by PMA in mammals, in trout MCSF1 expression is down-regulated by PMA treatment. As in mammals, recombinant trout MCSF1 can promote the growth of head kidney leukocytes, and it up-regulates the expression of CXCR3 in head kidney macrophages, with the latter suggesting a role of MCSF in the trafficking of macrophages to sites of inflammation or injury where the CXCR3 ligands are expressed. Thus MCSF has an important role in the immune system of fish as in mammals.

Bioactivity studies of rainbow trout (Oncorhynchus mykiss) interleukin-6: Effects on macrophage growth and antimicrobial peptide gene expression

Interleukin-6 (IL-6) is a pleiotropic cytokine that regulates hematopoiesis, inflammation, immune responses and bone homeostasis in mammals. Fish IL-6 has been cloned in recent years but to date no functional studies have been reported. Thus, in this paper we present for the first time in fish the functional characterisation of IL-6, using rainbow trout (Oncorhynchus mykiss) as the fish model and with a focus on macrophage effects. Trout IL-6 (tIL-6) expression in macrophages could be induced by proinflammatory agents (LPS, polyI:C, and IL-1β) and recombinant tIL-6 (rtIL-6) rapidly induced STAT3 phosphorylation and expression of SOCS-1 to -3, CISH and IRF-1, as seen in mammals. However, three findings in this study suggest a novel role of tIL-6 in fish. Firstly, macrophage growth was enhanced by rtIL-6 in vitro, suggesting that IL-6 produced during inflammatory events may promote macrophage proliferation locally. Secondly, rtIL-6 induced the expression of cathelicidin-2, an antimicrobial peptide with immune-modulatory function, but down-regulated the expression of IL-1β and TNF-α, indicating a role of IL-6 in host defence and also in limiting inflammation. Thirdly, rtIL-6 induced the expression of hepcidin in macrophages. In mammals hepcidin is antimicrobial but also regulates iron homeostasis by inhibiting iron absorption, and its expression is induced by IL-6 only in hepatocytes but not macrophages. Thus, in fish if IL-6 is induced in patrolling macrophages during sepsis this may act to reduce iron availability by induction of hepcidin expression and lead to iron deficiency, as a means to limit the spread of infection.

Identification of a Novel IL-1 Cytokine Family Member in Teleost Fish

A novel IL-1 family member (nIL-1F) has been discovered in fish, adding a further member to this cytokine family. The unique gene organization of nIL-1F, together with its location in the genome and low homology to known family members, suggests that this molecule is not homologous to known IL-1F. Nevertheless, it contains a predicted C-terminal β-trefoil structure, an IL-1F signature region within the final exon, a potential IL-1 converting enzyme cut site, and its expression level is clearly increased following infection, or stimulation of macrophages with LPS or IL-1β. A thrombin cut site is also present and may have functional relevance. The C-terminal recombinant protein antagonized the effects of rainbow trout rIL-1β on inflammatory gene expression in a trout macrophage cell line, suggesting it is an IL-1β antagonist. Modeling studies confirmed that nIL-1F has the potential to bind to the trout IL-1RI receptor protein, and may be a novel IL-1 receptor antagonist.

Characterization of three novel β-defensin antimicrobial peptides in rainbow trout (Oncorhynchus mykiss).

An initial bioinformatics investigation followed by cloning and sequencing analysis, has led to the identification of three novel members (omDB-2, omDB-3, omBD-4) of the beta-defensin family in rainbow trout (Oncorhynchus mykiss). The contiguous sequences could be translated to give predicted peptides of 62 (omDB-2), 63 (omDB-3) and 68 (omDB-4) amino acids (aa) in length, with mature peptides of 43 (omDB-2), 39 (omDB-3) and 42 (omDB-4) aa, with no obvious proregion present. Analysis of the gene organization found that all three new genes contained three exons divided by two introns, as seen in defensin genes of other fish species. Constitutive expression of all the trout defensins was detected by RT-PCR in a wide range of mucosal and systemic tissues from healthy fish, with omDB-3 and omDB-4 showing the highest expression levels. Following bacterial challenge in vivo, the defensin genes were induced at the three mucosal sites examined (skin, gill, gut), with levels of omDB-2 and omDB-3 increased some 16-fold in gut and gill respectively. Using polyinosinic polycytosinic RNA (polyI:C) as a viral mimic, all of the four trout beta-defensin genes were induced in head kidney primary leucocyte cultures at 4h post-stimulation, with omDB-1 and omDB-3 particularly highly expressed. These data suggest that beta-defensins are likely an important component of the innate defences of fish, and reveal an added level of antimicrobial peptide complexity in fish to that known previously.