Inflammasome-dependent IL-1β release depends upon membrane permeabilisation

Interleukin-1β (IL-1β) is a critical regulator of the inflammatory response. IL-1β is not secreted through the conventional ER–Golgi route of protein secretion, and to date its mechanism of release has been unknown. Crucially, its secretion depends upon the processing of a precursor form following the activation of the multimolecular inflammasome complex. Using a novel and reversible pharmacological inhibitor of the IL-1β release process, in combination with biochemical, biophysical, and real-time single-cell confocal microscopy with macrophage cells expressing Venus-labelled IL-1β, we have discovered that the secretion of IL-1β after inflammasome activation requires membrane permeabilisation, and occurs in parallel with the death of the secreting cell. Thus, in macrophages the release of IL-1β in response to inflammasome activation appears to be a secretory process independent of nonspecific leakage of proteins during cell death. The mechanism of membrane permeabilisation leading to IL-1β release is distinct from the unconventional secretory mechanism employed by its structural homologues fibroblast growth factor 2 (FGF2) or IL-1α, a process that involves the formation of membrane pores but does not result in cell death. These discoveries reveal key processes at the initiation of an inflammatory response and deliver new insights into the mechanisms of protein release.

The tumor necrosis factor alpha of the bony fish seabream exhibits the in vivo proinflammatory and proliferative activities of its mammalian counterparts, yet it functions in a species-specific manner

Information on the bioactivities of non-mammalian cytokines is scant due to the lack of the recombinant molecules and specific antibodies. We produced the mature predicted peptide of tumor necrosis factor alpha (TNF alpha) from the bony fish gilthead seabream (Sparus aurata L.) (sbTNF alpha), and its biological role was determined in vitro and in vivo. We first demonstrated by analytical size-exclusion chromatography that sbTNF alpha is an oligomeric protein but the dimer appears to predominate over the trimeric form, in contrast to mammalian TNF alpha. Intraperitoneal injection of native sbTNF alpha resulted in (i) priming of the respiratory burst of the peritoneal exudate and head-kidney (HK) leukocytes, the latter being the bone marrow equivalent in fish; (ii) rapid recruitment of phagocytic granulocytes to the injection site, and (iii) induction of granulopoiesis in the HK. Interestingly, sbTNF alpha was able to induce a strong proliferation of HK cells in vitro, whereas human TNF alpha did not. Conversely, sbTNF alpha was not cytotoxic for murine L929 fibroblasts.

A role for acidophilic granulocytes in the testis of the gilthead seabream (Sparus aurata L., Teleostei)

In mammals, a complex interaction between the immune and the reproductive systems has been described, in which testicular immune cells produce cytokines and growth factors which modulate gonad functions, while specific gonad cells influence the immune response in this organ. In this study we describe the presence of acidophilic granulocytes in the testis of the hermaphrodite teleost fish gilthead seabream (Sparus aurata L.) by using a specific monoclonal antibody. During the post-spawning stage of the testis, this cell type appears in the germinal compartment, accumulates interleukin (IL)-1beta and does not seem to be involved in the phagocytosis of degenerating cells. Moreover, in vitro, 11-ketotestosterone and 17beta-oestradiol, the principal fish sexual steroids, regulate the respiratory burst activity of acidophilic granulocytes obtained from the head-kidney (the bone marrow equivalent in fish) and the intracellular accumulation of IL-1beta by these cells. It is likely, therefore, that IL-1beta produced by testicular acidophilic granulocytes regulates important functions of the testis in fish.

Characterisation of gilthead seabream acidophilic granulocytes by a monoclonal antibody unequivocally points to their involvement in fish phagocytic response

The various cell types involved in fish phagocytic defence have not been properly established because of the morphological heterogeneity of leucocytes and the lack of appropriate cell-surface markers. In this study, we report the production and characterisation of a monoclonal antibody, G7, which specifically recognises gilthead seabream acidophilic granulocytes, as assayed by immunofluorescence and immunoelectron microscopy. The antibody reacted with 40%-50% of head-kidney and peritoneal exudate leucocytes and 10%-20% of spleen and peripheral blood leucocytes. More importantly, G7(+) acidophils constituted 85% of the head-kidney leucocytes showing phagocytic activity towards the fish pathogenic bacterium Vibrio anguillarum. The results are discussed in relation to the role played by this cell type in fish immune responses.

Interleukin-1beta isolated from a marine fish reveals up-regulated expression in macrophages following activation with lipopolysaccharide and lymphokines

The gilthead seabream IL-1beta gene consists of five exons/four introns. The complete coding sequence contains a 102 bp 5' untranslated region (UTR), a single open reading frame of 762 bp which translates into a 253 amino acid molecule, and a 407 bp 3'UTR with a polyadenylation signal 14 nucleotides upstream of the poly(A)tail. The seabream sequence has the highest degree of nucleotide (61.7%) and amino acid (53%) identity with the trout IL-1beta sequences. The IL-1beta message was detected by RT-PCR in head-kidney, blood, spleen, liver, gill and peritoneal exudate of both non-infected and Vibrio anguillarum-challenged fish. More importantly, IL-1beta was highly expressed by purified macrophage monolayers and was up-regulated by lipopolysaccharide and lymphocyte-derived macrophage-activating factor stimulation.

A fish cell surface receptor defined by a mAb mediates leukocyte aggregation and deactivation

Cell adhesion molecules play a key role in the inflammatory response. Selectins, integrins and immunoglobulin gene superfamily adhesion receptors mediate the different steps of leukocyte migration from the blood-stream towards inflammatory foci. In addition to their adhesive function, these receptors modulate major cellular processes such as cell activation, growth, differentiation and death. To characterise the fish molecules involved in cell adhesion, a panel of mAbs was raised by immunising mice with macrophages from the marine fish gilthead seabream (Sparus aurata L.). One of these mAbs, which we named anti-Aggregatin, was found to induce a rapid heterotypic aggregation of seabream leukocytes. Anti-Aggregatin defined a 140-kDa cell surface receptor which was highly expressed by macrophages and was up-regulated after co-stimulation with LPS and MAF. Functionally, the cell adhesion which occurred upon exposure to anti-Aggregatin required Ca(2+), an intact cytoskeleton and an active cell metabolism. More importantly, Aggregatin engagement resulted in strong inhibition of the phagocyte respiratory burst, although the cells showed neither loss of viability nor DNA fragmentation. The results are discussed in relation to the potential role of cell adhesion molecules in fish immune responses.