Among the 11 members of the IL-1 family cytokines, the precursors of IL-1α, IL-1β, and IL-33 have relatively long N-terminal pro-sequences of approximately 100 amino acid residues prior to the N-terminus of the mature forms. Compared to the mature forms secreted from the cell, 80–90% of the primary translation product is in the intracellular compartment in the precursor form. However, the precursors are readily released from cells during infections but also with non-infectious conditions such a hypoxia and trauma. In this setting, the precursors act rapidly as “alarmins” in the absence of a processing mechanism to remove the pro-sequence and generate a mature form. In the case of IL-1α, the release of the precursor activates adjacent cells via receptor-mediated signaling. However, there are no data comparing the specific activity of the IL-1α precursor to the mature form. In the present study, we compared the precursor and mature forms of recombinant human IL-1α, IL-1β, and IL-33 proteins on the induction of cytokines from A549 cells as well as from human peripheral blood mononuclear cells (PBMC). Similar to the mature form, the IL-1α precursor was active in inducing IL-6 and TNFα, whereas the precursor forms of IL-1β and IL-33 were not active. On PBMC, precursor and mature IL-1α at 0.04 and 0.2 nM were equally active in inducing IL-6. Given the fact that during necrotic cell death, the IL-1α precursor is released intact and triggers IL-1 receptors on tissue macrophages, these data identify the precursor form of IL-1α as a key player in sterile inflammation.

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Background:The maturation process of IL-33 (IL-1F11) remains unclear. Results: IL-33 ligand affinity column isolates neutrophil proteinase 3. Conclusion: PR3 is an IL-33-processing enzyme. Significance: PR3 has a dual function in IL-33 biological activity.

The interleukin (IL)-1 family is the largest family of interleukins. Eleven members of the IL-1 family of ligands are intracellular molecules, except a single isoform of an IL-1 receptor antagonist (IL-1Ra; also known as IL-1RN), which contains a signal peptide at the N-terminus for effective secretion. The inflammasome is a complex of intracellular molecules that is responsible for the processing of IL-1β and IL-18, whereas the remaining IL-1 family members, including IL-1α, are processed in an inflammasome caspase-1-independent pathway. Among the eleven members of the IL-1 family ligands, precursor IL-1α, IL-1β, and IL-33 have comparatively long pro-peptides of approximately 110 amino acid residues at the N-terminus. However, the other IL-1 members, except for IL-37 (also known as IL-1F7), have relatively short propeptides with fewer than 40 amino acid residues at the N-terminus. Most cytokines, including interferons and interleukins, possess a hydrophobic signal sequence for secretion. Therefore, soluble cytokines readily act on cell surface receptors immediately after their release from cells. Unlike other cytokine families, IL-1 family ligands exhibit two-step regulation: transcriptional induction at the mRNA level and post-translational modification at the protein level because of the lack of a hydrophobic signal sequence at the N-terminus. Various processing enzymes involved in the activation of intracellular IL-1 family cytokines likely provide effective immune regulation to protect the host from infections. In this review, we describe all eleven IL-1 family ligand processing enzymes, mature ligand functions, and mode of receptor conformation.

α 1 -Antitrypsin (AAT) is a member of the serine proteinase inhibitor family that impedes the enzymatic activity of serine proteinases, including human neutrophil elastase, cathepsin G and neutrophil proteinase 3. Here, we expressed recombinant AAT by fusing the intact AAT gene to the constant region of IgG1 to generate soluble recombinant AAT-Fc protein. The recombinant AATFc protein was produced in Chinese hamster ovary (CHO) cells and purified using mini-protein A affinity chromatography. Recombinant AAT-Fc protein was tested for antiinflammatory function and AAT-Fc sufficiently suppressed tumor necrosis factor (TNF)-α-induced interleukin (IL)-6 in human peripheral blood mononuclear cells (PBMCs) and inhibited cytokine-induced TNFα by different cytokines in mouse macrophage Raw 264.7 cells. However, AAT-Fc failed to suppress lipopolysaccharide-induced cytokine production in both PBMCs and macrophages. In addition, our data showed that AAT-Fc blocks the development of hyperglycemia in a streptozotocin-induced mouse model of diabetes. Interestingly, we also found that plasma-derived AAT specifically inhibited the enzymatic activity of elastase but that AAT-Fc had no inhibitory effect on elastase activity.