Role of caspase-1 in nuclear translocation of IL-37, release of the cytokine, and IL-37 inhibition of innate immune responses

Abstract

IL-37 is a fundamental inhibitor of innate immunity. Human IL-37 has a caspase-1 cleavage site and translocates to the nucleus upon LPS stimulation. Here, we investigated whether caspase-1 processing affects IL-37-mediated suppression of LPS-induced cytokines and the release from cells by analyzing a caspase-1 cleavage site mutant IL-37 (IL-37D20A). Nuclear translocation of IL-37D20A is significantly impaired compared with WT IL-37 in transfected cells. LPS-induced IL-6 was decreased in cells expressing WT IL-37 but not IL-37D20A. The function of IL-37 in transfected bone marrowderived macrophages is nucleotide-binding oligomerization domain-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome-dependent, because IL-37 transfection in apoptosis-associated speck-like protein containing a carboxyl-terminal caspase recruitment domain-and NLRP3-deficient cells does not reduce levels of IL-6 and IL-1β upon LPS stimulation. IL-37-expressing macrophages release both precursor and mature IL-37, but only the externalization of mature IL-37 was dependent on ATP. Precursor and mature IL-37 was also secreted from human dendritic cells and peripheral blood mononuclear cells. To determine whether IL-37 is active in the extracellular compartment, we pretreated IL-37 transgenic mice with IL-37-neutralizing antibodies before LPS challenge. In IL-37-expressing mice, neutralizing IL-37 antibodies reversed the suppression of LPS-induced serum IL-6. In contrast, the addition of neutralizing antibody did not reverse suppression of LPS-induced IL-6 in mouse macrophages transfected with IL-37. Although caspase-1 is required for nuclear translocation of intracellular IL-37 and for secretion of mature IL-37, the release of the IL-37 precursor is independent of caspase-1 activation. IL-37 now emerges as a dual-function cytokine with intra-and extracellular properties for suppressing innate inflammation. W ith the exception of the IL-1 receptor antagonist, members of the IL-1 family are first synthesized as precursor molecules containing a propeptide domain lacking a classical signal sequence (1). Caspase-1 has emerged as the main intracellular processing enzyme responsible for maturation of active IL-1β and IL-18, which are then released into the extracellular space, as shown for IL-1β and IL-18 (2, 3). The IL-1 family member IL-37 is also synthesized as a precursor and is processed to its mature form upon LPS treatment (4, 5). Caspase-1 seems to be the main enzyme responsible for the in vitro maturation of IL-37 in comparison to caspase-4 and granzyme B (4). A putative cleavage site for caspase-1 is located in exon 1 between residues D20 and E21 of IL-37 (4). HEK 293 or CHO cells transfected with the IL-37 precursor release IL-37 starting at amino acid V46, suggesting a second cleavage site in the sequence encoded by exon 2 (6). We previously demonstrated that processing of IL-37 is only partially inhibited by caspase-1 inhibitors, indicating that caspase-1 is not the only enzyme responsible for the processing of IL-37 (5)...

Extracellular forms of IL-37 inhibit innate inflammation in vitro and in vivo but require the IL-1 family decoy receptor IL-1R8

“…However, in the presence of a caspase-1 inhibitor, there is no translocation to the nucleus and no reduction in LPS-induced cytokines (7). Mutation of aspartic acid at the caspase-1 cleavage position 20 to alanine also results in failure to translocate to the nucleus and loss of the suppression of cytokine production (8). Thus, as with IL-1α and IL-33, IL-37 is the third member of the IL-1 family that translocates to the nucleus and affects cellular responses.…”

Interleukin-37 gene variants segregated anciently coexist during hominid evolution

“…Therefore, we propose that the biological functions of IL37 variants need to be verified and compared with earlier experimental results using NCBI reference protein. 4,8,22 Second, our data provides valuable guidance on picking SNPs for disease association studies. Previous reports suggested that IL37 was involved in the development of various inflammatory conditions, including ankylosing spondylitis and rheumatoid arthritis.…”

“…1,3 The IL37b is a precursor, which is thought to be processed by caspase-1 into the mature form and translocates actively into the cell nucleus. 4 IL37b mRNA has been found in multiple human tissues, including the lymph node, thymus, bone marrow, placenta, lung and testis. 5 IL37 protein level in PBMCs and dendritic cells is upregulated when stimulated by TLR ligands or proinflammatory cytokines.…”

Targeting cytokines in asthma therapy: could IL-37 be a solution?

“…These studies demonstrated that IL-37 is capable of inhibiting proinflammatory effects that are mediated through activation of receptors belonging to the interleukin-1 receptor/toll-like receptor (TIR) superfamily like TIRs 2 and 4 and the IL-1 receptor [30,31]. As shown in in vitro and in a wide range of animal models for chronic inflammatory diseases, this inhibition on the one hand results in further diminished production of proinflammatory cytokines such as IL-1α, IL-1β, IL-6, IL-17, IL-23, GM-CSF, and TNF-α and chemokines like CCL12, CXCL1, CXCL2, CXCL8, and CXCL13 [32][33][34]. On the other hand, IL-37 has been described to support IL-10 production, tolerogenic dendritic cell, and regulatory T cell activity [34,35].…”

“…On the other hand, IL-37 has been described to support IL-10 production, tolerogenic dendritic cell, and regulatory T cell activity [34,35]. So far, its mode of action includes interference with the inflammasome, an intracellular activity as well as extracellular binding to IL-18 receptor α and the regulatory orphan receptor SIGIRR/IL-1R8 [31,32,36,37]. All these observations suggested that IL-37 could be a potent regulator of local innate immunity.…”