Interleukin-18 (IL 18) is a member of the IL 1 family of cytokines. Increasing reports have expanded the role of IL 18 in mediating inflammation in animal models of disease using IL 18 deficient mice, neutralization of IL 18 or deficiency in the IL 18 receptor alpha chain. Similar to IL 1β, IL 18 is synthesized as an inactive precursor requiering processing by caspase 1 into an active cytokine but unlike IL 1β, the IL 18 precursor is constitutively present in nearly all cells in healthy humans and animals. The activity of IL 18 is balanced by the presence of a high-affinity naturally occuring IL 18 binding protein (IL 18BP). In humans, disease increased disease severity can be associated with an imbalance of IL 18 to IL 18BP such that the levels of free IL 18 are elevated in the circulation. A role for IL 18 has been implicated in several autoimmune diseases, myocardial function, emphysema, metabolic syndromes, psoriasis, inflammatory bowel disease, hemophagocytic syndromes, macrophage activation syndrome, sepsis and acute kidney injury, although in some diseases, IL 18 is protective. IL 18 plays a major role in the production of interferon-g from natural killer cells. The IL 18BP has been used safely in humans and clinical trials of IL 18BP as well as neutralizing anti-IL 18 antibodies are in clinical trials. This review updates the biology of IL 18 as well as its role in human disease

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.

After emigration from the bone marrow to the peripheral blood, monocytes enter tissues and differentiate into macrophages, the prototype scavenger of the immune system. By ingesting and killing microorganisms and removing cellular debris, macrophages also process antigens as a first step in mounting a specific immune response. IL-32 is a cytokine inducing proinflammatory cytokines and chemokines via p38-MAPK and NF-κB. In the present study, we demonstrate that IL-32 induces differentiation of human blood monocytes as well as THP-1 leukemic cells into macrophage-like cells with functional phagocytic activity for live bacteria. Muramyl dipepide (MDP), the ligand for the intracellular nuclear oligomerization domain (NOD) 2 receptor, has no effect on differentiation alone but augments the monocyte-to-macrophage differentiation by IL-32. Unexpectedly, IL-32 reversed GM-CSF/IL-4-induced dendritic cell differentiation to macrophage-like cells. Whereas the induction of TNFα, IL-1β, and IL-6 by IL-32 is mediated by p38-MAPK, IL-32-induced monocyte-to-macrophage differentiation is mediated through nonapoptotic, caspase-3-dependent mechanisms. Thus, IL-32 not only contributes to host responses through the induction of proinflammatory cytokines but also directly affects specific immunity by differentiating monocytes into macrophage-like cells.

Summary: Cytokines are crucial in host defence against pathogens such as bacteria, viruses, fungi and parasites. A newly described cytokine, interleukin‐32 (IL‐32), induces various proinflammatory cytokines (tumour necrosis factor‐α, IL‐1β, IL‐6) and chemokines in both human and mouse cells through the nuclear factor‐κB and p38 mitogen‐activated protein kinase inflammatory signal pathway. The IL‐32 primarily acts on monocytic cells rather than T cells. In an attempt to isolate the IL‐32 soluble receptor, we used an IL‐32 ligand‐affinity column to purify neutrophil proteinase 3, which is a serine proteinase involved in many inflammatory diseases. IL‐32 has biological activity associated with Mycobacterium tuberculosis and chronic proinflammatory diseases such as rheumatoid arthritis. IL‐32 is transcribed as six alternative splice variants and the biological activity of each individual isoform remains unknown. Here, we cloned the complementary DNA of the four IL‐32 isoforms (α, β, γ and δ) that are the most representative IL‐32 transcripts. To produce recombinant protein with a high yield, the amino acids of two cysteine residues were mutated to serine residues, because serine residues are not conserved among different species. The multi‐step purified recombinant IL‐32 isoform proteins were assessed for their biological activities with different cytokine assays. The γ isoform of IL‐32 was the most active, although all isoforms were biologically active. The present study will provide a specific target to neutralize endogenous IL‐32, which may contribute to basic and clinical immunology.

Inflammatory cytokines mediate inflammatory bowel diseases (IBDs) and cytokine blocking therapies often ameliorate the disease severity. IL-32 affects inflammation by increasing the production of IL-1, TNFα, and several chemokines. Here, we investigated the role of IL-32 in intestinal inflammation by generating a transgenic (TG) mouse expressing human IL-32γ (IL-32γ TG). Although IL-32γ TG mice are healthy, constitutive serum and colonic tissue levels of TNFα are elevated. Compared with wild-type (WT) mice, IL-32γ TG mice exhibited a modestly exacerbated acute inflammation early following the initiation of dextran sodium sulfate (DSS)-induced colitis. However, after 6 d, there was less colonic inflammation, reduced tissue loss, and improved survival rate compared with WT mice. Associated with attenuated tissue damage, colonic levels of TNFα and IL-6 were significantly reduced in the IL-32γ TG mice whereas IL-10 was elevated. Cultured colon explants from IL-32γ TG mice secreted higher levels of IL-10 compared with WT mice and lower levels of TNFα and IL-6. Constitutive levels of IL-32γ itself in colonic tissues were significantly lower following DSS colitis. Although the highest level of serum IL-32γ occurred on day 3 of colitis, IL-32 was below constitutive levels on day 9. The ability of IL-32γ to increase constitutive IL-10 likely reduces TNFα, IL-6, and IL-32 itself accounting for less inflammation. In humans with ulcerative colitis (UC), serum IL-32 is elevated and colonic biopsies contain IL-32 in inflamed tissues but not in uninvolved tissues. Thus IL-32γ emerges as an example of how innate inflammation worsens as well as protects intestinal integrity.

IL-33/IL-1F11 is a new member of the IL-1 family ligand and provokes T helper-type immune responses. IL-33 is the ligand of ST2 and IL-1 receptor accessory protein (IL-1RAcP) that triggers nuclear factor-κ light chain enhancer of activated B cells (NF-κB) and MAPK signaling. We discovered a novel short splice variant of IL-33 that was termed spIL-33. The new spIL-33 lacks exon 3 containing a proposed caspase-1 cleavage site. We isolated spIL-33 cDNA from the Huh7 human hepatocarcinoma cell line and expressed the recombinant spIL-33 protein in Escherichia coli. The recombinant spIL-33 and pro-IL-33 were not cleaved by caspase-1, unlike IL-18 (IL-1F4). The recombinant spIL-33 was constitutively active, and spIL-33-induced inflammatory cytokine production was caspase-1-independent in HMC-1 and Raw 264.7 cells. The recombinant spIL-33 induced the phosphorylation of IL-1 receptor-associated kinase (IRAK1), NF-κB, p38 MAPK, p44/42 MAPK, and JNK in a time- and dose-dependent manner. Anti-ST2 monoclonal antibody specifically blocked the spIL-33-induced cytokine production. In this study, we identified and characterized a new IL-33 splice variant, which was a constitutively active IL-33 isoform. The existence of constitutively active spIL-33 suggests that the biological activity of IL-33 could be triggered by diverse stimulations during immune responses. Further investigation of the spIL-33 expression pattern may contribute to understanding the involvement of IL-33 in inflammatory disorders.