Novel IL-1 Family Members

Until recently, there were four members of the IL-1 family: IL-1 alpha, IL-1 beta, IL-1ra and IL-18. Six additional members of this family have since been described based on conservation of amino acid (aa) sequence, identity of gene structure, and three-dimensional structure. All of the new genes map to the region of chromosome 2 between the IL-1 beta and IL-1ra loci,¹ thus suggesting that each IL-1 family member arose from a common ancestral gene that later became duplicated. Each gene codes for a protein that contains a single structural domain formed from 12 beta strands connected by loop regions arranged in a beta-trefoil structure. Similar to IL-1 beta and IL-1ra, the new IL-1 family members differ most from each other within these loop regions. As with so many cytokines, the novel IL-1 family members have been described by several groups using their own nomenclature, thus resulting in a number of different names for the same molecule. Table 1 presents a simple nomenclature that has been proposed for the IL-1 family.² The numeric order reflects the date of publication for each gene. The biological activities of the new IL-1 ligands have not been clearly identified. Further studies need to be performed in order to identify whether they are agonists or antagonists, which receptors they bind, and if they possess any of the characteristic activities so far ascribed to IL-1 alpha and IL-1 beta.

IL-1F5

IL-1F5^3-8 contains 155 aa, lacks a signal sequence and glycosylation sites, and produces a 17 kDa band upon immunoprecipitation. IL-1F5 is highly expressed in keratinocytes, psoriatic skin, placenta, uterus, brain, thymus, heart, kidney, monocytes, B cells, and dendritic cells. In binding studies, IL-1F5 does not co-precipitate with IL-1R1, R3, R4, R5, R6, or R7 Fc fusions.⁵ The degree of sequence similarity between IL-1F5 and IL-1ra indicates that it may be a novel IL-1 receptor antagonist. Although the similarity with IL-1ra over the entire sequence is 52%, it is only 24% within the loop regions. Molecular modeling predicts that the loop structures (between strands 4 and 5 and between strands 7 and 8) are important determinants of the agonist properties of IL-1. Without functional studies and based on structure alone, however, it is very difficult to predict whether IL-1F5 has agonist or antagonist functions. Functional studies that have been performed have focused on the inflammatory properties of IL-1 by using T cells, fibroblasts, and endothelial cells to determine if IL-1F5 can mimic or antagonize IL-1 activity. So far, none have provided a definitive answer. Either IL-1F5 is an antagonist for an as yet untested agonist, or the IL-1F5 receptor(s) are not present on the cell types tested to date.

IL-1F6

This new member of the IL-1 family has been described by only one group.⁵ It is expressed in spleen, lymph node, tonsil, bone marrow, monocytes, B cells, and T cells. IL-1F6 is unique, as it is the only new IL-1 family member synthesized by T cells. In binding studies, IL-1F6 does not co-precipitate with IL-1R1, R3, R4, R5, R6, or R7 Fc fusions.⁵ IL-1F6 is most closely related to IL-1F7 and IL-1F8.⁵

IL-1F7

IL-1F7 is expressed in most tissues, with relatively high levels in testis, thymus, and uterus. IL-1F7 is one of the first examples of a mature cytokine of the IL-1 family to form homodimers.⁹ It is also the only novel IL-1 family member to possess a pro domain⁷ (similar to IL-18, IL-1 alpha, and IL-1 beta), yet it has more aa identity with IL-1ra than any of the agonists. Both IL-1 beta and IL-18 are cleaved by caspase-1 to generate the mature agonist form. Caspase-1 and, to a lesser extent, caspase-4 cleave pro-IL-1F7.⁹ It is difficult to speculate whether this molecule is an agonist or antagonist, however, without functional data. Pan et al.¹⁰ have shown that IL-1F7 binds IL-18 R alpha (IL-1R5), although this binding did not elicit a functional response. Additionally, Kumar et al.⁹ have demonstrated both pro and mature forms of IL-1F7 bind IL-18 R alpha, but not IL-1R1 or IL-1R5. Unlike IL-18, IL-1F7 does not induce IFN-gamma secretion in KG-1 cells and neither the pro or mature form of IL-1F7 binds IL-18 BP.⁹

IL-1F8

IL-1F8 has been cloned by two groups.^5,7 Like IL-1 and IL-18, the IL-1F8 protein lacks a hydrophobic leader sequence, yet unlike these molecules, it also lacks a pro domain.⁵ It is expressed in tonsil, bone marrow, heart, placenta, lung, testis, colon, monocytes, and B cells. IL-1F8 is most closely related to IL-1F6⁵ based on aa sequence alignment. A receptor for IL-1F8 has not been found to date.

IL-1F9

Keratinocytes are the main producers of IL-1F9. Production is greatly increased by stimulation with IL-1 beta and TNF-alpha, but not IL-4 or IFN-gamma.⁶ In contrast, Kumar et al.⁷ demonstrated an up-regulation of keratinocyte IL-1F9 by IFN-gamma.⁷ IL-1F9 is also up-regulated in models of contact hypersensitivity infection with HSV-1⁷ and in psoriasis,⁶ suggesting that IL-1F9 plays a role in immunity and inflammation of the skin. IL-1F9 does not bind IL-1R1, IL-1R4, or IL-1R5, but induces NF-kappa B activation in IL-1R6 transfected cells.⁶ The addition of IL-1F5 to this reaction inhibits the effect, suggesting IL-F9 and IL-1F5 are the agonist and antagonist (respectively) for IL-1R6. IL-1F9 does not contain a hydrophobic leader sequence or a pro form, yet it is secreted as a 20 kDa molecule.

IL-1F10

Although IL-1F10 binds to soluble IL-1R1 (albeit with a lower affinity than IL-1 beta or IL-1ra), the functional significance of this is unclear. Accessory proteins may affect the binding affinity of IL-1F10 to sIL-1R1. Similar to other novel IL-1 family members, IL-1F10 is expressed in skin, but it is also expressed on activated B cells in tonsil. The DNA sequence demonstrates the conserved intron placement of the IL-1 family genes and corresponds to a 152 aa protein with a predicted molecular mass of 17 kDa.¹¹

References

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5. Smith, D.E. et al. (2000) J. Biol. Chem. 275:1169.
6. Debets, R. et al. (2001) J. Immunol. 167:1440.
7. Kumar, S. et al. (2000) J. Biol. Chem. 275:10308.
8. Busfield, S.J. et al. (2000) Genomics 66:213.
9. Kumar, S. et al. (2002) Cytokine 18:61.
10. Pan, G. et al. (2001) Cytokine 13:1.
11. Lin, H. et al. (2001) J. Biol. Chem. 276:20597.