Identification and initial characterization of four novel members of the interleukin-1 family

Interleukin-1 (IL-1), fibroblast growth factors (FGFs), and their homologue s are secreted factors that share a common p-barrel structure and act on ta rget cells by binding to cell surface receptors with immunoglobulinlike fol ds in their extracellular domain. While numerous members of the FGF family have been discovered, the IL-1 family has remained small and outnumbered by IL-1 receptor homologues, From expressed sequence tag data base searches, we have now identified four additional IL-1 homologues, IL-1H1, IL-1H2, IL- 1H3, and IL-1H4. Like most other IL-1/FGFs, these proteins do not contain a hydrophobic leader sequence. IL-1H4 has a propeptide sequence, while IL-1H 1, IL-1H2, and IL-1H3 encode only the mature protein. Circular dichroism sp ectra and thermal stability analysis suggest that 1H1 folds similarly to IL -1ra. The novel homologues are not widely expressed in mammals. IL-1H1 is c onstitutively expressed only in placenta and the squamous epithelium of the esophagus, However, IL-1H1 could be induced in vitro in keratinocytes by i nterferon-gamma and tumor necrosis factor-alpha and in vivo via a contact h ypersensitivity reaction or herpes simplex virus infection. This suggests t hat IL-1H1 may be involved in pathogenesis of immune mediated disease proce sses. The addition of four novel IL-1 homologues suggests that the IL-1 fam ily is significantly larger than previously thought.