Dinarello C A
University of Colorado Health Sciences Center, Division Infectious Diseases, B168, 4200 East Ninth Avenue, Denver, CO 80262, USA.
Ann Rheum Dis. 2000 Nov;59 Suppl 1(Suppl 1):i17-20. doi: 10.1136/ard.59.suppl_1.i17.
A novel, constitutively expressed and secreted interleukin 18 (IL18) binding protein (IL18BP) neutralises IL18. IL18BP shares many characteristics with soluble cytokine receptors of the IL1 family in that the protein exhibits specificity for IL18, belongs to the immunoglobulin-like class of receptors and has limited amino acid sequences with those of the IL1 receptor type II. However, unlike soluble cytokine receptors, IL18BP does not have a transmembrane domain and hence is not anchored to the cell membrane. IL18BP is a secreted protein and not cleaved from the cell surface. IL18BP is naturally occurring and was isolated from the urine of healthy subjects. Because IL18 is an important inducer of interferon gamma (IFNgamma), IL18BP suppresses the production of IFNgamma resulting in reduced T-helper type 1 immune responses. There are four human and two mouse isoforms-resulting from mRNA splicing and found in various cDNA libraries. Each of these IL18BP isoforms have been expressed, purified and assessed for binding and neutralisation of IL18 biological activities. Two human IL18BP isoforms exhibited the greatest affinity for IL18 with a rapid on-rate, a slow off-rate and a dissociation constant (kDa) of 399 pM. The two other isoforms with an incomplete immunoglobulin domain were unable to neutralise IL18. The two human isoforms that possess a complete immunoglobulin domain, neutralise >95% IL18 at a molar excess of two. Molecular modelling identified a large mixed electrostatic and hydrophobic binding site in the immunoglobulin domain of IL18BP, which could account for its high affinity binding to the ligand. These high affinity forms may be ideally suited for blocking IL18 in human disease. It is likely that preferential secretion of high affinity functional and non-functional isoforms of IL18BP affect the immune response and the outcome of disease.
一种新的、组成性表达和分泌的白细胞介素18（IL18）结合蛋白（IL18BP）可中和IL18。IL18BP与IL1家族的可溶性细胞因子受体有许多共同特征，即该蛋白对IL18具有特异性，属于免疫球蛋白样受体类别，并且与II型IL1受体的氨基酸序列有限。然而，与可溶性细胞因子受体不同，IL18BP没有跨膜结构域，因此不锚定在细胞膜上。IL18BP是一种分泌蛋白，不是从细胞表面裂解而来。IL18BP是天然存在的，从健康受试者的尿液中分离得到。由于IL18是γ干扰素（IFNγ）的重要诱导剂，IL18BP抑制IFNγ的产生，从而导致1型辅助性T细胞免疫反应降低。通过mRNA剪接产生了四种人类和两种小鼠异构体，并在各种cDNA文库中发现。这些IL18BP异构体中的每一种都已被表达、纯化，并对其结合和中和IL18生物学活性进行了评估。两种人类IL18BP异构体对IL18表现出最大的亲和力，具有快速的结合速率、缓慢的解离速率和399 pM的解离常数（kDa）。另外两种具有不完整免疫球蛋白结构域的异构体无法中和IL18。具有完整免疫球蛋白结构域的两种人类异构体在摩尔过量为两倍时可中和>95%的IL18。分子建模在IL18BP的免疫球蛋白结构域中确定了一个大的混合静电和疏水结合位点，这可以解释其与配体的高亲和力结合。这些高亲和力形式可能非常适合在人类疾病中阻断IL18。IL18BP高亲和力功能性和非功能性异构体的优先分泌可能会影响免疫反应和疾病的结果。
