ICOS ligand expression is essential for allergic airway hyperresponsiveness (1)
In order to be activated, T cells need to receive three signals: TCR engagement, co-stimulatory molecule stimulation and cytokine stimulation. Inducible co-stimulator (ICOS), a relatively newly defined member of the CD28 family, is constitutively expressed in low levels by naive Th cells but is significantly increased after T-cell activation. Inducible costimulator ligand (ICOSL) also named B7RP-1, is a member of the B7 family of co-stimulatory ligands and shares nearly 20% sequence homology with B7.1 and B7.2. ICOSL is constitutively expressed in B-cell areas of secondary lymphoid organs, on most of APCs, including dendritic cells (DCs), B cells and monocyte/macrophages. IL-17 has an inhibitory effect on established ovalbumin (OVA)-induced allergic hyperreactivity., Mucosal IL-17 administration inhibited the asthma- like reaction by reducing the pulmonary production of IL-5 and the chemokines thymus and activation-regulated che- mokine (TARC) and eotaxin, which mediate eosinophilic inflammation in asthma. Blockade of ICOS–ICOSL interaction in animal models of experimental allergic encephalomyelitis and of cardiac allograft rejection results in altered local inflammation.
Female C57BL/6 Wt mice (7–10 weeks old) were intra-peritoneally immunized with OVA (2 ug) in 0.5ml alum adjuvant. After 2 weeks, mice were intra-nasally re-challenged with OVA (50 ug in 40 ul PBS). One week after re-challenge, spleens were collected.
The OVA exposure protocol induced marked airway hyperresponsiveness in C57BL/6 mice. Naive ICOSL KO mice exhibited responsiveness to MCh that mimicked naive C57BL/6 animals, OVA sensitization and challenge had no impact on MCh responses in ICOSL mice.
The cellular pattern in each mouse strain was dramatically different. BAL infiltrates in Wt mice were composed mainly of eosinophils, while ICOSL KO mice showed more neutrophils and mononuclear cells.
H&E staining also confirmed that there was significantly much more eosinophilic infiltration into the lung tissues of Wt mice compared with KO mice. HMI, a quantitation of mucus producing airway epithelial cell revealed by PAS staining, was significantly higher in Wt mice than that of ICOSL KO mice.
Although both groups exhibited an increase in OVA-specific antibody responses following OVA treatment, the titers for specific IgG1, IgG2a and IgE in ICOSL KO mice were significantly lower than those of Wt controls.
Th1 cytokine (IFN-g) production by splenocytes from OVA-treated ICOSL KO mice was significantly higher than those of Wt controls either with or without OVA re-stimulation in vitro. In contrast, the production of Th2 cytokines (IL-4, IL-5, IL-10 and IL-13) in ICOSL KO mice was significantly lower than those of Wt controls.
The KO mice showed significantly lower Th2 cytokine levels but higher Th1 (IFN-g) level in the BAL fluids than Wt mice. ICOSL KO mice produced significantly higher TGF-b1 and IL-6 than those of Wt mice in both splenic cell culture and BAL fluid. In addition, IL-12 p40, the a-chain of IL-23, was also significantly higher in the lung of KO mice.
Wt mice showed 2.41 times (10.99% versus 4.55%) higher IL-4 production but 4.13 times (2.61% versus 11.07%) lower IFN-g production by CD4+ T cells than those of ICOSL KO mice. The KO mice also showed 4.06 times higher (10.5% versus 2.5%) intracellular IL-17F production than their Wt counterparts.
Upon both antigen-specific and polyclonal T-cell stimulation (anti-CD3e antibody), splenic cells from ICOSL KO mice showed significantly lower proliferation capability (i.e. less MTT uptake). The production of IL-2, an important cytokine for T-cell proliferation, was not significantly different between the KO and Wt mice.
As a newly defined co-stimulatory molecule, ICOS is expressed on the surface of T cells, especially CD4+ T cells and is elevated during the late phases of the immune response. Due to the nature of the model (allergic Th2 response), most of the proliferating cells in Wt mice are Th2 cells, thus a small proportion of proliferating Th1 and Th17 cells having no significant impact in total proliferation levels observed in the assay. Moreover, since Th2 cells express higher ICOS than Th1 cell, it is logical to assume that ICOS–ICOSL interaction is particularly important for Th2 cell proliferation.