TARC: novel mediator of allergic inflammation

Chemokines are a large family of 8–14 KDa heparin-binding peptides, which have been subdivided into four subfamilies on the basis of the position of either one or two cysteine residues located near the amino terminus of the protein (CXCL, CCL, CL and CX3CL) [1, 2]. Imai et al. in 1996 first described the isolation and molecular characterization of a new CC chemokine by means of a signal sequence trap method and a cDNA expression library from phytohemagglutinin (PHA)-stimulated peripheral blood mononuclear cells (PBMC) [3]. The gene was designated TARC, from thymus and activation regulated chemokine (CCL17). It is located on chromosome region 16q13 [4, 5], and it is 2176 base pairs in length, coding a highly basic preprotein of 94 amino acid residues with a cleavage site between Ala 23 and Ala 24. After post-translational processing the mature protein is a 71 amino acid residues polypeptide [3]. TARC is constitutively present in thymus and upon activation is produced by a number of cellular sources, including PBMCs, monocytes, macrophages, thymic cells, dendritic cells, endothelial cells and human bronchial epithelial cells [3, 6-9]. For example, TARC mRNA expression is undetectable in fresh PBMC. However, a maximum mRNA expression of TARC is detected 24 h after stimulation of PBMC with PHA. TARC gene expression appears to be controlled at both transcriptional and post-transcriptional levels. TARC induces chemotaxis in certain human T cell lines, but it is not chemotactic for either monocytes or granulocytes [3, 10]. In vitro studies have shown that TARC induces selective migration of lymphocytes, especially of the Th2 phenotype [6, 10-12]. In addition, TARC induces integrin-dependent adhesion to the intercellular adhesion molecule 1 (ICAM-1) of skin memory T cells subset causing a rapid arrest of such cells under physiological flow conditions [8]. TARC has also been found to activate platelets, the effects of TARC on these cells include shape change, adhesion to collagen or fibrinogen, aggregation, and calcium influx [13, 14]. Chemokines activate leucocytes through seven transmembrane G protein coupled receptors. The chemokine receptors have been classified in subfamilies according to whether they are activated by the different chemokine subfamilies (XCR, CXCR, CCR and CX3CR) [1]. CCR4 is the specific high-affinity functional receptor for TARC [10]. It was proposed that in addition to CCR4, TARC utilizes CCR8 receptor to induce chemotaxis [15]. However, Garlisi et al. have subsequently shown that TARC is not a ligand for CCR8. They demonstrated that TARC did not bind to/or induce chemotaxis through CCR8 on a stable transfected cell line (1D-21) and did not bind to CCR8 on in vitro differentiated human CD4+ Th2 cell [16]. CCR4 is also expressed in skin-homing cutaneous lymphocyte-associated antigen (CLA+) T cells, NK cells, thymocytes and platelets [8, 14, 17]. It has been proposed that polarization of T cells into Th1 and Th2 is characterized by differences in chemokine receptor expression [11, 12, 18]. Th1 cells express CXCR3 and CCR5, while the Th2 phenotypes express CCR3, CCR4 and CCR8 [6, 12, 19]. In contrast, CD4+ T cells (ThIL−10) express CXCR3, CCR5, CCR3, CCR4 and CCR8 [20]. In vivo, it has been shown that repeated antigen stimulation of mice transferred with effector Th cells show a progressive increase in CCR4+ cells which then results in long-term recruitment of Th2 cells [21]. Asthma is a chronic inflammatory lung disease characterized by infiltration of both eosinophils and Th2 lymphocytes. Moreover, production of Th2 cytokines, such as IL-4 and IL-5, correlates with disease severity and infiltration of these cells in allergic asthmatics [22] Evidence for an in vivo role of TARC in allergic inflammation derives from a study showing that pretreatment of sensitized mice with an anti-TARC antibody not only prevents airway eosinophilia induced by ovalbumin challenge, but also the degree of airway eosinophilia with a concomitant decrease of both infiltrating CD4+ lymphocytes and levels of Th2-type cytokines including IL-4 and IL-13 [23]. In a separate study however, it was shown that deletion of CCR4 has no effect on a Th2-dependent model of allergic airway inflammation [24]. Discrepancies between these two studies may be explained by the fact that mature knockout mice may have different pathophysiology from wild-type mice. The initial observation for the participation of TARC in humans was made by Sekiya et al. [9]. Using immunohistochemistry this group showed that the bronchial epithelium of asthmatics expressed higher TARC immunoreactivity compared with normal subjects. Moreover, this study demonstrated that IL-4 stimulated the bronchial cell line A549 to express TARC mRNA and a combination of TNF-α plus IL-4 further enhanced the ability of the A549 cells to produce TARC. In contrast, IFN-γ but not IL-4 stimulated the bronchial epithelial cell line BEAS-2B to express TARC mRNA. More recently, it has been shown that asthmatic patients exposed to a relevant allergen release large quantities of TARC in bronchoalveolar lavage (BAL) fluid [25]. In this study, it was found that IFN-γ, but not IL-4 or IL-13, stimulated human bronchial airway epithelial cells to express TARC. TNF-α enhanced the ability of IFN-γ to induce TARC. Allergic rhinitis is also characterized by infiltration of eosinophils, mast cells and Th2 lymphocytes into the nasal mucosa. In this issue of Clinical and Experimental Allergy Terada et al. show that nasal epithelial cells (NAECs) derived from allergic individuals release higher concentrations of TARC than those derived from normal subjects [26]. By immunohistochemistry applied to nasal biopsies, the authors demonstrate that TARC immunoreactivity is mainly localized to the nasal airway epithelium, and to a lesser extent to mononuclear cells in the nasal submucosa. In vitro stimulation of NAECs with the combination of TNF-α with either IL-4 or IL-13 induces both TARC mRNA and protein production. In contrast, nasal microvascular endothelial and nasal fibroblasts produce minimal concentrations of TARC. These findings suggest that TARC may play a major role in allergic rhinitis. TARC has also been implicated in atopic dermatitis (AD). Indeed, elevated production of TARC has been documented in skin lesions in a mouse model of AD associated with infiltration of IL-4 producing cells [27]. By applying immunohistochemistry to skin biopsies from AD patients, two separate reports have demonstrated increased TARC immunoreactivity predominantly localized to keratinocytes and to a lesser extent to endothelial cells and dermal infiltrating cells including CD3 lymphocytes and CD1a dendritic cells [28, 29]. In one study, elevated levels of TARC were found in the serum of AD patients compared with both patients suffering from psoriasis and also normal controls [28] and levels of serum TARC correlated with the severity of AD as assessed by SCORAD (SCORing atopic dermatitis system) as well as with the blood eosinophil numbers and serum E-selectin. Similarly the CD4+CD45RO+ cells expressing CCR4 in PBMCs of subjects with AD were higher than in healthy control subjects [28]. Consistent with these observations it was found that the proportion of CLA+CCR4+ lymphocytes is up-regulated in PBMCs derived from AD patients [29]. Interestingly, treated patients with local corticosteroids and oral antihistaminics show decreased levels of serum TARC; this was associated with reduction on the SCORAD score, decreased in blood eosinophil numbers as well as reduction of soluble E-selectin. Allergic contact dermatitis (ACD) results from a T cell response to harmless, low molecular weight chemicals (haptens) applied to skin. In the sensitization phase, haptens penetrating the skin are collected by resident dendritic cells that migrate to the regional lymph nodes to activate and clonally expand specific T cells precursors [30]. Re-exposure to the relevant hapten initiates the efferent phase and clinical expression of ACD, characterized by the rapid recruitment and activation of specific T cells at the sites of hapten challenge. T cells involved in ACD are extremely heterogeneous in their cytokine profile and function, with CD4+ and CD8+ T cells playing distinct roles in the expression of the disease. The recruitment of T cells in the skin is regulated by the expression of proper homing receptors, such as the CLA, which mediates rolling of T cells over activated endothelial cells expressing E-selectin. Recently, it has been shown that skin-seeking CLA+ T cells coexpress CCR4 [8]. CCR4 triggered by TARC exposed on the endothelial cell surface during inflammatory skin disorders is thought to augment integrin-dependent firm adhesion of T cells to endothelial ICAM-1. Nickel is frequently caused of ACD. Nickel-reactive CD4+ cells that produce high levels of IL-10, low levels of IFN-γ and no IL-4 have been isolated from the blood and skin of allergic individuals [8]. These nickel CD4 ThIL−10 cells express a broad range of chemokine receptors including CCR4 and migrate in response to TARC [20]. Analysis of skin biopsies obtained 12–72 h following epicutaneous application of nickel showed increased mRNA expression for TARC; this was associated with the infiltration of T cells into the epidermal compartment [31]. Over the last three years evidence has accumulated to suggest that TARC may play a major role in allergic inflammation. The use of a monoclonal antibody to neutralize TARC in a mice model of asthma has shown an important contribution for this cytokine in inducing the infiltration of both CD4+ lymphocytes and eosinophils in response to allergen challenge. This observation is further supported by the finding of increased TARC into the airways of asthmatic patients. Interestingly, the airway epithelium appears to be the major cellular source of this chemokine in asthmatic airways. Moreover, TARC is implicated in other allergic diseases including allergic rhinitis, AD and ACD. High circulating levels of TARC have been reported in the serum of patients with chronic hypereosinophilia [32]. The demonstration that CCR4, the major chemokine receptor in the CD4+ lymphocyte, serves as a receptor for TARC makes this receptor a target for therapeutic intervention. Indeed, the study conducted by Kawasaki et al. shows that preadministration of CCR4 neutralizing antibody to mice prior to antigen challenge not only prevents eosinophil recruitment into the airways after allergen challenge but also decreases the number of CD4+ lymphocytes and prevents airway hyper-reactivity [23]. Taking these observations all together suggest that the use of small-CCR4 antagonists with clinical efficacy may have a substantial impact in treating allergic disease.