Expression of CXCL9, -10, -11, and CXCR3 in the Tear Film and Ocular Surface of Patients with Dry Eye Syndrome

Dry eye syndrome is a multifactorial disease of the tears and ocular surface, resulting in symptoms of ocular discomfort, visual disturbance, and tear film instability, with potential damage to the ocular surface.1 It is caused by aqueous-deficient conditions, including Sjogren's syndrome, non-Sjogren's dry eye, and evaporative conditions. The pathogenesis of dry eye syndrome has not been clearly established. However, there is increasing evidence to suggest that it is associated with ocular surface inflammation and that the inflammatory component contributes to the dry eye symptoms as well as the disease process itself. Immunopathologic changes in the conjunctival epithelium of dry eyes include inflammatory cell infiltration; increased expression of immune activation and adhesion molecules, such as HLA-DR, intercellular adhesion molecule (ICAM)-1, and CD 40; elevated levels of inflammatory cytokines or chemokines, such as interleukin (IL)-1, -6, and -8 and tumor necrosis factor (TNF)-α; and increased expression of chemokine receptor CCR5.2–6 The ocular surface in dry eye syndrome also manifests increased concentration and activity of matrix metalloproteinases (MMPs), such as MMP-9, and increased apoptosis.7–9 Chemokines are low-molecular-mass (7- to 12-kDa) secreted proteins that play an important role in the recruitment of leukocytes to sites of inflammation.10,11 They are produced by a variety of cell types and are divided into four subgroups based on the position of the first two cysteines at the N-terminal region of the protein: CXC (α), CC (β), C (γ), and CXXXC (δ).12 CXC chemokines can be further divided into two groups. The first group, termed ELR+, contains a glutamic acid, leucine, and arginine tripeptide motif. The ELR motif has high affinity for CXCR1 and -2 chemokine receptors on neutrophils. The second group of CXC chemokines lacks the ELR motif and has a high affinity for the CXCR3 receptor found on activated T cells and NK cells.11–13 Members of the ELR– chemokine subset, which are readily induced by interferon (IFN)-γ, include monokine induced by interferon-γ (MIG/CXCL9), interferon-γ–inducible protein 10 kDa (IP-10/CXCL10), and interferon-inducible T-cell α chemoattractant (I-TAC/CXCL11). Chemokines are involved in specific attraction and expansion of T-cell subtypes based on their complement of chemokine receptors.14,15 Chemokine receptors CCR5 and CXCR3 are expressed primarily on T cells that mediate type 1 inflammatory responses, whereas CCR3 and -4 are expressed primarily on T cells that mediate type 2 responses.6,16–18 Although there is considerable evidence of the roles of chemokines and chemokine receptors in the pathogenesis of many acute and chronic inflammatory diseases, such as asthma and rheumatoid arthritis, very little is known about chemokines and their receptors in dry eye syndrome.19 Experimental studies have established that human corneal keratocytes and epithelial cells have the potential to produce ELR− chemokines, such as CXCL9, -10, and -11.20,21 In a mouse model of experimental dry eye, desiccating stress potently stimulated the expression of T-helper type 1 (Th-1) cell–attracting chemokines and their receptors on the ocular surface of C57BL/6 mice.22 In human studies, CCR5 expression has been shown to increase in the conjunctival epithelium of patients with dry eye syndrome.6,18 In the present study, we investigated the CXCL9, -10, and -11 levels in tears of patients with Sjogren's syndrome and those with non-Sjogren's dry eye, and compared the results with those of normal control subjects. We also analyzed the correlation between CXCL9, -10, and -11 levels and tear film and ocular surface parameters in patients with dry eye syndrome. The relative expressions of these chemokines and their receptor CXCR3 were evaluated in conjunctival biopsy specimens using immunohistochemistry and flow cytometry.