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Background: Cytokines and chemokines are known to be involved in tumor growth and progression of disease. Sialyl LewisX (sLeX), a ligand for adhesion molecule E-selectin, is known to affect inflammatory processes and an elevated level is associated with tumor metastasis. Therefore, we assessed serum levels of sLeX and cytokines/chemokines in patients with non-invasive ductal carcinoma in situ (DCIS), early invasive breast cancer (EBC), or metastatic breast cancer (MBC). Patients and Methods: Sera from 250 patients (26 DCIS, 157 EBC, 67 MBC) and 43 healthy donors (HD) were assayed for sLeX using an immunoassay kit (CSLEX; Nittobo Medical Co. Ltd., Japan) and a panel of cytokines and chemokines using a multiplex assay kit. Differences in serum markers between patients and HD, and among patient groups were determined using the Kruskal-Wallis and Mann-Whitney tests. Spearman's correlation determined the non-parametric correlation between the serum levels of sLeX and the inflammatory mediators. The receiver operating characteristic (ROC) curves and the corresponding area under the curve (AUC) analyses were used to determine the sensitivity and specificity of a given cut-off value for a particular serum marker. Results: The median sLeX level tended to increase with the stage of disease: MBC > EBC > DCIS albeit without significant differences among the disease stages. Among MBC patients, patients with sLeX below 1.75 U/mL had significantly improved overall survival (OS, mean survival 11.1 vs. 33.7 months, P = 0.002) and progression-free survival (PFS, mean survival 9.7 vs. 20.9 months, p = 0.042). The Hazard Ratio of high sLeX for OS was 5.5 (95% CI 1.6 to 18.9, p = 0.007) and 2.3 for PFS (95% CI 1.0 to 5.2, P = 0.048). EBC and MBC patients have significantly higher serum levels of IL-1, IL-1RA, IL-6, IL-8, MCP-1, MCP-3, and MIP-1βthan those of hD. In addition, there were positive correlations between the serum levels of sLeX and cytokines IL-1β, IL-1RA, IL-2, IL-8, MIP-1β, and MCP-3. The AUC for sLeX was 0.598 (P = 0.016), and a cut-off of 3.13 pg/mL distinguished hormone receptor (HR)-positive from HR-negative patients ({chi} 2 = 4.0, P = 0.045). Likewise, the AUC for TNF-α was 0.620 (P = 0.003), and a cut-off 7.18 pg/mL distinguished HR-positive from HR-negative patients ({chi} 2 = 12.6, P < 0.001). Using a cut-off value established by ROC curves, few MBC patients (9 of 66, 13.6%) had a serum IL-2 level > 7.1 pg/mL compared to 57 of 185 (30.8%) non-MBC patients (χ 2 = 7.4, P = 0.007), suggesting that metastatic disease may be associated with immune suppression related to low serum IL-2. Conversely, 31 of 66 (47%) MBC patients had a serum MCP-1 level > 750 pg/mL vs. 37 of 185 non-MBC patients (20%) ({chi} 2 = 23.8, P < 0.0001), suggesting that a high level of MCP-1 may play an important role in metastasis. Conclusion: Serum levels of sLeX were able to distinguish HR-positive from HR-negative patients and predict overall survival in metastatic patients. Serum sLeX and some inflammatory mediators tended to increase with the severity of disease, and together may facilitate local invasion of tumor cells. Furthermore, serum levels of MCP-1 and IL-2 may have prognostic value in breast cancer patients.χ [ABSTRACT FROM AUTHOR]