Molecular analysis of melanoma-induced sentinel lymph node immune dysfunction

Introduction: Sentinel lymph nodes (SLNs) of melanoma patients show evidence of tumor-induced immune dysfunction. Our previous works have shown that IL-10 and IFNγ co-regulate indoleamine-2,3-dioxygenase (IDO)-expressing immunosuppressive dendritic cells (DCs) in melanoma SLNs. The goal of this study is to examine the relationship between melanoma SLN tumor burden and the degree of SLN immune dysfunction as a model to study tumor-induced immune dysfunction. We hypothesize that SLN tumor burden correlates with the degree of SLN immune dysfunction. Methods: Patients undergoing SLN biopsy for clinical stages I and II melanomas were enrolled in the study under an IRB-approved protocol. During the SLN biopsy, non-hot and non-blue portion of the SLN was harvested, flash-frozen in liquid nitrogen, and mRNA was extracted. By using quantitative real-time PCR, gene expressions of cytokines (IL-4, IL-10, IFNγ, TGFβ, GM-CSF) and the surrogates of immunosuppressive regulatory and effector cells (IDO-expressing DCs and Foxp3-expressing T-regs, respectively) were measured and correlated against the SLN tumor burden (MART1) and against each other. The data were log transformed for normalization. Statistical test used Student's t-test and stepwise multivariate regression analysis. Statistical significance was determined at P < 0.05. Results: SLNs of 74 patients were analyzed in this analysis. Ten of seventy-four patients (13.5%) had tumor-positive SLNs. MART1 gene expression showed a significant difference between the SLN (+) and SLN (-) groups (P = 0.04). Among the various cytokines, multivariate analysis showed that only IFNγ gene expression correlated independently with MART1 gene expression (P < 0.0001, r = 0.91). Similar multivariate analyses show that IFNγ (P < 0.0001, r = 0.78), IL-10 (P = 0.0037, r = 0.60), and TGFβ (P < 0.0001, r = 0.95) gene expressions correlated independently with IDO gene expression. IFNγ (P < 0.0001, r = 0.87) and GM-CSF (P = 0.042, r = 0.76) gene expressions correlated independently with Foxp3 gene expression. MART1 gene expression showed independent correlation with IDO (P = 0.0002, r = 0.75) and Foxp3 (P = 0.0002, r = 0.75) gene expressions. Conclusion: SLN tumor burden correlates with immunosuppressive IDO and Foxp3 expressions within the SLNs of melanoma patients. Our data are consistent with our theory that melanoma induces expressions of specific cytokines, which in turn, stimulate immune suppressors within the SLN. This study also supports our previous finding that IL-10 and IFNγ co-regulate IDO within the SLN. In our data, IFNγ is the sole cytokine that correlates with the SLN tumor burden and seems to play a central role in tumor-induced immunological changes in the SLN immune microenvironment.