PURPOSE
Galectin-9, a β-galactoside binding protein, is defined as a negative regulator of T helper 1 (Th1) immune responses, favoring Th2 bias. Systemic immunity in patients with metastatic melanoma is predominately Th2 biased. We hypothesized galectin-9 could modulate systemic immunity toward Th2 polarization in patients with advanced melanoma.
EXPERIMENTAL DESIGN
Presence/concentration of galectin-9 was assessed in tumors and plasma, respectively, in patients with metastatic melanoma. Immunomodulatory function of galectin-9 was determined by exposing human peripheral blood mononuclear cells (PBMC) to galectin-9 in vitro.
RESULTS
Galectin-9 was expressed in 57% of tumors and was significantly (3.6-fold) increased in the plasma of patients with advanced melanoma compared to healthy controls (p<0.001). High plasma galectin-9 concentration was associated with systemic Th2 polarization and reduced two-year survival compared to low/no galectin-9 expression. In vitro, galectin-9 reduced proliferation of healthy PBMCs, promoted Th1 cell apoptosis, and encouraged Th2 biased cell phenotypes and cytokine secretion. Galectin-9 also stimulated monocyte differentiation towards an M2 macrophage phenotype, as assessed by chemokine/cytokine secretion and CD206 expression, observed both in vitro as well as in patients with metastatic melanoma.
CONCLUSION
Elevated galectin-9 in patient plasma correlates with Th2 systemic bias and less favorable clinical outcomes in metastatic melanoma. This Th2 bias appears to be not only a feature of the known mechanisms of Th1 apoptosis via Tim3 binding, but also mediated via myeloid cell differentiation towards a M2 phenotype, potentially favoring tumor progression. These data support galectin-9 as a novel therapeutic target for patients with metastatic melanoma.