Human Tumor-Derived Exosomes Selectively Impair Lymphocyte Responses to Interleukin-2

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Abstract
Exosomes are nanometer-sized vesicles, secreted by normal and neoplastic cells. The outcome following interaction between the cellular immune system and cancer-derived exosomes is not well understood. Interleukin-2 (IL-2) is a key factor supporting expansion and differentiation of CTL and natural killer (NK) cells but can also support regulatory T cells and their suppressive functions. Our study examined whether tumor-derived exosomes could modify lymphocyte IL-2 responses. Proliferation of healthy donor peripheral blood lymphocytes in response to IL-2 was inhibited by tumor exosomes. In unfractionated lymphocytes, this effect was seen in all cell subsets. Separating CD4+ T cells, CD8+ T cells, and NK cells revealed that CD8+ T-cell proliferation was not inhibited in the absence of CD4+ T cells and that NK cell proliferation was only slightly impaired. Other exosome effects included selective impairment of IL-2–mediated CD25 up-regulation, affecting all but the CD3+CD8− T-cell subset. IL-2–induced Foxp3 expression by CD4+CD25+ cells was not inhibited by tumor exosomes, and the suppressive function of CD4+CD25+ T cells was enhanced by exosomes. In contrast, exosomes directly inhibited NK cell killing function in a T-cell–independent manner. Analysis of tumor exosomes revealed membrane-associated transforming growth factor β1 (TGFβ1), which contributed to the antiproliferative effects, shown by using neutralizing TGFβ1-specific antibody. The data show an exosome-mediated mechanism of skewing IL-2 responsiveness in favor of regulatory T cells and away from cytotoxic cells. This coordinated “double hit” to cellular immunity strongly implicates the role of exosomes in tumor immune evasion. [Cancer Res 2007;67(15):7458–66]