Identification of a new subset of myeloid suppressor cells in peripheral blood of melanoma patients and modulation by GM-CSF-based anti-tumor vaccine

Abstract

21082 Background: Phenotypic and functional features of myeloid suppressor cells (MSC), known to serve as critical regulators of anti-tumor T cell responses in tumor-bearing mice, are still poorly defined in human cancers. Here we analyzed myeloid subsets with suppressive activity present in peripheral blood of metastatic melanoma patients (MM) and evaluated their modulation by a GM-CSF-based anti- tumor vaccine. Methods: Stage IV AJCC MM patients (n=16) vaccinated with autologous tumor-derived heat-shock protein peptide complexes gp96 (HSPPC-96) and low dose GM-CSF provided pre- and post-treatment whole blood samples. Peripheral blood mononuclear cells (PBMC) were analyzed by flow cytometry, separated into cellular subsets and used for in vitro proliferation assays. PBMC from stage- matched melanoma patients (n=12) treated with non-GM-CSF-based vaccines (i.e. HSPPC-96 alone or IFNa/melanoma-derived peptides) or gender and age-matched healthy donors (n=16) were also analyzed for comparison. Results: The lack or low levels of HLA-DR expression was found to identify a CD14+ cell subset with high suppressive activity on lymphocyte proliferation and functions. CD14+HLA-DR-/lo cells were significantly expanded in all MM patients, while undetectable in healthy donors. Suppressive activity was mediated by TGFβ, as suggested by functional experiments with neutralizing specific antibodies. In contrast, no involvement of arginase and iNOS pathways could be detected. CD14+HLA-DR-/lo cells, as well as spontaneous ex- vivo release and plasma levels of TGFβ, were augmented after administration of the HSPPC-96/GM-CSF vaccine. Interestingly, the expansion of suppressive CD14+ monocytes was associated to the inability to mount a significant CD8-mediated T cell response upon vaccination. On the other hand, no quantitative or qualitative enhancement of the CD14+HLA-DR-/lo suppressive cell population was observed in patients receiving a non-GM-CSF based vaccine. Conclusions: CD14+HLA-DR- /lo cells exerting TGFβ-mediated immune suppression may represent a new subset of myeloid suppressive cells, potentially expandable by the administration of GM-CSF-based vaccines in melanoma patients. [Table: see text]