CD14⁺ hla-DR^-/Lo Myeloid-Derived Suppressor Cells Express Immunosuppressive B7-H Family Members and Are Depleted Following Taxane-Based Chemotherapy in Melanoma.

Abstract 464

Myeloid-derived suppressor cells (MDSC) are important, albeit incompletely characterized, immune regulatory cells which suppress host anti-tumor immunity in malignancies like melanoma. We sought to address 3 outstanding questions in the present study: i) does tumor burden drive the expansion of MDSC; ii) do MDSC express immunosuppressive B7-H family members,;iii) are MDSC depleted following chemotherapy administration. As MDSC expand in response to tumor-derived factors, including those elevated in melanoma patients, we hypothesized that the frequency of MDSC present in the peripheral blood of melanoma patients may be dependent on tumor stage. Therefore, PBMC were obtained from both normal donors and melanoma patients with all stages of disease (stage I, n=10; stage II, n=10; stage III, n=10; stage IV, n=47). A discreet population of CD14⁺HLA-DR^-/lo MDSC was appreciated in samples obtained from melanoma patients, but was largely absent in normal donors. Approximately 0.5% of PBMC were CD14⁺HLA-DR^-/lo in normal donors. In contrast, the frequency of these cells progressively increased with tumor stage in melanoma. While a two-fold increase in CD14⁺HLA-DR^-/lo MDSC was observed in stage I patients, this difference did not reach statistical significance. In contrast, a significant increase in CD14⁺HLA-DR^-/lo MDSC was observed in patients with stage II-IV disease (2.2% Stage II, p<.01; 3.4% stage III, p<.001; 13.2% stage IV, p<.001). While the observed range (range: 0.8-66.3%) in frequency of CD14⁺HLA-DR^-/lo MDSC was broad in stage IV melanoma, these cells comprised more than 10% of total PBMC in the majority of patients. Various B7 homologues (e.g. B7-H1, B7-H2, B7-H3 and B7-H4) may inhibit T-cell activation, proliferation and cytokine production or promote the induction of T-cell apoptosis and unresponsiveness. Due to their immunosuppressive affects, and expression by tumor cells or immune regulatory cells (e.g. Treg, MDSC), these B7-H members are associated with adverse outcomes in multiple human malignancies. As the expression of these important immunoregulatory molecules on MDSC is unknown, we sought to determine whether CD14⁺HLA-DR^-/lo MDSC in melanoma may express B7-H family members. Therefore, PBMC from patients with metastatic melanoma were stained with either an isotype control or antibodies against B7-H1, B7-H2, B7-H3 or B7-H4. In all cases, B7-H family members were more highly expressed on CD14⁺HLA-DR^-/lo MDSC when compared with the expression observed on CD14⁺ cells from normal donors. While B7-H1 (p<.01), B7-H2 (p<.001) and B7-H4 (p<.01) were preferentially expressed on HLA-DR^-/lo cells in melanoma patients, B7-H3 was more highly expressed on both HLA-DR⁺ (p<.05) and HLA-DR^-/lo (p<.001) CD14⁺ cells. Serial blood samples, both at baseline and at least 12 weeks after therapy, were availabel for 10 patients with metastatic melanoma undergoing therapy wither either carboplatin and nanoparticle albumin bound paclitaxel (nab paclitaxel; n=6) or carboplatin, paclitaxel and bevacizumab (n=4). The frequency of both CD14⁺HLA-DR^-/lo MDSC and conventional CD14⁺HLA-DR⁺ monocytes was determined both before and after chemotherapy. An approximately 50% reduction in the frequency of CD14⁺HLA-DR^-/lo MDSC was observed following chemotherapy (p=.004), with no significant reduction in the frequency of conventional HLA-DR⁺ monocytes (p=.08). Collectively, the data presented implicates multiple B7-H family members in MDSC-mediated immune suppression and demonstrates that the expansion of B7-H⁺CD14⁺HLA-DR^-/lo MDSC in melanoma is directly associated with the tumor burden, but may be abrogated by the administration of taxane-based chemotherapy.