A population of immunosuppressive CD14+ monocytes found in glioblastoma patients inhibit DC differentiation and T-cell function Free

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Myeloid suppressors of various phenotypes are more frequent in many tumor models and have been observed in some cancer patients. They are thought to play a role in tumor mediated immune evasion. One phenotype of myeloid suppressor (CD14⁺/HLA-DR^-) is associated with adverse response to sepsis and has been identified in patients with advanced melanoma. We have found elevated percentage of CD14⁺/HLA-DR^- cells in monocytes from glioblastoma (GBM) patients when compared to normal donors (p<0.0001). Isolated CD14⁺ cells from GBM patients were assessed for their ability to inhibit proliferation of autologous T cells stimulated with CD3/28 beads. We found that monocytes from GBM patients suppressed T cell proliferation by 60.0% at 1 monocyte to 10 T cells ratio compared to T cells alone (preliminary data, n = 2). The suppression of T cell proliferation was dependent on the number of monocytes, reaching 98.9% at 1:1 ratio. The observed suppression was up to 30 fold more than that of normal donors. To further characterize the role of these myeloid suppressors within the monocyte population, CD14⁺/HLA-DR^- cells were enriched from monocytes of normal donors by immunomagnetic beads. Flow cytometry confirmed that DR^--enriched monocytes were distinct from immunosuppressive monocytes previously described in literature (Lin-/CD33+, CD16+). Selected monocytes were combined with autologous peripheral blood mononuclear cells and stimulated with CD3/28 beads. DR^--enriched monocytes inhibited T cell proliferation compared to the DR^--reduced counterpart (67.5 ± 16.7% vs 81.2 ± 12.7% T cell proliferation; n = 6 from 3 donors, p = 0.002). Furthermore, DR^--enriched monocytes were a poor source of dendritic cell (DC) progenitors compared to culture from DR^--reduced monocytes. The purity (CD80+/CD83+) of the final preparation was inversely correlated with increasing percentage of DR^- cells in the initial monocyte population (n = 24 from 9 donors; p = 0.02). The reduction in DC purity was observed in a correlative reduction in the ability to stimulate T cell proliferation (85.5 ± 3.74% DR^--enriched vs 89.5 ± 3.69% DR^--depleted; n = 5; p = 0.03). This is the first study to characterize the functional consequence of CD14+/HLA-DR^- myeloid suppressors in GBM. Our findings show for the first time that these cells can affect both the adaptive immune response via inhibition of DC differentiation and the effector immune response by interfering with T cell proliferation. This work was supported through a Development Research grant from the Mayo Clinic Brain Tumor SPORE CA 108961-03.