Cancer Vaccines in the World of Immune Suppressive Monocytes (CD14(+)HLA-DR(lo/neg) Cells): The Gateway to Improved Responses

Abstract

Dendritic cells are an important target in cancer immunotherapy based on their critical role in antigen presentation and response to tumor development. The capacity of dendritic cells to stimulate anti-tumor immunity has led investigators to use these cells to mediate anti-tumor responses in a number of clinical trials. However, these trials have had mixed results. The typical method for generation of ex vivo dendritic cells starts with the purification of CD14(+) cells. Our studies identified a deficiency in the ability to generate mature dendritic cell using CD14(+) cells from cancer patients that corresponded with an increased population of monocytes with altered surface marker expression (CD14(+)HLA-DR(lo/neg)). Further studies identified systemic immune suppression and increased concentrations of CD14(+)HLA-DR(lo/neg) monocytes capable of inhibiting T-cell proliferation and DC maturation. Together, these findings strongly suggest that protocols aimed at immune stimulation via monocytes/dendritic cells, if optimized on normal monocytes or in systems without these suppressive monocytes, are unlikely to engender effective DC maturation in vitro or efficiently trigger DC maturation in vivo. This highlights the importance of developing optimal protocols for stimulating DCs in the context of significantly altered monocyte phenotypes often seen in cancer patients.

Keywords: CD14+HLA-DRlo/neg; MDSC; dendritic cells; immunotherapy; monocytes.

Figure 1

Monocyte and dendritic cell defects in cancer. (A) Cancer patients have an increased percentage of CD14⁺HLA-DR^lo/neg monocytes in circulation. Peripheral blood of healthy volunteers and cancer patients was analyzed by flow cytometry for immune phenotype. (B) Monocytes from cancer patients have decreased capacity to differentiate to mDC (CD83⁺) under a variety of stimulation conditions. Monocytes were selected from blood of healthy volunteers (HV) or cancer patients (GBM, glioblastoma multiforme LYM, B-cell lymphoma; RCC, renal cell carcinoma; SR, sarcoma) by CD14⁺ immunomagnetic beads and cultured under different methods as labeled in X axis. Method A, fast-DC (28); B, ex vivo media with 5 days culture as described (29, 30); C, 5 days culture in StemLine media and GM-CSF, maturation factors TNFα and PGE2 added in the last 2 days of culture; D, method C with IL-4 added for 5 days of culture; E, method D with poly I:C added to maturation factors; F, method D with CpG used as maturation factor in place of TNF-α (*p < 0.05). (C) Decreased generation of mDC correlates with increased percentage of CD14⁺HLA-DR^lo/neg in the monocytes selected for culture (Method B).