Abnormal dendritic cell differentiation and accumulation of immature myeloid suppressor cells (ImC) is one of the major mechanisms of tumor escape. We tested the possibility of pharmacologic regulation of myeloid cell differentiation using all-trans-retinoic acid (ATRA). Eighteen patients with metastatic renal cell carcinoma were treated with ATRA followed by s.c. interleukin 2 (IL-2). Eight healthy individuals comprised a control group. As expected, the cancer patients had substantially elevated levels of ImC. We observed that ATRA dramatically reduced the number of ImC. This effect was observed only in patients with high plasma concentration of ATRA (>150 ng/mL), but not in patients with lower ATRA concentrations (<135 ng/mL). Effects of ATRA on the proportions of different dendritic cell populations were minor. However, ATRA significantly improved myeloid/lymphoid dendritic cell ratio and the ability of patients' mononuclear cells to stimulate allogeneic T cells. This effect was associated with significant improvement of tetanus-toxoid–specific T-cell response. During the IL-2 treatment, the ATRA effect was completely eliminated. To assess the role of IL-2, specimens from 15 patients with metastatic renal cell carcinoma who had been treated with i.v. IL-2 alone were analyzed. In this group also, IL-2 significantly reduced the number and function of dendritic cells as well as T-cell function. These data indicate that ATRA at effective concentrations eliminated ImC, improved myeloid/lymphoid dendritic cell ratio, dendritic cell function, and antigen-specific T-cell response. ATRA treatment did not result in significant toxicity and it could be tested in therapeutic combination with cancer vaccines. (Cancer Res 2006; 66(18): 9299-307)

Purpose: The initial goal of this study was to test the immunologic and clinical effects of a new cancer vaccine consisting of dendritic cells (DC) transduced with the full-length wild-type p53 gene delivered via an adenoviral vector in patients with extensive stage small cell lung cancer.

Purpose: Induction of antitumor immune responses requires adequate function of dendritic cells. Dendritic cell defects in cancer patients have been implicated in tumor escape and the limited efficacy of cancer vaccines. Previous studies have shown that vascular endothelial growth factor (VEGF) plays a major role in abnormal dendritic cell differentiation and function in cancer. It has been proposed that inhibition of VEGF may result in improved immune responses. The goal of this study was to test this hypothesis. Experimental Design: Fifteen patients with refractory solid tumors were enrolled into a phase I clinical trial of VEGF-Trap. Phenotype and function of different subsets of mononuclear cells were measured before and at different time points after the start of treatment. Results: VEGF-Trap treatment did not affect the total population of dendritic cells, their myeloid or plasmacytoid subsets, myeloid-derived suppressor cells (MDSC), or regulatory T cells. It significantly increased the proportion of mature dendritic cells. However, that improvement was not associated with an overall increase in immune responses to various antigens and mitogens. A subset analysis revealed significant improvement in immune responses in patients who had no increase in the proportion of MDSC. An improvement in immune responses was absent in patients with an increase in the proportion of MDSC. Conclusions: Inhibition of VEGF signaling may improve differentiation of dendritic cells in cancer patients. However, it was not sufficient to improve immune responses. This shows multifaceted nature of immune deficiency and points out to the need for complex approach to modulation of immune reactivity in cancer.

The fact that the immune response to cancer is compromised has been convincingly demonstrated in murine tumor models as well as in cancer patients. The unresponsiveness of the host immune system is one of the major mechanisms of tumor escape as well as an important factor that limits the success of cancer immunotherapy. Inadequate function of professional antigen presenting cells dendritic cells (DC) in cancer is one of the major elements of compromised anti-tumor immune response. Despite substantial progress in recent years, the mechanism of inadequate DC function in cancer still remains unclear. The tumor microenvironment has emerged as an important component contributing to DC malfunction. In this review we will discuss the potential role of tumor microenvironment in DC dysfunction.

IFN-γ is of central importance for the induction of robust cell-mediated immunity and for the activation of APC. Recent studies using experimental murine systems have now suggested a fundamental role for APC-derived IFN-γ during infection with intracellular pathogens. It is currently unknown whether human dendritic cells (DC) can respond to bacterial stimulation with production of IFN-γ. To test this question, we used human monocyte-derived DC stimulated by Mycobacterium bovis bacillus Calmette-Guérin as a model system. We demonstrate production of IFN-γ mRNA and protein on the single cell level. IFN-γ in DC cultures was not simply produced by contaminating lymphocytes because production of DC-IFN-γ could also be demonstrated in highly purified DC cultures containing virtually no T, B, and NK cells. TLR2 was identified as a key receptor involved in triggering production of DC-IFN-γ. Interestingly, DC-IFN-γ seems to participate in an autocrine DC activation loop, and production of DC-IFN-γ could be enhanced by costimulation of DC with IL-12/IL-15/IL-18. In conclusion, we have demonstrated production of IFN-γ by human DC on the single cell level, identified TLR2 as a pattern recognition receptor involved in this process, and elucidated some of the functional consequences of autocrine IFN-γ production by human DC.

Recent evidence suggests that platelets are not only involved in haemostatic processes but also modulate immune responses. As antigen‐presenting cells (APC) are of crucial importance for the regulation of immunity, in this study we wanted to define the role of platelet factor 4 (PF‐4) as one of the major platelet‐derived chemokines on the transition of monocytes into APCs. Our experiments show that within 3 days PF‐4 in conjunction with IL‐4 induces a rapid differentiation of monocytes into APC. These PFAPC (PF‐4/IL‐4 differentiated APC) display unique phenotypical and functional characteristics setting them apart from macrophages and conventional dendritic cells. Functional studies revealed that PFAPC preferentially stimulated proliferation of lymphocytes and lytic NK activity while they induced only moderate cytokine responses. Beyond day 3 of differentiation, PFAPC became less immunostimulatory and maintained their capacity to phagocytose particulate material even after LPS‐induced maturation. These experiments uncover a previously unknown role for the platelet‐derived CXC‐chemokine PF‐4 in differentiation of human APC. Our data further support the newly discovered function of platelets in immunomodulation and provide new evidence for a rapid transition of monocytes into APC under the influence of inflammatory stimuli.