In cancer therapy, dendritic cell (DC) vaccination is still being explored. Clinical responses, however, are diverse and there is a lack of immunologic readout systems that correspond with clinical outcome. Only in the minority of patients, T-cell responses correlate with clinical outcome, indicating that other immune cells also gain anticancer activity. We still have limited knowledge of the effect of DC vaccination on different immune effector cells. However, it has been shown that bidirectional cross-talk between natural killer (NK) cells and DCs is responsible for enhanced activation of both cell types and increases their antitumor activity. In this review, we postulate the possibility that NK cells are the secret weapons in DC vaccination and studying their behavior together with T-cell activation in vaccinated individuals might predict clinical outcome.

Among prostaglandins (PGs), PGE2 is abundantly expressed in various malignancies and is probably one of many factors promoting tumor growth by inhibiting tumor immune surveillance . In the current study, we report on a novel mechanism by which PGE2 inhibits in vitro natural killer-dendritic cell (NK-DC) crosstalk and thereby innate and adaptive immune responses via its effect on NK-DC crosstalk. The presence of PGE2 during IFN-␥/membrane fraction of Klebsiella pneumoniae DC maturation inhibits the production of chemokines (CCL5, CCL19, and CXCL10) and cytokines (IL-12 and IL-18), which is cAMP-dependent and imprinted during DC maturation. As a consequence, these DCs fail to attract NK cells and show a decreased capacity to trigger NK cell IFN-␥ production, which in turn leads to reduced T-helper 1 polarization. In addition, the presence of PGE2 during DC maturation impairs DC-mediated augmentation of NK-cell cytotoxicity. Op- IntroductionProstaglandins (PGs) are potent immune modulators that are produced during inflammation after the conversion of arachidonic acid by cyclooxygenase (COX). 1 Furthermore, PGs are also abundantly produced by various types of tumors. 2 COX2 expression, which is correlated with a poor prognosis, is induced in a variety of human premalignant and malignant tumors, including solid tumors as well as hematologic malignancies. [3][4][5][6] Several lines of evidence demonstrate that COX2-derived PGs are involved in the promotion of tumor growth by regulation of cancer cell proliferation, apoptosis, migration, and invasion. [7][8][9][10][11] PGs are also produced by tumor-surrounding cells, creating a tumor-supporting environment by enhancing angiogenesis and inhibiting tumor immune surveillance. 2,[11][12][13][14] Of all prostaglandins, PGE2 has a pivotal role in tumor immunosuppression. It has been hypothesized that this effect is caused by induction of a permanent state of inflammation, 2 resulting in phenotypic and functional changes of T-helper (T H ) cells, cytotoxic T-lymphocyte (CTL) cells, dendritic cells (DCs), natural killer (NK) cells, and myeloid-derived suppressor cells. 12 PGE2 has been shown to deviate T H cell skewing from an antitumor T H 1 response toward a T H 2/T H 17 response by direct binding to these cells. [15][16][17] In addition, PGE2 is responsible for shifting the balance of IL-12/IL-23 production by DCs toward IL-23, which is a very potent cytokine responsible for T H 17 expansion and survival. 18,19 As a consequence, less IL-12 and other proinflammatory cytokines are produced, thereby inhibiting T H 1 polarization. 17,20 PGE2 also decreases the cytotoxic capacity of CTLs directly by inducing the expression of inhibitory receptors on CTLs 21 and indirectly by inhibiting DC maturation and antigen presentation. 22,23 Moreover, tumor-associated PGE2 has been reported to be responsible for the preferential attraction and induction of regulatory T cells, creating an immune-regulatory microenvironment. 24,25 Next to the modulating effects of PGE2 on the effector mech...

Accumulating evidence indicates that fractionated radiotherapy (RT) can result in distant non-irradiated (abscopal) tumour regression. Although preclinical studies indicate the importance of T cells in this infrequent phenomenon, these studies do not preclude that other immune mechanisms exhibit an addition role in the abscopal effect. We therefore addressed the question whether in addition to T cell mediated responses also humoral anti-tumour responses are modulated after fractionated RT and whether systemic dendritic cell (DC) stimulation can enhance tumour-specific antibody production. We selected the 67NR mammary carcinoma model since this tumour showed spontaneous antibody production in all tumour-bearing mice. Fractionated RT to the primary tumour was associated with a survival benefit and a delayed growth of a non-irradiated (contralateral) secondary tumour. Notably, fractionated RT did not affect anti-tumour antibody titers and the composition of the immunoglobulin (Ig) isotypes. Likewise, we demonstrated that treatment of tumour-bearing Balb/C mice with DC stimulating growth factor Flt3-L did neither modulate the magnitude nor the composition of the humoral immune response. Finally, we evaluated the immune infiltrate and Ig isotype content of the tumour tissue using flow cytometry and found no differences between treatment groups that were indicative for local antibody production. In conclusion, we demonstrate that the 67NR mammary carcinoma in Balb/C mice is associated with a pre-existing antibody response. And, we show that in tumour-bearing Balb/C mice with abscopal tumour regression such pre-existing antibody responses are not altered upon fractionated RT and/or DC stimulation with Flt3-L. Our research indicates that evaluating the humoral immune response in the setting of abscopal tumour regression is not invariably associated with therapeutic effects.

A coordinated cellular interplay is of crucial importance in both host defense against pathogens and malignantly transformed cells. The various interactions of Dendritic Cells (DC), Natural Killer (NK) cells, and T helper (Th) cells can be influenced by a variety of pathogen-associated molecular patterns (PAMPs) and will lead to enhanced CD8+ effector T cell responses. Specific Pattern Recognition Receptor (PRR) triggering during maturation enables DC to enhance Th1 as well as NK helper cell responses. This effect is correlated with the amount of IL-12p70 released by DC. Activated NK cells are able to amplify the proinflammatory cytokine profile of DC via the release of IFN-γ. The knowledge on how PAMP recognition can modulate the DC is of importance for the design and definition of appropriate therapeutic cancer vaccines. In this review we will discuss the potential role of specific PAMP-matured DC in optimizing therapeutic DC-based vaccines, as some of these DC are efficiently activating Th1, NK cells, and cytotoxic T cells. Moreover, to optimize these vaccines, also the inhibitory effects of tumor-derived suppressive factors, for example, on the NK-DC crosstalk, should be taken into account. Finally, the suppressive role of the tumor microenvironment in vaccination efficacy and some proposals to overcome this by using combination therapies will be described.

Besides T helper (Th) cells, natural killer (NK) cells have also been described to participate in the shaping of dendritic cell (DC)-mediated adaptive immune responses. At present, it remains unclear to what extent the induction of these NK helper cell immune mechanisms is coupled with Th responses and whether both helper immune responses are induced by the same DC upon specific pathogen recognition receptor (PRR) stimulation. In this study, we demonstrate that maturation of DCs with a cocktail containing FMKp (membrane fragments of Klebsiella pneumoniae) mounts both Th cell and NK cell helper responses in a PRR-triggered dose-dependent manner as determined by the capacity of the helper cells to produce IFN-γ. Furthermore, by triggering an additional PRR pathway [FMKp in combination with poly(I:C) lyovec], we reveal that both approaches modulate the amount of DC-derived IL-12p70 and that this cytokine is the key determinant of the DC-induced Th1 and NK cell helper responses. Moreover, all PRR triggers able to induce IL-12-producing mature DCs are sufficient to induce these helper responses. We propose the existence of a single program used by DCs to induce potent cellular immune responses by stimulating both T helper and NK cell helper processes. This knowledge can help to select the proper PRR triggers in preventive and therapeutic vaccine design.