To prime immune responses, dendritic cells (DCs) need to be activated to acquire T cell stimulatory capacity. Although some stimuli trigger interleukin 12 (IL-12) production that leads to T helper cell type I (TH1) polarization, others fail to do so and favor TH2 polarization. We show that after activation by lipopolysaccharide, DCs produced IL-12 only transiently and became refractory to further stimulation. The exhaustion of cytokine production impacted the T cell polarizing process. Soon after stimulation DCs primed strong TH1 responses, whereas at later time points the same cells preferentially primed TH2 and nonpolarized T cells. These findings indicate that during an immune response, T cell priming conditions may change in the lymph nodes, suggesting another mechanism for the regulation of effector and memory T cells.

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Cholera toxin (CT) is a potent mucosal adjuvant that amplifies B and T cell responses to mucosally co‐administered antigens, stimulating predominant Th2‐type responses. However, little is known about the mechanism of adjuvanticity of CT and on the influence this toxin may have on Th2 cell development during the priming of an immune response. We analyzed the effect of CT on dendritic cells (DC), which are responsible for the priming of immune responses at the systemic as well as at the mucosal level. We found that CT induces phenotypic and functional maturation of blood monocyte‐derived DC. Indeed, CT‐treated DC up‐regulate expression of HLA‐DR molecules, B7.1 and B7.2 co‐stimulatory molecules, and are able to prime naive CD4+CD45RA+ T cells in vitro, driving their polarization towards the Th2 phenotype. Furthermore, CT‐matured DC express functional chemokine receptors CCR7 and CXCR4 which may render them responsive to migratory stimuli towards secondary lymphoid organs. Interestingly, the maturation program induced by CT is unique since CT does not induce but rather inhibits cytokine (IL‐12p70 and TNF‐α) and chemokine (RANTES, MIP‐1α and MIP‐1β) secretion by lipopolysaccharide‐ or CD40 ligand‐activated DC. Our results help to elucidate the mechanism of action of CT as an adjuvant and highlight a new stimulus of bacterial origin that promotes maturation of DC.

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Dendritic cells (DC) of hepatitis B virus (HBV) carriers have been reported to exhibit functional impairment. Possible explanations for this phenomenon are infection of HBV by DC or alteration of DC function by HBV. We therefore analyzed whether DC support the different steps of HBV infection and replication: uptake, deposition of the HBV genome in the nucleus, antigen expression, and progeny virus release. When HBV genomes were artificially introduced into monocyte-derived DC by adenoviral vectors, low-level expression of hepatitis B surface antigen (HBsAg) and hepatitis B e antigen (HBeAg) but no HBV replication was detected. When monocyte-derived DC were subjected to wild-type HBV or a recombinant HBV expressing Renilla luciferase under a non-liver-specific promoter, intracellular HBV DNA was detected in a low percentage of cells. However, neither nuclear cccDNA was formed nor luciferase activity was detected, indicating that either uncoating or nucleocytoplasmic transport were blocked. To verify our observation in the in vivo situation, myeloid and plasmacytoid DC were isolated from blood of high viremic HBV carriers, and analyzed by quantitative polymerase chain reaction (PCR) and electron microscopy. Although circulating DC had in vivo been exposed to more than 10 4 HBV virions per cell, HBV genomic DNA was hardly detected, and no nuclear cccDNA was detected at all. By using electron microscopy, subviral particles were found in endocytic vesicles, but virions were undetectable as were viral capsids in the cytoplasm. H epatitis B virus (HBV) infection represents a major health problem worldwide. Over 350 million individuals are chronically infected with HBV and are at high risk to develop liver cirrhosis or hepatocellular carcinoma. To eliminate the virus after infection, a strong humoral and cellular immune response is required. 1 Thus, control of HBV infection is associated with a multispecific and polyclonal cytotoxic T-cell response and a strong type 1 T helper cell response. 2,3 In contrast, chronically infected patients display oligoclonal T helper cell responses with weak or undetectable cytotoxic T-cell activity. 4 Dendritic cells (DC) are the most important professional antigen-presenting cells. They act as key players in initiating virus-specific T-cell responses. 5 This is reflected by the fact that viruses can evade immune responses by impairing DC function. [6][7][8] The role of DC during HBV infection was intensively studied during the last years, but whether total numbers of DC are reduced during chronic infection is discussed controversary. [9][10][11] In some studies, functional deficits of these cells were reported on contact of moDC with HBV, 14 whereas they remained minor in others. 12 Whether the weak or absent T-cell response described in chronic hepatitis B patients results from a defect in the DC compartment, which is caused by the virus itself, is unclear. Theoretically, numbers or functionality of DC subsets could be affected by interaction of surface receptors on DC with either vi...