Spleen dendritic cells exhibit altered morphology and increased allostimulatory capacity after short-term culture*

Conversion of Tolerogenic CD4¯8¯ Dendritic Cells to Immunogenic Ones Inducing Efficient Antitumor Immunity

Culturing conditions may affect dendritic cell (DC) maturation status and functional effects. We have previously demonstrated that different DC subsets play distinct roles in immune responses. The splenic CD4-8- DC subset that secretes transforming growth factor (TGF)-beta stimulates CD4+ regulatory T type 1 (Tr1) cell responses, and this leads to antitumor immune tolerance. In this study, we investigated the potential effect of culturing conditions, namely: (1) duration of culturing and (2) the dose of antigen ovalbumin (OVA) for DC pulsing, respectively, in the conversion of tolerogenic CD4-8- DC into immunogenic DCs. Our data showed that isolated CD4-8- DCs cultured for an additional 18 hours in medium containing 15-20 ng/mL granulocyte macrophage colony-stimulating factor (GM-CSF) became more mature compared to the freshly isolated CD4-8- DCs. When pulsed with OVA at the relatively high concentration of 1 mg/mL, but not at 0.1 mg/mL, the CD4-8- DCs could be converted into immunogenic CD4-8- DCs, which stimulated CD4+ T-cell differentiation into type 1 helper T (Th1) cells. Vaccination of mice with converted CD4-8- DCs induced strong OVA-specific cytotoxic T-lymphocyte (CTL) responses and protective immunity against OVA-expressing BL6-10OVA B16 melanoma. Taken together, our findings indicate that the conversion of DCs from a tolerogenic to an immunogenic state can be achieved by the elongation of DC culturing time in combination with a high-dose antigen for DC pulsing. Therefore, our results may have a significant impact in designing DC-based antitumor vaccines.

Cross-priming as a predominant mechanism for inducing CD8(+) T cell responses in gene gun DNA immunization

DNA immunization induces CD8(+) CTL responses by bone marrow-derived APCs, which are directly transfected with a plasmid DNA and/or acquire Ags from DNA-transfected non-APCs. To investigate the relative contribution of DNA-transfected APCs vs non-APCs to the initiation of CD8(+) T cell responses, we used tissue-specific promoter-directed gene expression and adoptive transfer systems in gene gun DNA immunization. In this study, we demonstrated that non-APC-specific gene expressions induced significant CD8(+) CTL and IFN-gamma-producing cells and Ab responses, whereas APC-specific gene expressions led to moderate CTL and IFN-gamma-producers, but no Ab responses. Interestingly, mice immunized with a non-APC-specific plasmid induced more rapid, vigorous, and prolonged proliferation of adoptively transferred Ag-specific CD8(+) T cells than APC-specific plasmid-immunized mice. In addition, the in vivo proliferative responses elicited by a non-APC-specific plasmid administration were dependent on TAP, but were independent of CD4(+) T cell help. Collectively, our results suggest that cross-priming, in which Ags expressed in non-APCs are taken up, processed, and presented by APCs, plays an important role in the initiation, magnitude, and maintenance of CD8(+) T cell responses in gene gun DNA immunization.

2,3,7,8-Tetrachlorodibenzo-p-dioxin affects the number and function of murine splenic dendritic cells and their expression of accessory molecules

Primary T cell-mediated immune responses are highly susceptible to suppression by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure, yet direct effects of TCDD on T cells have been difficult to demonstrate. Since the activation of naive T cells has been shown to be initiated primarily by dendritic cells (DC), these cells represent a potential target for TCDD immunotoxicity. In this report, we have examined the influence of TCDD exposure on splenic DC phenotype and function in the absence of antigenic stimulation. Results showed that DC from TCDD-treated mice expressed higher levels of several accessory molecules including ICAM-1, CD24, B7-2, and CD40, whereas the expression of LFA-1 was significantly reduced. These effects were dose-dependent and persisted for at least 14 days after exposure. The effects were also dependent upon the aryl hydrocarbon receptor (AhR), as similar effects were observed in AhR+/+ C57Bl/6 and Balb/c mice but not in AhR-/- mice. When DC from TCDD-treated mice were cultured with allogeneic T cells, the proliferative response and production of IL-2 and IFN-gamma by the T cells were increased. Production of IL-12 by the DC was likewise enhanced in comparison to cells from vehicle-treated mice. Interestingly, however, the number of DC recovered from TCDD-treated mice was significantly decreased. Taken together, these results suggest that, in the absence of antigen, TCDD provides an activation stimulus to DC that may lead to their premature deletion. Since the survival of DC has been shown to influence the strength and duration of the immune response, these results suggest a possible novel mechanism for TCDD-induced immune suppression.

Expression of B7 costimulatory molecule in cultured human epidermal Langerhans cells is regulated at the mRNA level

Langerhans cells (LC) belong to the dendritic cell lineage and are the principal antigen-presenting cells of squamous epithelia. Short-term cultured LC (cLC) exhibit a marked augmented capacity to stimulate allogeneic T cells and acquire the ability to activate naive T cells, probably in relation to enhanced expression of accessory signals. In this study, we evaluated the expression of B7 costimulatory molecule (CD80) in human freshly isolated (fLC) and cLC at both the protein and mRNA level. Staining of frozen skin sections did not reveal any epidermal dendritic cell reactive with either of two different anti-B7 monoclonal antibodies. fLC in suspension did not exhibit any B7 staining as evaluated by two-color flow-cytometry analysis and immunoelectron microscopy. In contrast, LC that were cultured for 24-72 h displayed strong surface B7 reactivity with a characteristic patchy pattern. Treatment with dispase and trypsin did not reduce B7 staining of cLC. Following warming to 37 degrees C, cLC tagged with anti-B7 monoclonal antibody and gold-conjugated secondary antibody could internalize surface B7 by using the organelles of receptor-mediated endocytosis. B7 mRNA, detected by the reverse-transcriptase polymerase chain reaction technique, was expressed at a low level in purified (> 90% HLA-DR+) fLC but not in LC-depleted epidermal cells, and was markedly upregulated in purified cLC. The results indicate that 1) fLC do not express B7 protein on their surface, but acquire B7 during culture, 2) surface B7 is not sensitive to trypsin, 3) B7 expression is regulated primarily at the mRNA level, and 4) membrane B7 can be internalized within cLC. B7 molecule on CLC may be relevant to their increased antigen-presenting cell potency and ability to stimulate naive T lymphocytes.