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Murine lymph node antigen presenting cells are the main source of interleukin-6 in the initiation of delayed-type
Abstract:
Interleukin-6 (IL-6) and Interferon gamma (IFN-gamma) production was analyzed in mice after topical exposure of the animals to Oxazolone, a well-known contact sensitizer. Since both IL-6 and IFN-gamma had been shown to be involved in the initiation of delayed-type hypersensitivity (DTH) reactions, and especially in contact sensitization (CS), we focussed our analysis on the cellular source of the two cytokines in local lymph nodes draining the site of exposure. We have demonstrated that IL-6 is found exclusively in lymph node antigen presenting cells (LNAPC), using three different approaches: i) in vitro restimulation of CD4-positive cells, obtained from Oxazolone-treated mice, in the presence of I-A-positive LNAPC, led to a strong IL-6 response, measured in culture supernatants by ELISA. Depletion of LNAPC in these suspensions prior to cultivation diminished IL-6 secretion, indicating that the LNAPC were the sole source of IL-6, ii) Staining of restimulated LNC for intracellular cytokines confirmed that LNAPC are the only source of IL-6 at various time-points during cultivation. iii) Competitive PCR analysis of cDNA, derived from freshly isolated lymph node cells (LNC) depleted either in CD8/B220- or CD8/B220/I-A-positive cells, showed that ex vivo IL-6-specific mRNA was found exclusively in the LNAPC. In contrast IFN-gamma is produced by CD4+ cells, although in some experiments CD8+ cells were also positive. Time-course analysis of the secretion of the two cytokines and their relation to lymphocyte blastosis in vitro showed that IL-6 peaked during the first 6 hrs of restimulation, whereas the number of IFN-gamma producing cells reached a maximum after 24 hrs and were closely correlated with the increasing number of in vitro blastocytes. Our data corroborate with other authors' investigations of DTH reactions, showing that IL-6, provided by LNAPC during primary responses in vivo, may serve as a co-stimulating factor for T cells.