The experiments described in this article characterize the phenotypic and functional changes in afferent lymph cell populations that occur as a result of in vivo immune stimulation. During the primary immune response (in antigen-naive sheep) there are very transient increases in level of CD1 expression by subpopulations of dendritic cells (DC) but no alterations in cell kinetics or MHC class II expression. In contrast, secondary antigenic challenge (in primed sheep) into the drainage area of an afferent lymphatic causes profound changes in the cell output, characterized by a greater than threefold drop in total cell output on days 1-3 followed by an approximate fivefold rise on day 5. There is also a substantial increase in both the proportion of MHC class II-positive T lymphocytes (from 28 to 54%) and in the quantitative expression of class II by both DC and lymphocytes. Class II expression by DC increases five- to sixfold by day 5, while the level of expression of class II on lymphocytes approximately doubles. The increase in CD1 expression during the secondary response is more prolonged than during the primary response, being detectable between days 2 and 6 after challenge. The rise in class II affects the whole DC population, in contrast to CD1 where the increase affects only a subpopulation of cells. In terms of functional properties, afferent lymph DC isolated during a primary response show no alteration of their activity, whereas DC taken 4-5 d after secondary challenge are up to fivefold more active in their ability to present soluble antigen to primed autologous T cells and to antigen-specific cell lines as well as to stimulate in the MLR. The relative expression of class II correlates temporally with an increased capacity of DC to present antigen. Monoclonal anti-class II antibodies totally inhibit the in vitro assays but anti-CD1 antibodies have no effect. The previous paper has demonstrated that afferent DC can associate with antigen in vivo and can present that antigen to antigen-specific T cells. This article extends our knowledge of DC biology and demonstrates that DC, activated during secondary in vivo immune responses, have an enhanced ability to present an antigen, unrelated to that used for challenge, to specific T cell lines. This enhancement correlates directly with quantitative variation of expressed class II and not CD1 and suggests that this variation in class II expression plays a physiological role in in vivo immune regulation.

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