Increased level of intracellular MHC class II molecules in murine Langerhans cells following in vivo and in vitro administration of contact allergens.

Treatment of murine Langerhans cells (LC) with contact allergens results in increased internalization of cell membrane constituents and therefore in depressed cell-surface expression of major histocompatibility complex (MHC) class II molecules during the first hours after haptenization. In this presentation we show that this downregulation of cell-surface-expressed Ia-antigens is accompanied by an augmentation of the intracellular pool of MHC class II molecules. Rat MoAb 2G9 was developed, which recognizes IA and IE molecules of the d-haplotype. This MoAb competes with the murine MoAb MK-D6 for binding sites to IAd-molecules. After blocking the cell-surface-expressed molecules with 2G9 and permeabilizing the cell membranes this allowed us to measure selectively the intracellular amount of IA molecules by double immunofluorescence staining and flow cytometric analysis. Cell-surface expression of IA molecules was found to be depressed but their internal pool was significantly elevated following in vivo treatment with the contact allergens DNFB, DNCB, oxazolone, and K2Cr2O7 for 3 h. In vitro culture of freshly prepared LC in the presence of 1 microgram/ml DNFB yielded similar results. Blocking of protein biosynthesis with cycloheximide did not prevent this intracellular class II accumulation. An augmented representation of internalized class II molecules in haptenized LC might play an important role in the presentation of contact allergens.