MOABS MAS516 AND 5B5, 2 FIBROBLAST MARKERS, RECOGNIZE HUMAN FOLLICULAR DENDRITIC CELLS

Follicular dendritic cells (FDC) are only located within follicles of secondary lymphoid tissues. The origin of this peculiar cell type is n ot clearly defined. To contribute to this study, we applied two monocl onal antibodies (MAS516 and 5B5) considered as specific for fibroblast s to tonsil cryosections and to isolated follicular dendritic cells. O n the basis of an enzyme cocktail digestion of human tonsils and a fra ctionation procedure on albumin gradients, FDC can be prepared in the form of cell aggregates with associated lymphoid cells. MAS516 reacts with surface membrane molecules expressed by human fibroblasts, tissue macrophages and peripheral blood monocytes. With immunoperoxidase ass ays on tonsil cryosections connective tissue cells and macrophages are stained. Inside germinal centres, heavy labelling of the light zone w as found. The MAS516 staining pattern is very similar to that of speci fic FDC markers DRC-1 or BU10. All isolated FDC reacted with MAS516 an tibody. 5B5, considered as a typical fibroblast marker, reacts with hu man prolyl-4-hydroxylase which is an intracellular enzyme related to c ollagen biosynthesis. In cryosections, interfollicular and capsular ar eas showed 5B5 positive connective tissue fibroblasts. In germinal cen tres, some cells presenting features of FDC were 5B5 positive. After c ell separation, 25%-50% of the isolated FDC were labelled with this an tibody. This positivity of some FDC for 5B5 antibody may support the i dea of their fibroblastic origin. The combination of observations real ized in situ and after cell purification ensured an unequivocal recogn ition and identification of FDC. In consequence, the analysis of FDC r eactivity to Moabs allows us to propose that precursor cells with a ph enotype intermediate to that of fibroblasts and FDC would have preserv ed their capacity to produce prolyl-4-hydroxylase whereas fully differ entiated FDC would have cost it.