S U M M A R Y Fas (CD95, APO-1) is widely expressed on lymphatic cells, and by interacting with its natural ligand (Fas-L), Fas induces apoptosis through a complex caspase cascade. In this study we sought to survey Fas-L expression in vascular and sinusoidal structures of human reactive lymph nodes. Immunohistochemical Fas-L expression was present in all paracortical high endothelial venules (HEVs), in cells lining the marginal sinus wall, and in a few lymphocytes, but only occasionally in non-HEV vascular endothelium. In the paracortical zone over 60% of all vessels and all paracortical HEVs showed Fas-L expression, whereas in the medullary zone less than 10% of the blood vessels were stained with Fas-L. Normal vessels outside lymph nodes mostly showed no Fas-L expression. We show that in human reactive lymph nodes Fas-L expression is predominantly present in HEVs. Because the circulating lymphocytes gain entry to nodal parenchyma by transendothelial migration through HEVs, the suggested physiological importance of Fas-L expression in these vessels lies in the regulation of lymphocyte access

We have optimized an immunohistochemical double-staining method combining immunohistochemical lymphocyte lineage marker detection and apoptosis detection with terminal deoxyribonucleotidyl transferase–mediated dUTP nick end labeling. The method was used to trace Fas-mediated apoptosis in human reactive lymph nodes according to cell lineage and anatomical location. In addition to Fas, we also studied the expression of Fas ligand (FasL), CD3, CD20, CD19, CD23, and CD68 of apoptotic cells. The presence of simultaneous Fas and FasL positivity indicated involvement of activation-induced death in the induction of paracortical apoptosis. FasL expression in the high endothelial venules might be an inductor of apoptosis of Fas-positive lymphoid cells. In addition to B-lymphocyte apoptosis in the germinal centers, there was often a high apoptosis rate of CD23-expressing follicular dendritic cells. In summary, our double-staining method provides valuable new information about the occurrence and mechanisms of apoptosis of different immune cell types in the lymph node compartments. Among other things, we present support for the importance of Fas/FasL–mediated apoptosis in lymph node homeostasis.

Serum sCD23 shows a trend of increment in CMSE and CD, and in the latter, sCD23 level may provide information about the severity of villous atrophy. In CMSE, high serum sFasL indicates both LNH and an increase of IELs, suggesting importance of FasL-mediated mechanisms in the pathogenesis of these features characteristic of CMSE. Further studies are necessary to evaluate whether intensive FasL expression in mucosal high endothelial venules presents a regulatory element in mucosal immunity.

Fas-mediated induction of apoptosis is a major factor in the selection of lymphocytes and downregulation of immunological processes. In the present study, we have assessed endothelial Fas-ligand (FasL) expression in normal human ileum, appendix, and colon, and compared the expression levels with that in inflammatory bowel disease and in acute appendicitis. In a normal appendix, endothelial FasL levels were constant in almost half of the mucosal vessels; but, in the normal ileum and colon, endothelial FasL was practically restricted to areas in close proximity to lymphatic follicles, and was expressed mainly in the submucosal aspect of the follicles in the vessels with high endothelium. In samples from subjects with either Crohn's disease or ulcerative colitis, the extent of endothelial FasL expression was elevated in the submucosa and associated with an elevated number of lymphoid follicles. In inflammatory bowel disease, ulcers and areas with a high density of mononuclear cells expressing FasL also showed an elevated density of blood vessels with endothelial FasL expression. Although the function of endothelial FasL remains unclear, such a specific expression pattern suggests that endothelial FasL expression has a role in the regulation of lymphocyte access to the peripheral lymphoid tissues, including the intestinal mucosa.