EXPRESSION OF THE INTERCELLULAR-ADHESION MOLECULE-1 ON HIGH ENDOTHELIAL VENULES AND ON NONLYMPHOID ANTIGEN HANDLING CELLS - INTERDIGITATINGCELLS, ANTIGEN TRANSPORTING CELLS AND FOLLICULAR DENDRITIC CELLS

Intercellular adhesion molecule-1 (ICAM-1)(I) has been implicated in t he development of germinal center reactions in vitro, and the present study was undertaken to determine the distribution of ICAM-1 in active germinal centers in vivo and in murine secondary lymphoid tissues in general. Anti-ICAM-1-specific monoclonal antibodies were used in conju nction with immunohistochemistry at both the light and ultrastructural levels of resolution. Examination of cryostat sections of lymph nodes , spleens, and Peyer's patches revealed that anti-ICAM-1 distinctly la beled cells in the light zones of germinal centers, a few cells in the T cell zones (e.g. paracortex of lymph nodes), cells in the sinus flo or of the subcapsular sinuses of lymph nodes, and high endothelial ven ules (HEV). Ultrastructural studies revealed that the cells labeling w ith anti-ICAM-1 in germinal centers were follicular dendritic cells (F DC) which appeared to have more ICAM-1 than any other cell type. The s urfaces of well-developed, intricate, convoluted FDC processes were in tensely labeled even under conditions where B cells appeared negative. Interdigitating cells (IDC) were also labeled as were certain endothe lial cells in the HEV. The cells in the subcapsular sinus floor labeli ng with anti-ICAM-1 were the ''antigen transporting cells'' (ATC) that carry antigen-antibody complexes into lymph node follicles. We suspec t ATC are FDC precursors which mature into FDC in the follicles. Inter estingly, FDC, IDC, and ATC are 3 important accessory cells known to h andle antigens in specific compartments of lymphoid tissues. The marke d localization of this adhesion molecule on these critical antigen han dling cells supports the concept that ICAM-1 is important in providing the intercellular adhesion necessary for optimal initiation of immune responses in vivo.