Immunohistological detection of gap junctions in human lymphoid tissue: connexin43 in follicular dendritic and lymphoendothelial cells.

We investigated the expression of gap junction connexins26, -32, and -43 in normal, reactive, and diseased human lymphoid tissue with single and double immunolabeling and confocal laser scanning microscopy. In all tissues, connexin43 positivity was detected in follicular dendritic cells positive for CD21 and CD35 antigens, around lymphoendothelial cells moderately positive for Factor VIII, CD31 and cathepsin-D antigens; and somewhat in vascular endothelia including high endothelial venules strongly positive for Factor VIII and CD31 antigens. The ultrastructural hallmark of gap junctions, pentalaminar structures with appropriate spacing, was found in follicular dendritic cell processes. Connexin43 was also detected between smooth muscle and stromal cells of the gut, in capsular fibroblasts, and in tonsil epithelium. Neither connexin32 nor -26 was revealed, except for connexin26 in the tonsil epithelium. In follicular dendritic cells, connexin43 co-localized closely with the desmosomal proteins desmoplakin and desmoglein, suggesting that cell adherence has a role in gap junction formation. Most connexin43 was observed in sinus lining cells of lymph nodes involved in malignancies and in follicular dendritic cells in the light zone of germinal centers where maturing but still proliferating lymphocytes are situated. In the light of their distribution, gap junctions may play a part in regulating the growth of germinal centers and in integrating activating or controlling signals in follicular dendritic and sinus lining cell networks. Because connexin43 is the connexin of stromal cells, finding it in follicular dendritic cells in consistent with the proposal that these cells originate from resident stromal cells.