Lymphoid follicles in extranodal sites: an immunohistochemical study

Lymphoid follicles in extranodal sites involved in chronic inflammatory reactions are the subject of this study. Immunohistochemistry was focused on associated lymphoid formations considered as incomplete or prefollicular structures because they were composed only of some types of the germinal centre cells among demonstrated dendritic reticulum cells (DRC) and thin vessels. Paraffin embedded tissues were stained with current histological methods and immunohistochemically processed with a panel of antibodies against DRC (CD21, CD35, IgM); endothelial cells (factor VIII R. Ag., EUA); B lymphoid cells (L26, MB2); T lymphoid cells (UCHL1, MT1): intercellular adhesion molecule (ICAM-1); histiocytic/monocytic cells (KP1, PS100, delta-1-antitrypsin, delta-1-antichymotrypsin); smooth muscle (actin 1A4); basement membranes (collagen IV). Prefollicular structures showed positivity for B cells markers among a network of DRC and thin long processes. Furthermore, scattered T lymphoid and histiocytic cells such as collagen IV fragments and some pericytic cells were demonstrated. Some sections with Emat-eos. were destained and submitted to immunohistochemistry analyses: in prefollicular structures, histologically referred endothelial cells were positive for dendritic reticulum markers and ICAM-1. These findings appear to favor the hypothesis that DRC could be transformed endothelial cells which therefore represent an important component in follicular formation.