Immunohistochemical study of tonsils from newborn infants with emphasis on follicular dendritic reticulum cells.

The origin of follicular dendritic reticulum cells (FDRCs) is still debated in the literature, although their derivation from local transformed mesenchymal cells is now generally accepted. The purpose of this immunohistochemical study was to further define the nature of the FDRC, for which an endothelial cell derivation was proposed in previous papers. Palatine tonsils were removed at autopsy from eight newborn infants ranging from a few hours to four days of age. Paraffin embedded tonsil specimens were stained by current histochemical methods and immunohistochemically processed with a panel of antibodies against follicular dendritic reticulum cells (CD21 and CD35), endothelial cells (Factor VIII R. Ag, CD31, CD34), B lymphoid cells (L26, LN1, MB2), T lymphoid cells (UCHL1, DF-T1, OPD4, CD8, CD3), monocytic cells (PGM1 and KP1), interdigitating dendritic cells (S100 Protein, PGM1), intercellular adhesion molecule (ICAM-1 (CD54). Double immunostainings for endothelial markers and for FDRCs were performed. In newborn infant tonsils, lymphoid follicles are absent. T lymphocytes appear to represent the largest component whereas B lymphocytes are seen in small aggregates between blood capillary vessels. Monocytes and interdigitating dendritic cells are also present. The capillary endothelial cells within the B lymphoid aggregates are positive for endothelial and FDRCs markers; double immunostaining for Factor VIII R Ag (or other endothelial markers) and CD21 was present within the same capillary endothelial cells. Further they were positive for ICAM-1. These observations lend further evidence that the derivation of FDRCs may be from transformed endothelial cells and on this way they act as enhancing microenvironment for B lymphocyte expansion.