Immunohistochemistry of Nasopharyngeal (Waldeyer's ring equivalent) lymphoid tissue in the rat

Previously we have described the presence of Waldeyer's ring equivalent (WRE) lymphoid tissue in the rat, and pointed out the importance of such an experimental model for studying the immunological role of nasopharyngeal lymphoid tissue. Here we extend this work with immunohistological data in terms of compartmentalization and distribution of the various lymphoid and non-lymphoid cells of this WRE lymphoid tissue in situ. WRE tissue consists of distinct T cell and B cell areas. B cell areas predominate; they are located directly under the mucosal epithelium and consist mainly of follicles. These follicles frequently contain a germinal center with IgD negative B cells interspersed with scattered CD4 (helper/inducer) T cells. Follicular dendritic cells are present in the germinal centers. T cell areas, on the other hand, are predominantly present at the abluminal side of the WRE in interfollicular areas. In these areas high endothelial venules and both CD4 and CD8 (suppressor/cytotoxic) T cell populations with a clear preponderance of CD4 over CD8 cells can be observed. MHC class II positive interdigitating dendritic cells are also scattered throughout these T cell areas. Mononuclear phagocytes (ED1-positive monocytes/macrophages) are scattered throughout the WRE, but especially in the T cell areas. A subpopulation of (ED3-positive) mononuclear phagocytes, e.g., the lymphoid tissue macrophage, is exclusively scattered between the small blood vessels along the abluminal side of the lymphoid tissue. Here, plasma cells, including those of the IgA type, are located. The data show that nasopharyngeal lymphoid tissue in the rat can be considered as an immunologically fully equipped and active mucosal lymphoid organ presumably executing similar immune functions as the tonsils in the human Waldeyer's ring.