Kompartimente der Weißen Milzpulpa bei Ratten - Immunhistologische Darstellung von Stromazellen, Lymphozyten und Makrophagen

The microscopic anatomy of rat spleens has been described by immunohistology 30 years ago shortly after monoclonal antibody (mAb) technology came up. Up-to-date surveys of rat splenic cell composition are lacking. In the present investigation a highly amplifying immunohistological technique, the ABC method, is applied to visualise stromal cells, B- and T-lymphocytes, macrophages and plasma cells/plasmablasts in spleens of the inbred LEW rat strain. Special emphasis is put on analysing cells in the splenic marginal zone (MZ), a border region between white and red pulp, which has originally been defined in rats. The antibodies used confirmed the well-known distribution of B- and T-lymphocytes in the rat spleen. In addition, two mAbs, His57 and His52, stained the majority of B-lymphocytes in the MZ and B-cells in follicles. Both mAbs have been described not to react with small recirculating B-lymphocytes in the original publication. The staining pattern observed may be due to MZ B-lymphocytes migrating between MZ and follicles. The present investigation defines the phenotype of MZ B-lymphocytes in LEW rats as His57++His52+IgM+IgD+/-. Stromal cells of the MZ could not be immunohistologically defined in rats. The phenotype of these cells obviously differs decisively from that of fibroblasts at the surface of human splenic follicles. Detection of ICAM-1 (CD54) was not informative, because all MZ B-lymphocytes expressed this adhesion molecule. In contrast, VCAM-1 (CD106) was almost absent in the MZ. Anti-pan-laminin showed an extracellular network in the MZ, which appeared to be produced by stromal cells. This network was also visualised by mAb Ox43. In addition, Ox43 targeted all parts of the open splenic circulation system. However, in other organs Ox43 produced typical membrane staining of different cell types. The so far uncharacterised Ox43 antigen could thus be a membrane molecule, which is also cleaved and circulates in the blood. It may have a coagulation-inhibiting function. Plasmablasts and plasma cells with high amounts of intracellular IgM formed clusters in the outer T-cell zone and near side branches of the central arteries termed "marginal zone bridging channels". About half of these cells expressed Ki-67. This suggests that not only proliferating plasmablasts, but also newly formed non-proliferating plasma cells may migrate in the spleen for longer distances. It cannot be excluded that IgM+ plasma cells are formed in the outer T-cell zone. The MZ clearly belongs to the open splenic circulation, because it harbours scattered erythrocytes and granulocytes. The presence of MZ B-cells, VCAM-1- stromal cells and CD169++ macrophages characterises the MZ as a unique species-specific splenic compartment. Rat MZs contain small recirculating B-lymphocytes and a predominant population of large pre-activated B- lymphocytes which most likely correspond to memory B cells. Whether these cells exhibit species-specific traits similar to the stromal cells is not clear. The phenotype of MZ stromal cells remains to be established in rats and mice.