Germinal centre formation and follicular antigen trapping in the spleen of lethally X-irradiated and reconstituted rats.

In this study, the relationship between germinal centre formation and the follicular trapping of immune complexes in the rat spleen was investigated. Lethally (9 Gy) X-irradiated rats were reconstituted with thoracic duct lymphocytes and subsequently challenged with sheep red blood cells to induce germinal centre formation. Rats were killed at daily intervals from 1 to 8 days after reconstitution and antigenic stimulation. Antigen trapping capacity during this interval was assessed by intravenous injection of HRP-anti-HRP immune complexes, 24 hr before killing of the animals. Germinal centre formation could be observed from Day 4 onwards. The follicular trapping capacity, which had been abolished by the X-irradiation, however, returned 2 days later (i.e. by Day 6). Apparently, in these experiments, early germinal centre formation could occur without an intact follicular trapping mechanism. It was, therefore, concluded that in this transfer system follicular immune complex trapping is not a prerequisite for the induction of de novo germinal centre formation. Previous studies have shown that both follicular dendritic cells and marginal zone B cells may play a role in the follicular antigen trapping mechanism. FDCs, as detectable with mAB MRC OX 2 in immunoperoxidase-stained frozen sections, were present in spleens at any time after irradiation and reconstitution; the impaired follicular trapping of HRP-anti-HRP immune complexes was therefore not due to an absence of FDCs. Marginal zone B cells, however, were almost absent until 5 days after reconstitution, as observed by immunohistochemical staining with anti-B cell mABs (HIS 14 and HIS 22) and a polyclonal anti-IgD antiserum. Our data suggest that regeneration of follicular antigen trapping capacity is dependent on the presence of marginal zone B cells.