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This paper describes a new, less toxic and more selective approach to study the adult thymus. An adriamycin (ADR), sparing bone marrow (BM) stem cells and nontoxic to cells that are not in cycle during treatment, was used as a depleting agent in conjunction with vascular thymus transplantation. We were able to deplete the thymus of thymocytes without damaging its microenvironment as witnessed by intact antigen profiles of stromal cells. Two models were used in this study, (1) regeneration after ADR induced depletion with or without BM reconstitution either systemically or intrathymically and (2) thymocyte turnover or regeneration in vascularly transplanted thymi. In the latter model either normal thymus was grafted into ADR treated recipient or ADR depleted thymus was grafted into normal recipient. These experiments clearly show that intact BM function is a prerequisite for intact continued cellularity of the adult thymus. Although the resident thymocyte population possesses some limited proliferating potential, it clearly does not seem to have a permanent self-renewing capacity of intrathymic stem cells.

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

In non-specifically immunized rats, bred under conventional conditions, the first 'spontaneous' germinal centres were observed by 21 days after birth. Deliberate antigenic stimulation led to an earlier appearance of germinal centres in neonatal spleen: immunization with sheep red blood cells as early as 7 days after birth resulted in germinal centre formation in the spleen as observed 7 days later. By that time the first primary follicles could also be observed, in both immunized and non-immunized rats. Although 3-day-old rats upon antigenic stimulation failed to generate germinal centres in their spleen, transfer experiments of 3-day-old spleen cells to lethally X-irradiated syngeneic adult recipients indicated that 3-day-old spleens at least contained all the essential lymphoid elements (B and T cells) needed for germinal centre formation. These results strongly suggest that the failure to induce germinal centres in 3-day-old rats is most probably due to an immaturity of their splenic microenvironment. Immunohistochemical staining of frozen sections of neonatal rat spleen using mAb ED 5 and MRC OX-2 showed that follicular dendritic cells (FDC) were found as soon as primary follicles were found (i.e. by 14 days after birth). The appearance of FDC in neonatal spleens was not influenced by deliberate antigenic stimulation nor by the administration of adult spleen cells. We postulate that, during the development of FDC, a splenic microenvironment is created that allows primary follicle formation and the generation of germinal centres