The ontogeny of germinal centre forming capacity of neonatal rat spleen.

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.