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Complement‐mediated follicular localization of T‐independent type‐2 antigens: the role of marginal zone macrophages revisited

In this study we demonstrate a hitherto undescribed phenomenon, namely that thymus‐independent type‐2 antigens (TI‐2 Ag) localize in splenic follicles within 1 h after administration. The follicular localization of 2,4,6‐trinitrophenyl (TNP)‐Ficoll was not antibody mediated. In addition in case of high‐dose administration we observed a relatively large amount of TI‐2 Ag in marginal zone macrophages. However, after low‐dose administration we observed a preferential localization of TNP‐Ficoll in the splenic follicles. Detection of TNP‐haptenated Ag in cryostat sections of murine spleens was performed with a high‐affinity TNP‐specific monoclonal antibody conjugated to β‐galactosidase. Within minutes after injection the TI‐2 Ag localized in the marginal zone, attached to marginal zone macrophages and B cells. Twenty minutes after injection the Ag was also detected in the follicles and gradually accumulated there until 7 h after injection. Thereafter, the amount of follicular Ag gradually decreased but was still detectable up to 14 days after immunization. The follicular localization of TNP‐Ficoll was complement dependent in contrast to the binding to and uptake by marginal zone macrophages. Double staining revealed that Ag was bound by macrophages, B cells and follicular dendritic cells. Haptenated thymus‐dependent (TD) Ag localized exclusively in the red pulp macrophages. In vivo macrophage elimination drastically increased the amount of TNP‐Ficoll in the follicles, and enhanced the humoral immune response at low doses of Ag. Moreover, complement deprivation of mice abrogated the localization of TI‐2 Ag in the follicles, and led to a decreased humoral TI‐2 immune response. In conclusion, we demonstrate for the first time that TI‐2 Ag localize in follicles. Moreover, the presented results provide further evidence that B cells and follicular localized Ag play an important role in the induction of humoral TI‐2 immune responses.