Long-term antigen retention by dendritic cells in the popliteal lymph node of immunized mice

Antigen retention by follicular dendritic cells (FDC) was studied in the popliteal lymph nodes (PLN) of mice actively or passively immunized against human serum albumin (HSA) or horse spleen ferritin. Electron microscopic autoradiography was used to locate a challenge dose (1 microgram) of 125I-labelled HSA in the draining PLN following injection into the hind footpad of specifically immune mice. In both actively and passively immunized mice, the radiolabelled antigen was localized to the follicles in the cortex of the draining node. In actively immunized mice, the antigen formed a crescent of label on the superficial aspect of germinal centres, while in passively immunized mice label was seen in primary follicles. In electron microscope autoradiographs, the silver grains were concentrated in areas of dendritic cell processes which emanated from a cell body containing a characteristic irregular nucleus. The size and complexity of the dendritic cell processes increased in actively immunized mice suggesting that the FDC could hypertrophy. High resolution studies using the electron-dense antigen, ferritin, showed that it was localized to the extracellular space of the FDC processes and was associated with amorphous electron-dense material; presumably immune complexes. Antigen was not present uniformly distributed in the extracellular material but rather it was in discrete patches occupying small segments of the FDC processes. Large amounts of label-free electron-dense material were present suggesting that immune complexes of various specificities were present on each DC. Ferritin was seen more than 3 months after challenge but only on the FDC. The data suggest that the antigen retaining mechanism in the lymph node of immune mice is antigen non-specific, is capable of hypertrophy in response to active immunization and provides a mechanism for stable long-term retention of antigen.