C1q production and C1q-mediated immune complex retention in lymphoid follicles of rat spleen

Involvement of C1q in retaining immune complexes in germinal centers in rat spleen was studied in vivo and in vitro. C1q production was found in fibroblastic reticulum cells in the peripheral mantle zone, in follicular dendritic cells in germinal centers, and in transitional forms between these two cells in the inner mantle zone. In passively immunized animals, immune complexes were found transiently on fibroblastic reticulum cells, then on the transitional forms and follicular dendritic cells. Extracellular C1q was detected by the presence of immune complexes on both the transitional forms and follicular dendritic cells, but not on fibroblastic reticulum cells. Thus, the fibroblastic reticulum cell appeared to trap immune complexes but not to retain either immune complexes or C1q. The morphology and function of the fibroblastic reticulum cell and the follicular dendritic cell suggest that they belong to the same lineage. Immune complexes were bound in vitro to germinal centers in cryostat spleen sections in the same manner as those retained in vivo. The binding required no complement in the incubation medium and was inhibited by C1q-suppressing factors. The extracellular C1q originating from the follicular cells may therefore play a role in retaining immune complexes in the germinal center.