REACTION OF GERMINAL-CENTERS IN THE T-CELL-INDEPENDENT RESPONSE TO THE BACTERIAL POLYSACCHARIDE-ALPHA (1-]6)DEXTRAN

Primary immunization of BALB/c mice with a(1-->6)dextran (DEX), a nati ve bacterial polysaccharide, induces an unexpected pattern of splenic B-cell responses. After a peak of antibody-secreting B-cell response a t day 4, deposition of dextran-anti-dextran immune complexes, as revea led by staining with both dextran and antibodies to dextran, occurs an d persists in splenic follicles until at least the fourth week after i mmunization. Antigen-specific B cells appear and proliferate in such f ollicles, leading by day 11 to development of DEX-specific germinal ce nters as characterized by the presence of distinct regions of DEX+ pea nut agglutinin-positive (PNA+) cells. At this time, fluorescence-activ ated cell sorter analysis also reveals the appearance of a distinct po pulation of DEX+ PNA+ splenic B cells. In contrast, DEX+ PNA- cells, c haracterized by intense cytoplasmic staining, are present outside of s plenic follicles, peak at day 4 to day 5, and persist until at least d ay 28. The frequency of these cells correlates with DEX-specific antib ody-secreting cells, as detected by the ELISA-spot assay. Thus, in add ition to the expected plasma cellular response, the typical T-cell-ind ependent type II antigen, DEX, surprisingly also elicits the formation of antigen-specific germinal centers. These observations raise fundam ental questions about the roles of germinal centers in T-cell-independ ent immune responses.