The functions of immune T and B rosette-forming cells.

From 7 to 35 days after CBA mice were primed with SRBC their spleens were removed and anti-SRBC rosettes were formed. The rosettes were purified from other spleen cells by velocity sedimentation at 4° and rosette-enriched, rosette-depleted and various control populations were injected into lethally irradiated CBA recipients. These were challenged with SRBC and their spleens analysed for direct (IgM) and enhanced (IgG) PFC 7 days later. Removal of RFC depleted the primed spleen cells of their capacity adoptively to transfer an immune response. This depletion was antigen-specific. Purified rosettes alone prepared 7–8 days after priming transferred significant (relative to controls) immune reactivity to the irradiated recipients. Both B and T RFC were present at this stage and the response was dependent upon cell collaboration between these two populations. Later in the primary response (35 days) purified rosettes transferred negligible immune reactivity. But these RFC (90 per cent B) collaborated with rosette-depleted cells to restore full reactivity. B memory lymphocytes (AFCP) form rosettes from 7 to 35 days after immunization but T memory cells only do so for a limited stage during the peak of the primary response. The majority of T memory cells probably never form rosettes in this system. It is suggested that most T RFC may be cells mediating delayed hypersensitivity or are `passive' rosettes.