Does the availability of either B cells or CD4+ cells limit germinal centre formation?

To determine whether the availability of either B cells or T cells regulates the number and size of germinal centres observed following antigenic challenge, irradiated PVG rats were reconstituted with limiting numbers of thoracic duct B cells (with excess T cells) or with limiting numbers of thoracic duct CD4+ cells (with sufficient B cells). Germinal centre formation was measured histologically in the spleens of reconstituted rats 7 days after immunization with 2 x 10(9) sheep erythrocytes (at the height of the germinal centre reaction). Although reconstitution with B cells and antigen alone, or with thymocytes and antigen alone, produced no germinal centres, saturating numbers of germinal centres on the order observed for normal, non-irradiated rats were observed in irradiated rats reconstituted with only 10(7) B cells and 5 x 10(6) CD4+ cells. Germinal centres in the spleens of minimally reconstituted rats were also comparable in size to those observed in normal rats. Reconstitution with either 4 x 10(7) thoracic duct lymphocytes (including 2 x 10(7) B cells, as well as CD8+ and CD4+ cells) or with 4 x 10(7) thoracic duct lymphocytes and 2 x 10(8) thymocytes also led to saturating numbers of germinal centres. It is concluded therefore that (i) CD4+ cells are sufficient for the T-cell contribution required for germinal centre formation, and (ii) some factor other than the availability of B cells or CD4+ cells normally limits germinal centre formation.