The Differences in Survival and Phenotype Between Centroblasts and Centrocytes

Affinity maturation of T cell-dependent antibody responses is achieved by two basic mechanisms within the germinal centers (G.C) : somatic mutation in Ig V genes (1-4) and subsequent positive selection based on the affinity for antigen binding (5). Since all these experiments were performed on mouse or human total G.C. B cells, there are several further questions needed to be addressed now: a) at what stage is somatic mutation machinery operating ? b) are somatic mutations and selection occuring at the same time or does selection occur after mutation? is there a negative selection at certain stages of G.C. reaction, to deal with auto-reactive B cell clones generated by somatic mutations ? Isolation of G.C. B cell subsets seems to be a key step to answer these questions. Classically, G.C. are divided into a dark zone and a light zone (6). The dark zone contains centroblasts which are large pyroninophilic or basophilic cells. Their nuclei are large round with multiple nucleoli. The light zone contains centrocytes which are large to medium sized cells and they have nuclei with distinct clefts (7). Using these criteria, the survival of these two types of G.C. B cells were studied during a short-term culture and they were finally isolated by centrifugal elutriation.