Death of germinal center B cells without DNA fragmentation

Abstract

During the selection of B cells within germinal centers (GC) on the basis of their affinity for T‐dependent antigen, B cells not positively selected are eliminated within GC. This process of B cell death has been considered to be apoptosis. In a recent study, we have reported that, although a substantial number of thymocytes were considered to be dead because of their extremely small cell size and heavy chromatin condensation even though they were not yet phagocytosed (pyknosis), they were devoid of DNA fragmentation, the most characteristic feature for apoptosis. In this study, we examined in vivo the mechanism of B cell death within GC by using the terminal deoxynucleotidyl transferase (TdT)‐mediated dUTP‐biotin nick end‐labeling (TUNEL) method to detect DNA double‐strand breaks. TUNEL⁺ B cells were scattered throughout the upper dark and the light zones of GC. Double staining of the sections by the TUNEL method and acid phosphatase (AcP) activities showed that all the TUNEL⁺ B cells were phagocytosed by macrophages. Light microscopic and ultrastructural studies revealed the presence of small unphagocytosed B cells within the light zone. These cells are undoubtedly dead because they were much smaller than surrounding lymphoid cells and have a heavy chromatin condensation. Furthermore, ultrastructural detection of DNA fragmentation confirmed that these small unphagocytosed B cells were TUNEL⁻, implying that DNA fragmentation is not primarily involved in the cell death process of these small dead B cells. These results indicate that most B cells, not positively selected and thus destined to be eliminated, die within GC without DNA fragmentation, and are subsequently phagocytosed by macrophages and become TUNEL⁺. Typical apoptosis, characterized by DNA fragmentation in situ, is not the predominant type of cell death that occurs during the selection of B cells in GC.