In Situ Observation of Germinal Center Cell Apoptosis During a Secondary Immune Response

Germinal centers are highly organized anatomic structures essential for the clonal expansion of germinal center (GC) B-cells and associated somatic hypermutation, isotype switching, selection of the high-affinity B-cells (affinity maturation), and elimination of irrelevant or autoreactive clones. The identification of cellular interactions and regulatory mechanisms controlling apoptosis within GCs is essential for a complete understanding of the cellular and molecular dynamics of the GC reaction. We performed a kinetic analysis of the apoptotic activity occurring within GCs of draining lymph nodes of mice immunized with sheep red blood cells (SRBC) after secondary stimulation. The apoptotic activity of GC cells can be divided into three distinct phases : 1) initial phase (within the first days after immunization), 2) reactive phase (from the 5th day to 15th day after secondary immunization), and 3) late phase (after the 15th day). Apoptosis decreased shortly after secondary immunization followed by an increase to peak after an additional 10 days. Finally, apoptosis of GC cells decreased to basal levels. Administration of apoptosis inhibitors decreased the amount of apoptosis during the reactive phase. These results suggest that the reactive phase may be the critical period in which clonal selection and cellular differentiation to antibody forming cells take place. [J Clin Exp Hematopathol 46(2) : 73-82, 2006]