Relaxed Negative Selection in Germinal Centers and Impaired Affinity Maturation in bcl-x_L Transgenic Mice

The role of apoptosis in affinity maturation was investigated by determining the affinity of (4-hydroxy-3-nitrophenyl)acetyl (NP)-specific antibody-forming cells (AFCs) and serum antibody in transgenic mice that overexpress a suppressor of apoptosis, Bcl-x_L, in the B cell compartment. Although transgenic animals briefly expressed higher numbers of splenic AFCs after immunization, the bcl-x_L transgene did not increase the number or size of germinal centers (GCs), alter the levels of serum antibody, or change the frequency of NP-specific, long-lived AFCs. Nonetheless, the bcl-x_L transgene product, in addition to endogenous Bcl-x_L, reduced apoptosis in GC B cells and resulted in the expansion of B lymphocytes bearing VDJ rearrangements that are usually rare in primary anti-NP responses. Long-lived AFCs bearing these noncanonical rearrangements were frequent in the bone marrow and secreted immunoglobulin G₁ antibodies with low affinity for NP. The abundance of noncanonical cells lowered the average affinity of long-lived AFCs and serum antibody, demonstrating that Bcl-x_L and apoptosis influence clonal selection/maintenance for affinity maturation.