In Situ Studies of the Primary Immune Response to (4-Hydroxy-3-Nitrophenyl)Acetyl. V. Affinity Maturation Develops in Two Stages of Clonal Selection

Abstract

To examine the role of germinal centers (GCs) in the generation and selection of high affinity antibody-forming cells (AFCs), we have analyzed the average affinity of (4-hydroxy-3-nitrophenyl)acetyl (NP)-specific AFCs and serum antibodies both during and after the GC phase of the immune response. In addition, the genetics of NP-binding AFCs were followed to monitor the generation and selection of high affinity AFCs at the clonal level. NP-binding AFCs gradually accumulate in bone marrow (BM) after immunization and BM becomes the predominant locale of specific AFCs in the late primary response. Although the average affinity of NP-specific BM AFCs rapidly increased while GCs were present (GC phase), the affinity of both BM AFCs and serum antibodies continued to increase even after GCs waned (post-GC phase). Affinity maturation in the post-GC phase was also reflected in a shift in the distribution of somatic mutations as well as in the CDR3 sequences of BM AFC antibody heavy chain genes. Disruption of GCs by injection of antibody specific for CD154 (CD40 ligand) decreased the average affinity of subsequent BM AFCs, suggesting that GCs generate the precursors of high affinity BM AFCs; inhibition of CD154-dependent cellular interactions after the GC reaction was complete had no effect on high affinity BM AFCs. Interestingly, limited affinity maturation in the BM AFC compartment still occurs during the late primary response even after treatment with anti-CD154 antibody. Thus, GCs are necessary for the generation of high affinity AFC precursors but are not the only sites for the affinity-driven clonal selection responsible for the maturation of humoral immune responses.

IL-21 Receptor Is Critical for the Development of Memory B Cell Responses

“…The memory B cell response to the T-dependent Ag NP-CGG develops via a GC-dependent process culminating in the formation of Ag-experienced memory B cells and PCs (31,39). Classswitched NIP + IgG1 + GC B cells were present in normal numbers from days 14-35 in NP-CGG-immunized IL-21R.KO mice, further highlighting that signals delivered via IL-21R are not required for GC maintenance.…”

A genetic lesion that arrests plasma cell homing to the bone marrow

“…Recently, it has been shown that the inappropriate distribution of B cells outside the GC may result in the loss of censoring the development of autoreactive B lymphocytes (50). Work by Kelsoe and coworkers (51) have shown that affinity selection can occur outside the GC. Whereas it is clear that the BM plays a critical role in positive and negative selection during B cell ontogeny, the question emerges as to whether the BM also plays a role in the censorship of affinity-matured B cells before terminal differentiation to PCs.…”

Changing responsiveness to chemokines allows medullary plasmablasts to leave lymph nodes

“…The interaction of B cells presenting antigen with primed T cells occurs in the outer T zone and the adjacent regions of the B cell follicles [24,25]. Activated B cells either migrate to follicles, where they form germinal centers and undergo affinity maturation [30], or to medullary cords where they differentiate to plasmablasts and then plasma cells. The medullary cord plasmablasts to not undergo Ig V-region gene mutation [24,28].…”