Scientific Report 2005

Immunology

Analysis of Immune Learning in B Lymphocytes

D. Nemazee, A. Gavin, D. Aït-Azzouzene, C. Huber, L. Verkoczy, J. Vela, B. Duong, P. Skog, M. Lim

The main goal of our research is to understand how lymphocytes distinguish between self and nonself antigens. Because antigen receptors on lymphocytes are assembled from component parts through an essentially random mechanism, many lymphocytes have self-reactive receptors. Regulation of such autoreactive specificities may be important to prevent autoimmune disease and to ensure efficient response to microbes.

The development of B lymphocytes is a multistep process punctuated by the somatic generation of genes for antibody heavy and light chains through DNA recombination, which is catalyzed by the products of recombinase activator gene 1 (RAG-1) and RAG-2. Because V(D)J recombination is imperfect and error prone, pre-B and B cells are endowed with sensing mechanisms to detect protein expression of heavy chains and assembled heavy and light chains (i.e., intact surface IgM). A major function of the expression of immunoglobulin in immature B cells is signaling to downregulate recombinase activity and to stimulate developmental progression. Newly formed B-cell receptors are also screened for autoreactivity. These quality control mechanisms rely on signaling by antigen receptors.

Previously, we showed that B cells with autoreactive receptors do not downregulate recombination because of excessive signaling through the antigen receptor, resulting in “receptor editing,” a process in which previously expressed genes for antibody light chains are inactivated and replaced by secondary DNA recombination. More recent data indicated that editing can also play an important role in inactivating and replacing receptor genes that are underexpressed at the protein level. In this situation, subnormal expression of unligated surface immunoglobulin does not provide a needed signal.

These recent results suggest that quality control of newly formed B lymphocytes is surprisingly stringent and that through recombinase regulation, B cells are often able to “repair” unacceptable light-chain genes by replacing the unacceptable genes with new genes. Because of the apparent efficiency of the editing process, we suspect that we have uncovered a major cellular “proofreading” pathway.

A key question of current interest is how signaling through the antigen receptor regulates editing. A major nuclear end point is the regulation of RAG transcription. We are assessing the biochemical signaling pathways by which the signal from antigen receptors regulates RAG transcription. Recent results suggested that NF-κB and rel transcription factors may be involved in both positive and negative regulation of the RAG genes. In addition, we are using DNA array analysis and other screening methods to look more closely at changes in gene expression during and subsequent to the receptor editing response. We found that a relatively small fraction of genes is differentially expressed, including a handful of previously uncharacterized mRNAs, which we are analyzing further.

In other studies, we focused on the cues that mature B cells use to distinguish self from nonself. Fully mature recirculating B cells can be rapidly inactivated and induced to apoptosis when confronted with tissue antigen, whereas the same cells are able to respond to antigens expressed by microbes. We are investigating both the death pathway involved in self-tolerance and the nature of the signals that prevent this pathway in responses to nonself antigens. Recently, we found that the ability of B cells to distinguish self from nonself in this setting is independent of T lymphocytes and instead most likely involves a novel pathway of self-recognition. We are exploring the idea that immune tolerance in mature B cells depends on specific costimulation by self-tissue.

Publications

Aït-Azzouzene, D., Verkoczy, L., Peters, J., Gavin, A., Skog, P., Vela, J.L., Nemazee, D. An immunoglobulin C_κ-reactive single chain antibody fusion protein induces tolerance through receptor editing in a normal polyclonal immune system. J. Exp. Med. 201:817, 2005.

Gavin, A., Aït-Azzouzene, D., Mårtensson, A., Duong, B., Verkoczy, L., Skog, J.L., Skog, P., Nemazee, D. Peripheral B lymphocyte tolerance. Keio J. Med. 53:151, 2004.

Gavin, A.L., Duong, B., Skog, P., Aït-Azzouzene, D., Greaves, D.R., Scott, M.L., Nemazee, D. ΔBAFF, a splice isoform of BAFF, opposes full-length BAFF activity in vivo in transgenic mouse models. J. Immunol. 175:319, 2005.

Peters, B., Sidney, J., Bourne, P., Bui, H.H., Buus, S., Doh, G., Fleri, W., Kronenberg, M., Kubo, R., Lund, O., Nemazee, D., Ponomarenko, J.V., Sathiamurthy, M., Schoenberger, S.P., Stewart, S., Surko, P., Way, S., Wilson, S., Sette, A. The immune epitope database and analysis resource: from vision to blueprint. PLoS Biol. 3:e91, 2005.

Verkoczy, L., Aït-Azzouzene, D., Skog, P., Mårtensson, A., Lang, J., Duong B., Nemazee, D. A role for nuclear factor κB/rel transcription factors in the regulation of the recombinase activator genes. Immunity 22:519, 2005.

Verkoczy, L.K., Mårtensson, A.S., Nemazee, D. The scope of receptor editing and its association with autoimmunity. Curr. Opin. Immunol. 16:808, 2004.