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The Nemazee Lab

RESEARCH

Analysis of Immune Learning in B Lymphocytes

Our research goal is to understand how lymphocytes distinguish between self and nonself antigens. Because antigen receptors on lymphocytes are generated through an essentially random process, the generation of self-reactive cells most likely is a frequent occurrence. Regulation of such autoreactive specificities may be important to prevent autoimmune disease and to ensure efficient response to foreign microbes.

The development of B lymphocytes is a multistep process punctuated by the somatic generation of antibody heavy- and light-chain genes through DNA recombination, which is catalyzed by the recombinase activator gene-1 and recombinase activator gene-2 products. Because the 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 are 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 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.

Current studies aim to determine the signaling pathways involved in translating a B cell receptor signal to the recombinase machinery and in determining the immunological consequences of defects in receptor editing. Mice genetically deficient in editing are being developed. These studies may shed light on the origins of autoimmune diseases that involve autoantibody production.

Because receptor editing is associated with a developmental block, we are using DNA array analysis and other screening methods to look more closely at changes in gene expression during and after receptor editing. 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 use of this pathway in responses to nonself antigens. 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.

In collaboration with colleagues from TSRI, we are also engaged in studies assessing the role of immune tolerance in HIV vaccinology. Specifically, we have generated gene targeted mice expressing broadly neutralizing HIV antibodies 4E10 and b12 to assess their propensity to be self reactive and as a platform to test and rank HIV vaccine candidates for the ability to stimulate B cells capable of producing neutralizing antibodies.