Jason Cyster, Ph.D.
The rapid induction of protective antibodies is critical for host defense against pathogens. Reciprocally, unwanted antibody responses against self-components (antigens) are a cause of autoimmune disease. To mount antibody responses, antigen-specific B and T cells that may be as rare as 1 in 100,000 cells must first encounter the antigen and then interact with each other. These encounters occur within peripheral lymphoid organs - lymph nodes, spleen, and mucosal lymphoid tissues - but the mechanisms that control lymphoid cell migration and that promote interactions between rare antigen-specific cells are far from understood.
(i) define the molecular cues that guide B and T cell migration and interactions in lymphoid organs during the immune response; (ii) characterize selection events required for induction of high affinity antibody responses; (iii) determine how autoreactive B cells are regulated.
Chemokines as lymphoid tissue organizers:
Chemokines are small secreted chemoattractive proteins that signal via heterotrimeric G-protein coupled receptors (GPCRs). We have demonstrated that several chemokines are expressed in lymphoid organs and function in guiding lymphocyte migration. Transgenic and gene knockout experiments in mice are further defining the organizing functions of these chemokines. Efforts are on-going using microarray, candidate gene and expression cloning approaches to define novel chemoattractant factors involved in promoting immune cell interactions during the immune response. Based on our work so far, groups in industry are developing small molecule inhibitors of lymphoid chemokine receptors to be tested as modulators of the immune response.
Lymphocyte Egress and the role of S1P:
A lot is known about how cells get into tissues from blood, but little is understood about how they get out. Yet they must get out to mediate their effector function at sites of infection or autoimmune inflammation. Evidence has emerged that the blood lipid, sphingosine-1-phosphate (S1P), is involved. Like chemokines, S1P signals via GPCRs, and we have found that if lymphocytes lack one of the five known S1P receptors, S1P1, they are unable to leave the thymus or peripheral lymphoid organs. A compound, FTY720, that disrupts the function of this receptor inhibits lymphocyte egress and is currently in clinical trial as an immunosuppressant for treatment of multiple sclerosis. On-going studies, involving molecular, biochemical and genetic approaches, are aimed at defining how this GPCR instructs lymphocytes to exit lymphoid organs. Findings from this work are likely to point to new therapeutic targets for inhibition of unwanted immune responses.
Affinity Maturation and the Germinal Center response:
Although first identified in the 1800's, the inner workings of the germinal center - the site of antibody affinity maturation - are still poorly understood. We have found a role for chemokines in organizing the structure into light and dark zones. Using 2-photon microscopy and real-time imaging of intact lymph nodes we have begun to characterize B and T cell migration and interaction dynamics during the selection events associated with this tightly regulated response. Future studies will combine perturbations in organizing molecules with methods to measure affinity maturation and imaging approaches to further define how selection of high affinity clones - and elimination of low affinity clones - occurs. This work has direct implications for methods to improve vaccine design.
Elimination of autoreactive B cells:
Lymphocytes are dependent on survival factors to protect them from programmed cell death. A major B cell survival factor is the cytokine, BAFF. We have found that autoantigen-engaged B cells have increased dependence on BAFF and as a consequence they compete poorly with other B cells for survival. These findings help explain why elevations in BAFF production are associated with autoimmune disease in humans. The increased BAFF dependence is due to elevated expression of the pro-apoptotic bcl-2 family member, BIM. In future studies we aim to define how BAFF receptor signaling antagonizes BIM function.