Leukocyte Migration and Selection Mechanisms in Lymphoid Organs
Background: 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 antigen-specific cells are far from understood.
Major goals: (i) define the molecular cues that guide immune cell migration and interactions in lymphoid organs; (ii) visualize immune response dynamics using advanced imaging approaches; (iii) define the selection mechanisms that underlie the antibody affinity maturation program and that help prevent autoantibody production; (iv) characterize the requirements for mounting mucosal IgA responses.
Chemokines and Oxysterols 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. Recently we identified a role for an intercellular signaling lipid, the oxysterol 7a,25-dihydroxycholesterol, in guiding B cell movements and supporting humoral immune responses. Efforts are ongoing to understand how production of this oxysterol is regulated and what additional influences it has on immune function. We also continue a discovery program to identify unaccounted for cues that control several immune cell migration and interaction processes.
Antigen Encounter Dynamics: Using real-time 2-photon microscopy we have visualized the dynamics of B cell–antigen encounters in intact lymphoid organs. This work highlighted new roles for sinus-lining macrophages and follicular dendritic cells (FDCs). The properties of these accessory cells that facilitate their roles in antigen display and as niche organizers are under continued investigation.
Lymphocyte Egress and S1P: A lot is known about how cells get into tissues from blood, but much less is understood about how they get out. Yet they must get out to mediate their effector function at sites of infection or autoimmune inflammation. We have shown 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, S1PR1, they are unable to leave the thymus or peripheral lymphoid organs. Our studies helped define the mode of action of a compound, FTY720, that inhibits lymphocyte egress and was recently approved for treatment of multiple sclerosis. On-going studies, involving genetic screens and cell biological and real-time imaging approaches, are aimed at defining how S1PR1 instructs lymphocytes to exit lymphoid organs.
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 and for S1P, acting via the migration inhibitory S1PR2 receptor, in promoting niche confinement. Using real-time imaging 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 and autoreactive clones - occurs. We are also aiming to apply our developing knowledge about B cell response dynamics to improving vaccine design, focusing on influenza and HIV-1 antigens.
Mucosal Immunity: IgA is the major antibody isotype produced in the body, yet the mechanism of mucosal B cell encounter with gut antigens and the factors controlling IgA isotype switching are incompletely understood. We are applying approaches used to study splenic and lymph node B cell responses to characterize the requirements for mounting IgA responses against commensal flora and intestinal pathogens.