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An effective immune system requires maximal B and T cell receptor diversity to eliminate a wide array of foreign antigens. Inherent in the generation of such diversity is the danger of failing to eliminate or silence autoreactive cells that may lead to autoimmune disease. Tolerance mechanisms such as failure to recruit antigen-specific T-help (due to T cell tolerance mechanisms), clonal deletion, receptor editing, and anergy prevent autoreactive B cells from participating in antigen-mediated responses. A main focus in the laboratory is to understand the mechanisms of tolerance in systemic lupus erythematosus (SLE) and to elucidate how these mechanisms fail during the onset of autoimmune disease. SLE is an autoimmune disease characterized by the production of autoantibodies specific for nuclear antigens such as double-stranded DNA, single-stranded DNA, histones and ribonucleoproteins such as Sm. Typically, B cells expressing autoreactive receptors are either eliminated or silenced thereby ensuring that autoantibodies are not produced. We are interested in defining how autoreactive B cells overcome the tolerance mechanisms that normally prevent their development into antibody secreting cells.
Recent studies have described that tolerance to the nuclear antigen Sm is reversible and mediated by dendritic cells. Although dendritic cells are known to mediate T cell tolerance, a role in B cell tolerance has not been previously described. We find that regulatory dendritic cells (rDCs) repress Sm-specific B cells by a dual mechanism. First, rDCs secrete IL-6 upon ligation of Toll Like Receptors. IL-6 selectively represses B cells that have continuously engaged self-antigen (chronically antigen experienced autoreactive B cells) but does not affect naïve B cells. This mechanism of selective repression is important during times of polyclonal activation when bacterial or viral infection activates the innate immune response to promote polyclonal T cell activation and polyclonal expansion of B cells. Under these conditions, selective repression allows the humoral immune response to occur in the absence of autoimmunity. A second mechanism of repression occurs when autoreactive B cells engage self-antigen on the surface of rDCs. It is widely known that dendritic cells process and present peptide that are recognized by T cells. Our data indicate that rDCs also express nuclear self-antigen that is recognized by B cells. Expression of self-antigen by the rDCs co-localizes B cells and regulatory dendritic cells and provides a source of antigen to desensitize the B cell antigen receptor (BCR).
Current studies in the lab are focused on identifying how IL-6 represses immunoglobulin secretion by autoreactive B cells. In addition, we are assessing if IL-6-mediated repression is defective in lupus prone mice (MRL/lpr and NZB x NZWF1) and in patients suffering from SLE. Another project focuses on characterizing partial tolerance and addressing why B cells specific for some antigens are completely tolerized while B cells specific for other antigens are prone to dysregulation.
A second focus within the laboratory is to understand the molecular basis of B cell receptor desensitization. The B cell receptor complex enters a desensitized state following antigen ligation. This state is characterized by receptors that remain competent to bind antigen but fail to renew signal transduction. Receptor desensitization maintains cellular unresponsiveness while the B cell awaits costimulation by T cells. We have recently described a potential mechanism of receptor desensitization wherein antigen ligation of the BCR leads to destabilization of the BCR complex. This results in surface mIg that is capable of binding antigen but lacks the associated Ig-alpha/Ig-beta transducer complexes. This observation suggests that BCR destabilization may be responsible for the desensitized phenotype of B cells that have encountered antigen in the absence of costimulation. We hypothesize that following antigen ligation of the BCR, activated kinases phosphorylate the BCR components (mIg, Ig-alpha, Ig-beta) leading to destabilization or dissociation of the complex. This results in a cell that is unresponsive to further antigen ligation while awaiting coreceptor stimulation.
Our current work centers on defining the molecular basis of BCR destabilization, and characterizing the mechanism by which the stability of BCR complex influences the ability of B cells to transduce signals. Understanding how the BCR complex becomes dissociated has potential therapeutic application for intervening in B cell-mediated autoimmune diseases and leukemia's such as lupus erythematosus, rheumatoid arthritis and Hodgkin's lymphoma. Ongoing research projects include assessing whether BCR destabilization represents a complete dissociation of mIg from Ig-alpha/Ig-beta or a biochemical destabilization of the complex. These studies will use biochemical techniques such as chemical crosslinkers and immunoprecipitation of receptor complexes. In addition we are using fluorescence microscopy and electron microscopy to visualize the components of the receptor complex following receptor aggregation and to assess the role of BCR destabilization in receptor-mediated endocytosis and antigen processing.
Williams JC, Wagner NJ, Earp HS, Vilen BJ, Matsushima GK (2010). Increased hematopoietic cells in the mertk-/- mouse peritoneal cavity: a result of augmented migration. J Immunol. 184(12):6637-48.
Cambier, J.C., S.B. Gauld, K.T. Merrell and B.J. Vilen. Understanding B-cell anergy: from mouse models to naturally occurring anergic B cells. Nature Reviews Immunology.
Carnathan, D.G, M.A. Kilmon, M. Borrero and B.J. Vilen. (2006) Autoreactive B cells recognize nuclear self-antigen displayed on dendritic cells and macrophages. submitted.
Kim J.-H., Rutan, J.A. and Vilen B.J. The transmembrane tyrosine of u-heavy chain is required for BCR destabilization and entry of antigen into clathrin-coated vesicles. submitted.
Kilmon M.A., A. Garland, K. Aviszus, L. Wysoki, and B.J. Vilen. Macrophages prevent the differentiation of autoreactive B cells by secreting CD40 ligand and IL-6. Blood, in press.
Gilbert, M.R., D.G. Carnathan, P.C. Cogswell, A.S. Baldwin Jr., B.J. Vilen. (2007) Dendritic cells from lupus-prone mice are defective in repressing autoreactive B cells. J. Immunol 178:4803-4810.
M.A. Kilmon, J.A. Rutan, S.H. Clarke, and B.J. Vilen. 2005. Low-affinity, Smith antigen-specific B cells are tolerized by dendritic cells and macrophages. J. Immunol. (Cutting Edge) 175:37-41.
J.-H. Kim, L.A. Cramer, H.A. Mueller, B.S. Wilson and B.J. Vilen. 2005. Independent trafficking of Ig-alpha/Ig-beta and mu-heavy chain is facilitated by dissociation of the BCR complex. J. Immunol. 175:147-154.