Differences in Human Germinal Center B Cell Selection Revealed by Analysis of IgVH Gene Hypermutation and Lineage Trees in Inflammatory Bowel Disease

Overview

Our overall objective in this study is to study the role of B cells in inflammatory bowel disease (IBD), using a combination of high-throughput experimental and novel bioinformatical techniques. Idiopathic IBD includes Crohn's disease (CD) and Ulcerative Colitis (UC), which are chronic inflammatory disorders of the intestine. IBD is common in developed countries, with up to 1 in 200 of individuals affected by theses diseases. It is currently thought that the disease arises owing to a complex array of genetic, environmental and immunologic susceptibility factors. T cells are thought to cause the lesions, but the B cell population apparently has a significant role as well, through secreting antibodies against certain self-antigens. We believe that a major contribution to the understanding of the pathogenesis of IBD, and especially of the immune pathway leading to CD, can be achieved by analysis of the B cell clones participating in immune responses in the gut, in particular their immunoglobulin (Ig) variable region gene diversity, which has never before been studied in the context of IBD. The adaptive immune system is one of the only two biological systems capable of continuously learning and memorizing its experiences. This is a highly complex, distributed system, in which pathogen recognition, decision-making and action are performed by an interacting network of diverse lymphocytes. Immune learning and memory are embedded in the dynamical states of the complete lymphocyte repertoire, and cannot be understood by studying the behavior of single cell types. This complexity, further increased by the non-linear behavior of each component, can only be elucidated by using theoretical tools to complement experimental and clinical studies. Needless to say, many aspects of the deregulation of lymphocyte clones are not evident in the phenotype of the single cell but rather in the population dynamics of a whole clone (or many clones) of cells, as in B cell lymphomas. Such aspects are best elucidated by studies of the population dynamics and genetics of the relevant B cell clone(s). In this study, we propose to utilize a novel bioinformatical approach - the analysis of the shapes of Ig gene mutational lineage trees. This is the main innovative feature in our proposal, as it taps into parameters that have never before been measured or analyzed with respect to B lymphocytes in IBD. While the method is new, it has already been shown that graphical analysis of B cell lineage trees and mathematical quantification of tree properties provide novel insights into the mechanisms of normal and malignant B cell clonal evolution. A preliminary analysis of lineage trees from other autoimmune diseases (shown below) indicates that, given sufficient amounts of data, the method could elucidate changes in Ig gene diversification and selection in IBD patients. Moreover, we aim to search for correlations between the parameters characterizing Ig gene diversification and parameters characterizing patients, disease history and severity, and histological markers, as this has the potential of yielding novel diagnostic and prognostic tools.