Investigation of Disease-Related T Cell-B Cell Collaboration Events

Abstract

Orchestrated collaboration between T cells and B cells is crucial for antibody-mediated protection against pathogens but can also underpin the generation of pathogenic B cells that drive chronic inflammatory diseases. The extent of T-B collaboration is tightly regulated to promote affinity maturation and prevent autoreactivity, however the mechanisms by which this regulation occurs remain unclear. T-B collaboration also generates a range of pathogenic B cells that vary in phenotype and function, and a more comprehensive understanding of specific B cell subsets is needed to develop targeted treatments for B cell-mediated diseases. By studying the humoral response in mice in which the phosphatase PTEN is selectively deleted in the regulatory T cell (Treg) compartment, we have demonstrated that Tregs are required for stringent T follicular helper (Tfh) cell-mediated selection of germinal center (GC) B cells. Immunization of these mice with a model T-dependent antigen revealed a defect in affinity maturation caused by aberrant antigen-driven selection, likely due to uncontrolled and abundant Tfh cell help. Promiscuous selection of GC B cells resulted in enhanced somatic hypermutation and a clonally diverse repertoire, suggesting that specifically inhibiting Tregs could broaden the responding B cell repertoire during vaccination when recognition of subdominant epitopes is critical for protection. T-B collaboration events outside of the GC, even though they do not result in high affinity antibodies, may nevertheless also contribute to disease. We have identified a potential murine counterpart of human disease-related IgD- CD27- “double negative” (DN) B cells. These IgD-CD148lo murine B cells are expanded in chronic infection and autoimmunity but not after immunization of healthy mice with T-dependent antigens; they are class switched to IgG and exhibit low levels of somatic hypermutation and aberrant selection, suggestive of an extrafollicular origin. These murine B cells closely resemble a previously defined subset of IgD-CD27-CD11c+ DN B cells that are expanded in lupus and also presumed to be of extrafollicular origin. By using TCRb-/-TCRd-/- mice we show that IgD-CD148lo B cells are dependent on T cell help and highlight the utility of manipulating murine counterparts of human immune cells to answer fundamental questions about human disease.