Ronald B. Corley, Ph.D.
Professor and Chairman of Microbiology
B.S. Duke University
Ph.D. Duke University
Our laboratory is interested in the functions of secretory antibody and in the changing functions of B lymphocytes during their development and differentiation. Our studies on secretory antibody structure and function focus on IgM, the first antibody secreted during adoptive immune responses. IgM is a polymeric antibody that is normally secreted by plasma cells in the pentameric form, but IgM can be secreted in other polymeric forms as well, IgM monomers and IgM hexamers. These latter two polymers are associated with certain disease states, but their role is not understood. We have been interested in the roles of these different polymeric forms, and in understanding the unique features of pentameric IgM that not only make it the “favored” IgM product, but distinguish its function from other Ig isotypes. Using both in vitro and in vivo systems, we have identified the elements that control IgM assembly, defined the subunits that modulate its assembly, and begun to identify the distinct functions of the different polymeric forms of IgM.
Figure 1. Structure of polymeric IgM. The primary structure of the secretory µ heavy chain and the associated light chain is shown, together with the 3 forms of IgM polymers found in vivo.
Our current efforts focus on the role of IgM in initiation the immune response. Mice that are deficient in secretory IgM give a delayed protective response to pathogens and respond poorly to low doses of antigens. To help to understand the immunodeficient phenotype of these animals, we are investigating the role that IgM plays in facilitating the trapping of antigen in secondary lymphoid organs, and in enhancement of primary and secondary immune responses. We are defining the requirements for antigen trapping in lymphoid follicles, and the transport of immune complexes onto the follicular dendritic cells, cells that sequester antigen and serve as focal points for germinal center responses. As part of these studies, we are constructing mouse models in which complement-fixing and non-fixing secretory IgM is expressed in order to understand more precisely why secretory IgM cannot be entirely compensated by other immunoglobulin isotypes, as well as to understand its function in disease.
Figure 2. Trapping of IgM-containing immune complexes in wildtype (B6), secretory µ-deficient (µs+) and complement receptor-deficient (Cr2+) mice. NP + P: immune complexes of antigen and IgM pentamers. NP: antigen alone. Note antigen trapping (NP; blue stain) in follicles on FDC in B6 and µs+ mice, while trapped antigen remains within the marginal zones in Cr2+ mice, indicating a role for complement receptors in FDC localization. Staining for the B cell marker B220 and the T cell marker CD3 are shown in red.
As part of our studies of B lymphocyte development and differentiation, we have been analyzing the changing roles of the NF-kB complex of transcription factors at different times during B cell differentiation. Using model B cell lines, we have found that the NF-kB complex is activated differently depending on the differentiation status of the cells. In primary B cells, the activation of NF-kB uses the “classical” pathway, where the IkB kinase (IKK) complex phosphorylates IkBalpha and IkBbeta, resulting in their degradation. However, more differentiated B cells, characterized by expression of the IgG B cell receptor, do not involve this mechanism, but rather may rely on the change in steady state levels of a third IkB component, Ikepsilon. We are determining the role that this “switch” in NF-kB activation plays in the immune system in conventional B cells, and defining the functional consequences of this switch for gene expression in B cells.
Figure 3. Degradation of IkBalpha in IgM+ (WEHI231, CH31, and CH12) compared with IgG+ (M12, A20) B cell lines. A. Pulse chase analysis of IkBalpha proteins, resolved by SDS-PAGE. B. Calculated half-life of IkBalpha in each of the cell lines, determined by phosphoimaging.