J chain gene transfer: Evidence that J chain regulates IgM assembly and secretion

• Main Author: Niles, Mary Jane
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1992
• Subjects: Cellular biology
• ISBN: 9798208707234

In the primary humoral response, mature resting B cells are stimulated by antigen and T cell-derived cytokines to divide and to differentiate into plasma cells secreting pentamer IgM, $\rm(\mu\sb2L\sb2)\sb5J.$ The immunoglobulin J (joining) chain is the third component of the IgM pentamer and its gene is expressed in response to signals from interleukin-2 and interleukin-5; its regulated expression is therefore a critical event in the development of an IgM-secreting plasma cell. Indeed, the synthesis of J chain is the only differentiative change detected in cell lines responsive to these interleukins. While the requirement for J chain in pentamer assembly has been demonstrated by in vitro studies of IgM polymerization, by analyses of pentamer assembly in normal B lymphocytes and by somatic cell hybrids, questions remain. First, given intracellular secretory IgM (IgM$\sb{\rm s}),$ is J chain the final component required for secretion, or are other, as yet undefined, lymphokine-induced events required? Second, does J chain itself induce changes which contribute to IgM secretion? In order to address these questions and to directly evaluate the role of J chain in pentamer assembly and secretion, the murine J chain cDNA was cloned, placed in the context of an expression plasmid and introduced by stable transfection into mouse cell lines representing pre-secretor, mature and immature stages of B cell development. Analysis of J chain transfectants and their non-transfected counterparts revealed that expression of the J chain induces secretion of pentameric IgM, even at the immature B cell stage, as long as the cell is synthesizing some monomer of the secreted form. The level of IgM secretion achieved by each transfectant was found to be limited by the amount of J chain provided by the integrant gene. Introduction of J chain, then, mimics interleukin-mediated signals that establish IgM secretion: the primary effect of IL-2 and IL-5 is activation of the J chain gene, and J chain, in turn, is critical to the assembly and transport of IgM. Analyses of J chain transfectants also revealed that the expression of J chain mRNA and protein induces no change in immunoglobulin levels nor alterations in $\rm\mu\sb{m}{:}\mu\sb{s}$ ratios. Furthermore, all of the B cell lines examined displayed at least a rudimentary capacity for secretion; given J chain and IgM$\sb{\rm s},$ the cells assemble and secrete pentamer IgM without any detectable change in the secretory apparatus. Thus J chain induces pentamer IgM secretion but does not itself direct any other developmental changes characteristic of terminal differentiation.