Functional analysis of the mouse J chain

We have in this study analyzed the function of the murine Joining chain (J chain) in vivo. J chain is a polypeptide found in sera and secretions in complex with pentameric IgM and dimeric IgA. J chain is expressed in terminally differentiated B lymphocytes and needed for the transport of dimeric IgA and pentameric IgM over mucosal surfaces via the poly Ig receptor. A mouse deficient for J chain was generated and it was shown that J chain in vivo seems to be necessary for efficient assembly and secretion of polymeric IgM. Secondly, in the absence of J chain, antigen-specific IgA immunity in the lamina propria of the mucosa cannot mediate protection on the mucosal surface of the intestinal lumen; thereby demonstrating in vivo a direct relationship between mucosal transport of secretory IgA and intestinal immune protection. A second mouse was generated where the Diphtheria toxin A gene was introduced into the J chain locus, thereby ablating J chain expressing cells. Evidence was found for the existence of two separate plasma cell populations distinguishable by the absence or the presence of J chain. The dogma that in mice the J chain is expressed in all plasma cells irrespective of isotype has been challenged.