Mucosal IgA is the most abundantly produced Ig upon colonization of the intestinal tract with commensal organisms in the majority of mammals. The repertoire of these IgA molecules is still largely unknown; a large amount of the mucosal IgA cannot be shown to react with the inducing microorganisms. Analysis of the repertoire of used H chain Ig (VH) genes by H-CDR3 spectrotyping, cloning, and sequencing of VH genes from murine intestinal IgA-producing plasma cells reveals a very restricted usage of VH genes and multiple clonally related sequences. The restricted usage of VH genes is a very consistent observation, and is observed for IgA plasma cells derived from B-1 or conventional B-2 cells from different mouse strains. Clonal patterns from all analyzed VH gene sequences show mainly independently acquired somatic mutations in contrast to the clonal evolution patterns often observed as a consequence of affinity maturation in germinal center reactions in peripheral lymphoid organs and Peyer’s patches. Our data suggest a model of clonal expansion in which many mucosal IgA-producing B cells develop in the absence of affinity maturation. The affinity of most produced IgA might not be the most critical factor for its possible function to control the commensal organisms, but simply the abundance of large amounts of IgA that can bind with relatively unselected affinity to redundant epitopes on such organisms.

We transferred peritoneal cells from BALB/c mice into C.B17 scid/scid mice. Six to eight months after injection, only cells with the B1 phenotype were retained in the spleens and peritoneal cavities of these mice. The lamina propria of the intestine contained many peritoneal, donor-derived, immunoglobulin A (IgA)-producing cells. The mesenteric lymph nodes of these mice were found to be a major site of proliferation and generation of IgA plasmablasts. We established eight IgA-producing hybridomas from the mesenteric lymph nodes of such mice, and all the hybridomas reacted with different but partially overlapping fecal bacterial populations. Cloning and sequencing of the V H genes of these hybridomas showed that two hybridomas utilized germ line-encoded V H genes while the V H genes of the other six hybridomas showed somatic mutations, some of which are indicative of an antigen-driven selection process.

These findings demonstrate that the expression of epithelial iNOS is highly bacterium-specific and correlates with the severity of disease, suggesting an important role for this enzyme in the development of IBD.

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