Dissertations, Theses, and Capstone Projects

Date of Degree

2-2014

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biochemistry

Advisor

Laurel A. Eckhardt

Subject Categories

Biochemistry | Immunology and Infectious Disease

Keywords

Allelic exclusion, Autoimmunity, B lymphocyte

Abstract

The immunoglobulin heavy chain locus (Igh ) intronic enhancer, Eμ, enhances transcription of recombined Igh genes. We have previously shown that in mice with an Eμ-deficient Igh allele (V H δa ), Igμ is expressed at half of the wild-type levels in pre-B cells. We also described an Eμ-dependent "check-point", operating at the pre-B to immature B cell transition, for heavy chain allelic exclusion. We now show that deletion of Eμ results in a smaller immature B cell compartment, and the pre-BCR/BCR signaling is diminished in pre-B cells as a result of the reduced Igμ levels, making it difficult for emerging BCRs to reach the signaling threshold required for positive selection of pre-B cells to the immature B cell stage. Our hypothesis is that, to circumvent the problem of inadequate signaling, E-μdeficient B cells either 1) expand the rare precursor B cells seemingly breaking the rules of allelic exclusion to express a second IgH allele as "double-producers", to achieve higher levels of Ig μ-chain and hence higher pre-BCR and BCR levels, or 2) undergo heightened light-chain editing to create an IgH/IgL combination with superior signaling properties to make up for the lower Ig μ-chain (lower BCR) levels and signaling. To test these hypotheses and to determine whether escape from the developmental defects in Eμ-deficient B cells is dependent upon light chain, we provided the Eμ-deficient mice with a pre-assembled VL gene (3-83Vκ). This led to not only a larger immature B cell compartment, but also a decrease in "double-producers". We suggest that an IgH/IgL combination with superior signaling properties may compensate for the reduced BCR levels and eliminate the selective advantage of "double-producers". We also find that "double-producers" in Eμ-deficient heterozygous mice (V H δa /WT b ), include a subpopulation with autoreactive BCRs. We infer that the BCRs with IgH from the VH δa allele are ignored during negative selection at the pre-B to immature B cell transition, due to their low density. Instead, the double-producers are both positively and negatively selected on the basis of BCRs with IgH from the alternate allele (with Eμ intact). Taken together, these results suggest that Eμ functions to ensure sufficient Igμ(IgH) levels at the pre-B to immature B transition, which has an important impact on the maintenance of heavy chain allelic exclusion, the breadth of the BCR repertoire, and the elimination of autoreactive B cells.

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