Abstract
It has been proposed that autoreactivity of modest affinity contributes to positive selection of a preimmunization B cell repertoire, whereas high-affinity autoreactivity leads to negative selection. This hypothesis predicts that a B cell producing a physiologically selected unmutated ssDNA-binding Ab should be a precursor of cells that respond to diverse exogenous Ags. To test this prediction, we prepared transgenic mice bearing the rearranged V(H) domain of an IgM Ab from a nonautoimmune mouse immunized with a DNA-protein complex, poly(dC)-methylated BSA. The Ab, dC1, binds both poly(dC) and ssDNA. It is encoded by V(H) and V(L) gene segments with no mutations, suggesting that the producing cell may have been selected before and activated during immunization. The dC1V(H) transgene was targeted to the IgH locus. In heterozygous mice, on a nonautoimmune C57BL/6 background, the transgene allotype was expressed on B cell surfaces and in serum Ig, but about one-third of B cells expressed the endogenous allele instead. Total serum Ig concentrations were normal and included both transgene- and endogenous gene-coded IgM and IgG. The transgene V(H) D(H)J(H) was expressed in splenic IgM cDNA with few or no mutations, and in IgG cDNA with multiple mutations. The transgene allotype was also expressed in Abs formed on immunization with thyroglobulin, pneumococcal polysaccharide, and ssDNA-methylated BSA. Consistent with the hypothesis, cells with a rearranged autoreactive V(H) domain selected for reactivity with a form of ssDNA did serve as precursors for cells producing IgM and IgG Abs to diverse Ags.
Highlights
It has been proposed that autoreactivity of modest affinity contributes to positive selection of a preimmunization B cell repertoire, whereas high-affinity autoreactivity leads to negative selection
The VH domain alone bound only weakly in the filter-binding assay but, at submicromolar concentrations, was able to bind to immobilized poly(dC), ssDNA, or poly(dT) (Fig. 3b)
The dC1VH was originally part of an anti-poly(dC) Ab, immunization with ssDNA yielded transgene-coded IgM that reacted selectively with ssDNA in preference to poly(dC), perhaps as a result of association with a different VL domain. These experiments reveal that B cells expressing the rearranged VH domain of a physiologically selected autoreactive Ab, with ssDNA-binding activity, can be precursors of cells that participate represent data from three mice of each group
Summary
It has been proposed that autoreactivity of modest affinity contributes to positive selection of a preimmunization B cell repertoire, whereas high-affinity autoreactivity leads to negative selection. Even some selective IgG Abs to Z-DNA (Ab Z22) [24] or poly(dC) [25] can be formed with few or no complementarity-determining region (CDR) mutations These findings probably reflect the nucleic acid-binding potential of numerous B cells that were selected even before immunization with exogenous Ag. For many anti-DNA Abs, the VH domain alone can bind ssDNA and polypyrimidines, with selectivity that is similar to that of the natural autoantibodies from LPS-stimulated cells [26, 27]. Stollar, unpublished data) can bind ssDNA and polypyrimidines This DNA reactivity of the VH domain may be a factor in positive selection of developing B cells in bone marrow
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