SummaryWe have analyzed B cell tolerance in a rheumatoid factor (RF) transgenic mouse model. The model is based on AM14, a hybridoma originally isolated from an autoimmune MRL/lpr mouse that has an affinity and specificity typical of disease-related RFs from this strain. AM14 binds to immunoglobulin (Ig)G2a of the "a" allotype (IgG2a a) and not to IgG2a u. Thus, by crossing the transgenes onto an IgH a (BALB/c) background or to a congenic IgH b (CB.17) background, we could study the RF-expressing B cells when they were self-specific (IgH 9 or when they were not self-specific (IgHb). These features make the AM14 model unique in focusing on a true autoantibody specificity while at the same time allowing comparison of autoreactive and nonautoreactive transgenic B cells, as was possible in model autoantibody systems such as anti-hen egg lysozyme. Studies showed that AM14 RF B cells can make primary immune responses and do not downregulate slgM, indicating that the presence of self-antigen does not induce anergy of these cells. In fact, IgH a AM14 transgenic mice have higher serum levels of transgene-encoded RF than their IgH b counterparts, suggesting that self-antigen-specific activation occurs even in the normal mouse background. Since AM14 B cells made primary responses, we had the opportunity to test for potential blocks to self-reactive cells entering the memory compartment. We did not find evidence of this, as AM14 B cells made secondary immune responses as well. These data demonstrate that a precursor of a disease-specific autoantibody can be present in the preimmune repertoire and functional even to the point of memory cell development of normal mice. Therefore, immunoregulatory mechanisms that normally prevent autoantibody production must exert their effects later in B cell development or through T cell tolerance. Conversely, the data suggest that it is not necessary to break central tolerance, even in an autoimmune mouse, to generate pathologic, disease-associated autoantibodies.

We have studied the regulation of anti–DNA B cells in transgenic mice with a heavy chain transgene (3H9H/56R). This transgene codes for a heavy chain that forms anti–double-stranded DNA (dsDNA) antibody when paired with most members of the endogenous Vκ repertoire, but certain L chains, referred to as Vκ editors, do not sustain dsDNA binding in combination with 3H9H/56R. In the nonautoimmune 3H9H/56R BALB/c, most B cells generated do not bind DNA because the transgene itself is edited or is associated with a Vκ editor. A minor population of B cells (30%) bind dsDNA and express the λ1 light chain (known to sustain 3H9H/56R DNA binding). These 3H9/56R/λ1 B cells coexpress a κ editor, and we propose that the down-regulation of the anti-DNA BCR caused by the dual L chain expression may prevent activation of this κ/λ population. These κ/λ B cells are sequestered in the marginal zone. Here, we studied the influence of autoimmunity on expression and regulation of 3H9H/56R. In 3H9H/56R MRL/lpr mice, the expression of anti-dsDNA is vastly accelerated. Anti–dsDNA B cells use noneditor κs but, in addition, most anti–dsDNA B cells have edited the heavy chain transgene. λ1 B cells (without the coexpression of a κ editor) are found and the κ/λ1 MZ population is absent. Our results suggest that improper editing and failure to sequester autoreactive B cells may contribute to the breakdown of tolerance in MRL/lpr mice.

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The nature of the immunoglobulin light chain affects peripheral B cell tolerance and autoreactivity.