Deletion of IgG-switched autoreactive B cells and defects in Fas(lpr) lupus mice.

During a T cell-dependent Ab response, B cells undergo Ab class switching and V region hypermutation, with the latter process potentially rendering previously innocuous B cells autoreactive. Class switching and hypermutation are temporally and anatomically linked with both processes dependent on the enzyme, activation-induced deaminase, and occurring principally, but not exclusively, in germinal centers. To understand tolerance regulation at this stage, we generated a new transgenic mouse model expressing a membrane-tethered gamma2a-reactive superantigen (gamma2a-macroself Ag) and assessed the fate of emerging IgG2a-expressing B cells that have, following class switch, acquired self-reactivity of the Ag receptor to the macroself-Ag. In normal mice, self-reactive IgG2a-switched B cells were deleted, leading to the selective absence of IgG2a memory responses. These findings identify a novel negative selection mechanism for deleting mature B cells that acquire reactivity to self-Ag. This process was only partly dependent on the Bcl-2 pathway, but markedly inefficient in MRL-Fas(lpr) lupus mice, suggesting that defective apoptosis of isotype-switched autoreactive B cells is central to Fas mutation-associated systemic autoimmunity.

Autoreactivity and allelic inclusion in a B cell nuclear transfer mouse.

Lymphocytes typically express only one functional antigen receptor, a restriction contributed to by allelic exclusion. Here we have analyzed B lymphocyte development in offspring of a mouse generated by nuclear transfer using a single donor B lymphocyte. In this mouse, all immunoglobulin alleles were inherited as found in the donor lymphocyte. This donor cell had two rearranged immunoglobulin light chain alleles, both directing the synthesis of light chains that could form functional antigen receptors, one of which was autoreactive. Progeny mice carrying this immunoglobulin light chain allele produced mature B cells, some of which continued to express the autoreactive receptor but required another rearrangement to rescue them from negative selection. Such receptor editing failed to destroy expression of one original light chain allele, thereby recreating dual receptor expression on these surviving B cells. We suggest that autoreactive antibodies in serum of mice and humans are due in part to such 'passenger' receptors.

Physical map of the mouse lambda light chain and related loci.

The lambda light chain genes and the related loci lambda 5, V(preB)1 and V(preB)2 are selectively expressed in the development of B lymphocytes. The sequences and the order of the gene segments are known, but a precise physical map of the lambda locus has not yet been published. Using the Celera mouse genome database we constructed such a map. In this paper, we confirm the order of the gene segments as being V lambda 2 - V lambda X - J lambda 2 - C lambda 2 - J lambda 4 - C lambda 4 - V lambda 1 - J lambda 3 - C lambda 3 - J lambda 1 - C lambda 1 and give precise germ line distances. We located the V(preB)2 locus 3' to the lambda locus, 1,077,001 bp downstream; the V(preB)1 locus, 2,180,618 bp downstream of the lambda locus; and the lambda 5 gene, 4,667 bp downstream of the V(preB)1 locus. Except for the V(preB)2 locus all other loci are in the same transcriptional orientation.