CD22-deficient mice are characterized by B cell hyperactivity and autoimmunity. We have constructed knock-in CD22 -/-mice, expressing an anti-DNA heavy (H) chain (D42), alone or combined with Vj1-Jj1 or Vj8-Jj5 light (L) chains. The Ig-targeted mice produced a lupus-like serology that was age-and sex-dependent. High-affinity IgG autoantibodies were largely dependent on the selection of B cells with a particular H/L combination, in which a non-transgenic, endogenous L chain was assembled by secondary rearrangements through the mechanism of receptor editing. Moreover, we present evidence that these secondary rearrangements are very prominent in splenic peripheral B cells. Since CD22 is primarily expressed on the surface of peripheral B cells, we propose a model for the development of a lupus-like autoimmune disease by a combination of peripheral receptor editing and abnormal B cell activation. IntroductionSystemic lupus erythematosus (SLE) is a rheumatic disease of unknown etiology, usually occurring in young women of childbearing age. The presence of serum antibodies to a variety of self components and their possible involvement in the development of severe kidney disease (glomerulonephritis) has made SLE a prototype of systemic autoimmune diseases [1]. The autoantibodies in lupus sera react with a large variety of nuclear antigens, including DNA, RNA and nuclear proteins. The antibodies to dsDNA are mostly highaffinity IgG that appear almost exclusively in SLE and very rarely in other diseases [2, 3].Several mouse models which spontaneously develop a lupus-like disease have been studied extensively [4]. The NZB/NZW F1 mouse is considered to be the murine model most closely resembling human SLE [5]. The lupus-like disease in these mice is more severe in females and is accompanied by high-affinity IgG anti-dsDNA autoantibodies. Both NZB and NZW parents contribute multiple susceptibility genes to the immune abnormalities of the F1 hybrid mouse [6]. Additional mouse models of SLE include mouse strains with deficiencies in antigen disposal mechanisms and mice that are deficient in proteins regulating the thresholds for tolerance and activation of B and T lymphocytes [7].CD22 is a B cell-specific surface glycoprotein of the Ig superfamily expressed on mature B cells [8, 9]. It functions as an adhesion molecule, as a signal-transducing molecule and as a modulator of intracellular signaling through the B cell receptor (BCR) complex. Negative regulatory roles for CD22 in BCR signaling are proposed to be critical for normal B cell activation, since immunoreceptor tyrosine-based inhibitory motifs with- in CD22 recruit SHP-1, a potent intracellular phosphatase with inhibitory functions in BCR signaling [10,11]. CD22-deficient mice [12][13][14][15] were reported to have decreased numbers of circulating B cells and slightly increased numbers of peritoneal B-1 cells, as well as higher levels of serum IgM [12,14]. Significantly, mature B cells from CD22-deficient mice had decreased expression of cell surface IgM and augmented i...

A nti-DNA autoantibodies (1, 2) are the hallmark of human systemic lupus erythematosus (3) and of mouse models of this prototypic autoimmune disease (4). The extensively studied anti-DNA Abs resemble secondary response Igs in experimental animals mostly comprising somatically mutated IgG with high affinity for DNA (5). In contrast, mostly lowaffinity, germline-encoded IgM "natural" anti-DNA Abs can be found in the sera of healthy individuals (6). This difference has been attributed to mechanisms of self-tolerance, leading to negative selection of autoreactive B and T cells (7). Several recent studies have suggested that intrinsic B cell defects may bear the prime responsibility for the loss of self-tolerance in systemic lupus erythematosus (8). Therefore, the mechanisms of B cell tolerance, particularly those involving clonal deletion (9), clonal anergy (10), and receptor editing (11) have become the focus of intensive investigation. Receptor editing provides the cell with a new receptor through replacement of the autoreactive L chain with a new one by continued rearrangement and is the primary mechanism for removal of a self-reactive BCR in the bone marrow (12).Recent studies have suggested that the process of secondary rearrangement is not necessarily accompanied by inactivation of the autoreactive L chain and may lead to a state of allelic or isotype inclusion (13)(14)(15)(16)(17)(18)(19). This results in the expression of more than one functional BCR on the same B cell and challenges the dogma of "one lymphocyte, one receptor" (20). Estimates of L chain-included B cell frequency in nonautoimmune, wild-type mice, using different methodologies, range from 3 to 10% of all B cells (17, 18). However, in transgenic (Tg) 3 animals expressing an autoreactive BCR, this frequency could reach 40% (15). It has been argued that coexpression of a nonself-reactive or L chain can rescue the B cell from negative selection by diluting out the selfreactive receptor (13)(14)(15)(16)18). Additionally, it was suggested that under certain circumstances, such as in aging mice, these B cells are potentially functional and can differentiate into autoantibodysecreting plasma cells (15).All of the studies so far have been conducted in wild-type or Tg normal mice. However, this newly described mechanism of L chain allelic inclusion could have particular implications for specific autoimmune responses (e.g., anti-DNA) in susceptible animals, such as the lupus-prone NZB/NZW F1 mouse (4). On one hand, these animals could use allelic or isotype inclusion to avoid B cell tolerance and thus significantly increase the frequency and selection of included B cells, especially in the fraction of autoantibody producing cells. Alternatively, an opposite scenario could also be envisaged, where the lack of effective tolerance mechanisms would reduce the need for L chain inclusion and their frequency would therefore be diminished.To test these different possibilities, we used our well-defined, anti-DNA, knock-in mouse lines (21,22) backcrossed to the NZ...

Clonal anergy has been well recognized as an important mechanism of B cell immunologic tolerance. However, the properties of anergic B cells and especially their role in the development of autoimmune disease in susceptible animals have been controversial. Here we show that low-affinity anti-DNA anergic B cells populate the mature B-cell compartment in the mouse spleen in excessive numbers and display paradoxical behavior in response to a combined B-cell receptor/TLR9 activation. Surprisingly, B-cell anergy was maintained in aged NZB/NZW F1 mice that develop a systemic lupus erythematosus (SLE)-like autoimmune disease. In several parameters of anergy, such as calcium mobilization and antibody secretion, the lupus-prone mice appeared more anergic than their non-autoimmune counterparts. We conclude that low-affinity anergic B cells are unlikely to serve as precursors for the high-affinity autoreactive B cells that give rise to pathogenic anti-DNA auto-antibodies in SLE.