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Light chain editing generates polyreactive antibodies in chronic graft-versus-host reaction

The chronic graft-versus-host (cGvH) reaction is a model of induced lupus caused by alloreactive CD4+ T cells from a Bm-12 mouse in a C57BL/6 recipient. We used this cGvH reaction in C57BL/6 anti-DNA H chain transgenic mice, 56R/B6, to understand the structure, specificity, and origin of the induced autoantibodies (auto-Abs). We found anti-DNA Abs that reacted to several different antigens, such as phosphatidylserine, myelin basic protein, thyroglobulin, histone, insulin, cytochrome C, and β-galactosidase. This polyreactivity was found for Abs from B cells that expressed the 56R H chain transgene with “editor” L chains that did not completely veto autoreactivity. We suggest that such incomplete editing results in polyreactivity and that incompletely edited polyreactive B cells influence the subsequent expression of pathogenic auto-Abs in disease. We also found B cells that coexpress κ and λ L chain. These B cells contributed to the autoimmune response and are possibly in the marginal zone of the spleen.

Systemic lupus erythematosus patients produce a broad spectrum of autoantibodies (auto-Abs), including anti-double-stranded (ds)DNA (1,2). In healthy individuals these auto-Abs are regulated at multiple checkpoints during B cell development (3,4), and defects, such as loss of T cell tolerance (5), late-stage B cell deregulation (6), and clearance defects of nuclear and cytoplasmic antigens, can lead to autoimmunity.
A reproducible appearance of auto-Abs can be achieved by induction of a systemic lupus erythematosus-like syndrome by a chronic graft-versus-host (cGvH) reaction (7). To initiate cGvH, lymphocytes from a Bm-12 mouse are transferred into a C57BL/6 (B6) recipient (8,9). The Bm-12 and the B6 mouse diff er at the MHC class II protein by three amino acids; hence, the cGvH is thought to be mediated by Bm-12 CD4 + T cells (10). cGvH leads to B cell hyperactivation followed by the production of a variety of auto-Abs directed to chromatin, Sm, DNA, and other nuclear antigens. In addition, the mice develop glomerulonephritis (8,11). The cGvH is thought to require B cell receptor cross-linking (12). The spectrum of auto-Abs induced by cGvH is narrowed in an anti-DNA transgenic mouse, referred to as 56R/B6 (13)(14)(15). This 56R H chain sitedirected knock-in mouse was generated using the VH from an anti-DNA Ab isolated from a diseased MRL/lpr mouse (16). The DNA reactivity of the 56R H chain is due to positive charges of arginines (Args) in the complementarity-determining regions (CDRs) (17).
The 3H9 and 56R transgenic models have been useful for studying the mechanism of receptor editing (18,19). L chain rearrangement in the 56R model seems to persist until the self-reactivity of the transgenic 56R H chain is altered. Such B cells bearing the 56R transgenic H chain are allowed to migrate into the periphery when paired with so called "editor" L chains. Such editor L chains have low isoelectric points due to a high frequency of aspartates (Asps) in their CDRs, and we think they block the interaction between H chain Arg and DNA (17).
The 56R H chain on a BALB/c background pairs mainly with the κ L chain Vκ21D, an effi cient editor of anti-DNA activity (17). On a B6 background, however, less effi cient editors Vκ38C and Vκ20 L chains are used (17,20, and unpublished data). Among λ chains, λx has been shown to modify DNA binding of the 56R H chain (21,22) and may result in altered self-reactivity. In 56R/BALB/c mice, a new population of edited 56R H chain transgenic B cells has been identifi ed. In this mouse, B cells express two receptors of which one is κ and the other is λ1 (17), and these "partially edited" B cells are thought to reside in the marginal zone (MZ) (23,24).
Using cGvH, we asked whether incompletely and partially edited B cells were activated and secreted auto-Abs in cGvH. We fi nd that the induction of anti-DNA Abs is accompanied by the induction of other auto-specifi cities. Hybridoma panels derived from cGvH-induced mice were analyzed and showed that auto-Abs arose from B cells that were polyreactive and bound to self-antigens and proteins, such as dsDNA, phosphatidylserine (PS), myelin basic protein (MBP), thyroglobulin, histone, insulin, cytochrome C, and β-galactosidase. We found that the observed polyreactivity was a feature of the 56R H chain in combination with the editor L chains Vκ20 and Vκ38C. Consistent with the expression of both the κ and λ L chain in the serum after cGvH disease, hybridoma panels derived from cGvH-induced mice included partially edited B cells with λ1 L chain that coexpressed κ L chain. The B cells with the λ and κ L chain bound to dsDNA, MBP, and PS and could account for some of the auto-Ab expression in cGvH. In addition, those double positive B cells may be located in the MZ.

Induction of auto-Abs in cGvH
The cGvH reaction induces a lupus-like syndrome that includes activated lymphocytes and the production of auto-Abs (8,9). In previous studies, cGvH was induced by the transfer of splenocytes (7-11, 14, 15, 25, 26) or partially purifi ed CD4 + T cells (27) from Bm-12 mice to B6 recipients. The cGvH induces a variety of auto-Abs in B6 mice (8,10), but in the case of 56R/B6, the auto-Ab repertoire is enriched in anti-DNA specifi cities as has been observed in studies on 56R Abs from unmanipulated and cGvH 56R/B6 mice (13)(14)(15)28). To rule out possible donor B cell response in the cGvH, we used purifi ed Bm-12 CD4 + T cells (CD4 + CD3 + > 96%; not depicted). Under these conditions, we found that cGvH caused an increase of anti-dsDNA Abs (Fig. 1 A), accompanied by an increase of anti-PS Abs (Fig. 1 B), but we also fi nd induction of anti-MBP Abs (Fig. 1 C).
The cGvH also activated peripheral B cells in the 56R/B6 recipient as seen by the transient expression of CD69 and CD86 as well as by the increased levels of MHC class II and Fas (CD95). A slight decrease of CD24 was also observed (Fig. S1, available at http://www.jem.org/cgi/content/full/ jem.20060075/DC1). We conclude that cGvH induced by purifi ed CD4 + T cells leads to secretion of anti-DNA Ab and activation of B cells in the 56R/B6 recipient. The induction of diff erent auto-Ab specifi cities in 56R transgenic mice raised the following questions: Do these specifi cities come from diff erent nontransgenic B cell populations, or can these specifi cities be attributed to polyreactive Abs expressed by 56R H chain with editor L chains as has been found to be the case for Vκ20 and Vκ38C? The 56R transgene in B6 mice both accelerates the onset and increases the titers of anti-dsDNA Abs in cGvH as reported previously (14,15). To study the role of 56R in auto-Ab induction, we tested the expression of 56R H chain by fl ow cytometry and in hybridoma panels.

Role of 56R H chain in auto-Ab induction
To test whether the anti-DNA, anti-PS, and anti-MBP Ab specifi cities (Fig. 1) were associated with the 56R H chain, we used allotype-specifi c Abs that distinguish the transgenic allele Igh a (16,29) and the nontransgenic endogenous allele For induction of cGvH, Bm12 CD4 + T cells were injected into 56R/B6 (■). As a control, B6 CD4 + T cells were injected into 56R/B6 (△). Anti-DNA, anti-PS, and anti-MBP Abs in sera were tested at the indicated time points by ELISA at a serum dilution of 1:180. IgM and IgG Abs to dsDNA (n = 6 for control group and n = 7 for cGvH group) (A), to PS (n = 5 for control and cGvH group) (B), and to MBP (n = 3 for control group and n = 6 for cGvH group) (C) were tested by ELISA. MRL/lpr sera were tested at the same concentration for comparison (•; n = 4). Results represent means ± SEM. Igh b of the 56R/B6 mouse. In the control mice, 45-48% of the B220 + splenic B cells were IgM a+ and IgD a+ at day 60. An almost identical number (44-45%) was found in the cGvH group. Of the B220 + B cells, 8-10% expressed endogenous H chain according to allotype IgM b and IgD b expression. This may result from aberrant VH replacement followed by rearrangement of the untargeted allele. The IgM b and IgD b fraction did not change after cGvH (Fig. S2 A, available at http://www.jem.org/cgi/content/full/jem.20060075/ DC1). From the correlation of allotype expression and auto-Ab production, we conclude that 56R H chain is mainly associated with the cGvH-induced auto-Abs.

mAbs isolated after cGvH
To test whether κ-edited 56R Abs are responsible for the different autoreactivities found in the serum (Fig. 1), we studied mAbs secreted by the hybridomas in two independent fusion panels generated 4 and 10 wk after cGvH. We tested the hybridomas for several characteristics: binding to multiple selfantigens, the H chain isotype, and the type of L chains.
Specifi city and isotype. In the 4-wk fusion, 53 out of 78 hybridomas secreted IgM or IgG (Table I A and Table S1, which is available at http://www.jem.org/cgi/content/full/ jem.20060075/DC1). Similar results were obtained in the 10-wk fusion, as 42 out of 139 hybridomas secreted IgM ( Table S2). Both fusions yielded a large number of clones that were IgM -IgG -. These "nonsecretors" were not analyzed further. Supernatants from both fusions were tested for binding to dsDNA, PS, and MBP. Most of the mAbs in the 4-wk fusion bound dsDNA as well as PS. In addition, these mAbs bound MBP (Table I A). Polyreactive mAbs that bound to dsDNA, PS, and MBP were also found in the 10-wk fusion (see below; Table II).
H chain gene usage in polyreactive Abs. To determine whether these polyreactive mAbs were associated with 56R, we tested whether the hybridomas have an intact 56R gene. PCR products from primers complementary to the 3H9/56R (56R) H chain leader sequence and the CDR3 of the 56R transgene (16) indicate the presence of the complete 56R transgene. In the 4-wk fusion, we found 33 clones out of 53 secretors to be 56R + (Table I A). The remaining 20 clones did not amplify the 56R transgene by this assay and are classifi ed as 56R -. 11 of these 20 56Rclones did amplify a product with a 56R CDR3 reverse primer and the JHCH primer. This PCR product represents 56R VH genes that have been truncated or VH replaced (13). No PCR product was detected with either of the 56R PCRs in 9 out of 20 56Rclones, and they were not analyzed further (Table S1).
Most of the clones that bound dsDNA, PS, and MBP were 56R + (Table I A). A few 56Rclones bound to PS, dsDNA, and MBP, but their affi nity for these antigens relative to the 56R + mAbs was low (Table S1).
L chain gene usage. We determined the types of L chains of these polyreactive mAbs. The Vκ genes from both the 56R + group and the 11 56R -mAbs that have truncated or VH  56R tg -b (11) 2(J5) a 1(J2) a 1(J4) 8 56R + clones from fusion 4 wk after cGvH (Bm-12 into 56R/B6) are polyreactive (A). κ L chain usage in 56R + and 56Rclones from fusion 4 wk after cGvH (Bm-12 into 56R/B6). Only clones with H and L chain secretion are listed and were tested for rearrangement of κ L chains using specifi c primers for Vκ12-13, Vκ20, Vκ21D, and Vκ38C, and a degenerate primer Vκ(S) for other κ rearrangements (B). For more details see Table S1. a Groups that contain clones with two L chain rearrangements. b 56R H chain transgenic -/CDR3JHCH + . J, Jκ gene in rearrangement. 56R + IgM hybridomas are compared from two fusions. Clones were polyreactive to at least two antigens in ELISA assays testing for dsDNA, PS, and MBP. The mean of the MBP/dsDNA and PS/dsDNA was calculated from at least three clones, and ± indicates the error (SEM). The values show the binding to PS and MBP relative to DNA binding. Fusion panels are from 4 (A) and 10 wk (B) after GvH. For more details see Table S2.
replaced the 56R gene were identifi ed by PCR using primers specifi c for the editors Vκ12−13, Vκ20, Vκ21D, and Vκ38C (Table I B) (17). Other Vκ rearrangements were detected using a degenerate Vκ primer, Vκ(S) or the L5 primer. Jκ was also determined. Most clones with the 56R H chain had rearranged the L chain editors Vκ12-13, Vκ20, Vκ21D, and Vκ38C to Jκ4 or Jκ5 (15 rearrangements) and slightly less to Jκ1 or Jκ2 (10 rearrangements). Some clones in the 56R + and 56Rgroup had more than one editor rearrangement (marked with an asterisk in Table I B). Editor L chains were more abundant in the 56R + group, and Vk20 and Vk38C were rearranged more often than Vk21D as has been shown for 56R/B6 (20 and unpublished data). This is a diff erent distribution than in 56R/BALB/c, where Vκ21D is the predominant editor (17,20). We think this may be related to the susceptibility to autoimmunity of 56R/B6 mice (30)(31)(32)(33)(34).

Contribution of L chain to polyreactivity
The 56R + mAbs had distinguishable reactivities depending on the Vκ and Vκ/Jκ combination. Vκ21D vetoed binding to dsDNA and the other antigens: PS, MBP, thyroglobulin, cytochrome C, histone, β-galactosidase, and insulin. Both Vκ20 and Vκ38C Abs were polyreactive, but their binding pattern was diff erent (Fig. 2 A). 56R + clones with Vκ20 or Vκ38C bound dsDNA with higher relative affi nity than Vκ21D. In addition, Vκ20 and Vκ38C rearranged to J2 bound dsDNA with a higher relative affi nity than the same V regions rearranged to J4 or J5 ( Fig. 2 A). We studied the binding to PS, MBP, and the other proteins relative to their binding to dsDNA. We showed that binding to PS was favored over binding to dsDNA in clones with the L chain Vκ20, whereas Vκ38C L chains bound to dsDNA and PS with similar affi nity. Affi nity for MBP, thyroglobulin, cytochrome C, histone, and β-galactosidase was higher in Vκ38C as compared with Vκ20, and no diff erence was found between Vκ20 and Vκ38C for insulin ( Fig. 2 B). Relative binding to PS and MBP was also assessed by calculating the ratios of PS to DNA and MBP to DNA binding of the mAbs that expressed the 56R transgene from clones in both fusions. Again, PS/dsDNA binding relative to MBP/ dsDNA binding was greater in clones that expressed the Vκ20 as compared with the Vκ38C L chain, whereas binding to PS/dsDNA and to MBP/dsDNA was similar in clones that expressed the Vκ38C L chain (Table II).

Plasma cell differentiation in cGvH
We searched for the origin of polyreactivity by fl ow cytometry. After cGvH induction, there was an increase in B220 + IgG + splenocytes expressing the plasma cell marker CD138 (2.57 and 4.05% of the B220 + cells by days 28 and 60 after cGvH, respectively). We considered these surface Ig-expressing B cells to be pre-plasma cells (Fig. 3 A). We also found an increase of B220 + IgM a+ B cells expressing CD138 (0.37 and 0.75% of the B220 + cells by days 28 and 60 after cGvH, respectively), indicating that B cells with the H chain transgenic allele Igh a of the 56R/B6 mouse diff erentiated into plasma cells, and were giving rise to anti-DNA Abs. B220 + IgM a-B cells expressing CD138 (2.29 and 2.95% of the B220 + cells by days 28 and 60 after cGvH, respectively) were also increased and most likely to be of the IgG class (Fig. S2 B). . All clones tested were sequenced using specifi c primers for Vκ20, Vκ21D, and Vκ38C and were unmutated. All tested binding specifi cities are shown in comparison for one representative result for each group. SEM from triple values in the ELISA assay (A). To analyze the binding of the subclones (to PS and the proteins MBP, thyroglobulin, histone, insulin, cytochrome C, and β-galactosidase) relative to dsDNA binding, dsDNA binding of each subclone was set to 1; therefore, only clones with considerable binding to dsDNA were included in this representation. Relative binding to the tested antigens is shown for clones using the indicated L chain. Numbers show p-values determined by the Anova Single Factor Test (B).

ARTICLE
Potentially pathogenic auto-Abs are thought to home to the MZ (35,36); therefore, we tested whether MZ B cells were aff ected by cGvH in the 56R/B6. This is relevant because it has been shown that 56R/BALB/c has an enlarged MZ B cell compartment. Here we show that in the 56R/B6, the MZ B cell population (CD21 high and CD23 low ) comprised as much as 50% of the B220 + splenocytes. The percentage of the MZ B cell population decreased after cGvH, whereas the CD21 -CD23 -B cell population increased. The percentage of follicular B cells (CD21 int CD23 high ) slightly increased (Fig. 3 B, dot plots). The majority of the CD138 + B cells was found in a subpopulation of CD21 -CD23 -B cells (Fig. 3 B, histograms). Yet, during cGvH, the number of lymphocytes in the spleen increased on average 2.5-fold and the spleen weight increased 3.2-fold (not depicted); therefore, we cannot conclude that the absolute numbers of MZ B cells in cGvH were decreasing. However, we observed a decrease of CD21 expression by the mean fl uorescence intensity on MZ B cells (not depicted). This suggests that MZ B cells were activated during cGvH. The concommitant increase of plasma cell diff erentiation and the decrease of CD21 expression in MZ B cells suggest that potentially autoreactive B cells from the MZ were aff ected by the cGvH reaction and may have diff erentiated to become auto-Ab-producing B cells.  T cells in the spleen of 56R/B6 recipients were also activated as seen by the down-regulation of CD3 (not depicted) and CD4 as well as by the increased expression of CD69 (tested at day 14; Fig. 4). The expression of the T cell costimulatory molecules CD28 and ICOS also increased, and CD4 + T cells displayed a memory phenotype as shown by the expression of CD44 high , CD45RB low , and CD62L low (Fig. 4). We could not distinguish between donor and recipient CD4 + T cells in these studies, but the activation of T cells as late as day 60, by which time we would not expect Bm-12 donor T cells, indicates a role of endogenous T cells in antigen activation. We conclude from these data that cGvH induced by purifi ed Bm-12 CD4 + T cells leads to plasma cell diff erentiation (Fig. 3) and a sustained activation profi le of T cells in the 56R/B6 recipient (Fig. 4). The role of endogenous T cells in the cGvH has been established by studies showing that CD4 knockout mice are unresponsive to cGvH (37).

Expression of L chain in cGvH
We tested anti-DNA Abs for the usage of κ and λ L chain in the serum after cGvH and developed our anti-dsDNA assay with anti-λ and anti-κ reagents. We found λand κ-containing anti-dsDNA Abs (Fig. 5 A). The strong induction of λ anti-dsDNA Abs led us to ask whether these Abs come from isotypically included B cells that express λ1 and κ L chain (18). We also asked whether these isotypically included B cells are activated by cGvH and diff erentiate into anti-dsDNA-producing plasma cells. This is relevant because it has been shown that MZ B cells in the 56R/BALB/c mouse can coexpress a κ and a λ L chain. The λ1 L chain with 56R H chain binds DNA (38), but the κ L chain is an editor (17). These isotypically included B cells are thought to be sequestered in the MZ (23,24). We analyzed the composition of B cells in the MZ of a 56R/B6 mouse using a single cell PCR technique and tested for B cells with both λ1 and κ L chain (Table III,   and L chain usage in anti-dsDNA Abs in 56R/B6 after cGvH. For induction of cGvH, Bm-12 CD4 + T cells were injected into 56R/B6 (■). As a control, B6 CD4 + T cells were injected into 56R/B6 (△). Anti-DNA Abs were detected by ELISA with an anti-λ (n = 6 for cGvH group and n = 3 for control group) and anti-κ (n = 4 for cGvH group and n = 3 for control group) detection Ab by ELISA at a serum dilution of 1:180. Results represent means ± SEM. MRL/lpr sera were tested for comparison (•; n = 4) (A). Detection of L chain expression during cGvH in serum was analyzed by Western blot. Sera from mice after cGvH (Bm-12 CD4 + T cells injected into B6 or 56R/B6 recipients) or sera from control mice (B6 CD4 + T cells were injected into B6 or 56R/B6 recipients) were run under reducing conditions. λx was detected using a polyclonal anti-Vλx Ab (reference 22). Serum samples were diluted at 1:100 for λ and at 1:2,000 for κ L chain detection. The expression of λx after exposure for 5 min, λ 1,2,3 after exposure for 20 min, and κ after exposure for 30 s is shown. Results are representative of seven experiments. λx, λ1,2,3 and κ in the serum are shown from one mouse (B). ARTICLE using nested PCRs on cDNA from single cells. Specifi c primers were used for Vλ1/2 and Cλ1/2, Vκ21D and Cκ, and Vκ38C and Cκ (20). Other κ L chains were amplifi ed using the degenerate primer Vκ(S) (39). We found that all B cells with λ L chain also had a κ L chain rearrangement. Sequence analysis showed that B cells with in-frame λ1 L chain had rearranged the editor L chain Vk21D and Vk38C. Two examples with in-frame λ1 L chain had, in addition to the κ editor, another κ rearrangement. Other λ L chain PCR products had large deletions in the V or V/J regions. Rearrangement of λ L chain in-frame or out of frame was always accompanied by an in-frame κ L chain that was often the editor Vκ38C or Vκ20 (Table III B). Thus, the MZ contains a population of B cells that seems to have rearranged excessively and introduced λ deletions.
We tested whether κ/λ1 56R B cells were activated by cGvH by analysis of a fusion panel 10 wk after cGvH. We found isotypically "included" B cells that coexpressed a λ1 and a κ L chain. 5 out of 139 clones were λ1, and 4 of these expressed the 56R H chain. One λ1 clone did not express the 56R H chain and was negative for IgM and IgG. All of the 56R/λ1-expressing clones coexpressed a κ L chain by ELISA, and in three of those examples, we found rearrangement of λx by PCR. In addition, we found one clone that expressed the 56R H chain with λ2. All clones that expressed the 56R H chain with λ1/κ or λ2 L chain bound to dsDNA, PS, and MBP. None of the clones expressed only λ1, indicating that B cells that express the 56R H chain with λ1 alone are eliminated from the repertoire (Table IV). Thus, partially edited B cells coexpressing λ and κ L chain were activated in cGvH and could be fused. We also found κ/λ-expressing B cells from cGvH-induced mice by fl ow cytometry (Fig. 3 C); however, the number of isotypically included B cells was higher as found by single cell PCR analysis (Table III). We are considering that this phenomenon is due to the slight cross-reactivity of anti-λ and anti-κ Abs used in fl ow cytometry analysis (Fig. 3 C). The editor L chain, λx, is also induced by cGvH. λx is known to bind MBP both as free L chain and in association with H chains (40,41), including 56R (unpublished data). We tested the possibility that cGvH might induce λx-associated Abs by a Western blot of sera developed with a λx antiserum (Fig. 5 B). We were unable to detect λx in the serum of 56R/B6 mice or in nontransgenic B6 mice, but after initiation of the cGvH, we found that λx levels in both 56R/B6 and nontransgenic B6 mice rose rapidly and were maintained until day 60 (Fig. 5 B; not depicted for day 60). Expression of λx after cGvH was signifi cantly higher in the 56R transgenic than in the nontransgenic B6, suggesting that this L chain is associated with the 56R H chain. Consistent with the ELISA data ( Fig. 5 A), we also found increased levels of λ and κ; however, in 56R/B6, expression of those L chains did not increase to the same extent as λx (Fig. 5 B).

DISCUSSION
The 56R H chain transgene enhances the anti-DNA production in cGvH (14,15) and in the quasi-autoimmune B6 (30-34 and unpublished data). This is unexpected because Abs with the 56R H chain in BALB/c are edited. DNA binding is either vetoed by L chains, such as Vκ21D (17) and λx (21,22), or DNA binding on the cell surface is diluted because more than one receptor is expressed (17,23,24). In this study, we show that editing can also be incomplete (Fig. 2). Editors were originally defi ned in BALB/c transgenics, a strain that does not convert to autoimmunity even with an anti-DNA transgene. Most of the B cells from this mouse express the 56R H chain with Vκ21D, an L chain that vetoes DNA binding completely (17). In the B6 mouse, the L chain repertoire is reversed, very few 56R/Vκ21D combinations are found, and the majority of B cells express 56R H chain with incomplete editors, such as Vκ20 and Vκ38C (reference 20, unpublished data, and Table III). We think that the difference lies in the tolerance threshold of the strain: BALB/c, the prototypic "healthy" strain, tolerates only completely edited Abs, whereas B6, a lupus-susceptible strain (30)(31)(32)(33)(34), is more permissive. We show that the cGvH-induced autoimmunity in 56R/B6 mice activates B cells using the incomplete editors Vκ20 and Vκ38C ( Fig. 2 and Table I B). Thus, incomplete editing contributes to autoimmunity by determining an available repertoire that includes autoreactive B cells. In support of this idea is the fi nding that 56R/B6 and the congenic 56R/Sle2/B6 (expressing the additional lupus susceptibility gene Sle2) spontaneously produce anti-DNAs that include 56R/Vκ20 Abs (unpublished data). Evidence for such a "lupus repertoire" is also found in lupus-aff ected individuals (3,4).
Incomplete editing is inevitable. The ability of certain L chains to edit is correlated with certain structural features of these L chains, in particular, a high frequency of Asp residues in or near the CDRs (17). Hence, we think editing may be the result of an interaction between the VH Arg residues and the VL Asp residues to interfere with the VH Arg interaction with DNA (38,42,43). However, editor Asps may not neutralize every Arg and might account for residual anti-DNA activity (44). Similarly, not all Asps need to interact with Args and might be available for interactions with basic proteins such as MBP, as in the 56R/λx.
The incompletely edited Abs, such as 56R/Vκ20, retain activity for DNA and cardiolipin. This is thought to be a cross-reaction based on binding to a phosphate-containing moiety shared by both (45). But a novel source of polyreactivity comes from binding activities that some editors confer upon an anti-DNA VH. In this case, polyreactivity is not because of cross-reactivity but because of alternate binding sites.

ARTICLE
A relevant example has been described in which the 3H9 H chain gives diff erent patterns of antinuclear antigens depending on the L chain with which it is associated (46). We conclude from these results that editor L chains are correlated with anti-self activity. J gene usage also seemed to play a role in binding some proteins, namely MBP, thyroglobulin, cytochrome C, and histone, as shown for its relevance for Vκ20 and Vκ38C in binding to dsDNA (Fig. 2, A and B). Although polyreactivity can come about by the creation of different combining sites, it can also be due to enhanced cross-reactivity, as in the case of Vκ20. Receptor editing acts in B cells with autoreactive receptors; hence, rearrangement persists and can produce isotypically included B cells as we have described in the 56R/BALB/c transgenic mouse. One of the two receptors (56R/λ1) is anti-DNA (38), and the other is a 56R/κ editor (24). Such isotypically included κ/λ double-expressing B cells in 56R/ BALB/c are found in the MZ of the spleen (24) and in hybridomas from LPS-activated fusions. Fusion panels generated after cGvH include clones that coexpressed λ1 and κ L chain and secrete anti-DNA Abs. In this study, none of the clones (Table IV) expressed λ1 L chain only. Therefore, we think the κ/λ B cell may represent a state of tolerance. In this sense, a B cell may have a self-reactive receptor and expression of two L chains may alter or reduce the affi nity of the B cell to an antigen. Other examples of dual expression come from recent studies in 56R mice in which the κ locus has been knocked out. Hybridoma panels from these mice showed that the vast majority of the 56R/λ1 B cells coexpressed λx L chain, an effi cient editor, and only very few have rearranged λ1 alone (47 and unpublished data), but these 56R/λ1 clones bound DNA even when λx L chain was coexpressed. cGvH appeared to activate these κ/λ doubleexpressing B cells because we fi nd anti-DNA Abs with λ1 in the serum in cGvH (Fig. 5 A). By single cell analysis we tested whether these κ/λ B cells may be in the MZ of 56R/ B6 mice and confi rmed the existence of κ/λ-included B cells (Table III). Single cell analysis of MZ B cells also revealed interesting properties that may act on the λ locus. All λ L chain rearrangements, whether out-of-frame or in-frame, were accompanied by a κ L chain, which was (except in one case) in-frame (Table III).
As discussed above, we think that DNA and/or MBP binding can result from incomplete editing. For example, the Asp-rich λx editor in association with H chains including 56R binds MBP (40, 41, and unpublished data). The acidic property of λx to which we attribute the MBP activity is also a feature of κ editors, and these L chains might also confer MBP activity to 56R. And, as we show here, 56R with Vκ38C and Vκ20 also bound MBP as well as DNA and PS (Fig. 2, A and B) (28). Therefore, an incompletely edited Ab may have both Arg(s) and Asp(s) available for binding both to DNA and proteins.
During the cGvH reaction there was a change in the percentages of MZ B cells, follicular B cells, and CD21 -CD23 -B cells (Fig. 3 B). MZ B cells may have activated by alloreactive Bm-12 T cells, which in turn may have activated endogenous T cells of the 56R/B6 recipient. These T cells could give help to the diff erentiation of plasma cells. This idea is consistent with recent fi ndings that showed that MZ B cells, not follicular B cells, are potent activators of naive CD4 + T cells. MZ B cells migrate to T cell areas and diff erentiate upon interaction with T cells into plasma cells (48). This could be the case in the cGvH reaction, and plasma cells found in the CD21 -CD23population might be a source of auto-Abs to DNA (Fig. 3 B, histograms). Support for this idea comes from features of 56R/Sle2/B6. Sle2 further increases the tendency to autoimmunity of 56R/B6, and in 56R/Sle2/B6, the MZ and the CD21 -CD23 -B cell populations are increased as compared with 56R/B6. These CD21 -CD23 -B cells secreted auto-Abs of the IgM a allotype, the transgene-encoded allele, in culture (unpublished data). We also fi nd Abs reactive to DNA expressing the H chain of the Ig a allotype ( Table I). Assuming that both Sle2 and cGvH enhance autoimmunity, we think that the increase of the CD21 -CD23cells in 56R/Sle2/B6 is analogous to the increase of CD21 -CD23 -B cells in cGvH-induced autoimmunity. The question remains whether partially edited κ and λ double positive B cells are able to move out of MZ to become plasma cells.

Conclusions
Autoreactive and polyreactive Abs. Here we present a detailed description of the auto-Abs induced by cGvH. The study reveals the structural basis for autoreactivity of certain Abs and the origin of these auto-Abs.
Origin. Natural auto-Abs are found in abundance in both healthy and autoimmune individuals, and there is a vast and confusing literature on their origin, rationale, and implications for autoimmunity and immune protection (49). Our study answers these questions by showing that at least some if not all "natural" auto-Abs result from receptor editing. Of course, the abundance of these autoAbs in our model is in part because of the anti-DNA transgene. This produces an immature B cell population, most of which must be edited to survive. But even nontransgenics must have a substantial repertoire that is shaped by editing. Based on the mechanisms that generate the immature VH repertoire, we estimate that at least 50% of this repertoire is anti-DNA (50,51). Thus, editing of this population will have a profound infl uence on the frequency of natural auto-Abs in the peripheral B cell repertoire.

Relevance.
A question raised by natural auto-Abs is how to explain their presence in a self-tolerant individual. This dilemma is usually explained by invoking affi nity diff erences; i.e., natural auto-Abs are low affi nity anti-self and are ignored by tolerance mechanisms (52). But the question remains: Why is there a large abundance of these auto-Abs? The only explanation is positive selection (53) of this population, and several roles for natural auto-Abs have been proposed. These roles range from housekeeping Abs to stimulators of the repertoire to anti-virus Abs (49). If indeed natural auto-Abs are important in normal immunity, their existence in the repertoire can be understood. It has been shown that the B cell repertoire that is induced by the cGvH is not random but a selective process and requires B cell receptor cross-linking (12) and is supported by the involvement of endogenous T cells (as seen in this study [ Fig. 4] and by others [37]). In addition, certain edited auto-Abs appear to be selected as suggested by diff erences in the repertoire of editors between strains; for example, the distribution of editors in B6 as compared with BALB/c.

Implications.
We show here that the peripheral repertoire is shaped by receptor editing and selection. Negative selection can continue to act on edited B cells if they retain self-reactivity after the initial editing event. In this sense, a healthy BALB/c mouse may only tolerate perfectly edited, nonautoreactive B cells. Other strains may be more permissive and allow incompletely edited B cells to enter the periphery. The ability to regulate this population may determine lupus susceptibility by supplying the peripheral B cell population with the precursors to pathogenic auto-Abs. That these B cells are involved in the onset of disease is demonstrated by these fi ndings.

Mice.
The generation of site-directed 3H9/56R knock-in mice has been described previously (13,17). The 3H9/56R was backcrossed onto the C57/BL6 background for 14 generations to engender 3H9/56R/B6. The 3H9/56R transgene was determined by PCR amplifi cation of tail DNA (16,17). B6 and coisogenic Bm-12 (B6.C-H2bm12/KhEg) mice were obtained from The Jackson Laboratory. All mice were maintained in our mouse colony at the University of Chicago. All animal care and procedures were conducted in accordance with the Animal Welfare Act.
cGvH. Mice were 2-8 mo old at time of cGvH initiation. All cGvH experiments for fl ow cytometry, serum data, and the fusion experiment at 10 wk after cGvH were performed with 10 7 Bm-12 (for cGvH) or B6 (for controls) CD4 + T cells. For the fusion at 4 wk, cGvH was performed as described previously (9). In brief, recipient mice were injected i.p. with Bm-12 (for cGvH) or B6 (for controls) splenocytes prepared using standard techniques, or with 10 7 CD4 + T cells (purity >96%) purifi ed using the CD4 (L3T4) MicroBead MACS system (Miltenyi Biotec). Experimental mice were bled before onset of cGvH and weekly thereafter. and anti-IgM-FITC (SouthernBiotech). Detection of biotinylated Abs was done using streptavidin-FITC, streptavidin-PE, or streptavidin-PE-Cy5 (all from BD Biosciences). Data were generated on a FACSCanto (BD Biosciences) and analyzed using FlowJo software.
Hybridomas. 3H9/56R/B6 mice were injected with Bm-12 splenocytes (fusion 4 wk after injection as described previously [14,15]) or with Bm-12 CD4 + T cells (fusion 10 wk after injection). Hybridomas were generated in the Monoclonal Antibody Facility of the University of Chicago as described previously (21). In brief, spleen cells from cGvH mice were fused without further manipulation to Sp2/0 myeloma cells. Hybridomas were plated at limiting dilution, wells bearing single colonies on 96-well plates were expanded, and hybridomas were grown 90% confl uent in six-well plates for ELISA assays.

Auto-Abs in serum.
Unless otherwise indicated, sera were diluted at 1:180 in the appropriate blocking buff er of the ELISA assay. Immulon 2 HBX was used for PS, and Immulon 4 HBX ELISA plates were used for all other ELISA assays (Thermo-Lab Systems).
DsDNA. Anti-dsDNA Abs were detected as described previously (19). Plates were coated with 10 μg avidin D (Vector Laboratories) in PBS at 4°C, blocked in PBS with 1% BSA at 37°C for 2 h, and washed with PBS with 0.05% Tween (PBS-T). Biotinylated dsDNA was bound to avidin-coated plates at 37°C for 1.5 h. Sera were applied, plates were washed three times, and DNA serum complexes were detected with anti-mouse IgM-Ap, IgG-AP, κ-AP, or λ-AP (all from SouthernBiotech). After absorption for 60 min, plates were washed and the retaining anti-dsDNA IgM, IgG, κ, or λ Abs were quantifi ed using AP substrate (Sigma-Aldrich). The OD was determined at 450 nm.

PS.
Anti-PS Abs were detected as described previously (45). Plates were coated with PS (Sigma-Aldrich) in 100% EtOH, dried, and blocked in 0.1% gelatin/1% BSA at 37°C for 2 h. Plates were washed with PBS. Sera were applied, plates were washed, and PS serum complexes were detected as described for dsDNA binding.

MBP.
Anti-MBP Abs were detected as described previously (40). Plates were coated with 10 μg human MBP in PBS at 4°C overnight and blocked in 0.5% casein at 37°C for 2 h. Plates were washed with PBS-T. Diluted sera were applied, plates were washed, and MBP serum complexes were detected as described for dsDNA binding.
Auto-Abs in hybridoma supernatants. Supernatants were tested for dsDNA and PS binding as described above for serum. Blocking buff er for MBP and all other protein binding assays contained 8% casein in PBS. Supernatants from fusions were tested for binding to dsDNA, PS, and to proteins MBP, thyroglobuline, cytochrome C, histone, β-galactosidase, and insulin. The OD 405 of the binding assays was normalized based on the Ig concentration of each supernatant.
Isotype and Ig concentration in hybridoma supernatants. Isotype and concentration in culture supernatants were determined as described previously (47). In brief, plates were coated with unconjugated anti-mouse Ig, supernatants from hybridomas were added, and binding was detected with goat anti-IgM-AP or anti-IgG-AP and developed with AP substrate. The Ig concentration was determined by comparing samples to a standard curve generated by an isotype-matched Ab (Sigma-Aldrich).