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* Department of Molecular Preventive Medicine, School of Medicine, University of Tokyo, Tokyo, Japan;
Second Department of Internal Medicine, Nara Medical University, Nara, Japan; and
Department of Anatomy and Developmental Biology, School of Medicine, Tokyo Womens Medical University, Tokyo, Japan
| Abstract |
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| Introduction |
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B1 cells are a specialized cell population that are distinguished from conventional B cells (B2 cells) by their origin, cell surface phenotype, unique tissue distribution, and capacity for self-renewal, and have also been considered to be involved in autoantibody production in the development of autoimmune diseases (5, 6, 7). Involvement of B1 cells in IgM-mediated autoimmune diseases such as autoimmune hemolytic anemia was also demonstrated (8, 9). However, it remains to be elucidated whether B1 cells class-switch from IgM to IgG in the development of autoimmune diseases. Accumulating data also suggest that B1 cells contribute to the innate immunity by producing natural IgM Ab in the circulation and by secreting IgA Ab in the intestinal mucosa (10, 11, 12, 13). It has been recently reported that BLC is essential for B1 cell homing to the peritoneal cavity and for body cavity immunity (14). In the present study, we investigated a role of BLC expression in aged BWF1 on aberrant B1 cell trafficking, including B1 cell homing to the peritoneal cavity. We found that B1 cell homing to the peritoneal cavity was impaired and preferential B1 cell trafficking to the target organs in aged BWF1 mice. We also demonstrated that the number of BLC-producing peritoneal macrophages, which were reported to be a major cell source for BLC in the peritoneal cavity (14), was markedly decreased in aged BWF1 mice. Pathological significance of abnormal B1 cell trafficking in the development of murine lupus is discussed.
| Materials and Methods |
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(NZB x NZW)F1 (BWF1), NZB, NZW, and BALB/c mice, originally obtained from the Shizuoka Laboratory Animal Center (Shizuoka, Japan), were maintained in our animal facility at University of Tokyo. Female BWF1 and BALB/c mice aged 810 wk were used as young mice, and BWF1 mice aged 812 mo with moderate-to-severe proteinuria and BALB/c mice aged 8 mo were used as aged mice.
Antibodies
Rat mAbs specific for mouse CD11b (M1/70), CD11c (HL-3), B220/CD45R (RA3-6B2), CD5 (53-7.3RRH), MHC class II (ER-TR3), and CD16/32 (2.4G2) were purchased from BD PharMingen (San Diego, CA). Anti-F4/80 (CI: A3-1) mAb and goat anti-BLC polyclonal Ab (pAb) were purchased from Serotec (Raleigh, CA) and Genzyme Techne (Minneapolis, MN), respectively.
Cell preparation
Peritoneal lavage cells were isolated by flushing the peritoneal cavity with 46 ml of RPMI 1640 supplemented with 5% FCS. Cell suspensions prepared from mouse spleen and lymph nodes were passed through nylon mesh. Peripheral blood leukocytes (PBLk) were obtained by centrifugation on Lymphorite-M (Cedarlane, Hornby, Ontario, Canada). RBC in the spleen and PBLk were lysed by ammonium chloride solution.
Flow cytometry
Flow-cytometric analyses of peritoneal cells, spleen cells, lymphoid cells, and PBLk were performed using an Epics Elite cell sorter (Coulter Electronics, Hialeah, FL) as described previously (3). After blocking FcR with anti-CD16/32 for 10 min, peritoneal cells were stained with FITC-conjugated anti-CD11b mAb and PE-conjugated anti-CD11c mAb, and biotin-conjugated anti-B220, F4/80, or class II mAb followed by allophycocyanin-streptavidin (BD PharMingen). CD11bhighCD11clowF4/80+ macrophages were sorted to >95% purity using an Epics Elite cell sorter. Sorted macrophages were spun down onto a slide glass and stained with Giemsa solution (Merck, Tokyo, Japan). In some experiments, peritoneal cells were strained with FITC-conjugated anti-CD5 mAb, PE-conjugated anti-B220 mAb, and biotin-conjugated anti-F4/80 mAb followed by allophycocyanin-streptavidin. CD5+B220+F4/80- B1 cells in the peritoneal cavity and CD5-B220+F4/80- B2 cells in the spleen were sorted on an Epics Elite cell sorter. More than 95% of B1 and B2 cells were CD19 positive.
Injection of CFSE- and 5-(and-6)-(((4-chloromethyl)benzoyl) amino)tetramethylrhodamine (CMTMR)-labeled B cells
Cell labeling of B1 cells with CFSE (Molecular Probes, Eugene, OR) and B2 cells with CMTMR (Molecular Probes) was performed according to the manufacturers instructions. These cells were all labeled in 10 µM at 37°C for 15 min. The cell viability after CFSE and CMTMR staining and before injection was >99% in dye exclusion tests using trypan blue. The mixture of 4 x 106 CFSE-labeled peritoneal B1 cells and/or 4 x 106 CMTMR-labeled splenic B2 cells suspended in 0.2 ml of PBS were injected i.v. into BWF1 or BALB/c mice. In some experiments, the mixture of B1 cells from aged BWF1 mice labeled with CFSE and B1 cells from young BWF1 mice labeled with CMTMR suspended in 0.2 ml of PBS was injected i.v. into young BWF1 mice. Mice were sacrificed 24 h after i.v. injection and subjected to flow-cytometric and immunofluorescent analysis. The number of cells localized in the various organs was calculated by the percentage of labeled cells in each organ, and the percentage of recruited cells to the total number of injected cells was presented using an Epics Elite cell sorter. In nonlymphoid organs, 10 nonconsecutive microscopic fields of cryostat section were examined, and the numbers of CFSE-labeled B1 cells and CMTMR-labeled B2 cells in each field were counted. The mean percentage of B1 cells among total number of fluorescent cells was presented.
Immunofluorescent staining
For immunofluorescent analysis of target organs, spleen, lung, kidney, and thymus from aged BWF1 mice were fixed in paraformaldehyde, embedded in Tissue-Tek OCT compound (Miles, Elkhart, IN), and then frozen in liquid nitrogen. Six-micron cryostat sections were then incubated with biotin-conjugated anti-B220 mAb followed by allophycocyanin-streptavidin. Avidin/biotin blocking solution (Vector Laboratories, Burlingame, CA) was used to decrease nonspecific staining. Finally, the sections were analyzed by Olympus IX-70 confocal laser-scanning microscope system (Olympus Optical, Tokyo, Japan).
Histological and immunocytochemical examination
Peritoneal macrophages obtained by cytospin centrifugation were stained with goat anti-BLC pAb, followed by biotinylated rabbit anti-goat IgG (DAKO, Carpinteria, CA) and HRP-labeled streptavidin (DAKO). Avidin/biotin blocking solution and normal rabbit IgG were used to decrease nonspecific staining.
Reverse transcription and real-time quantitative PCR analysis
Total RNA was isolated from peritoneal cells by using RNAzol B (Tel-Test, Friendswood, TX) according to manufacturers instructions. It was then reversely transcribed into cDNA using Superscript II preamplification kit (Invitrogen, Carlsbad, CA) and amplified with specific oligonucleotide primers. The sense and the antisense primers used were as follows: BLC primer, 5'-TCTCTCCAGGCCACGGTATTCT-3' and 5'-ACCATTTGGCACGAGGATTCAC-3', and GAPDH primer, 5'-AGTATGACTCCACTCACGGCAA-3' and 5'-TCTCGCTCCTGGAAGATGGT-3'. PCR was performed by thermal cycler for 35 cycles of 94°C for 30 s, 58°C for 45 s, and 72°C for 45 s, and final extension was done at 72°C for 10 min. The PCR products of GAPDH and BLC were examined by 2.5% agarose gel electrophoresis. Real-time quantitative PCR analysis was performed by using ABI 7700 sequence detector system (PE Applied Biosystems, Foster City, CA). FAM-labeled primers were used as target hybridization probes for BLC and GAPDH (BLC, 5'-CATCATAGTTCGGATTCAAGTTACGCCCCC-3'; GAPDH, 5'-AAGGGACACAGTCAAGGCCGAGAAT-3'). The thermal cycling condition included 50°C for 2 min and 95°C for 10 min, followed by 45 cycles of amplification at 95°C for 15 s and 55°C for 1.5 min for denaturing and annealing, respectively. PCR were run in triplicate. BLC quantity was normalized by the level of GAPDH.
Statistical analysis
Statistical analysis was performed using Students t test. The 95% confidence limit was taken as significant.
| Results |
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We conducted B1 cell homing experiments where CFSE-labeled B1 or CMTMR-labeled B2 cells were injected i.v. into young or aged BWF1 mice to examine the change in B1 cell trafficking during the development of lupus nephritis in these mice. A significant number of B1 cells homed to the peritoneal cavity in young BWF1 mice when injected i.v. with CFSE-labeled B1 cells (Fig. 1A). However, in aged BWF1 mice, B1 cells failed to home to the peritoneal cavity (Fig. 1B). In contrast, B1 cells did home to the peritoneal cavity in aged BALB/c mice as well as in young BALB/c mice (Fig. 1, C and D) We next investigated the accumulation of B1 cells in the milky spots, which are reported to be the entry sites for B1 cells into the peritoneum (14). The decreased number of CFSE-labeled B1 cells accumulated in the omentum milky spots in aged BWF1 mice compared with young mice (Fig. 2A). In contrast, B1 cell accumulation in the omentum milky spots was intact in aged BALB/c mice (Fig. 2B). B1 cell homing to the peritoneal cavity was also normal in aged BALB/c mice. A similar number of B1 cells was detected in the peritoneal cavity and spleen in aged BALB/c mice compared with that in young BWF1 mice. Furthermore, there was no difference in homing ability to the peritoneal cavity between B1 cells obtained from young BWF1 mice and those from aged mice. A similar number of B1 cells from either origin was detected in the milky spots and peritoneal cavity when injected together into young BWF1 mice (Fig. 2C).
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When the mixture of CFSE-labeled B1 cells and CMTMR-labeled B2 cells was injected i.v. into the same aged BWF1 mice, B1 cells were preferentially recruited in the cellular infiltrates (B220 positive) in the target organs such as the kidney, lung, and thymus, whereas they were similarly recruited to B cell follicles in the spleen (Fig. 3A). The mean percentages of B1 cells among total number of fluorescent cells were 50.15 ± 3.44, 62.33 ± 10.60 (p < 0.05), 88.81 ± 6.88 (p < 0.001), and 83.00 ± 3.91 (p < 0.001) in the spleen, thymus, kidney, and lung, respectively (Fig. 3B). These data confirmed that the percentage of B1 cells recruited to the target organs was significantly higher than that of B2 cells in aged BWF1 mice.
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To elucidate the mechanism for defective B1 cell homing, RT-PCR and real-time quantitative PCR analysis on BLC expression in the peritoneum were performed. BLC was expressed in the peritoneal cells in both young and aged BWF1 mice, although BLC gene expression was significantly lower in aged BWF1 mice than in young mice (Fig. 4A, a and b). It contrasted with our previous study demonstrating that BLC gene expression was markedly increased in the target organs including the kidney, lung, and thymus in aged BWF1 mice (3). RT-PCR analysis on highly purified leukocyte subpopulations demonstrated that BLC was highly expressed in peritoneal CD11bhighCD11clow cells (Fig. 4B). Sorted CD11bhighCD11clow cells showed a morphological appearance of macrophages and expressed F4/80, a cell surface marker for macrophage while they were low in class II expression and negative for B220 (Fig. 5A). These findings were consistent with previous reports (14). Furthermore, immunocytochemical analysis confirmed BLC protein expression by peritoneal macrophages both in young and aged BWF1 mice (Fig. 5B).
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Although the number of total peritoneal cells and B1 cells in the peritoneal cavity was significantly increased in aged BWF1 mice compared with those in young BWF1 mice (Fig. 6, A and B), the number of peritoneal macrophages in aged BWF1 mice was markedly reduced compared with those in young BWF1 mice (Fig. 6C).
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| Discussion |
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Coinjection of B1 and B2 cells into aged BWF1 mice resulted in preferential recruitment of B1 cells in the target organs such as the thymus, kidney, and lung, whereas a similar number of B1 and B2 cells was recruited to the spleen. These results are also well explained by our previous findings that B1 cells are more efficiently chemoattracted toward BLC than B2 cells (3). Because BLC expression in high endothelial venules is reported in chronically activated lymphoid tissues such as tonsils and Peyers patches (16, 17), BLC expression in high endothelial venules in inflammatory cellular infiltrates is also possibly involved in preferential B1 cell recruitment to the target organs. Chemotaxis assay showed that B1 cells migrated toward stromal cell-derived factor 1 (SDF-1) and secondary lymphoid tissue chemokine (SLC) in addition to BLC, and that only BLC showed preferential chemotactic activity to B1 cells (data not shown). It has been reported recently that SDF-1 expression is enhanced in the glomeruli in the kidney in aged BWF1 mice (18). However, RT-PCR analysis showed no increase in the level of SDF-1 expression in the target organs between young and aged BWF1 mice (data not shown). SLC is another chemokine that may possibly affect B1 cell homing to the peritoneal cavity. However, little SLC expression was detected in peritoneal cells both in young and aged BWF1 mice (data not shown). Constitutive SLC expression was observed in the target organs such as the kidney as well as in lymphoid tissues in both young and aged BWF1 mice. These results suggest that preferential recruitment of B1 cells in the target organs such as the kidney and lung compared with B2 cells is most likely attributed to high and ectopic expression of BLC in these organs. It is unlikely that migrated cells in the target organs were contaminated macrophages or other cell type, because the purity of B1 cells (CD5+B220+F4/80-) was >95%. It was also confirmed that all CD5+B220+F4/80- B cells were CD19 positive. We, therefore, favor the idea that ectopic high expression of BLC in the target organs and decreased number of BLC-producing macrophages in the peritoneal cavity contribute to aberrant B1 cell trafficking in aged BWF1 mice.
Although we do not know the physiological significance of B1 cell homing to the peritoneal cavity at this moment, it is possibly required to maintain the level of IgM natural Abs, because B1 cells differentiate into IgM plasmablasts in the presence of Ag-primed myeloid dendritic cells or peritoneal macrophages (19). Natural IgM Abs, which are secreted mainly by B1 cells, constitute most of circulating IgM and protect systemic bacterial infection (20, 21, 22). Unlike mammalian DNA, bacterial DNA has potent immunostimulatory effects that lead to polyclonal B cell activation as well as the production of specific Abs in mice (23). It is reported that bacterial DNA induces anti-dsDNA Ab cross-reactive to mammalian dsDNA only in autoimmune-prone mice such as in BWF1 mice (24). IgG anti-DNA Ab are a serologic hallmark for SLE and important mediators for kidney damage (25, 26). Furthermore, accumulating data suggest that B1 cells play an important role in mucosal immunity in the gut (13). Macpherson et al. (12) suggested that IgA produced by B1 cells in the gut contributed to the defense against direct penetration of commensal bacteria into the systemic circulation. Therefore, it is tempting to speculate that aberrant B1 cell trafficking in aged BWF1 mice may result in penetration of pathogens into systemic circulation and induction of vigorous IgG anti-DNA Ab production that cross-reacts with mammalian DNA.
It has been a matter of debate whether B1 cells are involved in IgG autoantibody production in aged BWF1 mice because of the lack of firm evidence for IgG class switching in vivo. Recent study has suggested that CXCR5+CD4+ T cells designated as follicular Th cells enhanced IgG and IgA response in the absence of APCs (16, 27). Preliminary experiments showed that the number of CXCR5+CD4+ T cells with the phenotype of follicular Th cells was increased in aged BWF1 mice, and that CD4+ T cells obtained from aged BWF1 mice enhanced IgG Ab production by B1 cells (data not shown). We are now extensively investigating the role of CXCR5+CD4+ T cells in IgG autoantibody production by B1 cells.
Collectively, we have demonstrated that B1 cells fail to home to the peritoneal cavity and are preferentially recruited to the target organs in aged BWF1 mice developing murine lupus. These findings would provide a new insight into the pathological significance of B1 cells in the development of SLE and a novel way for the regulation of murine lupus by targeting B1 cell trafficking.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Kouji Matsushima, Department of Molecular Preventive Medicine, School of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail address: koujim{at}m.u-tokyo.ac.jp ![]()
3 Abbreviations used in this paper: SLE, systemic lupus erythematosus; BLC, B lymphocyte chemoattractant; pAb, polyclonal Ab; PBLk, peripheral blood leukocyte; CMTMR, 5-(and-6)-(((4-chloromethyl)benzoyl)amino)tetramethylrhodamine; SDF-1, stromal cell-derived factor 1; SLC, secondary lymphoid tissue chemokine. ![]()
Received for publication July 9, 2003. Accepted for publication January 20, 2004.
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