Human CD72 splicing isoform responsible for resistance to systemic lupus erythematosus regulates serum immunoglobulin level and is localized in endoplasmic reticulum
© Hitomi et al.; licensee BioMed Central Ltd. 2012
Received: 17 September 2012
Accepted: 21 December 2012
Published: 26 December 2012
CD72 is an inhibitory co-receptor expressed on B cells. We previously demonstrated significant association of the polymorphism of the CD72 gene with susceptibility to human systemic lupus erythematosus (SLE) in individuals carrying a SLE-susceptible FCGR2B genotype (FCGR2B-232Thr/Thr). The human CD72 locus generates a splicing isoform that lacks exon 8 (CD72Δex8) as well as full-length CD72 (CD72fl), and the CD72 polymorphism regulates exon 8 skipping.
Here we demonstrated that individuals carrying the disease-protective CD72 genotype exhibit significantly lower serum immunoglobulin levels than do individuals carrying other CD72 genotypes (P < 0.05). Although expression level of CD72fl in the peripheral blood B cells was similar regardless of CD72 genotype, the protein level of CD72Δex8 was increased in individuals carrying the disease-protective CD72 genotype, suggesting a crucial role of CD72Δex8 in regulation of antibody production. By expressing these human CD72 isoforms in mouse cell lines, we further demonstrated that CD72Δex8 is accumulated in endoplasmic reticulum (ER) and fails to regulate BCR signaling whereas human CD72fl is efficiently transported to the cell surface and inhibits signaling through the B cell antigen receptor (BCR), as is the case for mouse CD72.
Human CD72 polymorphism appears to regulate antibody production as well as susceptibility to SLE by regulating expression of ER-localizing CD72Δex8.
KeywordsPolymorphism Exon skipping C-type lectin domain
CD72, a 45 kDa type II membrane protein expressed on B cells, is an inhibitory co-receptor that regulates signaling through the B cell antigen receptor (BCR) [1–6]. Both human and mouse CD72 contains a C-type lectin-like domain in the extracellular region and an immunoreceptor tyrosine-based inhibition motif (ITIM) in the cytoplamic region [1–3]. Mouse CD72 negatively regulates BCR signaling by recruiting Src homology 2 domain-containing protein tyrosine phosphatase-1 (SHP-1) at ITIM [4–6]. However, the signaling function of human CD72 remains unknown.
Four human CD72 polymorphisms have been identified in the upstream regulatory region and introns . These CD72 polymorphisms constitute two major haplotypes, CD72*1 and CD72*2. We previously demonstrated that FCGR2B-Ile232Thr, a gene polymorphism of FcγRIIb, is significantly associated with SLE in Asian populations, and is a risk factor for SLE only in individuals with CD72*1/1[7, 8]. This finding indicates that CD72*2 confers resistance to SLE in individuals carrying FCGR2B-Ile232Thr. Two polymorphisms in intron 8 regulate generation of an alternative splicing isoform (CD72Δex8) that skips exon 8 independently; probably act in combination as cis-acting intronic splicing enhancer (ISE) or silencer (ISS) . Exon 8 encodes the C-terminal part of the C-type lectin-like domain and the stop codon, and skipping of it results in replacement of the C-terminal part of the C-type lectin-like domain by a sequence encoded in exon 9 in CD72Δex8. The ratio of mRNA level of CD72Δex8 to that of full-length CD72 (CD72fl) is strikingly higher in B cells from individuals with the CD72*2/2 or CD72*1/2 genotype than in those with CD72*1/1. Because there are no substitutions in exons among different CD72 haplotypes, these findings strongly suggest that increased CD72Δex8 level, decreased CD72fl level, or both are responsible for the resistance of CD72*2-carrying individuals to SLE.
In the present study, we addressed the functional properties of CD72 isoforms. Analysis of healthy individuals revealed that those with CD72*1 express a significantly lower level of the CD72Δex8 protein in B cells, and show the higher level of serum immunoglobulins than those carrying CD72*2, suggesting that CD72Δex8 regulates the immunoglobulin level as well susceptibility to SLE. Analysis using B cell transfectants expressing CD72Δex8 demonstrated that CD72Δex8 does not regulate BCR signaling but accumulates in the endoplasmic reticulum (ER). Thus, the CD72 polymorphism regulates antibody production and autoimmunity by modulating the level of ER-localizing CD72Δex8.
cDNAs including the entire coding region of human CD72fl or CD72Δex8 but not nucleotides for the stop codon were obtained by RT-PCR from peripheral blood mononuclear cells (PBMCs) with a pair of specific primers (5′-GCA GAG CTG CTC AGG ACC AT-3′ and 5′-ACC CCA TTC TAC CAT GGG AA-3′). The cDNAs encoding CD72fl and CD72Δex8 were inserted with a pair of oligonucleotides encoding FLAG-tag into the retrovirus expression vector pMX-ires-GFP, and the resulting plasmids were termed pMX-CD72fl and pMX-CD72Δex8, respectively.
The retrovirus expression plasmids pMX-CD72flYF and pMX-CD72Δex8YF encoding the mutants of CD72fl and CD72Δex8, in which tyrosine7 is replaced by phenylalanine, were generated by PCR-based site-directed mutagenesis using a specific primer set (5′- GCA GAT CTG AGG TTT GTG AA -3′ and 5′- AAA GGT GAT GGC CTC AGC CA -3′).
The mouse B cell lines WEHI-231 and K46μv and the human B cell line Raji were described previously [9, 10], and cultured in RPMI 1640 medium supplemented with 10% FCS, 50 μM 2-ME, and 1 mM glutamine. The mouse fibroblast cell line Balb/c-3T3 was cultured in DMEM medium supplemented with 10% FCS and 1 mM glutamine. Retrovirus-mediated gene transfer was performed as described previously .
PBMCs were obtained from unrelated healthy Japanese living in the central part of Japan where genetic background is shown to be relatively homogeneous . Informed consents were obtained from these indiciduals prior to collecting samples. Peripheral B lymphocytes were isolated from PBMCs by an autoMACS cell sorter (Miltenyi Biotec, Auburn, CA) using the B cell isolation kit II. This study was approved by the Research Ethics Committees of the Graduate School of Medicine, The University of Tokyo.
Human CD72 haplotype was determined by genotyping CD72-VNTR, the tag polymorphism, by PCR-SSLP method as described previously . FCGR2B-Ile232Thr was genotyped by nested PCR and fluorescence resonance energy transfer (FRET) technology as described previously .
Serum IgG level
Serum IgG levels in healthy individuals were measured by turbidimetric immunoassay.
Peripheral blood B cells were incubated with FITC-labeled anti-human CD72 mAb J4-117 (BD Biosciences, San Jose, CA). B cell transfectants were incubated with rabbit anti-FLAG Ab (Cell Signaling Technology, Danvers, MA), followed by reaction with PE-labeled goat anti-rabbit IgG (Southern Biotech, Birmingham, AL). Alternatively, transfectants were stained with NP-conjugated PE. Cells were then analyzed by flow cytometry using a FACSCalibur (BD Biosciences).
Generation of anti-CD72Δex8 antibody
Rabbits were immunized with the peptide specific for CD72Δex8 (Ala-Asp-Pro-His-Leu-Thr-Leu), and serum IgG was purified from rabbit serum by ammonium sulfate precipitation. The specificity of the antibody was confirmed by Western blotting of total cell lysates of COS-7 CD72Δex8 transfectant using pre-immune and immune serum.
Immunoprecipitation and Western blotting
Cells were stimulated with either 0.2 μg/ml NP15-coupled BSA (NP-BSA) or 10 μg/ml anti-IgM Ab, and were lysed in Triton X-100 lysis buffer [9, 10]. Lysates were immunoprecipitated with anti-FLAG Ab (Sigma-Aldrich, St. Louis, MO) together with protein G-Sepharose (GE Healthcare Life Sciences, Piscataway, NJ). Total cell lysates or immunoprecipitates were separated on SDS-PAGE and were transferred to polyvinylidene difluoride membranes. Membranes were incubated with anti-phosphotyrosine mAb 4G10 (Millipore), anti-β-tubulin mAb TUB2.1 (Seikagaku Kogyo, Tokyo, Japan), rabbit anti-FLAG Ab, rabbit anti-CD72 Ab (Santa Cruz Biotechnology, Santa Cruz, CA), rabbit anti-phospho ERK Ab (Cell Signaling Technology), rabbit anti-SHP-1 Ab (Santa Cruz Biotechnology), or rabbit anti-CD72Δex8 Ab. Proteins were visualized with an ECL system.
Measurement of intracellular calcium mobilization
Cells were incubated in culture medium containing 5 μg/ml fluo-4-AM (Molecular Probes) for 30 min. Cells were stimulated with 0.2 μg/ml NP-BSA, and were analyzed by flow cytometry using a FACSCalibur.
Assay of apoptotic cells
Cells were incubated with thapsigargin or staurosporine for 24 h and 48 h, and cells containing hypodiploid DNA were measured by flow cytometry .
Cells were fixed in 4% PFA for 15 min, washed with PBS, and permeabilized with 0.1% Triton X-100 for 15 min. Cells were then washed with PBS, blocked with 2% BSA for 30 min, and stained for FLAG-tagged human CD72 and organelle-specific marker proteins in ER, mitochondria, Golgi apparatus, early endosomes, and late endosomes using rabbit anti-FLAG Ab, anti-KDEL mAb 10C3 (Stressgen, MI), anti-cytochrome C mAb 6H2.B4 (BD Biosciences), anti-GM130 mAb 35 (BD Biosciences), anti-transferrin receptor mAb H68.4 (Invitrogen-Life Technologies), and anti-lamp1 mAb 1D4B (Southern Biotech), respectively. Cells were then analyzed by laser scanning confocal microscopy (Model LSM 510; Carl Zeiss Inc., Jena, Germany).
The serum IgG level among CD72 genotypes was analyzed using regression analysis and Kruskal-Wallis rank sum test. Relative expression of CD72Δex8 protein product between CD72 genotype was analyzed using Mann–Whitney U test. Otherwise, Student’s t-test was used for statistical analysis.
Decreased serum immunoglobulin levels in CD72*2-carrying individuals
Human CD72fl but not CD72Δex8 regulates BCR signaling
In K46μv transfectants, surface expression of CD72Δex8 isoform was much lower than that of CD72fl (Figure 2A), although the amount of CD72Δex8 in total cell lysates was much larger than that of CD72fl (Figure 2E), suggesting poor efficiency of transport of CD72Δex8 to the cell surface. Equivalent ERK phosphorylation and Ca2+ influx by NP-BSA treatment in K46μv CD72Δex8 cells indicate that CD72Δex8 does not regulate BCR signaling. CD72Δex8 is thus poorly expressed on the cell surface and does not regulate BCR signaling.
Increased CD72Δex8 expression in CD72*2-carrying peripheral B-cells
CD72Δex8 accumulates in ER
Here we addressed humoral immunity and expression of the two isoforms of human CD72, i.e., CD72fl and CD72Δex8, in healthy individuals carrying different CD72 haplotypes as the coding sequences of these isoforms were the same regardless of different CD72 haplotypes. Individuals carrying higher numbers of CD72*2 allele showed lower serum levels of IgG, clearly demonstrating association of, CD72*2 with low serum Ig as well as reduced risk for SLE . The cell surface expression level of CD72fl on peripheral B cells was similar regardless of the number of CD72*2 allele. This is in agreement with the previous study in which flow cytometry demonstrated relatively homogeneous CD72 expression on B cell surface in healthy individuals . Thus, BCR signal regulation mediated by CD72fl in B cells does not differ among each CD72 genotypes because BCR regulation appears to occur on the cell surface. In contrast, Individuals carrying higher numbers of CD72*2 allele showed higher levels of CD72Δex8 in peripheral B cells. Although the sample size of this analysis is relatively small, this result is in agreement with our previous study with a larger sample size (n=32) demonstrating that the ratio of CD72Δex8 to CD72fl mRNA correlates to the number of CD72*2 allele , and suggests that presence of CD72*2 allele increases the protein level of CD72Δex8 but not CD72fl. Taken together, CD72Δex8 but not CD72fl is responsible for difference in serum Ig and risk for SLE among individuals carrying different CD72 genotypes.
We further examined the functional activity of CD72fl and CD72Δex8, in signal regulation and cellular localization by expressing these isoforms in mouse B cell lines K46μv and WEHI-231, human B cell line Raji and fibroblasts. As is the case for mouse CD72, CD72fl negatively regulated BCR signaling in ITIM-dependent fashion. In contrast, CD72Δex8 did not regulate BCR signaling, probably due to its poor surface expression. CD72Δex8 was accumulated in ER in fibroblasts and augmented apoptosis induced by ER-stress in WEHI-231 and Raji, demonstrating ER localization of CD72Δex8. Thus, ER-localizing CD72Δex8 is responsible for the regulation of antibody production as well as susceptibility to SLE mediated by CD72 polymorphism.
CD72Δex8 is generated by alternative splicing that skips exon 8, causing replacement of an exon 8-encoded stretch of 42 amino acids in the C-type lectin domain in the extracellular region by a totally different sequence of 49 amino acids encoded by exon 9 . This extensive change in the extracellular C-type lectin domain may cause accumulation of CD72Δex8 in ER. The change in the C-type lectin domain may induce misfolding of the CD72Δex8 protein, which causes ER retention of the protein. Splicing isoforms of surface proteins such as human FcγRIb, mouse glutamate receptor 7b (GluR7b), and rat voltage-dependent and Ca2+−activated K+ channel (MaxiK) are mostly retained in the ER [20–22]. Whether these ER-localized isoforms play a role in cellular functions need to be elucidated in future studies.
We previously demonstrated that CD72*2 specifically abrogates the susceptibility to SLE conferred by the allele of FcγRIIb carrying the substitution of 232Ile by 232Thr (FCGR2B-Ile232Thr) , which is suggested to reduce the inhibitory function of FcγRIIb . FcγRIIb-deficient mice exhibit loss of B cell self-tolerance at the IgG+ B cell and plasma cell stages, and development of SLE-like disease , demonstrating that FcγRIIb provides a distal peripheral checkpoint limiting the accumulation of self-reactive plasma cells and thereby preventing SLE. The Ile232Thr substitution may thus reduce the activity of FcγRIIb in maintaining self-tolerance, and thereby increase risk for SLE. Our findings suggest that CD72*2 augments apoptosis of B cells by enhancing expression of CD72Δex8 (Figure 6). Augmented B cell apoptosis may reduce the number of self-reactive B cells, and autoantibody-producing plasma cells may not be generated at high frequency in CD72*2-carrying individuals even in the presence of inefficient self-tolerance caused by FCGR2B-Ile232Thr. Recently, over 30 loci were reported as the genes significantly associated with susceptibility to SLE by genome-wide association study (GWAS) . However, most of these loci carry very small effect, and the proportion of heritability explained by these variants is modest [26, 27]. Other than the effect of causal rare variant, it is possible to explain a part of “missing heritability” by the gene-gene interaction. This study, which shows the functional interaction between CD72 and FCGR2B, will be a functional evidence of such a gene-gene interaction.
Increasing numbers of studies have identified disease-susceptibility alleles associated with specific splicing patterns . Our previous  and the present studies have demonstrated that the genetic polymorphism of human CD72 associated with susceptibility to SLE regulates the efficiency of alternative splicing, although how the polymorphism affects alternative splicing is not known. Further elucidation of the molecular mechanisms of splice variations and their functional significance will provide important clues to the pathogenesis of autoimmune diseases and development of new therapeutic approaches.
Our finding that serum IgG levels in individuals carrying the CD72*2 allele including both individuals with CD72*1/2 and those with CD72*2/2 are significantly lower than those in individuals carrying CD72*1/1 clearly demonstrates that human CD72 polymorphism regulates humoral immunity as well as risk for SLE . Of the two splicing isoforms of human CD72, CD72Δex8 appears to be responsible for the regulation of Ig production and autoimmunity, as the expression level of CD72Δex8 but not the other isoform CD72fl is different between B cells from individuals with CD72*2 and those with CD72*1/1. Because CD72Δex8 is accumulated in ER and poorly transported to the cell surface, CD72Δex8 may regulate B cell activity by a mechanism distinct from CD72fl that is expressed on the surface and regulate signaling through BCR as is the case for mouse CD72. Although overexpression of CD72Δex8 enhances ER stress-induced apoptosis, further studies are required to elucidate how CD72Δex8 in ER regulates B cell activity.
B cell antigen receptor
Extracellular signal-regulated kinase
Genome-wide association study
Immunoreceptor tyrosine-based inhibition motif
Hapten (4-hydroxy-3-nitrophenyl) acetyl
Src homology 2 domain-containing protein tyrosine phosphatase-1
Systemic lupus erythematosus.
We thank Dr. T. Kitamura (The University of Tokyo, Japan) for providing Plat-E and Plat-A cells, Drs. M. Hagiwara, S. Shimizu, T. Mizuta (Tokyo Medical and Dental University, Japan), T. Suzuki (Mitsui Memorial Hospital, Japan), D. Sakurai (Dokkyo Medical University, Japan) and A. Fujimoto (RIKEN, Japan) for valuable suggestions, and K. Mizuno and A. Yoshino (Tokyo Medical and Dental University, Japan) for technical assistance.
This work was supported in part by grants from the Ministry of Education, Culture, Sports, Science, and Technology of Japan and the Japan Society for the Promotion of Science.
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