CEACAM1 is expressed on B-cell subsets
We first analysed CEACAM1 expression on various cell populations in the peripheral blood of wild-type (WT) mice. Erythrocytes stained negative for CEACAM1 (Supplementary Fig. 1). As previously reported343536, high levels of CEACAM1 expression were detected on blood granulocytes (Ly6G+) and monocytes (CD115+) with the anti-CEACAM1-specific monoclonal antibody (clone CC1, Fig. 1a). Cells from Ceacam1−/− mice stained negative for CEACAM1 (Fig. 1a). Next, we analysed CEACAM1 expression on lymphoid cells in the blood. CD90.2 cells, representing primarily T cells, showed weak CEACAM1 expression by individual cells (Fig. 1b), a finding suggesting that various T-cell subpopulations may differentially express CEACAM1. B cells in peripheral blood expressed CEACAM1 at high levels (Fig. 1b). Evaluation of precursor (B220+CD43−IgM+IgDlow) and mature B cells (B220+CD43−IgDhigh) in the bone marrow revealed strong CEACAM1 expression in the B-cell lineage (Fig. 1c). In line with this finding, lymph node B cells, follicular B cells (FO, B220+CD19+CD21/35loCD23hi) and MZ B cells (B220+CD19+CD21/35hiCD23lo) in the spleen expressed CEACAM1 at high levels (Fig. 1d,e). Furthermore, quantitative RT–PCR analyses of expression of long and short isoforms of CEACAM1 showed that murine splenic B cells express more long than short isoforms (Fig. 1f). In conclusion, strong CEACAM1 expression was detected in all B-cell subpopulations tested, a finding implying that CEACAM1 is functionally relevant in B cells.
CEACAM1 induces survival genes via Syk and Erk and NF-κB
Recently, expression of CEACAM1 in granulocytes was shown to lead to Syk phosphorylation37. In addition, expression of CEACAM1 participates in Btk phosphorylation38. In B cells, Syk and Btk are essential signalling molecules, which are phosphorylated after BCR or BAFFR crosslinkage and play an essential role in induction of survival signals1539. We thus focused on the influence of CEACAM1 on Syk activity during CEACAM1 and BCR crosslinkage. Indeed, in B-cell lymphoma immunoprecipitation of CEACAM1 pulled down Syk (Supplementary Fig. 2), which confirms the recently published interaction of Syk and CEACAM1 (ref. 37). To get insights into the functional relevance of CEACAM1 in B cells, we isolated splenocytes from WT and Ceacam1−/− mice and challenged them with anti-IgM, anti-CEACAM1 activating antibody (monoclonal antibody, CC1) or lipopolysaccharide (LPS). Ig-α, which associated with BCR, showed slight tyrosine-phosphorylation after IgM crosslinkage (Fig. 2a, left panel). Neither anti-CEACAM1 monoclonal antibody challenge nor LPS treatment induced phosphorylation of Ig-α (Fig. 2a, right panel). We subsequently analysed Syk tyrosine phosphorylation in WT and Ceacam1−/− mice. As expected BCR crosslinkage induced strong Syk phosphorylation. Absence of CEACAM1 strongly reduced Syk phosphorylation after BCR crosslinkage (Fig. 2b, left panel). Challenge with anti-CEACAM1 monoclonal antibody also triggered phosphorylation of Syk, which was stronger than LPS-induced Syk phosphorylation (Fig. 2b, right panel). Therefore, we concluded that CEACAM1 enhances Syk phosphorylation not only directly, but also after BCR crosslinkage. Next we analysed the downstream target of Syk, Btk, another member of the Tec family of non-transmembrane PTKs. Flow cytometric analysis showed that Btk is transiently phosphorylated in B cells derived from WT but not in Ceacam1−/− mice after challenge with anti-IgM (Fig. 2c, upper panel) antibody. Similarly, anti-CEACAM1 monoclonal antibody challenge induced Btk phosphorylation in WT B cells (Fig. 2c, lower panel). We therefore conclude that CEACAM1 directly influences the phosphorylation of Syk and Btk, after BCR and CEACAM1 crosslinkage. To gain insight into downstream targets of CEACAM1 activation, we analysed specific Syk substrates. We observed that the CEACAM1 signal phosphorylate Erk1 (Erk-42) but not Erk2 (Erk-44) and, to a lesser extent, p-38. LPS could phosphorylate both Erk1/2 and p-38 (Fig. 2d,e). Triggering with the anti-CEACAM1 monoclonal antibody induced degradation of IκBα and slight phosphorylation of p65, a finding suggesting that CEACAM1 signals activate the canonical NF-κB pathway (Fig. 2f).
Tonic BCR signalling followed by Syk phosphorylation induces several survival genes, which are essential for B cell survival12. To see whether lack of CEACAM1 influences expression of those genes, we cultured purified splenic B cells from WT and Ceacam1−/− mice for 24 h without further stimulation and measured mRNA expression of various NF-κB-regulated genes. Indeed, lack of CEACAM1 led to reduced expression of Bcl6, Pax5, Bcl2a1 and Xiap (Fig. 2g). In line with the known positive feedback loop of NF-κB, we found reduced expression of NF-κB p65, Rel-B, Nfatc1 and Nfatc2 in Ceacam1−/− B cells (Fig. 2h,i). c-Jun, c-Fos and Ap1 were not influenced by CEACAM1 expression (Fig. 2j). Blimp1, which is another key regulator of plasma cell differentiation, was not affected by CEACAM1 expression (Fig. 2k). Taken together, we found that CEACAM1 contributed to the Syk phosphorylation during BCR and/or CEACAM1 crosslinkage, leading to the enhanced expression of NF-κB, NFAT and survival genes.
CEACAM1 promotes survival of B cells in vitro
We found that expression of CEACAM1 enhanced BCR-dependent Syk phosphorylation. Therefore, absence of CEACAM1 reduced expression of survival genes, which are induced via the tonic BCR signal. To see how lack of CEACAM1 influenced proliferation and survival of B cells, we labelled WT and Ceacam1−/− B cells with carboxyfluorescein succinimidyl ester (CFSE) and challenged them with anti-CEACAM1 monoclonal antibody or the BCR-independent B-cell activator such as LPS, or CD40 ligand (CD40L) in combination with mouse interleukin (IL)-4. Challenge with anti-CEACAM1 (monoclonal antibody, CC1) induced some proliferation in WT but not in Ceacam1−/− B cells (Fig. 3a). Challenge with LPS (TLR4-ligand) or CD40/IL-4 induced proliferation that was comparable between WT and Ceacam1−/− B cells (Fig. 3a). Therefore, we conclude that the induction of proliferation via TLR4 or CD40 is not affected by CEACAM1.
Next, we analysed the impact of CEACAM1 on B-cell survival. First, we quantified the total number of living B cells with diamidino-2-phenylindole (DAPI) staining and fluorescence-activated cell sorting (FACS) beads. Without any further challenge, WT B cells consistently died under our in vitro culture conditions (Fig. 3b), a finding that agrees with that of a previous study40. Lack of CEACAM1 enhanced death of B cells (Fig. 3b). When challenged with CD40/IL-4, the numbers of WT B cells were higher than those of non-activated B cells after 24 h (Fig. 3b). The numbers of Ceacam1−/− B cells 24 h after stimulation with CD40/IL-4 increased similarly to those of WT B cells, however, declined rapidly 24 h after challenge with CD40/IL-4 (Fig. 3b). This rapid decrease was probably due to apoptosis and cell death, because Annexin-V staining was significantly enhanced in none activated and activated DAPI−, B cells (Fig. 3c). We postulate that defective survival of Ceacam1−/− B cells was due to limited Syk and Btk phosphorylation. To test this hypothesis, we treated WT and Ceacam1−/− B cells with the Btk inhibitor Ibrutinib41. We observed that inhibition of Btk in WT B cells led to the rapid death of B cells. This effect was less pronounced in Ceacam1−/− B cells (Fig. 3d). In conclusion, we found that lack of CEACAM1 signal limited survival of resting and activated B cells.
CEACAM1 promotes B-cell differentiation and survival in vivo
We found that CEACAM1 is expressed on all B-cell subpopulations analysed and that it influences the survival of B cells, especially after activation. During maturation, several activating stimuli allow B cells to progress through various stages (Fig. 4a). To determine the influence of CEACAM1 on survival during B-cell proliferation, we analysed B-cell subsets in bone marrow, peripheral blood, lymph node and spleen of WT and Ceacam1−/− mice. We found that B-cell precursor subsets in the bone marrow are reduced only in stages B, C and C′ (Fig. 4b). However, after stage C′ they expand more rapidly; therefore, the numbers of newly formed B cells in stages D and E are similar in WT and Ceacam1−/− mice (Fig. 4b). This finding is concordant with the similar numbers of newly formed (immature, B220+IgM+) B cells in the peripheral blood (Fig. 4c). These findings are also in line with previously published experiments showing that there is no difference in B-cell numbers in the peripheral blood of Ceacam1−/− mice27. Therefore, we conclude that the generation of newly formed B cells is hardly affected by CEACAM1. B-cell analysis in the peritoneal cavity showed that the frequency of B2 (CD19+B220+CD43−) B cells was slightly reduced. B1a (CD19+B220+CD43+CD5+) B-cell frequency was significantly reduced, whereas the B1b (CD19+B220+CD43+CD5−) B-cell population was higher in Ceacam1−/− mice (Supplementary Fig. 4).
Next, we examined whether CEACAM1 signals are essential for B cells in secondary lymphoid organs, where newly formed B cells undergo further proliferation and differentiation. In lymph nodes, the numbers of newly formed (CD19+IgM+IgDlow) B cells in WT and Ceacam1−/− mice were similar (Fig. 4d). In contrast, the numbers of matured (M, CD19+IgDhigh) B cells were significantly reduced in the lymph nodes of Ceacam1−/− mice (Fig. 4d). In the spleen, the T1, T2 and T3 populations of transitional B cells were reduced in the absence of CEACAM1 (Fig. 4e). The numbers of follicular B cells I (Fol I) and II (Fol II), MZ precursor cells and MZ B cells were dramatically reduced in the spleen of Ceacam1−/− mice (Fig. 4e). In line with this finding, the numbers of mature B cells (stage F, B220+CD43−IgDhigh) in the bone marrow were reduced in Ceacam1−/− mice (Fig. 4b). Similarly, histologic analysis showed lower numbers of B cells in the spleen and lymph node (Fig. 4f) and the absence of MZ B cells as well as low numbers of metallophilic (CD169+) macrophages in Ceacam1−/− mice (Fig. 4g). To strengthen our hypothesis that reduced Syk phosphorylation after B-cell-activating factor receptor (BAFFR) or BCR activation, but not TLR or Fcgr2b signalling, is responsible for reduced B-cell numbers in Ceacam1−/− mice, we performed histology of Jh−/−, Baffr−/−, Fcgr2b−/− and Myd88/Trif−/− mice. Histological analysis showed that a lack of BCR and BAFFR signalling, but neither TLR nor Fcgr2b signalling, reproduces the B-cell deficiency seen in Ceacam1−/− mice (Supplementary Fig. 5).
Next, we determined whether defective intrinsic survival signals are responsible for reduced B-cell numbers in Ceacam1−/− B cells, we transferred the same number of B cells from WT or Ceacam1−/− mice into B-cell-deficient (Jh−/−) mice. Adoptively transferred WT B cells survived in the Jh−/− mice and repopulated the spleen within 30 days (Fig. 4h). In contrast, Ceacam1−/− B cells were not detectable after 30 days (Fig. 4h), a finding suggesting that the absence of intrinsic CEACAM1 limits the survival of B cells after proliferation in Jh−/− mice. To gauge the direct influence of CEACAM1 expressed on B cells, we made use of mixed bone marrow chimeras, where the irradiation of WT (CD45.2) mice was followed by reconstitution with bone marrow from WT (CD45.1) and WT (CD45.2, mixed 1:1) or WT (CD45.1) and Ceacam1−/− (CD45.2, mixed 1:1) mice. Analysis of peripheral blood showed some, but not statistically significant differenc in blood B-cell levels between WT and Ceacam1−/− cells (Fig. 4i). In contrast, splenic MZ B cells were only derived from WT (CD45.1) mice (Fig. 4j). FO B cells were also mainly from WT (CD45.1) origin (Fig. 4j). This suggests that CEACAM1 B-cell intrinsic signals are essential for B-cell survival after B cells home to the spleen. Next, we analysed how the lack of CEACAM1 contributes to antibody production in unchallenged mice. Analysis of various immunoglobulin (Ig) subtypes in the serum of naïve WT and Ceacam1−/− mice showed a reduction in IgM, IgG1, IgG2a, IgG2b, IgG3, IgA and IgE in Ceacam1−/− mice (Supplementary Fig. 6). In conclusion, CEACAM1 expression influences survival in mature B cells.
CEACAM1 ensures mouse survival during VSV challenge
Next, we analysed the role of CEACAM1 during antigen challenge. We infected mice with VSV, a cytopathic pathogen for which systemic control strongly depends on the rapid induction of neutralizing antibodies42. We found strong expression of CEACAM1 in the MZ (Fig. 5a), a site at which VSV-specific B cells become activated4344. Because CEACAM1 is a natural CEACAM1 ligand, this finding suggests that CEACAM1 may release an important survival signal during antigenic challenge. To determine the role of CEACAM1 during B-cell activation, we transferred same number of B cells from Vi10 mice (WT × Vi10), which express a VSV-specific BCR or from Vi10 mice crossed to Ceacam1−/− mice (Ceacam1−/− × Vi10), into WT mice and infected them with VSV. After 3 days, the number of WT × Vi10 B cells but not Ceacam1−/− × Vi10 B cells was expanded (Fig. 5b). In the absence of stimulation, Ceacam1−/− × Vi10 B cells were reduced as compared to WT × Vi10 B cells (Fig. 5b), a finding indicating that CEACAM1 provides an important survival signal in antigen-activated and non-activated B cells. In line with these data, Ceacam1−/− mice displayed delayed induction of total VSV-neutralizing antibodies (Fig. 5c, left panel). In agreement with this finding, Ceacam1−/− mice failed to generate neutralizing IgG antibodies (Fig. 5c, right panel). ELISA confirmed that VSV-specific IgG levels were significantly low in Ceacam1−/− mice (Supplementary Fig. 7, right panel). Notably, anti-VSV IgM levels were significantly reduced in Ceacam1−/− mice (Supplementary Fig. 7, left panel). During challenge with non-replicating ultraviolet-inactivated VSV, Ceacam1−/− mice did not secrete neutralizing antibodies (Fig. 5d), a finding suggesting that Ceacam1−/− mice exhibit strongly impaired B-cell functions. To demonstrate that CEACAM1 expression by B cells is responsible for poor VSV-neutralizing antibody responses, we adoptively transferred CEACAM1-competent VSV-specific B cells into WT and Ceacam1−/− mice. This adoptive transfer of B cells rescued VSV-neutralizing antibody responses (Fig. 5e). Detailed analysis showed that both MZ and FO B cells could rescue the VSV-neutralizing antibody response in the Ceacam1−/− mice (Supplementary Fig. 8). Absence of early neutralizing antibodies can result in the spread of VSV to the nervous system45. To determine whether a reduction in the levels of neutralizing antibodies in Ceacam1−/− mice affected the immune response of virus control, we analysed organ virus titres 8 days after systemic VSV infection. Indeed, Ceacam1−/− mice exhibited VSV replication in the brain and spinal cord, whereas WT mice controlled VSV replication in all organs (Fig. 5f). VSV replication in Ceacam1−/− brains was associated with the death of mice (Fig. 5g). To determine whether a diminished B-cell response contributed to mortality, we performed survival experiments. Ceacam1−/− mice with adoptively transferred VSV-specific WT B cells survived VSV infection (Fig. 5h). Therefore, B-cell intrinsic expression of CEACAM1 is essential for initiating a protective antiviral immune response after exposure to VSV.
CEACAM1 facilitates LCMV-dependent B-cell activation
Next, we infected WT and Ceacam1−/− mice with non-cytopathic lymphocytic choriomeningitis virus (LCMV) to confirm whether antigen-specific B-cell responses are impaired in Ceacam1−/− mice. Early control of an infection dose of 200 plaque-forming units (PFUs) of LCMV depends only on CD8+ T cells. However, at higher infection doses, B cells are important for LCMV control46. WT mice produce higher titres of LCMV-specific antibodies than do Ceacam1−/− mice during LCMV infection (Fig. 6a). In line with this finding, LCMV replication in the liver of Ceacam1−/− mice was prolonged (Fig. 6b).
Human B-cell subpopulations express CEACAM1
To determine whether CEACAM1 also plays a potential role in human B cells, we measured CEACAM1 expression by naïve B cells. Previous publications reported that most B cells isolated from peripheral blood express CEACAM1 (ref. 32). Also, we found that naive human B cells (IgDhighCD27−) and memory B cells (CD27+) express substantial levels of CEACAM1 (Fig. 7a). The expression of memory B cells is slightly higher than that of naive B cells (Fig. 7b), a finding suggesting that the CEACAM1 signal in humans may be crucial for the survival of memory B cells rather than naïve B cells.
The results of this study demonstrate that CEACAM1 expression is essential for the survival of murine B cells. The absence of CEACAM1 expression on murine B cells is associated with reduced numbers of B cells and a defective immune response after viral antigen challenge.
Greicius et al.31 demonstrated that binding of CEACAM1 with an anti-CEACAM1 monoclonal antibody induces strong B-cell proliferation. Other studies showed that BCR activation in the presence of another anti-CEACAM1 monoclonal antibody (clone T84.1) limits B-cell proliferation4748. In the current study, we demonstrate that CEACAM1 crosslinkage had some effect on proliferation. However, in vitro and in vivo we found a strong influence of CEACAM1 on the survival of activated B cells. Therefore, we consider that a prolonged signal via CEACAM1 ligation is essential for promoting survival signals and maintaining B-cell numbers, which could explain reduced B-cell numbers in Ceacam1−/− mice.
BCR crosslinking by auto-antigens has an essential role in the differentiation of transitional B cells into MZ B cells and follicular B cells6. In addition, this tonic BCR signal, together with constant signalling via the BAFF receptor, is the most important survival signal for naïve B cells. How this diverse process can be regulated by BCR signals remains unexplained. We suggest that CEACAM1 is another strong regulator of B-cell survival. This is supported by previous findings that CEACAM1 regulates apoptosis in granulocytes49. Because CEACAM1 is expressed most strongly by B cells, and because CEACAM1 is its own ligand, we suggest that CEACAM1 induces positive survival signals primarily on B cells within the B-cell follicles and that CEACAM1 acts as a positive feedback loop once B cells reach the B-cell follicle. Therefore, in addition to chemokine gradients50, CEACAM1 expression appears to contribute to the generation of B-cell follicles.
We found that Syk phosphorylation after BCR crosslinkage was dependent on expression of CEACAM1. In line with these results, monoclonal antibody activation of CEACAM1 led to Syk phosphorylation. These data suggest that CEACAM1 directly interacts with BCR signalling and therefore would influence the tonic BCR signal as well as antigen-dependent BCR activation. Tonic BCR signalling, but also BAFFR signalling, induces several survival genes via Syk. Therefore, lack of BCR and BAFFR signalling limits B-cell development and survival. Indeed, in line with data from Jh−/− or Baffr−/− mice, Ceacam1−/− mice showed strongly reduced B-cell numbers in naïve mice. Therefore, we concluded that CEACAM1, in addition to BAFFR, is another important membrane molecule influencing BCR signalling via Syk.
We found that CEACAM1 activation leads to Btk phosphorylation. This finding is related to the enhanced survival of B cells. Loss-of-function mutations in BTK lead to X-linked agammaglobulinemia because of a complete absence of mature B cells51. Mice deficient in CD19 or BCR exhibit a strongly reduced number of mature B cells52. This reduction in the numbers of MZ B cells in CEACAM1-deficient mice is also found in mice and humans lacking the Wiskott-Aldrich syndrome protein (WASp)53. WASp is also involved in BTK phosphorylation54. As we have shown for CEACAM1, the necessity for WASp in the generation of the MZ B-cell compartment is B-cell intrinsic55. Whether CEACAM1 affects WASP function must be investigated in future studies.
We found that CEACAM1 is expressed on human B cells. This could suggest that also in human antiviral immune response, CEACAM1 signalling may play a role in inducing sufficient antibody responses against different viruses. Therefore, we would suggest that lack of CEACAM1 could be another factor explaining B-cell deficiency in humans. If indeed CEACAM1 deficiency occurs in humans, remains to be fully elucidated.
Recently, we found that CD169+ macrophages enforce viral replication and therefore are essential for initiating an antiviral immune response4456. B cells play an important role in recruiting CD169+ macrophages3. Therefore, B-cell deficiency leads to defects in enforced virus replication (Supplementary Fig. 9) and a reduction in the innate immune response. Indeed, we found that in Ceacam1−/− mice the ability of CD169+ macrophages to enforce virus replication was impaired (Supplementary Fig. 10). Which could be an additional mechanism how CEACAM1 deficiency in B cells contributes to defective anti-VSV immune response.
In conclusion, we found that CEACAM1 expressed on murine B cells is an important regulator of B-cell homeostasis. During exposure with cytopathic virus, CEACAM1 was essential for inducing an efficient antiviral antibody response and thereby reducing mouse mortality.
All mice used in this study, including Ceacam1−/−, Jh−/−, Tcrab−/−, Aid−/−, sIgM−/−, Myd88/Trif−/−, Baffr−/− and Fcgr2b−/− mice, were maintained on the C57BL/6 genetic background (back-crossed at least 8 times and as many as 16 times) and were bred as homozygotes. Vi10/CD45.1 mice expressing VSV-specific BCR as a transgene were used for cell transfer studies, and mice expressing the CD45.1 transgene were used for reconstitution of bone marrow. Six- to eight-week-old, age- and sex-matched mice were used for all the studies. All animals were housed in single ventilated cages. During survival experiments, the health status of the mice was checked twice daily. Animal experiments were authorized by the Nordrhein Westfalen Landesamt für Natur, Umwelt und Verbraucherschutz (Recklinghausen, Germany), and in accordance with the German law for animal protection or according to institutional guidelines at the Ontario Cancer Institute of the University Health Network and at McGill University. Animals exhibiting severe symptoms of sickness or paralysis or showing substantial weight loss during VSV infection were put to death and were considered dead for statistical analysis.
Bone marrow chimeras
For generation of bone marrow chimeras, C57BL/6 mice were irradiated twice for 7 min each with a total of 1,050 rad. After 24 h, mice were reconstituted intravenously with 5 × 106 bone marrow cells from each donor for mixed bone marrow chimeras. Mice were analysed 40–45 days after reconstitution.
Virus and plaque assays
VSV, Indiana strain (VSV-IND, Mudd-Summers isolate), was originally obtained from Professor D. Kolakofsky (University of Geneva, Switzerland). Virus was propagated on BHK-21 cells at a multiplicity of infection of 0.01. VSV concentration was determined as described below, and was then plaque-purified on Vero cells57. Mice were infected intravenously with VSV at the indicated doses. Virus titres were measured with a plaque-forming assay. For this assay, organs were smashed in Dulbecco’s modified Eagle medium (DMEM) containing 2% fetal calf serum (FCS), titrated 1:3 over 12 steps, and plaqued on Vero cells. After a 2-h incubation at 37 °C, overlay was added, and the virus preparation was again incubated at 37 °C. Plaques were counted 24 h later by the use of crystal violet staining. The LCMV strain WE was originally obtained from F. Lehmann-Grube (Heinrich Pette Institute, Hamburg, Germany) and was propagated on L929 cells, MC57 cells or both. Mice were infected intravenously at the indicated dose. LCMV viral titres were detected by a plaque-forming assay on MC57 fibroblasts as previously described58. In short, smashed organs were plaqued with MC57 cells as described above and incubated at 37 °C. After a 3-h incubation at 37 °C, overlay was added, and the virus preparation was again incubated at 37 °C. Plaques were counted 72 h later by LCMV NP staining. Cells were fixed (with 4% formaldehyde solution), permeabilized (with 1% Triton-X solution), blocked (with 10% FCS in phosphate-buffered saline) and stained with anti-LCMV NP antibody (made in house). ECL-conjugated anti-rabbit-IgG antibody was used as a secondary antibody. Plaques were detected by colour reaction (0.2 M Na2HPO4+0.1 M citric acid+30% H2O2+o-phenylenediamine dihydrochloride), all chemicals from Sigma-Aldrich.
Neutralizing antibody assay
Serum was prediluted (1:40). The complement system was inactivated at 56 °C for 30 min. For analysis of IgG kinetics, diluted samples were treated with 2-mercaptoethanol (0.1 M) for removal of IgM. Serum was titrated 1:2 over 12 steps and was incubated with 1 × 103 PFU of VSV. After a 90-min incubation at 37 °C, the virus–serum mixture was plaqued onto Vero cells. Overlay was added after 1 h, and the mixture was incubated again for 24 h at 37 °C. Plaques were counted by crystal violet staining. Antibody titres are presented as two- or threefold dilution steps (−log2 and −log3) times the predilution (that is, × 40).
Spleens retrieved from WT and Ceacam1−/− mice were homogenized in magnetic-activated cell sorting (MACS) buffer (1% FCS and 0.8% 0.5 M EDTA). B220+ B cells were isolated by positive selection with CD45R-conjugated magnetic beads (MACS Miltenyi Biotech). Flow cytometry confirmed that the purity of B220+ cells was higher than 95%. For proliferation assays, cells were labelled with 5 μM carboxyfluorescein succinimidyl ester, and 2 × 105 cells per well were cultured in 96-well flat bottom plates in RPMI 1640 medium supplemented with 10% LPS-free FCS, 1% antibiotics and 0.1% 50 mM 2-mercaptoethanol. They were then challenged with anti-CEACAM1 (20 μg ml−1, clone CC1; a kind gift from Dr Kathryn V. Holmes, University of Colorado, Denver, CO) antibody or recombinant mouse CD40 ligand (1 μg ml−1) in combination with mouse IL-4 (10 ng ml−1; R&D Systems) or LPS (100 ng ml−1; Sigma-Aldrich) for 48 h. Similarly, for inhibition experiments, purified B cells (as described above) were cultured in the medium described above with recombinant mouse CD40 ligand (1 μg ml−1) in combination with mouse IL-4 (10 ng ml−1; R&D Systems) and were treated with 10 μM Ibrutinib (Selleck). Control wells were supplemented with equal amounts of DMSO used to dissolve Ibrutinib. Cell death was measured by DAPI (Life Technologies) staining. For survival experiments, B cells were cultured as mentioned above, and cells were stained with Annexin-V (BD Biosciences) followed by DAPI. Proliferation and survival were determined by FACS at indicated time points. For cell signalling experiments, splenocytes were dissociated in VLE-DMEM supplemented with 10% LPS-free FCS and 1% antibiotics and were challenged with anti-CEACAM1 (clone CC1, 20 μg ml−1; K. Holmes) and anti-IgM (10 μg ml−1) antibody (Jackson ImmunoResearch Laboratories, Inc.) for various time periods at 37 °C.
Histologic analyses of snap-frozen tissue were performed with a monoclonal antibody to VSV glycoprotein (Vi10; made in-house). Anti-CD45R (B220; RA3-6B2), anti-CD90.2 (53-2.1), anti-CD19 (1D3), anti-CD1d (1B1) and anti-CEACAM1 (CC1) monoclonal antibodies were purchased from eBioscience. Biotin-anti-CD169/SIGLEC1 (MOMA-1) was purchased from Acris. Sections were washed and stained with streptavidin (eBioscience). In short, sections were fixed with acetone, and nonspecific antigens were blocked in PBS containing 2% FCS for 15 min, followed by various stainings of antibodies, diluted 1:100 in blocking solution for 45 min. Images of stained sections were acquired with a fluorescence microscope (KEYENCE BZ II analyzer).
Peripheral blood cells were stained with anti-Ly6G (RB6-8C5), anti-CD115 (AFS98), anti-CD45R (B220; RA3-6B2), anti-CD90.2 (30-H12), anti-CEACAM1 (CC1; with corresponding isotype control anti-IgG1 (M1-14D12)), anti-IgD (11-26c), anti-CD93 (AA4.1), anti-CD19 (1D3) (all from eBioscience) and anti-IgM (II/41; BD Biosciences) antibodies. Recovered bone marrow cells were resuspended in FACS buffer (0.5 M EDTA, 0.1% sodium azide, 1% FCS in PBS) and incubated with anti-CD45R (B220; RA3-6B2), anti-IgD (11-26c), anti-IgM (II-41), anti-CEACAM1 (CC1), anti CD24 (M1/69; all from eBioscience), anti-CD43 (1B-11), and anti-BP1 (6C3; from BioLegend) antibodies. Inguinal lymph nodes were disaggregated in FACS buffer, and cells were stained for anti-CD45R (B220; RA3-6B2), anti-CD19 (1D3), anti-IgD (11-26c; all from eBioscience), and anti-IgM (II/41; BD Biosciences) antibodies. Spleens were dissociated in FACS buffer, and splenocytes were incubated with anti-CD45R (B220; RA3-6B2), anti-CD19 (1D3), anti-CD93 (AA4.1), anti-CD21/35 (8D9), anti-CD23 (B3B4), anti-IgD (11-26c), anti-CEACAM1 (CC1), anti-CD45.1 (A20), anti-CD45.2 (104; all from eBioscience) and anti-IgM (II/41; BD) antibodies. Human peripheral blood samples were stained with anti-IgD (IA6-2) and anti-CD27 (M-T271) antibodies (both from BD Biosciences) and anti-CEACAM1 (monoclonal antibody, B3-17) antibody (from Dr Singer, Essen). Peritoneal B cells were stained for anti-CD19 (1D3), anti-CD45R (B220; RA3-6B2), anti-IgM (II/41; all from eBioscience), and anti-CD5 (53-7.3; BD Biosciences) and anti-CD43 (1B-11; from BioLegend) antibodies. Dead cells were discriminated by staining with propidium iodide (PI, eBioscience) and/or DAPI and were excluded from all analyses except for blood. For cell signalling experiments, cells were fixed and permeabilized according to the manufacturer’s instructions (BD Phosflow, BD Biosciences). The cells were stained with anti-Btk (pY223)/Itk (pY180) (BD Phosflow) antibody. All antibodies were diluted 1:100 to their original concentration in FACS buffer. For quantification of total cell numbers, FACS beads were used (BD Biosciences). All stained cells were analysed on an LSRII or a FACSFortessa (BD Biosciences) flow cytometer, and data were analysed with Flowjo software.
Spleens were dissociated, and splenocytes were subjected to erythrocyte lysis buffer (MORPHISTO). Next, 10 × 106 cells were challenged with or without anti-CEACAM1 (clone CC1, 20 μg ml−1) antibodies, anti-IgM (Jackson ImmunoResearch Laboratories, Inc., 10 μg ml−1) antibodies, and LPS (Sigma-Aldrich, 100 ng ml−1) treatment for indicated time points. Cells were lysed with boiling SDS buffer (1.1% SDS, 11% glycerol, 0.1 M Tris; pH 6.8) with 10% 2-mercaptoethanol. Total cell extracts were examined by 10% SDS–PAGE gels and transferred onto Whatman nitrocellulose membrane (GE Healthcare) by standard techniques. Membranes were blocked for 1 h in 5% BSA (PAA Laboratories) in TBS supplemented with 1% Tween-20 and incubated with the following antibodies: anti-phospho-Igα (Y182); anti-phospho-Syk/ZAP (Y352/Y319); anti-phospho-p44/42 (p-Erk1/2); anti-phopshpo-p38; anti-phospho-NF-κB p65; anti-Syk; anti-p44/42 (Erk1/2), anti-p38; anti-IκBα (all from Cell Signalling Technologies); anti-Igα (Thermo Scientific); and anti-NF-κB p65 (Santa Cruz). The secondary antibodies anti-actin (Cell Signalling Technologies) and anti-GAPDH (Meridian Life Science) were detected by horse radish peroxidase (HRP)-conjugated anti-mouse IgG (Bio-Rad) or anti-rabit IgG (GE Healthcare) antibodies, or both. Signals were detected with the BIO RAD ChemiDoc imaging system and analysed with the manufacturer’s software. Blots were quantified with KODAK MI software. Images have been cropped for presentation purpose. Full-size images are presented in Supplementary Fig. 3.
Mouse B-lymphocyte H16-L10-4R5 cells were cultured in RPMI 1660 medium supplemented with 10% FCS and 1% antibiotics. 25 × 106 cells were lysed in dulbecco's phosphate-buffered saline (DPBS) supplemented with 1% Triton-X and protease inhibitor cocktail (both from Sigma-Aldrich). Lysates were incubated with anti-CEACAM1 antibody (CC1, Novus Biologicals, 5 μg ml−1) overnight at 4°. CEACAM1 was immunoprecipitated with protein G dynabeads (Life technologies).
Total RNA was extracted from MACS-sorted pure B cells with TRIZOL reagent (Ambion) according to the manufacturer’s instructions. RNA was transcribed with a QuantiTect Reverse Transcription Kit (Qiagen). Quantitative real-time PCR amplification of single genes was performed with SYBR Green quantitative PCR master mix in a Light Cycler 480 (Roche). QuantiTect Primer assays for Bcl-6, Pax-5, Bcl2a1, Xiap, NF-κB, Rel-B, Nfatc1, Nfatc2, c-Jun, c-Fos, Ap1S1 and Blimp-1 (Qiagen) were used for quantification of mRNA expression of the respective genes. The following oligonucleotide primers that detect murine CEACAM1 isoforms were used for detection: mouse CEACAM1, a common sense primer that detects both long and short isoform FP-5′- GCCATGCAGCCTCTAACCCACC -3′; and two antisense primers that detect specific isoforms, mouse CEACAM1-L BP-5′- CTGGAGGTTGAGGGTTTGTGCTC -3′ and mouse CEACAM1-S BP-5′- TCAGAAGGAGCCAGACCCGCC -3′. The product was analysed on 3% agarose gels in Tris-borate-EDTA buffer and visualized by ethidium bromide staining. For analysis, expression levels of target genes were normalized to glyceraldehyde 3-phosphate dehydrogenase, 18S r-RNA (Qiagen), or both as an internal control gene (ΔCt). Gene expression values were then calculated with the ΔΔCt method; the mean of four untreated WT B cells (0 h after sorting) was used as a calibrator to which all other samples were compared. Relative quantities (RQs) were determined with the equation RQ=2−ΔΔCt.
LCMV-glycoprotein GP1-specific IgG measurements
The detection of LCMV-glycoprotein GP-1-specific IgG by ELISA has been previously described59. In short, 96-well flat-bottom Nunc Immuno Plates (Thermo Scientific) were coated with anti-human IgG (Jackson ImmunoResearch Laboratories, Inc.) in coating buffer (0.1 M Na2CO3+0.1 M NaHCO3; pH 9.6) overnight at 4 °C. On the next day, plates were washed with washing buffer (PBS with 0.05% Tween-20), and unspecific binding was blocked with 2% FCS in PBS for 2 h. Plates were incubated with LCMV Gp-Fc supernatant (made in-house) for 3 h at room temperature. Plates were washed and titrated with pre-diluted (1: 20) serum over 12 wells with 1: 3 dilutions in successive wells. After a 90-min incubation, plates were incubated with HRP-conjugated anti-mouse-IgG antibody (Sigma). After a 1-h incubation, plates were developed as described below.
For detection of VSV-specific anti-IgG and anti-IgM antibodies, 96-well flat-bottom Nunc Immuno Plates (Thermo Scientific) were coated with baculovirus VSV-GP42 in coating buffer 0.1 M NaCO3 (0.1 M Na2CO3+0.1 M NaHCO3; pH 9.6) overnight at 4 °C. On the next day, plates were washed with washing buffer (PBS with 0.05% Tween-20), and unspecific binding was blocked with 2% FCS in PBS for 1–2 h. Plates were washed once and titrated with prediluted (1:15) serum over 12 wells with 1:3 dilutions in successive wells. Plates were incubated at room temperature for 2 h. Plates were washed with washing buffer and incubated with HRP-conjugated anti-mouse-IgG (Sigma) or anti-mouse-IgM (Sigma) antibody for 30–60 min. Plates were washed and incubated with 1 × TMB Substrate solution (eBioscience) in the dark, after which 10% H2SO4 solution was added to stop the colour reaction. Optical density was measured at 450 nm (FLUOstar Omega, BMG LABTECH). Various immunoglobulin isotypes and subtypes were measured in naïve serum of WT and Ceacam1−/− mice as described55.
Data are expressed as mean±s.e.m. Student’s t-test was used to detect statistically significant differences between groups. Significant differences between several groups were detected by one-way analysis of variance (ANOVA) with Bonferroni or Dunnett post hoc tests. Survival was compared with log-rank (Mantel-Cox) tests. The level of statistical significance was set at P<0.05.
V.K. and V.D. designed, planned and performed the experiments, analysed data and wrote the paper. S.K.M. helped in experiments and was involved in data analysis. N.H. and N.S. were involved in data discussion. A.P. helped in experiments. M.S. performed studies on human lymphocytes. V.P., H.C.X. and P.S. helped in experiments. F.B. and F.M. performed immunoglobulin isotype and subtype ELISAs. K.M. and E.L. helped in experiments. C.K., A.M.W., D.H., F.L., U.D., R.K., M.R., C.H. and I.S. were involved in the data discussion and in drafting the manuscript. N.B. provided the Ceacam1−/− mice and was involved in the data discussion and in drafting the manuscript. J.R.G. performed the B-cell analysis, provided the reagents, discussed the data and wrote the paper. B.B.S. initiated the study, provided reagents, organized and discussed the data and wrote the paper. P.A.L. discussed the data and wrote the paper. K.S.L. initiated, organized and designed the study, wrote the paper and completed the manuscript.
How to cite this article: Khairnar, V. et al. CEACAM1 induces B-cell survival and is essential for protective antiviral antibody production. Nat. Commun. 6:6217 doi: 10.1038/ncomms7217 (2015).