Quantitative determination of the induction of apoptosis in a murine B cell line using flow cytometric bivariate cell cycle analysis

WEHI-231 cells have been used extensively as a model of tolerance induction in B cells. Recent evidence has shown that anti-IgM treatment of WEHI-231 cells resulted in the induction of apoptosis. In this study, using acridine orange staining and flow cytometric analysis, we demonstrated that apoptotic cells are detected as a distinct population of cells separate from the cells in normal cell cycle progression. The validity of analysis gates was confirmed by cell sorting of the apoptotic population versus normal cells and subsequent gel analysis. Using this technique, we have demonstrated that F(ab')2 anti-mu, A23187, or PMA induced apoptosis in the WEHI-231 cells. The addition of LPS reversed apoptotic induction as seen previously with the WEHI-231 cell line. In contrast, however, PMA did not prevent the induction of apoptosis in anti-mu-treated cells. Additionally, we were interested in determining if the induction of apoptosis was restricted to a specific phase of cell cycle. Since growth inhibition results in most cells arresting in the G1 phase of cell cycle, we wanted to demonstrate apoptosis as a G1-dependent event. This was examined with WEHI-231 cells treated with known cell cycle inhibitors. Interestingly, inhibition of cells in each phase of cycle resulted in the induction of apoptosis. LPS was able to inhibit the induction of apoptosis with each of the cell cycle inhibitors except actinomycin D. Furthermore, we have demonstrated that the WEHI-231 cells contain a Ca(2+)-Mg(2+)-dependent preexisting endonuclease.

Tumor biomarker glycoproteins in the seminal plasma of healthy human males are endogenous ligands for DC-SIGN

DC-SIGN is an immune C-type lectin that is expressed on both immature and mature dendritic cells associated with peripheral and lymphoid tissues in humans. It is a pattern recognition receptor that binds to several pathogens including HIV-1, Ebola virus, Mycobacterium tuberculosis, Candida albicans, Helicobacter pylori, and Schistosoma mansoni. Evidence is now mounting that DC-SIGN also recognizes endogenous glycoproteins, and that such interactions play a major role in maintaining immune homeostasis in humans and mice. Autoantigens (neoantigens) are produced for the first time in the human testes and other organs of the male urogenital tract under androgenic stimulus during puberty. Such antigens trigger autoimmune orchitis if the immune response is not tightly regulated within this system. Endogenous ligands for DC-SIGN could play a role in modulating such responses. Human seminal plasma glycoproteins express a high level of terminal Lewis(x) and Lewis(y) carbohydrate antigens. These epitopes react specifically with the lectin domains of DC-SIGN. However, because the expression of these sequences is necessary but not sufficient for interaction with DC-SIGN, this study was undertaken to determine if any seminal plasma glycoproteins are also endogenous ligands for DC-SIGN. Glycoproteins bearing terminal Lewis(x) and Lewis(y) sequences were initially isolated by lectin affinity chromatography. Protein sequencing established that three tumor biomarker glycoproteins (clusterin, galectin-3 binding glycoprotein, prostatic acid phosphatase) and protein C inhibitor were purified by using this affinity method. The binding of DC-SIGN to these seminal plasma glycoproteins was demonstrated in both Western blot and immunoprecipitation studies. These findings have confirmed that human seminal plasma contains endogenous glycoprotein ligands for DC-SIGN that could play a role in maintaining immune homeostasis both in the male urogenital tract and the vagina after coitus.

Inducible DNA-protein interactions of the murine kappa immunoglobulin enhancer in intact cells: comparison with in vitro interactions

The large intron of the kappa immunoglobulin gene contains a cis-acting enhancer element, which is important in the tissue-specific expression of the gene. We have confirmed the binding activity of a sequence-specific factor present in lymphoid extracts derived from cell lines expressing, or induced to express, the kappa gene. We have extended these studies to show the binding activity is present in normal activated splenic B cells as well as lambda producing cells, and have demonstrated by DNAse footprint analysis full protection of a sequence containing the 11 bp homology to the SV-40 core enhancer. We have compared these in vitro binding studies with an analysis of protein-DNA interactions in intact murine cell lines using genomic sequencing techniques. We demonstrate significant alterations in DMS reactivity of DNA in the murine 70Z/3 cell line after it is induced to kappa expression. These alterations occur at guanine residues which are part of the the 11 bp core sequence, and are identical to those observed in cells constitutively expressing kappa. This provides direct evidence for the induced binding of the tissue specific factor to intact chromatin. In intact chromatin we also observed significant alteration in the reactivity of a guanine, 3' of the core sequence, which is part of a potential secondary DNA structure, and protection of four residues that are part of a region homologous to the heavy chain enhancer.

Effects of anti-IgM suppression on polyclonally activated murine B cells: analysis of immunoglobulin mRNA, gene specific nuclear factors and cell cycle distribution

Polyclonal activation of murine B cells with bacterial lipopolysaccharide (LPS) and dextran sulfate (DxS) results in cell proliferation as well as increased immunoglobulin gene transcription and antibody secretion. When added to B cell cultures during mitogen activation, anti-mu antibody suppresses the rate of proliferation and immunoglobulin gene expression. Using this model system we show that co-cultures of B cells with LPS/DxS and anti-mu resulted in a decrease of both mu and kappa chain mRNA. Suppression did not prevent B cell entry into cycle nor a significant alteration in the distribution of cells in phases of cell cycle, although it did prolong the cycle transit time in a dose dependent fashion as determined by bromodeoxyuridine pulse labelling. Analysis of B cell specific nuclear binding factors, which previously have been shown to be important in regulating immunoglobulin gene transcription were examined. Results show that the kappa-specific enhancer binding activity of NF-kappa B was induced in activated as well as suppressed cultures. The lymphoid specific factor NF-A2, which recognizes the octamer sequence motif in the promoters of immunoglobulin genes, was induced by the polyclonal activation but was selectively lost in extracts from suppressed cells. Thus, specific regulation of the nuclear factor which plays a critical role in both heavy and light chain immunoglobulin gene expression may contribute to the transcriptional suppression observed in anti-mu treated B cells.

Fluorescence-activated cell sorting-derived clones of Babesia bigemina show karyotype polymorphism

Use of the fluorescence-activated cell sorter proved to be an accurate and highly efficient means for cloning Babesia parasites. These qualities were examined by separating a mixed population of Babesia-infected bovine erythrocytes composed of two isolates with different karyotypes. Direct evidence of polymorphism was detected during comparison of the resultant clones.

Alteration of cell cycle kinetics and immunoglobulin gene transcription as the result of multiple agonist stimulation of murine B cells

Several laboratories have established that anti-IgM can inhibit polyclonal B cell activation by LPS or LPS/DxS. The use of intact anti-IgM results in an inhibition of both proliferation and differentiation, whereas F(ab')2 fragments inhibit only differentiation. Since signal transduction by both alpha-Ig's (intact and F(ab')2 fragments) is known to be mediated by PIP2 hydrolysis, we have investigated the effects of A23187 and PMA on LPS/DxS activation of splenic B cells. These agents mimic the second messengers generated as the results of PIP2 hydrolysis. As with intact alpha-IgM, either agent in conjunction with LPS/DxS resulted in an inhibition of proliferation as assessed by [3H]thymidine uptake. However, when proliferation was assessed by acridine orange (AO) staining and flow cytometric analysis, cells were observed to have entered cell cycle. This disparity between AO staining and proliferation was resolved by using BrDu/Hoechst quenching analysis and revealed a delay in cell cycle transit time as the result of multiple agent stimulation. Since both anti-IgM's result in the inhibition of differentiation, we also investigated the effects of these agents on differentiation normally observed with LPS/DxS alone activation of B cells. A23187 and PMA, either alone or in combination, were observed to result in a decrease in mRNA-encoding mu immunoglobulin of the 2.4-kb mRNA for secreted IgM.

LPS-induced differentiation of a murine B cell leukemia (BCL1): changes in surface and secreted IgM

We examined the effect of LPS-induced differentiation on surface and secreted IgM in a cloned BCL1 in vitro cell line. Incubation of this cell line with LPS resulted in a decrease in the amount of membrane IgM, measured by both immunofluorescence and immunoprecipitation, and an increase in IgM secretion, measured by plaque-forming cells (PFC). Activation to high rate secretion was independent of cell cycle in synchronized cells and was independent of DNA synthesis because PFC formation was not inhibited by hydroxyurea. Almost all cells in the in vitro line were shown to contain large quantities of intracytoplasmic IgM before LPS activation. Thus, it would appear that the in vitro cell line represents a partially activated stage of differentiation compared to normal resting B cells or to the in vivo line of BCL1. Analysis of the two forms of mRNA coding for membrane and secreted IgM showed that, at least for cells at the level of differentiation examined here, the control of membrane IgM expression is post-transcriptional. The differentiation of resting B cells to the plasma cell level appears to consist of multiple stages of differentiation. The present data suggest that LPS provides at least two signals of activation. One induces the resting cell to synthesize cytoplasmic IgM, increase surface IgM, and to begin cell division. The second induces the secretion of intracytoplasmic IgM associated with a decrease in surface IgM.

The relationship between surface immunoglobulin isotype and the immune function of murine B lymphocytes. V. High affinity secondary antibody responses are transferred by both IgD-positive and IgD-negative memory B cells

We examined the adoptive secondary anti-DNP responses restored by surface IgD+ and IgD- memory B cells. Several parameters that might affect the affinity and magnitude of the adoptive responses were studied: 1) time after priming of cell donors, 2) source of anti-IgD antibodies used for immunofluorescent cell staining, 3) adjuvant used for priming, 4) carrier protein used for priming, 5) amount of antigen used for the challenge of adoptive hosts, and 6) the strain of ice used as donors and recipients. In contrast to previous reports, the present results demonstrate that the selection of cells with high affinity antigen receptors can occur to the same extent in both the delta + and delta - memory cell pools. This suggests that the loss of surface IgD is not a necessary intermediate stage in the maturation of memory B cells.

Surface IgG-bearing cells retain the capacity to secrete IgM

Our previous studies indicated that a large proportion of surface IgG+-primed B cells give rise to clones secreting IgM antibodies. Due to the implications of this result relative to molecular mechanisms of class switching, it was important to document that the surface IgG had been endogenously synthesized by the surface IgG+ cells and was not present as a result of cytophilic IgG. Therefore, spleen cells from immunized mice were treated sequentially with anti-immunoglobulin and protease which removed greater than 99% of surface immunoglobulin. After overnight incubation to allow resynthesis of surface immunoglobulin, the treated cells were sorted for surface IgG-bearing cells and were transferred to carrier-primed, irradiated adoptive recipients for analysis in the splenic focus assay. It was found that the majority of antibody-secreting clones derived from these surface IgG+ B cells still synthesized IgM. These data are discussed relative to current concepts of molecular mechanisms of immunoglobulin class switching.

Immunoglobulin class commitment exhibited by B lymphocytes separated according to surface isotype

In order to determine the extent to which B cells exhibit class commitment, the isotype(s) secreted by individual B cells and their progeny was studied in relation to the isotype(s) expressed on the surface of the B cell. Both primary and secondary B cells were separated according to cell surface isotypes and analyzed under optimal stimulation conditions using the T-dependent splenic focus assay. From these studies the following conclusions conclude be made: a) essentially all mature primary B cells appear to express mu on their surface; b) accordingly, the primary B cell population appears to be devoid of B cells expressing solely IgG on their surface; c) the majority of primary B cells in the presence of optimal T cell help have the potential for multiple class production; d) the secretion of IgA coincides with that of IgM and not IgG in its synthesis by immature B cells; e) a substantial proportion of memory B cells bear mu on their surface; f) memory B cells that lack s-mu are unable to yield clones secreting IgM; and g) a substantial proportion of memory B cells have the potential for multiple class production.

Surface IgG-bearing cells retain the capacity to secrete IgM

Our previous studies indicated that a large proportion of surface IgG+-primed B cells give rise to clones secreting IgM antibodies. Due to the implications of this result relative to molecular mechanisms of class switching, it was important to document that the surface IgG had been endogenously synthesized by the surface IgG+ cells and was not present as a result of cytophilic IgG. Therefore, spleen cells from immunized mice were treated sequentially with anti-immunoglobulin and protease which removed greater than 99% of surface immunoglobulin. After overnight incubation to allow resynthesis of surface immunoglobulin, the treated cells were sorted for surface IgG-bearing cells and were transferred to carrier-primed, irradiated adoptive recipients for analysis in the splenic focus assay. It was found that the majority of antibody-secreting clones derived from these surface IgG+ B cells still synthesized IgM. These data are discussed relative to current concepts of molecular mechanisms of immunoglobulin class switching.

LPS-induced differentiation of a murine B cell leukemia (BCL1): changes in surface and secreted IgM

We examined the effect of LPS-induced differentiation on surface and secreted IgM in a cloned BCL1 in vitro cell line. Incubation of this cell line with LPS resulted in a decrease in the amount of membrane IgM, measured by both immunofluorescence and immunoprecipitation, and an increase in IgM secretion, measured by plaque-forming cells (PFC). Activation to high rate secretion was independent of cell cycle in synchronized cells and was independent of DNA synthesis because PFC formation was not inhibited by hydroxyurea. Almost all cells in the in vitro line were shown to contain large quantities of intracytoplasmic IgM before LPS activation. Thus, it would appear that the in vitro cell line represents a partially activated stage of differentiation compared to normal resting B cells or to the in vivo line of BCL1. Analysis of the two forms of mRNA coding for membrane and secreted IgM showed that, at least for cells at the level of differentiation examined here, the control of membrane IgM expression is post-transcriptional. The differentiation of resting B cells to the plasma cell level appears to consist of multiple stages of differentiation. The present data suggest that LPS provides at least two signals of activation. One induces the resting cell to synthesize cytoplasmic IgM, increase surface IgM, and to begin cell division. The second induces the secretion of intracytoplasmic IgM associated with a decrease in surface IgM.

Immunoglobulin class commitment exhibited by B lymphocytes separated according to surface isotype

In order to determine the extent to which B cells exhibit class commitment, the isotype(s) secreted by individual B cells and their progeny was studied in relation to the isotype(s) expressed on the surface of the B cell. Both primary and secondary B cells were separated according to cell surface isotypes and analyzed under optimal stimulation conditions using the T-dependent splenic focus assay. From these studies the following conclusions conclude be made: a) essentially all mature primary B cells appear to express mu on their surface; b) accordingly, the primary B cell population appears to be devoid of B cells expressing solely IgG on their surface; c) the majority of primary B cells in the presence of optimal T cell help have the potential for multiple class production; d) the secretion of IgA coincides with that of IgM and not IgG in its synthesis by immature B cells; e) a substantial proportion of memory B cells bear mu on their surface; f) memory B cells that lack s-mu are unable to yield clones secreting IgM; and g) a substantial proportion of memory B cells have the potential for multiple class production.

The relationship between surface immunoglobulin isotype and the immune function of murine B lymphocytes. V. High affinity secondary antibody responses are transferred by both IgD-positive and IgD-negative memory B cells

We examined the adoptive secondary anti-DNP responses restored by surface IgD+ and IgD- memory B cells. Several parameters that might affect the affinity and magnitude of the adoptive responses were studied: 1) time after priming of cell donors, 2) source of anti-IgD antibodies used for immunofluorescent cell staining, 3) adjuvant used for priming, 4) carrier protein used for priming, 5) amount of antigen used for the challenge of adoptive hosts, and 6) the strain of ice used as donors and recipients. In contrast to previous reports, the present results demonstrate that the selection of cells with high affinity antigen receptors can occur to the same extent in both the delta + and delta - memory cell pools. This suggests that the loss of surface IgD is not a necessary intermediate stage in the maturation of memory B cells.

Alkaline phosphatase on activated B cells characterization of the expression of alkaline phosphatase on activated B cells. Kinetics and membrane anchor

Recently we reported that the expression of the enzyme alkaline phosphatase (APase) is a marker for B cell activation. Enzymatic activity was found only in activated B cells and not T cells. Using flow cytometry we showed that some of the APase was found on the cell membranes (mAPase) and by functional assays, some was spontaneously released into the tissue culture medium. In the present report the expression of mAPase on activated B lymphocytes is more fully characterized. Two mAb specific for rat APase were used to measure the kinetics of the membrane expression of mAPase. Within 48 h of activation, mAPase is detected by flow cytometry and increases coordinately with both the transferrin receptor and IL-2R. Maximal membrane expression of mAPase in terms of number of positive cells and mean fluorescent intensity, is detected by day 4 to 5 of culture. Using hydroxyurea and demecolcine to block cells at G1/S and G2/M, respectively, it appeared that the initial expression of mAPase occurred as cells progressed into S phase of the cell cycle. This was confirmed using two-color flow cytometric analysis with the Hoechst DNA stain 33342 and the FITC-labeled APase-specific mAb. Finally, using phosphatidylinositol-specific phospholipase C we were able to show that 60 to 80% of the mAPase is linked to the membrane via a glycosyl-phosphatidylinositol linkage. From this we have concluded that mAPase can be added to a growing list of glycoproteins that are anchored to the membrane by the glycosyl-phosphatidylinositol linkage and are expressed on differentiating B cells. This list now includes Thy-1, BLAST-1, Jlld, and mAPase.

Alkaline phosphatase on activated B cells characterization of the expression of alkaline phosphatase on activated B cells. Kinetics and membrane anchor

Recently we reported that the expression of the enzyme alkaline phosphatase (APase) is a marker for B cell activation. Enzymatic activity was found only in activated B cells and not T cells. Using flow cytometry we showed that some of the APase was found on the cell membranes (mAPase) and by functional assays, some was spontaneously released into the tissue culture medium. In the present report the expression of mAPase on activated B lymphocytes is more fully characterized. Two mAb specific for rat APase were used to measure the kinetics of the membrane expression of mAPase. Within 48 h of activation, mAPase is detected by flow cytometry and increases coordinately with both the transferrin receptor and IL-2R. Maximal membrane expression of mAPase in terms of number of positive cells and mean fluorescent intensity, is detected by day 4 to 5 of culture. Using hydroxyurea and demecolcine to block cells at G1/S and G2/M, respectively, it appeared that the initial expression of mAPase occurred as cells progressed into S phase of the cell cycle. This was confirmed using two-color flow cytometric analysis with the Hoechst DNA stain 33342 and the FITC-labeled APase-specific mAb. Finally, using phosphatidylinositol-specific phospholipase C we were able to show that 60 to 80% of the mAPase is linked to the membrane via a glycosyl-phosphatidylinositol linkage. From this we have concluded that mAPase can be added to a growing list of glycoproteins that are anchored to the membrane by the glycosyl-phosphatidylinositol linkage and are expressed on differentiating B cells. This list now includes Thy-1, BLAST-1, Jlld, and mAPase.