Role of cell surface immunoglobulin in B-lymphocyte activation

PDGFA-associated protein 1 protects mature B lymphocytes from stress-induced cell death and promotes antibody gene diversification

The establishment of protective humoral immunity is dependent on the ability of mature B cells to undergo antibody gene diversification while adjusting to the physiological stressors induced by activation with the antigen. Mature B cells diversify their antibody genes by class switch recombination (CSR) and somatic hypermutation (SHM), which are both dependent on efficient induction of activation-induced cytidine deaminase (AID). Here, we identified PDGFA-associated protein 1 (Pdap1) as an essential regulator of cellular homeostasis in mature B cells. Pdap1 deficiency leads to sustained expression of the integrated stress response (ISR) effector activating transcription factor 4 (Atf4) and induction of the ISR transcriptional program, increased cell death, and defective AID expression. As a consequence, loss of Pdap1 reduces germinal center B cell formation and impairs CSR and SHM. Thus, Pdap1 protects mature B cells against chronic ISR activation and ensures efficient antibody diversification by promoting their survival and optimal function.

Aryl hydrocarbon receptor-induced activation of the human IGH hs1.2 enhancer: Mutational analysis of putative regulatory binding motifs

The human hs1.2 enhancer within the Ig heavy chain gene (IGH) is polymorphic and associated with a number of autoimmune diseases. The polymorphic region is characterized by tandem repeats of an ∼53-bp invariant sequence containing possible binding sites for several transcription factors. Our previous studies suggest the human hs1.2 enhancer is sensitive to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an environmental toxicant and high affinity ligand of the aryl hydrocarbon receptor (AhR). TCDD induced hs1.2 enhancer activity in an AhR-dependent manner and the number of invariant sequences influenced the magnitude of activity. To better understand the regulation of human hs1.2 enhancer activity, the objective of the current study was to utilize mutational analysis and luciferase reporter constructs to evaluate the contribution of putative transcription factor binding sites to overall hs1.2 enhancer activity and modulation by TCDD. Basal and LPS-induced activity of the hs1.2 enhancer appeared to be most affected by mutation of sites outside of the invariant sequence or deletion of the entire invariant sequence; whereas sites influencing the effect of TCDD were dependent on the cellular activation state (i.e. unstimulated vs. LPS stimulation) and relatively independent of the putative AhR binding site within the invariant sequence. These results suggest that AhR activation affects human hs1.2 activity through an as yet undetermined non-canonical pathway. A better understanding regarding the role of the hs1.2 enhancer in human Ig expression and how AhR ligands modulate its activity may lead to insights into overall Ig regulation and mechanisms of dysfunction.

Phospholipid Biosynthesis Program Underlying Membrane Expansion during B-lymphocyte Differentiation

Stimulated B-lymphocytes differentiate into plasma cells committed to antibody production. Expansion of the endoplasmic reticulum and Golgi compartments is a prerequisite for high rate synthesis, assembly, and secretion of immunoglobulins. The bacterial cell wall component lipopolysaccharide (LPS) stimulates murine B-cells to proliferate and differentiate into antibody-secreting cells that morphologically resemble plasma cells. LPS activation of CH12 B-cells augmented phospholipid production and initiated a genetic program, including elevated expression of the genes for the synthesis, elongation, and desaturation of fatty acids that supply the phospholipid acyl moieties. Likewise, many of the genes in phospholipid biosynthesis were up-regulated, most notably those encoding Lipin1 and choline phosphotransferase. In contrast, CTP:phosphocholine cytidylyltransferase alpha (CCTalpha) protein, a key control point in phosphatidylcholine biosynthesis, increased because of stabilization of protein turnover rather than transcriptional activation. Furthermore, an elevation in cellular diacylglycerol and fatty acid correlated with enhanced allosteric activation of CCTalpha by the membrane lipids. This work defines a genetic and biochemical program for membrane phospholipid biogenesis that correlates with an increase in the phospholipid components of the endoplasmic reticulum and Golgi compartments in LPS-stimulated B-cells.

Expression of protein tyrosine kinases in the Ig complex of anti-mu-sensitive and anti-mu-resistant B-cell lymphomas: role of the p55blk kinase in signaling growth arrest and apoptosis

The src family of non-receptor protein tyrosine kinases (PTKs), including the blk, fyn, lyn and lck kinases, is expressed in B-lineage cells, may associate with the immunoglobulin receptor complex and, therefore, play a role in signal transduction via membrane IgM. To establish which of these PTKs is involved in growth inhibition of B-cell lymphomas by anti-mu, we examined the expression pattern and state of activation of these kinases in nine B-cell lymphomas. Tyrosine-phosphorylated p55blk was constitutively expressed in all growth-inhibitable lymphomas; furthermore, anti-mu caused a relative increase of tyrosine phosphorylation in p55blk and a 2- to 3-fold increase in its kinase activity in these cells within minutes. In contrast, p55blk was not present in three of five anti-mu-resistant lymphomas and there was no detectable increase of blk activity in one of the resistant cell lines tested. Thus, we proposed that activatable blk kinase in the IgM complex is essential for the growth inhibitory effect of anti-mu. To test this hypothesis, CH31 lymphoma cells were treated with antisense oligos for the blk kinase and found to be resistant to anti-mu-mediated growth inhibition and subsequent apoptosis. These studies implicate the blk kinase as an integral part of the growth inhibitory pathway leading to arrest and apoptosis. Transfectants of blk gene constructs are being generated to further test this hypothesis.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Membrane biogenesis during B cell differentiation: most endoplasmic reticulum proteins are expressed coordinately

The induction of high-rate protein secretion entails increased biogenesis of secretory apparatus organelles. We examined the biogenesis of the secretory apparatus in the B cell line CH12 because it can be induced in vitro to secrete immunoglobulin (Ig). Upon stimulation with lipopolysaccharide (LPS), CH12 cells increased secretion of IgM 12-fold. This induced secretion was accompanied by preferential expansion of the ER and the Golgi complex. Three parameters of the rough ER changed: its area and volume increased 3.3- and 3.7-fold, respectively, and the density of membrane-bound ribosomes increased 3.5-fold. Similarly, the area of the Golgi stack increased 3.3-fold, and its volume increased 4.1-fold. These changes provide sufficient biosynthetic capacity to account for the increased secretory activity of CH12. Despite the large increase in IgM synthesis, and because of the expansion of the ER, the concentration of IgM within the ER changed less than twofold during the differentiation process. During the amplification of the rough ER, the expression of resident proteins changed according to one of two patterns. The majority (75%) of rough microsomal (RM) proteins increased in proportion to the increase in rough ER size. Included in this group were both lumenal proteins such as Ig binding protein (BiP), and membrane proteins such as ribophorins I and II. In addition, the expression of a minority (approximately 9%) of RM polypeptides increased preferentially, such that their abundance within the RM of secreting CH12 cells was increased. Thus, the expansion of ER during CH12 differentiation involves preferential increases in the abundance of a few resident proteins, superimposed upon proportional increases in most ER proteins.

Expression of intracisternal A-type particles is increased when a B-cell lymphoma differentiates into an immunoglobulin-secreting cell

The murine B-cell lymphoma CH12, like many other murine cells, expresses intracisternal A-type particles (IAPs). When CH12 cells are treated with lipopolysaccharide, the cells differentiate and secrete immunoglobulin M. The expression of IAP genes was also increased, in parallel with the increased expression of immunoglobulin genes. The amount of IAP mRNA increased within 48 h of lipopolysaccharide treatment and reached a level fivefold higher than that in unactivated CH12 cells. The two major IAP transcripts (7.2 and 5.4 kilobases) were induced to similar extents. The increased level of mRNA was reflected in higher levels of the major IAP structural protein p70, whose abundance increased 3.6-fold. The number of IAP particles per cell also increased upon activation of CH12, from 67 in nonsecreting CH12 to 290 in secreting cells. All of the IAPs were confined to the cisternae of the endoplasmic reticulum (ER), regardless of the differentiation state of the cell. Accompanying the induction of p70 was the induction of the related IAP polypeptide p102. A third viral polypeptide, p120, was also made in CH12; its abundance was almost unchanged. Localization of the IAP proteins on ultrathin frozen sections showed that most were assembled into particles in the ER. However, there were small pools of unassembled proteins both in the ER and on the plasma membrane. p70 and p120 could be detected, by iodination, on the surfaces of both secreting and nonsecreting CH12 cells. Unlike p70 and p120, p102 did not seem to be membrane associated. Taken together, these observations show that IAP expression is regulated developmentally in B lymphocytes. Also, these observations point to possible interactions between IAP genes and other cellular genes.

Signaling to a B-cell clone by Ek, but not Ak, does not reflect alteration of Ak genes

The mouse B-cell clone, CH12.LX (Iak, Ly-1+, mu+, delta+), can be induced to differentiate and secrete antibody in an antigen-specific, H-2-restricted manner. Induction requires two signals. One must be provided by the binding of specific antigen to the membrane IgM; the other is delivered by the binding of Ek-specific T-cell hybridomas to the Ek molecules of CH12.LX (Bishop and Haughton 1986). Previous studies demonstrated that Ek-specific monoclonal antibodies (mAbs) could substitute for T cells in delivering the second differentiative signal (Bishop and Haughton 1986). Although CH12.LX cells present Ak to Ak-restricted or alloreactive T-helper cells, neither T cells nor mAbs specific for Ak induce differentiation (Bishop and Haughton 1986). However, since the Akspecific mAbs tested previously were beta-chain-specific and the Ia epitope specificity of the T cells used was unknown, it is possible that the differentiative signal delivered to the CH12.LX class II molecule is chain-specific. Here we report the effects of ten additional Iak-specific mAbs upon the differentiation of CH12.LX. In addition, a cDNA library was prepared from CH12.LX cells, clones corresponding to the alpha and beta chains of the Ak molecule were isolated, and their nucleotide sequences were determined. Finally, the Ak and Ek molecules of CH12.LX and H-2k spleen cells were compared by two-dimensional gel electrophoresis to examine possible post-translational differences in the Iak molecules of CH12.LX.

Studies of surface immunoglobulin-dependent B cell activation

Studies from a number of laboratories have firmly established the potential of surface immunoglobulin-generated signals in B lymphocyte activation. While clearly there are multiple ways of activating B lymphocytes, some of which may not involve surface immunoglobulin, it is clear that crosslinking of surface immunoglobulin whether by antigen or antireceptor antibody can generate signals relevant to B cell activation. Although considerable insight into the mechanism of transduction of mIg-generated signals across the plasma membrane has been realized, a molecular explanation for linking inositol phospholipid hydrolysis to changes within the cytoplasm and nucleus of the B cell is still speculative. A more rigorous definition of the PKC and calcium components of the mIg signal transduction pathway are critical for a thorough understanding of the mechanism of signal transduction by this receptor. The use of tumor cell models allowing selection of mutants within the signalling pathway(s) will be invaluable to fully defining the critical molecular and biochemical events involved in B cell activation.

The specific direct interaction of helper T cells and antigen-presenting B cells. II. Reorientation of the microtubule organizing center and reorganization of the membrane-associated cytoskeleton inside the bound helper T cells

We have produced and investigated cell couples formed between cloned Th cells or T hybridoma cells, and either Ag-presenting B hybridoma or B lymphoma cells. The specific direct interaction between a Th and B-APC is here demonstrated by two rearrangements occurring inside the bound Th cell; the MTOC (and presumably the GA) is oriented to face the cell contact region with the B cell, and a membrane-associated cytoskeletal protein, talin, becomes concentrated under the contacting Th membrane. In the absence of the specific Ag or the correct Ia determinant, nonspecific T-B cell couples form that are morphologically indistinguishable from specific cell couples in the light microscope, but neither the MTOC nor the talin rearrangement occurs inside the bound T cell of such nonspecific couples. Furthermore, Ag processing by the B cell is required to produce the MTOC and talin rearrangements within the T cell in specific T-B couples. In the case of allogeneic Th-B cell couples, similar specific MTOC and talin rearrangements are observed inside the Th. Extracellular Ca2+ is required for the MTOC orientation to occur inside the specifically bound Th cell, but not for the talin rearrangement. It is proposed that the MTOC (and GA) reorientation and the talin rearrangement are involved in the directed secretion of GA-derived lymphokines from the Th cell to the bound B cell.

Expression of the Fc gamma receptor on Ly-1+ B lymphocytes

The expression of IgG Fc receptor (FcR) molecules was examined on Ly-1+ B cells and B cells tumors. FcR molecules were found on a representative Ly-1+ B cell lymphoma of the pre-B and as well as of the mature cell B phenotypes. The expression of the FcR did not change on these cells during their differentiation to B cells or to antibody-secreting cells, respectively. Ly-1+ B cells were found at low frequency (approximately 2%) in the spleens of normal mice, and in the peritoneal cavity where their representation was greater. The frequency of Ly-1+ B cells declined after birth, although their numbers increased in both organs. These Ly-1+ B cells expressed the FcR molecule throughout ontogeny. Furthermore, the amount of FcR expressed on Ly-1+ B cells was similar to the levels expressed on their "conventional" (Ly-1-) B cell counterparts. The FcR was also found on Ly-1+ B cells from autoimmune mice. The significance of the expression of FcR molecules on Ly-1+ B cells is discussed.

Induced differentiation of a transformed clone of Ly-1+ B cells by clonal T cells and antigen

The present study used cocultures of clonally derived B and T cells, together with an antigen reactive with the membrane Ig of the clonal B cells, to address the issue of B-cell differentiation requirements. The B cells were CH12.LX, an in vitro grown subclone of a murine B-cell lymphoma, which bears surface IgM reactive with sheep erythrocytes. The T cells were alloreactive T-helper-cell hybridomas. Very small numbers of T-helper cells could induce differentiation of cloned B cells without the presence of accessory cells, but such induction was dependent upon the presence of the antigen recognized by the B cell. Induced differentiation of the B cells did not depend on metabolic activity of the T cells, and metabolically active T cells could be replaced by fixed cells or by monoclonal antibody reactive with the class II molecule of the B cell to deliver an Ia-specific differentiative signal. T cells, or alloantibody that reacted with the I-E molecule, induced differentiation of the B cells; those that reacted with the I-A molecule did not. These results define the minimal requirements for major histocompatibility complex-restricted, T-cell-mediated induction of B-cell differentiation.

Three classes of signalling molecules on B-cell membranes

The question of whether surface immunoglobulin and Ia molecules have a signalling function in helper T cell-dependent activation of B cells has been evaluated. Two sources of B cells have been used, one a purified population of hapten-binding B cells, the other a B-cell lymphoma, CH12, with known antigen specificity. Evidence is presented that both immunoglobulin and Ia molecules are receptors actively involved in the initial activation of resting B cells. Nevertheless, the requirements for ligand binding to either receptor can be bypassed under appropriate conditions, and the implications of this result for the function of these molecules is discussed. With respect to B-cell Ia, the authors present data that demonstrate two distinct functions of this molecule, one as a restricting element for T-cell activation, the second as a signalling receptor for B-cell excitation. On the CH12 surface, the I-A molecule fulfills the former function, but T-cell interactions with I-A fail to result in B-cell stimulation, suggesting that B-cell Ia may limit helper T cell-B cell interactions. We suggest that the binding of antigen surface immunoglobulin and binding of helper T-cell receptors to the appropriate Ia molecule(s) results in the activation of genes that encode for a third class of membrane B-cell receptors, those that bind B-cell stimulating factors.

Two separate functions of class II (Ia) molecules: T-cell stimulation and B-cell excitation

We have evaluated the role of major histocompatibility complex-encoded class II (Ia) molecules as transmembrane signaling receptors in the T helper cell-dependent activation of B lymphocytes. For these studies, we utilized the murine B-cell lymphoma CH12, which expresses both I-A and I-E class II molecules. In addition, CH12 cells carry IgM of known antigen specificity and require both specific antigen and Ia-restricted T-cell help for the induction of antibody secretion. In this respect, they resemble normal resting B cells. We have studied the ability of antigen-specific or alloreactive T helper cells reactive with either the I-A or the I-E molecules on CH12 to be activated and their ability to stimulate antibody production by CH12. The results show that, although CH12 cells present antigen to T helper cells that interact with either the I-A or the I-E molecules, CH12 cells are stimulated to secrete antibody only by T helper cells reactive with their I-E molecules. Our data demonstrate that class II molecules are transducers of signals for B-cell excitation in addition to serving a restricting function for helper T-cell stimulation. Moreover, the data demonstrate that these two functions, T-cell stimulation and B-cell excitation, are discrete and need not be expressed by the same Ia molecule.

Major histocompatibility complex-restricted and unrestricted interactions in the T cell-dependent activation of hapten-binding B cells

The requirements for linked recognition and major histocompatibility complex-restricted interactions in helper T cell-dependent activation of purified unprimed and primed hapten-binding B cells have been investigated. The activation of unprimed hapten-binding B cells required specific antigen and restricted interactions between helper T cells and B cells. As expected, specific hapten-carrier conjugates were essential for the activation of B cells at low antigen doses. Interestingly, however, supraoptimal concentrations (125 micrograms/ml) of carrier protein alone could substitute for specific conjugates in the activation of unprimed B cells. The requirement for restricted helper T cell-B interactions was unchanged under these conditions. These results are discussed in terms of the signalling requirements for B cell activation. In contrast to the results using hapten-specific B cells from unprimed mice, the further stimulation of B cells prepared from mice primed 5 to 7 days earlier with immunogenic forms of the hapten was limited only by the requirements for helper T cell activation. The transition in the activation properties of B cells following in vivo stimulation was not solely a result of the binding of B cell membrane immunoglobulin to specific antigen, since the interaction of unprimed B cells with hapten during their purification, even for extended periods of time, did not alter the requirements for their further stimulation. These results demonstrate that the recent antigenic experience of B cells determines their activation state which, in turn, dictates the further requirements for helper T cell function in B cell stimulation. This implies that specificity of the immune response is determined in the early stages of B cell activation.