A different sort of Mott cell

TFG is required for autophagy flux and to prevent endoplasmic reticulum stress in CH12 B lymphoma cells

Plasma cells depend on quality control of newly synthesized antibodies in the endoplasmic reticulum (ER) via macroautophagy/autophagy and proteasomal degradation. The cytosolic adaptor protein TFG (Trk-fused gene) regulates ER-Golgi transport, the secretory pathway and proteasome activity in non-immune cells. We show here that TFG is upregulated during lipopolysaccharide- and CpG-induced differentiation of B1 and B2 B cells into plasmablasts, with the highest expression of TFG in mature plasma cells. CRISPR-CAS9-mediated gene disruption of tfg in the B lymphoma cell line CH12 revealed increased apoptosis, which was reverted by BCL2 but even more by ectopic TFG expression. Loss of TFG disrupted ER structure, leading to an expanded ER and increased expression of ER stress genes. When compared to wild-type CH12 cells, tfg KO CH12 cells were more sensitive toward ER stress induced by tunicamycin, monensin and proteasome inhibition or by expression of an ER-bound immunoglobulin (Ig) μ heavy (µH) chain. CH12 tfg KO B cells displayed more total LC3, lower LC3-II turnover and increased numbers and size of autophagosomes. Tandem-fluorescent-LC3 revealed less accumulation of GFP-LC3 in starved and chloroquine-treated CH12 tfg KO B cells. The GFP:RFP ratio of tandem-fluorescent-LC3 was higher in tunicamycin-treated CH12 tfg KO B cells, suggesting less autophagy flux during induced ER stress. Based on these data, we suggest that TFG controls autophagy flux in CH12 B cells and propose that TFG is a survival factor that alleviates ER stress through the support of autophagy flux in activated B cells and mature plasma cells.Abbreviations: Ab, antibody; Ag, antigen; ASC, antibody-secreting cells; ATG, autophagy-related; BCR, B cell receptor; COPII, coat protein complex II; CpG, non-methylated CpG oligonucleotide; ER, endoplasmic reticulum; ERAD, ER-associated degradation; FO, follicular; GFP, green fluorescent protein; HC, heavy chain; Ig, immunoglobulin; IRES, internal ribosomal entry site; LC, light chain; MZ, marginal zone; NFKB, nuclear factor of kappa light polypeptide gene enhancer in B cells; TLR, toll-like receptor; UPR, unfolded protein response.

Functional Roles of the IgM Fc Receptor in the Immune System

It is now evident from studies of mice unable to secrete IgM that both non-immune “natural” and antigen-induced “immune” IgM are important for protection against pathogens and for regulation of immune responses to self-antigens. Since identification of its Fc receptor (FcμR) by a functional cloning strategy in 2009, the roles of FcμR in these IgM effector functions have begun to be explored. Unlike Fc receptors for switched Ig isotypes (e.g., FcγRs, FcεRs, FcαR, Fcα/μR, pIgR, FcRn), FcμR is selectively expressed by lymphocytes: B, T, and NK cells in humans and only B cells in mice. FcμR may have dual signaling ability: one through a potential as yet unidentified adaptor protein non-covalently associating with the FcμR ligand-binding chain via a His in transmembrane segment and the other through its own Tyr and Ser residues in the cytoplasmic tail. FcμR binds pentameric and hexameric IgM with a high avidity of ~10 nM in solution, but more efficiently binds IgM when it is attached to a membrane component via its Fab region on the same cell surface (cis engagement). Four different laboratories have generated Fcmr-ablated mice and eight different groups of investigators have examined the resultant phenotypes. There have been some clear discrepancies reported that appear to be due to factors including differences in the exons of Fcmr that were targeted to generate the knockouts. One common feature among these different mutant mice, however, is their propensity to produce autoantibodies of both IgM and IgG isotypes. In this review, we briefly describe recent findings concerning the functions of FcμR in both mice and humans and propose a model for how FcμR plays a regulatory role in B cell tolerance.

Enhanced auto-antibody production and Mott cell formation in FcμR-deficient autoimmune mice

The IgM-Fc receptor (FcμR) is involved in IgM homeostasis as evidenced by increased pre-immune serum IgM and natural auto-antibodies of both IgM and IgG isotypes in Fcmr-deficient C57BL/6 (B6) mice. To determine the impact of Fcmr-ablation on autoimmunity, we introduced the Fcmr null mutation onto the Fas-deficient autoimmune-prone B6.MRL Fas (lpr/lpr) mouse background (B6/lpr). Both IgM and IgG auto-antibodies against dsDNA or chromatin appeared earlier in FcμR(-) B6/lpr than FcμR(+) B6/lpr mice, but this difference became less pronounced with age. Splenic B2 cells, which were 2-fold elevated in FcμR(+) B6/lpr mice, were reduced to normal B6 levels in FcμR(-) B6/lpr mice, whereas splenic B1 cells were comparable in both groups of B6/lpr mice. By contrast, marginal zone (MZ) B cells were markedly reduced in FcμR(-) B6/lpr mice compared with either FcμR(+) B6/lpr or wild type (WT) B6 mice. This reduction appeared to result from rapid differentiation of MZ B cells into plasma cells in the absence of FcμR, as IgM antibody to a Smith (Sm) antigen, to which MZ B cells are known to preferentially respond, was greatly increased in both groups (B6/lpr and B6) of FcμR(-) mice compared with FcμR(+) B6/lpr or B6 mice. Mott cells, aberrant plasma cells with intra-cytoplasmic inclusions, were also increased in the absence of FcμR. Despite these abnormalities, the severity of renal pathology and function and survival were all indistinguishable between FcμR(-) and FcμR(+) B6/lpr mice. Collectively, these findings suggest that FcμR plays important roles in the regulation of auto-antibody production, Mott cell formation and the differentiation of MZ B cells into plasma cells in B6.MRL Fas (lpr/lpr) mice.

Aggregates, crystals, gels, and amyloids: intracellular and extracellular phenotypes at the crossroads of immunoglobulin physicochemical property and cell physiology

Recombinant immunoglobulins comprise an important class of human therapeutics. Although specific immunoglobulins can be purposefully raised against desired antigen targets by various methods, identifying an immunoglobulin clone that simultaneously possesses potent therapeutic activities and desirable manufacturing-related attributes often turns out to be challenging. The variable domains of individual immunoglobulins primarily define the unique antigen specificities and binding affinities inherent to each clone. The primary sequence of the variable domains also specifies the unique physicochemical properties that modulate various aspects of individual immunoglobulin life cycle, starting from the biosynthetic steps in the endoplasmic reticulum, secretory pathway trafficking, secretion, and the fate in the extracellular space and in the endosome-lysosome system. Because of the diverse repertoire of immunoglobulin physicochemical properties, some immunoglobulin clones' intrinsic properties may manifest as intriguing cellular phenotypes, unusual solution behaviors, and serious pathologic outcomes that are of scientific and clinical importance. To gain renewed insights into identifying manufacturable therapeutic antibodies, this paper catalogs important intracellular and extracellular phenotypes induced by various subsets of immunoglobulin clones occupying different niches of diverse physicochemical repertoire space. Both intrinsic and extrinsic factors that make certain immunoglobulin clones desirable or undesirable for large-scale manufacturing and therapeutic use are summarized.

Russell body inducing threshold depends on the variable domain sequences of individual human IgG clones and the cellular protein homeostasis

Russell bodies are intracellular aggregates of immunoglobulins. Although the mechanism of Russell body biogenesis has been extensively studied by using truncated mutant heavy chains, the importance of the variable domain sequences in this process and in immunoglobulin biosynthesis remains largely unknown. Using a panel of structurally and functionally normal human immunoglobulin Gs, we show that individual immunoglobulin G clones possess distinctive Russell body inducing propensities that can surface differently under normal and abnormal cellular conditions. Russell body inducing predisposition unique to each immunoglobulin G clone was corroborated by the intrinsic physicochemical properties encoded in the heavy chain variable domain/light chain variable domain sequence combinations that define each immunoglobulin G clone. While the sequence based intrinsic factors predispose certain immunoglobulin G clones to be more prone to induce Russell bodies, extrinsic factors such as stressful cell culture conditions also play roles in unmasking Russell body propensity from immunoglobulin G clones that are normally refractory to developing Russell bodies. By taking advantage of heterologous expression systems, we dissected the roles of individual subunit chains in Russell body formation and examined the effect of non-cognate subunit chain pair co-expression on Russell body forming propensity. The results suggest that the properties embedded in the variable domain of individual light chain clones and their compatibility with the partnering heavy chain variable domain sequences underscore the efficiency of immunoglobulin G biosynthesis, the threshold for Russell body induction, and the level of immunoglobulin G secretion. We propose that an interplay between the unique properties encoded in variable domain sequences and the state of protein homeostasis determines whether an immunoglobulin G expressing cell will develop the Russell body phenotype in a dynamic cellular setting.

Ig Heavy Chain Promotes Mature B Cell Survival in the Absence of Light Chain

Survival of mature B cells is thought to depend on the BCR signaling (BCR) because ablation of either H chain (HC) expression or BCR signaling causes B cells to rapidly disappear. Whether a complete BCR is required for survival of mature B cells is not known. To address this question, we generated a mouse in which we can repress the expression of a transgenic Ig L chain (IgL) by doxycycline (IgL-repressible mouse). Repression of IgL abrogated expression. Surprisingly, however, IgL-negative B cells survived longer than 14 wk, expressed signal-competent HC on the cell's surface, and active unfolded protein response factors. Like postgerminal center B cells, IgL-negative B cells were small lymphocytes, not dividing and expressed Bcl-6. Our results indicate that expression of unpaired HC, as it may occur as a consequence of Ag ligation, somatic hypermutation, or receptor editing, facilitates the survival of cells either by inducing receptor signaling or by inducing unfolded protein response and/or the expression of survival genes such as Bcl-6.

Extensive Immunoglobulin Production Sensitizes Myeloma Cells for Proteasome Inhibition

Multiple myeloma is an incurable plasma cell neoplasia characterized by the production of large amounts of monoclonal immunoglobulins. The proteasome inhibitor bortezomib (PS-341, Velcade) induces apoptosis in various malignant cells and has been approved for treatment of refractory multiple myeloma. Inhibition of the antiapoptotic transcription factor nuclear factor-kappaB (NF-kappaB) apparently contributes to the antitumor effects of bortezomib; however, this mechanism cannot fully explain the exceptional sensitivity of myeloma cells. Extensive protein synthesis as in myeloma cells is inherently accompanied by unfolded proteins, including defective ribosomal products (DRiPs), which need to be degraded by the ubiquitin-proteasome system. Therefore, we hypothesized that the proapoptotic effect of bortezomib in multiple myeloma is mainly due to the accumulation of unfolded proteins in cells with high protein biosynthesis. Using the IgG-secreting human myeloma cell line JK-6L and murine muH-chain-transfected Ag8.H myeloma cells, apoptosis induction upon proteasome inhibition was clearly correlated with the amount of immunoglobulin production. Preferentially in immunoglobulin-high myeloma cells, bortezomib triggered activation of caspases and induction of proapoptotic CHOP, a component of the terminal unfolded protein response induced by endoplasmic reticulum (ER) stress. In immunoglobulin-high cells, bortezomib increased the levels of proapoptotic Bax while reducing antiapoptotic Bcl-2. Finally, IgG-DRiPs were detected in proteasome inhibitor-treated cells. Hence, proteasome inhibitors induce apoptosis preferentially in cells with high synthesis rate of immunoglobulin associated with accumulation of unfolded proteins/DRiPs inducing ER stress. These findings further elucidate the antitumor activities of proteasome inhibitors and have important implications for optimizing clinical applications.

VH replacement rescues progenitor B cells with two nonproductive VDJ alleles

Inaccurate VDJ rearrangements generate a large number of progenitor (pro)-B cells with two nonproductive IgH alleles. Such cells lack essential survival signals mediated by surface IgM heavy chain (muH chain) expression and are normally eliminated. However, secondary rearrangements of upstream VH gene segments into assembled VDJ exons have been described in mice transgenic for productive muH chains, a process known as VH replacement. If VH replacement was independent of muH chain signals, it could also modify nonproductive VDJ exons and thus rescue pro-B cells with unsuccessful rearrangements on both alleles. To test this hypothesis, we homologously replaced the JH cluster of a mouse with a nonproductive VDJ exon. Surprisingly, B cell development in IgHVDJ-/VDJ- mice was only slightly impaired and significant numbers of IgM-positive B cells were produced. DNA sequencing confirmed that all VDJ sequences from muH chain-positive B lymphoid cells were generated by VH replacement in a RAG-dependent manner. Another unique feature of our transgenic mice was the presence of IgH chains with unusually long CDR3-H regions. Such IgH chains were functional and only modestly counter-selected, arguing against a strict length constraint for CDR3-H regions. In conclusion, VH replacement can occur in the absence of a muH chain signal and provides a potential rescue mechanism for pro-B cells with two nonproductive IgH alleles.

Immunologic basis for the rare occurrence of true nonsecretory plasma cell dyscrasias

Lymphocytes and plasma cells are major actors of the adaptive immune response and can rightly be considered as human health keepers. However, recombination and mutation events occurring at high rate in the B cell lineage also expose these cells to gene alterations, potentially resulting in uncontrolled and life-threatening cell proliferation. Although in cultured cell lines, such gene alterations frequently generate nonsecretory variants, most immunoproliferative B cell disorders feature in vivo immunoglobulin (Ig) secretion. In this paper, we review the molecular mechanisms involved in various instances of the rare, nonsecretory myelomas, in light of current notions about the molecular control of Ig production, assembly, and secretion in normal B cells. We finally document the attractive hypothesis that B cell clones, which retain nonsecretable, intracellular Igs, may be ideal, in vivo targets for efficient anti-idiotypic immune responses, and clones featuring an abundant secretion may by contrast easily induce T cell anergy and escape the anti-tumoral immune surveillance.

Immunoglobulin mu heavy chains do not mediate tyrosine phosphorylation of Ig alpha from the ER-cis-Golgi

Signals delivered by Ig receptors guide the development of functional B lymphocytes. For example, clonal expansion of early mu heavy chain ( mu HC)-positive pre-B cells requires the assembly of a signal-competent pre-B cell receptor complex (pre-BCR) consisting of a mu HC, a surrogate L chain, and the signal dimer Ig alpha beta. However, only a small fraction of the pre-BCR is transported to the cell surface, suggesting that pre-BCR signaling initiates already from an intracellular compartment, e.g., the endoplasmic reticulum (ER). The finding that differentiation of pre-B cells and allelic exclusion at the IgH locus take place in surrogate L chain-deficient mice further supports the presence of a mu HC-mediated intracellular signal pathway. To determine whether a signal-competent Ig complex can already be assembled in the ER, we analyzed the consequence of pervanadate on tyrosine phosphorylation of Ig alpha in J558L plasmacytoma and 38B9 pre-B cells transfected with either a transport-competent IgL chain-pairing or an ER-retained nonpairing micro HC. Flow cytometry, combined Western blot-immunoprecipitation-kinase assays, and confocal microscopy revealed that both the nonpairing and pairing mu HC assembled with the Ig alpha beta dimer; however, in contrast to a pairing mu HC, the nonpairing mu HC was retained in the ER-cis-Golgi compartment, and neither colocalized with the src kinase lyn nor induced tyrosine phosphorylation of Ig alpha after pervanadate treatment of cells. On the basis of these findings, we propose that a signal-competent Ig complex consisting of mu HC, Ig alpha beta, and associated kinases is assembled in a post-ER compartment, thereby supporting the idea that a pre-BCR must be transported to the cell surface to initiate pre-BCR signaling.

Pre-B Cell Receptor-Mediated Selection of Pre-B Cells Synthesizing Functional μ Heavy Chains

Ig gene rearrangements could generate VH-D-JH joining sequences that interfere with the correct folding of a μ-chain, and thus, its capability to pair with IgL chains. Surrogate light (SL) chain might be the ideal molecule to test the capacity of a μ-chain to pair with a L chain early in development, in that only pre-B cells that assemble a membrane μ-SL complex would be permitted to expand and further differentiate. We have previously identified two SL chain nonpairing VH81X-μ-chains with distinct VH-D-JH joining regions. Here, we show that one of these VH81X-μ-chains does not rescue B cell development in JH knock-out mice, because flow cytometric analysis of bone marrow cells from VH81X-μ transgenic JH knock-out mice revealed normal numbers of pro-B cells, but essentially no pre-B and surface IgM+ B cells. Immunoprecipitation analysis of transfected pre-B and hybridoma lines revealed that the same μ-chain fails to pair not only with SL chain but also with four distinct κ L chains. These findings demonstrate that early pre-B cells are selected for maturation on the basis of the structure of a μ-chain, in particular its VH-D-JH joining or CDR3 sequence, and that one mechanism for this selection is the capacity of a μ-chain to assemble with SL chain. Therefore, we propose a new function of SL chain in early B cell development: SL chain is part of a quality control mechanism that tests a μ-chain for its ability to pair with conventional L chains.

Cloning and characterization of HUPF1, a human homolog of the Saccharomyces cerevisiae nonsense mRNA-reducing UPF1 protein

Levels of most nonsense mRNAs are normally reduced in prokaryotes and eukaryotes when compared with that of corresponding functional mRNAs. Genes encoding polypeptides that selectively reduce levels of nonsense mRNA have so far only been identified in simple eukaryotes. We have now cloned a human cDNA whose deduced amino acid sequence shows the highest degree of homology to that of UPF1, a bona fide Saccharomyces cerevisiae group I RNA helicase required for accelerated degradation of nonsense mRNA. Based on the total sequence of the shorter yeast UPF1 protein, the overall identity between the human protein and UPF1 is 51%. Besides NTPase and other RNA helicase consensus motifs, UPF1 and its human homolog also share similar putative zinc finger motifs that are absent in other group I RNA helicases. Northern blot analysis with the human cDNA probe revealed two transcripts in several human cell lines. Further, antibodies raised against a synthetic peptide of the human polypeptide detected a single 130 kDa polypeptide on Western blots from human and mouse cells. Finally, immunofluorescence and Western blot analyses revealed that the human and mouse polypeptides, like yeast UPF1, are expressed in the cytoplasm, but not in the nucleus. We have thus identified the first mammalian homolog of yeast UPF1, a protein that regulates levels of nonsense mRNA, and we tentatively name this protein human HUPF1 (for human homolog of UPF1).

Roles of heavy and light chains in IgM polymerization

IgM antibodies are secreted as multisubunit polymers that consist of as many as three discrete polypeptides: mu heavy chains, light (L) chains, and joining (J) chains. We wished to determine whether L chains that are required to confer secretory competence on immunoglobulin molecules must be present for IgM to polymerize--that is, for intersubunit disulfide bonds to form between mu chains. Using a L-chain-loss variant of an IgM-secreting hybridoma, we demonstrated that mu chains were efficiently polymerized independent of L chains, in a manner similar to that observed for conventional microL complexes, and that the mu polymers incorporated J chain. These mu polymers were not secreted but remained associated with the endoplasmic reticulum-resident chaperone BiP (GRP78). This finding is consistent with the endoplasmic reticulum being the subcellular site of IgM polymerization. We conclude that mu chain alone has the potential to direct the polymerization of secreted IgM, a process necessary but not sufficient for IgM to attain secretory competence.

A simpler sort of antibody

The monoclonal antibody NEMO is directed against a molecule expressed by human cells of the melanocytic lineage. Although obtained by conventional immunization and fusion procedures, NEMO consists solely of kappa light chain. SDS/PAGE analysis indicates that the kappa chains are present as both monomers and dimers. When these two forms were separated by gel filtration, only the monomeric form bound antigen. As kappa light chains from the myeloma MOPC-41 and the hybridoma MORK do not bind to the melanocytic cells, we conclude that the binding specificity of NEMO resides in the variable region.