Truncation of the mu heavy chain alters BCR signalling and allows recruitment of CD5+ B cells

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.

Characterization of immunoglobulin heavy chain knockout rats

The rat is a species frequently used in immunological studies but, until now, there were no models with introduced gene-specific mutations. In a recent study, we described for the first time the generation of novel rat lines with targeted mutations using zinc-finger nucleases. In this study, we compare immune development in two Ig heavy-chain KO lines; one with truncated Cμ and a new line with removed JH segments. Rats homozygous for IgM mutation generate truncated Cμ mRNA with a de novo stop codon and no Cγ mRNA. JH-deletion rats showed undetectable mRNA for all H-chain transcripts. No serum IgM, IgG, IgA and IgE were detected in these rat lines. In both lines, lymphoid B-cell numbers were reduced >95% versus WT animals. In rats homozygous for IgM mutation, no Ab-mediated hyperacute allograft rejection was encountered. Similarities in B-cell differentiation seen in Ig KO rats and ES cell-derived Ig KO mice are discussed. These Ig and B-cell-deficient rats obtained using zinc-finger nucleases-technology should be useful as biomedical research models and a powerful platform for transgenic animals expressing a human Ab repertoire.

Replacement of Imu-Cmu intron by NeoR gene alters Imu germ-line expression but has no effect on V(D)J recombination

The NeoR gene has often been used to unravel the mechanisms underlying long-range interactions between promoters and enhancers during V(D)J assembly and class switch recombination (CSR) in the immunoglobulin heavy chain (IgH) locus. This approach led to the notion that CSR is regulated through competition of germ-line (GL) promoters for activities displayed by the 3' regulatory region (3'RR). This polarized long-range effect of the 3'RR is disturbed upon insertion of NeoR gene in the IgH constant (C(H)) region, where only GL transcription derived from upstream GL promoters is impaired. In the context of V(D)J recombination, replacement of Emu enhancer or Emu core enhancer (cEmu) by NeoR gene fully blocked V(D)J recombination and mu0 GL transcription which originates 5' of DQ52 and severely diminished Imu GL transcription derived from Emu/Imu promoter, suggesting a critical role for cEmu in the regulation of V(D)J recombination and of mu0 and Imu expression. Here we focus on the effect of NeoR gene on mu0 and Imu GL transcription in a mouse line in which the Imu-Cmu intron was replaced by a NeoR gene in the sense-orientation. B cell development was characterized by a marked but incomplete block at the pro-B cell stage. However, V(D)J recombination was unaffected in sorted pro-B and pre-B cells excluding an interference with the accessibility control function of Emu. mu0 GL transcription initiation was relatively normal but the maturation step seemed to be affected most likely through premature termination at NeoR polyadenylation sites. In contrast, Imu transcription initiation was impaired suggesting an interference of NeoR gene with the IgH enhancers that control Imu expression. Surprisingly, in stark contrast with the NeoR effect in the C(H) region, LPS-induced NeoR expression restored Imu transcript levels to normal. The data suggest that Emu enhancer may be the master control element that counteracts the down-regulatory "Neo effect" on Imu expression upon LPS stimulation. More importantly, they reveal a complex and developmentally regulated interplay between IgH enhancers in the control of Imu expression.

Immunoglobulin aggregation leading to Russell body formation is prevented by the antibody light chain

Russell bodies (RBs) are intracellular inclusions filled with protein aggregates. In diverse lymphoid disorders these occur as immunoglobulin (Ig) deposits, accumulating in abnormal plasma or Mott cells. In heavy-chain deposition disease truncated antibody heavy-chains (HCs) are found, which bear a resemblance to diverse polypeptides produced in Ig light-chain (LC)-deficient (L(-/-)) mice. In L(-/-) animals, the known functions of LC, providing part of the antigen-binding site of an antibody and securing progression of B-cell development, may not be required. Here, we show a novel function of LC in preventing antibody aggregation. L(-/-) mice produce truncated HC naturally, constant region (C)gamma and Calpha lack C(H)1, and Cmicro is without C(H)1 or C(H)1 and C(H)2. Most plasma cells found in these mice are CD138(+) Mott cells, filled with RBs, formed by aggregation of HCs of different isotypes. The importance of LC in preventing HC aggregation is evident in knock-in mice, expressing Cmicro without C(H)1 and C(H)2, which only develop an abundance of RBs when LC is absent. These results reveal that preventing antibody aggregation is a major function of LC, important for understanding the physiology of heavy-chain deposition disease, and in general recognizing the mechanisms, which initiate protein conformational diseases.

Removal of the BiP-retention domain in Cmicro permits surface deposition and developmental progression without L-chain

Nascent, full length, immunoglobulin (Ig) heavy (H)-chains are post-translationally associated with H-chain-binding protein (BiP or GRP78) in the endoplasmic reticulum (ER). The first constant (C) domain, CH1 of a C gene (Cmu, Cgamma, Calpha), is important for this interaction. The contact is released upon BiP replacement by conventional Ig light (L)-chain (kappa or lambda). Incomplete or mutated H-chains with removed variable (VH) and/or C(H)1 domain, as found in H-chain disease (HCD), can preclude stable BiP interaction. Progression in development after the preB cell stage is dependent on surface expression of IgM when association of a micro H-chain with a L-chain overcomes the retention by BiP. We show that IgM lacking the BiP-binding domain is displayed on the cell surface and elicits a signal that allows developmental progression even without the presence of L-chain. The results are reminiscent of single chain Ig secretion in camelids where developmental processes leading to the generation of fully functional H-chain-only antibodies are not understood. Furthermore, in the mouse the largest secondary lymphoid organ, the spleen, is not required for H-chain-only Ig expression and the CD5 survival signal may be obsolete for cells expressing truncated IgM.

Heavy chain-only antibodies are spontaneously produced in light chain-deficient mice

In healthy mammals, maturation of B cells expressing heavy (H) chain immunoglobulin (Ig) without light (L) chain is prevented by chaperone association of the H chain in the endoplasmic reticulum. Camelids are an exception, expressing homodimeric IgGs, an antibody type that to date has not been found in mice or humans. In camelids, immunization with viral epitopes generates high affinity H chain-only antibodies, which, because of their smaller size, recognize clefts and protrusions not readily distinguished by typical antibodies. Developmental processes leading to H chain antibody expression are unknown. We show that L(-/-) (kappa(-/-)lambda(-/-)-deficient) mice, in which conventional B cell development is blocked at the immature B cell stage, produce diverse H chain-only antibodies in serum. The generation of H chain-only IgG is caused by the loss of constant (C) gamma exon 1, which is accomplished by genomic alterations in C(H)1-circumventing chaperone association. These mutations can be attributed to errors in class switch recombination, which facilitate the generation of H chain-only Ig-secreting plasma cells. Surprisingly, transcripts with a similar deletion can be found in normal mice. Thus, naturally occurring H chain transcripts without C(H)1 (V(H)DJ(H)-hinge-C(H)2-C(H)3) are selected for and lead to the formation of fully functional and diverse H chain-only antibodies in L(-/-) animals.

Expression of a Dromedary Heavy Chain-Only Antibody and B Cell Development in the Mouse

In mature B cells of mice and most mammals, cellular release of single H chain Abs without L chains is prevented by H chain association with Ig-specific chaperons in the endoplasmic reticulum. In precursor B cells, however, surface expression of mu-H chain in the absence of surrogate and conventional L chain has been identified. Despite this, Ag-specific single H chain Ig repertoires, using mu-, gamma-, epsilon-, or alpha-H chains found in conventional Abs, are not produced. Moreover, removal of H chain or, separately, L chain (kappa/lambda) locus core sequences by gene targeting has prevented B cell development. In contrast, H chain-only Abs are produced abundantly in Camelidae as H2 IgG without the C(H)1 domain. To test whether H chain Abs can be produced in mice, and to investigate how their expression affects B cell development, we introduced a rearranged dromedary gamma2a H chain into the mouse germline. The dromedary transgene was expressed as a naturally occurring Ag-specific disulphide-linked homodimer, which showed that B cell development can be instigated by expression of single H chains without L chains. Lymphocyte development and B cell proliferation was accomplished despite the absence of L chain from the BCR complex. Endogenous Ig could not be detected, although V(D)J recombination and IgH/L transcription was unaltered. Furthermore, crossing the dromedary H chain mice with mice devoid of all C genes demonstrated without a doubt that a H chain-only Ab can facilitate B cell development independent of endogenous Ig expression, such as mu- or delta-H chain, at early developmental stages.

Silencing of the immunoglobulin heavy chain locus by removal of all eight constant-region genes in a 200-kb region

Silencing or removal of individual C (constant)-region genes and/or adjacent control sequences did not generate fully deficient Ig (immunoglobulin)- mice. A reason is that different C genes share many functional tasks and most importantly are individually capable of ensuring lymphocyte differentiation. Nevertheless, incomplete arrests in B-cell development were found, most pronounced at the onset of H-chain expression. Here we show that removal of 200 kb accommodating all C genes, Cmu-Cdelta-Cgamma3-Cgamma1-Cgamma2b-Cgamma2a-Cepsilon-Calpha, stops antibody production. For this two loxP targeting constructs were introduced into the most 5' C gene and the distal alpha 3' enhancer. Cre-loxP-mediated in vivo deletion was accompanied by extensive germ-line mosaicism, which could be separated by breeding. Homozygous C-gene deletion mice did not express Ig H or L chains and flow cytometry revealed a complete block in B-cell development. However, C-gene removal did not affect DNA rearrangement processes following locus activation, as recombination efficacy appears to be similar to what is found in normal mice.

IL-9-Induced Expansion of B-1b Cells Restores Numbers but Not Function of B-1 Lymphocytes in xid Mice

Mice expressing the X-linked immunodeficiency (xid) mutation lack functional Bruton's tyrosine kinase and were shown to be specifically deficient in peritoneal B-1 lymphocytes. We have previously shown that IL-9, a cytokine produced by TH2 lymphocytes, promotes B-1 cell expansion in vivo. To determine whether IL-9 overexpression might compensate the xid mutation for B-1 lymphocyte development, we crossed xid mice with IL-9-transgenic mice. In this model, IL-9 restored normal numbers of mature peritoneal B-1 cells that all belonged to the CD5(-) B-1b subset. Despite this normal B-1 lymphocyte number, IL-9 failed to restore classical functions of B-1 cells, namely, the production of natural IgM Abs, the T15 Id Ab response to phosphorylcholine immunization, and the antipolysaccharide humoral response against Streptococcus pneumoniae. By using bromelain-treated RBC, we showed that the antigenic repertoire of these IL-9-induced B-1b lymphocytes was different from the repertoire of classical CD5(+) B-1a cells, indicating that the lack of B-1 function by B-1b cells is associated with distinct Ag specificities. Taken together, our data show that B-1b cell development can restore the peritoneal B-1 population in xid mice but that these B-1b cells are functionally distinct from CD5(+) B-1a lymphocytes.

Block in development at the pre-B-II to immature B cell stage in mice without Ig kappa and Ig lambda light chain

Silencing individual C (constant region) lambda genes in a kappa(-/-) background reduces mature B cell levels, and L chain-deficient (lambda(-/-)kappa(-/-)) mice attain a complete block in B cell development at the stage when L chain rearrangement, resulting in surface IgM expression, should be completed. L chain deficiency prevents B cell receptor association, and L chain function cannot be substituted (e.g., by surrogate L chain). Nevertheless, precursor cell levels, controlled by developmental progression and checkpoint apoptosis, are maintained, and B cell development in the bone marrow is fully retained up to the immature stage. L chain deficiency allows H chain retention in the cytoplasm, but prevents H chain release from the cell, and as a result secondary lymphoid organs are B cell depleted while T cell levels remain normal.

B cell development arrest upon insertion of a neo gene between JH and Emu: promoter competition results in transcriptional silencing of germline JH and complete VDJ rearrangements

Previous targeting experiments within the IgH locus have shown that V(D)J recombination was affected by an insertion of a neo gene within E(mu) upstream of the core enhancer, but not by insertions downstream of the enhancer. Similarly, class switch recombination to a given (C) gene was affected only by interposition of neo in between that gene and the 3' IgH enhancers. Here we show that insertion of neo upstream E(mu) only marginally impairs V(D)J recombination, but results in an altered D and J(H) gene usage and completely blocks transcription of the germline J(H) region and the rearranged VDJ segments. Although transcriptional silencing of J(H) occurs upstream of the insertion and results in the lack of mature B cells in homozygous mutant animals, IgH transcription is maintained downstream of the insertion together with neo transcription and can be up-regulated by LPS stimulation or upon fusion with plasmacytoma cells. Altogether these data argue for a polarized "neo effect" involving promoter competition and further show that V(D)J rearrangement can be uncoupled from transcription.