Al amyloidosis

Definition of the disease

AL amyloidosis results from extra-cellular deposition of fibril-forming monoclonal immunoglobulin (Ig) light chains (LC) (most commonly of lambda isotype) usually secreted by a small plasma cell clone. Most patients have evidence of isolated monoclonal gammopathy or smoldering myeloma, and the occurrence of AL amyloidosis in patients with symptomatic multiple myeloma or other B-cell lymphoproliferative disorders is unusual. The key event in the development of AL amyloidosis is the change in the secondary or tertiary structure of an abnormal monoclonal LC, which results in instable conformation. This conformational change is responsible for abnormal folding of the LC, rich in β leaves, which assemble into monomers that stack together to form amyloid fibrils.

Epidemiology

AL amyloidosis is the most common type of systemic amyloidois in developed countries with an estimated incidence of 9 cases/million inhabitant/year. The average age of diagnosed patients is 65 years and less than 10% of patients are under 50.

Clinical description

The clinical presentation is protean, because of the wide number of tissues or organs that may be affected. The most common presenting symptoms are asthenia and dyspnoea, which are poorly specific and may account for delayed diagnosis. Renal manifestations are the most frequent, affecting two thirds of patients at presentation. They are characterized by heavy proteinuria, with nephrotic syndrome and impaired renal function in half of the patients. Heart involvement, which is present at diagnosis in more than 50% of patients, leading to restrictive cardiopathy, is the most serious complication and engages prognosis.

Diagnostic methods

The diagnosis relies on pathological examination of an involved site showing Congo red-positive amyloid deposits, with typical apple-green birefringence under polarized light, that stain positive with an anti-LC antibody by immunohistochemistry and/or immunofluorescence. Due to the systemic nature of the disease, non-invasive biopsies such as abdominal fat aspiration should be considered before taking biopsies from involved organs, in order to reduce the risk of bleeding complications.

Differential diagnosis

Systemic AL amyloidosis should be distinguished from other diseases related to deposition of monoclonal LC, and from other forms of systemic amyloidosis. When pathological studies have failed to identify the nature of amyloid deposits, genetic studies should be performed to diagnose hereditary amyloidosis.

Management

Treatment of AL amyloidosis is based on chemotherapy, aimed at controlling the underlying plasma clone that produces amyloidogenic LC. The hematological response should be carefully checked by serial measurements of serum free LC. The association of an alkylating agent with high-dose dexamethasone has proven to be effective in two thirds of patients and is considered as the current reference treatment. New agents used in the treatment of multiple myeloma are under investigation and appear to increase hematological response rates. Symptomatic measures and supportive care is necessary in patients with organ failure. Noticeably, usual treatments for cardiac failure (i.e. calcium inhibitors, β-blockers, angiotensin converting enzyme inhibitors) are inefficient or even dangerous in patients with amyloid heart disease, that should be managed using diuretics. Amiodarone and pace maker implantation should be considered in patients with rhythm or conduction abnormalities. In selected cases, heart and kidney transplantation may be associated with prolonged patient and graft survival.

Prognosis

Survival in AL amyloidosis depends on the spectrum of organ involvement (amyloid heart disease being the main prognosis factor), the severity of individual organs involved and haematological response to treatment.

Role of the monoclonal chain V domain and reversibility of renal damage in a transgenic model of acquired Fanconi syndrome

Acquired Fanconi syndrome (FS) is a complication of monoclonal gammopathies featuring a generalized dysfunction of the proximal tubule of the kidney, due to the storage within proximal tubular cells of a monoclonal immunoglobulin light chain. We engineered transgenic mice in which the endogenous mouse Jkappa cluster was replaced by a human VkappaJkappa rearranged gene cloned from a patient with smoldering myeloma-associated FS. The V region belonged to the VkappaI subgroup and was related to the O2-O12 germ-line gene, a V segment previously found associated with FS and light-chain crystallization in several patients with myeloma. Association of the human VkappaI domain with a mouse kappa constant domain in transgenic animals yielded a nephrotoxicity pattern similar to that observed in patients, strongly suggesting that the whole pathogenic effect of FS light chains can be ascribed to a peculiar structure of the V domain. Morphologic alterations of the kidney tubular cells, which contained rhomboid-shape crystals, were observed in mice, together with alterations of the proximal tubule reabsorption function. Moreover, the number of renal crystalline inclusions was dramatically reduced after conditional deletion of the human VkappaI transgene, showing that proximal tubular lesions are reversible upon suppression of the nephrotoxic light chain secretion.

Fanconi’s syndrome induced by a monoclonal Vκ3 light chain in Waldenström’s macroglobulinemia

Fanconi's syndrome (FS) is a disorder of sodium-dependent proximal tubule reabsorption, which may complicate plasma cell disorders producing a free monoclonal light chain (LC). FS often occurs in the setting of smoldering myeloma and features cytoplasmic crystalline inclusions of monoclonal kappa LC in proximal tubular cells and malignant plasma cells. Although the clinical and pathological presentation may vary, including lack of crystal formation, monoclonal kappa LCs that underlie FS show a striking genetic and biochemical homogeneity: they almost always belong to the Vkappa1 subgroup of variability and originate from 2 germline genes, O2/O12 or O8/O18. Their variable domain sequences present unusual hydrophobic residues, responsible for the resistance to proteolysis, which leads to LC accumulation in the endocytic compartment of proximal tubule cells. We report a patient with slowly progressive Waldenstrom's macroglobulinemia and full-blown FS with accumulation of a monoclonal kappa LC within proximal tubules, but no detectable crystalline organization. This LC, which belonged to the unusual Vkappa3 subgroup and derived from the L2/L16 germline gene, showed no common substitution with previously described FS kappaI LC and was sensitive to trypsin digestion. These data show that molecular and biochemical characteristics of kappa LCs in patients with FS are more heterogeneous than initially suspected. Mechanisms other than resistance of LCs to endosomal proteolysis probably are involved in the pathogenesis of FS-associated plasma cell dyscrasias.

Kidney diseases associated with monoclonal immunoglobulin M-secreting B-cell lymphoproliferative disorders: a case series of 35 patients

BACKGROUND

Kidney diseases associated with immunoglobulin M (IgM) monoclonal gammopathy are poorly described, with few data for patient outcomes and renal response.

STUDY DESIGN

Case series.

SETTING & PARTICIPANTS

35 patients from 8 French departments of nephrology were retrospectively studied. Inclusion criteria were: (1) detectable serum monoclonal IgM, (2) estimated glomerular filtration rate (eGFR) < 60mL/min/1.73m(2) and/or proteinuria with protein excretion > 0.5g/d and/or microscopic hematuria, and (3) kidney biopsy showing monoclonal immunoglobulin deposits and/or lymphomatous B-cell renal infiltration. All patients received chemotherapy, including rituximab-based regimens in 8 cases.

PREDICTORS

Patients were classified into 3 groups according to renal pathology: glomerular AL amyloidosis (group 1; n=11), nonamyloid glomerulopathies (group 2; n=15, including 9 patients with membranoproliferative glomerulonephritis), and tubulointerstitial nephropathies (group 3; n=9, including cast nephropathy in 5, light-chain Fanconi syndrome in 3, and isolated tumor infiltration in 1).

OUTCOMES

Posttreatment hematologic response (≥50% reduction in serum monoclonal IgM and/or free light chain level) and renal response (≥50% reduction in 24-hour proteinuria or eGFR≥30mL/min/1.73m(2) in patients with glomerular and tubulointerstitial disorders, respectively).

RESULTS

Nephrotic syndrome was observed in 11 and 6 patients in groups 1 and 2, respectively. Patients in group 3 presented with acute kidney injury (n=7) and/or proximal tubular dysfunction (n=3). Waldenström macroglobulinemia was present in 26 patients (8, 12, and 6 in groups 1, 2, and 3, respectively). Significant lymphomatous interstitial infiltration was observed in 18 patients (4, 9, and 5 patients, respectively). Only 9 of 29 evaluable patients had systemic signs of symptomatic hematologic disease (2, 5, and 2, respectively). In groups 1, 2, and 3, respectively, hematologic response was achieved after first-line treatment in 3 of 9, 9 of 10, and 5 of 6 evaluable patients, while renal response occurred in 5 of 10, 9 of 15, and 5 of 8 evaluable patients.

LIMITATIONS

Retrospective study; insufficient population to establish the impact of chemotherapy.

CONCLUSIONS

IgM monoclonal gammopathy is associated with a wide spectrum of renal manifestations, with an under-recognized frequency of tubulointerstitial disorders.

Impaired Lysosomal Function Underlies Monoclonal Light Chain-Associated Renal Fanconi Syndrome

Monoclonal gammopathies are frequently complicated by kidney lesions that increase the disease morbidity and mortality. In particular, abnormal Ig free light chains (LCs) may accumulate within epithelial cells, causing proximal tubule (PT) dysfunction and renal Fanconi syndrome (RFS). To investigate the mechanisms linking LC accumulation and PT dysfunction, we used transgenic mice overexpressing human control or RFS-associated κLCs (RFS-κLCs) and primary cultures of mouse PT cells exposed to low doses of corresponding human κLCs (25 μg/ml). Before the onset of renal failure, mice overexpressing RFS-κLCs showed PT dysfunction related to loss of apical transporters and receptors and increased PT cell proliferation rates associated with lysosomal accumulation of κLCs. Exposure of PT cells to RFS-κLCs resulted in κLC accumulation within enlarged and dysfunctional lysosomes, alteration of cellular dynamics, defective proteolysis and hydrolase maturation, and impaired lysosomal acidification. These changes were specific to the RFS-κLC variable (V) sequence, because they did not occur with control LCs or the same RFS-κLC carrying a single substitution (Ala30→Ser) in the V domain. The lysosomal alterations induced by RFS-κLCs were reflected in increased cell proliferation, decreased apical expression of endocytic receptors, and defective endocytosis. These results reveal that specific κLCs accumulate within lysosomes, altering lysosome dynamics and proteolytic function through defective acidification, thereby causing dedifferentiation and loss of reabsorptive capacity of PT cells. The characterization of these early events, which are similar to those encountered in congenital lysosomal disorders, provides a basis for the reported differential LC toxicity and new perspectives on LC-induced RFS.

Toward understanding renal Fanconi syndrome: step by step advances through experimental models

Renal Fanconi syndrome (FS) is a generalized dysfunction of proximal tubular epithelial cells leading to the urinary leak of essential metabolites like phosphate, uric acid, glucose, amino acids and low molecular weight proteins. From inherited forms involving mutations on apparently unrelated genes to acquired forms induced by drugs, heavy metals or monoclonal immunoglobulin (Ig) light chains (LC), heterogeneous causalities of FS have complicated the understanding of this pathology for a long time. Experimental models of FS have allowed researchers to face the challenge and have helped unravel the main mechanisms disturbing proximal tubule reabsorption. Administration of cadmium to animals first demonstrated an inhibition of Na/K/ATPase activity, highlighting how a single toxic component could induce the general sodium-linked transport defect observed in FS. Today, genetically modified mice allow the development of reliable and reproducible experimental models for inherited or acquired forms of FS. One of the most exciting advances offered by these models is the unexpected major role of endocytosis in the function of the proximal tubule revealed by megalin and ClC-5 knockout mice. Using gene-targeted insertion, a transgenic mouse for LC-associated FS, the most frequent adult form of FS, has also been recently developed and represents a major step in the development of models of this pathology. Beyond deciphering molecular and cellular events at the origin of FS, these models also represent essential tools for the development of therapeutic strategies.

Unravelling the immunopathological mechanisms of heavy chain deposition disease with implications for clinical management

Randall-type heavy chain deposition disease (HCDD) is a rare disorder characterized by tissue deposition of a truncated monoclonal immunoglobulin heavy chain lacking the first constant domain. Pathophysiological mechanisms are unclear and management remains to be defined. Here we retrospectively studied 15 patients with biopsy-proven HCDD of whom 14 presented with stage 3 or higher chronic kidney disease, with nephrotic syndrome in 9. Renal lesions were characterized by nodular glomerulosclerosis, with linear peritubular and glomerular deposits of γ-heavy chain in 12 patients or α-heavy chain in 3 patients, without concurrent light chain staining. Only 2 patients had symptomatic myeloma. By serum protein electrophoresis/immunofixation, 13 patients had detectable monoclonal gammopathy. However, none of these techniques allowed detection of the nephrotoxic truncated heavy chain, which was achieved by immunoblot and/or bone marrow heavy chain sequencing in 14 of 15 patients. Serum-free kappa to lambda light chain ratio was abnormal in 11 of 11 patients so examined. Immunofluorescence studies of bone marrow plasma cells showed coexpression of the pathogenic heavy chain with light chain matching the abnormal serum-free light chain in all 3 tested patients. Heavy chain sequencing showed first constant domain deletion in 11 of 11 patients, with high isoelectric point values of the variable domain in 10 of 11 patients. All patients received chemotherapy, including bortezomib in 10 cases. Renal parameters improved in 11 patients who achieved a hematological response, as assessed by normalization of the free light chain ratio in 8 cases. Tissue deposition in HCDD relates to physicochemical peculiarities of both variable and constant heavy chain domains. Early diagnosis and treatment with bortezomib-based combinations appear important to preserve renal prognosis. Thus, monitoring of serum-free light chain is an indirect but useful method to evaluate the hematological response.

Interallelic class switch recombination contributes significantly to class switching in mouse B cells

Except for the expression of IgM and IgD, DNA recombination is constantly needed for the expression of other Ig classes and subclasses. The predominant path of class switch recombination (CSR) is intrachromosomal, and the looping-out and deletion model has been abundantly documented. However, switch regions also occasionally constitute convenient substrates for interchromosomal recombination, since it is noticeably the case in a number of chromosomal translocations causing oncogene deregulation in the course of lymphoma and myeloma. Although asymmetric accessibility of Ig alleles should theoretically limit its occurrence, interallelic CSR was shown to occur at low levels during IgA switching in rabbit, where the definition of allotypes within both V and C regions helped identify interchromosomally derived Ig. Thus, we wished to evaluate precisely interallelic CSR frequency in mouse B cells, by using a system in which only one allele (of b allotype) could express a functional VDJ region, whereas only interallelic CSR could restore expression of an excluded (a allotype) allele. In our study, we show that interchromosomal recombination of V(H) and Cgamma or Calpha occurs in vivo in B cells at a frequency that makes a significant contribution to physiological class switching: trans-association of V(H) and C(H) genes accounted for 7% of all alpha mRNA, and this frequency was about twice higher for the gamma3 transcripts, despite the much shorter distance between the J(H) region and the Cgamma3 gene, thus confirming that this phenomenon corresponded to site-specific switching and not to random recombination between long homologous loci.

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.

AID-induced remodeling of immunoglobulin genes and B cell fate

Survival and phenotype of normal and malignant B lymphocytes are critically dependent on constitutive signals by the B cell receptor (BCR) for antigen. In addition, either antigen ligation of the BCR or various mitogenic stimuli result in B cell activation and induction of activation-induced deaminase (AID). AID activity can in turn mediate somatic hypermutation (SHM) of immunoglobulin (Ig) V regions and also deeply remodel the Ig heavy chain locus through class switch recombination (CSR) or locus suicide recombination (LSR). In addition to changes linked to affinity for antigen, modifying the class/isotype (i.e. the structure and function) of the BCR or suddenly deleting BCR expression also modulates the fate of antigen-experienced B cells.

Transcription-dependent somatic hypermutation occurs at similar levels on functional and nonfunctional rearranged IgH alleles

Allelic exclusion of IgH chain expression is stringently established before or during early B cell maturation. It likely relies both on cellular mechanisms, selecting those cells in which a single receptor allows the best possible Ag response, and on molecular restrictions of gene accessibility to rearrangement. The extent to which transcriptional control may be involved is unclear. Transcripts arising from nonfunctional alleles would undergo nonsense-mediated degradation and their virtual absence in mature cells cannot ensure that transcription per se is down-regulated. By contrast, somatic hypermutation may provide an estimate of primary transcription in Ag-activated cells since both processes are directly correlated. For coding regions, the rate and nature of mutations also depend upon Ag binding constraints. By sequencing intronic sequence downstream mouse VDJ genes, we could show in the absence of such constraints that somatic hypermutation intrinsically targets nonfunctional rearranged alleles at a frequency approaching that of functional alleles, suggesting that transcription also proceeds on both alleles at a similar rate. By contrast and confirming the strong dependency of somatic hypermutation upon transcription, we show that artificial blockade of transcription on the nonfunctional allele by a knock-in neomycin resistance cassette keeps the VDJ region unmutated even when its promoter is intact and when it is fully rearranged.

RNA surveillance down-regulates expression of nonfunctional kappa alleles and detects premature termination within the last kappa exon

Random V(D)J junctions would generate nonfunctional and/or out-of-frame sequences in about two-thirds of cases and result in abundant transcripts encoding truncated proteins. Although allelic exclusion at the DNA recombination level ensures that a single allele is functional, the frequent biallelic rearrangements need additional mechanisms to down-regulate aberrant transcripts in those cells with both a functionally and a nonfunctionally rearranged allele. The process of nonsense-mediated decay targets aberrantly rearranged Ig heavy-chain transcripts, but the situation of light-chain mRNAs is more complex, because they do not meet the usual requirements for nonsense-mediated decay and most often lack a spliceable intron downstream of the premature termination. We studied immunoglobulin heavy-chain -/- pro-B cells in which light chain genes get rearranged and expressed in the absence of any selection for the assembly of a functional B cell receptor. Using this model, we show that the whole kappa locus is accessible in pro-B cells and allows the assembly of a broad spectrum of VkappaJkappa segments, most of which are out-of-frame. This model provides an evaluation of the in vivo efficiency of RNA surveillance toward aberrant kappa mRNAs produced in pro-B cells. Our data show that nonfunctional kappa transcripts are excluded from the mature mRNA pool not only by detecting termination in an upstream exon but also by detecting changes in the position of termination within the last exon. Similar mechanisms efficiently down-regulate nonfunctional kappa transcripts arising in normal mature B cells due to the biallelic transcription of rearranged kappa genes.

The Proximal Tubule Toxicity of Immunoglobulin Light Chains

Plasma and B cells dyscrasias that overproduce monoclonal immunoglobulin free light chains (FLCs) affect the kidney frequently in various ways. The hematologic dyscrasia responsible for the production of FLCs may or may not meet the criteria for cancer, such as multiple myeloma (MM) or lymphoma, or may remain subclinical. If there is overt malignancy, the accompanying kidney disorder is called myeloma- or lymphoma-associated. If the dyscrasia is subclinical, the associated kidney disorders are grouped as monoclonal gammopathy of renal significance. Glomeruli and tubules may both be involved. The proximal tubule disorders comprise a spectrum of interesting syndromes, which range in severity. This review focuses on the recent insights gained into the patterns and the mechanisms of proximal tubule toxicity of FLCs, including subtle transport disorders, such as proximal tubule acidosis, partial or complete Fanconi syndrome, or severe acute or chronic renal failure. Histologically, there may be crystal deposition in the proximal tubule cells, acute tubule injury, interstitial inflammation, fibrosis, and tubule atrophy. Specific structural alterations in the V domain of FLCs caused by somatic hypermutations are responsible for crystal formation as well as partial or complete Fanconi syndrome. Besides crystal formation, tubulointerstitial inflammation and proximal tubulopathy can be mediated by direct activation of inflammatory pathways through cytokines and Toll-like receptors due to cell stress responses induced by excessive FLC endocytosis into the proximal tubule cells. Therapy directed against the clonal source of the toxic light chain can prevent progression to more severe lesions and may help preserve kidney function.

Light chain only variant of proliferative glomerulonephritis with monoclonal immunoglobulin deposits is associated with a high detection rate of the pathogenic plasma cell clone

IgG (mainly IgG3) is the most commonly involved isotype in proliferative glomerulonephritis with monoclonal immunoglobulin deposits (PGNMID). Here we describe the first series of PGNMID with deposition of monoclonal immunoglobulin light chain only (PGNMID-light chain). This multicenter cohort of 17 patients presented with nephritic or nephrotic syndrome with underlying hematologic conditions of monoclonal gammopathy of renal significance (71%) or multiple myeloma (29%). Monoclonal immunoglobulin was identified by serum and urine immunofixation in 65% and 73%, respectively, with abnormal serum free light chain in 83%, and a detectable bone marrow plasma cell clone in 88% of patients. Renal biopsy showed a membranoproliferative pattern in most patients. By immunofluorescence, deposits were restricted to glomeruli and composed of restricted light chain (kappa in 71%) and C3, with granular appearance and subendothelial, mesangial and subepithelial distribution by electron microscopy. Proteomic analysis in four cases of kappa PGNMID-light chain revealed spectra for kappa constant and variable domains, without evidence of Ig heavy chains; spectra for proteins of the alternative pathway of complement and terminal complex were detected in three. The classical pathway was not detected in three cases. After median follow up of 70 months, the renal response was dependent on a hematologic response and occurred in six of ten patients treated with plasma cell-directed chemotherapy but none of five patients receiving other therapies. Thus, PGNMID-light chain differs from PGNMID-IgG by higher frequency of a detectable pathogenic plasma cell clone. Hence, proper recognition is crucial as anti-myeloma agents may improve renal prognosis. Activation of an alternative pathway of complement by monoclonal immunoglobulin light chain likely plays a role in its pathogenesis.

Heavy Chain Fibrillary Glomerulonephritis: A Case Report

Heavy chain amyloidosis and heavy chain deposition disease are the only known kidney diseases caused by the deposition of truncated immunoglobulin heavy chains. Fibrillary glomerulonephritis typically results from deposition of DNAJB9 (DnaJ heat shock protein family [Hsp40] member B9) and polytypic immunoglobulin G (IgG). We describe a patient with monoclonal gammopathy (IgG with λ light chain) who developed DNAJB9-negative fibrillary glomerulonephritis leading to end-stage kidney disease, with recurrence in 2 kidney allografts. Pre- and postmortem examination showed glomerular deposition of Congo red-negative fibrillar material that was determined to be immunoglobulin heavy chain. We propose the term "heavy chain fibrillary glomerulonephritis" to describe this lesion, which appears to be a rare kidney complication of monoclonal gammopathy. The diagnosis should be suspected when the kidney biopsy shows fibrillary glomerulonephritis with negative staining for immunoglobulin light chains and DNAJB9; the diagnosis can be confirmed using immunochemical and molecular studies.

Light chain inclusion permits terminal B cell differentiation and does not necessarily result in autoreactivity

Mice in which the Jkappa cluster was replaced with a VkappaJkappa rearranged gene were studied. More than 90% of B cells from homozygous mutant mice expressed the transgenic kappa chain but showed a slightly reduced level of kappa transcripts compared with WT B lymphocytes. Light chain inclusion was apparent in 10% of B cells from these mice and raised 25% in hemizygous mice with a still lower expression of the knockin kappa chain. Beyond the rules of clonal selection, peripheral B cells developed in such animals, with included cells being activated and differentiating into class-switched or antibody-secreting cells. The high amount of included mature B cells was associated with an increase of hybrid kappa/lambda immunoglobulins but not with the increased prevalence of autoantibodies. Altogether, these data suggest that light chain exclusion prevalent in normal B cells mostly results from ordered rearrangements and stochastic mechanisms but is neither tightly ensured by a stringent cell selection process nor absolutely required for normal B cell function.

Production of human or humanized antibodies in mice

Mice are widely available laboratory animals that can easily be used for the production of antibodies against a broad range of antigens, using well-defined immunization protocols. Such an approach allows optimal in vivo affinity maturation of the humoral response. In addition, high-affinity antibodies arising in this context can readily be further characterized and produced as monoclonals after immortalizing and selecting specific antibody-producing cells through hybridoma derivation. Using such conventional strategies combined with mice that are either genetically engineered to carry humanized immunoglobulin (Ig) genes or engrafted with a human immune system, it is thus easy to obtain and immortalize clones that produce either fully human Ig or antibodies associating variable (V) domains with selected antigen specificities to customized human-like constant regions, with defined effector functions. In some instances, where there is a need for in vivo functional assays of a single antibody with a known specificity, it might be of interest to transiently express that gene in mice by in vivo gene transfer. This approach allows a rapid functional assay. More commonly, mice are used to obtain a diversified repertoire of antibody specificities after immunization by producing antibody molecules in the mouse B cell lineage from mouse strains with transgene Ig genes which are of human, humanized, or chimeric origin. After in vivo maturation of the immune response, this will lead to the secretion of antibodies with optimized antigen binding sites, associated to the desired human constant domains. This chapter focuses on two simple methods: (1) to obtain such humanized Ig mice and (2) to transiently express a human Ig gene in mice using hydrodynamics-based transfection.

Efficient AID targeting of switch regions is not sufficient for optimal class switch recombination

Antibody affinity maturation relies on activation-induced cytidine deaminase (AID)-dependent somatic hypermutation (SHM) of immunoglobulin (Ig) loci. Class switch recombination (CSR) can in parallel occur between AID-targeted, transcribed, spliced and repetitive switch (S) regions. AID thus initiates not only mutations but also double-strand breaks (DSBs). What governs the choice between those two outcomes remains uncertain. Here we explore whether insertion of transcribed intronic S regions in a locus (Igκ) strongly recruiting AID is sufficient for efficient CSR. Although strongly targeted by AID and carrying internal deletions, the knocked-in S regions only undergo rare CSR-like events. This model confirms S regions as exquisite SHM targets, extending AID activity far from transcription initiation sites, and shows that such spliced and repetitive AID targets are not sufficient by themselves for CSR. Beyond transcription and AID recruitment, additional IgH elements are thus needed for CSR, restricting this hazardous gene remodelling to IgH loci.

A plasma cell differentiation quality control ablates B cell clones with biallelic Ig rearrangements and truncated Ig production

Aberrantly rearranged immunoglobulin (Ig) alleles are frequent. They are usually considered sterile and innocuous as a result of nonsense-mediated mRNA decay. However, alternative splicing can yield internally deleted proteins from such nonproductively V(D)J-rearranged loci. We show that nonsense codons from variable (V) Igκ exons promote exon-skipping and synthesis of V domain-less κ light chains (ΔV-κLCs). Unexpectedly, such ΔV-κLCs inhibit plasma cell (PC) differentiation. Accordingly, in wild-type mice, rearrangements encoding ΔV-κLCs are rare in PCs, but frequent in B cells. Likewise, enforcing expression of ΔV-κLCs impaired PC differentiation and antibody responses without disturbing germinal center reactions. In addition, PCs expressing ΔV-κLCs synthesize low levels of Ig and are mostly found among short-lived plasmablasts. ΔV-κLCs have intrinsic toxic effects in PCs unrelated to Ig assembly, but mediated by ER stress-associated apoptosis, making PCs producing ΔV-κLCs highly sensitive to proteasome inhibitors. Altogether, these findings demonstrate a quality control checkpoint blunting terminal PC differentiation by eliminating those cells expressing nonfunctionally rearranged Igκ alleles. This truncated Ig exclusion (TIE) checkpoint ablates PC clones with ΔV-κLCs production and exacerbated ER stress response. The TIE checkpoint thus mediates selection of long-lived PCs with limited ER stress supporting high Ig secretion, but with a cost in terms of antigen-independent narrowing of the repertoire.

The characteristics of patients with kidney light chain deposition disease concurrent with light chain amyloidosis

The type of monoclonal light chain nephropathy is thought to be largely a function of the structural and physiochemical properties of light chains; hence most affected patients have only one light chain kidney disease type. Here, we report the first series of kidney light chain deposition disease (LCDD) concomitant with light chain amyloidosis (LCDD+AL), with or without light chain cast nephropathy (LCCN). Our LCDD+AL cohort consisted of 37 patients (54% females, median age 70 years (range 40-86)). All cases showed Congo red-positive amyloid deposits staining for one light chain isotype on immunofluorescence (62% lambda), and LCDD with diffuse linear staining of glomerular and tubular basement membranes for one light chain isotype (97% same isotype as the amyloidogenic light chain) and ultrastructural non-fibrillar punctate deposits. Twelve of 37 cases (about 1/3 of patients) had concomitant LCCN of same light chain isotype. Proteomic analysis of amyloid and/or LCDD deposits in eight revealed a single light chain variable domain mutable subgroup in all cases (including three with separate microdissections of LCDD and amyloid light chain deposits). Clinical data on 21 patients showed proteinuria (100%), hematuria (75%), kidney insufficiency and nephrotic syndrome (55%). Extra-kidney involvement was present in 43% of the patients. Multiple myeloma occurred in 68% (about 2/3) of these patients; none had lymphoma. On follow up (median 16 months), 63% developed kidney failure and 56% died. The median kidney and patient survivals were 12 and 32 months, respectively. LCDD+AL mainly affected patients 60 years of age or older. Thus, LCDD+AL could be caused by two pathological light chains produced by subclones stemming from one immunoglobulin light chain lambda or kappa rearrangement, with a distinct mutated complementary determining region.