A rheostat sets B-cell receptor repertoire selection to distinguish self from non-self

Functions of IgM fc receptor (FcµR) related to autoimmunity

Unlike Fc receptors for switched immunoglobulin (Ig) isotypes, Fc receptor for IgM (FcµR) is selectively expressed by lymphocytes. The ablation of the FcµR gene in mice impairs B cell tolerance as evidenced by concomitant production of autoantibodies of IgM and IgG isotypes. In this essay, we reiterate the autoimmune phenotypes observed in mutant mice, ie IgM homeostasis, dysregulated humoral immune responses including autoantibodies, and Mott cell formation. We also propose the potential phenotypes in individuals with FCMR deficiency and the model for FcµR-mediated regulation of self-reactive B cells.

Interleukin 4-driven reversal of self-reactive B cell anergy contributes to the pathogenesis of systemic lupus erythematosus

OBJECTIVES

Reactivation of anergic autoreactive B cells (BND cells) is a key aetiological process in systemic lupus erythematosus (SLE), yet the underlying mechanism remains largely elusive. This study aimed to investigate how BND cells participate in the pathogenesis of SLE and the underlying mechanism.

METHODS

A combination of phenotypical, large-scale transcriptome and B cell receptor (BCR) repertoire profiling were employed at molecular and single cell level on samples from healthy donors and patients with SLE. Isolated naïve B cells from human periphery blood were treated with anti-CD79b mAb in vitro to induce anergy. IgM internalisation was tracked by confocal microscopy and was qualified by flow cytometer.

RESULTS

We characterised the decrease and disruption of BND cells in SLE patients and demonstrated IL-4 as an important cytokine to drive such pathological changes. We then elucidated that IL-4 reversed B cell anergy by promoting BCR recycling to the cell surface via STAT6 signalling.

CONCLUSIONS

We demonstrated the significance of IL-4 in reversing B cell anergy and established the scientific rationale to treat SLE via blocking IL-4 signalling, also providing diagnostic and prognostic biomarkers to identify patients who are most likely going to benefit from such treatments.

Physiological and Pathophysiological Roles of IgM Fc Receptor (FcµR) Isoforms

IgM is the first antibody to emerge during phylogeny, ontogeny, and immune responses and serves as a first line of defense. Effector proteins interacting with the Fc portion of IgM, such as complement and its receptors, have been extensively studied for their functions. IgM Fc receptor (FcµR), identified in 2009, is the newest member of the FcR family and is intriguingly expressed by lymphocytes only, suggesting the existence of distinct functions as compared to the FcRs for switched Ig isotypes, which are expressed by various immune and non-hematopoietic cells as central mediators of antibody-triggered responses by coupling the adaptive and innate immune responses. Results from FcµR-deficient mice suggest a regulatory function of FcµR in B cell tolerance, as evidenced by their propensity to produce autoantibodies of both IgM and IgG isotypes. In this article, we discuss conflicting views about the cellular distribution and potential functions of FcµR. The signaling function of the Ig-tail tyrosine-like motif in the FcµR cytoplasmic domain is now formally shown by substitutional experiments with the IgG2 B cell receptor. The potential adaptor protein associating with FcµR and the potential cleavage of its C-terminal cytoplasmic tail after IgM binding are still enigmatic. Critical amino acid residues in the Ig-like domain of FcµR for interacting with the IgM Cµ4 domain and the mode of interaction are now defined by crystallographic and cryo-electron microscopic analyses. Some discrepancies on these interactions are discussed. Finally, elevated levels of a soluble FcµR isoform in serum samples are described as the consequence of persistent B cell receptor stimulation, as seen in chronic lymphocytic leukemia and probably in antibody-mediated autoimmune disorders.

[B lymphocytes and plasma cells as drivers of rheumatic diseases]

Verschiedene Arbeitsgruppen am Deutschen Rheuma-Forschungszentrum Berlin haben in enger Zusammenarbeit mit der Medizinischen Klinik mit Schwerpunkt Rheumatologie und Klinische Immunologie an der Charité wichtige Beiträge zur Bedeutung der B‑Zellen und Plasmazellen bei rheumatischen Erkrankungen geleistet, die nicht nur für die Rheumatologie, sondern für alle klinischen Fachgebiete, in denen antikörpervermittelte Erkrankungen eine Rolle spielen, relevant sind. Insbesondere wird auf die gestörte B‑Zell-Homöostase, die Bedeutung des Immunglobulin M(IgM)-Fc-Rezeptors für die Regulation der Autoimmunität, die Rolle der langlebigen Gedächtnis-Plasmazelle bei der Aufrechterhaltung der Autoimmunität sowie die Sicherung ihres Überlebens in speziellen, von Stromazellen organisierten Nischen im Knochenmark und in entzündeten Geweben eingegangen. Die Forschungsergebnisse haben zu einem besseren Verständnis der immunologischen und molekularen Mechanismen bei rheumatischen Erkrankungen und ihrer Therapie beigetragen. Die Identifizierung der langlebigen Gedächtnis-Plasmazelle hat zu vielversprechenden therapeutischen Ansätzen mit kurativem Potenzial bei Autoimmunerkrankungen geführt.