Hypomorphic Mutations in the BCR Signalosome Lead to Selective Immunoglobulin M Deficiency and Impaired B-cell Homeostasis

Abstract

B cell activation via the B cell receptor (BCR) signalosome involves participation of signaling molecules such as BTK and BLNK. Genetic defects in these molecules are known to impair B cell differentiation and subsequently lead to agammaglobulinemia. Here we identified novel mutations in BTK and BLNK in two unrelated patients that perturb the intrinsic B-cell receptor signaling pathway and lead to selective IgM deficiency, whereas production of other immunoglobulin isotypes and IgG antibody response remain intact. Currently it is unknown how BCR signaling strength affects mature B cell development in humans. Both patients show reduced levels of BCR signalosome phosphorylation as well as impaired BCR-dependent Ca2+ influx, which was accompanied by a marked decrease in IgD+IgM+CD27+ MZ-like B-cells. We further describe reduced expression of essential B cell differentiation factors such as BAFF-R and T-Bet in the patients' B-cells, which might contribute to the observed deficiency of MZ-like B cells. MZ-like B cells are known to produce natural IgM antibodies that play an essential role in immune homeostasis. By using surface plasmon resonance (SPR) technology and a synthetic blood group A trisaccharide as antigen we were able to show that both patients lack the presence of anti-blood group A IgM considered to be prototypical natural antibodies whereas IgG levels were normal. Antibody binding dynamics and binding affinity of anti-blood group A IgG were comparable between patients and healthy controls. These results indicate that human IgM deficiency can be associated with signaling defects in the BCR signalosome, defective production of natural IgM antibodies in the blood group A/B/0 system and abnormalities in B cell development.

Complement Factor H Modulates Splenic B Cell Development and Limits Autoantibody Production

“…Therefore, to study the influence of CFH deficiency on BCR signaling, we chose to quantify phosphorylated Syk and Btk levels of splenic B cell subsets in Cf h +/+ and Cf h −/− mice using a flow cytometry-based approach. First, we confirmed previous findings by our lab and others (24, 28, 29) that strong BCR signaling promotes MZ over FO B cell differentiation, as MZ B cells showed higher phosphorylated Btk levels compared to FO/T2 B cells of Cf h +/+ mice (FO/T2 B cells, MFI = 2206 ± 113; MZ B cells MFI = 4783 ± 310, p < 0.0001). Moreover, we found that both MZ and FO/T2 B cells display elevated levels of phosphorylated Btk and Syk in Cf h −/− mice compared to control mice (Figures 2A,B), which could provide an explanation for the accumulation of MZ B cells as a result of CFH deficiency.…”

“…Initially, BCR signaling strength was thought to be directly associated with FO B cell differentiation (39, 40). We and others have provided direct evidence that strong BCR signaling favors MZ B cell over FO cell development (24, 28, 29). Mice deficient in secreted IgM displayed increased MZ B cell and decreased FO B cell numbers and low-dose treatment of sIg M −/− mice with the Btk inhibitor Ibrutinib promoted differentiation of FO B cells while restricting MZ B cell formation.…”

A Novel BLNK Gene Mutation in a Four-Year-Old Child Who Presented with Late Onset of Severe Infections and High IgM Levels and Diagnosed and Followed as X-Linked Agammaglobulinemia for Two Years

“…Five of them had homozygous, one patient had heterozygous, and two patients had compound heterozygous mutations. As far as we know, our patient is the 9th patient with BLNK mutation reported in the literature and is the first patient reported from Turkey [5,[7][8][9][10][11] (Table 2). Two of the previously reported cases were Chinese, two were Arab, and two were Turkish.…”

Selective immunoglobulin M deficiency in a patient with celiac disease and recurrent pneumonia

“…2 Although the etiology of SIgMD is not well identified yet, several genetic and environmental factors have been proposed to its pathogenesis. [5][6][7] Deletion in 22q11.2 chromosome is largely related to SIgMD, especially in subjects suffering from chronic otitis, developmental delay, velopharyngeal insufficiency, and dysmorphic features. 8 This disorder is mainly divided into primary and secondary SIgMD.…”