IgD shapes the pre-immune naïve B cell compartment in humans.

B cell maturation and immunoglobulin (Ig) repertoire selection are governed by expression of a functional B cell receptor (BCR). Naïve B cells co-express their BCR as IgM and IgD isotype. However, the role of the additionally expressed IgD on naïve B cells is not known. Here we assessed the impact of IgD on naïve B cell maturation and Ig repertoire selection in 8 individuals from 3 different families with heterozygous loss-of-function or loss-of expression mutations in IGHD. Although naïve B cells from these individuals expressed IgM on their surface, the IGHD variant in heterozygous state entailed a chimeric situation by allelic exclusion with almost half of the naïve B cell population lacking surface IgD expression. Flow cytometric analyses revealed a distinct phenotype of IgD-negative naïve B cells with decreased expression of CD19, CD20 and CD21 as well as lower BAFF-R and integrin-ß7 expression. IgD-negative B cells were less responsive in vitro after engaging the IgM-BCR, TLR7/9 or CD40 pathway. Additionally, a selective disadvantage of IgD-negative B cells within the T2 transitional and mature naïve B cell compartment as well as reduced frequencies of IgMlo/- B cells within the mature naïve B cell compartment lacking IgD were evident. RNA-Ig-seq of bulk sorted B cell populations showed an altered selection of distinct VH segments in the IgD-negative mature naïve B cell population. We conclude that IgD expression on human naïve B cells is redundant for generation of naïve B cells in general, but further shapes the naive B cell compartment starting from T2 transitional B cells. Our observations suggest an unexpected role of IgD expression to be critical for selection of distinct Ig VH segments into the pre-immune Ig repertoire and for the survival of IgMlo/- naïve B cells known to be enriched in poly-/autoreactive B cell clones.

Further evidence for POMK as candidate gene for WWS with meningoencephalocele.

BACKGROUND: Walker-Warburg syndrome (WWS) is a rare form of alpha-dystroglycanopathy characterized by muscular dystrophy and severe malformations of the CNS and eyes. Bi-allelic pathogenic variants in POMK are the cause of a broad spectrum of alpha-dystroglycanopathies. POMK encodes protein-O-mannose kinase, which is required for proper glycosylation and function of the dystroglycan complex and is crucial for extracellular matrix composition. RESULTS: Here, we report on male monozygotic twins with severe CNS malformations (hydrocephalus, cortical malformation, hypoplastic cerebellum, and most prominently occipital meningocele), eye malformations and highly elevated creatine kinase, indicating the clinical diagnosis of a congenital muscular dystrophy (alpha-dystroglycanopathy). Both twins were found to harbor a homozygous nonsense mutation c.640C>T, p.214* in POMK, confirming the clinical diagnosis and supporting the concept that POMK mutations can be causative of WWS. CONCLUSION: Our combined data suggest a more important role for POMK in the pathogenesis of meningoencephalocele. Only eight different pathogenic POMK variants have been published so far, detected in eight families; only five showed the severe WWS phenotype, suggesting that POMK-associated WWS is an extremely rare disease. We expand the phenotypic and mutational spectrum of POMK-associated WWS and provide evidence of the broad phenotypic variability of POMK-associated disease.