Pten controls B-cell responsiveness and germinal center reaction by regulating the expression of IgD BCR

In contrast to other B‐cell antigen receptor (BCR) classes, the function of IgD BCR on mature B cells remains largely elusive as mature B cells co‐express IgM, which is sufficient for development, survival, and activation of B cells. Here, we show that IgD expression is regulated by the forkhead box transcription factor FoxO1, thereby shifting the responsiveness of mature B cells towards recognition of multivalent antigen. FoxO1 is repressed by phosphoinositide 3‐kinase (PI3K) signaling and requires the lipid phosphatase Pten for its activation. Consequently, Pten‐deficient B cells expressing knock‐ins for BCR heavy and light chain genes are unable to upregulate IgD. Furthermore, in the presence of autoantigen, Pten‐deficient B cells cannot eliminate the autoreactive BCR specificity by secondary light chain gene recombination. Instead, Pten‐deficient B cells downregulate BCR expression and become unresponsive to further BCR‐mediated stimulation. Notably, we observed a delayed germinal center (GC) reaction by IgD‐deficient B cells after immunization with trinitrophenyl‐ovalbumin (TNP‐Ova), a commonly used antigen for T‐cell‐dependent antibody responses. Together, our data suggest that the activation of IgD expression by Pten/FoxO1 results in mature B cells that are selectively responsive to multivalent antigen and are capable of initiating rapid GC reactions and T‐cell‐dependent antibody responses.

Minimal residual disease assessment in B-cell precursor acute lymphoblastic leukemia by semi-automated identification of normal hematopoietic cells: A EuroFlow study

Presence of minimal residual disease (MRD), detected by flow cytometry, is an important prognostic biomarker in the management of B-cell precursor acute lymphoblastic leukemia (BCP-ALL). However, data-analysis remains mainly expert-dependent. In this study, we designed and validated an Automated Gating & Identification (AGI) tool for MRD analysis in BCP-ALL patients using the two tubes of the EuroFlow 8-color MRD panel. The accuracy, repeatability, and reproducibility of the AGI tool was validated in a multicenter study using bone marrow follow-up samples from 174 BCP-ALL patients, stained with the EuroFlow BCP-ALL MRD panel. In these patients, MRD was assessed both by manual analysis and by AGI tool supported analysis. Comparison of MRD levels obtained between both approaches showed a concordance rate of 83%, with comparable concordances between MRD tubes (tube 1, 2 or both), treatment received (chemotherapy versus targeted therapy) and flow cytometers (FACSCanto versus FACSLyric). After review of discordant cases by additional experts, the concordance increased to 97%. Furthermore, the AGI tool showed excellent intra-expert concordance (100%) and good inter-expert concordance (90%). In addition to MRD levels, also percentages of normal cell populations showed excellent concordance between manual and AGI tool analysis. We conclude that the AGI tool may facilitate MRD analysis using the EuroFlow BCP-ALL MRD protocol and will contribute to a more standardized and objective MRD assessment. However, appropriate training is required for the correct analysis of MRD data.

The significance of PAX5 in Merkel cell carcinoma

Merkel cell carcinoma (MCC) is a highly malignant skin cancer that expresses epithelial-, neuroendocrine-, and lymphoid-associated genes. Here, we focused on B-cell differentiation, which is characterised by the coexpression of PAX5 and TdT. PAX5 is the master regulator of B-cell commitment and is expressed in 65% of MCC cases. Yet little is known about the underlying molecular biology of the frequently reported PAX5 expression in MCC. Multi-omics analyses, including RNA next-generation sequencing, RT-qPCR, immunohistochemistry, and western blotting, were performed to assess PAX5 expression in MCC. Differential DNA methylation analysis at 61,043 PAX5 binding sites in enhancer and promoter elements was performed to detect differences between n = 14 MCC tissues and n = 91 various normal B-cell populations. RNA analysis revealed full-length PAX5 expression in MCC at the transcriptional level using both PAX5 transcription start sites. PAX5 protein expression was found in 40 of 41 MCCs and six out of seven MCC cell lines. DNA methylation array analysis revealed 1,084 hypermethylated loci of enhancer and promoter elements located in PAX5 binding sites in MCC. Of these, 702 loci were associated with 257 genes that are not expressed. The PAX5-associated regulatory elements of these 257 genes were enriched for interferon regulatory factor 4 (IRF4) and SPi-proto-oncogene (SPI1) binding motifs. Neither IRF4 or SPI1 could be detected in MCC on RNA or the protein level. Thus, because of the absence of these transcription factors, we conclude that full-length PAX5 alone cannot induce B-cell differentiation. © 2025 The Author(s). The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.