Overactive STAT3 drives accumulation of disease-associated CD21low B cells
Masle-Farquhar et al. (BioRxiv) doi: 10.1101/2021.12.20.473595
Age-associated B cells
Signal Transducer and Activator of Transcription (STAT) 3 is a cytosolic protein expressed ubiquitously and acting as a transcription factor following phosphorylation by Janus Kinases downstream of various cytokine and growth factor receptors (Ref 1). Stat3 gene polymorphism has been associated with autoimmune and inflammatory diseases. In addition, in many B cell lymphoma STAT3 is activated constitutively. Despite the recent clinical development of treatments aiming at STAT3 inhibition (Ref 2), the B cell intrinsic consequences of STAT3 activation remain elusive.
In this preprint (Ref 3), the authors showed that human suffering from STAT3 gain of function syndrome accumulate atypical CD21low B cells, similar to cells previously associated with chronic and autoimmune diseases. Similar results were found in mice carrying gain of function mutations in Stat3. To decipher the intrinsic B cell effect, authors used mice chimeras with a mixture of STAT3 mutant and wild type bone marrow transplant into irradiated RAG-1 deficient mice. CD21lowCD23low B cells also accumulated in these animals, in blood, lymph nodes and spleen, with similar clonal diversity and CDR3 length as compared to follicular B cells. The use of transgenic mice expressing HEL allowed the authors to show that auto-reactive B cells (e.i. HEL-specific B cells) are not depleted when expressing STAT3 gain of function mutant and accumulate with a CD21lowCD23low phenotype. Importantly, single cell transcriptomic analyses of spleen CD21lowCD23low B cells showed that these cells shared gene expression with previously published data on aged-associated B cells (Ref 4). The STAT3 gain of function mutant B cells also displayed altered expression of genes shown by CHIPseq to bind phosphorylated STAT3 following IgM stimulation of human B cells or B lymphoma. Transcriptomic analyses showed that STAT3 mutant CD21lowCD23low B cells have higher expression of genes involved in BCR signalling pathway when compared to B cells expressing wild type STAT3. Finally, the authors could not observe any difference in calcium response between STAT3 wild type or mutant B cells. However, the activation of follicular B cells from STAT3 gain of function mutant with anti-IgM and anti-CD40 induce higher expression of CD86 and Tbet as compared to wild type B cells, in line with high Tbet expression of B cells during autoimmunity.
These results showed that STAT3 gain of function mutant led to the accumulation of CD21low B cells and prevented the depletion of autoreactive B cells with a CD21low phenotype. These CD21low cells shared transcriptomic gene signature of aged-associated B cells, with the expression of genes involved BCR signaling pathway. Thus, STAT3 sustained activation could explain the accumulation of CD21low B cells during autoimmune or chronic inflammatory diseases.
While the results presented are extensive, the transcriptomic analyses were performed only in mice. Data from CD21low B cells in human both in aging and during autoimmunity would be important, especially as they might display disease-specific features. (Ref 5)
The authors used RAG-1 deficient mice that, in addition to be depleted of B cells, are also devoid of T cells. Thus, it cannot be excluded that the results observed arise indirectly from the effect of STAT3 mutation on T cells. The use of mice models specifically deficient in only B cells, such as the µMT mice model, may be important to confirm the authors observations.
The results on accumulation of autoreactive B cells are very valuable. It would also be interesting to understand the impact of STAT3 gain of function in an autoimmune mice model.
B cell activation is the hallmark of various autoimmune and cancer disease. The authors showed that STAT3 constitutive activation led to the accumulation of CD21low B cells, also known to be increased in chronic inflammation and during autoimmunity. Those results confirm a key role of STAT3 in driving disease-associated B cells.
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Masle-Farquhar, E. et al. Overactive STAT3 drives accumulation of disease-associated CD21low B cells. bioRxiv2021.12.20.473595 (2021) doi:10.1101/2021.12.20.473595.
Russell Knode, L. M. et al. Age-Associated B Cells Express a Diverse Repertoire of V H and Vκ Genes with Somatic Hypermutation . J. Immunol. 198, 1921–1927 (2017).
Freudenhammer, M., Voll, R. E., Binder, S. C., Keller, B. & Warnatz, K. Naive- and Memory-like CD21 low B Cell Subsets Share Core Phenotypic and Signaling Characteristics in Systemic Autoimmune Disorders . J. Immunol. 205, 2016–2025 (2020).
Reviewed by Nicolas Ruffin as part of the cross-institutional journal club of the Immunology Institute of the Icahn School of Medicine, Mount Sinai, the Kennedy Institute of Rheumatology and the Oxford Centre for Immuno-Oncology (OXCIO) (University of Oxford, GB) and Karolinska Institute’s Center for Infectious Medicine (CIM) & Center for Molecular Medicine (CMM). Follow him on Twitter.