The trajectory of human B-cell function, immune deficiency, and allergy revealed by inborn errors of immunity.

The essential role of B cells is to produce protective immunoglobulins (Ig) that recognize, neutralize, and clear invading pathogens. This results from the integration of signals provided by pathogens or vaccines and the stimulatory microenvironment within sites of immune activation, such as secondary lymphoid tissues, that drive mature B cells to differentiate into memory B cells and antibody (Ab)-secreting plasma cells. In this context, B cells undergo several molecular events including Ig class switching and somatic hypermutation that results in the production of high-affinity Ag-specific Abs of different classes, enabling effective pathogen neutralization and long-lived humoral immunity. However, perturbations to these key signaling pathways underpin immune dyscrasias including immune deficiency and autoimmunity or allergy. Inborn errors of immunity that disrupt critical immune pathways have identified non-redundant requirements for eliciting and maintaining humoral immune memory but concomitantly prevent immune dysregulation. Here, we will discuss our studies on human B cells, and how our investigation of cytokine signaling in B cells have identified fundamental requirements for memory B-cell formation, Ab production as well as regulating Ig class switching in the context of protective versus allergic immune responses.

The ups and downs of STAT3 function: too much, too little and human immune dysregulation.

The STAT3 story has almost 30 years of evolving history. First identified in 1994 as a pro-inflammatory transcription factor, Signal Transducer and Activator of Transcription 3 (STAT3) has continued to be revealed as a quintessential pleiotropic signalling module spanning fields including infectious diseases, autoimmunity, vaccine responses, metabolism, and malignancy. In 2007, germline heterozygous dominant-negative loss-of-function variants in STAT3 were discovered as the most common cause for a triad of eczematoid dermatitis with recurrent skin and pulmonary infections, first described in 1966. This finding established that STAT3 plays a critical non-redundant role in immunity against some pathogens, as well as in the connective tissue, dental and musculoskeletal systems. Several years later, in 2014, heterozygous activating gain of function germline STAT3 variants were found to be causal for cases of early-onset multiorgan autoimmunity, thereby underpinning the notion that STAT3 function needed to be regulated to maintain immune homeostasis. As we and others continue to interrogate biochemical and cellular perturbations due to inborn errors in STAT3, we will review our current understanding of STAT3 function, mechanisms of disease pathogenesis, and future directions in this dynamic field.

STAT5B restrains human B-cell differentiation to maintain humoral immune homeostasis.

BACKGROUND: Lymphocyte differentiation is regulated by coordinated actions of cytokines and signaling pathways. IL-21 activates STAT1, STAT3, and STAT5 and is fundamental for the differentiation of human B cells into memory cells and antibody-secreting cells. While STAT1 is largely nonessential and STAT3 is critical for this process, the role of STAT5 is unknown. OBJECTIVES: This study sought to delineate unique roles of STAT5 in activation and differentiation of human naive and memory B cells. METHODS: STAT activation was assessed by phospho-flow cytometry cell sorting. Differential gene expression was determined by RNA-sequencing and quantitative PCR. The requirement for STAT5B in B-cell and CD4+ T-cell differentiation was assessed using CRISPR-mediated STAT5B deletion from B-cell lines and investigating primary lymphocytes from individuals with germline STAT5B mutations. RESULTS: IL-21 activated STAT5 and strongly induced SOCS3 in human naive, but not memory, B cells. Deletion of STAT5B in B-cell lines diminished IL-21-mediated SOCS3 induction. PBMCs from STAT5B-null individuals contained expanded populations of immunoglobulin class-switched B cells, CD21loTbet+ B cells, and follicular T helper cells. IL-21 induced greater differentiation of STAT5B-deficient B cells into plasmablasts in vitro than B cells from healthy donors, correlating with higher expression levels of transcription factors promoting plasma cell formation. CONCLUSIONS: These findings reveal novel roles for STAT5B in regulating IL-21-induced human B-cell differentiation. This is achieved by inducing SOCS3 to attenuate IL-21 signaling, and BCL6 to repress class switching and plasma cell generation. Thus, STAT5B is critical for restraining IL-21-mediated B-cell differentiation. These findings provide insights into mechanisms underpinning B-cell responses during primary and subsequent antigen encounter and explain autoimmunity and dysfunctional humoral immunity in STAT5B deficiency.

Human STAT3 variants underlie autosomal dominant hyper-IgE syndrome by negative dominance.

Most patients with autosomal dominant hyper-IgE syndrome (AD-HIES) carry rare heterozygous STAT3 variants. Only six of the 135 in-frame variants reported have been experimentally shown to be dominant negative (DN), and it has been recently suggested that eight out-of-frame variants operate by haploinsufficiency. We experimentally tested these 143 variants, 7 novel out-of-frame variants found in HIES patients, and other STAT3 variants from the general population. Strikingly, all 15 out-of-frame variants were DN via their encoded (1) truncated proteins, (2) neoproteins generated from a translation reinitiation codon, and (3) isoforms from alternative transcripts or a combination thereof. Moreover, 128 of the 135 in-frame variants (95%) were also DN. The patients carrying the seven non-DN STAT3 in-frame variants have not been studied for other genetic etiologies. Finally, none of the variants from the general population tested, including an out-of-frame variant, were DN. Overall, our findings show that heterozygous STAT3 variants, whether in or out of frame, underlie AD-HIES through negative dominance rather than haploinsufficiency.