Elevated unphosphorylated STAT1 and IRF9 in T and B cells of primary sjögren's syndrome: Novel biomarkers for disease activity and subsets.

OBJECTIVES: Autoreactive B cells and interferon (IFN) signature are hallmarks of primary sjögren's syndrome (pSS), but how IFN signaling pathways influence autoantibody production and clinical manifestations remain unclear. More detailed studies hold promise for improved diagnostic methodologies and personalized treatment. METHODS: We analyzed peripheral blood T and B cell subsets from 34 pSS patients and 38 healthy donors (HDs) at baseline and upon stimulation regarding their expression levels of type I and II IFN signaling molecules (STAT1/2, IRF1, IRF9). Additionally, we investigated how the levels of these molecules correlated with serological and clinical characteristics and performed ROC analysis. RESULTS: Patients showed elevated IFN pathway molecules, including STAT1, STAT2 and IRF9 among most T and B cell subsets. We found a reduced ratio of phosphorylated STAT1 and STAT2 in patients in comparison to HDs, although B cells from patients were highly responsive by increased phosphorylation upon IFN stimulation. Correlation matrices showed further interrelations between STAT1, IRF1 and IRF9 in pSS. Levels of STAT1 and IRF9 in T and B cells correlated with the IFN type I marker Siglec-1 (CD169) on monocytes. High levels of STAT1 and IRF9 within pSS B cells were significantly associated with hypergammaglobulinemia as well as anti-SSA/anti-SSB autoantibodies. Elevated STAT1 levels were found in patients with extraglandular disease and could serve as a biomarker for this subgroup (p < 0.01). Notably, IRF9 levels in T and B cells correlated with EULAR Sjögren's syndrome disease activity index (ESSDAI). CONCLUSION: Here, we provide evidence that in active pSS patients, enhanced IFN signaling incl. unphosphorylated STAT1 and STAT2 with IRFs entertain chronic T and B cell activation. Furthermore, increased STAT1 levels candidate as biomarker of extraglandular disease, while IRF9 levels can serve as biomarker for disease activity.

B and T Cell Responses after a Third Dose of SARS-CoV-2 Vaccine in Kidney Transplant Recipients.

BACKGROUND: Accumulating evidence sugges ts solid organ transplant recipients, as opposed to the general population, show strongly impaired responsiveness toward standard SARS-CoV-2 mRNA-based vaccination, demanding alternative strategies for protectio n o f this vulnerable group. METHODS: In line with recent recommendations, a third dose of either heterologous ChAdOx1 (AstraZeneca) or homologous BNT162b2 (BioNTech) was administered to 25 kidney transplant recipients (KTR) without humoral response after two doses of BNT162b2, followed by analysis of serological responses and vaccine-specific B- and T-cell immunity. RESULTS: Nine out of 25 (36%) KTR under standard immunosuppressive treatment seroconverted until day 27 after the third vaccination, whereas one patient developed severe COVID-19 infection immediately after vaccination. Cellular analysis 7 days after the third dose showed significantly elevated frequencies of viral spike-protein receptor-binding domain-specific B cells in humor al responders as compared with nonresponders. Likewise, portions of spike-reactive CD4 + T helper cells were significantly elevated in patients who were seroconverting. Furthermore, overall frequencies of IL-2 + , IL-4 + , and polyfunctional CD4 + T cells significantly increased after the third dose, whereas memory/effector differentiation remained unaffected. CONCLUSIONS: Our data suggest a fraction of transplant recipients benefit from triple vaccination, where seroconversion is associated with quantitative and qualitative changes of cellular immunity. At the same time, the study highlights that modified vaccination approaches for immunosuppressed patients remain an urgent medical need. PODCAST: This article contains a podcast at https://www.asn-online.org/media/podcast/JASN/2021_11_23_briggsgriffin112321.mp3.

Immune checkpoints and the multiple faces of B cells in systemic lupus erythematosus.

PURPOSE OF REVIEW: B-lymphocytes are crucial in the pathogenesis of systemic lupus erythematosus (SLE), including autoantibody production, antigen presentation, co-stimulation, and cytokine secretion. Co-stimulatory and co-inhibitory molecules control interactions between B and T cells during an inflammatory response, which is essential for an appropriate host protection and maintenance of self-tolerance. Here, we review recent findings about checkpoint molecules and SLE B cells including their potential therapeutic implications and experiences from clinical trials. RECENT FINDINGS: Most prominent checkpoint molecules involved in pathologic B and T cell interaction in SLE are CD40/CD40L and inducible co-stimulator/ICOSL, both also intimately involved in the formation of germinal centers and ectopic lymphoid tissue. Dysregulations of inhibitory checkpoint molecules, like programmed death-1/programmed death-ligand 1 and B- and T-lymphocyte attenuator have been suggested to impair B cell functions in SLE recently. SUMMARY: Accumulating evidence indicates that dampening immune responses by either blocking co-activating signals or enhancing co-inhibitory signals in different cell types is a promising approach to treat autoimmune diseases to better control active disease but may also allow resolution of chronic autoimmunity.

Postactivated B cells in systemic lupus erythematosus: update on translational aspects and therapeutic considerations.

PURPOSE OF REVIEW: This review summarizes recent insights and current understanding of the role of postactivated B cells in SLE and related pathogenic and potential therapeutic implications. RECENT FINDING: B cells are considered key players in SLE and experience from various B-cell-targeted therapies underlines their clinical relevance. In the last years, new insights have been obtained on B-cell abnormalities within the complex pathophysiology of SLE. These insights involve a revised understanding of BCR signaling, that has been reported to be hyperresponsive in the past, but newer studies suggest a postactivation functiotype in terms of reduced BCR and TLR signaling. Despite comprehensive efforts to delineate B-cell abnormalities on assessing large-scale genomic, epigenomic and proteomic data, understanding functional impairments of cellular interactions and subcellular functions remains crucial. A recently identified enhanced protein tyrosine phosphatase (PTP) activity was found in relation to diminished BCR responses in SLE. This finding together with reduced cytokine production upon TLR9 activation appears to mark postactivated lupus B cells. Other studies identified increased PTP activity in line with a gain-of-function mutation of phosphatase PTPN22, one of the strongest SLE risk alleles. Improved understanding of these B cell abnormalities in SLE holds promise to gain further insights in mechanisms of autoimmunity and pave the way for selective therapies targeting key principles of chronic autoimmunity. SUMMARY: SLE B cells (similar as previously described for lupus T cells) are characterized by a postactivation (exhausted) functiotype mandating consideration for innovative therapies.

Recruitment of plasma cells from IL-21-dependent and IL-21-independent immune reactions to the bone marrow.

Bone marrow plasma cells (BMPC) are the correlate of humoral immunity, consistently releasing antibodies into the bloodstream. It remains unclear if BMPC reflect different activation environments or maturation of their precursors. Here we define human BMPC heterogeneity and track the recruitment of antibody-secreting cells (ASC) from SARS-CoV-2 vaccine immune reactions to the bone marrow (BM). Trajectories based on single-cell transcriptomes and repertoires of peripheral and BM ASC reveal sequential colonisation of BMPC compartments. In activated B cells, IL-21 suppresses CD19 expression, indicating that CD19low-BMPC are derived from follicular, while CD19high-BMPC originate from extrafollicular immune reactions. In primary immune reactions, both CD19low- and CD19high-BMPC compartments are populated. In secondary immune reactions, most BMPC are recruited to CD19high-BMPC compartments, reflecting their origin from extrafollicular reactivations of memory B cells. A pattern also observable in vaccinated-convalescent individuals and upon diphtheria/tetanus/pertussis recall-vaccination. Thus, BMPC diversity reflects the evolution of a given humoral immune response.

Translational implications of newly characterized pathogenic pathways in systemic lupus erythematosus.

Improved characterization of relevant pathogenic pathways in systemic lupus erythematosus (SLE) has been further delineated over the last decades. This led to the development of targeted treatments including belimumab and anifrolumab, which recently became available in clinics. Therapeutic targets in SLE encompass interferon (IFN) signaling, B-T costimulation including immune checkpoints, and increasing modalities of B lineage targeting, such as chimeric antigen receptor (CAR) T cells directed against CD19 or sequential anti-B cell targeting. Patient profiling based on characterization of underlying molecular abnormalities, often performed through comprehensive omics analyses, has recently been shown to better predict patients' treatment responses and also holds promise to unravel key molecular mechanisms driving SLE. SLE carries two key signatures, namely the IFN and B lineage/plasma cell signatures. Recent advances in SLE treatments clearly indicate that targeting innate and adaptive immunity is successful in such a complex autoimmune disease. Although those signatures may interact at the molecular level and provide the basis for the first selective treatments in SLE, it remains to be clarified whether these distinct treatments show different treatment responses among certain patient subsets. In fact, notwithstanding the remarkable amount of novel clues for innovative SLE treatment, harmonization of big data within tailored treatment strategies will be instrumental to better understand and treat this challenging autoimmune disorder. This review will provide an overview of recent improvements in SLE pathogenesis, related insights by analyses of big data and machine learning as well as technical improvements in conducting clinical trials with the ultimate goal that translational research results in improved patient outcomes.

Transcriptional profiling upon T cell stimulation reveals down-regulation of inflammatory pathways in T and B cells in SLE versus Sjögren's syndrome.

Systemic lupus erythematosus (SLE) and primary Sjögren's syndrome (pSS) share clinical as well as pathogenic similarities. Although previous studies suggest various abnormalities in different immune cell compartments, dedicated cell-type specific transcriptomic signatures are often masked by patient heterogeneity. Here, we performed transcriptional profiling of isolated CD4, CD8, CD16 and CD19 lymphocytes from pSS and SLE patients upon T cell stimulation, in addition to a steady-state condition directly after blood drawing, in total comprising 581 sequencing samples. T cell stimulation, which induced a pronounced inflammatory response in all four cell types, gave rise to substantial re-modulation of lymphocyte subsets in the two autoimmune diseases compared to healthy controls, far exceeding the transcriptomic differences detected at steady-state. In particular, we detected cell-type and disease-specific down-regulation of a range of pro-inflammatory cytokine and chemokine pathways. Such differences between SLE and pSS patients are instrumental for selective immune targeting by future therapies.

Current and future treatment in primary Sjögren's syndrome - A still challenging development.

Primary Sjögren's syndrome (pSS) is a chronic autoimmune disease characterized by sicca symptoms, systemic manifestations and constitutional symptoms substantially diminishing patient's quality of life. In this review, we summarize recent recommendations for management of pSS patients and current clinical studies in pSS addressing unmet medical needs. Expanding knowledge about disease pathogenesis and the introduction of validated outcome measures, such as capturing disease activity (ESSDAI) and patient-reported outcomes (ESSPRI) have shaped recent developments. In contrast, lack of evidence for current treatment options remarkably limits the management of pSS patients as reflected by the 2019 updated EULAR recommendations for management of Sjögren's syndrome. In this context, symptomatic treatment is usually appropriate for sicca symptoms, whereas systemic treatment is reserved for moderate to severe organ manifestations including care by a multidisciplinary team in centers of expertise. Most promising targets for new treatment modalities are based on immunopathological insights and include direct B cell targeting strategies, targeting co-stimulation by CD40/CD40L blocking, inhibition of key cytokine activity (BLyS/BAFF, type I interferon) and intracellular signaling pathways.

B- and Plasma Cell Subsets in Autoimmune Diseases: Translational Perspectives.

B lymphocytes play a central role in immunity owing to their unique antibody-producing capacity that provides protection against certain infections and during vaccination. In autoimmune diseases, B cells can gain pathogenic relevance through autoantibody production, antigen presentation, and proinflammatory cytokine secretion. Recent data indicate that B and plasma cells can function as regulators through the production of immunoregulatory cytokines and/or employing checkpoint molecules. In this study, we review the key findings that define subsets of B and plasma cells with pathogenic and protective functions in autoimmunity. In addition to harsh B-cell depletion, we discuss the strategies that have the potential to reinstall the balance of pathogenic and protective B cells with the potential of more specific and personalized therapies.

CD39 and CD326 Are Bona Fide Markers of Murine and Human Plasma Cells and Identify a Bone Marrow Specific Plasma Cell Subpopulation in Lupus.

Antibody-secreting cells (ASCs) contribute to immunity through production of antibodies and cytokines. Identification of specific markers of ASC would allow selective targeting of these cells in several disease contexts. Here, we performed an unbiased, large-scale protein screening, and identified twelve new molecules that are specifically expressed by murine ASCs. Expression of these markers, particularly CD39, CD81, CD130, and CD326, is stable and offers an improved resolution for ASC identification. We accessed their expression in germ-free conditions and in T cell deficient mice, showing that at least in part their expression is controlled by microbial- and T cell-derived signals. Further analysis of lupus mice revealed the presence of a subpopulation of LAG-3- plasma cells, co-expressing high amounts of CD39 and CD326 in the bone marrow. This population was IgM+ and correlated with IgM anti-dsDNA autoantibodies in sera. Importantly, we found that CD39, CD81, CD130, and CD326 are also expressed by human peripheral blood and bone marrow ASCs. Our data provide innovative insights into ASC biology and function in mice and human, and identify an intriguing BM specific CD39++CD326++ ASC subpopulation in autoimmunity.

B Cell Characteristics at Baseline Predict Vaccination Response in RTX Treated Patients.

Background: Vaccination is considered as most efficient strategy in controlling SARS-CoV-2 pandemic spread. Nevertheless, patients with autoimmune inflammatory rheumatic diseases receiving rituximab (RTX) are at increased risk to fail humoral and cellular responses upon vaccination. The ability to predict vaccination responses is essential to guide adequate safety and optimal protection in these patients. Methods: B- and T- cell data before vaccination were evaluated for characteristics predicting vaccine responses in altogether 15 patients with autoimmune inflammatory rheumatic diseases receiving RTX. Eleven patients with rheumatoid arthritis (RA) on other therapies, 11 kidney transplant recipients (KTR) on regular immunosuppression and 15 healthy controls (HC) served as controls. A multidimensional analysis of B cell subsets via UMAP algorithm and a correlation matrix were performed in order to identify predictive markers of response in patients under RTX therapy. Results: Significant differences regarding absolute B cell counts and specific subset distribution pattern between the groups were identified at baseline. In this context, the majority of B cells from vaccination responders of the RTX group (RTX IgG+) were naïve and transitional B cells, whereas vaccination non-responders (RTX IgG-) carried preferentially plasmablasts and double negative (CD27-IgD-) B cells. Moreover, there was a positive correlation between neutralizing antibodies and B cells expressing HLA-DR and CXCR5 as well as an inverse correlation with CD95 expression and CD21low expression by B cells among vaccination responders. Summary: Substantial repopulation of the naïve B cell compartment after RTX therapy appeared to be essential for an adequate vaccination response, which seem to require the additional capability of antigen presentation and germinal center formation. Moreover, expression of exhaustion markers represent negative predictors of vaccination responses.

Plasmablast-like Phenotype Among Antigen-Experienced CXCR5-CD19low B Cells in Systemic Lupus Erythematosus.

OBJECTIVE: Altered composition of the B cell compartment in the pathogenesis of systemic lupus erythematosus (SLE) is characterized by expanded plasmablast and IgD-CD27- double-negative B cell populations. Previous studies showed that double-negative B cells represent a heterogeneous subset, and further characterization is needed. METHODS: We analyzed 2 independent cohorts of healthy donors and SLE patients, using a combined approach of flow cytometry (for 16 healthy donors and 28 SLE patients) and mass cytometry (for 18 healthy donors and 24 SLE patients) and targeted RNA-Seq analysis. To compare B cell subset formation during the acute immune response versus that during autoimmune disease, we investigated healthy donors at various time points after receipt of the BNT162b2 messenger RNA COVID-19 vaccine and patients with acute SARS-CoV-2 infection, using flow cytometry. RESULTS: We found that IgD-CD27+ switched and atypical IgD-CD27- memory B cells, the levels of which were increased in SLE patients, represented heterogeneous populations composed of 3 different subsets each. CXCR5+CD19intermediate , CXCR5-CD19high , and CXCR5-CD19low populations were found in the switched memory and double-negative compartments, suggesting the relatedness of IgD-CD27+ and IgD-CD27- B cells. We characterized a hitherto unknown and antigen-experienced CXCR5-CD19low subset that was enhanced in SLE patients, had a plasmablast phenotype with diminished B cell receptor responsiveness, and expressed CD38, CD95, CD71, PRDM1, XBP1, and IRF4. Levels of CXCR5-CD19low subsets were increased and correlated with plasmablast frequencies in SLE patients and in healthy donors who received BNT162b2, suggesting their interrelationship and contribution to plasmacytosis. The detection of CXCR5-CD19low B cells among both CD27+ and CD27- populations calls into question the role of CD27 as a reliable marker of B cell differentiation. CONCLUSION: Our data suggest that CXCR5-CD19low B cells are precursors of plasmablasts. Thus, cotargeting this subset may have therapeutic value in SLE.

Three-Month Follow-Up of Heterologous vs. Homologous Third SARS-CoV-2 Vaccination in Kidney Transplant Recipients: Secondary Analysis of a Randomized Controlled Trial.

Response to SARS-CoV-2-vaccines in kidney-transplant recipients (KTR) is severely reduced. Heterologous3rd vaccination combining mRNA and vector vaccines did not increase seroconversion at 4 weeks after vaccination, but evolution of antibody levels beyond the first month remains unknown. We have recently completed a randomized-controlled trial on heterologous (Ad26COVS1) vs. homologous (BNT162b2 or mRNA-1273) 3rd vaccination in 201 KTR not developing SARS-CoV-2-spike-protein antibodies following two doses of mRNA vaccine (EurdraCT: 2021-002927-39). Here, we report seroconversion at the second follow-up at 3 months after the 3rd vaccination (prespecified secondary endpoint). In addition, higher cut-off levels associated with neutralizing capacity and protective immunity were applied (i.e., > 15, > 100, > 141, and > 264 BAU/ml). A total of 169 patients were available for the 3-month follow-up. Overall, seroconversion at 3 months was similar between both groups (45 vs. 50% for mRNA and the vector group, respectively; p = 0.539). However, when applying higher cut-off levels, a significantly larger number of individuals in the vector group reached antibody levels > 141 and > 264 BAU/ml at the 3-month follow-up (141 BAU/ml: 4 vs. 15%, p = 0.009 and 264 BAU/ml: 1 vs. 10%, p = 0.018 for mRNA vs. the vector vaccine group, respectively). In line, antibody levels in seroconverted patients further increased from month 1 to month 3 in the vector group while remaining unchanged in the mRNA group (median increase: mRNA = 1.35 U/ml and vector = 27.6 U/ml, p = 0.004). Despite a similar overall seroconversion rate at 3 months following 3rd vaccination in KTR, a heterologous 3rd booster vaccination with Ad26COVS1 resulted in significantly higher antibody levels in responders.

Persistent but atypical germinal center reaction among 3rd SARS-CoV-2 vaccination after rituximab exposure.

Background: Durable vaccine-mediated immunity relies on the generation of long-lived plasma cells and memory B cells (MBCs), differentiating upon germinal center (GC) reactions. SARS-CoV-2 mRNA vaccination induces a strong GC response in healthy volunteers (HC), but limited data is available about response longevity upon rituximab treatment. Methods: We evaluated humoral and cellular responses upon 3rd vaccination in seven patients with rheumatoid arthritis (RA) who initially mounted anti-spike SARS-CoV-2 IgG antibodies after primary 2x vaccination and got re-exposed to rituximab (RTX) 1-2 months after the second vaccination. Ten patients with RA on other therapies and ten HC represented the control groups. As control for known long-lived induced immunity, we analyzed humoral and cellular tetanus toxoid (TT) immune responses in steady-state. Results: After 3rd vaccination, 5/7 seroconverted RTX patients revealed lower anti-SARS-CoV-2 IgG levels but similar neutralizing capacity compared with HC. Antibody levels after 3rd vaccination correlated with values after 2nd vaccination. Despite significant reduction of circulating total and antigen-specific B cells in RTX re-exposed patients, we observed the induction of IgG+ MBCs upon 3rd vaccination. Notably, only RTX treated patients revealed a high amount of IgA+ MBCs before and IgA+ plasmablasts after 3rd vaccination. IgA+ B cells were not part of the steady state TT+ B cell pool. TNF-secretion and generation of effector memory CD4 spike-specific T cells were significantly boosted upon 3rd vaccination. Summary: On the basis of pre-existing affinity matured MBCs within primary immunisation, RTX re-exposed patients revealed a persistent but atypical GC immune response accompanied by boosted spike-specific memory CD4 T cells upon SARS-CoV-2 recall vaccination.

Corrigendum: Three-Month follow-up of heterologous vs. homologous third SARS-CoV-2 vaccination in kidney transplant recipients: Secondary analysis of a randomized controlled trial.

[This corrects the article DOI: 10.3389/fmed.2022.936126.].

B Cell Numbers Predict Humoral and Cellular Response Upon SARS-CoV-2 Vaccination Among Patients Treated With Rituximab.

OBJECTIVE: Patients with autoimmune inflammatory rheumatic diseases receiving rituximab (RTX) therapy are at higher risk of poor COVID-19 outcomes and show substantially impaired humoral immune response to anti-SARS-CoV-2 vaccine. However, the complex relationship between antigen-specific B cells and T cells and the level of B cell repopulation necessary to achieve anti-vaccine responses remain largely unknown. METHODS: Antibody responses to SARS-CoV-2 vaccines and induction of antigen-specific B and CD4/CD8 T cell subsets were studied in 19 patients with rheumatoid arthritis (RA) or antineutrophil cytoplasmic antibody-associated vasculitis receiving RTX, 12 patients with RA receiving other therapies, and 30 healthy controls after SARS-CoV-2 vaccination with either messenger RNA or vector-based vaccines. RESULTS: A minimum of 10 B cells per microliter (0.4% of lymphocytes) in the peripheral circulation appeared to be required for RTX-treated patients to mount seroconversion to anti-S1 IgG upon SARS-CoV-2 vaccination. RTX-treated patients who lacked IgG seroconversion showed reduced receptor-binding domain-positive B cells (P = 0.0005), a lower frequency of Tfh-like cells (P = 0.0481), as well as fewer activated CD4 (P = 0.0036) and CD8 T cells (P = 0.0308) compared to RTX-treated patients who achieved IgG seroconversion. Functionally relevant B cell depletion resulted in impaired interferon-γ secretion by spike-specific CD4 T cells (P = 0.0112, r = 0.5342). In contrast, antigen-specific CD8 T cells were reduced in both RA patients and RTX-treated patients, independently of IgG formation. CONCLUSION: In RTX-treated patients, a minimum of 10 B cells per microliter in the peripheral circulation is a candidate biomarker for a high likelihood of an appropriate cellular and humoral response after SARS-CoV-2 vaccination. Mechanistically, the data emphasize the crucial role of costimulatory B cell functions for the proper induction of CD4 responses propagating vaccine-specific B cell and plasma cell differentiation.

Temporary antimetabolite treatment hold boosts SARS-CoV-2 vaccination-specific humoral and cellular immunity in kidney transplant recipients.

Transplant recipients exhibit an impaired protective immunity after SARS-CoV-2 vaccination, potentially caused by mycophenolate (MPA) immunosuppression. Recent data from patients with autoimmune disorders suggest that temporary MPA hold might greatly improve booster vaccination outcomes. We applied a fourth dose of SARS-CoV-2 vaccine to 29 kidney transplant recipients during a temporary (5 weeks) MPA/azathioprine hold, who had not mounted a humoral immune response to previous vaccinations. Seroconversion until day 32 after vaccination was observed in 76% of patients, associated with acquisition of virus-neutralizing capacity. Interestingly, 21/25 (84%) calcineurin inhibitor-treated patients responded, but only 1/4 belatacept-treated patients responded. In line with humoral responses, counts and relative frequencies of spike receptor binding domain-specific (RBD-specific) B cells were markedly increased on day 7 after vaccination, with an increase in RBD-specific CD27++CD38+ plasmablasts. Whereas overall proportions of spike-reactive CD4+ T cells remained unaltered after the fourth dose, frequencies were positively correlated with specific IgG levels. Importantly, antigen-specific proliferating Ki67+ and in vivo-activated programmed cell death 1-positive T cells significantly increased after revaccination during MPA hold, whereas cytokine production and memory differentiation remained unaffected. In summary, antimetabolite hold augmented all arms of immunity during booster vaccination. These data suggest further studies of antimetabolite hold in kidney transplant recipients.

BTLA Expression and Function Are Impaired on SLE B Cells.

B- and T-lymphocyte attenuator (BTLA/CD272) is an inhibitory checkpoint molecule expressed on T and B cells. Prior studies reported defective function of BTLA by T cells in patients with systemic lupus erythematosus (SLE), whereas nothing is known about its role on B cells in SLE, a disease with various B cell abnormalities. Peripheral blood mononuclear cells (PBMCs) from 23 healthy donors (HD) and 34 SLE patients were stained for BTLA and its expression on B cells was assessed. PBMCs or CD27-IgD+ naive B cells were stimulated together with an activating anti-BTLA antibody or an inhibitor of spleen tyrosine kinase (SYK) and differentiation as well as the expression of activation markers CD71, PD-1 and CD86 were analyzed. Our phenotypic and functional studies revealed reduced BTLA expression on CD27-IgD+ naïve B cells from SLE patients (p=0.0017) related to anti-dsDNA antibody titers (p=0.0394) and SIGLEC-1/CD169 expression on monocytes (p=0.0196), a type I interferon marker related to disease activity. BTLA engagement was found to control CpG/TLR9 activation limiting plasmablast (p=0.0156) and B cell memory induction (p=0.0078) in normal B cells in contrast to other B cell activation pathways (CD40, BCR). These BTLA functions were impaired in SLE B cells. Inhibition of SYK was found to mimic the effects of BTLA activity in vitro. Thus, is it possible that reduced BTLA expression and function of CD27-IgD+ antigen- and T cell-inexperienced SLE B cells could be overcome by SYK inhibition which should be tested in future studies as potential therapeutic principle.

Deep Phenotyping of CD11c+ B Cells in Systemic Autoimmunity and Controls.

Circulating CD11c+ B cells are a key phenomenon in certain types of autoimmunity but have also been described in the context of regular immune responses (i.e., infections, vaccination). Using mass cytometry to profile 46 different markers on individual immune cells, we systematically initially confirmed the presence of increased CD11c+ B cells in the blood of systemic lupus erythematosus (SLE) patients. Notably, significant differences in the expression of CD21, CD27, and CD38 became apparent between CD11c- and CD11c+ B cells. We observed direct correlation of the frequency of CD21-CD27- B cells and CD21-CD38- B cells with CD11c+ B cells, which were most pronounced in SLE compared to primary Sjögren's syndrome patients (pSS) and healthy donors (HD). Thus, CD11c+ B cells resided mainly within memory subsets and were enriched in CD27-IgD-, CD21-CD27-, and CD21-CD38- B cell phenotypes. CD11c+ B cells from all donor groups (SLE, pSS, and HD) showed enhanced CD69, Ki-67, CD45RO, CD45RA, and CD19 expression, whereas the membrane expression of CXCR5 and CD21 were diminished. Notably, SLE CD11c+ B cells showed enhanced expression of the checkpoint molecules CD86, PD1, PDL1, CD137, VISTA, and CTLA-4 compared to HD. The substantial increase of CD11c+ B cells with a CD21- phenotype co-expressing distinct activation and checkpoint markers, points to a quantitative increased alternate (extrafollicular) B cell activation route possibly related to abnormal immune regulation as seen under the striking inflammatory conditions of SLE which shows a characteristic PD-1/PD-L1 upregulation.