Lipopolysaccharide (LPS) stimulates dual receptor signaling by bridging the B cell receptor and Toll-like receptor 4 (BCR/TLR4). B cells from IκBNS-deficient bumble mice treated with LPS display reduced proliferative capacity and impaired plasma cell differentiation. To improve our understanding of the regulatory role of IκBNS in B cell activation and differentiation, we investigated the BCR and TLR4 signaling pathways separately by using dimeric anti-IgM Fab (F(ab')2) or lipid A, respectively. IκBNS-deficient B cells exhibited reduced survival and defective proliferative capacity in response to lipid A compared to B cells from wildtype (wt) control mice. In contrast, anti-IgM stimulation of bumble B cells resulted in enhanced viability and increased differentiation into CD138+ cells compared to control B cells. Anti-IgM-stimulated IκBNS-deficient B cells also showed enhanced cycle progression with increased levels of c-Myc and cyclin D2, and augmented levels of pCD79a, pSyk, and pERK compared to control B cells. These results suggest that IκBNS acts as a negative regulator of BCR signaling and a positive regulator of TLR4 signaling in mouse B cells.

Mutations causing loss of the NF-κB regulator IκBNS, result in impaired development of innate-like B cells and defective plasma cell (PC) differentiation. Since productive PC differentiation requires B cell metabolic reprogramming, we sought to investigate processes important for this transition using the bumble mouse strain, deficient for IκBNS. We report that LPS-activated bumble B cells exhibited elevated mTOR activation levels, mitochondrial accumulation, increased OXPHOS and mROS production, along with a reduced capacity for autophagy, compared to wildtype B cells. Overall, our results demonstrate that PC differentiation in the absence of IκBNS is characterized by excessive activation during early rounds of B cell division, increased mitochondrial metabolism and decreased autophagic capacity, thus improving our understanding of the role of IκBNS in PC differentiation.

Mice lacking the atypical inhibitory kappa B (IκB) protein, IκBNS, a regulator of the NF-κB pathway encoded by the nfkbid gene, display impaired antibody responses to both T cell-independent (TI) and T cell-dependent (TD) antigens. To better understand the basis of these defects, we crossed mice carrying floxed nfkbid alleles with mice expressing Cre under the transcriptional control of the Cd79a gene to create mice that lacked IκBNS expression only in B cells. Analyses of these conditional knock-out mice revealed intact CD4+ and CD8+ T cell populations, including preserved frequencies of FoxP3+ regulatory T cells, which are known to be reduced in IκBNS knock-out mice. Like IκBNS knock-out mice, mice with conditional IκBNS ablation in B cells displayed defective IgM responses to TI antigens and a severe reduction in peritoneal B-1a cells. However, in contrast to mice lacking IκBNS altogether, the conditional IκBNS knock-out mice responded well to TD antigens compared to the control mice, with potent IgG responses following immunization with the viral antigen, rSFV-βGal or the widely used hapten-protein model antigen, NP-CGG. Furthermore, B cell intrinsic IκBNS expression was dispensable for germinal center (GC) formation and T follicular helper cell responses to NP-CGG immunization. The results presented here suggest that the defect in antibody responses to TD antigens observed in IκBNS knock-out mice results from a B cell extrinsic defect.

Serological studies are critical for understanding pathogen-specific immune responses and informing public health measures (1,2). By developing highly sensitive and specific trimeric spike (S)-based antibody tests, we report IgM, IgG and IgA responses to SARS-CoV-2 in COVID-19 patients (n=105) representing different categories of disease severity. All patients surveyed were IgG positive against S. Elevated anti-SARS-CoV-2 antibody levels were associated with hospitalization, with IgA titers, increased circulating IL-6 and strong neutralizing responses indicative of intensive care status. Antibody-positive blood donors and pregnant women sampled during the pandemic in Stockholm, Sweden (weeks 14-25), displayed on average lower titers and weaker neutralizing responses compared to patients; however, inter-individual anti-viral IgG titers differed up to 1,000-fold. To provide more accurate estimates of seroprevalence, given the frequency of weak responders and the limitations associated with the dichotomization of a continuous variable (3,4), we used a Bayesian approach to assign likelihood of past infection without setting an assay cut-off. Analysis of blood donors (n=1,000) and pregnant women (n=900) sampled weekly demonstrated SARS-CoV-2-specific IgG in 7.2% (95% Bayesian CI [5.1-9.5]) of individuals two months after the peak of spring 2020 COVID-19 deaths. Seroprevalence in these otherwise healthy cohorts increased steeply before beginning to level-off, following the same trajectory as the Stockholm region deaths over this time period.

Serology is critical for understanding pathogen-specific immune responses, but is fraught with difficulty, not least because the strength of antibody (Ab) response varies greatly between individuals and mild infections generally generate lower Ab titers1–3. We used robust IgM, IgG and IgA Ab tests to evaluate anti-SARS-CoV-2 responses in individuals PCR+ for virus RNA (n=105) representing different categories of disease severity, including mild cases. All PCR+ individuals in the study became IgG-positive against pre-fusion trimers of the virus spike (S) glycoprotein, but titers varied greatly. Elevated IgA, IL-6 and neutralizing responses were present in intensive care patients. Additionally, blood donors and pregnant women (n=2,900) sampled throughout the first wave of the pandemic in Stockholm, Sweden, further demonstrated that anti-S IgG titers differed several orders of magnitude between individuals, with an increase of low titer values present in the population at later time points4,5. To improve upon current methods to identify low titers and extend the utility of individual measures6,7, we used our PCR+ individual data to train machine learning algorithms to assign likelihood of past infection. Using these tools that assigned probability to individual responses against S and the receptor binding domain (RBD), we report SARS-CoV-2-specific IgG in 13.7% of healthy donors five months after the peak of spring COVID-19 deaths, when mortality and ICU occupancy in the country due to the virus were at low levels. These data further our understanding of antibody responses to the virus and provide solutions to problems in serology data analysis. Significance statement Antibody testing provides critical clinical and epidemiological information during an emerging disease pandemic. We developed robust SARS-CoV-2 IgM, IgG and IgA antibody tests and profiled COVID-19 patients and exposed individuals throughout the outbreak in Stockholm, Sweden, where full societal lockdown was not employed. As well as elucidating several disease immunophenotypes, our data highlight the challenge of identifying low IgG titer individuals, who comprise a significant proportion of the population following mild/asymptomatic infection, especially as antibody titers wane following peak responses. To provide a solution to this, we used SARS-CoV-2 PCR+ individual data to develop machine learning approaches that assigned likelihood of past infection to blood donors and pregnant women, improving the accuracy and utility of individual and population-level Ab measures.

The levels of the antibody response against SARS-CoV-2 varies widely between individuals, which together with the decline of antibody responses over time, complicates the correct classification of seropositivity using conventional assay cut-offs. All subjects in a cohort of SARS-CoV-2 PCR+ individuals representing different disease severity categories (n=105), and a group of PCR+ hospital staff (n=33), developed IgG against pre-fusion-stabilized spike (S) trimers and 97% did against the receptor-binding domain (RBD). The levels differed by several orders of magnitude and associated with disease phenotype. Concomitant analysis of a cohort of healthy blood donors and pregnant women (n=1,000), representing individuals who had undergone milder infections, demonstrated highly variable IgG titers, including several that scored between the classical 3SD and 6SD cut-offs. Since the correct classification of seropositivity is critical for epidemiological estimates, we trained probabilistic algorithms to assign likelihood of past infection using anti-S and -RBD IgG data from PCR+ individuals and a large cohort of historical negative controls (n=595). Applied to blood donors and pregnant women, this probabilistic approach provided a more accurate way to interpret antibody titers spread over a large continuum offering a probability-based diagnosis. The methods described here are directly applicable to serological measurements following natural infection and vaccination.

Humoral immunity is established after differentiation of antigen-specific B cells into plasma cells (PCs) that produce antibodies of relevant specificities. Defects in the development, activation, or differentiation of B cells severely compromises the immune response. Primary immunodeficiencies are often characterized by hypogammaglobulinemia and the inability to mount effective antigen-specific antibody responses, resulting in increased susceptibility to infections. After IgA deficiency, which is most often asymptomatic, common variable immunodeficiency (CVID) is the most prevalent symptomatic primary immunodeficiency, but in most cases the underlying genetic causes are unknown or their roles in disease pathogenesis are poorly understood. In this study, we developed a protocol for in vitro stimulation of primary human B cells for subsequent analyses of PC differentiation and antibody production. With this approach, we were able to detect a population of CD38+ IRF4+ Blimp-1+ cells committed to PC fate and IgG production, including when starting from cryopreserved samples. The application of functional assays to characterize PC differentiation and possible defects therein in B cells from patients suffering from primary antibody deficiencies with late B cell defects could increase our understanding of the disease pathophysiology and underlying mechanisms.

Antibody responses vary widely between individuals1, complicating the correct classification of low-titer measurements using conventional assay cut-offs. We found all participants in a clinically diverse cohort of SARS-CoV-2 PCR+ individuals (n=105) – and n=33 PCR+ hospital staff – to have detectable IgG specific for pre-fusion-stabilized spike (S) glycoprotein trimers, while 98% of persons had IgG specific for the receptor-binding domain (RBD). However, anti-viral IgG levels differed by several orders of magnitude between individuals and were associated with disease severity, with critically ill patients displaying the highest anti-viral antibody titers and strongest in vitro neutralizing responses. Parallel analysis of random healthy blood donors and pregnant women (n=1,000) of unknown serostatus, further demonstrated highly variable IgG titers amongst seroconverters, although these were generally lower than in hospitalized patients and included several measurements that scored between the classical 3 and 6SD assay cut-offs. Since the correct classification of seropositivity is critical for individual- and population-level metrics, we compared different probabilistic algorithms for their ability to assign likelihood of past infection. To do this, we used tandem anti-S and -RBD IgG responses from our PCR+ individuals (n=138) and a large cohort of historical negative controls (n=595) as training data, and generated an equal-weighted learner from the output of support vector machines and linear discriminant analysis. Applied to test samples, this approach provided a more quantitative way to interpret anti-viral titers over a large continuum, scrutinizing measurements overlapping the negative control background more closely and offering a probability-based diagnosis with potential clinical utility. Especially as most SARS-CoV-2 infections result in asymptomatic or mild disease, these platform-independent approaches improve individual and epidemiological estimates of seropositivity, critical for effective management of the pandemic and monitoring the response to vaccination.

Serological studies are critical for understanding pathogen−specific immune responses and informing public health measures (1,2). By developing highly sensitive and specific trimeric spike (S)−based antibody tests, we report IgM, IgG and IgA responses to SARS−CoV−2 in COVID−19 patients (n=105) representing different categories of disease severity. All patients surveyed were IgG positive against S. Elevated anti−SARS−CoV−2 antibody levels were associated with hospitalization, with IgA titers, increased circulating IL−6 and strong neutralizing responses indicative of intensive care status. Antibody−positive blood donors and pregnant women sampled during the pandemic in Stockholm, Sweden (weeks 14−25), displayed on average lower titers and weaker neutralizing responses compared to patients; however, inter−individual anti−viral IgG titers differed up to 1,000−fold. To provide more accurate estimates of seroprevalence, given the frequency of weak responders and the limitations associated with the dichotomization of a continuous variable (3,4), we used a Bayesian approach to assign likelihood of past infection without setting an assay cut−off. Analysis of blood donors (n=1,000) and pregnant women (n=900) sampled weekly demonstrated SARS-CoV-2-specific IgG in 7.2% (95% Bayesian CI [5.1−9.5]) of individuals two months after the peak of spring 2020 COVID−19 deaths. Seroprevalence in these otherwise healthy cohorts increased steeply before beginning to level−off, following the same trajectory as the Stockholm region deaths over this time period.

Impaired classical NF-κB pathway signaling causes reduced antibody responses to T-independent (TI) antigens. We investigated the potential reasons for defective TI responses in mice lacking the atypical inhibitory kappa B (IκB) protein of the NF-κB pathway, IκBNS. Analyses of the plasma cell compartment in vitro and in vivo after challenge with lipopolysaccharide (LPS) showed significant decreases in the frequencies of plasma cells in the absence of IκBNS. In vitro activation of B cells via the B cell receptor or via Toll-like receptor 4 revealed that early activation events were unaffected in IκBNS-deficient B cells, while proliferation was reduced compared to in similarly stimulated wildtype (wt) B cells. IκBNS-deficient B cells also displayed impaired upregulation of the transmembrane activator and calcium modulator cyclophilin ligand interactor (TACI), which is essential for TI responses, and decreased sensitivity to TACI ligands upon stimulation. Furthermore, IκBNS-deficient B cells, in contrast to wt B cells, displayed altered expression of IRF4, Blimp-1 and Pax5 upon LPS-induced differentiation, indicating impaired transcriptional regulation of plasma cell generation.

B-1a cells are mainly generated from fetal liver progenitor cells, peri- and neonatally. The developmental steps and anatomical sites required for these cells to become mature B-1a cells remain elusive. We recently described a phenotypically distinct transitional B cell subset in the spleen of neonatal mice that generated B-1a cells when adoptively transferred. This, in combination with findings demonstrating that B-1a cells are lacking in congenitally asplenic mice, led us to hypothesize that the neonatal spleen is required for B-1a cell development. In accordance with previous reports, we found that B-1a cell numbers were reduced in adult mice that had undergone splenectomy compared to after sham surgery. In contrast, neonatal splenectomy led to peritoneal B-1a cell frequencies comparable to those observed in sham-operated mice until 6 weeks after surgery, suggesting that an intact spleen is required for B-1a cell maintenance rather than development. To study the role of the prenatal spleen in generating B-1a cells, we transferred fetal liver cells from pre-splenic embryos [embryonic age 11 (E11) days] into splenectomized recipient mice. B-1a cells were generated in the absence of the spleen, albeit at slightly reduced frequencies, and populated the peritoneal cavity and bone marrow. Lower bone marrow B-1a cell frequencies were also observed both after neonatal and adult splenectomy. These results demonstrated that B-1a cells could be generated in the complete absence of an intact spleen, but that asplenia led to a decline in these cells, suggesting a role of the spleen for maintaining the B-1a compartment.

Impaired classical NF-kappa B pathway signaling causes reduced antibody responses to T-independent (TI) antigens. We investigated the potential reasons for defective TI responses in mice lacking the atypical inhibitory kappa B (I kappa B) protein of the NF-kappa B pathway, I kappa BNS. Analyses of the plasma cell compartment in vitro and in vivo after challenge with lipopolysaccharide (LPS) showed significant decreases in the frequencies of plasma cells in the absence of I kappa BNS. In vitro activation of B cells via the B cell receptor or via Toll-like receptor 4 revealed that early activation events were unaffected in I kappa BNS-deficient B cells, while proliferation was reduced compared to in similarly stimulated wildtype (wt) B cells. I kappa BNS-deficient B cells also displayed impaired upregulation of the transmembrane activator and calcium modulator cyclophilin ligand interactor (TACI), which is essential for TI responses, and decreased sensitivity to TACI ligands upon stimulation. Furthermore, I kappa BNS-deficient B cells, in contrast to wt B cells, displayed altered expression of IRF4, Blimp-1 and Pax5 upon LPS-induced differentiation, indicating impaired transcriptional regulation of plasma cell generation.

Marginal zone (MZ) B cells reside in the splenic MZ and play important roles in T cell–independent humoral immune responses against blood-borne pathogens. IκBNS-deficient bumble mice exhibit a severe reduction in the MZ B compartment but regain an MZ B population with age and, thus, represent a valuable model to examine the biology of MZ B cells. In this article, we characterized the MZ B cell defect in further detail and investigated the nature of the B cells that appear in the MZ of aged bumble mice. Flow cytometry analysis of the splenic transitional B cell subsets demonstrated that MZ B cell development was blocked at the transitional-1 to transitional-2–MZ precursor stage in the absence of functional IκBNS. Immunohistochemical analysis of spleen sections from wild-type and bumble mice revealed no alteration in the cellular MZ microenvironment, and analysis of bone marrow chimeras indicated that the MZ B cell development defect in bumble mice was B cell intrinsic. Further, we demonstrate that the B cells that repopulate the MZ in aged bumble mice were distinct from age-matched wild-type MZ B cells. Specifically, the expression of surface markers characteristic for MZ B cells was altered and the L chain Igλ+ repertoire was reduced in bumble mice. Finally, plasma cell differentiation of sorted LPS-stimulated MZ B cells was impaired, and aged bumble mice were unable to respond to NP-Ficoll immunization. These results demonstrate that IκBNS is required for an intact MZ B cell compartment in C57BL/6 mice.

Mice deficient in central components of classical NF-κB signaling have low levels of circulating natural IgM antibodies and fail to respond to immunization with T-independent type 2 (TI-2) antigens. A plausible explanation for these defects is the severely reduced numbers of B-1 and marginal zone B (MZB) cells in such mice. By using an ethyl-N-nitrosourea mutagenesis screen, we identified a role for the atypical IκB protein IκBNS in humoral immunity. IκBNS-deficient mice lack B-1 cells and have severely reduced numbers of MZB cells, and thus resemble several other strains with defects in classical NF-κB signaling. We analyzed mice heterozygous for the identified IκBNS mutation and demonstrate that these mice have an intermediary phenotype in terms of levels of circulating IgM antibodies and responses to TI-2 antigens. However, in contrast to mice that are homozygous for the IκBNS mutation, the heterozygous mice had normal frequencies of B-1 and MZB cells. These results suggest that there is a requirement for IκBNS expression from two functional alleles for maintaining normal levels of circulating natural IgM antibodies and responses to TI-2 antigens.

SignificanceA subpopulation of antibody-secreting cells, B-1 cells, provides early protection against several types of pathogens. Both the development and function differ between B-1 cells and the better known B-2 cells, and exclusively B-1 cells are lacking in mice deficient for the nuclear inhibitory κB protein, IκBNS. B-1 cells mature similar to B-2 cells via a transitional stage. We demonstrate here the existence of a phenotypically distinct B-1 transitional B (TrB)-cell population in the neonatal spleen of wild-type mice. This TrB-1a–cell subset was lost in the absence of IκBNS, thus revealing a requirement for intact NF-κB signaling via IκBNS during this stage of the development of B-1 cells. Learning more about the development of B-1 cells may reveal new targets for therapeutic intervention.