RIPK1 deficiency prevents thymic NK1.1 expression and subsequent iNKT cell development

Receptor Interacting Protein Kinase 1 (RIPK1) and caspase-8 (Casp8) jointly orchestrate apoptosis, a key mechanism for eliminating developing T cells which have autoreactive or improperly arranged T cell receptors. Mutations in the scaffolding domain of Ripk1 gene have been identified in humans with autoinflammatory diseases like Cleavage Resistant RIPK1 Induced Autoinflammatory (CRIA) and Inflammatory Bowel Disease. RIPK1 protein also contributes to conventional T cell differentiation and peripheral T cell homeostasis through its scaffolding domain in a cell death independent context. Ripk1 deficient mice do not survive beyond birth, so we have studied the function of this kinase in vivo against a backdrop Ripk3 and Casp8 deficiency which allows the mice to survive to adulthood. These studies reveal a key role for RIPK1 in mediating NK1.1 expression, including on thymic iNKT cells, which is a key requirement for thymic stage 2 to stage 3 transition as well as iNKT cell precursor development. These results are consistent with RIPK1 mediating responses to TcR engagement, which influence NK1.1 expression and iNKT cell thymic development. We also used in vivo and in vitro stimulation assays to confirm a role for both Casp8 and RIPK1 in mediating iNKT cytokine effector responses. Finally, we also noted expanded and hyperactivated iNKT follicular helper (iNKTFH) cells in both DKO (Casp8-, Ripk3- deficient) and TKO mice (Ripk1-, Casp8-, Ripk3- deficient). Thus, while RIPK1 and Casp8 jointly facilitate iNKT effector function, RIPK1 uniquely influenced thymic iNKT cell development most likely by regulating molecular responses to T cell receptor engagement. iNKT developmental and functional aberrances were not evident in mice expressing a kinase-dead version of RIPK1 (RIPK1kd), indicating that the scaffolding function of this protein exerts the critical regulation of iNKT cells. Our findings suggest that small molecule inhibitors of RIPK1 could be used to regulate iNKT cell development and effector function to alleviate autoinflammatory conditions in humans.

Continuous germinal center invasion contributes to the diversity of the immune response

Antibody responses are characterized by increasing affinity and diversity over time. Affinity maturation occurs in germinal centers by a mechanism that involves repeated cycles of somatic mutation and selection. How antibody responses diversify while also undergoing affinity maturation is not as well understood. Here, we examined germinal center (GC) dynamics by tracking B cell entry, division, somatic mutation, and specificity. Our experiments show that naive B cells continuously enter GCs where they compete for T cell help and undergo clonal expansion. Consistent with late entry, invaders carry fewer mutations but can contribute up to 30% or more of the cells in late-stage germinal centers. Notably, cells entering the germinal center at later stages of the reaction diversify the immune response by expressing receptors that show low affinity to the immunogen. Paradoxically, the affinity threshold for late GC entry is lowered in the presence of high-affinity antibodies.

T-bet+ B cells accumulate in adipose tissue and exacerbate metabolic disorder during obesity

Obesity is accompanied by inflammation in adipose tissue, impaired glucose tolerance, and changes in adipose leukocyte populations. These studies of adipose tissue from humans and mice revealed that increased frequencies of T-bet⁺ B cells in adipose tissue depend on invariant NKT cells and correlate with weight gain during obesity. Transfer of B cells enriched for T-bet⁺ cells exacerbates metabolic disorder in obesity, while ablation of Tbx21 specifically in B cells reduces serum IgG2c levels, inflammatory cytokines, and inflammatory macrophages in adipose tissue, ameliorating metabolic symptoms. Furthermore, transfer of serum or purified IgG from HFD mice restores metabolic disease in T-bet⁺ B cell-deficient mice, confirming T-bet⁺ B cell-derived IgG as a key mediator of inflammation during obesity. Together, these findings reveal an important pathological role for T-bet⁺ B cells that should inform future immunotherapy design in type 2 diabetes and other inflammatory conditions.

Diet-induced obesity promotes CD11c+ T-bet+ B cell expansion in liver and adipose tissue

Immune cells are increasingly appreciated to play a role in inflammation in adipose and hepatic tissue during obesity and may contribute to the development of non-alcoholic fatty liver disease (NAFLD). Both invariant Natural Killer T (iNKT) cells and B cells are enriched in the liver and adipose tissues of humans and mice, but the degree and nature of their influence in the development of NAFLD remains unclear. iNKT cells interact with B cells in many contexts, including during the chronic inflammation associated with autoimmune disease and infection, so it is likely they interact during obesity. Emerging evidence implicates intrahepatic B cells in the progression of human NAFLD, and proinflammatory B cells increase in the steatotic livers of HFD-fed mice. Our preliminary studies demonstrate an iNKT cell-dependent expansion of inflammatory CD11c+ T-bet+ B cells in the subcutaneous adipose tissue of obese humans and perigonadal adipose tissue of obese mice. We now extend those findings to other iNKT cell-rich fatty depots. Flow cytometry identified increased frequencies of T-bet+ B cells in the livers, but not the omenta or mesenteric adipose tissue, of obese mice. In parallel to the in vivo studies, in vitro co-cultures revealed that iNKT cells are sufficient to mediate expansion of T-bet+ B cells. Ongoing experiments will examine the potential role and mechanism of iNKT cells in mediating hepatic Tbet+ B cell expansion. Characterization of the specific contribution of immune inflammation in the progression NAFLD to NASH will allow for the design of successful therapeutic interventions for this increasingly frequent public health problem.

Funding support from NIH K12GM111726 (BTE), NIH TL1 TR002647 (TS), Swedish Research Council (TH), and NIH R01 AI32798-01A1 (EAL).

Dysfunction in B cell tolerance and activation in obesity

Obesity is a complex condition that affects a large part of the global population, making it a major risk factor for premature mortality and medical complications. Obesity profoundly engages the immune system, impacting cancer immunity, infections, and autoimmunity. Additionally, obesity is associated with a low-grade chronic inflammation which exacerbates metabolic dysfunction. B cells play a significant role in obesity-associated inflammation and metabolic dysfunction, but their effect is subset-dependent and can be either protective or pathogenic. Therefore, understanding how different B cell populations contribute to these phenotypes represent a crucial goal.

Our lab has identified a subset of B lymphocytes which is expanded in the adipose tissue of humans and mice during obesity. These B cells co-express the transcription factor T-bet and the integrin CD11c, and they produce IgG2c antibodies. In vivo studies with mice fed a high fat diet revealed that these Tbet+ B cells promote inflammation through the production of proinflammatory antibodies. Additionally, these B cells express high levels of the scavenger receptor CD36, a lipid-transport molecule associated with inflammation and atherosclerosis. As a potential consequence of the CD36 expression, the Tbet+ B cells maintain a higher intracellular lipid content and are capable of increased uptake of oxidized LDL compared to conventional B cells, suggesting a unique lipid metabolism.

Because of their unique nature, T-bet+ B cells represent a potential link between obesity, inflammation, and autoimmunity. Identifying the mechanisms regulating their function will provide new potential targets to reduce inflammation and immunological dysfunction in obese patients.

Supported by R01 AI132798

Receptor Interacting Protein Kinase Pathways Regulate Innate B Cell Developmental Checkpoints But Not Effector Function in Mice

Mutations in the scaffolding domain of Receptor Interacting Protein kinases (RIP) underlie the recently described human autoimmune syndrome, CRIA, characterized by lymphadenopathy, splenomegaly, and autoantibody production. While disease mechanisms for CRIA remain undescribed, RIP kinases work together with caspase-8 to regulate cell death, which is critical for normal differentiation of many cell types. Here, we describe a key role for RIP1 in facilitating innate B cell differentiation and subsequent activation. By comparing RIP1, RIP3, and caspase-8 triple deficient and RIP3, caspase-8 double deficient mice, we identified selective contributions of RIP1 to an accumulation of murine splenic Marginal Zone (MZ) B cells and B1-b cells. We used mixed bone-marrow chimeras to determine that innate B cell commitment required B cell-intrinsic RIP1, RIP3, and caspase-8 sufficiency. RIP1 regulated MZ B cell development rather than differentiation and RIP1 mediates its innate immune effects independent of the RIP1 kinase domain. NP-KLH/alum and NP-Ficoll vaccination of mice doubly deficient in both caspase-8 and RIP3 or deficient in all three proteins (RIP3, caspase-8, and RIP1) revealed uniquely delayed T-dependent and T-independent IgG responses, abnormal splenic germinal center architecture, and reduced extrafollicular plasmablast formation compared to WT mice. Thus, RIP kinases and caspase-8 jointly orchestrate B cell fate and delayed effector function through a B cell-intrinsic mechanism.

Tbet+ B cells facilitate iNKT cell expansion in murine liver and omentum during obesity

Obesity is a burgeoning public health crisis, responsible for increasing chronic systemic inflammation which leads to metabolic disease in humans and mice. Understanding the contribution of immune inflammation to metabolic disease is critical for designing effective therapeutic interventions. Invariant Natural Killer T (iNKT) cells are enriched in the adipose tissue, liver, and omenta of both humans and mice, but their frequency is reduced in adipose during obesity. iNKT cells negatively regulate or positively activate other immune cells, including B cells, depending on the immunologic context. Adipose B regulatory cells protect against inflammation, but they are not the only subset of B cells in adipose tissue. Our preliminary studies find increased frequencies of inflammatory Tbet+ B cells in adipose tissue of obese humans and mice which correlates with weight gain in mice and increasing BMI in humans. We now extend those findings to consider iNKT cell interaction with B cells during obesity in the adipose, liver, and omental tissues. Flow cytometry confirmed iNKT cell-dependent expansion of Tbet+ CD11c+ B cells in adipose tissue of obese mice. Interestingly, similar expansion was not observed in liver or omental tissue from the same obese mice. Instead, the livers and omenta of obese mice demonstrated significant increases in iNKT cell frequency compared with lean mice. In contrast to the adipose tissue, the increase in liver and omental iNKT cells during obesity was missing in mice genetically deficient in Tbet+ B cells. Thus, we find that iNKT cell-dependent expansion of Tbet+ CD11c+ B cells within adipose tissue of obese mice is counter-balanced by a reciprocal Tbet+ B cell-dependent expansion of iNKT cells in liver and omentum.

Persistent cellular immunity to SARS-CoV-2 infection

SARS-CoV-2 is responsible for an ongoing pandemic that has affected millions of individuals around the globe. To gain further understanding of the immune response in recovered individuals, we measured T cell responses in paired samples obtained an average of 1.3 and 6.1 mo after infection from 41 individuals. The data indicate that recovered individuals show persistent polyfunctional SARS-CoV-2 antigen–specific memory that could contribute to rapid recall responses. Recovered individuals also show enduring alterations in relative overall numbers of CD4⁺ and CD8⁺ memory T cells, including expression of activation/exhaustion markers, and cell division.

Structures of Human Antibodies Bound to SARS-CoV-2 Spike Reveal Common Epitopes and Recurrent Features of Antibodies

Neutralizing antibody responses to coronaviruses mainly target the receptor-binding domain (RBD) of the trimeric spike. Here, we characterized polyclonal immunoglobulin Gs (IgGs) and Fabs from COVID-19 convalescent individuals for recognition of coronavirus spikes. Plasma IgGs differed in their focus on RBD epitopes, recognition of alpha- and beta-coronaviruses, and contributions of avidity to increased binding/neutralization of IgGs over Fabs. Using electron microscopy, we examined specificities of polyclonal plasma Fabs, revealing recognition of both S1A and RBD epitopes on SARS-CoV-2 spike. Moreover, a 3.4 Å cryo-electron microscopy (cryo-EM) structure of a neutralizing monoclonal Fab-spike complex revealed an epitope that blocks ACE2 receptor binding. Modeling based on these structures suggested different potentials for inter-spike crosslinking by IgGs on viruses, and characterized IgGs would not be affected by identified SARS-CoV-2 spike mutations. Overall, our studies structurally define a recurrent anti-SARS-CoV-2 antibody class derived from VH3-53/VH3-66 and similarity to a SARS-CoV VH3-30 antibody, providing criteria for evaluating vaccine-elicited antibodies.

Convergent antibody responses to SARS-CoV-2 in convalescent individuals

During the coronavirus disease-2019 (COVID-19) pandemic, severe acute respiratory syndrome-related coronavirus-2 (SARS-CoV-2) has led to the infection of millions of people and has claimed hundreds of thousands of lives. The entry of the virus into cells depends on the receptor-binding domain (RBD) of the spike (S) protein of SARS-CoV-2. Although there is currently no vaccine, it is likely that antibodies will be essential for protection. However, little is known about the human antibody response to SARS-CoV-21-5. Here we report on 149 COVID-19-convalescent individuals. Plasma samples collected an average of 39 days after the onset of symptoms had variable half-maximal pseudovirus neutralizing titres; titres were less than 50 in 33% of samples, below 1,000 in 79% of samples and only 1% of samples had titres above 5,000. Antibody sequencing revealed the expansion of clones of RBD-specific memory B cells that expressed closely related antibodies in different individuals. Despite low plasma titres, antibodies to three distinct epitopes on the RBD neutralized the virus with half-maximal inhibitory concentrations (IC50 values) as low as 2 ng ml-1. In conclusion, most convalescent plasma samples obtained from individuals who recover from COVID-19 do not contain high levels of neutralizing activity. Nevertheless, rare but recurring RBD-specific antibodies with potent antiviral activity were found in all individuals tested, suggesting that a vaccine designed to elicit such antibodies could be broadly effective.

RIP Kinase death pathways regulate B cell homeostasis and splenic architecture

Receptor-interacting protein kinase (RIPK) 1, RIPK3, and Caspase-8, are essential adapter proteins in death receptor-induced apoptosis and programmed necrosis. Apoptosis and necroptosis are crucial for normal cell development, homeostasis, and immune responses. RIP kinases and Caspase-8 form parts of multiprotein complexes like the Death-Inducing Signaling Complex (DISC) and the necrosome, thus mediating signaling events in these cell death pathways. We compared RipK3−/−, RipK1−/−RipK3−/−Caspase-8−/−, RipK3−/−Caspase-8−/−, and RIPK1 kinase-dead (RIPK1kd) mice to define the role of RIP kinase inflammatory pathways and Caspase-8 in immune responses and lymphocyte development.

Preliminary studies show that lack of RIPK1 and Caspase-8, but not RIPK3, leads to the accumulation of Marginal Zone (MZ) B cells. Evidence from mixed-bone marrow chimera experiments reveals an intrinsic requirement for RIPK1 and caspase 8 in normal B cell differentiation post-T2 B cell status. Immunofluorescence showed splenic architecture disorganization in the T cell zone and MZB populations in RIPK-deficient mice. Surprisingly, humoral immune responses to T-dependent and T-independent antigens were largely normal, but immunization-induced splenic GC formation was diminished in mice lacking both Caspase-8 and RIPK3. Elucidation of cell development/death in the context of RIPK1 (whose ubiquitination status influences cellular homeostasis) could lead to better priming of B cells for chemotherapy-induced apoptosis. Previous studies also suggest human RIP kinases are potential therapeutic targets for treating a wide range of autoimmune, inflammatory, and neurodegenerative diseases.

A Link Between a Common Mutation in CFTR and Impaired Innate and Adaptive Viral Defense

Acute respiratory virus infections predispose the cystic fibrosis (CF) lung to chronic bacterial colonization, which contributes to high mortality. For reasons unknown, respiratory virus infections have a prolonged duration in CF. Here, we demonstrate that mice carrying the most frequent cystic fibrosis transmembrane conductance regulator (CFTR) mutation in humans, ΔF508, show increased morbidity and mortality following infection with a common human enterovirus. ΔF508 mice demonstrated impaired viral clearance, a slower type I interferon response and delayed production of virus-neutralizing antibodies. While the ΔF508 mice had a normal immune cell repertoire, unchanged serum immunoglobulin concentrations and an intact immune response to a T-cell-independent antigen, their response to a T-cell-dependent antigen was significantly delayed. Our studies reveal a novel function for CFTR in antiviral immunity and demonstrate that the ΔF508 mutation in cftr is coupled to an impaired adaptive immune response. This important insight could open up new approaches for patient care and treatment.

Neutrophils license iNKT cells to regulate self-reactive mouse B cell responses

The innate responsiveness of the immune system is important not only for quick responses to pathogens but also for the initiation and shaping of the subsequent adaptive immune response. Activation via the cytokine IL-18, a product of inflammasomes, gives rise to a rapid response that includes the production of self-reactive antibodies. As increased concentrations of this cytokine are found in inflammatory diseases, we investigated the origin of the B cell response and its regulation. We identified an accumulation of B cell-helper neutrophils in the spleen that interacted with innate-type invariant natural killer T cells (iNKT cells) to regulate B cell responses. We found that neutrophil-dependent expression of the death-receptor ligand FasL by iNKT cells was needed to restrict autoantibody production. Neutrophils can thus license iNKT cells to regulate potentially harmful autoreactive B cell responses during inflammasome-driven inflammation.

Sensitivity of dendritic cells to NK-mediated lysis depends on the inflammatory environment and is modulated by CD54/CD226-driven interactions

Previous studies have suggested that NK cells may limit T cell responses by their ability to eradicate dendritic cells, as demonstrated by NK cell-mediated killing of dendritic cells generated from mouse bone marrow cells or human monocytes with GM-CSF. In the present study, we demonstrated that conventional dendritic cells, generated in vitro with Flt3 ligand or from spleens, were resistant to NK cell-mediated lysis. However, upon stimulation with GM-CSF, NK cells could mediate lysis of these dendritic cells. GM-CSF-stimulated Flt3 ligand dendritic cells or splenic dendritic cells increased surface expression of costimulatory molecules and known NK cell ligands. Likewise, NK cells could target dendritic cells in vivo, which could be inhibited, in part, by anti-GM-CSF antibodies. The blocking of CD54 or CD226 inhibited NK cell-mediated cytotoxicity of the GM-CSF-stimulated Flt3 ligand conventional dendritic cells. Furthermore, the CD226⁺NKG2A^- subset of NK cells was selectively better at targeting GM-CSF-stimulated Flt3 ligand conventional dendritic cells. However, CD155, a known ligand for CD226, could also act as an inhibitor of NK cell-mediated lysis, as dendritic cells lacking CD155 were more sensitive to NK cell-mediated lysis than wild-type dendritic cells. We hypothesize that by only permitting a subset of NK cells to target activated dendritic cells during inflammation, this would allow the immune system to balance between dendritic cells able to drive adaptive immune responses and dendritic cells targeted for elimination by NK cells to hinder, e.g., spread of infection.

IL-18 in inflammatory and autoimmune disease

Inflammation serves as the first line of defense in response to tissue injury, guiding the immune system to ensure preservation of the host. The inflammatory response can be divided into a quick initial phase mediated mainly by innate immune cells including neutrophils and macrophages, followed by a late phase that is dominated by lymphocytes. Early in the new millennium, a key component of the inflammatory reaction was discovered with the identification of a number of cytosolic sensor proteins (Nod-like receptors) that assembled into a common structure, the 'inflammasome'. This structure includes an enzyme, caspase-1, which upon activation cleaves pro-forms of cytokines leading to subsequent release of active IL-1 and IL-18. This review focuses on the role of IL-18 in inflammatory responses with emphasis on autoimmune diseases.