IgM marks persistent IgG anti-human leukocyte antigen antibodies in highly sensitized heart transplant patients

BACKGROUND

Sensitization to human leukocyte antigens (HLA) is a persistent problem in heart transplant (HT) candidates. We sought to characterize the anti-HLA antibody and circulating B cell repertoire in a cohort of highly sensitized HT candidates.

METHODS

We assessed immunoglobulin G (IgG) and immunoglobulin M (IgM) anti-HLA antibodies using Luminex single antigen bead assays in a cohort of 11 highly sensitized (HS; calculated panel reactive antibody ≥ 90%) and 3 mildly sensitized (MS) candidates. We also performed B cell receptor repertoire sequencing (BCRseq) in HS candidates and 33 non-candidate controls. HLA antibody strength was measured by mean fluorescence intensity (MFI).

RESULTS

We found that IgM anti-HLA antibodies were present in all HS candidates, but with a lower breadth and strength as compared to IgG. When anti-HLA IgG specificities intersected with IgM, binding strength was higher. In contrast, there were IgM but no intersecting IgG specificities for the MS group. In four candidates in the HS group, IgG anti-HLA antibodies decreased in both breadth and strength after HT, but the decrease in strength was smaller if the IgG possessed a specificity that intersected with pre-transplant IgM. BCRseq revealed larger B cell clonotypes in HS candidates but similar diversity as compared to controls.

CONCLUSIONS

IgM marks IgG anti-HLA antibodies with higher strength before HT and persistence after HT. The presence of IgM intersecting IgG for an anti-HLA specificity may be a useful approach to determine which donor HLA should be avoided for a sensitized candidate.

Distinct RBC alloantibody responses in type 1 interferon-dependent and -independent lupus mouse models

During transfusion of red blood cells (RBCs), recipients are exposed to both ABO and non-ABO 'minor' antigens. RBC donor units and recipient RBCs are not routinely matched for non-ABO antigens. Thus, recipients are exposed to many RBC alloantigens that can lead to RBC alloantibody production and subsequent clinically significant hemolysis. RBC alloantibodies also significantly limit the provision of compatible RBC units for recipients. Prior studies indicate that the frequency of RBC alloimmunization is increased during inflammatory responses and in patients with autoimmune diseases. Still, mechanisms contributing to alloimmune responses in patients with autoimmunity are not well understood. More than half of adult patients with systemic lupus erythematosus (SLE) produce type 1 interferons (IFNα/β) and express IFNα/β stimulated genes (ISGs). Previously, we reported that IFNα/β promote RBC alloimmune responses in the pristane mouse model, which develops a lupus-like phenotype that is dependent on IFNα/β signaling. However, it is unclear whether IFNα/β or the lupus-like phenotype induces alloimmunization in lupus models. Therefore, we tested the hypothesis that IFNα/β promotes RBC alloimmune responses in lupus by examining alloimmune responses in IFNα/β-independent (MRL-lpr) and IFNα/β-dependent (pristane) lupus models. Whereas pristane treatment significantly induced interferon-stimulated genes (ISGs), MRL-lpr mice produced significantly lower levels that were comparable to levels in untreated WT mice. Transfusion of murine RBCs that express the KEL antigen led to anti-KEL IgG production by pristane-treated WT mice. However, MRL-lpr mice produced minimal levels of anti-KEL IgG. Treatment of MRL-lpr mice with recombinant IFNα significantly enhanced alloimmunization. Collectively, results indicate that a lupus-like phenotype in pre-clinical models is not sufficient to induce RBC alloantibody production, and IFNα/β gene signatures may be responsible for RBC alloimmune responses in lupus mouse models. If these findings are extended to alternate pre-clinical models and clinical studies, patients with SLE who express an IFNα/β gene signature may have an increased risk of developing RBC alloantibodies and may benefit from more personalized transfusion protocols.

Durable remission of thrombotic thrombocytopenic purpura in the setting of pembrolizumab therapy

BACKGROUND

There is a small but growing number of thrombotic thrombocytopenic purpura (TTP) cases attributed to immune checkpoint inhibitor therapy, with nivolumab and ipilimumab therapy being the most frequently described in the literature.

STUDY DESIGN AND METHODS

This report evaluates the course of a patient with a history of metastatic adenocarcinoma of the lung who developed TTP following treatment with the PD-1 inhibitor Pembrolizumab. The patient was treated with six sessions of therapeutic plasma exchange and appeared to be in remission. Exacerbation occurred 4 days later, and seven more sessions of plasma exchange were performed along with four total doses of Rituximab, and a steroid taper with monitoring of platelet counts and ADAMTS13 activity.

RESULTS

His platelet count recovered to a peak of 318,000 UL with an ADAMTS13 activity of 77% at the time of discharge. The patient has been following up regularly for outpatient testing with no TTP relapse as of the completion of this report.

DISCUSSION

This is one of a few cases of Pembrolizumab-associated TTP reported in the literature with successful complete remission following treatment. Plasma exchange in this setting may be an especially beneficial therapeutic intervention because of the removal of both the anti-ADAMTS13 antibody as well as the immune system upregulating anti-PDL1 monoclonal antibody with replacement of ADAMTS13 from donor plasma. Longer duration of plasma exchange and monitoring for normalization of ADAMTS13 levels in addition to platelet count before cessation of treatment may improve durable remission rates in this entity.

Successful autologous hematopoietic stem cell transplants using Salmonella positive products collected from asymptomatic donors

BACKGROUND

Bacterial contamination of hematopoietic stem cell (HSC) products is most commonly due to normal skin flora. Salmonella in HSC products is rare, and to our knowledge safe administration of an autologous HSC product containing Salmonella has not been reported.

STUDY DESIGN AND METHODS

We describe two patients undergoing autologous HSC transplant: peripheral blood HSC collection was performed by leukapheresis, and samples were cultured according to standard institutional protocol. Subsequent microorganism identification was performed using MALDI-TOF (Bruker Biotyper). Strain-relatedness was investigated by infrared spectroscopy using the IR Biotyper (Bruker).

RESULTS

The patients were asymptomatic throughout the collection process; however, HSC products collected on two consecutive days from each patient were positive for Salmonella. Isolates from both cultures were further characterized as Salmonella enterica serovar Dublin by the local public health department. Antibiotic susceptibility testing revealed different sensitivity patterns for the two strains. IR Biotyper demonstrated significant discriminatory power among the clinically significant Salmonella enterica subspecies, serogroups B, C1, and D. The patient strains were similar as both belonged to Group D Salmonella enterica serovar Dublin but were not identical. The Salmonella positive autologous HSC products were infused to both patients following administration of empiric antibiotic therapy. Both patients successfully engrafted and did well.

CONCLUSION

Salmonella is rarely seen in cellular therapy products and positivity may be the result of asymptomatic bacteremia at the time of collection. We present two instances of autologous HSC products containing Salmonella that were infused, along with prophylactic antimicrobial therapy without significant adverse clinical effects.

Storage differentially impacts alloimmunization to distinct red cell antigens following transfusion in mice

INTRODUCTION

The impact of blood storage on red blood cell (RBC) alloimmunization remains controversial, with some studies suggesting enhancement of RBC-induced alloantibody production and others failing to observe any impact of storage on alloantibody formation. Since evaluation of storage on RBC alloimmunization in patients has examined antibody formation against a broad range of alloantigens, it remains possible that different clinical outcomes reflect a variable impact of storage on alloimmunization to specific antigens.

METHODS

RBCs expressing two distinct model antigens, HEL-OVA-Duffy (HOD) and KEL, separately or together (HOD × KEL), were stored for 0, 8, or 14 days, followed by detection of antigen levels prior to transfusion. Transfused donor RBC survival was assessed within 24 h of transfusion, while IgM and IgG antibody production were assessed 5 and 14 days after transfusion.

RESULTS

Stored HOD or KEL RBCs retained similar HEL or KEL antigen levels, respectively, as fresh RBCs, but did exhibit enhanced RBC clearance with increased storage age. Storage enhanced IgG antibody formation against HOD, while the oppositive outcome occurred following transfusion of stored KEL RBCs. The distinct impact of storage on HOD or KEL alloimmunization did not appear to reflect intrinsic differences between HOD or KEL RBCs, as transfusion of stored HOD × KEL RBCs resulted in increased IgG anti-HOD antibody development and reduced IgG anti-KEL antibody formation.

CONCLUSIONS

These data demonstrate a dichotomous impact of storage on immunization to distinct RBC antigens, offering a possible explanation for inconsistent clinical experience and the need for additional studies on the relationship between RBC storage and alloimmunization.

Investigating anti-D in an individual with the weak D type 2 genotype

Anti-D in individuals with a weak D phenotype is an unexpected finding that may require additional investigation to determine whether the anti-D is an autoantibody or alloantibody. Further investigation may also include assessment of the patient's RHD genotype and exclusion of anti-G. We present a case of an 84-year-old man with the weak D type 2 genotype who developed an unexpected anti-D along with anti-C. Individuals with the weak D type 2 genotype are thought not to be at risk for developing alloanti-D, although the distinction between alloanti-D and autoanti-D may be difficult to ascertain. Furthermore, investigations may affect transfusion recommendations. This patient was restricted to crossmatch-compatible, D-C- red blood cells even though the clinical significance of the anti-D was uncertain. This report is one of a few reported cases of an individual with the weak D type 2 genotype with demonstrable anti-D but without evidence for alloanti-D.

Retention of functional mitochondria in mature red blood cells from patients with sickle cell disease

Sickle cell disease (SCD) is an inherited blood disorder characterized by sickled red blood cells (RBCs), which are more sensitive to haemolysis and can contribute to disease pathophysiology. Although treatment of SCD can include RBC transfusion, patients with SCD have high rates of alloimmunization. We hypothesized that RBCs from patients with SCD have functionally active mitochondria and can elicit a type 1 interferon response. We evaluated blood samples from more than 100 patients with SCD and found elevated frequencies of mitochondria in reticulocytes and mature RBCs, as compared to healthy blood donors. The presence of mitochondria in mature RBCs was confirmed by flow cytometry, electron microscopy, and proteomic analysis. The mitochondria in mature RBCs were metabolically competent, as determined by enzymatic activities and elevated levels of mitochondria-derived metabolites. Metabolically-active mitochondria in RBCs may increase oxidative stress, which could facilitate and/or exacerbate SCD complications. Coculture of mitochondria-positive RBCs with neutrophils induced production of type 1 interferons, which are known to increase RBC alloimmunization rates. These data demonstrate that mitochondria retained in mature RBCs are functional and can elicit immune responses, suggesting that inappropriate retention of mitochondria in RBCs may play an underappreciated role in SCD complications and be an RBC alloimmunization risk factor.

Tubular IL-1β Induces Salt Sensitivity in Diabetes by Activating Renal Macrophages

BACKGROUND

Chronic renal inflammation has been widely recognized as a major promoter of several forms of high blood pressure including salt-sensitive hypertension. In diabetes, IL (interleukin)-6 induces salt sensitivity through a dysregulation of the epithelial sodium channel. However, the origin of this inflammatory process and the molecular events that culminates with an abnormal regulation of epithelial sodium channel and salt sensitivity in diabetes are largely unknown.

METHODS

Both in vitro and in vivo approaches were used to investigate the molecular and cellular contributors to the renal inflammation associated with diabetic kidney disease and how these inflammatory components interact to develop salt sensitivity in db/db mice.

RESULTS

Thirty-four-week-old db/db mice display significantly higher levels of IL-1β in renal tubules compared with nondiabetic db/+ mice. Specific suppression of IL-1β in renal tubules prevented salt sensitivity in db/db mice. A primary culture of renal tubular epithelial cells from wild-type mice releases significant levels of IL-1β when exposed to a high glucose environment. Coculture of tubular epithelial cells and bone marrow-derived macrophages revealed that tubular epithelial cell-derived IL-1β promotes the polarization of macrophages towards a proinflammatory phenotype resulting in IL-6 secretion. To evaluate whether macrophages are the cellular target of IL-1β in vivo, diabetic db/db mice were transplanted with the bone marrow of IL-1R1 (IL-1 receptor type 1) knockout mice. db/db mice harboring an IL-1 receptor type 1 knockout bone marrow remained salt resistant, display lower renal inflammation and lower expression and activity of epithelial sodium channel compared with db/db transplanted with a wild-type bone marrow.

CONCLUSIONS

Renal tubular epithelial cell-derived IL-1β polarizes renal macrophages towards a proinflammatory phenotype that promotes salt sensitivity through the accumulation of renal IL-6. When tubular IL-1β synthesis is suppressed or in db/db mice in which immune cells lack the IL-1R1, macrophage polarization is blunted resulting in no salt-sensitive hypertension.

Altered type 1 interferon responses in alloimmunized and nonalloimmunized patients with sickle cell disease

Patients with sickle cell disease (SCD) have a high prevalence of RBC alloimmunization. However, underlying mechanisms are poorly understood. Given that proinflammatory type 1 interferons (IFNα/β) and interferon stimulated genes (ISGs) promote alloimmunization in mice, we hypothesized that IFNα/β may contribute to the increased frequency of alloimmunization in patients with SCD. To investigate this, expression of ISGs in blood leukocytes and peripheral blood mononuclear cells (PBMCs) of previously transfused SCD patients with or without alloimmunization and race-matched healthy controls were quantified, and IFNα/β gene scores were calculated. IFNα/β gene scores of SCD leukocytes and plasma cytokines were elevated, compared to controls (gene score, p < 0.01). Upon stimulation with IFNβ, isolated PBMCs from patients with SCD had elevated ISGs and IFNα/β gene scores (p < 0.05), compared to stimulated PBMCs from controls. However, IFNβ-stimulated and unstimulated ISG expression did not significantly differ between alloimmunized and non-alloimmunized patients. These findings indicate that patients with SCD express an IFNα/β gene signature, and larger studies are needed to fully determine its role in alloimmunization. Further, illustration of altered IFNα/β responses in SCD has potential implications for IFNα/β-mediated viral immunity, responses to IFNα/β-based therapies, and other sequelae of SCD.

Potential Implications of a Type 1 Interferon Gene Signature on COVID-19 Severity and Chronic Inflammation in Sickle Cell Disease

At the onset of the corona virus disease 19 (COVID-19) pandemic, there were concerns that patients with sickle cell disease (SCD) might be especially vulnerable to severe sequelae of SARS-CoV-2 infection. While two reports support this conclusion, multiple studies have reported unexpectedly favorable outcomes in patients with SCD. However, mechanisms explaining these disparate conclusions are lacking. Here, we review recent studies indicating that the majority of patients with SCD express elevated levels of anti-viral type 1 interferons (IFNα/β) and interferon stimulated genes, independent of COVID-19, during their baseline state of health. We also present our data from the pre-COVID-19 era, illustrating elevated expression of a well-characterized interferon stimulated gene in a cohort of patients with SCD, compared to race-matched controls. These type 1 interferons and interferon stimulated genes have the potential to contribute to the variable progression of COVID-19 and other viral infections in patients with SCD. While the majority of evidence supports a protective role, the role of IFNα/β in COVID-19 severity in the general population remains an area of current investigation. We conclude that type 1 interferon responses in patients with SCD may contribute to the variable COVID-19 responses reported in prior studies. Additional studies investigating the mechanisms underlying IFNα/β production and other clinical consequences of IFNα/β-mediated inflammation in SCD disease are warranted.

Complement Plays a Critical Role in Inflammation-Induced Immunoprophylaxis Failure in Mice

Complement impacts innate and adaptive immunity. Using a model in which the human KEL glycoprotein is expressed on murine red blood cells (RBCs), we have shown that polyclonal immunoprophylaxis (KELIg) prevents alloimmunization to transfused RBCs when a recipient is in their baseline state of heath but with immunoprophylaxis failure occurring in the presence of a viral-like stimulus. As complement can be detected on antibody coated KEL RBCs following transfusion, we hypothesized that recipient complement synergizes with viral-like inflammation to reduce immunoprophylaxis efficacy. Indeed, we found recipient C3 and C1q were critical to immunoprophylaxis failure in the setting of a viral-like stimulus, with no anti-KEL IgG alloantibodies generated in C3^-/- or C1q^-/- mice following KELIg treatment and KEL RBC transfusion. Differences in RBC uptake were noted in mice lacking C3, with lower consumption by splenic and peripheral blood inflammatory monocytes. Finally, no alloantibodies were detected in the setting of a viral-like stimulus following KELIg treatment and KEL RBC transfusion in mice lacking complement receptors (CR1/2^-/-), narrowing key cells for immunoprophylaxis failure to those expressing these complement receptors. In-vitro studies showed complement fixed opsonized RBCs were significantly less likely to bind to B-cells from CR1/2^-/- than wild type mice, potentially implicating lowered B-cell activation threshold in the presence of complement as being responsible for these findings. We thus propose a two-hit model for inflammation-induced immunoprophylaxis failure, where the first “hit” is recipient inflammation and the second “hit” is complement production/sensing. These results may have translational relevance to antigen-antibody interactions in humans.

Renal Inflammation Induces Salt Sensitivity in Male db/db Mice through Dysregulation of ENaC

BACKGROUND

Hypertension is considered a major risk factor for the progression of diabetic kidney disease. Type 2 diabetes is associated with increased renal sodium reabsorption and salt-sensitive hypertension. Clinical studies show that men have higher risk than premenopausal women for the development of diabetic kidney disease. However, the renal mechanisms that predispose to salt sensitivity during diabetes and whether sexual dimorphism is associated with these mechanisms remains unknown.

METHODS

Female and male db/db mice exposed to a high-salt diet were used to analyze the progression of diabetic kidney disease and the development of hypertension.

RESULTS

Male, 34-week-old, db/db mice display hypertension when exposed to a 4-week high-salt treatment, whereas equivalently treated female db/db mice remain normotensive. Salt-sensitive hypertension in male mice was associated with no suppression of the epithelial sodium channel (ENaC) in response to a high-salt diet, despite downregulation of several components of the intrarenal renin-angiotensin system. Male db/db mice show higher levels of proinflammatory cytokines and more immune-cell infiltration in the kidney than do female db/db mice. Blocking inflammation, with either mycophenolate mofetil or by reducing IL-6 levels with a neutralizing anti-IL-6 antibody, prevented the development of salt sensitivity in male db/db mice.

CONCLUSIONS

The inflammatory response observed in male, but not in female, db/db mice induces salt-sensitive hypertension by impairing ENaC downregulation in response to high salt. These data provide a mechanistic explanation for the sexual dimorphism associated with the development of diabetic kidney disease and salt sensitivity.

Type 1 Interferon Gene Signature Promotes RBC Alloimmunization in a Lupus Mouse Model

Red blood cell (RBC) transfusion exposes recipients to hundreds of unmatched minor RBC antigens. This exposure can lead to production of alloantibodies that promote clinically significant hemolytic events. Multiple studies have reported an increased frequency of RBC alloimmunization in patients with autoimmunity. However, cellular and molecular mechanisms that underlie autoimmunity-induced alloimmunization have not been reported. Patients with systemic lupus erythematosus (SLE) have a high frequency of alloimmunization and express a type 1 interferon (IFNα/β) gene signature. Thus, we utilized the pristane-induced lupus mouse model to test the hypothesis that inflammation in lupus promotes RBC alloimmunization, and to examine the potential role of IFNα/β. Intraperitoneal injection of pristane, a hydrocarbon oil, led to autoantibody production, glomerulonephritis, and pulmonary hemorrhage in wild type (WT) mice. Pristane treatment significantly induced serum IFNα and expression of multiple interferon-stimulated genes (ISGs) in peripheral blood and peritoneal fluid cells, including inflammatory macrophages. Following transfusion with allogeneic RBCs expressing the KEL glycoprotein, pristane-treated WT mice produced significantly elevated levels of anti-KEL IgM and anti-KEL IgG, compared to untreated mice. Pristane induced comparable levels of inflammatory cells and cytokines in mice lacking the IFNα/β receptor (IFNAR1^–/–) or the IFNα/β-inducing transcriptions factors (IRF3/7^–/–), compared to WT mice. However, pristane-treated IFNAR1^–/– and IRF3/7^–/– mice failed to produce ISGs and produced significantly lower levels of transfusion-induced anti-KEL IgG, compared to WT mice. Thus, pristane induction of a lupus-like phenotype promoted alloimmunization to the KEL RBC antigen in an IFNα/β-dependent manner. To our knowledge, this is the first examination of molecular mechanisms contributing to RBC alloimmunization in a model of autoimmunity. These results warrant further investigation of the role of IFNα/β in alloimmunization to other RBC antigens and the contribution of the IFNα/β gene signature to the elevated frequency of alloimmunization in patients with SLE.

Characterization of circulating and cultured Tfh-like cells in sickle cell disease in relation to red blood cell alloimmunization status

BACKGROUND

People living with sickle cell disease (SCD) are prone to red blood cell (RBC) alloimmunization. We hypothesized that subjects with alloantibodies (responders) would have differences in circulating T-follicular helper (Tfh)-like cells compared to subjects without alloantibodies (non-responders).

MATERIALS AND METHODS

Peripheral blood mononuclear cells were collected from 28 subjects, including those with SCD and controls. Circulating CD4 T-cell subsets were first evaluated at baseline. CD4 T-cell subsets were also evaluated after naïve CD4 T-cells were differentiated into Tfh-like cells following in vitro culture with CD3/CD28 beads, IL-7, IL-12, and Activin A. Transfusion and alloantibody histories were extracted from the electronic medical record.

RESULTS

Non-responders had a lower percentage of CD45RA negative Tmemory cells than responders or controls (p<0.05). Notably, there were no differences in circulating Tfh-like cells between any group. However, naïve CD4 T-cells from subjects with SCD were more likely to express CXCR5 after in vitro culture than cells from controls. After culture, CXCR5 expressing cells from responders were more likely to express PD1 and ICOS (16.43 %, sd. 20.23) compared to non-responders (3.69 %, s.d. 3.09) or controls (2.78 %, s.d. 2.04).

DISCUSSION

The tendency for naïve CD4 T-cells from responders to differentiate into Tfh-like cells after in vitro culture may suggest these cells are prepared to assist B-cells with antibody production regardless of antigen specificity. Further studies are needed, but it is possible that these results may explain why some responders form RBC alloantibodies with multiple specificities, in addition to RBC autoantibodies and HLA alloantibodies.

Poly(I:C) causes failure of immunoprophylaxis to red blood cells expressing the KEL glycoprotein in mice

Polyclonal anti-D (Rh immune globulin [RhIg]) therapy has mitigated hemolytic disease of the newborn over the past half century, although breakthrough anti-D alloimmunization still occurs in some treated females. We hypothesized that antiviral responses may impact the efficacy of immunoprophylaxis therapy in a type 1 interferon (IFN)-dependent manner and tested this hypothesis in a murine model of KEL alloimmunization. Polyclonal anti-KEL immunoprophylaxis (KELIg) was administered to wild-type or knockout mice in the presence or absence of polyinosinic-polycytidilic acid (poly[I:C]), followed by the transfusion of murine red blood cells (RBCs) expressing the human KEL glycoprotein. Anti-KEL alloimmunization, serum cytokines, and consumption of the transfused RBCs were evaluated longitudinally. In some experiments, recipients were treated with type 1 IFN (IFN-α/β). Recipient treatment with poly(I:C) led to breakthrough anti-KEL alloimmunization despite KELIg administration. Recipient CD4+ T cells were not required for immunoprophylaxis efficacy at baseline, and modulation of the KEL glycoprotein antigen occurred to the same extent in the presence or absence of recipient inflammation. Under conditions where breakthrough anti-KEL alloimmunization occurred, KEL RBC consumption by inflammatory monocytes and serum monocyte chemoattractant protein-1 and interleukin-6 were significantly increased. Poly(I:C) or type I IFN administration was sufficient to cause breakthrough alloimmunization, with poly(I:C) inducing alloimmunization even in the absence of recipient type I IFN receptors. A better understanding of how recipient antiviral responses lead to breakthrough alloimmunization despite immunoprophylaxis may have translational relevance to instances of RhIg failure that occur in humans.

A potential association of an interferon gene signature with RBC alloimmunization in sickle cell disease

RBC transfusion can lead to the production of alloantibodies against RBC antigens. RBC alloimmunization can result in hemolytic events and severely limit the availability of compatible RBCs. Patients with sickle cell disease (SCD) have the highest rate of alloimmunization, compared to any other disease population. However, mechanisms underlying this increased frequency are poorly understood. Inflammation in the recipient has been shown to promote alloimmunization in mice and humans. In mouse transfusion models, type 1 interferons (IFNα/β) and interferon-stimulated genes (ISGs) have been shown to promote alloimmunization. Given the chronic inflammatory state in patients with SCD, we hypothesized that SCD patients may also have an IFNα/β gene signature that may contribute to the increased frequency of alloimmunization. To test this hypothesis, we measured the expression of multiple ISGs, including myxovirus resistance protein 1 (MxA) and Siglec-1 by whole blood immunoassay and flow cytometry, respectively. MxA and Siglec-1 levels were significantly elevated in SCD patients, compared to transfused controls. We then determined the degree to which ISG expression correlated with alloimmunization frequency by linear regression. There was a trend toward elevated MxA expression in patients with 1 or more alloantibodies. Patients with 2 or more alloantibodies had significantly elevated MxA compared to non-alloimmunized transfused patients. These findings suggest the presence of an IFNα/β gene signature in patients with SCD and warrant further investigation of the contribution of IFNα/β pathways toward the profoundly elevated frequency of RBC alloimmunization in patients with SCD.

Type 1 interferons promote allo-antibody responses to RBC transfusion in a lupus mouse model

Red blood cell (RBC) transfusion exposes recipients to hundreds of unmatched minor RBC antigens. This exposure can lead to production of alloantibodies that can cause potentially fatal hemolytic events. Prior studies have reported that inflammatory states, including autoimmune disease, in the transfusion recipient can promote alloimmunization. Recent studies reported that pro-inflammatory type 1 interferons (IFNα/β) regulated RBC alloimmunization in multiple mouse transfusion models. Interestingly, patients with systemic lupus erythematosus (SLE) have an increased frequency of alloimmunization and express an IFNα/β gene signature. Thus, we tested the hypothesis that IFNα/β activation in lupus promotes RBC alloimmunization by utilizing the pristane-induced lupus mouse model. Pristane significantly induced serum IFNα and expression of multiple interferon-stimulated genes (ISG) in peritoneal fluid cells, including inflammatory macrophages. Following transfusion with allogeneic RBCs expressing the KEL antigen, pristane-treated WT mice produced significantly elevated levels of anti-KEL IgM and anti-KEL IgG, compared to untreated mice. Pristane-treated mice lacking the IFNα/β receptor (IFNAR1−/−) or IFNα/β-inducing transcription factors (IRF3/7−/−) mice failed to produce ISGs and produced significantly lower levels of anti-KEL IgG compared to WT mice. In conclusion, pristane induction of a lupus-like phenotype promoted alloimmunization to the KEL RBC antigen in an IFNα/β-dependent manner. These results warrant further investigation of the role of IFNα/β in alloimmunization to other RBC antigens and the contribution of IFNα/β responses to the increased frequency of alloimmunization in patients with SLE.

Red blood cell alloimmunization is associated with lower expression of FcγR1 on monocyte subsets in patients with sickle cell disease

BACKGROUND

Despite the clinical significance of red blood cell (RBC) alloantibodies, there are currently no laboratory tests available to predict which patients may be at risk of antibody formation after transfusion exposure. Given their phagocytic and inflammatory functions, we hypothesized that differences in circulating monocytes may play a role in alloimmunization.

STUDY DESIGN AND METHODS

Forty-two adults with sickle cell disease (SCD) were recruited, with data extracted from the electronic medical record and peripheral blood analyzed by flow cytometry for total monocytes, monocyte subsets (CD14 high/CD16 low+ classical monocytes, CD14 high/CD16 high+ intermediate monocytes, and CD14 intermediate/CD16 high+ non-classical/inflammatory monocytes), and FcγR1 (CD64) expression. Thirteen "non-responder" patients (non-alloimmunized patients with documented RBC transfusion at the study institution) were compared to 20 alloimmunized "responder" patients, who had a total of 44 RBC alloantibodies identified.

RESULTS

There were no significant differences in the percentages of total monocytes, monocyte subsets, or measured cytokines between non-responders and responders. However, non-responders had higher CD64 expression on classical monocytes (MFI mean 3424 ± standard deviation 1141) compared to responders (MFI mean 2285 ± 1501), p = 0.029, and on intermediate monocytes (MFI mean 3720 ± 1191) compared to responders (MFI mean 2497 ± 1640), p = 0.033.

CONCLUSIONS

Monocytes and the inflammatory milieu increasingly are being appreciated to play a role in some complications of SCD. The differences in FcγR1 expression on monocyte subsets noted between responders and non-responders, which cannot be directly explained by the serum cytokines evaluated, warrant further investigation.

Type 1 IFN signaling critically regulates influenza-induced alloimmunization to transfused KEL RBCs in a murine model

BACKGROUND

Only a fraction of red blood cell (RBC) transfusion recipients form alloantibodies, and variables determining responsiveness or nonresponsiveness are poorly understood. We and others have previously shown in animal models that pretreatment with toll-like receptor agonists that mimic different types of infections impacts the magnitude or frequency of RBC alloantibody responses. We hypothesized that influenza infection, coexistent with transfusion, would impact responses to transfused RBCs in a manner dependent on Type 1(α/β) interferon (IFN) signaling and tested this in a murine model.

STUDY DESIGN AND METHODS

Wild-type mice or mice lacking the ability to respond to Type 1 IFN were infected with influenza prior to the transfusion of transgenic murine RBCs (K1) expressing the human KEL glycoprotein or the triple fusion HOD protein. Alloantibody responses were measured longitudinally after transfusion by flow cytometric crossmatch, and posttransfusion RBC recovery and survival was evaluated.

RESULTS

Influenza-infected mice transfused with K1 RBCs developed robust anti-KEL alloantibodies, whereas animals transfused in the absence of infection remained nonresponders; influenza-associated RBC alloimmunization was also observed after transfusion of HOD RBCs. Recipient Type 1 IFN production was critical to the mechanism of action of influenza-induced RBC alloimmunization, with alloimmunization being significantly decreased in mice unable to sense Type 1 IFN (through antibody blockade or genetic approaches).

CONCLUSION

These and other data suggest that Type 1 IFN responses to toll-like receptor agonists or infections regulate RBC alloantibody responses. Studies investigating whether such a correlation exists in humans may be informative.

Marginal Zone B Cells Induce Alloantibody Formation Following RBC Transfusion

Red blood cell (RBC) alloimmunization represents a significant immunological challenge for some patients. While a variety of immune constituents likely contribute to the initiation and orchestration of alloantibodies to RBC antigens, identification of key immune factors that initiate alloantibody formation may aid in the development of a therapeutic modality to minimize or prevent this process. To define the immune factors that may be important in driving alloimmunization to an RBC antigen, we determined the specific immune compartment and distinct cells that may initially engage transfused RBCs and facilitate subsequent alloimmunization. Our findings demonstrate that the splenic compartment is essential for formation of anti-KEL antibodies following KEL RBC transfusion. Within the spleen, transfused KEL RBCs are found within the marginal sinus, where they appear to specifically co-localize with marginal zone (MZ) B cells. Consistent with this, removal of MZ B cells completely prevented alloantibody formation following KEL RBC transfusion. While MZ B cells can mediate a variety of key downstream immune pathways, depletion of follicular B cells or CD4 T cells failed to similarly impact the anti-KEL antibody response, suggesting that MZ B cells may play a key role in the development of anti-KEL IgM and IgG following KEL RBC transfusion. These findings highlight a key contributor to KEL RBC-induced antibody formation, wherein MZ B cells facilitate antibody formation following RBC transfusion.

B cells require Type 1 interferon to produce alloantibodies to transfused KEL-expressing red blood cells in mice

BACKGROUND

Alloantibodies to red blood cell (RBC) antigens can cause significant hemolytic events. Prior studies have demonstrated that inflammatory stimuli in animal models and inflammatory states in humans, including autoimmunity and viremia, promote alloimmunization. However, molecular mechanisms underlying these findings are poorly understood. Given that Type 1 interferons (IFN-α/β) regulate antiviral immunity and autoimmune pathology, the hypothesis that IFN-α/β regulates RBC alloimmunization was tested in a murine model.

STUDY DESIGN AND METHODS

Leukoreduced murine RBCs expressing the human KEL glycoprotein were transfused into control mice (WT), mice lacking the unique IFN-α/β receptor (IFNAR1^-/- ), or bone marrow chimeric mice lacking IFNAR1 on specific cell populations. Anti-KEL IgG production, expressed as mean fluorescence intensity (MFI), and B-cell differentiation were examined.

RESULTS

Transfused WT mice produced anti-KEL IgG alloantibodies (peak response MFI, 50.4). However, the alloimmune response of IFNAR1^-/- mice was almost completely abrogated (MFI, 4.2; p < 0.05). The response of bone marrow chimeric mice lacking IFNAR1 expression in all hematopoietic cells or specifically in B cells was also diminished (MFI, 3.8 and 5.4, respectively, compared to control chimeras, MFI, 65.6; p < 0.01). Accordingly, transfusion-induced differentiation of IFNAR1^-/- B cells into germinal center B cells and plasma cells was significantly reduced, compared to WT B cells.

CONCLUSIONS

This study demonstrates that B cells require signaling from IFN-α/β to produce alloantibodies to the human KEL glycoprotein in mice. These findings provide a potential mechanistic basis for inflammation-induced alloimmunization. If these findings extend to human studies, patients with IFN-α/β-associated conditions may have an elevated risk of alloimmunization and benefit from personalized transfusion protocols.

CD4 Depletion or CD40L Blockade Results in Antigen-Specific Tolerance in a Red Blood Cell Alloimmunization Model

Approximately 3–10% of human red blood cell (RBC) transfusion recipients form alloantibodies to non-self, non-ABO blood group antigens expressed on donor RBCs, with these alloantibodies having the potential to be clinically significant in transfusion and pregnancy settings. However, the majority of transfused individuals never form detectable alloantibodies. Expanding upon observations that children initially transfused with RBCs at a young age are less likely to form alloantibodies throughout their lives, we hypothesized that “non-responders” may not only be ignorant of antigens on RBCs but instead tolerized. We investigated this question in a reductionist murine model, in which transgenic donors express the human glycophorin A (hGPA) antigen in an RBC-specific manner. Although wild-type mice treated with poly IC and transfused with hGPA RBCs generated robust anti-hGPA IgG alloantibodies that led to rapid clearance of incompatible RBCs, those transfused in the absence of an adjuvant failed to become alloimmunized. Animals depleted of CD4⁺ cells or treated with CD40L blockade prior to initial hGPA RBC exposure, in the presence of poly IC, failed to generate detectable anti-hGPA IgG alloantibodies. These non-responders to a primary transfusion remained unable to generate anti-hGPA IgG alloantibodies upon secondary hGPA exposure and did not prematurely clear transfused hGPA RBCs even after their CD4 cells had returned or their CD40L blockade had resolved. This observed tolerance was antigen (hGPA) specific, as robust IgG responses to transfused RBCs expressing a third-party antigen occurred in all studied groups. Experiments completed in an RBC alloimmunization model that allowed evaluation of antigen-specific CD4⁺ T-cells (HOD (hen egg lysozyme, ovalbumin, and human duffy^b)) demonstrated that CD40L blockade prevented the expansion of ovalbumin 323-339 specific T-cells after HOD RBC transfusion and also prevented germinal center formation. Taken together, our data suggest that recipients may indeed become tolerized to antigens expressed on RBCs, with the recipient’s immune status upon initial RBC exposure dictating future responses. Although questions surrounding mechanism(s) and sustainability of tolerance remain, these data lay the groundwork for future work investigating RBC immunity versus tolerance in reductionist models and in humans.

Type I IFN Is Necessary and Sufficient for Inflammation-Induced Red Blood Cell Alloimmunization in Mice

During RBC transfusion, production of alloantibodies against RBC non-ABO Ags can cause hemolytic transfusion reactions and limit availability of compatible blood products, resulting in anemia-associated morbidity and mortality. Multiple studies have established that certain inflammatory disorders and inflammatory stimuli promote alloimmune responses to RBC Ags. However, the molecular mechanisms underlying these findings are poorly understood. Type I IFNs (IFN-α/β) are induced in inflammatory conditions associated with increased alloimmunization. By developing a new transgenic murine model, we demonstrate that signaling through the IFN-α/β receptor is required for inflammation-induced alloimmunization. Additionally, mitochondrial antiviral signaling protein-mediated signaling through cytosolic pattern recognition receptors was required for polyinosinic-polycytidylic acid-induced IFN-α/β production and alloimmunization. We further report that IFN-α, in the absence of an adjuvant, is sufficient to induce RBC alloimmunization. These findings raise the possibility that patients with IFN-α/β-mediated conditions, including autoimmunity and viral infections, may have an increased risk of RBC alloimmunization and may benefit from personalized transfusion protocols and/or targeted therapies.

Bortezomib decreases the magnitude of a primary humoral immune response to transfused red blood cells in a murine model

BACKGROUND

Few therapeutic options currently exist to prevent or to mitigate transfusion-associated red blood cell (RBC) alloimmunization. We hypothesized that bortezomib, a proteasome inhibitor currently being utilized for HLA alloantibody and ADAMTS13 autoantibody reduction, may be beneficial in a transfusion setting. Herein, we utilized a reductionist murine model to test our hypothesis that bortezomib would decrease RBC alloimmune responses.

STUDY DESIGN AND METHODS

Wild-type mice were treated with bortezomib or saline and transfused with murine RBCs expressing the human KEL glycoprotein. Levels of anti-KEL immunoglobulins in transfusion recipients were measured by flow cytometry. The impact of bortezomib treatment on recipient plasma cells (PCs) and other immune cells was also assessed by flow cytometry and immunofluorescence.

RESULTS

After bortezomib treatment, mice had a 50% reduction in splenic white blood cells and a targeted reduction in marrow PCs. Mice treated with bortezomib before the transfusion of KEL RBCs became alloimmunized in three of three experiments, although their serum anti-KEL IgG levels were 2.6-fold lower than those in untreated mice. Once a primary antibody response was established, bortezomib treatment did not prevent an anamnestic response from occurring.

CONCLUSION

To the extent that these findings are generalizable to other RBC antigens and to humans, bortezomib monotherapy is unlikely to be of significant clinical benefit in a transfusion setting where complete prevention of alloimmunization is desirable. Given the impact on PCs, however, it remains plausible that bortezomib therapy may be beneficial for RBC alloimmunization prevention or mitigation if used in combination with other immunomodulatory therapies.

The Nlrp3 Inflammasome Does Not Regulate Alloimmunization to Transfused Red Blood Cells in Mice

Red blood cell (RBC) transfusions are essential for patients with hematological disorders and bone marrow failure syndromes. Despite ABO matching, RBC transfusions can lead to production of alloantibodies against “minor” blood group antigens. Non-ABO alloimmunization is a leading cause of transfusion-associated mortality in the U.S. Despite its clinical importance, little is known about the immunological factors that promote alloimmunization. Prior studies indicate that inflammatory conditions place patients at higher risk for alloimmunization. Additionally, co-exposure to pro-inflammatory pathogen associated molecular patterns (PAMPs) promotes alloimmunization in animal models, suggesting that RBC alloimmunization depends on innate immune cell activation. However, the specific innate immune stimuli and sensors that induce a T cell-dependent alloantibody response to transfused RBCs have not been identified. The NLRP3 inflammasome senses chemically diverse PAMPs and damage associated molecular patterns (DAMPs), including extracellular ATP and iron-containing heme. We hypothesized that activation of the NLRP3 inflammasome by endogenous DAMPs from RBCs promotes the alloimmune response to a sterile RBC transfusion. Using genetically modified mice lacking either NLRP3 or multiple downstream inflammasome response elements, we ruled out a role for the NLRP3 inflammasome or any Caspase-1 or -11 dependent inflammasome in regulating RBC alloantibody production to a model antigen.

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Transfusion of stored red blood cells (RBCs) induces proinflammatory cytokine production and alloimmunization to an RBC antigen in mice.

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Transfusion of stored RBCs, regardless of alloantigen expression, activates conventional dendritic cells in the spleen.

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NOD-like receptor (NLR) inflammasomes, including NLRP3, do not regulate inflammation and alloimmunization induced by stored RBCs.

Following a blood transfusion, the immune system may produce antibodies that have detrimental effects. To understand how the immune system recognizes factors in transfused blood, we examined the immune response of mice lacking important inflammatory molecules, called inflammasomes. The results demonstrate that inflammasomes do not affect the production of potentially harmful antibodies that recognize transfused red blood cells.

Cutting edge: mast cells critically augment myeloid-derived suppressor cell activity

Myeloid-derived suppressor cells (MDSCs) are primarily recognized for their immunosuppressive properties in malignant disease. However, their interaction with other innate immune cells and their regulation of immune responses, such as in parasitic infection, necessitate further characterization. We used our previously published mouse model of MDSC accumulation to examine the immunoregulatory role of MDSCs in B16 melanoma metastasis and Nippostrongylus brasiliensis infection. In this study, we demonstrate that the activity of MDSCs is dependent on the immune stimuli and subset induced. Monocytic MDSCs predictably suppressed antitumor immune responses but granulocytic MDSCs surprisingly enhanced the clearance of N. brasiliensis infection. Intriguingly, both results were dependent on MDSC interaction with mast cells (MCs), as demonstrated by adoptive-transfer studies in MC-deficient (Kit(Wsh)(/)(Wsh)) mice. These findings were further supported by ex vivo cocultures of MCs and MDSCs, indicating a synergistic increase in cytokine production. Thus, MCs can enhance both immunosuppressive and immunosupportive functions of MDSCs.

A disintegrin and metalloproteinase 10 regulates antibody production and maintenance of lymphoid architecture

A disintegrin and metalloproteinase 10 (ADAM10) is a zinc-dependent proteinase related to matrix metalloproteinases. ADAM10 has emerged as a key regulator of cellular processes by cleaving and shedding extracellular domains of multiple transmembrane receptors and ligands. We have developed B cell-specific ADAM10-deficient mice (ADAM10(B-/-)). In this study, we show that ADAM10 levels are significantly enhanced on germinal center B cells. Moreover, ADAM10(B-/-) mice had severely diminished primary and secondary responses after T-dependent immunization. ADAM10(B-/-) displayed impaired germinal center formation, had fewer follicular Th cells, decreased follicular dendritic cell networks, and altered chemokine expression in draining lymph nodes (LNs). Interestingly, when spleen and LN structures from immunized mice were analyzed for B and T cell localization, tissues structure was aberrant in ADAM10(B-/-) mice. Importantly, when ADAM10-deficient B cells were stimulated in vitro, they produced comparable Ab as wild type B cells. This result demonstrates that the defects in humoral responses in vivo result from inadequate B cell activation, likely because of the decrease in follicular Th cells and the changes in structure. Thus, ADAM10 is essential for the maintenance of lymphoid structure after Ag challenge.

A potential new target for asthma therapy: a disintegrin and metalloprotease 10 (ADAM10) involvement in murine experimental asthma

BACKGROUND

Elevated levels of CD23, a natural regulator of IgE production, have been shown to decrease the signs of lung inflammation in mice. The aim of this study was to study the involvement of ADAM10, the primary CD23 sheddase, in experimental asthma.

METHODS

ADAM10 was blocked either by using mice with a B-cell-specific deletion of the protease or pharmacologically by intranasal administration of selective ADAM10 inhibitors. Airway hypersensitivity (AHR) and bronchoaveolar lavage fluid (BALF) eosinophilia and select BALF cytokine/chemokine levels were then determined.

RESULTS

Using an IgE and mast cell-dependent mouse model, B-cell-specific ADAM10(-/-) mice (C57B/6 background) exhibited decreased eosinophilia and AHR when compared with littermate (LM) controls. Treatment of C57B/6 mice with selective inhibitors of ADAM10 resulted in an even further decrease in BALF eosinophilia, as compared with the ADAM10(-/-) animals. Even in the Th2 selective strain, Balb/c, BALF eosinophilia was reduced from 60% to 23% respectively. In contrast, when an IgE/mast cell-independent model of lung inflammation was used, the B-cell ADAM10(-/-) animals and ADAM10 inhibitor treated animals had lung inflammation levels that were similar to the controls.

CONCLUSIONS

These results thus show that ADAM10 is important in the progression of IgE-dependent lung inflammation. The use of the inhibitor further suggested that ADAM10 was important for maintaining Th2 levels in the lung. These results thus suggest that decreasing ADAM10 activity could be beneficial in controlling asthma and possibly other IgE-dependent diseases.

ADAM10 overexpression shifts lympho- and myelopoiesis by dysregulating site 2/site 3 cleavage products of Notch

Although the physiological consequences of Notch signaling in hematopoiesis have been extensively studied, the differential effects of individual notch cleavage products remain to be elucidated. Given that ADAM10 is a critical regulator of Notch and that its deletion is embryonically lethal, we generated mice that overexpress ADAM10 (ADAM10 transgenic [A10Tg]) at early stages of lympho- and myeloid development. Transgene expression resulted in abrogated B cell development, delayed T cell development in the thymus, and unexpected systemic expansion of CD11b(+)Gr-1(+) cells, also known as myeloid-derived suppressor cells. Mixed bone marrow reconstitution assays demonstrated that transgene expression altered hematopoiesis via a cell-intrinsic mechanism. Consistent with previously reported observations, we hypothesized that ADAM10 overexpression dysregulated Notch by uncoupling the highly regulated proteolysis of Notch receptors. This was confirmed using an in vitro model of hematopoiesis via culturing A10Tg hematopoietic Lineage(-)Sca-1(+)c-Kit(+) cells with OP-9 stromal cells in the presence or absence of Delta-like 1, a primary ligand for Notch. Blockade of the site 2 (S2) and site 3 (S3) cleavage of the Notch receptor demonstrated differential effects on hematopoiesis. OP9-DL1 cultures containing the ADAM10 inhibitor (S2 cleavage site) enhanced and rescued B cell development from wild-type and A10Tg Lineage(-)Sca-1(+)c-Kit(+) cells, respectively. In contrast, blockade of γ-secretase at the S3 cleavage site induced accumulation of the S2 product and consequently prevented B cell development and resulted in myeloid cell accumulation. Collectively, these findings indicate that the differential cleavage of Notch into S2 and S3 products regulated by ADAM10 is critical to hematopoietic cell-fate determination.

The emergence of ADAM10 as a regulator of lymphocyte development and autoimmunity

Proteolytic processing of transmembrane receptors and ligands can have a dramatic impact on cell signaling processes and subsequent cellular responses, including activation and differentiation. A member of the disintegrin and metalloproteinase family, ADAM10, has emerged as a prominent regulator of numerous receptors and ligands, including Notch and CD23. Here, we review studies resulting from the recent generation of ADAM10 conditional knockout mice which revealed a critical role for ADAM10 in Notch-dependent lymphocyte development. Additionally, we discuss results of numerous in vitro and ex vivo studies indicating that ADAM10 regulates the production of multiple secreted factors that contribute to autoimmune reactions.

CD23 Sheddase A disintegrin and metalloproteinase 10 (ADAM10) is also required for CD23 sorting into B cell-derived exosomes

The low affinity receptor for IgE, CD23, is the natural regulator of IgE synthesis, and understanding both the synthesis and the catabolism of CD23 are, thus, important issues. Membrane CD23 is cleaved by a disintegrin and metalloproteinase 10 (ADAM10) and this cleavage influences the ability of CD23 to regulate IgE. In contrast to the belief that cleavage is a cell surface event, endosomal neutralization with NH(4)Cl was found to dramatically reduce CD23 cleavage, suggesting that the majority of CD23 cleavage occurred subsequent to internalization in the endosomal pathway and not at the cell surface. In line with this, full-length CD23 was shown to be sorted in an ADAM10-dependent manner into exosomes. Greatly increased ADAM10-mediated CD23 cleavage was seen at endosomal pH. Additionally, the stalk region of CD23 was found to interact with ADAM10 and ADAM10 binding of CD23 was found to be protease independent. SPR analysis of the interaction indicated about a 10-fold increase in the R(max) at endosomal pH (pH 5.8) compared with pH 7.4, whereas the affinity of the interaction was not significantly changed. The R(max) change, combined with the increased cleavage at endosomal pH, indicates greater accessibility of the CD23 stalk region for ADAM10 at the lower pH. These results indicate a model where CD23 internalization results in ADAM10-dependent incorporation into exosomes, followed by partial cleavage of CD23 by ADAM10 prior to being released from the cell. The increased cleavage at endosomal pH also has implications for other ADAM10 substrates.

Regulation of the IgE response

IgE was the last of the five immunoglobulin classes to be discovered and is the antibody that is responsible for much of human type I allergic disease. This review summarizes recent developments with respect to control of IgE synthesis with an emphasis on Th2 (T helper 2) control and regulation using IgE Fc receptors.

ADAM10 is essential for Notch2-dependent marginal zone B cell development and CD23 cleavage in vivo

The proteolytic activity of a disintegrin and metalloproteinase 10 (ADAM10) regulates cell-fate decisions in Drosophila and mouse embryos. However, in utero lethality of ADAM10^−/− mice has prevented examination of ADAM10 cleavage events in lymphocytes. To investigate their role in B cell development, we generated B cell–specific ADAM10 knockout mice. Intriguingly, deletion of ADAM10 prevented development of the entire marginal zone B cell (MZB) lineage. Additionally, cleavage of the low affinity IgE receptor, CD23, was profoundly impaired, but subsequent experiments demonstrated that ADAM10 regulates CD23 cleavage and MZB development by independent mechanisms. Development of MZBs is dependent on Notch2 signaling, which requires proteolysis of the Notch2 receptor by a previously unidentified proteinase. Further experiments revealed that Notch2 signaling is severely impaired in ADAM10-null B cells. Thus, ADAM10 critically regulates MZB development by initiating Notch2 signaling. This study identifies ADAM10 as the in vivo CD23 sheddase and an important regulator of B cell development. Moreover, it has important implications for the treatment of numerous CD23- and Notch-mediated pathologies, ranging from allergy to cancer.

ADAM10 regulates B cell development and immunoglobulin production (138.6)

Members of the disintegrin and metalloprotease (ADAM) family can regulate lymphocyte development and function by cleaving membrane receptors. Previous studies showed that ADAM10 cleaves the B cell receptors, CD23 and Notch2, in vitro. However, in the absence of in vivo models, the physiologic impact of these events has not been examined. Thus, we generated B cell-specific ADAM10 KO and transgenic (Tg) mice to determine the role of ADAM10 in B cell development and function. Notch signaling regulates the differentiation of common lymphoid progenitors (CLPs) by promoting T cell fate. Accordingly, overexpression of ADAM10 in CLPs completely blocked B cell development. Deletion of ADAM10 from B cells in ADAM10fl/fl, CD19-cre+ mice prevented differentiation of marginal zone B cells, which is dependent on Notch2 signaling. In addition, expression of CD23 on ADAM10null B cells was dramatically elevated while levels of soluble CD23 were reduced in serum. As a result, IgE production by ADAM10fl/fl, CD19-cre+ mice was remarkably suppressed upon immunization, mimicking the response of CD23Tg mice. Finally, production of IgG1 and IgG2b was profoundly inhibited, suggesting a novel role for ADAM10 in class-switching to IgG. These results illustrate the physiologic role of ADAM10 in Notch signaling and CD23 cleavage. Moreover, they identify ADAM10 as a critical regulator of B cell development and immunoglobulin production.

AHA 0815066E, NIAID R01AI18697, NIAID U19AI077435

ADAMs 10 and 17 represent differentially regulated components of a general shedding machinery for membrane proteins such as transforming growth factor alpha, L-selectin, and tumor necrosis factor alpha

Protein ectodomain shedding is a critical regulator of many membrane proteins, including epidermal growth factor receptor-ligands and tumor necrosis factor (TNF)-alpha, providing a strong incentive to define the responsible sheddases. Previous studies identified ADAM17 as principal sheddase for transforming growth factor (TGF)-alpha and heparin-binding epidermal growth factor, but Ca++ influx activated an additional sheddase for these epidermal growth factor receptor ligands in Adam17-/- cells. Here, we show that Ca++ influx and stimulation of the P2X7R signaling pathway activate ADAM10 as sheddase of many ADAM17 substrates in Adam17-/- fibroblasts and primary B cells. Importantly, although ADAM10 can shed all substrates of ADAM17 tested here in Adam17-/- cells, acute treatment of wild-type cells with a highly selective ADAM17 inhibitor (SP26) showed that ADAM17 is nevertheless the principal sheddase when both ADAMs 10 and 17 are present. However, chronic treatment of wild-type cells with SP26 promoted processing of ADAM17 substrates by ADAM10, thus generating conditions such as in Adam17-/- cells. These results have general implications for understanding the substrate selectivity of two major cellular sheddases, ADAMs 10 and 17.

CD23: an overlooked regulator of allergic disease

Given the importance of immunoglobulin (Ig) E in mediating type I hypersensitivity, inhibiting IgE production would be a general way of controlling allergic disease. The low-affinity IgE receptor (FceRII or CD23) has long been proposed to be a natural regulator of IgE synthesis. In vivo research supporting this concept includes the observation that mice lacking CD23 have increased IgE production whereas mice overexpressing CD23 show strongly suppressed IgE responses. In addition, the finding that mice injected with monoclonal antibody directed against the coiled-coil stalk of CD23 have enhanced soluble CD23 release and increased IgE production demonstrates that full-length, trimeric CD23 is responsible for initiating an IgE inhibitory signal. The recent identification of ADAM10 (a disintegrin and metalloprotease) as the CD23 metalloprotease provides an alternative approach for designing therapies to combat allergic disease. Current data suggest that stabilizing cell-surface CD23 would be a natural means to decrease IgE synthesis and thus control type I hypersensitivity.

Transient T cell accumulation in lymph nodes and sustained lymphopenia in mice treated with FTY720

FTY720 (2-amino-2-[2-(4-octylphenyl)ethyl]propane-1,3-diol hydrochloride) is an orally available immunomodulatory agent that induces severe peripheral blood lymphopenia. Most studies of these lymphopenic effects have been limited to short-term exposure to FTY720. FTY720 alters the ability of lymphocytes to respond to sphingosine-1-phosphate (S1P) through S1P receptors, particularly S1P1. FTY720 affects different leukocyte populations and their trafficking through major lymphoid organs. We show the dynamics of CD4 T, CD8 T, and B lymphocyte recirculation in all major lymphoid compartments during 21-day FTY720 treatment of normal C57BL/6 mice. Following a transient increase in peripheral lymph nodes and Peyer's patches, lymphocyte recirculation reaches a new steady state. Other lymphoid organs show transient changes in lymphocyte composition with various patterns. At 21 days of FTY720 treatment, total body lymphocyte content is reduced by 20% and blood lymphocytes by 80%. Modeling suggests that the new steady state is due to a combination of reduced naive lymphocyte release from the thymus and a transient reduction of lymphocyte egress from lymph nodes. Our data indicate that the commonly held belief that FTY720 blocks lymphocyte egress from lymph nodes cannot fully explain the lymphocyte dynamics observed with prolonged treatment.