A PDZ-kinase allosteric relay mediates Par complex regulator exchange.

The Par complex polarizes the plasma membrane of diverse animal cells using the catalytic activity of atypical Protein Kinase C (aPKC) to pattern substrates. Two upstream regulators of the Par complex, Cdc42 and Par-3, bind separately to the complex to influence its activity in different ways. Each regulator binds a distinct member of the complex, Cdc42 to Par-6 and Par-3 to aPKC, making it unclear how they influence one another's binding. Here we report the discovery that Par-3 binding to aPKC is regulated by aPKC autoinhibition and link this regulation to Cdc42 and Par-3 exchange. The Par-6 PDZ domain activates aPKC binding to Par-3 via a novel interaction with the aPKC kinase domain. Cdc42 and Par-3 have opposite effects on the Par-6 PDZ-aPKC kinase interaction: while the Par-6 kinase domain interaction competes with Cdc42 binding to the complex, Par-3 binding is enhanced by the interaction. The differential effect of Par-3 and Cdc42 on the Par-6 PDZ interaction with the aPKC kinase domain forms an allosteric relay that connects their binding sites and is responsible for the negative cooperativity that underlies Par complex polarization and activity.

Immune Cell Profiles of Patients with Sickle Cell Disease during Parvovirus B19-Induced Transient Red Cell Aplasia.

Parvovirus B19 frequently infects children and targets cells of the erythroid lineage. Although healthy children rarely suffer severe disease, children with sickle cell disease (SCD) can experience transient red cell aplasia (TRCA), hospitalization, and life-threatening anemia upon first virus exposure. Given that children with SCD can also suffer chronic inflammation and that parvovirus B19 has been associated with autoimmune disease in other patient populations, we asked if parvovirus B19 infections contributed to acute and chronic immune abnormalities in children with SCD. Nineteen hospitalized patients with SCD and parvovirus B19-induced TRCA were evaluated. Blood tests included CBC, flow cytometry, and total antibody isotype analyses. Cytokine/chemokine analyses were performed on nasal wash (NW) samples, representing a common site of viral entry. Unusually high white blood cell count (WBC) and absolute neutrophil count (ANC) values were observed in some patients. A correlation matrix with Day 0 values from the 19 patients then identified two mutually exclusive phenotype clusters. Cluster 1 included WBC, ANC, absolute reticulocyte count (ARC), absolute lymphocyte count (ALC), lactate dehydrogenase (LDH), NW cytokines/chemokines, % naïve cells among B cell and T cell populations, and parvovirus-specific IgG. This cluster was negatively associated with virus load, suggesting a signature of successful adaptive immunity and virus control. Cluster 2 included virus load, % CD38+CD24- cells among CD19+ B cells (termed 'plasmablasts' for simplicity), % HLA-DRlow cells among CD19+ B cells, IgG4, and % memory phenotypes among B cell and T cell populations. Plasmablast percentages correlated negatively with parvovirus-specific IgG, possibly reflecting a non-specific trigger of cell activation. All patients were released from the hospital within 1 week after admission, and the highest WBC and ANC values were eventually reduced. Nonetheless, a concern remained that the acutely abnormal immune profiles caused by parvovirus B19 infections could exacerbate chronic inflammation in some patients. To avoid the numerous sequelae known to affect patients with SCD following hospitalizations with parvovirus B19, rapid development of a parvovirus B19 vaccine is warranted.

The cytokinetic midbody mediates asymmetric fate specification at mitotic exit during neural stem cell division.

Asymmetric cell division (ACD) is a broadly used mechanism for generating cellular diversity. Molecules known as fate determinants are segregated during ACD to generate distinct sibling cell fates, but determinants should not be activated until fate can be specified asymmetrically. Determinants could be activated after cell division but many animal cells complete division long after mitosis ends, raising the question of how activation could occur at mitotic exit taking advantage of the unique state plasticity at this time point. Here we show that the midbody, a microtubule-rich structure that forms in the intercellular bridge connecting nascent siblings, mediates fate determinant activation at mitotic exit in neural stem cells (NSCs) of the Drosophila larval brain. The fate determinants Prospero (Pros) and Brain tumor (Brat) are sequestered at the NSC membrane at metaphase but are released immediately following nuclear division when the midbody forms, well before cell division completes. The midbody isolates nascent sibling cytoplasms, allowing determinant release from the membrane via the cell cycle phosphatase String, without influencing the fate of the incorrect sibling. Our results identify the midbody as a key facilitator of ACD that allows asymmetric fate determinant activation to be initiated before division.

Association of Vitamin A and D Deficiencies with Infectious Outcomes in Children Undergoing Intensive Induction Therapy for Acute Lymphoblastic Leukemia.

OBJECTIVE: To evaluate the association between deficiency of vitamin A or D at diagnosis of pediatric acute lymphoblastic leukemia (ALL) and subsequent infectious complications during induction therapy. STUDY DESIGN: We conducted an institutional review board-approved, retrospective cohort study of children with newly diagnosed ALL from 2007 to 2017 at St. Jude Children's Research Hospital. We measured vitamin D, vitamin D binding protein, retinol binding protein as a surrogate for vitamin A, and immunoglobulin isotypes in serum obtained at ALL diagnosis, and we assessed the association between vitamin deficiencies or levels and infection-related complications during the 6-week induction phase using Cox regression models. RESULTS: Among 378 evaluable participants, vitamin A and D deficiencies were common (43% and 17%, respectively). Vitamin D deficiency was associated with higher risks of febrile neutropenia (adjusted hazard ratio [aHR], 1.7; P = .0072), clinically documented infection (aHR, 1.73; P = .025), and likely bacterial infection (aHR, 1.86; P = .008). Conversely, vitamin A deficiency was associated solely with a lower risk of sepsis (aHR, 0.19; P = .027). CONCLUSIONS: In this retrospective study, vitamin D deficiency was associated with an increased risk of common infection-related complications during induction therapy for ALL. Additional studies are warranted to evaluate whether vitamin D supplementation could mitigate this effect.

The Drosophila neuroblast polarity cycle at a glance.

Drosophila neural stem cells, or neuroblasts, rapidly proliferate during embryonic and larval development to populate the central nervous system. Neuroblasts divide asymmetrically to create cellular diversity, with each division producing one sibling cell that retains the neuroblast fate and another that differentiates into glia or neurons. This asymmetric outcome is mediated by the transient polarization of numerous factors to the cell cortex during mitosis. The powerful genetics and outstanding imaging tractability of the neuroblast make it an excellent model system for studying the mechanisms of cell polarity. This Cell Science at a Glance article and the accompanying poster explore the phases of the neuroblast polarity cycle and the regulatory circuits that control them. We discuss the key features of the cycle - the targeted recruitment of proteins to specific regions of the plasma membrane and multiple phases of highly dynamic actomyosin-dependent cortical flows that pattern both protein distribution and membrane structure.

Cooperative regulation of C1-domain membrane recruitment polarizes atypical protein kinase C.

Recruitment of the Par complex protein atypical protein kinase C (aPKC) to a specific membrane domain is a key step in the polarization of animal cells. While numerous proteins and phospholipids interact with aPKC, how these interactions cooperate to control its membrane recruitment has been unknown. Here, we identify aPKC's C1 domain as a phospholipid interaction module that targets aPKC to the membrane of Drosophila neural stem cells (NSCs). The isolated C1 binds the NSC membrane in an unpolarized manner during interphase and mitosis and is uniquely sufficient among aPKC domains for targeting. Other domains, including the catalytic module and those that bind the upstream regulators Par-6 and Bazooka, restrict C1's membrane targeting activity-spatially and temporally-to the apical NSC membrane during mitosis. Our results suggest that aPKC polarity results from cooperative activation of autoinhibited C1-mediated membrane binding activity.

How Estrogen, Testosterone, and Sex Differences Influence Serum Immunoglobulin Isotype Patterns in Mice and Humans.

Females often exhibit superior immune responses compared to males toward vaccines and pathogens such as influenza viruses and SARS-CoV-2. To help explain these differences, we first studied serum immunoglobulin isotype patterns in C57BL/6 male and female mice. We focused on IgG2b, an isotype that lends to virus control and that has been previously shown to be elevated in murine females compared to males. Improvements in IgG2b serum levels, and/or IgG2b ratios with other non-IgM isotypes, were observed when: (i) wildtype (WT) female mice were compared to estrogen receptor knockout mice (IgG2b, IgG2b/IgG3, IgG2b/IgG1, and IgG2b/IgA were all higher in WT mice), (ii) unmanipulated female mice were compared to ovariectomized mice (IgG2b/IgA was higher in unmanipulated animals), (iii) female mice were supplemented with estrogen in the context of an inflammatory insult (IgG2b and IgG2b/IgG3 were improved by estrogen supplementation), and (iv) male mice were supplemented with testosterone, a hormone that can convert to estrogen in vivo (IgG2b, IgG2b/IgG3, IgG2b/IgG1, and IgG2b/IgA were all improved by supplementation). We next examined data from three sets of previously described male and female human blood samples. In each case, there were higher IgG2 levels, and/or ratios of IgG2 with non-IgM isotypes, in human females compared to males. The effects of sex and sex hormones in the mouse and human studies were subtle, but frequent, suggesting that sex hormones represent only a fraction of the factors that influence isotype patterns. Examination of the gene loci suggested that upregulation of murine IgG2b or human IgG2 could be mediated by estrogen receptor binding to estrogen response elements and cytosine-adenine (CA) repeats upstream of respective Cγ genes. Given that murine IgG2b and human IgG2 lend to virus control, the isotype biases in females may be sufficient to improve outcomes following vaccination or infection. Future attention to sex hormone levels, and consequent immunoglobulin isotype patterns, in clinical trials are encouraged to support the optimization of vaccine and drug products for male and female hosts.

Retinol Binding Protein, Sunlight Hours, and the Influenza Virus-Specific Immune Response.

Healthy pediatric immune responses depend on adequate vitamin A and D levels. Relationships between solar ultraviolet B (UVB) radiation and vitamin D are well understood, while relationships between sunlight, vitamin A, and its serum escort, retinol binding protein (RBP), are not. A pediatric clinical study enrolled 2-8-year-old children at various times between September 2016 and March 2017, inclusive, in Memphis, Tennessee. A serum sample from each child was then assayed to examine the influence of season on vitamin levels. We found that RBP and RBP/retinol molar ratios decreased in winter months and RBP/retinol ratios correlated positively with the average daily sunlight hours per month. A food frequency questionnaire given to parents/guardians indicated a shift in dietary intake from plant-based foods to animal-based foods by children between winter and spring months. This translated to higher retinol and zinc (integral to RBP-transthyretin-retinol complexes) in the spring, perhaps explaining the seasonal influence on RBP/retinol. RBP and retinol were associated positively with IgG/IgM and IgA/IgM ratios. RBP and retinol, but not 25(OH)D, also correlated positively with influenza virus-specific antibodies. Retinol correlated negatively, while 25(OH)D correlated positively, with certain serum cytokine/chemokine levels. Significant differences in 25(OH)D, immunoglobulin ratios, and cytokines/chemokines were observed between black and white children. In sum, seasonal changes in dietary foods rich in retinol and zinc may have influenced RBP levels, which in turn influenced innate and adaptive immune responses. Results encourage routine monitoring and reporting of season, RBP, and vitamin levels in future clinical studies, as seasons may affect sunlight exposures, diet, vitamin levels, and immune protection against infectious disease.

Hypothesis: Low Vitamin A and D Levels Worsen Clinical Outcomes When Children with Sickle Cell Disease Encounter Parvovirus B19.

Human parvovirus B19 causes life-threatening anemia due to transient red cell aplasia (TRCA) in individuals with sickle cell disease (SCD). Children with SCD experiencing profound anemia during TRCA often require red blood cell transfusions and hospitalization. The prevalence of vitamin deficiencies in SCD is high and deficiencies are associated with respiratory and pain symptoms, but the effects of vitamins on acute infection with parvovirus B19 remain unclear. We performed a clinical study in which 20 SCD patients hospitalized with parvovirus B19 infections (Day 0) were monitored over a 120-day time course to query relationships between vitamins A and D and clinical outcomes. There were significant negative correlations between Day 0 vitamin levels and disease consequences (e.g., red blood cell transfusion requirements, inflammatory cytokines). There were significant positive correlations (i) between Day 0 vitamins and peak virus-specific antibodies in nasal wash, and (ii) between Day 0 virus-specific serum plus nasal wash antibodies and absolute reticulocyte counts. There was a significant negative correlation between Day 0 virus-specific serum antibodies and virus loads. To explain the results, we propose circular and complex mechanisms. Low baseline vitamin levels may weaken virus-specific immune responses to permit virus amplification and reticulocyte loss; consequent damage may further reduce vitamin levels and virus-specific immunity. While the complex benefits of vitamins are not fully understood, we propose that maintenance of replete vitamin A and D levels in children with SCD will serve as prophylaxis against parvovirus B19-induced TRCA complications.

Energetic determinants of animal cell polarity regulator Par-3 interaction with the Par complex.

The animal cell polarity regulator Par-3 recruits the Par complex (consisting of Par-6 and atypical PKC, aPKC) to specific sites on the cell membrane. Although numerous physical interactions have been reported between Par-3 and the Par complex, it is unclear how each of these interactions contributes to the overall binding. Using a purified, intact Par complex and a quantitative binding assay, here, we found that the energy required for this interaction is provided by the second and third PDZ protein interaction domains of Par-3. We show that both Par-3 PDZ domains bind to the PDZ-binding motif of aPKC in the Par complex, with additional binding energy contributed from the adjacent catalytic domain of aPKC. In addition to highlighting the role of Par-3 PDZ domain interactions with the aPKC kinase domain and PDZ-binding motif in stabilizing Par-3-Par complex assembly, our results indicate that each Par-3 molecule can potentially recruit two Par complexes to the membrane during cell polarization. These results provide new insights into the energetic determinants and structural stoichiometry of the Par-3-Par complex assembly.

Novel Surrogate Neutralizing Assay Supports Parvovirus B19 Vaccine Development for Children with Sickle Cell Disease.

Children with sickle cell disease (SCD) suffer life-threatening transient aplastic crisis (TAC) when infected with parvovirus B19. In utero, infection of healthy fetuses may result in anemia, hydrops, and death. Unfortunately, although promising vaccine candidates exist, no product has yet been licensed. One barrier to vaccine development has been the lack of a cost-effective, standardized parvovirus B19 neutralization assay. To fill this void, we evaluated the unique region of VP1 (VP1u), which contains prominent targets of neutralizing antibodies. We discovered an antigenic cross-reactivity between VP1 and VP2 that, at first, thwarted the development of a surrogate neutralization assay. We overcame the cross-reactivity by designing a mutated VP1u (VP1uAT) fragment. A new VP1uAT ELISA yielded results well correlated with neutralization (Spearman's correlation coefficient = 0.581; p = 0.001), superior to results from a standard clinical diagnostic ELISA or an ELISA with virus-like particles. Virus-specific antibodies from children with TAC, measured by the VP1uAT and neutralization assays, but not other assays, gradually increased from days 0 to 120 post-hospitalization. We propose that this novel and technically simple VP1uAT ELISA might now serve as a surrogate for the neutralization assay to support rapid development of a parvovirus B19 vaccine.

Month of Influenza Virus Vaccination Influences Antibody Responses in Children and Adults.

The improvement of influenza virus vaccines and the development of a universal product have been long-standing goals in pre-clinical and clinical research. To meet these goals and to understand the strengths and weaknesses of current vaccine strategies, scientists routinely study human responses toward seasonal influenza vaccines. This research is frequently performed with clinical samples taken throughout an influenza season, often without strict attention to the month of inoculation for each study participant. Here, we ask how the timing of vaccination affects outcomes. Results demonstrate significant influences of inoculation month on the immune response. During the progression from fall to winter months, there are changes in host lifestyles and in the frequencies of clinical/sub-clinical viral infections that can significantly alter vaccine immunogenicity. We now recommend routine assessment of inoculation month during clinical studies to inform data interpretation and expedite the development of successful vaccines. This recommendation is pertinent to numerous vaccine development efforts within and outside the influenza virus field.

Effect of Vitamin A Deficiency in Dysregulating Immune Responses to Influenza Virus and Increasing Mortality Rates After Bacterial Coinfections.

BACKGROUND: Secondary bacterial coinfections are ranked as a leading cause of hospitalization and morbid conditions associated with influenza. Because vitamin A deficiency (VAD) and insufficiency are frequent in both developed and developing countries, we asked how VAD influences coinfection severity. METHODS: VAD and control mice were infected with influenza virus for evaluation of inflammatory cytokines, cellular immune responses, and viral clearance. Influenza-infected mice were coinfected with Streptococcus pneumoniae to study weight loss and survival. RESULTS: Naive VAD mouse lungs exhibited dysregulated immune function. Neutrophils were enhanced in frequency and there was a significant reduction in RANTES (regulated on activation of normal T cells expressed and secreted), a chemokine instrumental in T-cell homing and recruitment. After influenza virus infection, VAD mice experienced failures in CD4+ T-cell recruitment and B-cell organization into lymphoid structures in the lung. VAD mice exhibited higher viral titers than controls and slow viral clearance. There were elevated levels of inflammatory cytokines and innate cell subsets in the lungs. However, arginase, a marker of alternatively activated M2 macrophages, was rare. When influenza-infected VAD animals were exposed to bacteria, they experienced a 100% mortality rate. CONCLUSION: Data showed that VAD dysregulated the immune response. Consequently, secondary bacterial infections were 100% lethal in influenza-infected VAD mice.

Questioning Cause and Effect: Children with Severe Asthma Exhibit High Levels of Inflammatory Biomarkers Including Beta-Hexosaminidase, but Low Levels of Vitamin A and Immunoglobulins.

Asthma affects over 8% of the pediatric population in the United States, and Memphis, Tennessee has been labeled an asthma capital. Plasma samples were analyzed for biomarker profiles from 95 children with severe asthma and 47 age-matched, hospitalized nonasthmatic controls at Le Bonheur Children's Hospital in Memphis, where over 4000 asthmatics are cared for annually. Asthmatics exhibited significantly higher levels of periostin, surfactant protein D, receptor for advanced glycation end products and ß-hexosaminidase compared to controls. Children with severe asthma had lower levels of IgG1, IgG2 and IgA, and higher levels of IgE compared to controls, and approximately half of asthmatics exhibited IgG1 levels that were below age-specific norms. Vitamin A levels, measured by the surrogate retinol-binding protein, were insufficient or deficient in most asthmatic children, and correlated positively with IgG1. Which came first, asthma status or low levels of vitamin A and immunoglobulins? It is likely that inflammatory disease and immunosuppressive drugs contributed to a reduction in vitamin A and immunoglobulin levels. However, a nonmutually exclusive hypothesis is that low dietary vitamin A caused reductions in immune function and rendered children vulnerable to respiratory disease and consequent asthma pathogenesis. Continued attention to nutrition in combination with the biomarker profile is recommended to prevent and treat asthma in vulnerable children.

Consequences of Vitamin A Deficiency: Immunoglobulin Dysregulation, Squamous Cell Metaplasia, Infectious Disease, and Death.

Vitamin A is an important regulator of immune protection, but it is often overlooked in studies of infectious disease. Vitamin A binds an array of nuclear receptors (e.g., retinoic acid receptor, peroxisome proliferator-activated receptor, retinoid X receptor) and influences the barrier and immune cells responsible for pathogen control. Children and adults in developed and developing countries are often vitamin A-deficient or insufficient, characteristics associated with poor health outcomes. To gain a better understanding of the protective mechanisms influenced by vitamin A, we examined immune factors and epithelial barriers in vitamin A deficient (VAD) mice, vitamin D deficient (VDD) mice, double deficient (VAD+VDD) mice, and mice on a vitamin-replete diet (controls). Some mice received insults, including intraperitoneal injections with complete and incomplete Freund's adjuvant (emulsified with PBS alone or with DNA + Fus-1 peptide) or intranasal inoculations with Sendai virus (SeV). Both before and after insults, the VAD and VAD+VDD mice exhibited abnormal serum immunoglobulin isotypes (e.g., elevated IgG2b levels, particularly in males) and cytokine/chemokine patterns (e.g., elevated eotaxin). Even without insult, when the VAD and VAD+VDD mice reached 3-6 months of age, they frequently exhibited opportunistic ascending bacterial urinary tract infections. There were high frequencies of nephropathy (squamous cell hyperplasia of the renal urothelium, renal scarring, and ascending pyelonephritis) and death in the VAD and VAD+VDD mice. When younger VAD mice were infected with SeV, the predominant lesion was squamous cell metaplasia of respiratory epithelium in lungs and bronchioles. Results highlight a critical role for vitamin A in the maintenance of healthy immune responses, epithelial cell integrity, and pathogen control.

Vitamin A Corrects Tissue Deficits in Diet-Induced Obese Mice and Reduces Influenza Infection After Vaccination and Challenge.

OBJECTIVE: Individuals with obesity suffer from an increased susceptibility to severe respiratory viral infections and respond poorly to vaccinations, making it imperative to identify interventions. Recent evidence suggesting that obesity leads to tissue-specific vitamin A deficiency led to an investigation of whether high-dose oral vitamin A, a treatment used for remediating vitamin A deficiency in developing countries, could correct obesity-associated tissue deficits. METHODS: Adult C57BL/6 diet-induced obese mice were supplemented with vitamin A for 4 weeks. A subset of mice were then vaccinated with inactivated influenza virus and challenged. Following supplementation, tissue vitamin A levels, lung immune cell composition, blood inflammatory cytokines, antibody responses, and viral clearance were evaluated. RESULTS: Supplementation significantly improved vitamin A levels in lung and adipose tissues in diet-induced obese mice. Additionally, supplementation decreased inflammatory cytokines in the blood and altered the lung immune environment. Importantly, vaccinated, vitamin A-treated diet-induced obese mice exhibited improved antibody responses and significantly reduced viral loads post challenge compared with PBS-treated mice. CONCLUSIONS: Results demonstrate a low-cost intervention that may correct vitamin A tissue deficits and help control respiratory viral infections in individuals with obesity.

Nuclear Receptors, Ligands and the Mammalian B Cell.

Questions concerning the influences of nuclear receptors and their ligands on mammalian B cells are vast in number. Here, we briefly review the effects of nuclear receptor ligands, including estrogen and vitamins, on immunoglobulin production and protection from infectious diseases. We describe nuclear receptor interactions with the B cell genome and the potential mechanisms of gene regulation. Attention to the nuclear receptor/ligand regulation of B cell function may help optimize B cell responses, improve pathogen clearance, and prevent damaging responses toward inert- and self-antigens.

Persistent hypogammaglobulinemia in pediatric solid organ transplant recipients.

INTRODUCTION: Hypogammaglobulinemia has not been well studied in pediatric solid organ transplant (SOT) recipients. We evaluated plasma immunoglobulin (Ig) and lymphocyte phenotypes among 31 pediatric heart and kidney recipients for two years post-transplant and from 10 non-transplanted children. METHODS: Plasma IgM, IgG, and IgA were quantified by immunoturbidimetric assays, IgG subclasses were quantified by bead-based multiplex immunoassay, and lymphocyte phenotypes were assessed by flow cytometry. RESULTS: Median age at transplant for SOT recipients was similar to that of the control cohort (15 vs. 12.5 years, respectively; P = .61). Mean plasma IgG and IgM levels for SOT recipients fell significantly below the control cohort means by 1 month post-transplant (P < .001 for both) and remained lower than control levels at 12-18 months post-transplant. Heart recipients had lower frequencies of a CD4+ naïve T lymphocytes relative to kidney recipients. CONCLUSIONS: Hypogammaglobulinemia was prevalent and persistent among pediatric SOT recipients and may be secondary to immunosuppressive medications, as well as loss of thymus tissue and CD45RA+   CD4+ T cells in heart recipients. Limitations of our study include but are not limited to small sample size from a single center, lack of samples for all participants at every time point, and lack of peripheral blood mononuclear cell samples for the non-transplanted cohort.

Vaccine Design Informed by Virus-Induced Immunity.

When an individual is exposed to a viral pathogen for the first time, the adaptive immune system is naive and cannot prevent virus replication. The consequence may be severe disease. At the same time, the host may rapidly generate a pathogen-specific immune response that will prevent disease if the virus is encountered again. Parvovirus B19 provides one such example. Children with sickle cell disease can experience life-threatening transient aplastic crisis when first exposed to parvovirus B19, but an effective immune response confers lifelong protection. We briefly examine the induction and benefits of virus-induced immunity. We focus on three human viruses for which there are no licensed vaccines (respiratory syncytial virus, human immunodeficiency virus type 1, and parvovirus B19) and consider how virus-induced immunity may inform successful vaccine design.

From Influenza Virus Infections to Lupus: Synchronous Estrogen Receptor α and RNA Polymerase II Binding Within the Immunoglobulin Heavy Chain Locus.

Males and females respond to pathogens differently and exhibit significantly different frequencies of autoimmune disease. For example, vaccinated adult females control influenza virus better than males, but females suffer systemic lupus erythematosus at a 9:1 frequency compared to males. Numerous explanations have been offered for these sex differences, but most have involved indirect mechanisms by which estrogen, a nuclear hormone, modifies cell barriers or immunity. In search of a direct mechanism, we examined the binding of estrogen receptor α (ERα), a class I nuclear hormone receptor, to the immunoglobulin heavy chain locus. Here, we show that in purified murine B cells, ERα and RNA polymerase II (RNA Pol II) exhibit extraordinarily similar DNA binding patterns. We further demonstrate that ERα preferentially binds adenosine-cytidine (AC)-repeats in the immunoglobulin heavy chain locus when supplemental estrogen is added to purified, lipopolysaccharide-activated B cells. Based on these and previous data, we hypothesize that (i) estrogen guides the binding of ERα and its RNA Pol II partner within the locus, which in turn instructs sterile transcription and class switch recombination (CSR), (ii) ERα binding to AC-repeats modifies the DNA architecture and loops associated with CSR, and (iii) by these mechanisms, estrogen instructs antibody expression. By targeting ERα-DNA interactions in the immunoglobulin heavy chain locus, clinicians may ultimately enhance antibody responses in the context of infectious diseases and reduce antibody responses in the context of allergic or autoimmune reactions.