Progesterone inhibits activation-induced deaminase by binding to the promoter

A humanized mouse that mounts mature class-switched, hypermutated and neutralizing antibody responses

Humanized mice are limited in terms of modeling human immunity, particularly with regards to antibody responses. Here we constructed a humanized (THX) mouse by grafting non-γ-irradiated, genetically myeloablated KitW-41J mutant immunodeficient pups with human cord blood CD34+ cells, followed by 17β-estradiol conditioning to promote immune cell differentiation. THX mice reconstitute a human lymphoid and myeloid immune system, including marginal zone B cells, germinal center B cells, follicular helper T cells and neutrophils, and develop well-formed lymph nodes and intestinal lymphoid tissue, including Peyer's patches, and human thymic epithelial cells. These mice have diverse human B cell and T cell antigen receptor repertoires and can mount mature T cell-dependent and T cell-independent antibody responses, entailing somatic hypermutation, class-switch recombination, and plasma cell and memory B cell differentiation. Upon flagellin or Pfizer coronavirus disease 2019 mRNA vaccination, THX mice mount neutralizing antibody responses to Salmonella or severe acute respiratory syndrome coronavirus 2 Spike S1 receptor-binding domain, with blood incretion of human cytokines, including APRIL, BAFF, TGF-β, IL-4 and IFN-γ, all at physiological levels. These mice can also develop lupus autoimmunity after pristane injection. By leveraging estrogen activity to support human immune cell differentiation and maturation of antibody responses, THX mice provide a platform to study the human immune system and to develop human vaccines and therapeutics.

Hallmarks of sex bias in immuno-oncology: mechanisms and therapeutic implications

Sex differences are present across multiple non-reproductive organ cancers, with male individuals generally experiencing higher incidence of cancer with poorer outcomes. Although some mechanisms underlying these differences are emerging, the immunological basis is not well understood. Observations from clinical trials also suggest a sex bias in conventional immunotherapies with male individuals experiencing a more favourable response and female individuals experiencing more severe adverse events to immune checkpoint blockade. In this Perspective article, we summarize the major biological hallmarks underlying sex bias in immuno-oncology. We focus on signalling from sex hormones and chromosome-encoded gene products, along with sex hormone-independent and chromosome-independent epigenetic mechanisms in tumour and immune cells such as myeloid cells and T cells. Finally, we highlight opportunities for future studies on sex differences that integrate sex hormones and chromosomes and other emerging cancer hallmarks such as ageing and the microbiome to provide a more comprehensive view of how sex differences underlie the response in cancer that can be leveraged for more effective immuno-oncology approaches.

Mechanisms and consequences of sex differences in immune responses

Biological sex differences refer to differences between males and females caused by the sex chromosome complement (that is, XY or XX), reproductive tissues (that is, the presence of testes or ovaries), and concentrations of sex steroids (that is, testosterone or oestrogens and progesterone). Although these sex differences are binary for most human individuals and mice, transgender individuals receiving hormone therapy, individuals with genetic syndromes (for example, Klinefelter and Turner syndromes) and people with disorders of sexual development reflect the diversity in sex-based biology. The broad distribution of sex steroid hormone receptors across diverse cell types and the differential expression of X-linked and autosomal genes means that sex is a biological variable that can affect the function of all physiological systems, including the immune system. Sex differences in immune cell function and immune responses to foreign and self antigens affect the development and outcome of diverse diseases and immune responses.

Polycystic ovary syndrome and thyroid disorder: a comprehensive narrative review of the literature

Background

Published data on the relationship between polycystic ovary syndrome (PCOS) and thyroid dysfunction are sparse and confusing.

Objective

To comprehensively review data available in the literature regarding the relationship between PCOS and the thyroid function, and its abnormalities.

Methods

Nine main areas of interest were identified and analyzed according to the available evidence: 1) Evaluation of thyroid function for PCOS diagnosis; 2) Epidemiology data on thyroid function/disorders in patients with PCOS, and vice versa; 3) Experimental data supporting the relationship between thyroid function/disorders and PCOS; 4) Effects of thyroid function/disorders on PCOS features, and vice versa; 5) Effect of thyroid alterations on the cardiometabolic risk in women with PCOS; 6) Effect of thyroid abnormalities on reproductive outcomes in women with PCOS; 7) Relationship between thyroid function/abnormalities in patients with PCOS who are undergoing fertility treatment; 8) Effect of treatments for thyroid diseases on PCOS; and 9) Effect of treatments for PCOS on thyroid function. An extensive literature search for specific keywords was performed for articles published from 1970 to March 2023 using PubMed and Web of Science. Data were reported in a narrative fashion.

Results

PCOS is a diagnosis of exclusion for which diagnosis is possible only after excluding disorders that mimic the PCOS phenotype, including thyroid dysfunctions. However, the tests and the cutoff values used for this are not specified. Many experimental and clinical data suggest a relationship between perturbations of the thyroid function and PCOS. Direct and unequivocal evidence on the effects of thyroid function/disorders on PCOS features are lacking. High thyroid-stimulating hormone levels and subclinical hypothyroidism may be associated with significant worsening of several intermediate endpoints of cardiometabolic risk in women with PCOS. Thyroid abnormalities may worsen reproductive outcomes, especially in patients undergoing fertility treatment. To date, there are no data demonstrating the efficacy of thyroid medications on fertility and cardiometabolic risk in women with PCOS. Lifestyle modification changes, metformin, and vitamin D seem to improve thyroid function in the general population.

Conclusion

PCOS and thyroid disorders are closely related, and their coexistence may identify patients with a higher reproductive and metabolic risk. Regular screening for thyroid function and thyroid-specific autoantibodies in women with PCOS, particularly before and during pregnancy, is highly recommended.

The off-target effects of AID in carcinogenesis

Activation-induced cytidine deaminase (AID) plays a crucial role in promoting B cell diversification through somatic hypermutation (SHM) and class switch recombination (CSR). While AID is primarily associated with the physiological function of humoral immune response, it has also been linked to the initiation and progression of lymphomas. Abnormalities in AID have been shown to disrupt gene networks and signaling pathways in both B-cell and T-cell lineage lymphoblastic leukemia, although the full extent of its role in carcinogenesis remains unclear. This review proposes an alternative role for AID and explores its off-target effects in regulating tumorigenesis. In this review, we first provide an overview of the physiological function of AID and its regulation. AID plays a crucial role in promoting B cell diversification through SHM and CSR. We then discuss the off-target effects of AID, which includes inducing mutations of non-Igs, epigenetic modification, and the alternative role as a cofactor. We also explore the networks that keep AID in line. Furthermore, we summarize the off-target effects of AID in autoimmune diseases and hematological neoplasms. Finally, we assess the off-target effects of AID in solid tumors. The primary focus of this review is to understand how and when AID targets specific gene loci and how this affects carcinogenesis. Overall, this review aims to provide a comprehensive understanding of the physiological and off-target effects of AID, which will contribute to the development of novel therapeutic strategies for autoimmune diseases, hematological neoplasms, and solid tumors.

The hormonal physiology of immune components in breast milk and their impact on the infant immune response

During pregnancy, the maternal body undergoes a considerable transformation regarding the anatomy, metabolism, and immune profile that, after delivery, allows for protection and nourishment of the offspring via lactation. Pregnancy hormones are responsible for the development and functionality of the mammary gland for breast milk production, but little is known about how hormones control its immune properties. Breast milk composition is highly dynamic, adapting to the nutritional and immunological needs that the infant requires in the first months of life and is responsible for the main immune modeling of breastfed newborns. Therefore, alterations in the mechanisms that control the endocrinology of mammary gland adaptation for lactation could disturb the properties of breast milk that prepare the neonatal immune system to respond to the first immunologic challenges. In modern life, humans are chronically exposed to endocrine disruptors (EDs), which alter the endocrine physiology of mammals, affecting the composition of breast milk and hence the neonatal immune response. In this review, we provide a landscape of the possible role of hormones in the control of passive immunity transferred by breast milk and the possible effect of maternal exposure to EDs on lactation, as well as their impacts on the development of neonatal immunity.

Sex bias in lymphocytes: Implications for autoimmune diseases

Autoimmune diseases are characterized by a significant sex dimorphism, with women showing increased susceptibility to disease. This is, at least in part, due to sex-dependent differences in the immune system that are influenced by the complex interplay between sex hormones and sex chromosomes, with contribution from sociological factors, diet and gut microbiota. Sex differences are evident in the number and function of lymphocyte populations. Women mount a stronger pro-inflammatory response than males, with increased lymphocyte proliferation, activation and pro-inflammatory cytokine production, whereas men display expanded regulatory cell subsets. Ageing alters the immune landscape of men and women in differing ways, resulting in changes in autoimmune disease susceptibility. Here we review the current literature on sex differences in lymphocyte function, the factors that influence this, and the implications for autoimmune disease. We propose that improved understanding of sex bias in lymphocyte function can provide sex-specific tailoring of treatment strategies for better management of autoimmune diseases.

Greater Breadth of Vaccine-Induced Immunity in Females than Males Is Mediated by Increased Antibody Diversity in Germinal Center B Cells

Inactivated influenza vaccines induce greater antibody responses in females than males among both humans and mice. To test the breadth of protection, we used recombinant mouse-adapted A/California/2009 (maA/Cal/09) H1N1 viruses containing mutations at one (1M), two (2M), or three (3M) antigenic sites, in addition to a virus containing the 1M mutation and a substitution of the Ca2 antigenic site (Sub) with one derived from an H5 hemagglutinin (HA) to challenge mice of both sexes. Following maA/Cal/09 vaccination, females produced greater virus-specific, class-switched total IgG and IgG2c antibodies against the vaccine and all mutant viruses, and antibodies from females recognized a greater number of unique, linear HA epitopes than did antibodies from males. While females had greater neutralizing antibody titers against the vaccine virus, both sexes showed a lower neutralization capacity against mutant viruses. After virus challenge, vaccinated females had lower pulmonary virus titers and reduced morbidity than males for the 1M and 2M viruses, but not the Sub virus. Females generated greater numbers of germinal center (GC) B cells containing superior somatic hypermutation (SHM) frequencies than vaccinated males. Deletion of activation-induced cytidine deaminase (Aicda) eliminated female-biased immunity and protection against the 2M virus. Harnessing methods to improve GC B cell responses and frequencies of SHM, especially in males, should be considered in the development of universal influenza vaccines. IMPORTANCE Adult females develop greater antibody responses to influenza vaccines than males. We hypothesized that female-biased immunity and protection would be dependent on the extent of virus diversity as well as molecular mechanisms in B cells which constrain the breadth of epitope recognition. We developed a panel of mouse-adapted (ma) A/Cal/09 viruses that had mutations in the immunodominant hemagglutinin. Following vaccination against maA/Cal/09, females were better able to neutralize maA/Cal/09 than males, but neutralization of mutant maA/Cal/09 viruses was equally poor in both sexes, despite vaccinated females being better protected against these viruses. Vaccinated females benefited from the greater production of class-switched, somatically hypermutated antibodies generated in germinal center B cells, which increased recognition of more diverse maA/Cal/09 hemagglutinin antigen epitopes. Female-biased protection against influenza infection and disease after vaccination is driven by differential mechanisms in males versus females and should be considered in the design of novel vaccine platforms.

Sex Bias and Autoimmune Diseases

Sex bias in immune function has been well-described, and women have been shown to counter immunologically stimulating phenomena such as infection, malignancy, and trauma with more protective responses than men. Heightened immunity in women may also result in a predisposition for loss of self-tolerance and development of autoimmunity, reflected by the overwhelming female sex bias of patients with autoimmune diseases. In this review, we discuss the postulated evolutionary etiologies for sexual dimorphism in immunity. We also review the molecular mechanisms underlying divergent immune responses in men and women, including sex hormone effects, X chromosome dosage, and autosomal sex-biased genes. With increasing evidence that autoimmune disease susceptibility is influenced by numerous hormonal and genetic factors, a comprehensive understanding of these topics may facilitate the development of much-needed targeted therapeutics.

Sex steroids and autoimmune rheumatic diseases: state of the art

In autoimmune rheumatic diseases, oestrogens can stimulate certain immune responses (including effects on B cells and innate immunity), but can also have dose-related anti-inflammatory effects on T cells, macrophages and other immune cells. By contrast, androgens and progesterone have predominantly immunosuppressive and anti-inflammatory effects. Hormone replacement therapies and oral contraception (and also pregnancy) enhance or decrease the severity of autoimmune rheumatic diseases at a genetic or epigenetic level. Serum androgen concentrations are often low in men and in women with autoimmune rheumatic diseases, suggesting that androgen-like compounds might be a promising therapeutic approach. However, androgen-to-oestrogen conversion (known as intracrinology) is enhanced in inflamed tissues, such as those present in patients with autoimmune rheumatic diseases. In addition, it is becoming evident that the gut microbiota differs between the sexes (known as the microgenderome) and leads to sex-dependent genetic and epigenetic changes in gastrointestinal inflammation, systemic immunity and, potentially, susceptibility to autoimmune or inflammatory rheumatic diseases. Future clinical research needs to focus on the therapeutic use of androgens and progestins or their downstream signalling cascades and on new oestrogenic compounds such as tissue-selective oestrogen complex to modulate altered immune responses.

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.

Frontline Science: A reduction in DHA-derived mediators in male obesity contributes toward defects in select B cell subsets and circulating antibody

Obesity dysregulates B cell populations, which contributes toward poor immunological outcomes. We previously reported that differing B cell subsets are lowered in the bone marrow of obese male mice. Here, we focused on how lipid metabolites synthesized from docosahexaenoic acid (DHA) known as specialized pro-resolving lipid mediators (SPMs) influence specific B cell populations in obese male mice. Metabololipidomics revealed that splenic SPM precursors 14-hydroxydocosahexaenoic acid (14-HDHA), 17-hydroxydocosahexaenoic acid (17-HDHA), and downstream protectin DX (PDX) were decreased in obese male C57BL/6J mice. Simultaneous administration of these mediators to obese mice rescued major decrements in bone marrow B cells, modest impairments in the spleen, and circulating IgG2c, which is pro-inflammatory in obesity. In vitro studies with B cells, flow cytometry experiments with ALOX5^-/- mice, and lipidomic analyses revealed the lowering of 14-HDHA/17-HDHA/PDX and dysregulation of B cell populations in obesity was driven indirectly via B cell extrinsic mechanisms. Notably, the lowering of lipid mediators was associated with an increase in the abundance of n-6 polyunsaturated fatty acids, which have a high affinity for SPM-generating enzymes. Subsequent experiments revealed female obese mice generally maintained the levels of SPM precursors, B cell subsets, and antibody levels. Finally, obese human females had increased circulating plasma cells accompanied by ex vivo B cell TNFα and IL-10 secretion. Collectively, the data demonstrate that DHA-derived mediators of the SPM pathway control the number of B cell subsets and pro-inflammatory antibody levels in obese male but not female mice through a defect that is extrinsic to B cells.

Updated advances of linking psychosocial factors and sex hormones with systemic lupus erythematosus susceptibility and development

Systemic lupus erythematosus (SLE) is a systemic autoimmune disease that primarily affects women, especially those of reproductive age. Genetics, environment, and gene-environment interactions play key roles in the development of SLE. Despite the numerous susceptibility genes of SLE identified to date, gene therapy is far from a clinical reality. Thus, more attention should be paid to the risk factors and underlying mechanisms of SLE. Currently, it is reported that psychosocial factors and sex hormones play vital roles in patients with SLE, which still need further investigated. The purpose of this review is to update the roles and mechanisms of psychosocial factors and sex hormones in the susceptibility and development of SLE. Based on review articles and reports in reputable peer-reviewed journals and government websites, this paper summarized psychosocial factors (e.g., alexithymia, depression, anxiety, negative emotions, and perceived stress) and sex hormones (e.g., estrogens, progesterone, androgens, and prolactin) involved in SLE. We further explore the mechanisms linking these factors with SLE susceptibility and development, which can guide the establishment of practical measures to benefit SLE patients and offer new ideas for therapeutic strategies.

Progesterone-Related Immune Modulation of Pregnancy and Labor

Pregnancy involves a complex interplay between maternal neuroendocrine and immunological systems in order to establish and sustain a growing fetus. It is thought that the uterus at pregnancy transitions from quiescent to laboring state in response to interactions between maternal and fetal systems at least partly via altered neuroendocrine signaling. Progesterone (P4) is a vital hormone in maternal reproductive tissues and immune cells during pregnancy. As such, P4 is widely used in clinical interventions to improve the chance of embryo implantation, as well as reduce the risk of miscarriage and premature labor. Here we review research to date that focus on the pathways through which P4 mediates its actions on both the maternal reproductive and immune system. We will dissect the role of P4 as a modulator of inflammation, both systemic and intrinsic to the uterus, during human pregnancy and labor.

Sex Differences in Older Adults' Immune Responses to Seasonal Influenza Vaccination

Background: Sex differences in immune responses to influenza vaccine may impact efficacy across populations. Methods: In a cohort of 138 older adults (50-74 years old), we measured influenza A/H1N1 antibody titers, B-cell ELISPOT response, PBMC transcriptomics, and PBMC cell compositions at 0, 3, and 28 days post-immunization with the 2010/11 seasonal inactivated influenza vaccine. Results: We identified higher B-cell ELISPOT responses in females than males. Potential mechanisms for sex effects were identified in four gene clusters related to T, NK, and B cells. Mediation analysis indicated that sex-dependent expression in T and NK cell genes can be partially attributed to higher CD4+ T cell and lower NK cell fractions in females. We identified strong sex effects in 135 B cell genes whose expression correlates with ELISPOT measures, and found that cell subset differences did not explain the effect of sex on these genes' expression. Post-vaccination expression of these genes, however, mediated 41% of the sex effect on ELISPOT responses. Conclusions: These results improve our understanding of sexual dimorphism in immunity and influenza vaccine response.

Signaling control of antibody isotype switching

Class switch recombination (CSR) generates isotype-switched antibodies with distinct effector functions essential for mediating effective humoral immunity. CSR is catalyzed by activation-induced deaminase (AID) that initiates DNA lesions in the evolutionarily conserved switch (S) regions at the immunoglobulin heavy chain (Igh) locus. AID-initiated DNA lesions are subsequently converted into DNA double stranded breaks (DSBs) in the S regions of Igh locus, repaired by non-homologous end-joining to effect CSR in mammalian B lymphocytes. While molecular mechanisms of CSR are well characterized, it remains less well understood how upstream signaling pathways regulate AID expression and CSR. B lymphocytes express multiple receptors including the B cell antigen receptor (BCR) and co-receptors (e.g., CD40). These receptors may share common signaling pathways or may use distinct signaling elements to regulate CSR. Here, we discuss how signals emanating from different receptors positively or negatively regulate AID expression and CSR.

Hormonal Regulation of Physiology, Innate Immunity and Antibody Response to H1N1 Influenza Virus Infection During Pregnancy

In 2009, the H1N1 swine flu pandemic highlighted the vulnerability of pregnant women to influenza viral infection. Pregnant women infected with influenza A virus were at increased risk of hospitalization and severe acute respiratory distress syndrome (ARDS), which is associated with high mortality, while their newborns had an increased risk of pre-term birth or low birth weight. Pregnant women have a unique immunological profile modulated by the sex hormones required to maintain pregnancy, namely progesterone and estrogens. The role of these hormones in coordinating maternal immunotolerance in uterine tissue and cellular subsets has been well researched; however, these hormones have wide-ranging effects outside the uterus in modulating the immune response to disease. In this review, we compile research findings in the clinic and in animal models that elaborate on the unique features of H1N1 influenza A viral pathogenesis during pregnancy, the crosstalk between innate immune signaling and hormonal regulation during pregnancy, and the role of pregnancy hormones in modulating cellular responses to influenza A viral infection at mid-gestation. We highlight the ways in which lung architecture and function is stressed by pregnancy, increasing baseline inflammation prior to infection. We demonstrate that infection disrupts progesterone production and upregulates inflammatory mediators, such as cyclooxygenase-2 (COX-2) and prostaglandins, resulting in pre-term labor and spontaneous abortions. Lastly, we profile the ways in which pregnancy alters innate and adaptive cellular immune responses to H1N1 influenza viral infection, and the ways in which these protect fetal development at the expense of effective long-term immune memory. Thus, we highlight advancements in the field of reproductive immunology in response to viral infection and illustrate how that knowledge might be used to develop more effective post-infection therapies and vaccination strategies.

Gene Expression of ABHD6, a Key Factor in the Endocannabinoid System, Can Be Modulated by Female Hormones in Human Immune Cells

One of the main risk factors for the development of an autoimmune disease is to be a woman. Much attention has been given to the involvement of female hormones in their etiology and sexual bias, although the mechanisms behind this potentially strong contribution in disease susceptibility are poorly understood. ABHD6 gene was recently identified as a risk factor for system lupus erythematosus and the risk was correlated with overexpression of the gene. ABHD6 is an enzyme that degrades the 2-arachidonoylglycerol, an endocannabinoid with immunomodulatory effects. Thus its degradation could contribute to immune dysregulation and development of autoimmune reactions. Sex hormones, such as estrogens, are believed to regulate important genes in the endocannabinoid pathway. However, no study was available regarding the effect of these hormones in human immune cells. In this study, ABHD6 expression was evaluated by quantitative PCR in leukocytes from healthy male and females and in the presence of estrogen or progesterone (PG). A statistical increase in ABHD6 expression could be detected in women. In the presence of estrogen or PG, a statistical upregulation of ABHD6 was observed, and in a sex-dependent manner, as only female cells responded to stimulation. Our results suggest that female hormones can promote the overexpression of ABHD6 in immune cells. This can potentially contribute to a pro-inflammatory scenario and partially explain the association of this gene in the development of LES, a highly female-biased disease.

Progesterone-based compounds affect immune responses and susceptibility to infections at diverse mucosal sites

Over 100 million women worldwide are currently on progesterone-based contraceptives to improve their health outcomes through reduced maternal mortality and family planning. In addition to their role in reproduction, progesterone-based compounds modulate immune responses throughout the body, particularly at mucosal sites. By binding to receptors located in immune cells, including natural killer cells, macrophages, dendritic cells, and T cells, as well in non-immune cells, such as epithelial and endothelial cells, progesterone-based compounds alter cellular signaling and activity to affect the outcome of infections at diverse mucosal sites, including the genital, gastrointestinal, and respiratory tracts. As the use of progesterone-based compounds, in the form of contraceptives and hormone-based therapies, continue to increase worldwide, greater consideration should be given to how the immunomodulatory effects these compounds alter the outcome of diseases at mucosal sites beyond the reproductive tract, which has profound implications for women's health.

Progesterone-Based Contraceptives Reduce Adaptive Immune Responses and Protection against Sequential Influenza A Virus Infections

In addition to their intended use, progesterone (P4)-based contraceptives promote anti-inflammatory immune responses, yet their effects on the outcome of infectious diseases, including influenza A virus (IAV) infection, are rarely evaluated. To evaluate their impact on immune responses to sequential IAV infections, adult female mice were treated with placebo or one of two progestins, P4 or levonorgestrel (LNG), and infected with a mouse-adapted H1N1 (maH1N1) virus. Treatment with P4 or LNG reduced morbidity but had no effect on pulmonary virus titers during primary H1N1 infection compared to placebo treatment. In serum and bronchoalveolar lavage fluid, total anti-IAV IgG and IgA titers and virus-neutralizing antibody titers but not hemagglutinin stalk antibody titers were lower in progestin-treated mice than placebo-treated mice. Females were challenged 6 weeks later with either an maH1N1 drift variant (maH1N1dv) or maH3N2 IAV. The level of protection following infection with the maH1N1dv was similar among all groups. In contrast, following challenge with maH3N2, progestin treatment reduced survival as well as the numbers and activity of H1N1- and H3N2-specific memory CD8⁺ T cells, including tissue-resident cells, compared with placebo treatment. In contrast to primary IAV infection, progestin treatment increased the titers of neutralizing and IgG antibodies against both challenge viruses compared with those achieved with placebo treatment. While the immunomodulatory properties of progestins protected immunologically naive female mice from the severe outcomes from IAV infection, it made them more susceptible to secondary challenge with a heterologous IAV, despite improving their antibody responses against a secondary IAV infection. Taken together, the immunomodulatory effects of progestins differentially regulate the outcome of infection depending on exposure history.IMPORTANCE The impact of hormone-based contraceptives on the outcome of infectious diseases outside the reproductive tract is rarely considered. Using a mouse model, we have made the novel observation that treatment with either progesterone or a synthetic analog found in hormonal contraceptives, levonorgestrel, impacts sequential influenza A virus infection by modulating antibody responses and decreasing the numbers and activity of memory CD8⁺ T cells. Progestins reduced the antibody responses during primary H1N1 virus infection but increased antibody titers following a sequential infection with either an H1N1 drift variant or an H3N2 virus. Following challenge with an H3N2 virus, female mice treated with progestins experienced greater mortality with increased pulmonary inflammation and reduced numbers and activity of CD8⁺ T cells. This study suggests that progestins significantly affect adaptive immune responses to influenza A virus infection, with their effect on the outcome of infection depending on exposure history.

Sex differences in immune responses: Hormonal effects, antagonistic selection, and evolutionary consequences

Males and females differ in both parasite load and the strength of immune responses and these effects have been verified in humans and other vertebrates. Sex hormones act as important modulators of immune responses; the male sex hormone testosterone is generally immunosuppressive while the female sex hormone estrogen tends to be immunoenhancing. Different sets of T-helper cells (Th) have important roles in adaptive immunity, e.g. Th1 cells trigger type 1 responses which are primarily cell-mediated, and Th2 cells trigger type 2 responses which are primarily humoral responses. In our review of the literature, we find that estrogen and progesterone enhance type 2 and suppress type 1 responses in females, whereas testosterone suppresses type 2 responses and shows an inconsistent pattern for type 1 responses in males. When we combine these patterns of generally immunosuppressive and immunoenhancing effects of the sex hormones, our results imply that the sex differences in immune responses should be particularly strong in immune functions associated with type 2 responses, and less pronounced with type 1 responses. In general the hormone-mediated sex differences in immune responses may lead to genetic sexual conflicts on immunity. Thus, we propose the novel hypothesis that sexually antagonistic selection may act on immune genes shared by the sexes, and that the strength of this sexually antagonistic selection should be stronger for type 2- as compared with type 1-associated immune genes. Finally, we put the consequences of sex hormone-induced effects on immune responses into behavioral and ecological contexts, considering social mating system, sexual selection, geographical distribution of hosts, and parasite abundance.

Immunology of neuromyelitis optica during pregnancy

Anti–aquaporin-4 (AQP4) autoantibody plays a key role in the pathogenesis of neuromyelitis optica (NMO). Studies have shown increased relapse rates in patients with NMO during pregnancy and postpartum. High estrogen levels during pregnancy can increase activation-induced cytidine deaminase expression, which is responsible for immunoglobulin production. Additionally, sex hormones may influence antibody glycosylation, with effects on antibody function. Estrogen decreases apoptosis of self-reactive B cells, through upregulation of antiapoptotic molecules. Furthermore, high estrogen levels during pregnancy can boost B-cell activating factor and type 1 interferon (IFN) production, facilitating development of self-reactive peripheral B cells in association with increased disease activity. Elevated levels of estrogen during pregnancy decrease IFN-γ generation, which causes a shift toward T helper (Th) 2 immunity, thereby propagating NMO pathogenesis. Women with NMO have an elevated rate of pregnancy complications including miscarriage and preeclampsia, which are associated with increased Th17 cells and reduction of T-regulatory cells. These in turn can enhance inflammation in NMO. Increased regulatory natural killer cells (CD56⁻) during pregnancy can enhance Th2-mediated immunity, thereby increasing inflammation. In the placenta, trophoblasts express AQP4 antigen and are exposed to maternal blood containing anti-AQP4 antibodies. Animal models have shown that anti-AQP4 antibodies can bind to AQP4 antigen in placenta leading to complement deposition and placental necrosis. Reduction of regulatory complements has been associated with placental insufficiency, and it is unclear whether these are altered in NMO. Further studies are required to elucidate the specific mechanisms of disease worsening, as well as the increased rate of complications during pregnancy in women with NMO.

Binding of estrogen receptors to switch sites and regulatory elements in the immunoglobulin heavy chain locus of activated B cells suggests a direct influence of estrogen on antibody expression

Females and males differ in antibody isotype expression patterns and in immune responses to foreign- and self-antigens. For example, systemic lupus erythematosus is a condition that associates with the production of isotype-skewed anti-self antibodies, and exhibits a 9:1 female:male disease ratio. To explain differences between B cell responses in males and females, we sought to identify direct interactions of the estrogen receptor (ER) with the immunoglobulin heavy chain locus. This effort was encouraged by our previous identification of estrogen response elements (ERE) in heavy chain switch (S) regions. We conducted a full-genome chromatin immunoprecipitation analysis (ChIP-seq) using DNA from LPS-activated B cells and an ERα-specific antibody. Results revealed ER binding to a wide region of DNA, spanning sequences from the JH cluster to Cδ, with peaks in Eμ and Sμ sites. Additional peaks of ERα binding were coincident with hs1,2 and hs4 sites in the 3' regulatory region (3'RR) of the heavy chain locus. This first demonstration of direct binding of ER to key regulatory elements in the immunoglobulin locus supports our hypothesis that estrogen and other nuclear hormone receptors and ligands may directly influence antibody expression and class switch recombination (CSR). Our hypothesis encourages the conduct of new experiments to evaluate the consequences of ER binding. A better understanding of ER:DNA interactions in the immunoglobulin heavy chain locus, and respective mechanisms, may ultimately translate to better control of antibody expression, better protection against pathogens, and prevention of pathologies caused by auto-immune disease.

Mutations, kataegis and translocations in B cells: understanding AID promiscuous activity

As B cells engage in the immune response, they express activation-induced cytidine deaminase (AID) to initiate the hypermutation and recombination of immunoglobulin genes, which are crucial processes for the efficient recognition and disposal of pathogens. However, AID must be tightly controlled in B cells to minimize off-target mutations, which can drive chromosomal translocations and the development of B cell malignancies, such as lymphomas. Recent genomic and biochemical analyses have begun to unravel the mechanisms of how AID-mediated deamination is targeted outside immunoglobulin genes. Here, we discuss the transcriptional and topological features that are emerging as key drivers of AID promiscuous activity.

Altered IgG autoantibody levels and CD4(+) T cell subsets in lupus-prone Nba2 mice lacking the nuclear progesterone receptor

Important interactions between female reproduction and autoimmunity are suggested by the female-predominance of systemic lupus erythematosus (SLE) and other autoimmune diseases and the amelioration of certain autoimmune diseases during pregnancy. Sexually dimorphic risk of developing SLE involves modulation of genetic risk by environmental factors, sex hormones and non-hormonal factors encoded on the sex chromosomes. In some lupus models, estrogen, via estrogen receptor alpha (ER-α), enhances production of highly pathogenic IgG2a/c autoantibodies (autoAbs). Some studies indicate that treatment with progesterone, a chief female reproductive steroid, can suppress IgG2a/2c autoAb production. Little is known about how endogenous progesterone impacts lupus autoimmunity. To investigate this, we introduced a disruptive progesterone receptor (PR) gene mutation into lupus-prone mice and tracked the development of spontaneous IgG autoAbs. Here, we present evidence that PR can suppress the emergence of class-switched IgG2c autoAbs, suggesting that PR and ER-α counter-regulate a critical step in lupus autoimmunity. PR's control of IgG2c autoAb production correlates with alterations in the relative abundance of splenic T follicular helper (TFH) cells and non-TFH CD4(+) T cells, especially regulatory T cells (TREGS). Surprisingly, PR also appears to help to maintain sexually dimorphic abundance of splenic leukocytes, a feature common to many mouse models of SLE. Together our results identify a novel molecular link between female reproduction and lupus autoimmunity. Further investigation into the immunomodulatory functions of PR promises to inform reproductive health care in women and offers mechanistic insight into important immunologic phenomena of pregnancy.

Cyclin A2 and its associated kinase activity are required for optimal induction of progesterone receptor target genes in breast cancer cells

A role for the cell cycle protein cyclin A2 in regulating progesterone receptor (PR) activity is emerging. This study investigates the role of cyclin A2 in regulating endogenous PR activity in T47D breast cancer cells by depleting cyclin A2 expression and measuring PR target genes using q-RT-PCR. Targets examined included genes induced by the PR-B isoform more strongly than PR-A (SGK1, FKBP5), a gene induced predominantly by PR-A (HEF1), genes induced via PR tethering to other transcription factors (p21, p27), a gene induced in part via extra-nuclear PR signaling mechanisms (cyclin D1) and PR-repressed genes (DST, IL1R1). Progestin induction of target genes was reduced following cyclin A2 depletion. However, cyclin A2 depletion did not diminish progestin target gene repression. Furthermore, inhibition of the associated Cdk2 kinase activity of cyclin A2 also reduced progestin induction of target genes, while Cdk2 enhanced the interaction between PR and cyclin A2. These results demonstrate that cyclin A2 and its associated kinase activity are important for progestin-induced activation of endogenous PR target genes in breast cancer cells.

AIDing Chromatin and Transcription-Coupled Orchestration of Immunoglobulin Class-Switch Recombination

Secondary diversification of the antibody repertoire upon antigenic challenge, in the form of immunoglobulin heavy chain (IgH) class-switch recombination (CSR) endows mature, naïve B cells in peripheral lymphoid organs with a limitless ability to mount an optimal humoral immune response, thus expediting pathogen elimination. CSR replaces the default constant (C_H) region exons (Cμ) of IgH with any of the downstream C_H exons (Cγ, Cε, or Cα), thereby altering effector functions of the antibody molecule. This process depends on, and is orchestrated by, activation-induced deaminase (AID), a DNA cytidine deaminase that acts on single-stranded DNA exposed during transcription of switch (S) region sequences at the IgH locus. DNA lesions thus generated are processed by components of several general DNA repair pathways to drive CSR. Given that AID can instigate DNA lesions and genomic instability, stringent checks are imposed that constrain and restrict its mutagenic potential. In this review, we will discuss how AID expression and substrate specificity and activity is rigorously enforced at the transcriptional, post-transcriptional, post-translational, and epigenetic levels, and how the DNA-damage response is choreographed with precision to permit targeted activity while limiting bystander catastrophe.

Glucocorticoid inhibition of activation-induced cytidine deaminase expression in human B lymphocytes

We examined whether glucocorticoids could modulate the expression of activation-induced cytidine deaminase (AICDA), the principal regulator of the processes of immunoglobulin gene somatic hypermutation and class switch recombination in B lymphocytes. Treatment of human B cells with IL-4 and anti-CD40 antibody for 18-20h resulted in induction of expression of AICDA mRNA by over 10-fold. Dexamethasone at 10nM concentration inhibited AICDA induction by an average of 51.8% (p<0.0001). These effects of glucocorticoids were found to be dose dependent in the physiologic range and were reversible by co-treatment with a glucocorticoid receptor antagonist. Human B cell viability and proliferation were unaltered by glucocorticoid treatment. These data demonstrate that physiologic concentrations of glucocorticoids can act on human B lymphocytes through glucocorticoid receptor-mediated mechanisms to diminish the expression of AICDA, a key regulator of humoral immune responses.

Regulation of immunoglobulin class-switch recombination: choreography of noncoding transcription, targeted DNA deamination, and long-range DNA repair

Upon encountering antigens, mature IgM-positive B lymphocytes undergo class-switch recombination (CSR) wherein exons encoding the default Cμ constant coding gene segment of the immunoglobulin (Ig) heavy-chain (Igh) locus are excised and replaced with a new constant gene segment (referred to as "Ch genes", e.g., Cγ, Cɛ, or Cα). The B cell thereby changes from expressing IgM to one producing IgG, IgE, or IgA, with each antibody isotype having a different effector function during an immune reaction. CSR is a DNA deletional-recombination reaction that proceeds through the generation of DNA double-strand breaks (DSBs) in repetitive switch (S) sequences preceding each Ch gene and is completed by end-joining between donor Sμ and acceptor S regions. CSR is a multistep reaction requiring transcription through S regions, the DNA cytidine deaminase AID, and the participation of several general DNA repair pathways including base excision repair, mismatch repair, and classical nonhomologous end-joining. In this review, we discuss our current understanding of how transcription through S regions generates substrates for AID-mediated deamination and how AID participates not only in the initiation of CSR but also in the conversion of deaminated residues into DSBs. Additionally, we review the multiple processes that regulate AID expression and facilitate its recruitment specifically to the Ig loci, and how deregulation of AID specificity leads to oncogenic translocations. Finally, we summarize recent data on the potential role of AID in the maintenance of the pluripotent stem cell state during epigenetic reprogramming.

The requirement for p42/p44 MAPK activity in progesterone receptor-mediated gene regulation is target gene-specific

Recent studies have suggested that progestins play a role in the etiology of breast cancer; however, the mechanisms by which progestins promote tumor formation/progression have not been defined. Progestin action, in target tissues such as the breast, is mediated by the progesterone receptor (PR). PR signaling is complex and PR regulates transcription of target genes through a variety of mechanisms. Many cell signaling pathways are activated inappropriately in breast cancer cells and these pathways can regulate PR activity. For example, the p42/p44 MAPK pathway can regulate PR function by altering phosphorylation of PR, as well as its coregulators. We found that inhibition of the p42/p44 MAPK signaling pathway with a MEK inhibitor (U0126) impairs PR-mediated gene induction, but not gene repression. In addition, the effects of U0126 on PR-mediated gene transcription are much greater with long-term versus short-term inhibition and are gene-specific. Finally, treatment with U0126 delays phosphorylation of Ser294, but does not block phosphorylation completely, suggesting that p42/p44 MAPK kinase is not the dominant kinase responsible for phosphorylating this site. Collectively, these studies suggest that in addition to the p42/p44 MAPK pathway, other signaling pathways are also important for PR transcriptional activity in breast cancer cells. The integration of PR transcriptional effects and cell signaling pathways has implications for the initiation or progression of breast cancer. Understanding how these pathways interact may aid in the development of prevention and/or treatment strategies for the disease.

The intracellular progesterone receptor regulates CD4+ T cells and T cell-dependent antibody responses

Pg has distinct immunomodulatory properties involved in poorly understood immune phenomena, including maternal tolerance of the fetus, increased risk of certain infections during pregnancy or after Pg birth control, and pregnancy-associated remission of autoimmune disease. Several potential mechanisms have been identified, including alteration of Th1 and Treg activity, but the precise cellular and molecular targets of Pg immunomodulation in vivo remain obscure, partly because Pg can signal through several different receptor types. One such receptor, the iPR, encoded by the pgr gene, is essential for reproduction in female mice and is expressed in the thymus and CD4(+) T cells. We hypothesized that iPR regulates CD4(+) T cell activity and adaptive immune responses in vivo. With the use of iPR KO mice, we demonstrate that iPR specifically suppresses TD antibody responses, primarily by dampening CD4(+) Teff activity, likely via transcriptional repression of the IFN-γ gene and modulation of other programs regulating CD4(+) T cells. Our results highlight a novel mechanism linking the endocrine and immune systems, and they offer insight into important but poorly understood phenomena in women's health and autoimmunity.

Gonadal steroids and humoral immunity

Humoral immune responses are sexually dimorphic. Female individuals generally exhibit more-robust antibody responses to vaccines and, in the clinical setting as well as in experimental models, are more likely than male individuals to produce autoreactive antibodies of pathogenic potential. A number of differences between the sexes might account for these observations, including differences in the dosage of specific X-chromosome and Y-chromosomal genes, increased exposure of female individuals to antigenic stimulation in childbearing, and differences in circulating concentrations of gonadal steroid hormones. The role of gonadal steroids in modulating such humoral immune responses has been studied for nearly a century, but advances in our knowledge of B-lymphocyte development and function, the mechanisms of immune tolerance, and the molecular basis of gonadal steroid hormone action are now yielding new understanding of the influence of gonadal steroid hormones on the humoral immune system. This Review examines how oestrogens and androgens modulate B-lymphocyte development and function, focusing on the areas of B-cell production in the bone marrow, the maintenance of immune tolerance for self antigens, and the processes of immunoglobulin heavy chain gene somatic hypermutation and class switch recombination during maturation of cells involved in humoral immune responses.

Activation-induced Cytidine Deaminase in B Cell Immunity and Cancers

Activation-induced cytidine deaminase (AID) is an enzyme that is predominantly expressed in germinal center B cells and plays a pivotal role in immunoglobulin class switch recombination and somatic hypermutation for antibody (Ab) maturation. These two genetic processes endow Abs with protective functions against a multitude of antigens (pathogens) during humoral immune responses. In B cells, AID expression is regulated at the level of either transcriptional activation on AID gene loci or post-transcriptional suppression of AID mRNA. Furthermore, AID stabilization and targeting are determined by post-translational modifications and interactions with other cellular/nuclear factors. On the other hand, aberrant expression of AID causes B cell leukemias and lymphomas, including Burkitt's lymphoma caused by c-myc/IgH translocation. AID is also ectopically expressed in T cells and non-immune cells, and triggers point mutations in relevant DNA loci, resulting in tumorigenesis. Here, I review the recent literatures on the function of AID, regulation of AID expression, stability and targeting in B cells, and AID-related tumor formation.

Chromosome translocation, B cell lymphoma, and activation-induced cytidine deaminase

Studies of B cell lymphomas in the early 1980s led to the cloning of genes (c-MYC and IGH) at a chromosome translocation breakpoint. A rush followed to identify recurrently translocated genes in all types of cancer, which led to remarkable advances in our understanding of cancer genetics. B lymphocyte tumors commonly bear chromosome translocations to immunoglobulin genes, which points to a role for antibody gene diversification processes in tumorigenesis. The discovery of activation-induced cytidine deaminase (AID) and the use of murine models to study translocation have led to a new understanding of how these events contribute to the genesis of lymphomas. Here, we review these advances with a focus on AID and insights gained from the study of translocations in primary cells.

Combinatorial mechanisms regulating AID-dependent DNA deamination: interacting proteins and post-translational modifications

Protective humoral immune responses result from immunoglobulin (Ig) diversification reactions that proceed through programmed DNA double-strand breaks and mutations in developing or mature B cells. While primary Ig diversity is dependent on V(D)J recombination and the RAG proteins, secondary diversification is achieved through class switch recombination (CSR) and somatic hypermutation (SHM), which require AID (activation induced deaminase). Because aberrant AID activity can result in mutations in non-Ig loci and DNA translocations between the Ig locus and non-Ig genes, the activity of AID must be stringently regulated. AID mRNA expression is regulated transcriptionally by cytokine stimulation and post-transcriptionally by miRNAs. AID activity is regulated by post-translational modifications, subcellular localization, and interaction with other proteins. All of these molecular mechanisms have evolved to specifically induce AID-dependent mutations and DNA double-strand breaks at the Ig loci to promote maximal Ig gene diversification while limiting the access of this mutator to non-Ig regions.

Progesterone and autoimmune disease

Sexual dimorphism in human immune systems is most apparent in the female predominance of certain autoimmune diseases (ADs) like systemic lupus erythematosus (SLE). Epidemiologic, observational and experimental evidence strongly suggest sex steroids are important modulators of genetic risk in human AD. In this regard, the roles of progesterone (Pg), an immunomodulatory female sex steroid, are poorly understood. Several lines of investigation indicate Pg and synthetic progestins impact risk of AD and immune-mediated injury in different ways depending on their concentrations and their engagement of various Pg receptors expressed in immune organs, immune cells or tissues targeted by immune attack. At low physiologic levels, Pg may enhance interferon-alpha (IFN-α) pathways important in SLE pathogenesis. Commonly used synthetic progestins may have the opposite effect. At pregnancy levels, Pg may suppress disease activity in rheumatoid arthritis (RA) and multiple sclerosis (MS) via inhibition of T helper type 1 (Th1) and Th17 pathways and induction of anti-inflammatory molecules. Importantly, Pg's immunomodulatory effects differ from those of estrogens and androgens. An additional layer of complexity arises from apparent interdependence of sex hormone signaling pathways. Identifying mechanisms by which Pg and other sex steroids modulate risk of AD and immune-mediated injury will require clarification of their cellular and molecular targets in vivo. These future studies should be informed by recent genetic discoveries in human AD, particularly those revealing their sex-specific genetic associations.

Activation-induced cytidine deaminase structure and functions: a species comparative view

In the ten years since the discovery of activation-induced cytidine deaminase (AID) there has been considerable effort to understand the mechanisms behind this enzyme's ability to target and modify immunoglobulin genes leading to somatic hypermutation and class switch recombination. While the majority of research has focused on mouse and human models of AID function, work on other species, from lamprey to rabbit and sheep, has taught us much about the scope of functions of the AID mutator. This review takes a species-comparative approach to what has been learned about the AID mutator enzyme and its role in humoral immunity.

Expression of humoral autoimmunity is related to androgen receptor CAG repeat length in men with systemic lupus erythematosus

We sought to explore whether inherited differences in androgen sensitivity conferred by variation in the length of a CAG repeat in exon 1 of the androgen receptor gene could be correlated with differing manifestations of humoral autoimmunity in men with lupus. In a sample of 15 men with lupus, AR CAG repeat length was linearly correlated with levels of antibodies against extractable nuclear antigens and with the number of diagnostic criteria for lupus. Protein microarrays were used to assess levels of 86 different IgG and IgM autoantibodies in the sera of these patients. IgG autoantibodies were more frequently observed in male lupus patients with longer AR CAG repeat length (>23), while IgM autoantibodies were more prevalent in subjects with shorter CAG repeat length (≤23). These data support a potential role for androgen signaling in the modulation of immunoglobulin class switching processes, with consequent impact on the autoimmune phenotype in men with lupus.

Role of activation-induced cytidine deaminase in inflammation-associated cancer development

Human cancer is a genetic disease resulting from the stepwise accumulation of genetic alterations in various tumor-related genes. Normal mutation rates, however, cannot account for the abundant genetic changes accumulated in tumor cells, suggesting that certain molecular mechanisms underlie such a large number of genetic alterations. Activation-induced cytidine deaminase (AID), a nucleotide-editing enzyme that triggers DNA alterations and double-strand DNA breaks in the immunoglobulin gene, has been identified in activated B lymphocytes. Recent studies revealed that AID-mediated genotoxic effects target not only immunoglobulin genes but also a variety of other genes in both B lymphocytes and non-lymphoid cells. Consistent with the finding that several transcription factors including nuclear factor-κB (NF-κB) mediate AID expression in B cells, proinflammatory cytokine stimulation of several types of gastrointestinal epithelial cells, such as gastric, colonic, hepatic, and biliary epithelium, induces aberrant AID expression through the NF-κB signaling pathway. In vivo studies revealed that constitutive AID expression promotes the tumorigenic pathway by enhancing the susceptibility to mutagenesis in a variety of epithelial organs. The activity of AID as a genome mutator provides a new avenue for studies aimed at understanding mutagenesis mechanisms during carcinogenesis.

Estrogen receptors bind to and activate the HOXC4/HoxC4 promoter to potentiate HoxC4-mediated activation-induced cytosine deaminase induction, immunoglobulin class switch DNA recombination, and somatic hypermutation

Estrogen enhances antibody and autoantibody responses through yet to be defined mechanisms. It has been suggested that estrogen up-regulates the expression of activation-induced cytosine deaminase (AID), which is critical for antibody class switch DNA recombination (CSR) and somatic hypermutation (SHM), through direct activation of this gene. AID, as we have shown, is induced by the HoxC4 homeodomain transcription factor, which binds to a conserved HoxC4/Oct site in the AICDA/Aicda promoter. Here we show that estrogen-estrogen receptor (ER) complexes do not directly activate the AID gene promoter in B cells undergoing CSR. Rather, they bind to three evolutionarily conserved and cooperative estrogen response elements (EREs) we identified in the HOXC4/HoxC4 promoter. By binding to these EREs, ERs synergized with CD154 or LPS and IL-4 signaling to up-regulate HoxC4 expression, thereby inducing AID and CSR without affecting B cell proliferation or plasmacytoid differentiation. Estrogen administration in vivo significantly potentiated CSR and SHM in the specific antibody response to the 4-hydroxy-3-nitrophenylacetyl hapten conjugated with chicken γ-globulin. Ablation of HoxC4 (HoxC4(-/-)) abrogated the estrogen-mediated enhancement of AID gene expression and decreased CSR and SHM. Thus, estrogen enhances AID expression by activating the HOXC4/HoxC4 promoter and inducing the critical AID gene activator, HoxC4.

AID and somatic hypermutation

In response to an assault by foreign organisms, peripheral B cells can change their antibody affinity and isotype by somatically mutating their genomic DNA. The ability of a cell to modify its DNA is exceptional in light of the potential consequences of genetic alterations to cause human disease and cancer. Thus, as expected, this mechanism of antibody diversity is tightly regulated and coordinated through one protein, activation-induced deaminase (AID). AID produces diversity by converting cytosine to uracil within the immunoglobulin loci. The deoxyuracil residue is mutagenic when paired with deoxyguanosine, since it mimics thymidine during DNA replication. Additionally, B cells can manipulate the DNA repair pathways so that deoxyuracils are not faithfully repaired. Therefore, an intricate balance exists which is regulated at multiple stages to promote mutation of immunoglobulin genes, while retaining integrity of the rest of the genome. Here we discuss and summarize the current understanding of how AID functions to cause somatic hypermutation.