Laurent Delpy

17Beta-estradiol promotes TLR4-triggered proinflammatory mediator production through direct estrogen receptor alpha signaling in macrophages in vivo.

17β-estradiol (E2) has been shown to promote the expression of inflammatory mediators by LPS-activated tissue resident macrophages through estrogen receptor α (ERα) signaling. However, it remained to be determined whether E2 similarly influences macrophages effector functions under inflammatory conditions in vivo, and whether this action of E2 resulted from a direct effect on macrophages. We show in this study that chronic E2 administration to ovariectomized mice significantly increased both cytokine (IL-1β, IL-6, and TNF-α) and inducible NO synthase mRNA abundance in thioglycolate (TGC)-elicited macrophages. The proinflammatory action of E2 was also evidenced at the level of released IL-1β and IL-6 by ex vivo LPS-activated macrophages. E2 concomitantly inhibited PI3K activity as well as Akt phosphorylation in TGC-elicited macrophages, suggesting that E2 promoted TLR-dependent macrophage activation by alleviating this suppressive signaling pathway. Indeed, this effect was abolished in the presence of the inhibitor wortmannin, demonstrating a key functional link between inhibition of PI3K activity and the E2 action on macrophage functions. Endogenous estrogens levels circulating in ovary-intact mice were sufficient to promote the above described actions. Finally, thanks to a CreLox strategy, targeted disruption of ERα gene in macrophages totally abolished the effect of E2 on the expression of inflammatory mediators by both resident and TGC-elicited peritoneal macrophages. In conclusion, we demonstrate that estrogens, through the activation of ERα in macrophages in vivo, enhance their ability to produce inflammatory mediators and cytokines upon subsequent TLR activation.

Estrogen Receptor α Signaling in T Lymphocytes Is Required for Estradiol-Mediated Inhibition of Th1 and Th17 Cell Differentiation and Protection against Experimental Autoimmune Encephalomyelitis

Estrogen treatment exerts a protective effect on experimental autoimmune encephalomyelitis (EAE) and is under clinical trial for multiple sclerosis therapy. Estrogens have been suspected to protect from CNS autoimmunity through their capacity to exert anti-inflammatory as well as neuroprotective effects. Despite the obvious impacts of estrogens on the pathophysiology of multiple sclerosis and EAE, the dominant cellular target that orchestrates the anti-inflammatory effect of 17β-estradiol (E2) in EAE is still ill defined. Using conditional estrogen receptor (ER) α-deficient mice and bone marrow chimera experiments, we show that expression of ERα is critical in hematopoietic cells but not in endothelial ones to mediate the E2 inhibitory effect on Th1 and Th17 cell priming, resulting in EAE protection. Furthermore, using newly created cell type-specific ERα-deficient mice, we demonstrate that ERα is required in T lymphocytes, but neither in macrophages nor dendritic cells, for E2-mediated inhibition of Th1/Th17 cell differentiation and protection from EAE. Lastly, in absence of ERα in host nonhematopoietic tissues, we further show that ERα signaling in T cells is necessary and sufficient to mediate the inhibitory effect of E2 on EAE development. These data uncover T lymphocytes as a major and nonredundant cellular target responsible for the anti-inflammatory effects of E2 in Th17 cell-driven CNS autoimmunity.

Endothelial Estrogen Receptor-α Plays a Crucial Role in the Atheroprotective Action of 17β-Estradiol in Low-Density Lipoprotein Receptor–Deficient Mice

Background— The prevention of early atheroma by estrogens has been clearly demonstrated in all animal models and appears to be mediated through a direct action on the arterial wall rather than through an effect on the lipoprotein profile. The goal of the present study was to evaluate which cellular target is crucial in this beneficial action of estradiol. Methods and Results— We first confirmed the key role of estrogen receptor-&agr; (ER&agr;) in the atheroprotective effect of estradiol, because this action was completely abolished in mice deficient in both the low-density lipoprotein receptor (LDLr) and ER&agr;. Second, using chimeric mice with an ER&agr; deficiency in the hematopoietic lineage, we showed the persistence of the protective action of estradiol, which suggests the involvement of extrahematopoietic ER&agr;. Third, we showed that loxP-flanked ER&agr; mice (ER&agr;flox/flox) bred with Tie2-Cre+ mice on an LDLr−/− background had complete inactivation of ER&agr; in most hematopoietic and all endothelial cells. Remarkably, in this mouse model, the atheroprotective effect of estradiol was completely abolished. Fourth, the atheroprotective effect of estradiol remained abolished in Tie2-Cre+ ER&agr;flox/flox LDLr−/− mice transplanted with either Tie2-Cre+ ER&agr;flox/flox or ER&agr;−/− bone marrow, whereas it was present in analogous chimeric Tie2-Cre− ER&agr;flox/flox LDLr−/− receivers expressing endothelial ER&agr;. Conclusions— We demonstrate directly and for the first time that endothelial ER&agr; represents a key target of the atheroprotective effect of estradiol, whereas hematopoietic ER&agr; is dispensable. Selective estrogen receptor modulators that mimic the endothelial action of estradiol should now be considered in atheroprotection.

Premature replacement of μ with α immunoglobulin chains impairs lymphopoiesis and mucosal homing but promotes plasma cell maturation

Sequentially along B cell differentiation, the different classes of membrane Ig heavy chains associate with the Igα/Igβ heterodimer within the B cell receptor (BCR). Whether each Ig class conveys specific signals adapted to the corresponding differentiation stage remains debated. We investigated the impact of the forced expression of an IgA-class receptor throughout murine B cell differentiation by knocking in the human Cα Ig gene in place of the Sμ region. Despite expression of a functional BCR, homozygous mutant mice showed a partial developmental blockade at the pro-B/pre-BI and large pre-BII cell stages, with decreased numbers of small pre-BII cells. Beyond this stage, peripheral B cell compartments of reduced size developed and allowed specific antibody responses, whereas mature cells showed constitutive activation and a strong commitment to plasma cell differentiation. Secreted IgA correctly assembled into polymers, associated with the murine J chain, and was transported into secretions. In heterozygous mutants, cells expressing the IgA allele competed poorly with those expressing IgM from the wild-type allele and were almost undetectable among peripheral B lymphocytes, notably in gut-associated lymphoid tissues. Our data indicate that the IgM BCR is more efficient in driving early B cell education and in mucosal site targeting, whereas the IgA BCR appears particularly suited to promoting activation and differentiation of effector plasma cells.

Germ-line transcription occurs on both the functional and the non-functional alleles of immunoglobulin constant heavy chain genes.

In B cells undergoing class switching, recombination is usually directed to the same switch sequences on both homologous chromosomes and is generally preceded by germ‐line transcription initiated from I promoters upstream of immunoglobulin constant heavy chain genes. Whether germ‐line transcription occurs on both alleles or is restricted to the productive one has not been directly addressed before. To clearly distinguish between these two possibilities, a system is needed in which one might detect allele‐specific germ‐line transcripts. We used homologous recombination to set up twoin vivo systems in which one γ3 allele was marked by a loxP sequence whereas the other allele was silenced by inhibiting μ heavy chain production. Here, we provide evidence that germ‐line transcription occurs on both the functional and the non‐functional alleles.

Endogenous estrogens, through estrogen receptor α, constrain autoimmune inflammation in female mice by limiting CD4+ T-cell homing into the CNS

Sex hormones influence immune responses and the development of autoimmune diseases including MS and its animal model, EAE. Although it has been previously reported that ovariectomy could worsen EAE, the mechanisms implicated in the protective action of endogenous ovarian hormones have not been addressed. In this report, we now show that endogenous estrogens limit EAE development and CNS inflammation in adult female mice through estrogen receptor α expression in the host non‐hematopoietic tissues. We provide evidence that the enhancing effect of gonadectomy on EAE development was due to quantitative rather than qualitative changes in effector Th1 or Th17 cell recruitment into the CNS. Consistent with this observation, adoptive transfer of myelin oligodendrocyte glycoprotein‐specific encephalitogenic CD4+ T lymphocytes induced more severe EAE in ovariectomized mice as compared to normal female mice. Finally, we show that gonadectomy accelerated the early recruitment of inflammatory cells into the CNS upon adoptive transfer of encephalitogenic CD4+ T cells. Altogether, these data show that endogenous estrogens, through estrogen receptor α, exert a protective effect on EAE by limiting the recruitment of blood‐derived inflammatory cells into the CNS.

Interallelic Class Switch Recombination Contributes Significantly to Class Switching in Mouse B Cells

Except for the expression of IgM and IgD, DNA recombination is constantly needed for the expression of other Ig classes and subclasses. The predominant path of class switch recombination (CSR) is intrachromosomal, and the looping-out and deletion model has been abundantly documented. However, switch regions also occasionally constitute convenient substrates for interchromosomal recombination, since it is noticeably the case in a number of chromosomal translocations causing oncogene deregulation in the course of lymphoma and myeloma. Although asymmetric accessibility of Ig alleles should theoretically limit its occurrence, interallelic CSR was shown to occur at low levels during IgA switching in rabbit, where the definition of allotypes within both V and C regions helped identify interchromosomally derived Ig. Thus, we wished to evaluate precisely interallelic CSR frequency in mouse B cells, by using a system in which only one allele (of b allotype) could express a functional VDJ region, whereas only interallelic CSR could restore expression of an excluded (a allotype) allele. In our study, we show that interchromosomal recombination of VH and Cγ or Cα occurs in vivo in B cells at a frequency that makes a significant contribution to physiological class switching: trans-association of VH and CH genes accounted for 7% of all α mRNA, and this frequency was about twice higher for the γ3 transcripts, despite the much shorter distance between the JH region and the Cγ3 gene, thus confirming that this phenomenon corresponded to site-specific switching and not to random recombination between long homologous loci.

B Cell Development Arrest Upon Insertion of a neo Gene Between JH and Eμ: Promoter Competition Results in Transcriptional Silencing of Germline JH and Complete V(D)J Rearrangements

Previous targeting experiments within the IgH locus have shown that V(D)J recombination was affected by an insertion of a neo gene within Eμ upstream of the core enhancer, but not by insertions downstream of the enhancer. Similarly, class switch recombination to a given (C) gene was affected only by interposition of neo in between that gene and the 3′ IgH enhancers. Here we show that insertion of neo upstream Eμ only marginally impairs V(D)J recombination, but results in an altered D and JH gene usage and completely blocks transcription of the germline JH region and the rearranged VDJ segments. Although transcriptional silencing of JH occurs upstream of the insertion and results in the lack of mature B cells in homozygous mutant animals, IgH transcription is maintained downstream of the insertion together with neo transcription and can be up-regulated by LPS stimulation or upon fusion with plasmacytoma cells. Altogether these data argue for a polarized “neo effect” involving promoter competition and further show that V(D)J rearrangement can be uncoupled from transcription.

Multiple RNA Surveillance Mechanisms Cooperate to Reduce the Amount of Nonfunctional Igκ Transcripts

Random V(D)J junctions ensure that the diversity of the Ig primary repertoire is adapted to the vast heterogeneity of Ags. In two-thirds of cases, recombination between variable segments induces a frameshift in the open reading frame and generates a premature termination codon. In B cells harboring biallelic V(D)J rearrangement of Ig genes, transcription is known to occur on both the functional and nonfunctional alleles, generating considerable amounts of primary transcripts with out-of-frame V regions. In this study, we analyzed in cell lines and primary B cells the RNA surveillance of nonfunctional Igκ transcripts arising from nonproductive rearrangement. We demonstrated that splicing inhibition, nonsense-mediated decay and nonsense-altered splicing each have an individual partial effect that together associate into an efficient surveillance machinery, downregulating nonfunctional Igκ mRNA. Moreover, we provide evidence that the RNA surveillance efficiency increases throughout B cell development. Whereas splicing inhibition remains constant in most cell lines, differences in nonsense-mediated decay and nonsense-altered splicing are responsible for the higher RNA surveillance observed in plasma cells. Altogether, these data show that nonfunctionally rearranged alleles are subjected to active transcription but that multiple RNA surveillance mechanisms eradicate up to 90% of out-of-frame Igκ mRNA.

Transcription-Dependent Somatic Hypermutation Occurs at Similar Levels on Functional and Nonfunctional Rearranged IgH Alleles

Allelic exclusion of IgH chain expression is stringently established before or during early B cell maturation. It likely relies both on cellular mechanisms, selecting those cells in which a single receptor allows the best possible Ag response, and on molecular restrictions of gene accessibility to rearrangement. The extent to which transcriptional control may be involved is unclear. Transcripts arising from nonfunctional alleles would undergo nonsense-mediated degradation and their virtual absence in mature cells cannot ensure that transcription per se is down-regulated. By contrast, somatic hypermutation may provide an estimate of primary transcription in Ag-activated cells since both processes are directly correlated. For coding regions, the rate and nature of mutations also depend upon Ag binding constraints. By sequencing intronic sequence downstream mouse VDJ genes, we could show in the absence of such constraints that somatic hypermutation intrinsically targets nonfunctional rearranged alleles at a frequency approaching that of functional alleles, suggesting that transcription also proceeds on both alleles at a similar rate. By contrast and confirming the strong dependency of somatic hypermutation upon transcription, we show that artificial blockade of transcription on the nonfunctional allele by a knock-in neomycin resistance cassette keeps the VDJ region unmutated even when its promoter is intact and when it is fully rearranged.