Gravity changes during animal development affect IgM heavy-chain transcription and probably lymphopoiesis

Hindlimb unloading, a physiological model of microgravity, modifies the murine bone marrow IgM repertoire in a similar manner as aging but less strongly

BACKGROUND

The spaceflight environment is an extreme environment that affects the immune system of approximately 50% of astronauts. With planned long-duration missions, such as the deployment of the Lunar Gateway and possible interplanetary missions, it is mandatory to determine how all components of the immune system are affected, which will allow the establishment of countermeasures to preserve astronaut health. However, despite being an important component of the immune system, antibody-mediated humoral immunity has rarely been investigated in the context of the effects of the space environment. It has previously been demonstrated that 30 days aboard the BION-M1 satellite and 21 days of hindlimb unloading (HU), a model classically used to mimic the effects of microgravity, decrease murine B lymphopoiesis. Furthermore, modifications in B lymphopoiesis reported in young mice subjected to 21 days of HU were shown to be similar to those observed in aged mice (18-22 months). Since the primary antibody repertoire composed of IgM is created by V(D) J recombination during B lymphopoiesis, the objective of this study was to assess the degree of similarity between changes in the bone marrow IgM repertoire and in the V(D)J recombination process in 2.5-month-old mice subjected to 21 days of HU and aged (18 months) mice.

RESULTS

We found that in 21 days, HU induced changes in the IgM repertoire that were approximately 3-fold less than those in aged mice, which is a rapid effect. Bone remodeling and epigenetics likely mediate these changes. Indeed, we previously demonstrated a significant decrease in tibial morphometric parameters from day 6 of HU and a progressive reduction in these parameters until day 21 of HU, and it has been shown that age and microgravity induce epigenetic changes.

CONCLUSION

These data reveal novel immune changes that are akin to advanced aging and underline the importance of studying the effects of spaceflight on antibody-mediated humoral immunity.

Next generation of astronauts or ESA astronaut 2.0 concept and spotlight on immunity

Although we have sent humans into space for more than 50 years, crucial questions regarding immune response in space conditions remain unanswered. There are many complex interactions between the immune system and other physiological systems in the human body. This makes it difficult to study the combined long-term effects of space stressors such as radiation and microgravity. In particular, exposure to microgravity and cosmic radiation may produce changes in the performance of the immune system at the cellular and molecular levels and in the major physiological systems of the body. Consequently, abnormal immune responses induced in the space environment may have serious health consequences, especially in future long-term space missions. In particular, radiation-induced immune effects pose significant health challenges for long-duration space exploration missions with potential risks to reduce the organism's ability to respond to injuries, infections, and vaccines, and predispose astronauts to the onset of chronic diseases (e.g., immunosuppression, cardiovascular and metabolic diseases, gut dysbiosis). Other deleterious effects encountered by radiation may include cancer and premature aging, induced by dysregulated redox and metabolic processes, microbiota, immune cell function, endotoxin, and pro-inflammatory signal production^1,2. In this review, we summarize and highlight the current understanding of the effects of microgravity and radiation on the immune system and discuss knowledge gaps that future studies should address.

Stem Cell Therapy for Sequestration of Traumatic Brain Injury-Induced Inflammation

Traumatic brain injury (TBI) is one of the leading causes of long-term neurological disabilities in the world. TBI is a signature disease for soldiers and veterans, but also affects civilians, including adults and children. Following TBI, the brain resident and immune cells turn into a “reactive” state, characterized by the production of inflammatory mediators that contribute to the development of cognitive deficits. Other injuries to the brain, including radiation exposure, may trigger TBI-like pathology, characterized by inflammation. Currently there are no treatments to prevent or reverse the deleterious consequences of brain trauma. The recognition that TBI predisposes stem cell alterations suggests that stem cell-based therapies stand as a potential treatment for TBI. Here, we discuss the inflamed brain after TBI and radiation injury. We further review the status of stem cells in the inflamed brain and the applications of cell therapy in sequestering inflammation in TBI.

Chronic Hypergravity Induces a Modification of Histone H3 Lysine 27 Trimethylation at TCRβ Locus in Murine Thymocytes

Gravity changes are major stressors encountered during spaceflight that affect the immune system. We previously evidenced that hypergravity exposure during gestation affects the TCRβ repertoire of newborn pups. To identify the mechanisms underlying this observation, we studied post-translational histone modifications. We first showed that among the four studied post-translational histone H3 modifications, only lysine 27 trimethylation (H3K27me3) is downregulated in the thymus of mice exposed to 2× g for 21 days. We then asked whether the TCRβ locus chromatin structure is altered by hypergravity exposure. ChIP studies performed on four Vβ segments of the murine double-negative SCIET27 thymic cell line, which corresponds to the last maturation stage before V(D)J recombination, revealed increases in H3K27me3 after 2× g exposure. Finally, we evaluated the implication for the EZH2 methyltransferase in the regulation of the H3K27me3 level at these Vβ segments by treating SCIET27 cells with the GSK126-specific inhibitor. These experiments showed that the downregulation of H3K27me3 contributes to the regulation of the Vβ germline transcript expression that precedes V(D)J recombination. These data show that modifications of H3K27me3 at the TCRβ locus likely contribute to an explanation of why the TCR repertoire is affected by gravity changes and imply, for the first time, EZH2 in the regulation of the TCRβ locus chromatin structure.

B-Cell Homeostasis Is Maintained During Two Months of Head-Down Tilt Bed Rest With or Without Antioxidant Supplementation

Alterations of the immune system could seriously impair the ability to combat infections during future long-duration space missions. However, little is known about the effects of spaceflight on the B-cell compartment. Given the limited access to astronaut samples, we addressed this question using blood samples collected from 20 healthy male volunteers subjected to long-duration bed rest, an Earth-based analog of spaceflight. Hematopoietic progenitors, white blood cells, total lymphocytes and B-cells, four B-cell subsets, immunoglobulin isotypes, six cytokines involved in inflammation, cortisone and cortisol were quantified at five time points. Tibia microarchitecture was also studied. Moreover, we investigated the efficiency of antioxidant supplementation with a cocktail including polyphenols, omega 3, vitamin E and selenium. Our results show that circulating hematopoietic progenitors, white blood cells, total lymphocytes and B-cells, and B-cell subsets were not affected by bed rest. Cytokine quantification suggested a lower systemic inflammatory status, supported by an increase in serum cortisone, during bed rest. These data confirm the in vivo hormonal dysregulation of immunity observed in astronauts and show that bed rest does not alter B-cell homeostasis. This lack of an impact of long-term bed rest on B-cell homeostasis can, at least partially, be explained by limited bone remodeling. None of the evaluated parameters were affected by the administration of the antioxidant supplement. The non-effectiveness of the supplement may be because the diet provided to the non-supplemented and supplemented volunteers already contained sufficient antioxidants. Given the limitations of this model, further studies will be required to determine whether B-cell homeostasis is affected, especially during future deep-space exploration missions that will be of unprecedented durations.

From Target Identification to Drug Development in Space: Using the Microgravity Assist

The unique nature of microgravity encountered in space provides an opportunity for drug discovery and development that cannot be replicated on Earth. From the production of superior protein crystals to the identification and validation of new drug targets to microarray analyses of transcripts attenuated by microgravity, there are numerous examples which demonstrate the benefit of exploiting the space environment. Moreover, studies conducted on Space Shuttle missions, the International Space Station and other craft have had a direct benefit for drug development programmes such as those directed against reducing bone and muscle loss or increasing bone formation. This review will highlight advances made in both drug discovery and development and offer some future insight into how drug discovery and associated technologies may be further advanced using the microgravity assist.

Gravitational Experimental Platform for Animal Models, a New Platform at ESA's Terrestrial Facilities to Study the Effects of Micro- and Hypergravity on Aquatic and Rodent Animal Models

Using rotors to expose animals to different levels of hypergravity is an efficient means of understanding how altered gravity affects physiological functions, interactions between physiological systems and animal development. Furthermore, rotors can be used to prepare space experiments, e.g., conducting hypergravity experiments to demonstrate the feasibility of a study before its implementation and to complement inflight experiments by comparing the effects of micro- and hypergravity. In this paper, we present a new platform called the Gravitational Experimental Platform for Animal Models (GEPAM), which has been part of European Space Agency (ESA)’s portfolio of ground-based facilities since 2020, to study the effects of altered gravity on aquatic animal models (amphibian embryos/tadpoles) and mice. This platform comprises rotors for hypergravity exposure (three aquatic rotors and one rodent rotor) and models to simulate microgravity (cages for mouse hindlimb unloading and a random positioning machine (RPM)). Four species of amphibians can be used at present. All murine strains can be used and are maintained in a specific pathogen-free area. This platform is surrounded by numerous facilities for sample preparation and analysis using state-of-the-art techniques. Finally, we illustrate how GEPAM can contribute to the understanding of molecular and cellular mechanisms and the identification of countermeasures.

Plasticity of the human IgM repertoire in response to long-term spaceflight

Immune dysregulation is among the main adverse outcomes of spaceflight. Despite the crucial role of the antibody repertoire in host protection, the effects of spaceflight on the human antibody repertoire are unknown. Consequently, using high-throughput sequencing, we examined the IgM repertoire of five cosmonauts 25 days before launch, after 64 ± 11 and 129 ± 20 days spent on the International Space Station (ISS), and at 1, 7, and 30 days after landing. This is the first study of this kind in humans. Our data revealed that the IgM repertoire of the cosmonauts was different from that of control subjects (n = 4) prior to launch and that two out the five analyzed cosmonauts presented significant changes in their IgM repertoire during the mission. These modifications persisted up to 30 days after landing, likely affected the specificities of IgM binding sites, correlated with changes in the V(D)J recombination process responsible for creating antibody genes, and coincided with a higher stress response. These data confirm that the immune system of approximately half of the astronauts who spent 6 months on the ISS is sensitive to spaceflight conditions, and reveal individual responses indicating that personalized approaches should be implemented during future deep-space exploration missions that will be of unprecedented durations.

Hypergravity disrupts murine intestinal microbiota

During spaceflight, organisms are subjected to various physical stressors including modification of gravity (G) that, associated with lifestyle, could lead to impaired immunity, intestinal dysbiosis and thus potentially predispose astronauts to illness. Whether space travel affects microbiota homeostasis has not been thoroughly investigated. The aim of this study was to evaluate changes in intestinal microbiota and mucosa in a ground-based murine model consisting in a 21-days confinement of mice in a centrifuge running at 2 or 3G. Results revealed an increased α-diversity and a significant change in intracaecal β-diversity observed only at 3G, with profiles characterized by a decrease of the Firmicutes/Bacteroidetes ratio. Compared to 1G microbiota, 12.1% of the taxa were significantly impacted in 3G microbiota, most of them (78%) being enriched. This study shows a G-level-dependent disruption of intracaecal microbiota, without alteration of mucosal integrity. These first data reinforce those recently obtained with in-flight experimentations or microgravity models, and emphasize the critical need for further studies exploring the impact of spaceflight on intestinal microbiota in order to optimize long-term space travel conditions.

Modulation of Iberian Ribbed Newt Complement Component C3 by Stressors Similar to those Encountered during a Stay Onboard the International Space Station

The complement system plays an important role in inflammation, innate and acquired immunity, as well as homeostasis. Despite these functions, the effects of spaceflight conditions on the complement system have not yet been intensively studied. Consequently, we investigated the effects of five types of chronic stressors, similar to those encountered during a stay onboard the International Space Station, on C3 expression in larvae of the urodele amphibian Pleurodeles waltl. We focused on C3 because it is a critical component of this system. These studies were completed by the analysis of adult mice exposed to two models of inflight stressors. Our data show that simulating space radiation, or combining a modification of the circadian rhythm with simulated microgravity, affects the amount of C3 proteins. These results suggest that C3 expression could be modified under real spaceflight conditions, potentially increasing the risk of inflammation and associated tissue damage.

Analysis of femurs from mice embarked on board BION-M1 biosatellite reveals a decrease in immune cell development, including B cells, after 1 wk of recovery on Earth

Bone loss and immune dysregulation are among the main adverse outcomes of spaceflight challenging astronauts' health and safety. However, consequences on B-cell development and responses are still under-investigated. To fill this gap, we used advanced proteomics analysis of femur bone and marrow to compare mice flown for 1 mo on board the BION-M1 biosatellite, followed or not by 1 wk of recovery on Earth, to control mice kept on Earth. Our data revealed an adverse effect on B lymphopoiesis 1 wk after landing. This phenomenon was associated with a 41% reduction of B cells in the spleen. These reductions may contribute to explain increased susceptibility to infection even if our data suggest that flown animals can mount a humoral immune response. Future studies should investigate the quality/efficiency of produced antibodies and whether longer missions worsen these immune alterations.-Tascher, G., Gerbaix, M., Maes, P., Chazarin, B., Ghislin, S., Antropova, E., Vassilieva, G., Ouzren-Zarhloul, N., Gauquelin-Koch, G., Vico, L., Frippiat, J.-P., Bertile, F. Analysis of femurs from mice embarked on board BION-M1 biosatellite reveals a decrease in immune cell development, including B cells, after 1 wk of recovery on Earth.

IL-6 and the dysregulation of immune, bone, muscle, and metabolic homeostasis during spaceflight

We have previously reported that exercise-related secretion of IL-6 by peripheral blood mononuclear cells is proportionate to body weight, suggesting that IL-6 is gravisensitive and that suboptimal production of this key cytokine may contribute to homeostatic dysregulations that occur during spaceflight. This review details what is known about the role of this key cytokine in innate and adaptive immunity, hematopoiesis, and in bone, muscle and metabolic homeostasis on Earth and in the microgravity of space and suggests an experimental approach to confirm or disavow the role of IL-6 in space-related dysregulations.

Socioenvironmental stressors encountered during spaceflight partially affect the murine TCR-β repertoire and increase its self-reactivity

Spaceflights are known to affect the immune system. In a previous study, we demonstrated that hypergravity exposure during murine development modified 85% of the T-cell receptor (TCR)-β repertoire. In this study, we investigated whether socioenvironmental stressors encountered during space missions affect T lymphopoiesis and the TCR-β repertoire. To address this question, pregnant mice were subjected throughout gestation to chronic unpredictable mild stressors (CUMS), a model used to mimic socioenvironmental stresses encountered during space missions. Then, newborn T lymphopoiesis and the TCR-β repertoire were studied by flow cytometry and high-throughput sequencing, respectively. No change in thymocyte maturation or TCR expression were noted. TCR-β repertoire analysis revealed that 75% of neonate TCR-β sequences resulted from the expression of 3 variable (V)β segments and that this core repertoire was not affected by CUMS. However, the minor repertoire, representing 25% of the global repertoire, was sensitive to CUMS exposure. We also showed that the variable (diversity) joining [V(D)J] recombination process was unlikely to be affected. Finally, we noted that the CUMS neonatal minor repertoire was more self-reactive than the one of control pups. These findings show that socioenvironmental stressors such as those encountered during space missions affect a fraction (25%) of the TCR-β repertoire and that these stressors could increase self-reactivity.-Fonte, C., Kaminski, S., Vanet, A., Lanfumey, L., Cohen-Salmon, C., Ghislin, S., Frippiat, J.-P. Socioenvironmental stressors encountered during spaceflight partially affect the murine TCR-β repertoire and increase its self-reactivity.

Immune System Dysregulation During Spaceflight: Potential Countermeasures for Deep Space Exploration Missions

Recent studies have established that dysregulation of the human immune system and the reactivation of latent herpesviruses persists for the duration of a 6-month orbital spaceflight. It appears certain aspects of adaptive immunity are dysregulated during flight, yet some aspects of innate immunity are heightened. Interaction between adaptive and innate immunity also seems to be altered. Some crews experience persistent hypersensitivity reactions during flight. This phenomenon may, in synergy with extended duration and galactic radiation exposure, increase specific crew clinical risks during deep space exploration missions. The clinical challenge is based upon both the frequency of these phenomena in multiple crewmembers during low earth orbit missions and the inability to predict which specific individual crewmembers will experience these changes. Thus, a general countermeasure approach that offers the broadest possible coverage is needed. The vehicles, architecture, and mission profiles to enable such voyages are now under development. These include deployment and use of a cis-Lunar station (mid 2020s) with possible Moon surface operations, to be followed by multiple Mars flyby missions, and eventual human Mars surface exploration. Current ISS studies will continue to characterize physiological dysregulation associated with prolonged orbital spaceflight. However, sufficient information exists to begin consideration of both the need for, and nature of, specific immune countermeasures to ensure astronaut health. This article will review relevant in-place operational countermeasures onboard ISS and discuss a myriad of potential immune countermeasures for exploration missions. Discussion points include nutritional supplementation and functional foods, exercise and immunity, pharmacological options, the relationship between bone and immune countermeasures, and vaccination to mitigate herpes (and possibly other) virus risks. As the immune system has sentinel connectivity within every other physiological system, translational effects must be considered for all potential immune countermeasures. Finally, we shall discuss immune countermeasures in the context of their individualized implementation or precision medicine, based on crewmember specific immunological biases.

A Model of Chronic Exposure to Unpredictable Mild Socio-Environmental Stressors Replicates Some Spaceflight-Induced Immunological Changes

During spaceflight, astronauts face radiations, mechanical, and socio-environmental stressors. To determine the impact of chronic socio-environmental stressors on immunity, we exposed adult male mice to chronic unpredictable mild psychosocial and environmental stressors (CUMS model) for 3 weeks. This duration was chosen to simulate a long flight at the human scale. Our data show that this combination of stressors induces an increase of serum IgA, a reduction of normalized splenic mass and tends to reduce the production of pro-inflammatory cytokines, as previously reported during or after space missions. However, CUMS did not modify major splenic lymphocyte sub-populations and the proliferative responses of splenocytes suggesting that these changes could be due to other factors such as gravity changes. Thus, CUMS, which is an easy to implement model, could contribute to deepen our understanding of some spaceflight-associated immune alterations and could be useful to test countermeasures.

Effects of spaceflight on the immunoglobulin repertoire of unimmunized C57BL/6 mice

Spaceflight has been shown to suppress the adaptive immune response, altering the distribution and function of lymphocyte populations. B lymphocytes express highly specific and highly diversified receptors, known as immunoglobulins (Ig), that directly bind and neutralize pathogens. Ig diversity is achieved through the enzymatic splicing of gene segments within the genomic DNA of each B cell in a host. The collection of Ig specificities within a host, or Ig repertoire, has been increasingly characterized in both basic research and clinical settings using high-throughput sequencing technology (HTS). We utilized HTS to test the hypothesis that spaceflight affects the B-cell repertoire. To test this hypothesis, we characterized the impact of spaceflight on the unimmunized Ig repertoire of C57BL/6 mice that were flown aboard the International Space Station (ISS) during the Rodent Research One validation flight in comparison to ground controls. Individual gene segment usage was similar between ground control and flight animals, however, gene segment combinations and the junctions in which gene segments combine was varied among animals within and between treatment groups. We also found that spontaneous somatic mutations in the IgH and Igκ gene loci were not increased. These data suggest that space flight did not affect the B cell repertoire of mice flown and housed on the ISS over a short period of time.

Heat shock response and shape regulation during newt tail regeneration

Regenerating newt tail has recently been found to react to hypergravity in a stable and reproducible way - by curving downwards. Such morphogenetic effect of non-specific physical factor applied to a complex structure of an adult animal is a rare phenomenon with unknown molecular basis. For the first steps of unraveling this basis we've chosen heat shock proteins (HSPs) as promising candidates. Morphometrical analysis of tail regeneration was performed in aquarium (control), on substrate (relative hypergravity) and in aquarium under weekly application of heat shock. HSPs were inhibited pharmacologically during regeneration in aquarium and on substrate. Hsp70, 90 gene expression and protein localization were analyzed in the studied conditions. Weekly application of heat shock to newts regenerating tails in otherwise normal conditions led to development of curved tails (both upwards and downwards), suggesting that similar mechanisms are at play in both hypergravity-altered and heat shock-altered morphogenesis. Heat shock protein inhibitor KNK437 didn't affect tail shape during normal regeneration, but prevented the formation of tail curve in appropriate conditions. It was shown that HSP70 and HSP90 proteins are present in muscle and connective tissue of intact tails as well as regenerates, but only appear in epidermis in hypergravity-altered regenerates and heated tails. Based on our data, we hypothesize, that different external factors (e.g. hypergravity and heat shock) are received, analyzed and transmitted further to affect morphogenesis by similar mechanisms that utilize a set of HSP in epidermal cells.

Dynamic gene expression response to altered gravity in human T cells

We investigated the dynamics of immediate and initial gene expression response to different gravitational environments in human Jurkat T lymphocytic cells and compared expression profiles to identify potential gravity-regulated genes and adaptation processes. We used the Affymetrix GeneChip® Human Transcriptome Array 2.0 containing 44,699 protein coding genes and 22,829 non-protein coding genes and performed the experiments during a parabolic flight and a suborbital ballistic rocket mission to cross-validate gravity-regulated gene expression through independent research platforms and different sets of control experiments to exclude other factors than alteration of gravity. We found that gene expression in human T cells rapidly responded to altered gravity in the time frame of 20 s and 5 min. The initial response to microgravity involved mostly regulatory RNAs. We identified three gravity-regulated genes which could be cross-validated in both completely independent experiment missions: ATP6V1A/D, a vacuolar H + -ATPase (V-ATPase) responsible for acidification during bone resorption, IGHD3-3/IGHD3-10, diversity genes of the immunoglobulin heavy-chain locus participating in V(D)J recombination, and LINC00837, a long intergenic non-protein coding RNA. Due to the extensive and rapid alteration of gene expression associated with regulatory RNAs, we conclude that human cells are equipped with a robust and efficient adaptation potential when challenged with altered gravitational environments.

Towards human exploration of space: The THESEUS review series on immunology research priorities

Dysregulation of the immune system occurs during spaceflight and may represent a crew health risk during exploration missions because astronauts are challenged by many stressors. Therefore, it is crucial to understand the biology of immune modulation under spaceflight conditions in order to be able to maintain immune homeostasis under such challenges. In the framework of the THESEUS project whose aim was to develop an integrated life sciences research roadmap regarding human space exploration, experts working in the field of space immunology, and related disciplines, established a questionnaire sent to scientists around the world. From the review of collected answers, they deduced a list of key issues and provided several recommendations such as a maximal exploitation of currently available resources on Earth and in space, and to increase increments duration for some ISS crew members to 12 months or longer. These recommendations should contribute to improve our knowledge about spaceflight effects on the immune system and the development of countermeasures that, beyond astronauts, could have a societal impact.

Hypergravity exposure during gestation modifies the TCRβ repertoire of newborn mice

During spaceflight, organisms are subjected to mechanical force changes (gravity (G) changes) that affect the immune system. However, gravitational effects on lymphopoiesis have rarely been studied. Consequently, we investigated whether the TCRβ repertoire, created by V(D)J recombination during T lymphopoiesis, is affected by hypergravity exposure during murine development. To address this question, C57BL/6j mice were mated in a centrifuge so that embryonic development, birth and TCRβ rearrangements occurred at 2G. Pups were sacrificed at birth, and their thymus used to quantify transcripts coding for factors required for V(D)J recombination and T lymphopoiesis. We also created cDNA mini-libraries of TCRβ transcripts to study the impact of hypergravity on TCRβ diversity. Our data show that hypergravity exposure increases the transcription of TCRβ chains, and of genes whose products are involved in TCR signaling, and affects the V(D)J recombination process. We also observed that ~85% of the TCRβ repertoire is different between hypergravity and control pups. These data indicate that changing a mechanical force (the gravity) during ontogeny will likely affect host immunity because properties of loops constituting TCR antigen-binding sites are modified in hypergravity newborns. The spectrum of peptides recognized by TCR will therefore likely be different.

Hind limb unloading, a model of spaceflight conditions, leads to decreased B lymphopoiesis similar to aging

Within the bone marrow, the endosteal niche plays a crucial role in B-cell differentiation. Because spaceflight is associated with osteoporosis, we investigated whether changes in bone microstructure induced by a ground-based model of spaceflight, hind limb unloading (HU), could affect B lymphopoiesis. To this end, we analyzed both bone parameters and the frequency of early hematopoietic precursors and cells of the B lineage after 3, 6, 13, and 21 d of HU. We found that limb disuse leads to a decrease in both bone microstructure and the frequency of B-cell progenitors in the bone marrow. Although multipotent hematopoietic progenitors were not affected by HU, a decrease in B lymphopoiesis was observed as of the common lymphoid progenitor (CLP) stage with a major block at the progenitor B (pro-B) to precursor B (pre-B) cell transition (5- to 10-fold decrease). The modifications in B lymphopoiesis were similar to those observed in aged mice and, as with aging, decreased B-cell generation in HU mice was associated with reduced expression of B-cell transcription factors, early B-cell factor (EBF) and Pax5, and an alteration in STAT5-mediated IL-7 signaling. These findings demonstrate that mechanical unloading of hind limbs results in a decrease in early B-cell differentiation resembling age-related modifications in B lymphopoiesis.

Molecular cloning and expression analysis of Pleurodeles waltl complement component C3 under normal physiological conditions and environmental stresses

C3 is a component of the complement system that plays a central role in immunity, development and tissue regeneration. In this study, we isolated the C3 cDNA of the Iberian ribbed newt Pleurodeles waltl. This cDNA encodes a 1637 amino acid protein with an estimated molecular mass of 212.5 kDa. The deduced amino acid sequence showed that P. waltl C3 contains all the conserved domains known to be critical for C3 function. Quantitative real-time PCR (qRT-PCR) demonstrated that under normal physiological conditions, P. waltl C3 mRNA is expressed early during development because it is likely required for neurulation. Then, its expression increased as the immune system developed. In adults, the liver is the richest source of C3, though other tissues can also contribute. Further analysis of C3 expression demonstrated that C3 transcription increased when P. waltl larvae were exposed to pH or temperature stress, suggesting that environmental modifications might affect this animal's defenses against pathogens.

Three weeks of murine hindlimb unloading induces shifts from B to T and from th to tc splenic lymphocytes in absence of stress and differentially reduces cell-specific mitogenic responses

Extended space missions are known to induce stress and immune dysregulation. Hindlimb unloading is a ground-based model used to reproduce most spaceflight conditions. The aim of this study was to better characterize the consequences of prolonged exposure to hindlimb unloading on murine splenic lymphocyte sub-populations. To ensure that the observed changes were not due to tail restraint but to the antiorthostatic position, three groups of mice were used: control (C), orthostatic restrained (R) and hindlimb unloaded (HU). After 21 days of exposure, no difference in serum corticosterone levels nor in thymus and spleen weights were observed between HU mice and their counterparts, revealing a low state of stress. Interestingly, flow cytometric analyses showed that B cells were drastically reduced in HU mouse spleens by 59% and, while the T cells number did not change, the Th/Tc ratio was decreased. Finally, the use of a fluorescent dye monitoring lymphoproliferation demonstrated that lymphocyte response to mitogen was reduced in Th and Tc populations and to a greater extent in B cells. Thus, we showed for the first time that, even if restraint has its own effects on the animals and their splenic lymphocytes, the prolonged antiorthostatic position leads, despite the absence of stress, to an inversion of the B/T ratio in the spleen. Furthermore, the lymphoproliferative response was impaired with a strong impact on B cells. Altogether, these results suggest that B cells are more affected by hindlimb unloading than T cells which may explain the high susceptibility to pathogens, such as gram-negative bacteria, described in animal models and astronauts.

Modulation of Pleurodeles waltl DNA polymerase mu expression by extreme conditions encountered during spaceflight

DNA polymerase µ is involved in DNA repair, V(D)J recombination and likely somatic hypermutation of immunoglobulin genes. Our previous studies demonstrated that spaceflight conditions affect immunoglobulin gene expression and somatic hypermutation frequency. Consequently, we questioned whether Polμ expression could also be affected. To address this question, we characterized Polμ of the Iberian ribbed newt Pleurodeles waltl and exposed embryos of that species to spaceflight conditions or to environmental modifications corresponding to those encountered in the International Space Station. We noted a robust expression of Polμ mRNA during early ontogenesis and in the testis, suggesting that Polμ is involved in genomic stability. Full-length Polμ transcripts are 8–9 times more abundant in P. waltl than in humans and mice, thereby providing an explanation for the somatic hypermutation predilection of G and C bases in amphibians. Polμ transcription decreases after 10 days of development in space and radiation seem primarily involved in this down-regulation. However, space radiation, alone or in combination with a perturbation of the circadian rhythm, did not affect Polμ protein levels and did not induce protein oxidation, showing the limited impact of radiation encountered during a 10-day stay in the International Space Station.