Effect of SLS-2 spaceflight on immunologic parameters of rats

Short-Term Reloading After Prolonged Unloading Ensures Restoration of Stromal but Not Hematopoietic Precursor Activity in Tibia Bone Marrow of C57Bl/6N Mice

Bone and muscle tissues are mostly susceptible to different kinds of hypodynamia, including real and simulated microgravity (sμg). To evaluate the effect of sμg on bone marrow (BM), male C57Bl/6N mice were divided into three groups: vivarium control (VC), 30-day hindlimb suspension (HS), and subsequent 12-h short-term support reloading (RL). The effects on BM total mononucleated cells (MNCs) as well as stromal and hematopoietic progenitors from murine tibia were studied. The number of BM MNCs, immunophenotype, proliferation, colony-forming units (CFUs), differentiation and secretory activity of hematopoietic and stromal BM cells were determined. HS led to a twofold decrease in MNCs, alteration of surface molecule expression profiles, suppression of proliferative activity of BM cells, and change of soluble mediators' levels. The stromal compartment was characterized by a decrease of CFU of fibroblasts and suppression of spontaneous osteo-commitment after HS. Among the hematopoietic precursors, a decrease in the total number of CFUs was found mainly at the expense of suppression of CFU-GM and CFU-GEMM. After RL, restoration of the stromal precursor's functional activity to control levels and overabundance of paracrine mediator's production were detected, whereas the complete recovery of hematopoietic precursor's activity did not occur. These data demonstrate the fast functional reaction of the stromal compartment on restoration of loading support.

Advances in space microbiology

Microbial research in space is being conducted for almost 50 years now. The closed system of the International Space Station (ISS) has acted as a microbial observatory for the past 10 years, conducting research on adaptation and survivability of microorganisms exposed to space conditions. This adaptation can be either beneficial or detrimental to crew members and spacecraft. Therefore, it becomes crucial to identify the impact of two primary stress conditions, namely, radiation and microgravity, on microbial life aboard the ISS. Elucidating the mechanistic basis of microbial adaptation to space conditions aids in the development of countermeasures against their potentially detrimental effects and allows us to harness their biotechnologically important properties. Several microbial processes have been studied, either in spaceflight or using devices that can simulate space conditions. However, at present, research is limited to only a few microorganisms, and extensive research on biotechnologically important microorganisms is required to make long-term space missions self-sustainable.

Validation of Methods to Assess the Immunoglobulin Gene Repertoire in Tissues Obtained from Mice on the International Space Station

Spaceflight is known to affect immune cell populations. In particular, splenic B-cell numbers decrease during spaceflight and in ground-based physiological models. Although antibody isotype changes have been assessed during and after spaceflight, an extensive characterization of the impact of spaceflight on antibody composition has not been conducted in mice. Next Generation Sequencing and bioinformatic tools are now available to assess antibody repertoires. We can now identify immunoglobulin gene-segment usage, junctional regions, and modifications that contribute to specificity and diversity. Due to limitations on the International Space Station, alternate sample collection and storage methods must be employed. Our group compared Illumina MiSeq® sequencing data from multiple sample preparation methods in normal C57Bl/6J mice to validate that sample preparation and storage would not bias the outcome of antibody repertoire characterization. In this report, we also compared sequencing techniques and a bioinformatic workflow on the data output when we assessed the IgH and Igκ variable gene usage. Our bioinformatic workflow has been optimized for Illumina HiSeq® and MiSeq® datasets, and is designed specifically to reduce bias, capture the most information from Ig sequences, and produce a data set that provides other data mining options.

Effects of skeletal unloading on the antibody repertoire of tetanus toxoid and/or CpG treated C57BL/6J mice

Spaceflight affects the immune system, but the effects on the antibody repertoire, responsible for humoral immunity, has not been well explored. In particular, the complex gene assembly and expression process; including mutations, might make this process vulnerable. Complementarity determining region 3 (CDR3), composed of parts of the V-(D-)J-gene segments, is very important for antigen binding and can be used as an important measure of variability. Skeletal unloading, and the physiological effects of it, parallel many impacts of space flight. Therefore, we explored the impact of skeletal unloading using the antiorthostatic suspension (AOS) model. Animals were experimentally challenged with tetanus toxoid (TT) and/or the adjuvant CpG. Blood was analyzed for anti-TT antibody and corticosterone concentrations. Whole spleen tissue was prepared for repertoire characterization. AOS animals showed higher levels of corticosterone levels, but AOS alone did not affect anti-TT serum antibody levels. Administration of CpG significantly increased the circulating anti-TT antibody concentrations. AOS did alter constant gene usage resulting in higher levels of IgM and lower levels of IgG. CpG also altered constant gene region usage increasing usage of IgA. Significant changes could be detected in multiple V-, D-, and J-gene segments in both the heavy and light chains in response to AOS, TT, and CpG treatments. Analysis of class-switched only transcripts revealed a different pattern of V-gene segment usage than detected in the whole repertoire and also showed significant alterations in gene segment usage after challenge. Alterations in V/J pairing were also detected in response to challenge. CDR3 amino acid sequence overlaps were similar among treatment groups, though the addition of CpG lowered overlap in the heavy chain. We isolated 3,045 whole repertoire and 98 potentially TT-specific CDR3 sequences for the heavy chain and 569 for the light chain. Our results demonstrate that AOS alters the repertoire response to challenge with TT and/or CpG.

Proteomic analysis and bioluminescent reporter gene assays to investigate effects of simulated microgravity on Caco-2 cells

Microgravity is one of the most important features in spaceflight. Previous evidence from in-vitro studies has shown that significant changes occur under simulated microgravity. For this reason, human colon adenocarcinoma Caco-2 cells were selected as cell model of intestinal epithelial barrier and their response to altered gravity conditions was investigated, especially on the protein level. In this study, we combined label-free shotgun proteomics and bioluminescent reporter gene assays to identify key proteins and pathways involved in the response of Caco-2 cells under reference and microgravity conditions. A two-dimensional clinostat was modified with 3D-printed adaptors to hold conventional T25 culture flasks. The comparative proteome analysis led to identify 38 and 26 proteins differently regulated by simulated microgravity after 48 and 72 h, respectively. Substantial fractions of these proteins are involved in regulation, cellular and metabolic processes and localization. Bioluminescent reporter gene assays were carried out to investigate microgavity-induced alterations on the transcriptional regulation of key targets, such as NF-kB pathway and CYP27A1. While no significant difference was found in the basal transcription, a lower NF-kB basal activation in simulated microgravity conditions was reported, corroborating the hypothesis of reduced immunity in microgravity conditions.

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.

Hormetic Effect of Chronic Hypergravity in a Mouse Model of Allergic Asthma and Rhinitis

We aimed to evaluate the effect of chronic hypergravity in a mouse model of allergic asthma and rhinitis. Forty BALB/c mice were divided as: group A (n = 10, control) sensitized and challenged with saline, group B (n = 10, asthma) challenged by intraperitoneal and intranasal ovalbumin (OVA) to induce allergic asthma and rhinitis, and groups C (n = 10, asthma/rotatory control) and D (n = 10, asthma/hypergravity) exposed to 4 weeks of rotation with normogravity (1G) or hypergravity (5G) during induction of asthma/rhinitis. Group D showed significantly decreased eosinophils, neutrophils, and lymphocytes in their BAL fluid compared with groups B and C (p < 0.05). In real-time polymerase chain reaction using lung homogenate, the expression of IL-1β was significantly upregulated (p < 0.001) and IL-4 and IL-10 significantly downregulated (p < 0.05) in group D. Infiltration of inflammatory cells into lung parenchyma and turbinate, and the thickness of respiratory epithelium was significantly reduced in group D (p < 0.05). The expression of Bcl-2 and heme oxygenase-1 were significantly downregulated, Bax and extracellular dismutase significantly upregulated in Group D. Therefore, chronic hypergravity could have a hormetic effect for allergic asthma and rhinitis via regulation of genes involved in antioxidative and proapoptotic pathways. It is possible that we could use hypergravity machinery for treating allergic respiratory disorders.

Mechanotransduction as an Adaptation to Gravity

Gravity has played a critical role in the development of terrestrial life. A key event in evolution has been the development of mechanisms to sense and transduce gravitational force into biological signals. The objective of this manuscript is to review how living organisms on Earth use mechanotransduction as an adaptation to gravity. Certain cells have evolved specialized structures, such as otoliths in hair cells of the inner ear and statoliths in plants, to respond directly to the force of gravity. By conducting studies in the reduced gravity of spaceflight (microgravity) or simulating microgravity in the laboratory, we have gained insights into how gravity might have changed life on Earth. We review how microgravity affects prokaryotic and eukaryotic cells at the cellular and molecular levels. Genomic studies in yeast have identified changes in genes involved in budding, cell polarity, and cell separation regulated by Ras, PI3K, and TOR signaling pathways. Moreover, transcriptomic analysis of late pregnant rats have revealed that microgravity affects genes that regulate circadian clocks, activate mechanotransduction pathways, and induce changes in immune response, metabolism, and cells proliferation. Importantly, these studies identified genes that modify chromatin structure and methylation, suggesting that long-term adaptation to gravity may be mediated by epigenetic modifications. Given that gravity represents a modification in mechanical stresses encounter by the cells, the tensegrity model of cytoskeletal architecture provides an excellent paradigm to explain how changes in the balance of forces, which are transmitted across transmembrane receptors and cytoskeleton, can influence intracellular signaling pathways and gene expression.

Infection prevention and control during prolonged human space travel

Prolonged human spaceflight to another planet or an asteroid will introduce unique challenges of mitigating the risk of infection. During space travel, exposure to microgravity, radiation, and stress alter human immunoregulatory responses, which can in turn impact an astronaut's ability to prevent acquisition of infectious agents or reactivation of latent infection. In addition, microgravity affects virulence, growth kinetics, and biofilm formation of potential microbial pathogens. These interactions occur in a confined space in microgravity, providing ample opportunity for heavy microbial contamination of the environment. In addition, there is the persistence of aerosolized, microbe-containing particles. Any mission involving prolonged human spaceflight must be carefully planned to minimize vulnerabilities and maximize the likelihood of success.

The microbiome: the forgotten organ of the astronaut’s body – probiotics beyond terrestrial limits

Space medicine research has drawn immense attention toward provision of efficient life support systems during long-term missions into space. However, in extended missions, a wide range of diseases may affect astronauts. In space medicine research, the gastrointestinal microbiome and its role in maintaining astronauts’ health has received little attention. We would like to draw researchers’ attention to the significant role of microbiota. Because of the high number of microorganisms in the human body, man has been called a ‘supra-organism’ and gastrointestinal flora has been referred to as ‘a virtual organ of the human body’. In space, the lifestyle, sterility of spaceship and environmental stresses can result in alterations in intestinal microbiota, which can lead to an impaired immunity and predispose astronauts to illness. This concern is heightened by increase in virulence of pathogens in microgravity. Thus, design of a personal probiotic kit is recommended to improve the health status of astronauts.

The Rel/NF-κB pathway and transcription of immediate early genes in T cell activation are inhibited by microgravity

This study tested the hypothesis that transcription of immediate early genes is inhibited in T cells activated in μg. Immunosuppression during spaceflight is a major barrier to safe, long-term human space habitation and travel. The goals of these experiments were to prove that μg was the cause of impaired T cell activation during spaceflight, as well as understand the mechanisms controlling early T cell activation. T cells from four human donors were stimulated with Con A and anti-CD28 on board the ISS. An on-board centrifuge was used to generate a 1g simultaneous control to isolate the effects of μg from other variables of spaceflight. Microarray expression analysis after 1.5 h of activation demonstrated that μg- and 1g-activated T cells had distinct patterns of global gene expression and identified 47 genes that were significantly, differentially down-regulated in μg. Importantly, several key immediate early genes were inhibited in μg. In particular, transactivation of Rel/NF-κB, CREB, and SRF gene targets were down-regulated. Expression of cREL gene targets were significantly inhibited, and transcription of cREL itself was reduced significantly in μg and upon anti-CD3/anti-CD28 stimulation in simulated μg. Analysis of gene connectivity indicated that the TNF pathway is a major early downstream effector pathway inhibited in μg and may lead to ineffective proinflammatory host defenses against infectious pathogens during spaceflight. Results from these experiments indicate that μg was the causative factor for impaired T cell activation during spaceflight by inhibiting transactivation of key immediate early genes.

Stress response and humoral immune system alterations related to chronic hypergravity in mice

Spaceflights are known to induce stress and immune dysregulation. Centrifugation, as hindlimb unloading, is a good ground based-model to simulate altered gravity which occurs during space missions. The aim of this study was to investigate the consequences of a long-term exposure to different levels of hypergravity on the stress response and the humoral immunity in a mouse model. For this purpose, adult C57Bl/6J male mice were subjected for 21 days either to control conditions or to 2G or 3G acceleration gravity forces. Corticosterone level and anxiety behavior revealed a stress response which was associated with a decrease of body weight, after 21-day of centrifugation at 3G but not at 2G. Spleen lymphocyte lipopolysaccharide (LPS) responsiveness was diminished by 40% in the 2G group only, whereas a decrease was noted when cells were stimulated with concanavalin A for both 2G and 3G groups (about 25% and 20%, respectively) compared to controls. Pro-inflammatory chemokines (MCP-1 and IP-10) and Th1 cytokines (IFNγ and IL2) were slightly decreased in the 2G group and strongly decreased in the 3G mouse group. Regarding Th2 cytokines (IL4, IL5) no further significant modification was observed, whereas the immunosuppressive cytokine IL10 was slightly increased in the 3G mice. Finally, serum IgG concentration was twice higher whereas IgA concentration was slightly increased (about 30%) and IgM were unchanged in 2G mice compared to controls. No difference was observed in the 3G group with these isotypes. Consequently, functional immune dysregulations and stress responses were dependent of the gravity level.

Spaceflight effects on T lymphocyte distribution, function and gene expression

The immune system is highly sensitive to stressors present during spaceflight. The major emphasis of this study was on the T lymphocytes in C57BL/6NTac mice after return from a 13-day space shuttle mission (STS-118). Spleens and thymuses from flight animals (FLT) and ground controls similarly housed in animal enclosure modules (AEM) were evaluated within 3-6 h after landing. Phytohemagglutinin-induced splenocyte DNA synthesis was significantly reduced in FLT mice when based on both counts per minute and stimulation indexes (P < 0.05). Flow cytometry showed that CD3(+) T and CD19(+) B cell counts were low in spleens from the FLT group, whereas the number of NK1.1(+) natural killer (NK) cells was increased (P < 0.01 for all three populations vs. AEM). The numerical changes resulted in a low percentage of T cells and high percentage of NK cells in FLT animals (P < 0.05). After activation of spleen cells with anti-CD3 monoclonal antibody, interleukin-2 (IL-2) was decreased, but IL-10, interferon-gamma, and macrophage inflammatory protein-1alpha were increased in FLT mice (P < 0.05). Analysis of cancer-related genes in the thymus showed that the expression of 30 of 84 genes was significantly affected by flight (P < 0.05). Genes that differed from AEM controls by at least 1.5-fold were Birc5, Figf, Grb2, and Tert (upregulated) and Fos, Ifnb1, Itgb3, Mmp9, Myc, Pdgfb, S100a4, Thbs, and Tnf (downregulated). Collectively, the data show that T cell distribution, function, and gene expression are significantly modified shortly after return from the spaceflight environment.

Alterations in the virulence potential of enteric pathogens and bacterial-host cell interactions under simulated microgravity conditions

Host immune mechanisms were proposed to decline under microgravity conditions during spaceflights, which might result in severe infections in astronauts. Therefore, it was important to investigate the effects of microgravity on infecting organisms and their interaction with host cells. Data showed that simulated microgravity (SMG) conditions markedly increased production of the enterotoxigenic Escherichia coli (ETEC) heat-labile enterotoxin, which induced fluid secretory responses in a mouse model. SMG also enhanced production of tumor necrosis factor-alpha in murine macrophages infected with enteropathogenic E. coli (EPEC). In a similar fashion, simulated microgravity conditions augmented the invasive potential of Salmonella enterica serovar typhimurium and enhanced production of tumor necrosis-factor alpha in S. typhimurium-infected epithelial cells. Furthermore, coculturing of macrophages and S. typhimurium in a simulated microgravity environment resulted in activation of stress-associated mitogen-activated protein kinase kinase 4. Using the antiorthostatic tail suspension mouse model, which simulates some aspects of microgravity, oral inoculation of S. typhimurium markedly reduced the 50% lethal dose compared to mice infected under normal gravitational conditions. Microarray analysis revealed simulated microgravity-induced alterations in the expression of 22 genes in S. typhimurium, and protein expression profiles were altered in both EPEC and S. typhimurium, based on two-dimensional gel electrophoresis. These studies indicated alterations in the virulence potential of bacteria and in host responses to these pathogens under simulated microgravity conditions, which may represent an important environmental signal. Such studies are essential for better understanding bacterial-host cell interactions, particularly in the context of spaceflights and space habitations of long duration.

Mouse infection models for space flight immunology

Several immunological processes can be affected by space flight. However, there is little evidence to suggest that flight-induced immunological deficits lead to illness. Therefore, one of our goals has been to define models to examine host resistance during space flight. Our working hypothesis is that space flight crews will come from a heterogeneous population; the immune response gene make-up will be quite varied. It is unknown how much the immune response gene variation contributes to the potential threat from infectious organisms, allergic responses or other long term health problems (e.g. cancer). This article details recent efforts of the Kansas State University gravitational immunology group to assess how population heterogeneity impacts host health, either in laboratory experimental situations and/or using the skeletal unloading model of space-flight stress. This paper details our use of several mouse strains with several different genotypes. In particular, mice with varying MHCII allotypes and mice on the C57BL background with different genetic defects have been particularly useful tools with which to study infections by Staphylococcus aureus, Salmonella typhimurium, Pasteurella pneumotropica and Ehrlichia chaffeensis. We propose that some of these experimental challenge models will be useful to assess the effects of space flight on host resistance to infection.

Effects of a long-term spaceflight on immunoglobulin heavy chains of the urodele amphibian Pleurodeles waltl

A variety of immune parameters are modified during and after a spaceflight. The effects of spaceflights on cellular immunity are well documented; however, little is known about the effects of these flights on humoral immunity. During the Genesis space experiment, two adult Pleurodeles waltl (urodele amphibian) stayed 5 mo onboard Mir and were subjected to oral immunization. Animals were killed 10 days after their return to earth. IgM and IgY heavy-chain transcripts in their spleens were quantified by Northern blotting. The use of the different VH families (coding for antibody heavy-chain variable domains) in IgM heavy chain transcripts was also analyzed. Results were compared with those obtained with ground control animals and animals reared in classical conditions in our animal facilities. We observed that, 10 days after the return on earth, the level of IgM heavy-chain transcription was normal but the level of IgY heavy-chain transcription was at least three times higher than in control animals. We also observed that the use of the different VH families in IgM heavy-chain transcripts was modified by the flight. These data suggest that the spaceflight affected the antibody response against the antigens contained in the food.

Microbial responses to microgravity and other low-shear environments

Microbial adaptation to environmental stimuli is essential for survival. While several of these stimuli have been studied in detail, recent studies have demonstrated an important role for a novel environmental parameter in which microgravity and the low fluid shear dynamics associated with microgravity globally regulate microbial gene expression, physiology, and pathogenesis. In addition to analyzing fundamental questions about microbial responses to spaceflight, these studies have demonstrated important applications for microbial responses to a ground-based, low-shear stress environment similar to that encountered during spaceflight. Moreover, the low-shear growth environment sensed by microbes during microgravity of spaceflight and during ground-based microgravity analogue culture is relevant to those encountered during their natural life cycles on Earth. While no mechanism has been clearly defined to explain how the mechanical force of fluid shear transmits intracellular signals to microbial cells at the molecular level, the fact that cross talk exists between microbial signal transduction systems holds intriguing possibilities that future studies might reveal common mechanotransduction themes between these systems and those used to sense and respond to low-shear stress and changes in gravitation forces. The study of microbial mechanotransduction may identify common conserved mechanisms used by cells to perceive changes in mechanical and/or physical forces, and it has the potential to provide valuable insight for understanding mechanosensing mechanisms in higher organisms. This review summarizes recent and future research trends aimed at understanding the dynamic effects of changes in the mechanical forces that occur in microgravity and other low-shear environments on a wide variety of important microbial parameters.

Hypergravity-induced immunomodulation in a rodent model: hematological and lymphocyte function analyses

The major purpose of this study was to quantify hypergravity-induced changes in erythrocyte and thrombocyte characteristics, spontaneous and mitogen-induced lymphoblastogenesis, and capacity of splenocytes to secrete immunoregulatory cytokines. C57BL/6 mice were subjected to chronic 1, 2, and 3 G; subsets were euthanized after 1, 4, 7, 10, and 21 days of centrifugation. Erythrocyte counts, hematocrit, and hemoglobin were significantly reduced by day 21 in both centrifuged groups. Hemoglobin concentration and volume per red blood cell were generally low, but an early, transient spike above normal was noted in thrombocyte counts in the 3-G group. Fluctuations above and below normal in blood and spleen cell spontaneous blastogenesis were dependent on the length of centrifugation time and not on the level of gravity. Depression in splenocyte responses to phytohemagglutinin and lipopolysaccharide due to gravity were noted when the data were expressed as stimulation indexes. Cytokine production by spleen cells was primarily affected during the first week of centrifugation: IL-2, IL-4, and tumor necrosis factor-alpha increased, whereas interferon-gamma decreased. These findings, although not identical to those reported for spaceflight, indicate that altered gravity can influence both hematological and functional variables that may translate into serious health consequences during extended missions.

Genetic models in applied physiology: selected contribution: effects of spaceflight on immunity in the C57BL/6 mouse. II. Activation, cytokines, erythrocytes, and platelets

This portion of the study quantified the effects of a 12-day space shuttle mission (Space Transport System-108/UF-1) on body and lymphoid organ masses, activation marker expression, cytokine secretion, and erythrocyte and thrombocyte characteristics in C57BL/6 mice. Animals in flight (Flt group) had 10-12% lower body mass compared with ground controls housed either in animal enclosure modules or under standard vivarium conditions (P < 0.001) and the smallest thymus and spleen masses. Percentages of CD25(+) lymphocytes, CD3(+)/CD25(+) T cells, and NK1.1(+)/CD25(+) natural killer cells from Flt mice were higher compared with both controls (P < 0.05). In contrast, CD71 expression was depressed in the Flt and animal enclosure module control mice compared with vivarium control animals (P < 0.001). Secretion of interferon-gamma, IL-2, and IL-4, but not tumor necrosis factor-alpha and IL-5, by splenocytes from Flt mice was decreased relative to either one or both ground controls (P < 0.05). Flt mice also had high red blood cell and thrombocyte counts compared with both sets of controls; low red blood cell volume and distribution width, percentage of reticulocytes, and platelet volume were also noted (P < 0.05) and were consistent with dehydration. These data indicate that relatively short exposure to the spaceflight environment can induce profound changes that may become significant during long-term space missions.

Genetic models in applied physiology: selected contribution: effects of spaceflight on immunity in the C57BL/6 mouse. I. Immune population distributions

There are several aspects of the spaceflight environment that may lead to changes in immunity: mission-related psychological stress, radiation, and changes in gravity. On December 5, 2001, the space shuttle Endeavor launched for a 12-day mission to examine these effects on C57BL/6 mice for the first time. On their return, assays were performed on the spleen, blood, and bone marrow. In response to flight, there were no significant differences in the general circulating leukocyte proportions. In contrast, there was an increase in splenic lymphocyte percentages, with a corresponding decrease in granulocytes. There was an overall shift in splenic lymphocytes away from T cells toward B cells, and a decrease in the CD4-to-CD8 ratios due to a decrease in T helpers. In contrast, there were proportional increases in bone marrow T cells, with decreases in B cells. Although the blast percentage and count were decreased in flight mice, the CD34(+) population was increased. The data were more consistent with a shift in bone marrow populations rather than a response to changes in the periphery. Many of the results are similar to those using other models. Clearly, spaceflight can influence immune parameters ranging from hematopoiesis to mature leukocyte mechanisms.

Animal models for the study of the effects of spaceflight on the immune system

Animal models have been used to determine the effects of spaceflight on the immune system. Rats and rhesus monkeys have been the primary animals used for actual space flight studies, but mice have also been utilized for studies in ground-based models. The primary ground based model used has been hindlimb unloading of rodents, which is similar to the chronic bed-rest model for humans. A variety of immune responses have been shown to be modified when animals are hindlimb unloaded. These results parallel those observed when animals are flown in space. In general, immune responses are depressed in animals maintained in the hindlimb unloading model or flown in space. These results raise the possibility that spaceflight could result in decreased resistance to infection in animals.

Effects of the space flight environment on the immune system

Space flight conditions have a dramatic effect on a variety of physiologic functions of mammals, including muscle, bone, and neurovestibular function. Among the physiological functions that are affected when humans or animals are exposed to space flight conditions is the immune response. The focus of this review is on the function of the immune system in space flight conditions during actual space flights, as well as in models of space flight conditions on the earth. The experiments were carried out in tissue culture systems, in animal models, and in human subjects. The results indicate that space flight conditions alter cell-mediated immune responses, including lymphocyte proliferation and subset distribution, and cytokine production. The mechanism(s) of space flight-induced alterations in immune system function remain(s) to be established. It is likely, however, that multiple factors, including microgravity, stress, neuroendocrine factors, sleep disruption, and nutritional factors, are involved in altering certain functions of the immune system. Such alterations could lead to compromised defenses against infections and tumors.

The immune system in space and microgravity

Space flight and models that created conditions similar to those that occur during space flight have been shown to affect a variety of immunological responses. These have primarily been cell-mediated immune responses including leukocyte proliferation, cytokine production, and leukocyte subset distribution. The mechanisms and biomedical consequences of these changes remain to be established. Among the possible causes of space flight-induced alterations in immune responses are exposure to microgravity, exposure to stress, exposure to radiation, and many more as yet undetermined causes. This review chronicles the known effects of space flight on the immune system and explores the possible role of stress in contributing to these changes.

Immune function during space flight

It is very likely that the human immune system will be altered in astronauts exposed to the conditions of long-term space flight: isolation, containment, microgravity, radiation, microbial contamination, sleep disruption, and insufficient nutrition. In human and animal subjects flown in space, there is evidence of immune compromise, reactivation of latent virus infection, and possible development of a premalignant or malignant condition. Moreover, in ground-based space flight model investigations, there is evidence of immune compromise and reactivation of latent virus infection. All of these observations in space flight itself or in ground-based models of space flight have a strong resonance in a wealth of human pathologic conditions involving the immune system where reactivated virus infections and cancer appear as natural consequences. The clinical conditions of Epstein-Barr-driven lymphomas in transplant patients and Kaposi's sarcoma in patients with autoimmune deficiency virus come easily to mind in trying to identify these conditions. With these thoughts in mind, it is highly appropriate, indeed imperative, that careful investigations of human immunity, infection, and cancer be made by space flight researchers.

Insulin secretion and sensitivity in space flight: diabetogenic effects

Nearly three decades of space flight research have suggested that there are subclinical diabetogenic changes that occur in microgravity. Alterations in insulin secretion, insulin sensitivity, glucose tolerance, and metabolism of protein and amino acids support the hypothesis that insulin plays an essential role in the maintenance of muscle mass in extended-duration space flight. Experiments in flight and after flight and ground-based bedrest studies have associated microgravity and its experimental paradigms with manifestations similar to those of diabetes, physical inactivity, and aging. We propose that these manifestations are characterized best by an etiology that falls into the clinical category of "other" causes of diabetes, including, but not restricted to, genetic beta-cell defects, insulin action defects, diseases of the endocrine pancreas, endocrinopathies, drug or chemically induced diabetes, infections, immune-mediated metabolic alteration, and a host of genetic related diseases. We present data showing alterations in tumor necrosis factor-alpha production, insulin secretion, and amino acid metabolism in pancreatic islets of Langerhans cultured in a ground-based cell culture bioreactor that mimics some of the effects of microgravity. Taken together, space flight research, ground-based studies, and bioreactor studies of pancreatic islets of Langerhans support the hypothesis that the pancreas is unable to overcome peripheral insulin resistance and amino acid dysregulation during space flight. We propose that measures of insulin secretion and insulin action will be necessary to design effective countermeasures against muscle loss, and we advance the "disposition index" as an essential model to be used in the clinical management of space flight-induced muscle loss.

Dose and dose rate effects of whole-body proton-irradiation on lymphocyte blastogenesis and hematological variables: part II

The goal of part II of this study was to evaluate functional characteristics of leukocytes and circulating blood cell parameters after whole-body proton irradiation at varying doses and at low- and high-dose-rates (LDR and HDR, respectively). C57BL/6 mice (n=51) were irradiated and euthanized at 4 days post-exposure for assay. Significant radiation dose- (but not dose-rate-) dependent decreases were observed in splenocyte responses to T and B cell mitogens when compared to sham-irradiated controls (P<0.001). Spontaneous blastogenesis, also significantly dose-dependent, was increased in both blood and spleen (P<0.001). Red blood cell counts, hemoglobin concentration, and hematocrit were decreased in a dose-dependent manner (P<0.05), whereas thrombocyte numbers were only slightly affected. Comparison of proton- and gamma-irradiated groups (both receiving 3 Gy at HDR) showed a higher level of spontaneous blastogenesis in blood leukocytes and a lower splenocyte response to concanavalin A following proton irradiation (P<0.05). There were no dose rate effects. Collectively, the data demonstrate that the measurements in blood and spleen were largely dependent upon the total dose of proton radiation and that an 80-fold difference in the dose rate was not a significant factor. A difference, however, was found between protons and gamma-rays in the degree of change induced in some of the measurements.

Decreased non-MHC-restricted (CD56+) killer cell cytotoxicity after spaceflight

Cytotoxic activity of non-major histocompatibility complex-restricted (CD56+) (NMHC) killer cells and cell surface marker expression of peripheral blood mononuclear cells were determined before and after spaceflight. Ten astronauts (9 men, 1 woman) from two space shuttle missions (9- and 10-day duration) participated in the study. Blood samples were collected 10 days before launch, within 3 h after landing, and 3 days after landing. All peripheral blood mononuclear cell preparations were cryopreserved and analyzed simultaneously in a 4-h cytotoxicity (51)Cr release assay using K562 target cells. NMHC killer cell lytic activity was normalized per 1,000 CD56+ cells. When all 10 subjects were considered as one study group, NMHC killer cell numbers did not change significantly during the three sampling periods, but at landing lytic activity had decreased by approximately 40% (P < 0.05) from preflight values. Nine of ten astronauts had decreased lytic activity immediately after flight. NMHC killer cell cytotoxicity of only three astronauts returned toward preflight values by 3 days after landing. Consistent with decreased NMHC killer cell cytotoxicity, urinary cortisol significantly increased after landing compared with preflight levels. Plasma cortisol and ACTH levels at landing were not significantly different from preflight values. No correlation of changes in NMHC killer cell function or hormone levels with factors such as age, gender, mission, or spaceflight experience was found. After landing, expression of the major lymphocyte surface markers (CD3, CD4, CD8, CD14, CD16, CD56), as determined by flow cytometric analysis, did not show any consistent changes from measurements made before flight.

Spaceflight induces changes in splenocyte subpopulations: effectiveness of ground-based models

Spaceflight produces changes in the immune system. The mechanisms for the alterations in immune function after spaceflight remain unclear due in part to the difficulties associated with conducting spaceflight research. The purpose of the following studies, therefore, was to create a ground-based protocol that can reproduce the immunological changes found after spaceflight, i.e., changes in splenic lymphocyte populations. Rats were exposed to either flight aboard the Space Shuttle Endeavor (STS-77) or ground-based simulations of various components of the spaceflight experience. The ground-based mock spaceflight was comprised of exposure to launch and landing loads and unloading of the hindlimbs. In addition, each component of this ground-based mock spaceflight was tested separately. The results were that spaceflight reduced splenic CD4(+) T (helper/inducer) cells and CD11b(+) (neutrophils/macrophages) cells. The ground-based simulations of spaceflight did not reproduce the same pattern of splenocyte changes. In fact, exposure to landing loads alone increased splenic CD4(+) T (helper/inducer) cells. These findings support the conclusion that the ground models tested did not induce similar changes in the immune system as did spaceflight. It is possible, therefore, that stressors/factors unique to the spaceflight experience impact the immune system in ways that cannot be currently, fully modeled on the ground.

Effects of spaceflight and PEG-IL-2 on rat physiological and immunological responses

Sprague-Dawley rats were subjected to two 8-day spaceflights on the space shuttle. Rats housed in the National Aeronautics and Space Administration's animal enclosure were injected (iv or sc) with pegylated interleukin-2 (PEG-IL-2) or a placebo. We tested the hypothesis that PEG-IL-2 would ameliorate some of the effects of spaceflight. We measured body and organ weights; blood cell differentials; plasma corticosterone; colony-forming units (macrophage and granulocyte macrophage); lymphocyte mitogenic, superantigenic, and interferon-gamma responses; bone marrow cell and peritoneal macrophage cytokine secretion; and bone strength and mass. Few immunological parameters were affected by spaceflight. However, some spaceflight effects were observed in each flight. Specifically, peritoneal macrophage spontaneous secretion of tumor necrosis factor-alpha occurred in the first but not in the second flight. A significant monocytopenia and lymphocytopenia were detected in the second but not in the first flight. The second mission produced bone changes more consistent with past spaceflight investigations. PEG-IL-2 did not appear to be beneficial; however, this was mostly due to the lack of spaceflight effects. These studies reflect the difficulty in reproducing experimental models by using current space shuttle conditions.

Space flight, microgravity, stress, and immune responses

Exposure of animals and humans to space flight conditions has resulted in numerous alterations in immunological parameters. Decreases in lymphocyte blastogenesis, cytokine production, and natural killer cell activity have all been reported after space flight. Alterations in leukocyte subset distribution have also been reported after flight of humans and animals in space. The relative contribution of microgravity conditions and stress to the observed results has not been established. Antiorthostatic, hypokinetic, hypodynamic, suspension of rodents and chronic head-down tilt bed-rest of humans have been used to model effects of microgravity on immune responses. After use of these models, some effects of space flight on immune responses, such as decreases in cytokine function, were observed, but others, such as alterations in leukocyte subset distribution, were not observed. These results suggest that stresses that occur during space flight could combine with microgravity conditions in inducing the changes seen in immune responses after space flight. The biological/biomedical significance of space flight induced changes in immune parameters remains to be established. Grant Numbers: NCC2-859, NAG2-933.

Effects of space flight and IGF-1 on immune function

We tested the hypothesis that insulin-like growth factor-1 (IGF-1) would ameliorate space flight-induced effects on the immune system. Twelve male, Sprague-Dawley rats, surgically implanted with mini osmotic pumps, were subjected to space flight for 10 days on STS-77. Six rats received 10 mg/kg/day of IGF-1 and 6 rats received saline. Flight animals had a lymphocytopenia and granulocytosis which were reversed by IGF-1. Flight animals had significantly higher corticosterone levels than ground controls but IGF-1 did not impact this stress hormone. Therefore, the reversed granulocytosis did not correlate with serum corticosterone. Space flight and IGF-1 also combined to induce a monocytopenia that was not evident in ground control animals treated with IGF-1 or in animals subjected to space flight but given physiological saline. There was a significant increase in spleen weights in vivarium animals treated with IGF-1, however, this change did not occur in flight animals. We observed reduced agonist-induced lymph node cell proliferation by cells from flight animals compared to ground controls. The reduced proliferation was not augmented by IGF-1 treatment. There was enhanced secretion of TNF, IL-6 and NO by flight-animal peritoneal macrophages compared to vivarium controls, however, O2(-) secretion was not affected. These data suggest that IGF-1 can ameliorate some of the effects of space flight but that space flight can also impact the normal response to IGF-1. Grant Numbers: NAGW-1197, NAGW-2328.

Spaceflight and development of immune responses

The NIH.R1 Space Shuttle experiment was designed to study the effects of spaceflight on rodent development. Pregnant rats were flown on the Space Shuttle for 11 days, and pregnant control rats were maintained in animal enclosure modules in a ground-based chamber under conditions approximating those in flight. Additional controls were in standard housing. The effects of the flight on immunological parameters of dams, fetuses, and pups were determined. Blastogenesis of spleen cells in response to mitogen was inhibited in flown dams but was not inhibited in cells from their pups. Interferon-gamma production by spleen cells showed a trend toward inhibition in flown dams but not in their pups. The response of bone marrow cells to colony-stimulating factor showed a trend toward inhibition after spaceflight in dams, but the response of fetus and pup liver cells was not inhibited. Total serum IgG was not affected by spaceflight. None of the examined immune parameters that were altered in rat dams after spaceflight was found to be altered in their offspring.