Overview
Spaceflight Immunodeficiency
Principal Investigator:
William T. Shearer, M.D., Ph.D.
Organization:
Baylor College of Medicine
Technical Summary
1. Antarctic Winter-Over Model of Space Flight
In collaboration with Dr. Desmond J. Lugg, Head of Polar Medicine, Australian Antarctic Division, Australian National Antarctic Research Expedition (ANARE), we have performed the first assessment of human specific antibody response to a T-cell dependent neoantigen (i.e., bacteriophage φX-174) in a spaceflight model. Previous investigations of humans in spaceflight or in models of space flight measured only serum immunoglobulin concentrations that do not equate to antibody. The bacteriophage φX-174 vaccine is known as the gold standard for evaluating deficiencies of humoral (antibody) immune responses. Therefore, this study was particularly important in quantitating several aspects of human immune responses (e.g., viral clearance, primary IgM antibody, secondary IgG antibody, and helper T-cell-induced immunologic switching of IgM to IgG production). The conditions of the Antarctic winter-over include stress, isolation, containment, and microbial contamination, which are common to those of space flight but lack the conditions of microgravity and solar radiation. Nevertheless, the Antarctic model is one of the best ground-based models of space flight. With the collaboration of ANARE, we studied test subjects stationed at the Casey outpost in Antarctica and control subjects stationed on Macquarie Island. Macquarie Island subjects are considered suitable controls because they have access to the mainland during the winter, in contrast to those subjects at Casey. In the third month of the Antarctic winter-over, subjects were immunized with bacteriophage φX-174 and six weeks later were given a booster injection. At Casey, the test subjects' immune responses were equivalent to those of the control subjects on Macquarie Island and to the absolute control data in the testing laboratory of Dr. Hans Ochs at the University of Washington, Seattle. These humoral immune responses do not show the deficiencies of some cellular immune responses (delayed type hypersensitivity DTH skin test responses to recall antigens) observed by Dr. Lugg in previous ANARE expeditions. These data are important because they clearly define normal antibody responses to a neoantigen (bacteriophage φX-174) during the rigorous conditions of the Antarctic winter-over. Also, they clearly do NOT predict similar normal humoral immune responses in space flight because of the important lack of the space factors of microgravity and solar radiation (protons and gamma-rays). These results dictate the application of new models for the assessment of risks of space flight for producing antibody deficiencies. The animal model of irradiated mice challenged with vaccines or virus challenge would be one such example that will be explored in the next funding extension of the present grant. The Antarctic winter-over experiments will be featured in the January 2001 issue of the well-respected Journal of Allergy and Clinical Immunology that will introduce space flight immunology research to its readership, along with a cover illustration of space exploration and a National Space Biomedical Research Institute (NSBRI) videotape of the conditions of space flight and model systems utilized in the publication.
2. Sleep Deprivation Model of Space Flight
With the recommendation of the NSBRI External Advisory Council, we began a synergy project collaboration with another NSBRI Team (Human Performance Factors, Sleep and Chronobiology) and Dr. David F. Dinges, Professor of Experimental Psychiatry, University of Pennsylvania. Dr. Dinges had previously demonstrated alterations in certain immune cells and cytokines in human subjects deprived of sleep for 64 hours. The loss of sleep in space flight is a major problem for astronauts, as judged by the fact that the most commonly prescribed medication is sleeping pills. In the synergy project, we measured plasma cytokines in a cohort of human subjects with total sleep deprivation (TSD) or partial sleep deprivation (PSD) for 88 hours. Two sleep regulatory cytokines/cytokine receptors (soluble tumor necrosis factor-alpha receptor 1 sTNF-αRI and interleukin 6 IL-6) were shown to become elevated in the TSD subjects compared to the PSD subjects. The same two messenger molecules of the neuroendocrine-immune system have been shown to regulate sleep in small animal studies involving instillation in the cerebral cortex. The fact that the PSD subjects did not demonstrate an increase in sTNF-αRI and IL-6, suggests that the short naps interspersed throughout the sleep deprivation period might serve as the basis for a countermeasures approach to the problems of loss of cognitive and mechanical ability in sleep-deprived humans. These exciting results have led to plans for new synergy projects with Dr. Dinges (now Team Leader of the NSBRI Neurobehavioral and Psychosocial Factors Team), involving assessment of upregulation of target cell virus receptors by alterations in peripheral blood cytokines and chemokines. The results of the first study will be co-featured with those of the Antarctica study in the January 2001 issue of the Journal of Allergy and Clinical Immunology.
3. Anti-Orthostatic Suspension Murine Model of Space Flight
Dr. Wayne Smith and colleagues have made several important observations on acquired and innate immune responses using the anti-orthostatic suppression (AOS) mouse system, which mimics known effects of space flight (i.e., underuse of lower extremities, overuse of upper extremities, limited excursion, containment, and cephalad (head) fluid shift. Dr. Smith was able to show that the AOS mouse produced greater DTH skin test responses to a recall antigen than control mice (orthostatic suppression or untethered). Although these findings may be somewhat confounded by the cephalad fluid shift, they suggest that AOS suppression may accentuate hypersensitivity reactions as part of an induced imbalance in immune homeostasis. It is possible that these data indicate a need to assess by additional model studies the development of autoimmune disease in astronauts in long-term space flight.
In addition, Dr. Smith has demonstrated that endotoxin challenge of AOS mice induced a greater expression of the intracellular adhesion molecule type-1 (ICAM-1) in muscle tissue deprived of fluid. This situation is likely to lead to an influx of leukocytes by virtue of their attraction by ICAM-1 molecules on endothelial cell layers. The muscle atrophy observed in astronauts might be related in part to a possible reperfusion injury due to leukocyte infiltration of blood-deprived muscle tissue.
Finally, as a follow-on study of the conflicting studies in the space research literature where phagocytosis and microbicidal activity of neutrophils were examined in the AOS mouse, Dr. Smith was unable to demonstrate any clear effects of AOS upon oxidative function of peritoneal neutrophils. This study will be published in Aviation, Space, and Environmental Medicine.
Satisfaction of Hypotheses, Objectives, Specific Aims of Original Proposal
Specific Aim One had two objectives: 1) Evaluate ANARE subjects for evidence of immunodeficiency, and 2) Evaluate Johnson Space Center (JSC) capsule-isolated astronauts-in-training for evidence of immunodeficiency. Only the first objective was carried out, because the JSC capsule studies were postponed by the National Aeronautics and Space Administration (NASA). Of the studies proposed on ANARE subjects, we have completed the assessment of specific antibody function, but we are now assessing cellular immune responses: lymphocyte subsets, mitogen and antigen-induced lymphoproliferation, and cytokine production. The 1999 ANARE specimens of frozen plasma and cells reached us in January - March of 2000 (three separate shipments), so most of the cellular studies are in progress. Also, critical to the evaluation of cellular immunity is the viability of the cell specimens. Regardless of viability, it will be possible to measure cytokine expression by DNA/RNA technology in the cells.
Specific Aim One was modified by inclusion of sleep deprivation experiments in the synergy collaboration project described above. Both the Antarctic and sleep-deprivation studies were sustained by the same hypothesis, namely that conditions of space flight models on Earth would induce alterations in the human immune system that might indicate the need to anticipate defects of immunity developing in astronauts on long space voyages (e.g., three-year trip to Mars). In the first objective, the Antarctic antibody studies did not validate the hypothesis, but they pointed to the need to change the model so that space factors not present in the Antarctic winter-over model could be examined. Thus, in the new grant cycle, we will use irradiated mice challenged by latent virus (gammaherpesvirus and polyomavirus), in collaboration with Dr. Daila Gridley and Dr. Gregory Nelson, radiation biologists at Loma Linda University. The modified objective of the first specific aim (sleep- deprivation studies) did validate the hypothesis, because it showed that sleep deprivation of 88 hours induced significant alterations in cytokine messenger molecules that connect the neuroendocrine-immune system. These sleep deprivation-induced changes in TNF-αRI and IL-6 led to the development of the next hypothesis: altered chemokine levels in astronauts or in ground models of space flight will upregulate viral receptors on target cells and lead to chronic infection and possibly development of malignant clones of transformed lymphocytes, such as Epstein-Barr virus (EBV)-driven lymphomas. This new hypothesis will be tested in ongoing synergy projects, in collaboration with Dr. David Dinges.
Specific Aim Two of the original project had several components, but the basic objective was to determine whether biochemical or structural components of the inflammatory system and cellular molecule adhesion system are altered in the AOS murine model of space flight. The underlying hypothesis was that local tissue fluid shifts associated with the AOS model would affect the tissue distribution of adhesion molecules and, therefore, alter inflammatory responses. This hypothesis was partially validated in the experiments to date, in that an enhanced DTH skin test response to a recall antigen was seen in the AOS mouse, and an upregulation of endotoxin-induced ICAM-1 molecules was documented in fluid-deprived muscle tissue. These results indicate a need to pursue the important research area of muscle reperfusion, since it is well known that astronauts suffer from muscle atrophy.
Implications of Project Research for Risk Reduction in Critical Research Path
The Critical Research Path Risks addressed by Specific Aims One and Two in this research project were: 1) immunodeficiencies/infections (rank 1 high priority, type III problem suspected) and 2) carcinogenesis (rank 1, type III). Thus, we intended to demonstrate whether there should be concern for the possible harmful effects of known conditions of long-term space flight. Our progress to date leads us to believe that continuation of this plan is very important for the preparation for interplanetary space travel. The unknown effects of microgravity and solar radiation (estimated 2 Gy) upon the normal balance of immunity needs careful exploration with additional models of space flight. There is ample clinical evidence from patients on Earth that the immunosurveillance system, when suppressed by virus infection (e.g., AIDS), by therapeutics (e.g., corticosteroids, immunomodulators, cytotoxic agents), by radiation exposure, and by unremittant stress (e.g., family caregivers to patients with terminal diseases such as Alzheimers) begins to fail in protecting the host against opportunistic infections (e.g., Pneumocystis carinii pneumonia), reactivation of latent viral infections (e.g., EBV infection), and malignancies (e.g., lymphomas, leukemias). The purpose of pursuing these potential developments in astronauts subjected to the potential dangers of space flight is to first examine their feasibility, and to then devise a countermeasures program to negate their impact on astronauts. Currently, our countermeasures readiness level is one or two (basic research level), but our sleep-deprivation experiments have already suggested that short intermittent naps (two-hour naps every 12 hours) prevent the increased production of the neuroendocrine-immune system messengers involved in sleep regulation. This, at least in a preliminary sense, is a significant beginning to the development of a countermeasure for the immune consequences (infections, cancer) of sleep-deprivation in the long-term perspective. These findings also hold importance for a countermeasures program for diseases and conditions with immediate consequences on Earth (e.g., sleep-starved workers, truck drivers, and pilots who sustain accidents). The experiments with the AOS mouse model that demonstrated an upregulation of ICAM-1 molecules in fluid-deprived muscle tissue possibly may related to the muscle atrophy observed in astronauts. It is well known that in reperfused cardiac tissue after heart attacks, there is an upregulation of ICAM-1 molecules that leads to destructive infiltration of leukocytes. Possibly, the same mechanism operates in space-flight muscle atrophy as part of the muscle disuse mechanism.