RationaleIt is unknown whether a prolonged period of bed-rest or use of musculoskeletal countermeasures to mitigate the effects of space travel will affect human immune responses and resistance to infection, particularly in female subjects exposed to prolonged bed-rest.MethodsAdult (25-40 yrs) female volunteers were maintained in a head-down bed-rest (HDBR) position for 60 days: 8 controls, 8 with regular muscular exercise, and 8 with dietary protein supplementation (0.6 g/kg/day) at the Institut de Médecine et Physiologie Spatiale, Toulouse, France. After 21 and 42 days of HDBR, subjects were immunized with bacteriophage ϕX-174, a schedule shortened by half from standard. Blood specimens were obtained to determine ϕX-174 clearance, antibody production and immunoglobulin class switching, and to measure plasma cytokine levels.ResultsAll 24 subjects cleared ϕX-174 by 7 days, but had atypical ϕX-174-specific antibody responses with moderately depressed secondary antibody responses and reduced IgM to IgG class switching. No effects of countermeasures on antibody responses were observed. Exercise and diet supplements produced significant time-related and cross-sectional alterations in plasma tumor necrosis factor-receptor 1, interleukin-1 receptor antagonist or interleukin-12p40 levels.ConclusionsFemale subjects experiencing HDBR demonstrated atypical antibody responses to ϕX-174, most likely related to the shortened immunization schedule. Countermeasures had no effect on antibody titers, but produced changes in pro- and anti-inflammatory plasma cytokine levels, suggestive of T cell activation. Space flight conditions may alter adult female immune responses, but more extensive evaluations of host defense and immune regulation are needed to know their impact on health. RationaleIt is unknown whether a prolonged period of bed-rest or use of musculoskeletal countermeasures to mitigate the effects of space travel will affect human immune responses and resistance to infection, particularly in female subjects exposed to prolonged bed-rest. It is unknown whether a prolonged period of bed-rest or use of musculoskeletal countermeasures to mitigate the effects of space travel will affect human immune responses and resistance to infection, particularly in female subjects exposed to prolonged bed-rest. MethodsAdult (25-40 yrs) female volunteers were maintained in a head-down bed-rest (HDBR) position for 60 days: 8 controls, 8 with regular muscular exercise, and 8 with dietary protein supplementation (0.6 g/kg/day) at the Institut de Médecine et Physiologie Spatiale, Toulouse, France. After 21 and 42 days of HDBR, subjects were immunized with bacteriophage ϕX-174, a schedule shortened by half from standard. Blood specimens were obtained to determine ϕX-174 clearance, antibody production and immunoglobulin class switching, and to measure plasma cytokine levels. Adult (25-40 yrs) female volunteers were maintained in a head-down bed-rest (HDBR) position for 60 days: 8 controls, 8 with regular muscular exercise, and 8 with dietary protein supplementation (0.6 g/kg/day) at the Institut de Médecine et Physiologie Spatiale, Toulouse, France. After 21 and 42 days of HDBR, subjects were immunized with bacteriophage ϕX-174, a schedule shortened by half from standard. Blood specimens were obtained to determine ϕX-174 clearance, antibody production and immunoglobulin class switching, and to measure plasma cytokine levels. ResultsAll 24 subjects cleared ϕX-174 by 7 days, but had atypical ϕX-174-specific antibody responses with moderately depressed secondary antibody responses and reduced IgM to IgG class switching. No effects of countermeasures on antibody responses were observed. Exercise and diet supplements produced significant time-related and cross-sectional alterations in plasma tumor necrosis factor-receptor 1, interleukin-1 receptor antagonist or interleukin-12p40 levels. All 24 subjects cleared ϕX-174 by 7 days, but had atypical ϕX-174-specific antibody responses with moderately depressed secondary antibody responses and reduced IgM to IgG class switching. No effects of countermeasures on antibody responses were observed. Exercise and diet supplements produced significant time-related and cross-sectional alterations in plasma tumor necrosis factor-receptor 1, interleukin-1 receptor antagonist or interleukin-12p40 levels. ConclusionsFemale subjects experiencing HDBR demonstrated atypical antibody responses to ϕX-174, most likely related to the shortened immunization schedule. Countermeasures had no effect on antibody titers, but produced changes in pro- and anti-inflammatory plasma cytokine levels, suggestive of T cell activation. Space flight conditions may alter adult female immune responses, but more extensive evaluations of host defense and immune regulation are needed to know their impact on health. Female subjects experiencing HDBR demonstrated atypical antibody responses to ϕX-174, most likely related to the shortened immunization schedule. Countermeasures had no effect on antibody titers, but produced changes in pro- and anti-inflammatory plasma cytokine levels, suggestive of T cell activation. Space flight conditions may alter adult female immune responses, but more extensive evaluations of host defense and immune regulation are needed to know their impact on health.