Alterations in adaptive immunity persist during long-duration spaceflight

Abstract

Background: It is currently unknown whether immune system alterations persist during long-duration spaceflight. In this study various adaptive immune parameters were assessed in astronauts at three intervals during 6-month spaceflight on board the International Space Station (ISS).

Aims: To assess phenotypic and functional immune system alterations in astronauts participating in 6-month orbital spaceflight.

Methods: Blood was collected before, during, and after flight from 23 astronauts participating in 6-month ISS expeditions. In-flight samples were returned to Earth within 48 h of collection for immediate analysis. Assays included peripheral leukocyte distribution, T-cell function, virus-specific immunity, and mitogen-stimulated cytokine production profiles.

Results: Redistribution of leukocyte subsets occurred during flight, including an elevated white blood cell (WBC) count and alterations in CD8⁺ T-cell maturation. A reduction in general T-cell function (both CD4⁺ and CD8⁺) persisted for the duration of the 6-month spaceflights, with differential responses between mitogens suggesting an activation threshold shift. The percentage of CD4⁺ T cells capable of producing IL-2 was depressed after landing. Significant reductions in mitogen-stimulated production of IFNγ, IL-10, IL-5, TNFα, and IL-6 persisted during spaceflight. Following lipopolysaccharide (LPS) stimulation, production of IL-10 was reduced, whereas IL-8 production was increased during flight.

Conclusions: The data indicated that immune alterations persist during long-duration spaceflight. This phenomenon, in the absence of appropriate countermeasures, has the potential to increase specific clinical risks for crewmembers during exploration-class deep space missions.

Figure 1

Absolute peripheral blood levels (×1000 cells/μl) for the indicated leukocyte, lymphocyte, and T-cell subpopulations before, during (early, mid-mission, and late) and following spaceflight. (a) Leukocyte subsets; (b) lymphocyte subsets; (c) T cell subsets; (d) cytotoxic CD8⁺ T cell subsets; (e) central memory CD8⁺ T cell subsets; (f) senescent T cell subsets; (g and h) constitutively activated T cell subsets; (i) viral peptide-specific T cell subsets. Data are presented as mean±s.e. Significance was evaluated via a Student’s Paired t-test, by comparing all other data points to L-180 baseline data. Significant differences (P⩽0.05) are indicated (*). Sample size for all data is 23 astronaut subjects, except for EBV- and CMV-specific T cells, where the assay is restricted to HLA-A2-positive subjects (n=8). CMV, cytomegalovirus; EBV, Epstein–Barr virus.

Figure 2

Intracellular cytokine analysis; mean percentage of T cells in samples collected before, during (early, mid-mission, and late) and after spaceflight that were capable of being stimulated to produce either IL-2 (CD4⁺) or IFNγ (CD8⁺) following mitogenic stimulation. Purified peripheral mononuclear cells were stimulated with PMA+ionomycin for 4 h; data are expressed as percentage of T cells that were positive. Data are presented as mean±s.e. Significance was evaluated via a Student’s paired t-test by comparing all other data points to L-180 baseline data. Significant differences (P⩽0.05) are indicated (*). Sample size for all data is 23 ISS astronaut subjects.

Figure 3

T-cell function (early blastogenesis) data: expression of either CD69 or CD69/CD25 following 24-h culture in the presence of (a) staphylococcal enterotoxin (a and b); or (b) antibodies to CD3 and CD28. Data are presented as mean±standard error. Significance was evaluated via a Student’s paired t-test by comparing all other data points to L-180 baseline data. Significant differences (P⩽0.05) are indicated (*). Sample size for all data is 23 ISS astronaut subjects. ISS, International Space Station.

Figure 4

Quantification of virus-specific T-cell function. The frequency of CD69⁺/IFNγ⁺ positive (functional) CD8⁺ T cells following culture in the presence of A*0201-restricted viral peptides was evaluated by flow cytometry. The absolute and functional numbers of viral peptide-specific T cells were used to derive a functional percentage for (a) EBV-specific and (b) CMV-specific CD8⁺ T cells during spaceflight. Significance was evaluated via a Student’s paired t-test by comparing all other data points to L-180 baseline data. Significant differences (P⩽0.05) are indicated (*). This assay is restricted to HLA-A2-positive subjects (n=8). CMV, cytomegalovirus; EBV, Epstein–Barr virus.

Figure 5

Mean secreted cytokine levels following mitogenic stimulation. For secreted cytokine production, the mitogen was (a) antibodies to both CD3 and CD28, (b) PMA+ionomycin or (c) LPS. Cytokine concentration data are expressed mean concentration in pg/ml±s.e.m. The lone exception is IFNγ, for which data is presented as mean fluorescence intensity (MFI), which corresponds to concentration. Significance was evaluated via a Student’s paired t-test by comparing all other data points to L-180 baseline data. Significant differences (P⩽0.05) are indicated (*). Sample size for all data is 23 ISS astronaut subjects. For IL-4 (a), late time point only, one outlier value removed prior to statistical analysis. ISS, International Space Station; LPS, lipopolysaccharide.

Figure 5

Mean secreted cytokine levels following mitogenic stimulation. For secreted cytokine production, the mitogen was (a) antibodies to both CD3 and CD28, (b) PMA+ionomycin or (c) LPS. Cytokine concentration data are expressed mean concentration in pg/ml±s.e.m. The lone exception is IFNγ, for which data is presented as mean fluorescence intensity (MFI), which corresponds to concentration. Significance was evaluated via a Student’s paired t-test by comparing all other data points to L-180 baseline data. Significant differences (P⩽0.05) are indicated (*). Sample size for all data is 23 ISS astronaut subjects. For IL-4 (a), late time point only, one outlier value removed prior to statistical analysis. ISS, International Space Station; LPS, lipopolysaccharide.