In the investigator's previous studies of astronauts, investigators found altered peripheral leukocyte distribution, altered cytokine production profiles, elevated levels of latent viral reactivation and elevated levels of cortisol during or immediately following long and short duration space flight. In addition, significant increases in stress hormones (i.e., cortisol, catecholamines) were found after landing. Notably, investigators found increased shedding of Epstein-Barr virus (EBV) and cytomegalovirus (CMV), two medically important herpesviruses, during space flight along with evidence of decreased cellular immunity. These increases in stress hormones directly correlated with CMV and EBV reactivation. Thus, latent herpesvirus reactivation in these astronauts may have resulted from both direct (i.e., stress hormones) and indirect (i.e., decreased immune function) mechanisms stemming from launch and landing acceleration. These results underscore the need to determine the causes underlying immune suppression in space in order to develop countermeasures.
Plasma, urine, and saliva cortisol were measured by enzyme immunoassay using commercially available kits. Salivary cortisol, which reflects free (unbound) hormone levels, will also be measured using commercially available kits.
This experiment was performed during Bed Rest Campaigns 3A, 3B, 3C, 3D, and 5A. In general, subjects did not display altered peripheral leukocyte subsets, constitutive immune activation, altered T cell function, or significant latent viral reactivation (EBV, VZV). Levels of constitutively activated T cells (CD8+/CD69+) and virus-specific T cells (CMV and EBV) decreased during the study. Cortisol levels (plasma and saliva) did not vary significantly during 90-day bed rest.
Conclusions: These data demonstrate the absence of significant immune system alteration and physiological stress during 90-day bed rest, and establish control data against which future studies (including countermeasures) may be compared.