profiles in the thymus differed greatly between the FLT mice and the AEM groups. The data
obtained from this study collectively exhibits that T cell distribution, function, and gene
expression are significantly modified shortly after return from the spaceflight environment.
However, it remains to be determined whether the quantified changes are brief and primarily
due to the tremendous physiological stress of landing and readaptation or have an enduring
effect on risk for infection and/or cancer (Gridley 2009).
Studies have shown that the spaceflight environment can impact several physiological systems
potentially resulting in serious consequences for immunity. The primary aim of this study was
to investigate changes in immune parameters concerning the spleen, liver, and thymus in
response to flight. C57BL/6 mice (a common inbred strain of lab mice) were flown on a 13-day
space shuttle mission. In response to flight, the mice exhibited reductions in liver, spleen, and
thymus masses in comparison to ground controls. The changes in organ masses suggest that
the mice were subject to psychological and/or physiological stress inflight or during landing.
Splenic (pertaining to the spleen) white blood cells (WBCs) and numbers of leukocyte (cells in
the blood that destroy disease-causing microorganisms) subpopulations were significantly
reduced after flight. To determine the recovery and proliferative capacity of lymphocytes, this
study characterized spontaneous blastogenesis (unstimulated DNA synthesis). The observed
increase in [3H]-TdR incorporation into DNA (tritiated thymidine-method for estimating
capacity for cell regeneration) by splenic lymphocytes demonstrated that ex vivo (outside the
body) DNA synthesis was increased after flight and suggests that cells were capable of shifting
into a proliferative (or recovery) state once removed from any stress-induced inhibition in vivo
(inside the body). In contrast, LPS (lipopolysaccharides-large molecules found in the outer
membrane of many common bacteria) induced proliferation was decreased in the flight mice,
indicating that the ability to respond to a potent B cell mitogen (substance that indices cell
division) may be compromised. The flight mice demonstrated an increased capacity to produce
biological responses, interleukin-6 and interleukin 10 (IL-6 and IL-10- chemical messengers
secreted by cells of the immune system), but not TNF-α (Tumor Necrosis Factor- alpha- protein
that can cause tumor cell death when injected into tumor-bearing mice). The genes responsible
for scavenging ROS (Reactive Oxygen Species play vital roles in normal cell functions, but are
also sources of tissue and DNA damage) were shown to be up-regulated after flight. The data
confirm that immune parameters are influenced by the spaceflight environment. Furthermore,
these data also suggest that exposure to the spaceflight environment can increase anti-
inflammatory mechanisms and change the ex vivo response to LPS, which is a bacterial
component that typically indices a strong response from the immune system (Farnaz 2009).
Genetic analysis in young adult mice at 8 weeks of age after exposure to spaceflight aboard the
space shuttle for a period of 13-days demonstrate that spaceflight induces significant changes
in mRNA expression of genes in the thymus that regulate stress, hormone receptor metabolism,
and T white blood cell signaling activity. These data explain, in part, the reported systemic
compromise of the immune system after exposure to microgravity. The results of the study
provide insight into how spaceflight affects stress-related gene expression in addition to
influencing genes associated with specific immunological processes in the thymus itself. The