Effects of spaceflight on the number of rat peripheral blood leukocytes and lymphocyte subsets

Experiments were carried out on peripheral blood leukocytes and spleen lymphocytes from 29 male rats that were flown during the Spacelab Life Sciences 1 (SLS-1) nine-day mission on the shuttle Columbia in June 1991 and on appropriate ground controls. On the day of landing, there was a significant decrease in the total white blood cell counts (P < 0.0001) of flight animals in comparison to controls. There was also a significant decrease in the absolute number of lymphocytes (P < 0.0001) and monocytes (P < 0.0001) in the flight animals. A slight decrease in the absolute number of eosinophils and a slight increase in the number of neutrophils were observed at landing, compared with preflight values. Immunophenotyping of the peripheral blood and spleen lymphocytes of flight and control animals indicated that, on the day of landing, there was a decrease in the absolute number of CD4 and CD8 positive cells and B lymphocytes. However, relative percentages of peripheral blood CD4+, CD8+, and B cells were not found to be depressed. No differences were discerned in the percent reactivity of spleen lymphocytes of flight animals compared with controls. The observed decrease in the number of leukocytes and lymphocytes at the immediate postflight period was transient and all values returned to the control levels by nine days postflight.