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The Hematopoietic Stem Cell Therapy to Countermeasure Cancer in Astronauts during Exploration of Deep Space.  S. Ohi1, R.P. Kindred1, A-N. Roach1, A. Edossa1, B.C. Kim1, S.R. Gonda2, K. Emami3, 1Howard University and Hospital, Washington, DC, 2NASA Johnson Space Center, Houston, TX, 3Wyle Laboratories, Houston, TX.   Exposure to cosmic radiation can cause chromosomal mutations, which may lead to cancer in astronauts engaged in space exploration.  Therefore, our goals are to develop countermeasures to prevent the space-induced cancer using hematopoietic stem cell therapy (HSCT) and gene therapy.  This presentation focuses on the HSCT for cancer.  Our previous experiments on a simulated, space-induced immunodeficiency (mouse hindlimb unloading model) indicated that transplanted hematopoietic stem cells (HSCs) could enhance the host's immunity, effectively eliminating bacterial infection (Ohi S, et. al. J Grav Physiol 10, P63-64, 2003; Ohi S, et. al.  Proceedings of the Space Technology and Applications International Forum (STAIF). American Institute of Physics, New York, pp. 938-950, 2004).  Hence, we hypothesized that the HSCs might be effective in combating cancer as well, judging it as foreign.  Studies on co-cultured mouse HSCs and the в-galactosidase-marked, rat gliosarcoma spheroids (9L/lacZ), a cancer model, indeed indicated antagonistic interactions between them, resulting in destruction of the spheroids by HSCs.  Trypan Blue dye-exclusion assay was consistent with the conclusion.  These results show potential usefulness of the HSCT for cancer.  Currently, the NASA Hydrodynamic Focusing Bioreactor (HFB), a space analog tissue/cell culture system, is being used to study an invasion of the gliosarcoma (GS) spheroids into mouse brain with or without co-cultured HSCs.  This may simulate the metastasis of gliosarcoma to brain.  There is a tendency that the HSCs inhibit invasion of GS spheroids into brain, as evidenced by the X-gal staining of co-cultured brain.  (Supported by NASA: NCC9-142/ USRA-DSLS)