Technical Abstract: Selenium (Se) is known to regulate carcinogenesis and immunity at nutritional and 26 supranutritional levels. Because the immune system provides critical defenses against 27 cancer and the athymic, immune-deficient NU/J nude mice are known to gradually develop 28 CD8+ and CD4+ T cells extrathymically, we asked whether dietary Se can modulate T cell 29 maturation and tomirigenesis in nude mice. Thirty homozygous nude mice were fed a 30 Se-deficient, Torula yeast basal diet alone (Se-) or supplemented with 0.15 (Se+) or 1.0 31 (Se++) mg Se/kg (as Na2SeO4) for 6 months, followed by a 7-week time course of PC-3 32 prostate cancer cell xenograft (2 x 106 cells/site, 2 sites/mouse). Here, we show that the Se- 33 diet enhanced the initial tumor development on Day 11-17 whereas tumor growth was 34 suppressed with Se++ diet on Day 35-47 after the xenograft. Although dietary Se status did 35 not affect levels of CD4+ and CD8+ T cells during the time course as determined by flow 36 cytometric analyses, mice fed with the Se++ diet expressed increased amount of 37 CD25+CD4+ T cells on Day 9. Taken together, dietary Se at the nutritional and 38 supranutritional levels differentially regulate tumor development and T cell immunity in 39 adult nude mice engrafted with PC-3 prostate cancer cells.