In Vitro Immune Toxicity of Depleted Uranium: Effects on Murine Macrophages, CD[4.Sup.+] T Cells, and Gene Expression Profiles

Article excerpt

Depleted uranium (DU) is a by-product of the uranium enrichment process and shares chemical properties with natural and enriched uranium. To investigate the toxic effects of environmental DU exposure on the immune system, we examined the influences of DU (in the form of uranyl nitrate) on viability and immune function as well as cytokine gene expression in murine peritoneal macrophages and splenic CD[4.sup.+] T cells. Macrophages and CD[4.sup.+] T cells were exposed to various concentrations of DU, and cell death via apoptosis and necrosis was analyzed using annexin-V/propidium iodide assay. DU cytotoxicity in both cell types was concentration dependent, with macrophage apoptosis and necrosis occurring within 24 hr at 100 [micro]M DU exposure, whereas CD[4.sup.+] T cells underwent cell death at 500 [micro]M DU exposure. Nuncytotoxic concentrations for macrophages and CD[4.sup.+] T cells were determined as 50 and 100 [micro]M, respectively. Lymphoproliferation analysis indicated that macrophage accessory cell function was altered with 200 [micro]M DU after exposure times as short as 2 hr. Microarray and real-time reverse-transcriptase polymerase chain reaction analyses revealed that DU alters gene expression patterns in both cell types. The most differentially expressed genes were related to signal transduction, such as c-jug NF-[kappa] Bp65, neurotrophic factors (e.g., Mdk), chemokine and chemokine receptors (e.g., TECKI CCL25), and interleukins such as IL-10 and IL-5, indicating a possible involvement of DU in cancer development, autoimmune diseases, and T helper 2 polarization ofT cells. The results are a first step in identifying molecular targets for the toxicity of DU and the elucidation of the molecular mechanisms for the immune modulation ability of DU. Key words: apoptosis, CD[4.sup.+] T cell, cytokine gene expression, depleted uranium, macrophage function, necrosis. Environ Health Perspect 114:85-91 (2006). doi: 10.1289/ehp.8085 available via http://dx.doi.org/ [Online 17 August 2005]

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Depleted uranium (DU) is a by-product of the enrichment process of natural uranium (Priest 2001). The release of uranium into the environment presents a threat to human and ecologic health in many parts of world (Hass et al. 1998; Murray et al. 2002). DU shares chemical properties with natural or enriched uranium, but the major hazard rendered by DU results from its heavy metal toxicity rather than from radiologic toxicity (Fisenne and Welford 1986; Priest 2001). The adverse health effects of DU compounds are partially dependent on its chemical form. Uranium compounds in +2 to +4 valence states are essentially insoluble. However, in vivo soluble uranium is always hexavalent, regardless of the oxidation state of uranium compound taken up (Edison 1994). It is this form (+6) that is of toxicologic importance. Because of their high affinity for phosphate, carboxyl, and hydroxyl groups, uranyl compounds readily combine with proteins and nucleotides to form stable complexes (Moss 1985).

Serum uranium forms a variety of non-diffusible complexes such as uranium-albumin compounds and diffusible ones such as ionic uranyl hydrogen carbonate complex (Moss 1985). Although the most characteristic response to DU exposure either short or long term is renal dysfunction (Domingo 1995; Leggett 1989; Zamora et al. 1998), uranium is also localized within the central nervous system, testes, lymph nodes, and spleen, suggesting the potential for uranium to cause health problems at these sites (Domingo 2001; Pellmar et al. 1999; Wrenn et al. 1985). Uranium-induced pathological changes in the testes and thyroid glands have been documented (Malenchenko et al. 1978).

In vitro studies have examined the effects of DU on a variety of cell types. For example, Chinese hamster ovary cells exposed to DU exhibit lower cell viability, depressed cell cycle kinetics, and increased sister chromatid exchanges, micronuclei, and chromosomal aberrations after DU exposure (Lin et al. …