Research

¹National Institute of Environmental Health Sciences Center for Children's Environmental Health, ²Department of Veterinary Molecular Biosciences, and ³Department of Medical Pathology, University of California-Davis, Davis, California, USA; ⁴MIND (Medical Investigation of Neurodevelopmental Disorders) Institute, University of California-Davis, Sacramento, California, USA

http://www.ehponline.org/docs/2006/8881/abstract.html

Abstract
Dendritic cells (DCs) , a rare cell type widely distributed in the soma, are potent antigen-presenting cells that initiate primary immune responses. DCs rely on intracellular redox state and calcium (Ca²⁺) signals for proper development and function, but the relationship between these two signaling systems is unclear. Thimerosal (THI) is a mercurial used to preserve vaccines and consumer products, and is used experimentally to induce Ca²⁺ release from microsomal stores. We tested adenosine triphosphate (ATP) -mediated Ca²⁺ responses of DCs transiently exposed to nanomolar THI. Transcriptional and immunocytochemical analyses show that murine myeloid immature DCs (IDCs) and mature DCs (MDCs) express inositol 1,4,5-trisphosphate receptor (IP₃R) and ryanodine receptor (RyR) Ca²⁺ channels, known targets of THI. IDCs express the RyR1 isoform in a punctate distribution that is densest near plasma membranes and within dendritic processes, whereas IP₃Rs are more generally distributed. RyR1 positively and negatively regulates purinergic signaling because ryanodine (Ry) blockade a) recruited 80% more ATP responders, b) shortened ATP-mediated Ca²⁺ transients > 2-fold, and c) produced a delayed and persistent rise (≥ 2-fold) in baseline Ca²⁺. THI (100 nM, 5 min) recruited more ATP responders, shortened the ATP-mediated Ca²⁺ transient (≥ 1.4-fold) , and produced a delayed rise (≥ 3-fold) in the Ca²⁺ baseline, mimicking Ry. THI and Ry, in combination, produced additive effects leading to uncoupling of IP₃R and RyR1 signals. THI altered ATP-mediated interleukin-6 secretion, initially enhancing the rate of cytokine secretion but suppressing cytokine secretion overall in DCs.DCs are exquisitely sensitive to THI, with one mechanism involving the uncoupling of positive and negative regulation of Ca²⁺ signals contributed by RyR1. Key words: calcium, calcium channel, dendritic cell, ethyl mercury, immunotoxicity, interleukin-6, organic mercury, redox, thimerosal. Environ Health Perspect 114:1083-1091 (2006) . doi:10.1289/ehp.8881 available via http://dx.doi.org/ [Online 21 March 2006]

Address correspondence to I.N. Pessah, Department of Veterinary Medicine, Molecular Biosciences, 1311 Haring Hall, One Shields Ave., University of California, Davis, CA 95616 USA. Telephone: (530) 752-6696. Fax: (530) 752-4698. E-mail: inpessah@ucdavis.edu

Supplemental Material is available on online at http://www.ehponline.org/docs/2006/8881/suppl.pdf

We thank C. Kwong for cytokine analyses and J. van de Water (Department of Medicine, Division of Rheumatology, Allergy and Clinical Medicine, University of California-Davis) for helpful discussions and for reviewing the manuscript.

This work was supported by the National Institute of Environmental Health Sciences (NIEHS) Center for Children's Environmental Health (grant PO1 ES11269) , the U.S. Environmental Protection Agency through the Science to Achieve Results (STAR) program (grant R829388) , and the MIND Institute. Additional support came from the NIEHS Center for Environmental Health Sciences Cellular Imaging Core (grant ES05707) . This investigation was conducted in a facility contructed with support from Research Facilities Improvement Program grant C06 RR-12088-01 from the National Center for Research Resources, National Institutes of Health.

The authors declare they have no competing financial interests.

Received 28 November 2005 ; accepted 13 March 2006.

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