Nonprotective effects of extracellular metallothionein

Metallothionein (MT) is a small, cysteine-rich protein that is readily induced by exposure to heavy metal cations. In previous work, we have demonstrated that MT has several significant immunomodulatory properties. MT decreases antigen-specific humoral responses in vivo and inhibits the ability of T cells to proliferate in response to antigen presented in vitro. To further characterize the mechanism by which this protein inhibits responsiveness to antigen, we have examined the effects of MT on cell viability in an antigen-presentation assay. MT (20 microM) caused substantial death to both lymphocytes and monocytes after 3 days of culture. The observed toxicity cannot be attributed to either increased superoxide radical generation or to production of tumor necrosis factor by MT-treated macrophages. Fractionation of supernatants from MT-treated cells suggests that the agent responsible for causing cytotoxicity is a soluble factor of at least 30 kDa. These results counter the perception that metallothionein uniformly plays a protective role in metal-stressed individuals.