Methyl mercury (MeHg) in vitro exposure alters mitogen-induced lymphocyte proliferation and cytokine expression in Steller sea lion (Eumetopias jubatus) pups.

Steller sea lions (Eumetopias jubatus, SSLs) are managed as two distinct population segments within U.S. waters: the endangered western distinct population segment and the recently delisted eastern distinct population segment. Recent studies reported concentrations of mercury in several tissues collected from young SSLs in the Aleutian Islands that were at or above concentrations found to negatively impact health in other fish-eating mammals. However, there are limited studies which have investigated the range of mercury concentrations that may negatively influence the SSL immune system. This study assessed relationships between methyl mercury (MeHg+) concentrations and two immune functions, lymphocyte proliferation and cytokine expression. Peripheral blood mononuclear cells (PBMCs) were isolated and cryopreserved from pups on three rookeries within the western distinct population segment: Chiswell Island, Ulak, and Agattu Islands. Lymphocyte proliferation and cytokine expression were assessed in vitro using thawed PBMCs with exposure to MeHg+ (unexposed control, 0.001, 0.01, and 0.1 µg/ml). Lymphocyte proliferation was measured without and with stimulation with a T cell mitogen (ConA) and B cell mitogen (LPS) and the concentration of cytokines was measured in the cell culture supernatant (with and without ConA or LPS). Spontaneous lymphocyte proliferation was significantly increased at 0.01 and 0.1 µg/ml. T lymphocyte proliferation was significantly increased at 0.001 µg/ml and 0.1 µg/ml, while B lymphocyte proliferation was decreased at 0.1 µg/ml. Cytokine concentrations for INFγ, IL-10, IL-6, and TNFα were reduced at 0.1 µg/ml upon either T or B cell mitogen stimulation, with the exception for IL-10, where 0.1 µg/ml reduced IL-10 concentration compared to unstimulated cells. These data suggest immune functions were affected by MeHg+ exposure requiring in vivo follow up investigations. The observed modulation of immune functions is of concern as any toxicant-induced modulation may adversely affect the health of individuals, particularly younger animals undergoing periods of critical development.