Decreased Immune Response in Zebra Finches Exposed to Sublethal Doses of Mercury Catherine A. Lewis • Daniel A. Cristol • John P. Swaddle • Claire W. Varian-Ramos • Patty Zwollo Received: 2 May 2012 / Accepted: 22 October 2012 Ó Springer Science+Business Media New York 2012 Abstract Mercury (Hg) is a ubiquitous contaminant with deleterious effects on many wildlife species. Most studies to date have focused on fish-eating birds and mammals because much historical Hg pollution is aquatic. Recently, however, comparable blood-Hg levels have been found in terrestrial insectivorous songbirds. As a result, research is needed to clarify the effects of Hg exposure on songbirds. One fundamental end point that is still poorly understood is the effect of Hg on the songbird immune system. If Hg affects the functioning of the immune system, exposed songbirds may be less able to mount an appropriate immune response against invading pathogens. To gain insight into how Hg affects songbird immune function on a cellular level, a flow cytometric assay was developed to measure lipopolysaccharide-induced B-lymphocyte prolif- eration in zebra finches (Taeniopygia guttata). This is the first experimental (dosing) study of the potential effect of Hg on songbird immune system functioning. Decreased B cell proliferation was observed after lipopolysaccharide exposure in individuals with greater concentrations of Hg in their blood and tissues. In addition, these individuals had decreased ratios of proliferating-to-resting B cells. This decrease in lymphocyte proliferation in response to an effective mitogen suggests that environmental exposure to sublethal levels of Hg may inhibit or delay B cell proliferation in songbirds, potentially increasing suscepti- bility to disease and decreasing survivorship. Mercury (Hg) is a global contaminant that is toxic to wildlife and humans (Boening 2000; Holmes et al. 2009). Hg pollution usually enters waterways in an inorganic form and then can be bioconverted into methylmercury (MeHg), which has a greater bioavailability to organisms such as fish and birds (Boening 2000). In fish-eating birds, high accumulation of MeHg through biomagnification has been associated with endocrine disruption and decreased repro- duction (Frederick and Jayasena 2011; Scheulhammer et al. 2007). In addition, recent studies have showed that ter- restrial insectivorous songbirds (Passeriformes) can accu- mulate Hg concentrations in blood comparable with those found in fish-eating and aquatic species (Cristol et al. 2008; Rimmer et al. 2010). The apparent consequences of envi- ronmental exposure to MeHg on insectivorous songbirds include altered singing behavior (Gorissen et al. 2005; Hallinger et al. 2010), suppressed adrenocortical response (Wada et al. 2009), decreased reproductive success (Brasso and Cristol 2008; Hallinger and Cristol 2011), and altered immune function (Hawley et al. 2009). Although these field studies offer correlational evidence of the effects of Hg on avian fitness and condition, there is little or no evidence from experimental studies of songbirds pointing to Hg exposure as a cause of immune suppression. Studies of the immunomodulatory effects of MeHg on songbirds have been particularly limited, in part due to a lack of songbird-specific reagents. However, there is evi- dence in songbirds that high Hg exposure is associated with immune suppression using the phytohemmaglutinin (PHA) skin test, and the same pattern, albeit not statistically sig- nificant, was observed for decreased antibody production C. A. Lewis Á D. A. Cristol Á J. P. Swaddle Á C. W. Varian-Ramos Department of Biology, The College of William and Mary, Institute for Integrative Bird Behavior Studies, Williamsburg, VA 23187, USA P. Zwollo (&) Department of Biology, The College of William and Mary, Williamsburg, VA 23187, USA e-mail: pxzwol@wm.edu 123 Arch Environ Contam Toxicol DOI 10.1007/s00244-012-9830-z