Determining the immune status of Steller sea lions (Eumetopias jubatus): an environmental agents of disease perspective

Abstract

The integrity of the immune system is paramount for preserving overall health for many organisms. Investigating environmental and physiological factors that may be associated with alterations of the immune status in non-traditional sentinel species, like the Steller sea lion (SSL), is a prominent undertaking in eco-immunology research. Changes to immune homeostasis likely impacts the health and survival of SSLs. Recent studies have reported that mercury concentrations in hair in 24 to 36% of newborn SSLs of the Western Aleutian Islands (WAI) exceed thresholds (>30 ppm) for potential adverse effects. Many of these individuals were from WAI rookeries that have historically experienced significant population declines with some slow to recover. Retrospective, and ongoing, analyses of mercury in lanugo coats (natal hair) from young pups of the WAI demonstrate in utero exposure to relatively high levels of mercury during late gestation. Therefore, this dissertation focuses on the notion that dietary acquired mercury could potentially alter immune response in SSLs, especially young pups, and may contribute to the lack of recovery from population declines. In order to gain an understanding of the potential for mercury to adversely affect the immune response of SSLs, selected aspects of immunity were measured (blood cell counts, haptoglobin, immunoglobulins, and cytokines) and investigated within the context of body condition, age, mercury exposure and regional population dynamics. In Chapter Two, the acute phase response protein, haptoglobin, was found to vary significantly with age and region. Individual SSL pups with greater concentrations of mercury had lower predicted concentrations of haptoglobin. In Chapter Three, a colorimetric protein A enzyme-linked immunosorbent assay was modified for enhancing accurate measurement of immunoglobulin concentrations in SSL serum. This improved methodology was then used in Chapter Four for comparing immunoglobulins in young developing SSL pups and dams as a measure of maternal investment of immunity among different rookeries. Lastly, Luminex multiplex technologies were employed for quantifying cell-signaling proteins (cytokines) in SSL serum to compare associations among rookery pups in Chapter Five. Although mercury concentrations in some individuals exceed adverse effects thresholds that are defined for other mammals, no statistically significant associations were found between immunoglobulins and cytokines relative to mercury concentrations in young developing pups. These thesis chapters provide a powerful baseline and improved methods for ongoing and future assessments of haptoglobin, immunoglobulins, and cytokines (combined with traditional hematologic measures) observed in young developing SSL pups in regions experiencing population decline when compared with rookeries with stable or increasing pup production. Some of these findings, especially for haptoglobin, are indicative of alterations in immune status in young SSL pups born to dams from different natal rookeries with higher mercury exposure. Understanding the cause of the differences in the immune status of young SSLs will require additional assessments of the maternal-fetal interface of immunity and other factors like nutrition, metabolic status, and infectious disease that may shape neonatal immunity leading to the regional differences observed.