Ecoimmunology: What Unconventional Organisms Tell Us after Two Decades.

Abstract

Model organisms have long been used to characterize the vertebrate immune system, which is responsible for protecting the host from mechanical and pathogen-induced injuries, pathology, and disease development (Janeway et al. 2001). As a result, today we have a deep understanding of how vertebrate defensive responses can be mediated by innate (i.e., non-specific) or adaptive (i.e., specific) mechanisms, or a mixture of both. Innate responses involve a generalized activation of immune mechanisms that can operate in response to any infectious agent (i.e., they are actionable to a diverse range of pathogens and parasites; Pham and Schneider 2008). In contrast, adaptive responses entail an activation of a subset of cells and/or systems to defend a host against a precise pathogen. While many of these mechanisms are highly conserved across vertebrate classes, intriguing distinctions have been revealed by research on non-model organisms (Demas and Nelson 2012). Ecoimmunology, or ecological immunology, emerged as a research area in the late 1990s and early 2000s (Sheldon and Verhulst 1996; Norris and Evans 2000; Zuk and Stoehr 2002; Brock et al. 2014; Downs et al. 2014). In this field, researchers seek to understand the factors leading to variation in the function of the immune system and how this variation contributes to disease susceptibility in a wide variety of taxonomic groups (Sheldon and Verhulst 1996; Norris and Evans 2000; Zuk and Stoehr 2002; Brock et al. 2014; Downs et al. 2014). Since its inception, ecoimmunological studies have predominantly focused on birds, with relatively fewer studies focused on other taxonomic groups (Martin et al. 2003; Ashley et al. 2012; Evans et al. 2015; Haase et al. 2016; Taves et al. 2017; Tieleman 2018). More recently, however, there has been an expansion in the diversity of taxa studied from an ecoimmunological perspective, with studies ranging from plants to invertebrates, ectothermic vertebrates, and non-model (i.e., lab rodents) mammalian species (Rolff and Siva-Jothy 2003; Zimmerman et al. 2010; Ardia et al. 2012; Fisher et al. 2012; Pijanowski et al. 2015; Titon et al. 2019; Sueiro et al. 2020; Zimmerman 2020). In addition, there has been a closer examination and optimization of the immune methods and techniques that are needed to conduct ecoimmunology studies in diverse organisms (e.g., Claunch et al. 2022).