title: On the evolutionary origin of the adaptive immune system--the adipocyte hypothesis.
abstract: Jawless vertebrates utilize a form of adaptive immunity that is functionally based on molecular effectors that are completely different from those of vertebrates. This observation raises an intriguing question: why did vertebrates, representing only 5% of all animals, twice evolve a system as complex as adaptive immunity? Theories aimed at identifying a selective pressure that would 'drive' the development of an adaptive immune system (AIS) fail to explain why invertebrates would not similarly develop an AIS. We argue that an AIS can only be implemented in a certain physiological context, i.e., that an AIS represents an unevolvable trait for invertebrates. The immune system is functionally integrated with other systems; therefore a preexisting physiological innovation unique to vertebrates may have acted as the prerequisite infrastructure that allowed the development of an AIS. We propose that future efforts should be directed toward identifying the evolutionary release that allowed the development of an adaptive immune system in vertebrates. In particular, the advent of specialized adipocytes might have expanded the metabolic scope of vertebrates, allowing the opportunistic incorporation of an AIS. However, physiological innovations, unique to (or more developed in) vertebrates, support the implementation of an AIS. Thus, understanding the interaction between systems (e.g. neural-immune-adipose connection) may illuminate our understanding regarding the perplexing immunological dimorphism within the animal kingdom.
authors.affiliation: Department of Physiological Sciences, Stellenbosch University, South Africa. Electronic address: gustav.v.niekerk@gmail.com.
authors.forename: Gustav
authors.initials: G
authors.lastname: van Niekerk
authors.affiliation: Department of Physiological Sciences, Stellenbosch University, South Africa.
authors.forename: Anna-Mart
authors.initials: AM
authors.lastname: Engelbrecht
mesh_terms: D056704:Adaptive Immunity / Q000502:physiology*; D017667:Adipocytes / Q000502:physiology*; D000818:Animals; D005075:Biological Evolution; D006801:Humans
keywords: Adaptive immune system; Adipose tissue; Evolutionary constraint; Innate immunity; Intestinal biota; Whole genome duplication