Commentary on: “A common origin for immunity and digestion”

Citation: van Niekerk G and Engelbrecht AM (2015) Commentary on: " A common origin for immunity and digestion. " Front. Microbiol. 6:531. We read with great interest the recent hypothesis forwarded by Broderick (2015). The underlying question addressed in her thesis is central to the field of evolutionary immunology: What drives immunological innovation? This question finds its most intriguing manifestation in what has been called " arguably the most exciting finding of the past decade in immunology " (Flajnik, 2014): the discovery of a second, convergent evolved, adaptive immune system (AIS) in jawless vertebrates (hagfish and lampreys). This discovery raises the question, why did vertebrates, (representing about 1% of all animals that ever lived) evolve an AIS twice, whereas invertebrates failed to do so? Clearly, understanding the forces that drive immunological innovation will undoubtedly have a formative impact on our understanding of one of the most perplexing immunological dimorphisms in nature. Broderick argues that both immune and digestive systems might have shared a common origin and goal: " the quest for more efficient energy acquisition " (Broderick, 2015). The author draws on two lines of arguments to support her claim. Firstly, she points out that various " enzymes involved in immune responses also [perform] roles in digestion. " Secondly, she notes that the gut represented an early innovation in metazoans, from which, as is hypothesized, immune functionality developed. Indeed, the author notes that this connection between immunity and digestion might be ancient since, " for single-celled organisms like amoebae, the process of infection and food acquisition are indistinguishable in the initial stages, with the two being separable only by outcome. " Though, this may be true for single cell organism (and very primitive animals such as sea sponges), we are skeptical of the extent to which the sharing of similar components (i.e., digestive enzymes) between systems justifies an interpretation of common ontology between digestion and immune system in higher animals. Both immune and digestive systems make use of proteolytic enzymes, but often for vastly different reasons. Beyond playing a role in direct pathogen killing, proteolytic enzymes also perform a range of other functions within an immunological context. As an example, neutrophils failing to detect pathogens after being activated proceed to liquefy surrounding tissue by the release of proteases in order to gain access to pathogens (Nathan, 2006). Furthermore, proteolytic enzymes also activate pro-antimicrobial peptides [e.g., proteinase 3 processing pro-AMP into …