Abstract
Most, if not all, animals engage in associations with bacterial symbionts. Understanding the mechanisms by which host immune systems and beneficial bacteria communicate is a fundamental question in the fields of immunology and symbiosis. The Hawaiian bobtail squid (Euprymna scolopes) engages in two known symbioses; a binary relationship with the light organ symbiont Vibrio fischeri, and a bacterial consortium within a specialized organ of the female reproductive system, the accessory nidamental gland (ANG). E. scolopes has a well-developed circulatory system that allows immune cells (hemocytes) to migrate into tissues, including the light organ and ANG. In the association with V. fischeri, hemocytes are thought to have a number of roles in the management of symbiosis, including the recognition of non-symbiotic bacteria and the contribution of chitin as a nutrient source for V. fischeri. Hemocytes are hypothesized to recognize bacteria through interactions between pattern recognition receptors and microbe-associated molecular patterns. Colonization by V. fischeri has been shown to affect the bacteria-binding behavior, gene expression, and proteome of hemocytes, indicating that the symbiont can modulate host immune function. In the ANG, hemocytes have also been observed interacting with the residing bacterial community. As a model host, E. scolopes offers a unique opportunity to study how the innate immune system interacts with both a binary and consortial symbiosis. This mini review will recapitulate what is known about the role of hemocytes in the light organ association and offer future directions for understanding how these immune cells interact with multiple types of symbioses.
Highlights
Studies in a number of animal-microbe symbioses have revealed that bacteria are active in a wide range of biological processes (McFall-Ngai et al, 2013), including digestion, host defense, development, and even camouflage (Gil-Turnes et al, 1989; McFall-Ngai and Montgomery, 1990; Jones and Nishiguchi, 2004; Kau et al, 2011; Hooper et al, 2012; Fraune et al, 2015; Gromek et al, 2016)
Once the light organ was cured, hemocytes bound significantly more V. fischeri, while binding to all non-symbiotic bacteria remained unchanged. These findings suggest that hemocytes can distinguish between related bacteria and that colonization state influences the binding behavior of the hemocytes, toward V. fischeri
Previous studies show that V. fischeri can influence hemocyte dynamics in multiple ways
Summary
Studies in a number of animal-microbe symbioses have revealed that bacteria are active in a wide range of biological processes (McFall-Ngai et al, 2013), including digestion, host defense, development, and even camouflage (Gil-Turnes et al, 1989; McFall-Ngai and Montgomery, 1990; Jones and Nishiguchi, 2004; Kau et al, 2011; Hooper et al, 2012; Fraune et al, 2015; Gromek et al, 2016). Interactions between hosts’ immune systems and the resident microbiota are often essential in the establishment and maintenance of beneficial associations (Stappenbeck et al, 2002; Nyholm and Graf, 2012; Buchon et al, 2013)
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