Intestinal B cells in the red‐eared slider turtle, Trachemys scripta: Anatomical distribution and implications for ecological interactions with pathogenic microbes

Abstract

Disease is a significant threat in the global decline of reptile species. Many aquatic reptiles live in habitats with high levels of opportunistic microbial pathogens, yet little is known about their immune system. Gut‐associated lymphoid tissue is vital for protection against ingested pathogens and maintenance of normal gut microbiota. In mammals, gut mucosal immunity is well‐characterized and mucosal surfaces are coated in protective antibodies. However, reptiles lack lymph nodes and Peyer's patches, which are the major sites of mammalian B cell responses. The presence or distribution of mucosal B cells in reptiles is unknown. In this study, we first set out to determine if B cells could be detected in intestinal tissues of red‐eared slider turtles, Trachemys scripta. Using whole‐mount immunochemistry and a primary antibody to turtle antibody light chains, we identified widely distributed B cell aggregates within the small intestine of hatchling turtles. These aggregates appeared similar to isolated lymphoid follicles (ILFs) in mammals and the frequency was much higher in distal intestinal sections than in proximal sections. To determine if these structures were inducible in the presence of microbes, we introduced an enteric Salmonella species through oral gavage. Analysis of intestinal tissues revealed that hatchlings exposed to Salmonella exhibited significantly more of these aggregates when compared with those that did not receive bacteria. These studies provide the first evidence for B cell‐containing ILF‐like structures in reptiles and provide novel information about gut immunity in nonmammalian vertebrates that could have important implications for ecological interactions with pathogens.

From Species to Regional and Local Specialization of Intestinal Macrophages

“…As mentioned above, the gastrointestinal mucosa of vertebrates has specialized sites dedicated to the detection of pathogens in contaminated food and water. Indeed, in reptiles, amphibians, and lungfishes, the gut contains lymphoid aggregates resembling the ILFs found in mammals (Borysenko and Cooper, 1972 ; Zapata and Solas, 1979 ; Solas and Zapata, 1980 ; Tacchi et al, 2015 ; Ashford et al, 2019 ). Like mammal ILFs, the number and size of these aggregates increase with microbial challenges (Tacchi et al, 2015 ; Ashford et al, 2019 ).…”

“…Indeed, in reptiles, amphibians, and lungfishes, the gut contains lymphoid aggregates resembling the ILFs found in mammals (Borysenko and Cooper, 1972 ; Zapata and Solas, 1979 ; Solas and Zapata, 1980 ; Tacchi et al, 2015 ; Ashford et al, 2019 ). Like mammal ILFs, the number and size of these aggregates increase with microbial challenges (Tacchi et al, 2015 ; Ashford et al, 2019 ). Based on recent observations made on lungfish, it seems however that these lymphoid aggregates lack a well-structured organization, showing no segregation between B and T cells, no germinal center, no AID expression, and no somatic hypermutation (Tacchi et al, 2015 ).…”

“…Most studies on reptile immune function have focused on systemic immune responses, leaving an important knowledge gap in the mucosal immune responses. Indeed, literature on reptile intestinal immunity consists mainly of descriptive studies (Borysenko and Cooper, 1972 ; Zapata and Solas, 1979 ; Solas and Zapata, 1980 ; Ashford et al, 2019 ). Reptiles possess primary lymphoid organs such as bone marrow and thymus but lack secondary lymphoid tissues such as Peyer's patches (PPs) or mesenteric lymph nodes (MLNs).…”

“…Instead, numerous ILF-like lymphoid aggregates are located throughout the small and large intestines. These aggregates are enriched in small lymphocytes and MFs (Borysenko and Cooper, 1972 ; Zapata and Solas, 1979 ; Solas and Zapata, 1980 ; Ashford et al, 2019 ). MFs are dispersed in the lamina propria but can migrate to the intestinal epithelium in these lymphoid aggregates (Solas and Zapata, 1980 ).…”

Immunity in Sea Turtles: Review of a Host-Pathogen Arms Race Millions of Years in the Running

“…Studies of sea turtle immune function have been focused on the protection and conservation of turtle wildlife from threats such as FP, parasites, or environmental contaminants [42,[49][50][51][52][53][54][55][56]73,75,76,[88][89][90][91][92]. Functional studies of the turtle immune system have been carried out on red-eared slider turtles Trachemys scripta (Thunberg, 1792)) [19][20][21][22][23], Caspian turtle or striped-neck terrapin (Mauremys capsica (Gmelin, 1774)) [27][28][29][30][31] and Chinese softshelled turtle (Pelodiscus sinensis (Wiegmann, 1853)) [14,87,87,93].…”