Gut-associated lymphoid tissue (GALT) in reptiles: intraepithelial cells

From Species to Regional and Local Specialization of Intestinal Macrophages

Initially intended for nutrient uptake, phagocytosis represents a central mechanism of debris removal and host defense against invading pathogens through the entire animal kingdom. In vertebrates and also many invertebrates, macrophages (MFs) and MF-like cells (e.g., coelomocytes and hemocytes) are professional phagocytic cells that seed tissues to maintain homeostasis through pathogen killing, efferocytosis and tissue shaping, repair, and remodeling. Some MF functions are common to all species and tissues, whereas others are specific to their homing tissue. Indeed, shaped by their microenvironment, MFs become adapted to perform particular functions, highlighting their great plasticity and giving rise to high population diversity. Interestingly, the gut displays several anatomic and functional compartments with large pools of strikingly diversified MF populations. This review focuses on recent advances on intestinal MFs in several species, which have allowed to infer their specificity and functions.

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

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.

The primary humoral immune response of European green lizards (Lacerta viridis) to Leishmania agamae

European green lizards, Lacerta viridis, produced relatively thermostable, dithiothreitol-sensitive, non-precipitating, agglutinins and complement-fixing antibodies (CFA) to Leishmania agamae administered subcutaneously (SC), intraperitoneally (IP) or orally (OR). Antibodies were also detected by the immobilization test (IMM) and by enzyme-linked immunosorbent assay (ELISA). The most sensitive method for the detection of stimulated immunoglobulins was ELISA. Antibodies were detected as early as 3 days post-infection with ELISA and between 5 and 7 for CFA, direct agglutination (DA) and indirect haemagglutination (IHA). In the case of IMM, the times of first detection varied from 14 to 28 days. Maximum CFA (2(-8)), DA (2(-8)), IHA (2(-11)) and ELISA (2(-16)) titres were reached from 42 to 49 days with significantly higher values occurring in the OR and IP groups. With IMM, maxima occurred after 5 or 6 weeks. Following exposure, two- to five-fold significant increases in serum lysozyme levels were demonstrated but the concentrations in sera following SC, IP or OR routes of antigen administration were not significantly different when the groups were compared with each other. The highest lysozyme values (approximately 12.3 - 12.5 micrograms ml(-1)) were found in the SC and OR groups when compared to the IP (7.40 micrograms ml(-1)).

Ultrastructure of gut-associated lymphoid tissue (GALT) in the amphibian urodele, Pleurodeles waltlii

The ultrastructure of gut-associated lymphoid tissue (GALT) has been studied in the salamander, Pleurodeles waltlii. Lymphoid accumulations appear as true infiltrates scattered throughout the lamina propria cell elements. The most important components of these infiltrates are small and medium sized lymphocytes, and, in lesser amounts, developing and mature plasma cells, macrophages and granulocytes. Migrating lymphoid cells massively invade the intestinal epithelium inducing noticeable modifications, such as the disappearance of the basement membrane and decreased numbers of mucous cells. Thus, in its organization and cell composition, the GALT of P. waltlii appears to represent a primitive phylogenetic precursor of the mammalian "intestinal-immunologic" barrier.