Lymphoid organs and lymphocytes

The amphibian thymus anlagen arise from the dorsal epithelium of the second (anuran), or third, fourth and fifth (urodeles) pharyngeal pouches. In Xenopus, the thymus buds are rapidly colonized by hematopoietic precursors, and by day 6-8 the cortex-medulla architecture becomes visible. The larval thymus involutes during metamorphosis, but efficient regeneration occurs in the metamorphosed froglet. The thymus then regresses at the time of sexual maturity. In axolotl the thymic anlagen appear at the time of hatching, 3 weeks after fertilization. The thymus buds are invaded by hematopoietic precursors 12 days after hatching, and the thymus develops slowly up to sexual maturity (about 12 months), and then regresses. The urodele thymus shows no cortex-medulla differentiation. The axolotl and Xenopus thymus contains most of the cell types that constitute the stroma of mammalian thymus, such as macrophages, nurse-like cells, cortical large dendritic cells and different types of epithelial cells.

The spleen appears about 12-14 days following fertilization in Xenopus and the mature spleen has a clear-cut red and white pulp, but no structures reminiscent of the avian and mammalian germinal centers. The axolotl spleen anlagen appear at the time of hatching (about 3 weeks after fertilization). Although some red/white pulp-like organization can be seen macroscopically, there is no follicular-like structure and no germinal centers. In urodeles and anurans, the hematopoietic peripheral layer of the liver supports B cell lymphopoiesis and granulo-cytopoiesis.

Immunoglobulin M (IgM)-producing cells can be observed along the digestive tract in axolotl, and IgM-, IgX- but no IgY-producing cells are present in the digestive tract of Xenopus. The anuran kidneys contain B lymphocytes, and can retain antigens in the intertubular tissue. Amphibians possess B cells and T-like cells in the blood and in Xenopus these cells can collaborate in vitro in a major histocompatibility complex (MHC)-restricted manner for the production of antibodies to T cell dependent antigens.

B Cells can be recognized in axolotl and Xenopus by monoclonal antibodies (mAbs) specific for the different heavy (H) and light (L) Ig chain isotypes. More difficult was the production of mAbs specific for T cells. In the axolotl, mAb 34.38.6 labels all thymocytes, 60-63% of slg~ splenic lymphocytes, hematopoietic stem cells, granulocytes and macrophages of normal animals, but only 9% of splenic lymphocytes from thymectomized animals. A polyclonal antibody (L12) coprecipitates several (38, 43 and 22 kDa)

covalently linked molecules that form a multimeric complex on the axolotl T cell surface.

A membrane glycoprotein (120 kDa), which may be a CD8 equivalent, has been identified on Xenopus J strain T cells using mAbs XT-1 and AM22 which recognize Xenopus cortical thymocytes and a subpopulation of peripheral T cells. mAb AM15 recognizes a 18 kDa protein on a subpopulation of Xenopus T cells. Two mAbs (anti-CTX: IS9-2 and X71) stain a subpopulation (65-80%) of Xenopus cortical thymocytes which are also labeled by mAb AM22 (anti-CD8-like), and thus could be the Xenopus equivalent of the avian and mammalian cortical DP thymocytes.

In tadpoles and adult Xenopus, the use of classical mitogenic agents, such as lipopolysaccharide (LPS), phytohemagglutinin (PHA), concanavalin A (Con A) and purified protein derivative (PPD), stimulate in vitro proliferation of B cells and T cells with the same responses that are seen in mammals. The PHA and mixed leukocyte responses are sensitive to thymectomy. Furthermore, a T cell growth factor similar to interleukin 2 (IL-2) is produced by stimulated T lymphocytes.

The axolotl has a population of B lymphocytes that proliferate in the presence of LPS at all steps of ontogenesis. The splenic T cells from adult, but not from young axolotls, proliferate significantly in response to PHA and Con A. Axolotl lymphocytes are also stimulated in vitro by the enterotoxins A and B from Staphylococcus aureus.

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