Immunobiology of T and B cells

Thymectomy in Xenopus either decreases or abolishes allograft rejection, MLR, PHA responsiveness, IgY antibody synthesis and antibody responses to thymus-dependent antigens. It does not abolish in vivo or in vitro responses to thymus-independent antigens or B cell polyclonal activation (LPS). In vitro assays using purified T and B cells from carrier-or hapten-primed Xenopus of various MHC types indicate that T cell help is involved in the differentiation of thymus-dependent antigen-primed B cells and that T-B collaboration is MHC restricted. IgM is produced first following antigenic stimulation, and then is produced in conjunction with IgY. A second injection generates a significantly stronger (xlO-100), mainly IgY, secondary response. Although somatic mutation occurs in Xenopus VH segments at almost the same frequency found in mammals, these mutations may not be properly selected, perhaps because of the absence of germinal centers in the lymphoid organs.

Larval thymectomy suppresses allograft rejection in urodeles, but does not abolish in vivo responses to certain thymus-dependent antigens, such as sheep or horse erythrocytes. Larval and adult thymectomy, low dose (50-150 rads; 0.5-1.5 Gy) irradiation and hydrocortisone treatment enhance specific antibody synthesis against erythrocyte antigens. These observations suggest that T cell help is impaired in urodeles, but that some kind of T cell-dependent suppression acts on antibody production. Urodeles can be immunized against particulate but not against soluble antigens, and in normal and thymectomized axolotls IgM is the single antigen-sensitive Ig class, specific IgY is not produced. The kinetics of the antibody response is slow and there is no occurrence of a typical secondary response following hyperimmun-ization. Thus, although the Xenopus and axolotl IgY molecules are clearly homologous at the molecular level, their respective physiological functions are different: IgY are IgG-like (thymus dependent, sensitive to thymectomy) in anurans, but IgA-like in axolotl, at least in the first 7-8 months of development; most IgY are found associated with the digestive epithelium and are secreted into the gut lumen following transepithelial transport in association with secretory component-like molecules.

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