A cell surface marker of thymus-dependent lymphocytes in Xenopus laevis is identifiable by mouse monoclonal antibody

A mouse monoclonal antibody (mAb), named XT-1 (IgG2b,lambda), was prepared against J strain Xenopus laevis thymocytes. The determinant recognized by this mAb (XT-1 determinant) was expressed by 92-98% thymocytes and 22-37% splenic lymphocytes from intact J strain frogs and larvae, as analyzed by flow cytofluorometry. The XT-1 determinant was also expressed by 20-30% peripheral blood lymphocytes and 5-11% bone marrow lymphocytes from adult, and 11-17% liver lymphocytes from larval J strain Xenopus. Early larval thymectomy depleted XT-1-positive cells in the spleen to less than 2% of the total lymphocyte population. Double immunofluorescence experiments revealed that XT-1+ lymphocytes in the spleen are surface IgM-. The strain distribution study on the expression of XT-1 determinant suggests the allelic polymorphism of the XT-1 determinant. The surface antigen identified by the present XT-1 mAb may provide an important marker for lymphocytes in thymus-dependent lineage in Xenopus.