Expression of a Leukocyte-Specific Antigen During Ontogeny in Xenopus Laevis

The monoclonal antibody CL21 recognizes a determinant present on the surface of leukocytes, but not on erythrocytes or nonhemopoietic tissue. The CL21 antigen was first expressed at 48 hr of development at 20°C (stage 28) on embryonic cells cultured from lateral plate mesoderm. Based on immunofluorescence staining and flow cytometric analysis, the distribution of fluorescence intensity of larval thymocytes and splenocytes was unimodal. Distributions of dull and bright cells were detected in both adult thymocytes and splenocytes. These different subpopulations appeared during the late perimetamorphic period. Adult splenocytes were metabolically activated when cultured in the presence of mAb CL21 bound to a substrate but not in the presence of mAb CL21 in suspension. Immunoprecipitation and SDS-PAGE under nonreducing conditions revealed that a single 180-kD molecule was expressed on thymocytes. Analysis of splenocytes demonstrated the presence of two molecules having similar molecular mass that resolved to a single band under reducing conditions.