Immunoglobulin Fc receptor molecules on Xenopus laevis splenocytes

The existence of membrane-associated Fc receptors for IgY (Fc nu R) or IgM (Fc mu R) was demonstrated on a large percentage of Xenopus splenocytes. The Fc receptors were detected by direct fluorescent staining in which the spleen cells were incubated with fluorescein isothiocyanate (FITC)-conjugated antigen-complexed IgY antibodies or with FITC-conjugated heat-aggregated IgM. Results showed that 28.9% (SD +/- 5.1) and 5.3% (SD +/- 2.2) of the cells bear Fcnu or Fc mu receptors, respectively. The specificity of the receptors was tested after incubation of the cells in the presence of the following fluorochrome-conjugated reagents: non-aggregated Xenopus Igs and human IgG, aggregated Xenopus albumin, Fc6 nu and Fab mu fragments and human IgG, and antigen-complex Fab2 nu fragments. Results indicated that the receptors are specific for Xenopus immunoglobulins, with the restriction that the latter must be presented in a complexed or aggregated form to the cells and that the precise binding site is located on the Fc portion of IgY or IgM. The identity of a large percentage (22.4 +/- 2.8%) of cells bearing Fc nu R was established by direct simultaneous double-fluorescent staining of surface membrane IgM and Fc nu R. All Xenopus splenic B lymphocytes bearing sIgM carry also Fc nu R, while 8.8% of splenocytes, of yet unknown lineage, bear Fc nu R alone on their surface. The presence of Fc receptors on Xenopus B lymphocytes (leaving aside their eventual presence in other leucocyte types) suggests, once again, a high degree of evolution of the immune system in these lower vertebrates.