Nonspecies-specific receptors for the Fc portion of immunoglobulin (RFc) were detected in various rabbit lymphoid tissues by the ability of the RFc-bearing cells to form rosettes with ox red blood cells coated with the IgG fraction of a rabbit antiserum directed against ox red blood cell stroma. The rosette assay was shown to be specific for Fc receptor activity. In lymphocyte preparations containing less than 5% phagocytic cells the average percent of RFc-bearing lymphocytes in the various tissues was as follows: appendix 12%, lymph node 9%, peripheral blood 26%, spleen 31%, and thymus 0%. In double assay experiments, in which the lymphocytes were prestained with an FITC-labeled goat anti-rabbit Fab IgG fraction before rosetting, the RFc-bearing lymphocytes were found to constitute a subpopulation of the surface Ig (sIg)-bearing cells in the tissues examined when normal nonactivated rabbit lymphocytes were studied. After Pronase treatment, under conditions sufficient to remove cell surface Ig, and subsequent incubation to allow resynthesis of sIg, the percentage of sIg-bearing cells was restored to pretreatment levels. Conversely, although the Pronase treatment initially had no effect on RFc, there was a decline in detectable RFc-bearing cells after incubation in both treated and untreated preparations. Even with the loss of RFc+ lymphocytes during the incubation period, the percentage of sIg+ lymphocytes that expressed RFcγ remained fairly constant, suggesting the existence of an RFc+-sIg- subpopulation in the rabbit. Double assays employing FITC-labeled goat anti-rabbit antibody specific for µ and γ determinants were performed to determine if there was any relationship between the class of sIg expressed and the presence of Fc receptor. There was an overlap in IgM- and IgG-bearing lymphocytes, but no apparent overlap between IgM+ RFcγ+ and IgG+ RFcγ+ lymphocytes, suggesting that cells expressing both surface IgM IgG do not possess RFcγ. In the present study cells with receptors for the Fc portion of IgM were not detected in any of the tissues examined under the conditions employed.

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