T-cell differentiation in the rabbit. I. Cytofluorometric analysis of a T-cell antigen on lymphocytes from normal and dexamethasone-treated animals.

The specificity of a rat anti-rabbit thymocyte antiserum (ATS), as analysed with immunofluorescence techniques, is described. The antiserum was used in cytofluorometric studies to quantify the density of the corresponding T-cell antigen present on the various lymphocyte populations. On the basis of fluorescence intensity, the positive cells could be divided into four classes. Most of the thymus lymphocytes stained bright while minor subclasses were found with dull, medium and very bright staining. Steroid-resistant thymus cells stained medium and bright. Further analysis on buoyant density and characterization of ultrastructural nuclear morphology, combined with ultrastructural studies of control and steroid-resistant thymus lymphocytes in situ showed that the medium and bright fluorescent cells represent medullary and cortical cells respectively. Moreover, two types of large cells were observed. It is suggested that these two types represent different pools of cycling cells. One located in the outer cortical zone with light density and a medium fluorescence and the other located in the deep cortex with a heavy density and a very bright fluorescence for ATS; peripheral T cells express mainly dull fluorescence. A model for T-cell differentiation is proposed and discussed in respect to the density of T-cell antigen present on the thymocyte membrane.