Immunofluorescence studies of a possible prethymic T-cell differentiation in congenitally athymic (nude) mice.

A rabbit antimouse brain theta reagent was made specific for cells of the T lineage by absorption in vivo in nude mice. When used in double fluorescence together with an antimouse immunoglobulin reagent, four types of cells were found in spleen and lymph nodes of both normal and nude mice: Ig+thetaBr-, Ig-thetaBr+, Ig-thetaBr-, and Ig+thetaBr+. The data show that about 20% of nude mouse spleen lymphocytes are definitely of T lineage (Ig-thetaBr+). On these cells, the detection of the "thetaBr" determinant, which is identical or very close to the "theta" determinant, depends on the large amplification produced by indirect immunofluorescence, which suggests a low density of theta antigen. Similar experiments suggest the presence of cells that express some TL antigen in the spleen of nudes made congenic to a TL+ strain (BALB/c). It is proposed that the T-cell precursor that will further differentiate in the thymus already expresses a low density of theta and, in TL+ strains, TL antigen.