High-frequency, but reduced absolute numbers of recent thymic migrants among peripheral blood T lymphocytes in diabetes-prone BB rats.

In rats, RT6 and CD45RC are expressed by mature peripheral T cells. The underrepresentation of T cells expressing these markers in the T lymphocytopenic BB rat might therefore be a reflection of a relatively immature T cell population. With the use of Thy-1 as a marker for recent thymic migrants, it was demonstrated that BB rats indeed have a phenotypically less mature T cell population than age-matched control rats. However, this could not account for the reduced percentages of RT6+ and CD45RC+ T cells, as these were also decreased among mature Thy-1- T cells of BB rats. Although relatively overrepresented, absolute numbers of Thy-1+ T cells were reduced in BB rats. Absolute numbers of mature Thy-1- T cells were also reduced in BB rats, but to a much larger degree than would proportionally be expected. Our findings taken together led us to conclude that both reduced thymic output and a defect in peripheral expansion are involved in the T lymphocytopenia of BB rats.