Post-thymic T cell development in rats: an update.

Introduction Peripheral T cells in rats differ in their expression of the membrane determinants CD4, CD8, QCA-1, CD45R, RT6 and Thy-1 [l-51. So combining these markers makes it possible to identify a large number of phenotypically different subsets of peripheral T cells. The lineage relationships between these subsets are unknown. The presence or absence of CD4, CD8, CD45R or RT6 expression have been associated with differences in T cell functions (for references see [6]). So far, this has not been found for QCA-1 and Thy1. Besides indicating differences in function, some of these determinants (i.e. CD45R, RT6 and Thy-1) have been proposed to mark different T cell lineages or different stages of maturation [5, 7, 81. Relating to this subject numerous studies have been performed with CD45R. So far however, they have generated rather confusing and conflicting data. Some authors claim that the loss of CD45R is a uni-directional event and marks a subset of memory cells that has been developed from a population of naive CD45R+ T cells after activation by antigen [9]. Others suggest that the expression of CD45R is bi-directional, and that this marker identifies T cells that exist transiently in different functional or activated stages [lo, 111. A similar situation exists for Thy-1 and RT6, for which it is also still not clear whether cells expressing these markers represent different T cell lineages and/or different functions, and whether gaining or losing these markers are uni-directional maturation effects, or just reflect temporary stages of activation [8]. Theoretically, based on the different possibilities of the presence or absence of Thy-I, RT6 and CD45R expression on T cells, eight different subsets can be created. This number is increased to 27 when the level of expression intensity (i.e. dull or bright) is taken into account also. In previous experiments [6] we have shown that at least 11 of these latter possibilities exist in the peripheral