The active T rosette: an early marker for T-cell activation.

This review summarizes some of the recent observations regarding the active T-rosette assay. This test, performed under suboptimal technical conditions, identifies a subpopulation of T lymphocytes with high-avidity receptors for sheep red blood cells. They are Ia-positive but appear to lack an Fc receptor. They can be either OKT4-positive or OKT8-positive. Isolated active T-rosette populations are capable of recognizing and killing allogeneic cells and they also modulate B-cell immunoglobulin production. They are directly correlated in vivo with specific cutaneous reactivity to an antigen. More interestingly, the percentage of active rosettes specifically increases in vitro after one hour of incubation with soluble or cellular antigens. This test fully correlates with the lymphocyte stimulation test. In the case of human mixed lymphocyte culture, the increase in active T rosettes is due to antigenic differences in the HLA D-DR region. The neoformation of active T rosettes is due to the early release of a rosetting factor. Drugs which are able to modulate T-cell functions, such as thymosin, transfer factor, isoprinosine and NPT-15392, also increase the percentage of active T rosettes. Therefore, the newly formed active T rosettes appear to be an early marker for lymphocyte activation.