The persistent PHA-responsive population in the mouse thymus. II. Recirculatory characteristics and immunological properties.

Experiments were performed to ascertain whether the persistent PHA-responsive cell population found in mouse thymus grafts was capable both of localizing to peripheral lymphoid organs when deliberately injected into suitable recipient mice, and of responding to antigenic stimulation. It was shown that the persistent population could not be found in large numbers in recipient spleens or lymph nodes at late time after transfer. These cells could, however, be found in large numbers in the peripheral blood of recipients. Thus they appeared to be able to circulate in the blood but not be able to extravascularize in lymphoid organs. Since these cells did not localize to spleen or lymph nodes, it was not surprising to find that they did not respond to the skin sensitizing agent oxazolone in draining lymph nodes or spleen, or to histocompatibility antigens (graft-vs-host). When tested against allogeneic cells in vitro (mixed lymphocyte reaction), it was shown that the persistent population was over nine times less reactive than in its reactivity to PHA, when compared to the non-persistent population. The functional significance of these thymocyte populations is discussed.