The morphology of immune reactions in normal, thymectomized and reconstituted mice. II. The response to oxazolone.

The response to oxazolone, painted on the skin, has been studied in the regional lymph nodes of normal adult CBA/H mice and of thymectomized, X-irradiated, bone-marrow injected mice with or without a reconstituting thymus graft. The thymus grafts carry a marker chromosome so that, in a reconstituted animal, cells derived from bone marrow and from the thymus can be distinguished. The response to oxazolone appears to be biphasic and both thymus-derived and bone-marrow-derived cells participate in it. In the first stage, normal mice show a massive proliferation of thymus-derived cells in the paracortex which reaches a peak on day 4. This reaction is absent in thymectomized mice but is restored if animals are grafted with a syngeneic thymus. The second stage consists of proliferation of bone-marrow derived cells which is maximal on day 8 and is then sustained, at a slightly lower level, for the rest of the experiment. It coincides with germinal centre formation and hyperplasia of the medullary cords—changes which are best seen in normal and reconstituted mice though they are also present to a lesser extent in thymectomized animals. The part played by bone-marrow derived elements in the immune response to oxazolone is not known but that there is a considerable proliferation of these cells in the later stages in the present experimental model is certain.