Cells involved in the immune response. XXII. The demonstration of thymus-specific antigens in the rabbit.

Horse anti-rabbit thymus cell serum (HARTS) was obtained by immunizing a horse with rabbit thymocytes intravenously at weekly intervals for 3 weeks. The horse was bled 2 weeks later and the antiserum was analysed for its cytotoxic activity with respect to the lymphocytes of the various lymphoid organs. It was demonstrated that the cytotoxic activity of the antiserum was several orders of magnitude greater for thymus cells than for cells of the other organs tested. Only thymus and lymph node cells were capable of absorbing the thymocytotoxic activity of the antiserum; however, ten to fifteen times as many lymph node cells as thymus cells were required to neutralize the thymocytotoxic activity of the serum. Absorption of the antiserum with the cells of the other lymphoid organs (spleen, bone marrow, appendix, sacculus rotundus, Peyer's patches and circulating leucocytes) resulted in a slight but significant decrease in the thymocytotoxic activity. At no time was the thymocytotoxic activity completely absorbed with cells of these organs. The cytotoxic activity of the antiserum with respect to the cells of the different lymphoid organs other than the thymus could be abolished following absorption of the antiserum with the cells of any of the lymphoid organs. On the basis of our data, it is concluded that (a) the thymocytes possess two groups of antigens—one thymocyte specific and one common to all rabbit lymphocytes and (b) only the lymph nodes of all the lymphoid organs other than the thymus possess significant numbers of thymus-derived or T-cells. However, the proportion of these cells in the lymph node does not exceed 7–10 per cent, a figure much lower than that found in the lymph nodes of the mouse. Less than 1 per cent of the circulating lymphocytes in the rabbit are T-cells.