Nature and mode of action of antilymphocytic antiserum.

Antilymphocytic serum (ALS) is the name given to an antiserum raised in members of one species by the injection of lymphocytes or lymphoid cells taken from members of another species. It has the power not merely to prevent or delay the onset of immunological reactions leading to the rejection of homografts,1' 2 but also to arrest reactions already in progress.2' 3 This combination of properties is unique, and since ALS is devoid of acute toxicity, and is indeed, in transplantation systems, the most powerful immunosuppressive agent yet described, its clinical and experimental potentialities are worth close attention. The experiments reported here are intended to throw further light on the nature and mode of action of ALS. Materials and Methods.-Preparation of antiserum: Antisera were raised by giving New Zealand rabbits two successive intravenous injections 14 days apart of a single-celled suspension of 109 living thymocytes from young female CBA mice. The rabbits were bled 7 days after the second injection, and the serum was heated to 56° for 30 min, filtered, and stored at -20°. This simple regimen served also for lymphocyte fractions and for cells other than lymphocytes, and the antisera so prepared required no absorption with red cells. Longer courses of injection usually yielded less effective antisera, perhaps because of a change in the physical character of the operative antibodies, or because of interference by antibodies to minor or irrelevant constituents of the immunizing cells. Antisera were always injected by the subcutaneous (subintegumentary) route. Assay; Skin grafting: The strength of a sample of ALS was measured in terms of its power to prolong the life of A-strain tail skin homografts on adult male CBA mice. In a typical experiment, each member of a uniform panel of mice received 0.5 ml ALS on the second and again on the fifth day after grafting (i.e., on days +2 and +5 in the now widely used notation). The results are figured as survival curves showing, day by day, the number of mice still bearing surviving grafts, and are expressed numerically either as the mean expectation of life (MEL) of a graft at grafting, or as the median survival time (MST). The error function cited is the standard deviation (SD). The MEL of A-strain tail skin homografts on adult CBA males is 11.6 ± 1.3 days SD (MST 11.5 days).3 Abrogation of the Second-Set Response. The sensitivity aroused in CBA mice by grafts of A skin, as measured by the survival time of a second graft transplanted at various intervals after the rejection of a first, is still clearly in force after 35 weeks.4 For the first 10 weeks after the rejection of first-set grafts, the MST of second-set grafts does not exceed 6 days. We have already shown3 that under the protection afforded by 4 X 0.5 ml ALS given on days -1, +1, +3, +5, the MST of second-set grafts exceeded 40 days (maximum 210 days). However, when 11 days was allowed to pass between the completion of such a course of injections and the transplantation of second-set grafts, the MST fell to between 14 and 15 days (maximum 31 days). If the delay between antiserum injections and the trans-