CLINICAL IMMUNOLOGY AND IMMUNOPATHOLOGY 1, 533-546 (1973) Inhibition of Immune Cell-Mediated Killing by Heparinl ERIC MARTZ~ AND BARUJ BENACERRAF The Department of Pathology, Harvard Medical School, Boston, Massachusetts 02115 Received March 26,1973 Heparin retarded the cell-mediated lysis of mouse ascitic tumor cells or normal spleen cells by effector spleen cells from alloimmunized mice. The retardation was immediately reversible by removal of the heparin. Heparin reduced the rate of cell lysis without preventing the eventual complete destruction of the target cell popula- tion. The completion of killing was not due to loss of heparin activity or desensitiza- tion of the killer cells to heparin. At a given concentration of heparin, the amount of inhibition depended on the target cell type employed, but was relatively indepen- dent of the speed with which lysis progressed. Retardation was independent of divalent cation concentration and serum concentration, and was not affected by depletion of red blood cells or adherent cells. Heparin did not inhibit incorporation of tritiated thymidine in mixed lymphocyte cultures, and hence was not non- specifically toxic for lymphocytes. The dose responses of killing to three other highly sulfated polymers were similar to that for heparin. This indicates that the inhibition is due to the high charge density and polyanionic character of heparin, and not to an impurity. The anticoagulant activity of these polyanions was not correlated with their inhibitory activity for cell-mediated killing. When heparin is added to cultures after about 10% specific release of chromium has occurred, it has virtually no effect on subsequent killing. This is not due to desensitization of the killer cells to heparin as a result of the previous killing experi- ence. Hence, there is an early heparin-sensitive killer-target interaction which, once it has occurred, cannot be reversed by heparin. This sensitive interaction seems likely to be the formation of a close contact between the killer and target cells. There are a number of in vitro models for the several mechanisms involved in graft or tumor rejection in &IO. These involve cytotoxicity for target cells mediated by effector cells from lymphoid tissues as first described by Go- vaerts (1). Such cell-mediated toxicity can be divided into two categories, ac- cording to the type of effector cells responsible: the first category includes cy- totoxicity effected by non-thymus-derived cells, such as “B” lymphocytes (23) or monocyte-macrophage cells (4,5,42). The second concerns cell- mediated killing mediated by thymus-derived lymphocytes. This report deals only with the latter category. Using the DBA/2 mastocytoma as the target for alloimmunized killers, it has been shown that thymus cells can give rise to killer cells (6) and that effector cells bearing the theta antigen (a marker for thymus-derived lymphocytes) are ’ Supported by Grants from the New York Cancer Research Institute, Inc., and the Council for Tobacco Research, U. S. A., Inc. 2 Postdoctoral trainee of the National Institutes of Health. Copyright @ 1973 by Academic Press, Inc. 533 All rights of reproduction in any form reserved,