LYSIS OF ADULT VENTRICULAR MYOCYTES BY CELLS INFILTRATING REJECTING MURINE CARDIAC ALLOGRAFTS

Background Immunologic mechanisms that mediate myocardial cell injury during rejection are not fully understood. We therefore investigated w hether cells that infiltrate rejecting cardiac allografts are capable of directly injuring myocytes and whether this injury resembles that p roduced by cytotoxic T lymphocytes (CTLs) that are generated in a mixe d lymphocyte reaction (MLR). Methods and Results Heart-infiltrating ce lls (HICs) were isolated from murine heterotopic BALB/c cardiac allogr afts undergoing rejection 6 to 8 days after transplantation into C57BL /6 mice. An in vitro model system of cultured adult murine ventricular myocytes was developed to facilitate investigation of cell-mediated m yocyte injury. Isolated adult myocytes were incubated with either HICs or MLR effector cells, and myocyte death was quantified by counting t he number of rod-shaped myocytes excluding trypan blue. The frequency of donor-reactive CTLs was similar in the HIC and MLR populations, as assessed by limiting dilution analysis. However, HICs were less effici ent at killing donor-strain myocytes than were MLR cells. CTL-mediated cell lysis occurred by 6 hours, whereas myocyte injury produced by HI Cs was more gradual, with considerable cytotoxicity occurring between 12 and 24 hours. Furthermore, whereas MLR cells lysed only donor-strai n myocytes, HIC lysed donor, third-party, and syngeneic myocytes. Trea tment of MLR cells and HICs with anti-CD8 antibody plus complement pro duced a much greater inhibition of MLR cytotoxicity than of HIC cytoto xicity. Conclusions These data demonstrate that only a small component of myocyte injury mediated by allograft-infiltrating cells can be asc ribed to CTLs within the infiltrating cell population. These findings suggest that cell types associated with a delayed-type hypersensitivit y response, as well as CTLs, cause myocyte injury during cardiac rejec tion.