The role of mononuclear phagocytes in cardiac allograft rejection in the rat: II. Characterization of mononuclear phagocytes extracted from rat cardiac allografts

A method was developed for the extraction of leukocytes infiltrating rat cardiac allografts. Mononuclear phagocytes (MNP) comprised 52.4 ± 5.5% of the cells extracted from allografts at the time of rejection (Day 7). Day 4 allografts and Day 7 syngeneic grafts yielded considerably fewer MNP although numbers of lymphoid cells were similar in all three groups. Allograft MNP were phagocytic for latex particles but only very low numbers were adherent to a variety of surfaces. About 50% were positive for attachment and internalization of opsonized sheep red cells via the Fc receptor. However, fewer cells were able to internalize sheep red cells than were able to bind them when complement receptor-mediated phagocytosis was investigated. Large amounts of plasminogen activator were secreted by allograft MNP while cells from syngeneic grafts produced very little. The possible participation of MNP in the effector phase of a mechanism for allograft rejection similar to delayed-type hypersensitivity is discussed.