Synergistic or Antagonistic Effect of Different Antibody Concentrations on in Vitro Lymphocyte Cytotoxicity in the Moloney Sarcoma Virus System

Thein vitro s tudy of the combined effects of humoral antibody and lymphocytes has been essential in attempting to understand the paradoxical effect of humoral antibodies leading to the phenomenon of enhancement (1). Par t of the mechanism has been considered to be the shielding of cellular antigenic determinants by humoral antibody from the lymphocytes. This could mask antigenic sites preventing recognition and result in an afferent block and also interfere with the cytotoxic effect of specifically sensitized lymphocytes producing an efferent block. Support for both these hypotheses has been demonstrated (2-7). The blocking effect of humoral antibodies has been proposed to be of importance in the phenomena of tolerance, chimerism, and the protection of the fetus from maternal lymphocytes (8). However, it has been well documented in certain systems that humoral antibody and lymphocytes may act in concert to effect target cells' destruction (9, 10). Under different experimental conditions, pretreatment of target cells with humoral antibody may protect them from the cytotoxic effect of immune lymphocytes or mediate their destruction by a normal lymphocyte population. In vivo probably a balance between these two phenomena occurs. In experiments concerning the combined effects of sera and lymphocytes from Moloney sarcoma virus (MSV)l-infected rats vs. MSV-transformed target