Mechanisms of lymphocyte-mediated cytotoxicity. III. Characterization of the mechanism of inhibition of the human alloimmune lymphocyte-mediated cytotoxic reaction by polyspecific anti-lymphotoxin sera in vitro.
The mechanism of inhibition of one human cytotoxic T lymphocyte-mediated (CTL) reaction in vitro by a polyspecific antiserum directed against soluble products of activated lymphocytes was investigated. This antiserum was made against serum-free culture supernatants from lectin-inactivated human lymphocytes (anti-WS). The anti-WS shows strong neutralizing activities for both soluble and membrane forms of human lymphotoxins. The anti-WS was investigated for its site(s) of inhibition of the alloimmune CTL reaction relative to the calcium-dependent phase. The anti-WS was found to neutralize the CTL reaction subsequent to the calcium-dependent phase, suggesting the anti-WS neutralized a lytic effector mechanism. A fluid-phase immunoadsorption assay was developed to analyze the biochemical characteristics of the antigenic determinant(s) recognized by the anti-WS. Unfractionated supernatants or defined m.w. regions corresponding to the m.w. classes of the human LT system were used to adsorb the anti-WS. The data indicated that unfractionated supernatants and LT-Cx fractions (>200,000 m.w.) significantly reduced the capacity of the anti-WS to inhibit the alloimmune CTL reaction. α-LT fractions (70 to 90,000 m.w.) partially adsorbed the cytolytic inhibitory antibodies of the anti-WS, whereas β and γ LT fractions (40 to 50,000 and <20,000 m.w., respectively) were ineffective. These results support the concept that the mechanism of inhibition mediated by the anti-WS was by its capacity to interact with multiple antigenic species of LT molecules, and thus implies that lymphotoxins are involved as a multicomponent system of cytotoxins in the lytic effector mechanism of human alloimmune cytotoxic T lymphocytes. The LT-Cx appears to represent the most likely molecular candidate for the lytic effector mechanism. A model is presented that attempts to integrate the biochemical properties of the LT system with the cellular mechanism(s) of the alloimmune cytotoxic reaction scheme.