Molecular interactions in human T cell-mediated cytotoxicity to EBV II. Monoclonal antibody OKT3 inhibits a post-killer-target recognition/adhesion step.

Abstract

Monoclonal antibody OKT3 (pan-T) inhibits the function of human cytotoxic T lymphocytes (CTL) specific for autologous Epstein Barr virus-(EBV) infected B cells. The inhibition is at the level of the effector cells that bind the antibody, rather than the target cells that do not react with OKT3. The blockade can be bypassed by addition of concanavalin A. The objective of the present study was to define which steps of the killer-target interaction OKT3 inhibits. A previously established system that operationally divides the early events of killer-target interaction into 2 discrete steps was adapted. The 1st step, adhesion formation, was quantitated by the amount of radioisotope released from 51Cr-labeled targets after interruption of killer cell recirculation by dispersion of killer-target mixtures in high MW (500,000) dextran. The 2nd step, lethal hit, was quantitated by the amount of radioactivity released after detachment of killer-target conjugates with EDTA and prevention of reattachment by dispersion in dextran-containing medium. Both steps were temperature dependent. Adhesion formation was 8-fold more efficient at 37.degree. C than at 20.degree. C, and was 60-fold more efficient at 37.degree. C than at 15.degree. C. The lethal hit step proceeded only at 37.degree. C, and not at all at 20.degree. or 15.degree. C, for the periods of time tested. A significant degree of adhesion formation had occurred by 10 min at 37.degree. C with very few lethal hit events. By 20 min, most of the cell adhesions had proceeded to lethal hits. The identification of these 2 early steps in human killer-target interaction verifies similar findings in the murine system. Monoclonal antibody OKT3 did not inhibit the formation of visual adhesions, and did not detach preformed conjugates. OKT3 was inhibitory even when added after adhesion formation but before the irreversible initiation of lethal hit events. Among monoclonal anti-human lymphocyte antibodies, OKT3 is unique in its ability to inhibit human CTL-mediated lympholysis by interfering with a step subsequent to killer-target recognition and adhesion.