Intercellular Adhesion Molecule‐1 (ICAM‐1) Monoclonal Antibody Inhibits Cytotoxic T Lymphocyte Recognition

Recent studies have shown that cytotoxic T lymphocytes form conjugates with antigen-negative targets.' These studies have further demonstrated that this process of antigen-independent conjugate formation (AIC) is mediated by way of two adhesion pathways: one involving the T cell-CD2 receptor binding to the ligand LFA-3* on the target, and the other involving the T cell-LFA-I receptor interacting with ICAM-1 and probably other ligandsj on the target. While the roles of CD2, LFA-I , and LFA-3 in CTL-mediated lysis (CML) have been demonstrated a b ~ n d a n t l y , ~ very little is known about the role of ICAM-I in such lysis. ICAM-I was initially proposed as a ligand for LFA-I , based upon monoclonal antibody (mAb) inhibition of LFA-1-dependent T-cell adhesion to fibroblasts and the finding that inhibition occurred with pretreatment of the fibroblasts and not the T cell.5.h These findings have been extended to and confirmed in AIC using CTL c1ones.j We initially investigated the role of ICAM-I by mAb-inhibition studies of AIC formation, and showed that its utilization in LFAI-dependent conjugates varied significantly between target^,^ irrespective of the ICAM1 level expressed on the target. To analyze its role in CML, targets that predominantly used ICAM-I in AIC were selected. One such target is U937 (a prornonocytic line).3 The ability of the ICAM-1 mAb to inhibit antigen-independent, phorbol myristic acetate (PMA)/ ionophore-triggered lysis of U937 by a CTL clone was studied. The results in FIGURE 1A showed that ICAM-1 mAb inhibits lysis by 70%; lysis is also inhibited well by the LFA-1 mAb (go%), and to a limited extent by CD2 (27%) and LFA-3 (38%) mAb. Following this result, a bulk population of allospecific T cells were prepared by two cycles of in vitro stimulation of peripheral blood mononuclear leukocytes with irradiated U937 as stimulator cells. The bulk population was tested in a standard C M L assay. The result in FIGURE 1B showed 75% inhibition by the ICAM-1 mAb, compared with 95%, 2096, and 15% inhibition by LFA-I , CD2, and LFA-3 mAb, respectively. These results showed that ICAM-I is critical for antigen-specific C T L recognition in this effectoritarget combination. Studies with various effectoritarget combinations revealed differing utilization and requirements for ICAM-1 in C T L r e ~ o g n i t i o n . ~ The results are consistent with the interpretation that ICAM-I can serve as one ligand for LFA-I in CTL recognition. The role of ICAM-1 in CTL recognition correlates closely with its ability to inhibit