CELLULAR IMh4UNOLOGY 95, 75-83 (1985) Differential Effects of Monoclonal Antibodies Anti-L3T4 and Anti- LFAl on the Antigen-Induced Proliferation of T-Helper-Cell Clones: Correlation between Their Susceptibility to Inhibition and Their Affinity for Antigen’ MARIE-LISE GOUGEON, GEORGES BISMUTH, AND JACQUES THEZE~ Unit6 d’Immunog&t?ique Cellulaire, Institut Pasteur, 25 rue du Docteur Roux, 75724 Paris Ckdex 15. France Received March 18. 1985; accepted May 16, 1985 Recognition by specific T helper (Tu) cells of antigen presented by antigen-presenting cells (APC) involves, in addition to the antigen-specific receptor, non-antigen-specific molecules such as L3T4 and LFA 1. In the present study, we analyzed the relationship between the avidity for antigen presented by APC of three Tn cell lines and the participation of L3T4 and LFAl cell surface antigens. We found a correlation between the avidity of Tn cells for the complex GAT/Ia on APC measured by two independent assays and the participation of the cell-adhesion molecules L3T4 as measured by the ability of corresponding monoclonal antibody (MAb) to block the antigen-induced prohferation of Tn cells. In contrast to the situation found with cytolytic T-lymphocyte (CT L) clones, we also found a differential inhibiting effect of anti-LFA 1 MAb on the GAT-specific proliferation of the three Tu clones. The results indicate a direct correlation between the inhibitory effects of anti-LFAl and anti-L3T4 MAb and the affinity of Tu cells for the complex formed by antigen and Ia. Q 1985 Academic FWS, I~C. INTRODUCTION The processof antigen-specific T-helper (T~)~-cell recognition requires concomitant interaction of the specific T-cell receptor with both antigen (Ag) and Ia molecules expressed on antigen-presenting cells (APC) (1). In addition, recent studies have shown the functional involvement of differentiation antigens in Tu-cell activation. Two cell surface molecules, LFAl and L3T4, have been identified by antibody- blocking experiments, testing the ability of the corresponding monoclonal antibody (MAb) to inhibit T-cell functions (2-10). MAb to LFAl reversibly inhibited soluble ’ This work was supported in part by grants from the Institut National de la Sante et de la Recherche Medicale (PRC 12702 1). ’ To whom all correspondence should be addressed: Unite d’Immunogen&ique Cellulaire, LXpartement d’Immunologie, Institut Pasteur, 25 rue du Docteur Roux, 75724 Paris C&lex 15, France. 3 Abbreviations used: Ag/Ia, complex formed by antigen and Ia molecule; APC, antigen-presenting cells; CTL, cytolytic T lymphocytes; GAT, terpolymer of L-glutamic acida, L-alanine”, L-tyrosine’O; Hepes, N-2-hydroxyethylpiperazine-N’-2-ethanesulfonic acid; [‘H]TdR, [‘Hlthymidine; Ig, immunoglobulin; IL-2, interleukin 2; MAb, monoclonal antibody; OVA, ovalbumin; RIA, radioimmunoassay; SC, spleen cells; SE, standard error; SI, stimulation index; SN, supematant; Tu, T helper. 75 0008-8749185$3.00 Copyright Q 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.