CELLULAR IMMUNOLOGY 135, 534-540 (1991) SHORT COMMUNICATIONS Interactions between CD3 and THY1 T Cell Activation Pathways: Blockade of CD3-Mediated T Lymphocyte Activation Induced by Immobilized Anti-THY 1 Antibodies’ MARIA BELLIO,* LUDMILA M. C. LEAL,~ JULIO SCHARFSTEIN,* AND GEORGE A. DOS REIS$ *Carlos Chagas Filho Institute of Biophysics, Federal University of Rio de Janeiro, tGenetics Section, National Cancer Institute (INCa) and *Department of Immunology, Institute of Microbiology, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, 21941, Brazil Received August I, 1990; accepted February 13, 1991 Resting murine T cell activation induced by either CD3 complexes or Thy1 molecules was investigated in vitro, using surface-bound anti-CD3 mAb as the stimulus. One mitogenic anti- Thy1 mAb (G7) lost mitogenicity when presented to T cells immobilized on a plastic surface, even in the presence of phorbol ester. Moreover, T cell activation induced by immobilized anti- CD3 was potently blocked by coimmobilized anti-Thy1 mAb. Nonmitogenic anti-Thy1 mAb also blocked CDS-induced activation when coimmobilized with anti-CD3. Control experiments showed that anti-Thy1 specifically blocked T cell activation, even in the presence of measurable and functional concentrations of plastic-bound antiCD3. Coimmobilized anti-Thy1 potently blocked IL2 secretion stimulated by antXD3. Addition of exogenous rIL2 completely prevented anti-Thy I-mediated blockade. On the other hand, while completely blocking T cell proliferation, immobilized anti-Thy1 only partially blocked secretion of IL3-like activity by the T cells. One IgM anti-Thy1 mAb (2A3) induced secretion of IL34ike activity by T cells when immobilized in the absence of bound anti-CD3. These results indicate that extensive aggregation of Thy1 molecules delivers a potent negative signal which antagonizes CD3-mediated T cell activation and growth. 0 199 I Academic Press. Inc. 1. INTRODUCTION The process of T lymphocyte activation is initiated by signaling through the T cell antigen receptor (TcR), which is noncovalently associated to the CD3 molecular com- plex at the T cell surface ( 1, 2). This transduction system comprises the central acti- vation pathway regulating T cell behavior. However, other T cell surface proteins have been described as capable of initiating T cell activation when triggered by appropriate monoclonal antibodies. One of these alternative modes of T cell activation in the mouse includes the Thy1 molecules (3, 4) an ubiquitous T cell surface protein with structural features characteristic of the immunoglobulin gene superfamily (5). In the present study we have compared T cell activation through CD3 and Thy l-mediated pathways. Triggering of T cells with soluble antibodies directed to cell surface activation ’ This work was supported by the Rio de Janeiro Foundation for Scientific Support (FAPERJ), the Financing Agency of Studies and Projects (FINEP), and the National Council for Scientific and Technological Devel- opment (CNPq). 534 0008-8749/g 1 $3.00 Copyright 0 1991 by Academic Press, Inc. All rights of reproduction in any form reserved,