NONCYTOTOXIC HUMAN CD4(-CELL CLONES PRESENTING AND SIMULTANEOUSLY RESPONDING TO AN ANTIGEN DIE OF APOPTOSIS() T)

Activated T-cells expressing MHC class II surface antigens are able to present antigen and thus function as peptide-presenting cells (T-APCs ). In this study we investigated whether antigen presentation by T-cel ls induced programmed cell death. As a model we used tetanus p30 pepti de (aa 947-967)-specific, noncytotoxic CD4(+) T-cell clones (C11 and C 31). For experimental purposes these T-cell clones were stimulated (a) with p30 peptide-pulsed and fixed EBV-transformd antigen-presenting c ells (B-APCs), (b) with p30-pulsed and fixed activated T-cells as APCs (as T-APCs we used either the T-cell clones themselves or an autologo us T-cell clone (CT3) with p30 unrelated specificity), or (c) with sol uble p30 peptide. The efficiency of antigen presentation was monitored by measuring proliferation as [H-3]thymidine uptake. Apoptosis was me asured by quantifying fragmented, cytoplasm DNA with the fluorescent d ye 4,6-diamidino-2-phenylindole or by visualizing fragmented DNA by ge l electrophoresis. Stimulation with p30-pulsed and fixed B-APCs or T-A PCs induced proliferation but no apoptosis of the responding T-cells. However, stimulation of cloned T-cells with soluble peptide induced up -regulation of the FAS surface molecules and apoptosis, which was depe ndent on the peptide doses. Because cloned T-cells express HLA class I I molecules, they can theoretically exert both functions at once: anti gen presentation and antigen response when they are stimulated with so luble peptide. Because death by apoptosis is only seen under such circ umstances, we suggest that T-cells simultaneously presenting and respo nding to an antigen die of apoptosis and thus contribute to the down-r egulation of the immune response. Such phenomena might occur in HIV in fection when activated CD4(+) T-cells take up gp120 via their CD4 mole cules, present it on their HLA class II surface antigens, and are simu ltaneously stimulated via their TCR. (C) 1995 Academic Press, Inc.