Apoliprotein E (apoE) is a potent suppressor of interleukin 2- (IL2-) dependent T lymphocyte proliferation. In this study, we have used a range of monomeric and dimeric peptides encompassing amino acids 130-169 in human apoE to locate a region with both cytostatic and cytotoxic effects on IL2-dependent T lymphocytes. Monomeric peptides representing residues 130-149 or 130-155 inhibited the proliferation of the cells without causing loss of cell viability. However, cytostasis by a peptide representing the extended 130-169 domain or dimeric peptides of amino acids 141 - 155 or 141 - 149 was accompanied by potent cytotoxic activity. These results suggest that residues 141 - 149, which include the overlap between the functional peptides, are responsible for cytostasis and cytotoxicity. Complete ablation of both activities by the polyanionic agent heparin highlighted the important contribution of the positively charged amino acids in the 141 - 149 region to peptide bioactivity. Furthermore, the bioactive apoE peptides also had a relatively high helical content, suggesting that α-helical content is necessary for bioactivity. Cytotoxic apoE peptides were characterized by a high density of polar face positively charged residues together with a high nonpolar face hydrophobicity. This conclusion is supported by the reduced hydrophobicity and polar face positive charge density of the significantly less active E2130-169 peptide. The cytotoxic apoE peptides are structurally similar to previously characterized class L lytic peptides. They do not, however, exert their cytotoxic activity by destabilizing membrane bilayers as is the case with the class L peptides, as evidenced by their minimal hemolytic activity. We conclude that amino acids 141 - 149 in apoE, when present in an amphipathic α-helix conformation, represent an immunoregulatory sequence which has either cytostatic or cytotoxic activity, depending upon nonpolar face hydrophobicity and polar face positive charge density. © 1995, American Chemical Society. All rights reserved.