The principal neutralizing determinant of human immunodeficiency virus type 1 (IIV-1) is located in the external envelope protein, gpl20, and has previously been mapped to a 24-amino acid-long sequence (denoted RP135). We show here that deletion of this sequence renders the envelope unable to elicit neutralizing antibodies. In addition, using synthetic peptide fragments of RP135, we have mapped the neutralizing determinant to 8 amino acids and found that a peptide ofthis size elicits neutralizing antibodies. This sequence contains a central Gly-Pro-Gly that is generally conserved between different HIV-1 isolates and is flanked by amino acids that differ from isolate to isolate. Antibodies elicited by peptides from one isolate do not neutralize two different isolates, and a hybrid peptide, consisting of amino acid sequences from two isolates, elicits neutralizing antibodies to both isolates. By using a mixture of peptides of this domain or a mixture of such hybrid peptides the type-specificity of the neutralizing antibody response to this determinant can perhaps be overcome.

The salient features of this model for oncornavirus assembly are that uncleaved precursor molecules to the internal virus polypeptides possess specific recognition sites both for viral envelope constituents already inserted in the cell membrane and for the viral RNA. After orderly alignment of these components at the budding site, virus maturation proceeds through specific proteolytic cleavage of the precursor components and association of the resultant molecules into the characteristic type C virion substructures revealed by electron microscopy.

The lymphocyte differentiation antigen CD4 serves as a receptor for human retroviruses associated with acquired immunodeficiency syndrome (AIDS) through its interaction with the major envelope virion glycoprotein, gpl20, which is also expressed on the surface of infected cells. In these experiments, purified gpl20 was shown to bind to normal human T-lymphocyte populations. The gpl2O-CD4 complex served as a target antigen for antibody-dependent complementmediated cytolysis by a goat serum raised against native gpl20. However, patient sera that bound to gpl20-adsorbed cells failed to direct their destruction in the presence of complement. In contrast, these sera were potent mediators of antibodydependent cellular cytotoxicity. These studies demonstrate that gpl20 situated on the cell surface can serve as an effective target for immune destruction by patient antibodies and effector lymphocytes. The possible contribution of this type of immunity to control of disease progression, on the one hand, and to lymphocyte destruction and immunopathology observed in AIDS, on the other, is discussed.The most profound hematologic feature associated with acquired immunodeficiency syndrome (AIDS) is the functional impairment and quantitative depletion of the subset of lymphocytes that express the CD4 surface antigen (1-6). Infection of CD4 cells in vitro results in the rapid spread of virus throughout the culture and subsequent cell death due to the as yet undefined process of virus-induced cytopathology (7)(8)(9)(10)(11)(12) The current body of evidence suggests that the T-cell lymphotropic properties of the causative agent virus are due, in large part, to the interaction between the major virion envelope glycoprotein (gp120) and specific epitopes of the CD4 antigen complex. Monoclonal antibodies directed against particular determinants of CD4 have been shown to block both infection (14, 15) and multinucleated giant cell formation (16), in addition to precipitating gpl2O-CD4 complexes from infected cells (17). Recent studies revealed that purified gpl20 can effectively block cell fusion, most likely by competing for available CD4 sites on noninfected cells (18). In addition, purified gpl20 readily associates with the CD4 molecules and forms a stable complex on the cell surface (19). This latter finding suggested the possibility that free gpl20 bound to noninfected CD4-expressing lymphocytes could serve as a potential target for immune attack resulting in subsequent lympholysis. MATERIALS AND METHODSCells. Normal CD4-positive lymphocyte populations (CD4 cells) were obtained by stimulating normal donor peripheral blood mononuclear cells with tetanus toxoid for 5 days, sorting for CD4 expression, and expanding in the presence of recombinant interleukin 2. CEM cell lines (CEM) or human T-cell lymphotropic virus IIIB (HTLV-IIIB)t chronically infected CEM (CEM/IIIB) have been described (19). gpl20 and Goat gpl2O Antisera. The gp120 was isolated from HTLV-IIIn-infected H9 cells as described (18). Briefly, cells were lysed with...

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