Specific therapy is not available for hantavirus cardiopulmonary syndrome caused by Andes virus (ANDV).Peptides capable of blocking ANDV infection in vitro were identified using antibodies against ANDV surface glycoproteins Gn and Gc to competitively elute a cyclic nonapeptide-bearing phage display library from purified ANDV particles. Phage was examined for ANDV infection inhibition in vitro, and nonapeptides were synthesized based on the most-potent phage sequences. Three peptides showed levels of viral inhibition which were significantly increased by combination treatment with anti-Gn-and anti-Gc-targeting peptides. These peptides will be valuable tools for further development of both peptide and nonpeptide therapeutic agents.Andes virus (ANDV), an NIAID category A agent linked to hantavirus cardiopulmonary syndrome (HCPS), belongs to the family Bunyaviridae and the genus Hantavirus and is carried by Oligoryzomys longicaudatus rodents (11). HCPS is characterized by pulmonary edema caused by capillary leak, with death often resulting from cardiogenic shock (9, 16). ANDV HCPS has a case fatality rate approaching 40%, and ANDV is the only hantavirus demonstrated to be capable of direct personto-person transmission (15, 21). There is currently no specific therapy available for treatment of ANDV infection and HCPS.Peptide ligands that target a specific protein surface can have broad applications as therapeutics by blocking specific proteinprotein interactions, such as preventing viral engagement of host cell receptors and thus preventing infection. Phage display libraries provide a powerful and inexpensive tool to identify such peptides. Here, we used selection of a cyclic nonapeptide-bearing phage library to identify peptides capable of binding the transmembrane surface glycoproteins of ANDV, Gn and Gc, and blocking infection in vitro.To identify peptide sequences capable of recognizing ANDV, we panned a cysteine-constrained cyclic nonapeptide-bearing phage display library (New England Biolabs) against density gradient-purified, UV-treated ANDV strain CHI-7913 (a gift from Hector Galeno, Santiago, Chile) (17, 18). To increase the specificity of the peptides identified, we eluted phage by using monoclonal antibodies (Austral Biologicals) prepared against recombinant fragments of ANDV Gn (residues 1 to 353) or Gc (residues 182 to 491) glycoproteins (antibodies 6B9/F5 and 6C5/D12, respectively). Peptide sequences were determined for phage from iterative rounds of panning, and the ability of phage to inhibit ANDV infection of Vero E6 cells was determined by immunofluorescent assay (IFA) (7). Primary IFA detection antibodies were rabbit polyclonal anti-Sin Nombre hantavirus (SNV) nucleoprotein (N) antibodies which exhibit potent cross-reactivity against other hantavirus N antigens (3). ReoPro, a commercially available Fab fragment which partially blocks infection of hantaviruses in vitro by binding the entry receptor integrin ␤ 3 (5), was used as a positive control (80 g/ml) along with the original antibody used for p...