A Human Monoclonal Antibody with Neutralizing Activity against Highly Divergent Influenza Subtypes Nicola Clementi1., Donata De Marco1., Nicasio Mancini1., Laura Solforosi1, Guisella J. Moreno1, Larisa V. Gubareva2, Vasiliy Mishin2, Andrea Di Pietro3, Elisa Vicenzi3, Antonio G. Siccardi4,5, Massimo Clementi1, Roberto Burioni1* 1 Laboratorio di Microbiologia e Virologia, Universita` Vita-Salute San Raffaele, Milano, Italia, 2 Virus Surveillance and Diagnosis Branch, Influenza Division, NCIRD, Centers for Disease Control and Prevention, Atlanta, Georgia, United States of America, 3 Viral Pathogens and Biosafety Unit, Divisione di Immunologia, Trapianti e Malattie infettive, Istituto Scientifico San Raffaele, Milano, Italia, 4 Istituto Scientifico San Raffaele, Milano, Italia, 5 Dipartimento di Biologia e Genetica per le Scienze Mediche, Universita` di Milano, Milano, Italia Abstract The interest in broad-range anti-influenza A monoclonal antibodies (mAbs) has recently been strengthened by the identification of anti-hemagglutinin (HA) mAbs endowed with heterosubtypic neutralizing activity to be used in the design of ‘‘universal’’ prophylactic or therapeutic tools. However, the majority of the single mAbs described to date do not bind and neutralize viral isolates belonging to highly divergent subtypes clustering into the two different HA-based influenza phylogenetic groups: the group 1 including, among others, subtypes H1, H2, H5 and H9 and the group 2 including, among others, H3 subtype. Here, we describe a human mAb, named PN-SIA28, capable of binding and neutralizing all tested isolates belonging to phylogenetic group 1, including H1N1, H2N2, H5N1 and H9N2 subtypes and several isolates belonging to group 2, including H3N2 isolates from the first period of the 1968 pandemic. Therefore, PN-SIA28 is capable of neutralizing isolates belonging to subtypes responsible of all the reported pandemics, as well as other subtypes with pandemic potential. The region recognized by PN-SIA28 has been identified on the stem region of HA and includes residues highly conserved among the different influenza subtypes. A deep characterization of PN-SIA28 features may represent a useful help in the improvement of available anti-influenza therapeutic strategies and can provide new tools for the development of universal vaccinal strategies. Citation: Clementi N, De Marco D, Mancini N, Solforosi L, Moreno GJ, et al. (2011) A Human Monoclonal Antibody with Neutralizing Activity against Highly Divergent Influenza Subtypes. PLoS ONE 6(12): e28001. doi:10.1371/journal.pone.0028001 Editor: Ron A. M. Fouchier, Erasmus Medical Center, The Netherlands Received August 1, 2011; Accepted October 30, 2011; Published December 5, 2011 This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Funding: The authors have no support or funding to report. Competing Interests: The authors have declared that no competing interests exist. * E-mail: burioni.roberto@hsr.it . These authors contributed equally to this work. Introduction Influenza, one of the diseases that has shaped human history [1,2], still has an evident clinical and socio-economical impact [3,4]. The 2009 pandemic has raised several major concerns related to the few prophylactic and therapeutic measures avail- able. Antiviral compounds have drawbacks caused by the rapid emergence of drug-resistant isolates [5,6], require prompt admini- stration to be effective [7], and have several associated side-effects especially in high-risk categories, including children and pregnant women [8,9]. Additionally, the vaccinal strategy is exposed to the annual risk of being ineffective due to possible mismatches between the predicted strains included in the vaccine and those actually in circulation; moreover, it would not engender a prompt response in pandemic settings [10]. In this scenario, new broad– range ‘‘universal’’ anti-influenza strategies are required [11,12]. In particular, it would be important to identify and eventually elicit what has recently been described as an unusually extreme broad- range immunity directed against broadly conserved viral regions, differing from the more common and restricted immunity directed against highly variable regions [12]. A number of approaches have already been proposed in literature [10,12,13,14,15,16], but a pivotal role, both in the prophylactic and therapeutic field, may be played by the availability of broad-range neutralizing human monoclonal antibodies (mAbs) allowing the identification of human B epitopes widely shared among different influenza subtypes [11,12]. Indeed, it is accepted that antibodies are key players in natural protection against influenza viruses, and that hemagglutinin (HA) is the main target for the virus-neutralizing antibody response [17]. However, although a single influenza infection provides lifelong immunity against the infecting virus and a limited number of antigenically correlated strains, the host can remain susceptible to infection with an antigenically drifted variant due to HA variability [18]. HA is the major glycoprotein of the influenza virus; it binds sialic acid on the surface of the cells through its globular head (HA1 domain) and makes possible the fusion of the viral envelope with the endosomal membranes through its stalk region (mainly formed by the HA2 domain) [17]. The sixteen known subtypes of HA, sharing between 40% and 60% amino acid sequence identity, have been clustered in two distinct phylogenetic groups: group 1 (H1, H2, H5, H6, H8, H9, H11, H12, H13, and H16) and group 2 (H3, H4, H7, H10, H14, and H15) [12,14]. The subtypes, H1, H2, H5 and H9 in group 1, and H3 and H7 in group 2 have been PLoS ONE | www.plosone.org 1 December 2011 | Volume 6 | Issue 12 | e28001