Influenza virus remains a constant public health threat, owing to its ability to evade immune surveillance through rapid genetic drift and reassortment. Monoclonal antibody (mAb)-based immunotherapy is a promising strategy for disease control. Here we use a human Ab phage display library and H5 hemagglutinin (HA) ectodomain to select ten neutralizing mAbs (nAbs) with a remarkably broad range among Group 1 influenza viruses, including the H5N1 “bird flu” and the H1N1 “Spanish flu” strains. Notably, nine of the Abs utilize the same germline gene, VH1-69. The crystal structure of one mAb bound to H5N1 HA reveals that only the heavy chain inserts into a highly conserved pocket in the HA stem, inhibiting the conformational changes required for membrane fusion. Our studies indicate that nAbs targeting this pocket could provide broad protection against both seasonal and pandemic influenza A infections.

Human angiotensin‐converting enzyme 2 (ACE2) is a functional receptor for SARS coronavirus (SARS‐CoV). Here we identify the SARS‐CoV spike (S)‐protein‐binding site on ACE2. We also compare S proteins of SARS‐CoV isolated during the 2002–2003 SARS outbreak and during the much less severe 2003–2004 outbreak, and from palm civets, a possible source of SARS‐CoV found in humans. All three S proteins bound to and utilized palm‐civet ACE2 efficiently, but the latter two S proteins utilized human ACE2 markedly less efficiently than did the S protein obtained during the earlier human outbreak. The lower affinity of these S proteins could be complemented by altering specific residues within the S‐protein‐binding site of human ACE2 to those of civet ACE2, or by altering S‐protein residues 479 and 487 to residues conserved during the 2002–2003 outbreak. Collectively, these data describe molecular interactions important to the adaptation of SARS‐CoV to human cells, and provide insight into the severity of the 2002–2003 SARS epidemic.

Effective prophylaxis and antiviral therapies are urgently needed in the event of reemergence of the highly contagious and often fatal severe acute respiratory syndrome (SARS) coronavirus (SARSCoV) infection. We have identified eight recombinant human single-chain variable region fragments (scFvs) against the S1 domain of spike (S) protein of the SARS-CoV from two nonimmune human antibody libraries. One scFv 80R efficiently neutralized SARS-CoV and inhibited syncytia formation between cells expressing the S protein and those expressing the SARS-CoV receptor angiotensin-converting enzyme 2 (ACE2). Mapping of the 80R epitope showed it is located within the N-terminal 261-672 amino acids of S protein and is not glycosylation-dependent. 80R scFv competed with soluble ACE2 for association with the S1 domain and bound S1 with high affinity (equilibrium dissociation constant, K d ‫؍‬ 32.3 nM). A human IgG1 form of 80R bound S1 with a 20-fold higher affinity of 1.59 nM comparable to that of ACE2 (K d ‫؍‬ 1.70 nM), and neutralized virus 20-fold more efficiently than the 80R scFv. These data suggest that the 80R human monoclonal antibody may be a useful viral entry inhibitor for the emergency prophylaxis and treatment of SARS, and that the ACE2-binding site of S1 could be an attractive target for subunit vaccine and drug development.T he severe acute respiratory syndrome (SARS)-associated coronavirus (SARS-CoV), a newly emergent member in the family Coronaviridae, causes SARS for which there are no vaccines or effective therapies currently available (1-4). It has been reported that high titers of protecting IgG antibody to SARS-CoV are present in convalescent serum, and SARS patients show clinical improvement if they are given serum from previously infected patients (5, 6). These observations suggest that passive immunization with human monoclonal antibodies could be developed for the treatment of SARS (7). The spike (S) proteins of coronaviruses are large type-I transmembrane glycoproteins that are responsible for receptor binding and membrane fusion. Two functional domains at the amino (S1) and carboxy (S2) termini of the S protein are conserved among the coronaviruses. The S1 and S2 domain of SARS-CoV S protein can be identified by sequence alignment with other coronavirus S proteins, especially with the more conserved S2 domain (8-10). The S protein is also the major antigenic determinant for coronaviruses (9,(11)(12)(13)(14). It has recently been demonstrated that the binding of the S1 domain to its receptor angiotensinconverting enzyme 2 (ACE2) on host cells is responsible for SARS-CoV entry into cells (15). Therefore, we targeted the S1 protein for generation of neutralizing human monoclonal antibodies. Here we report the identification, production, and characterization of a neutralizing human monoclonal antibody 80R against SARS-CoV that blocks the binding of S1 to ACE2. Materials and MethodsExpression and Purification of SARS-CoV S1 and Truncated S1. Plasmids encoding SARS-CoV S protein residues 12-672, 12-327, o...

In this report, the antiviral activity of 80R immunoglobulin G1 (IgG1), a human monoclonal antibody against severe acute respiratory syndrome coronavirus (SARS-CoV) spike (S) protein that acts as a viral entry inhibitor in vitro, was investigated in vivo in a mouse model. When 80R IgG1 was given prophylactically to mice at doses therapeutically achievable in humans, viral replication was reduced by more than 4 orders of magnitude to below assay limits. The essential core region of S protein required for 80R binding was identified as a conformationally sensitive fragment (residues 324 to 503) that overlaps the receptor ACE2-binding

CD26 is a T cell activation antigen that contains dipeptidyl peptidase IV activity and is known to bind adenosine deaminase. The mechanism by which CD26 costimulation potentiates T cell receptor-mediated T cell activation, leading to subsequent exertion of T cell effector function, is still not clearly defined. In this article, we demonstrate that CD26 localizes into lipid rafts, and targeting of CD26 to rafts is necessary for signaling events through CD26. Importantly, aggregation of CD26 by anti-CD26 mAb crosslinking also causes coaggregation of CD45 into rafts. Moreover, we show that CD26 directly binds to the cytoplasmic domain of CD45. Our results therefore indicate a mechanism whereby CD26 engagement promotes aggregation of lipid rafts and facilitates colocalization of CD45 to T cell receptor signaling molecules p56 Lck , ZAP-70, and TCR, thereby enhancing protein tyrosine phosphorylation of various signaling molecules and subsequent interleukin-2 production.T he CD26 is a 110-kDa cell-surface glycoprotein that possesses dipeptidyl peptidase IV (EC 3.4.14.5) activity in its extracellular domain (1, 2) and has an important role in T cell activation (3). Although constitutively expressed in the liver, intestine, and kidney, CD26 expression level is tightly regulated on T cells, and its density is markedly enhanced after T cell activation (4). In the resting state of human T lymphocytes, CD26 is expressed on a subset of CD4 ϩ memory T cells, and this CD4 ϩ CD26 high T cell population has been shown to respond maximally to recall antigens (5). In fact, CD26 itself is involved in the processes of T cell signal transduction.Activation of resting T cells requires two independent signals, the first stemming from recognition by the T cell receptor (TCR) complex of processed antigen peptides plus major histocompatibility complex molecules on antigen-presenting cells, with the second signal being provided by other receptor-ligand interactions between T cells and antigen-presenting cells (6, 7). Intensive work over the past several years has characterized the T cell surface molecule CD26 as a receptor capable of generating T cell costimulatory signals (8, 9). Crosslinking of CD26 and CD3 with immobilized mAbs induced T cell activation and IL-2 production (5). Moreover, anti-CD26 antibody treatment of T cells led to decreased surface expression of CD26 via its internalization, and this modulation of CD26 resulted in an enhanced proliferative response to anti-CD3 or anti-CD2 stimulation (9). We also demonstrated previously that ligation of CD26 by the anti-CD26 mAb 1F7 induced increased tyrosine phosphorylation of signaling molecules such as c-Cbl, mitogen-activated protein kinase [MAP kinase, or extracellularregulated kinase (ERK1͞2)], Zap70, p56 Lck , and CD3 (10). However, the exact mechanism responsible for CD26-mediated tyrosine phosphorylation and costimulation is unclear, particularly in view of the fact that CD26 has a short conserved cytoplasmic domain consisting of only six amino acid residues, without intrins...

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Phylogenetic analyses have provided strong evidence that amino acid changes in spike (S) protein of animal and human SARS coronaviruses (SARS-CoVs) during and between two zoonotic transfers (2002/03 and 2003/04) are the result of positive selection. While several studies support that some amino acid changes between animal and human viruses are the result of inter-species adaptation, the role of neutralizing antibodies (nAbs) in driving SARS-CoV evolution, particularly during intra-species transmission, is unknown. A detailed examination of SARS-CoV infected animal and human convalescent sera could provide evidence of nAb pressure which, if found, may lead to strategies to effectively block virus evolution pathways by broadening the activity of nAbs. Here we show, by focusing on a dominant neutralization epitope, that contemporaneous- and cross-strain nAb responses against SARS-CoV spike protein exist during natural infection. In vitro immune pressure on this epitope using 2002/03 strain-specific nAb 80R recapitulated a dominant escape mutation that was present in all 2003/04 animal and human viruses. Strategies to block this nAb escape/naturally occurring evolution pathway by generating broad nAbs (BnAbs) with activity against 80R escape mutants and both 2002/03 and 2003/04 strains were explored. Structure-based amino acid changes in an activation-induced cytidine deaminase (AID) “hot spot” in a light chain CDR (complementarity determining region) alone, introduced through shuffling of naturally occurring non-immune human VL chain repertoire or by targeted mutagenesis, were successful in generating these BnAbs. These results demonstrate that nAb-mediated immune pressure is likely a driving force for positive selection during intra-species transmission of SARS-CoV. Somatic hypermutation (SHM) of a single VL CDR can markedly broaden the activity of a strain-specific nAb. The strategies investigated in this study, in particular the use of structural information in combination of chain-shuffling as well as hot-spot CDR mutagenesis, can be exploited to broaden neutralization activity, to improve anti-viral nAb therapies, and directly manipulate virus evolution.

Carbonic anhydrase IX (CAIX, gene G250/MN-encoded transmembrane protein) is highly expressed in various human epithelial tumors such as renal clear cell carcinoma (RCC), but absent from the corresponding normal tissues. Besides the CA signal transduction activity, CAIX may serve as a biomarker in early stages of oncogenesis and also as a reliable marker of hypoxia, which is associated with tumor resistance to chemotherapy and radiotherapy. Although results from preclinical and clinical studies have shown CAIX as a promising target for detection and therapy for RCC, only a limited number of murine monoclonal antibodies (mAbs) and one humanized mAb are available for clinical testing and development. In this study, paramagnetic proteoliposomes of CAIX (CAIX-PMPLs) were constructed and used for anti-CAIX antibody selection from our 27 billion human single-chain antibody (scFv) phage display libraries. A panel of thirteen human scFvs that specifically recognize CAIX expressed on cell surface was identified, epitope mapped primarily to the CA domain, and affinity-binding constants (KD) determined. These human anti-CAIX mAbs are diverse in their functions including induction of surface CAIX internalization into endosomes and inhibition of the carbonic anhydrase activity, the latter being a unique feature that has not been previously reported for anti-CAIX antibodies. These human anti-CAIX antibodies are important reagents for development of new immunotherapies and diagnostic tools for RCC treatment as well as extending our knowledge on the basic structure-function relationships of the CAIX molecule.