BackgroundThe hemagglutinin (HA) glycoprotein is the principal target of protective humoral immune responses to influenza virus infections but such antibody responses only provide efficient protection against a narrow spectrum of HA antigenic variants within a given virus subtype. Avian influenza viruses such as H5N1 are currently panzootic and pose a pandemic threat. These viruses are antigenically diverse and protective strategies need to cross protect against diverse viral clades. Furthermore, there are 16 different HA subtypes and no certainty the next pandemic will be caused by an H5 subtype, thus it is important to develop prophylactic and therapeutic interventions that provide heterosubtypic protection.Methods and FindingsHere we describe a panel of 13 monoclonal antibodies (mAbs) recovered from combinatorial display libraries that were constructed from human IgM+ memory B cells of recent (seasonal) influenza vaccinees. The mAbs have broad heterosubtypic neutralizing activity against antigenically diverse H1, H2, H5, H6, H8 and H9 influenza subtypes. Restriction to variable heavy chain gene IGHV1-69 in the high affinity mAb panel was associated with binding to a conserved hydrophobic pocket in the stem domain of HA. The most potent antibody (CR6261) was protective in mice when given before and after lethal H5N1 or H1N1 challenge.ConclusionsThe human monoclonal CR6261 described in this study could be developed for use as a broad spectrum agent for prophylaxis or treatment of human or avian influenza infections without prior strain characterization. Moreover, the CR6261 epitope could be applied in targeted vaccine strategies or in the design of novel antivirals. Finally our approach of screening the IgM+ memory repertoire could be applied to identify conserved and functionally relevant targets on other rapidly evolving pathogens.

BackgroundExperimental animal data show that protection against severe acute respiratory syndrome coronavirus (SARS-CoV) infection with human monoclonal antibodies (mAbs) is feasible. For an effective immune prophylaxis in humans, broad coverage of different strains of SARS-CoV and control of potential neutralization escape variants will be required. Combinations of virus-neutralizing, noncompeting mAbs may have these properties.Methods and FindingsHuman mAb CR3014 has been shown to completely prevent lung pathology and abolish pharyngeal shedding of SARS-CoV in infected ferrets. We generated in vitro SARS-CoV variants escaping neutralization by CR3014, which all had a single P462L mutation in the glycoprotein spike (S) of the escape virus. In vitro experiments confirmed that binding of CR3014 to a recombinant S fragment (amino acid residues 318–510) harboring this mutation was abolished. We therefore screened an antibody-phage library derived from blood of a convalescent SARS patient for antibodies complementary to CR3014. A novel mAb, CR3022, was identified that neutralized CR3014 escape viruses, did not compete with CR3014 for binding to recombinant S1 fragments, and bound to S1 fragments derived from the civet cat SARS-CoV-like strain SZ3. No escape variants could be generated with CR3022. The mixture of both mAbs showed neutralization of SARS-CoV in a synergistic fashion by recognizing different epitopes on the receptor-binding domain. Dose reduction indices of 4.5 and 20.5 were observed for CR3014 and CR3022, respectively, at 100% neutralization. Because enhancement of SARS-CoV infection by subneutralizing antibody concentrations is of concern, we show here that anti-SARS-CoV antibodies do not convert the abortive infection of primary human macrophages by SARS-CoV into a productive one.ConclusionsThe combination of two noncompeting human mAbs CR3014 and CR3022 potentially controls immune escape and extends the breadth of protection. At the same time, synergy between CR3014 and CR3022 may allow for a lower total antibody dose to be administered for passive immune prophylaxis of SARS-CoV infection.

SARS coronavirus continues to cause sporadic cases of severe acute respiratory syndrome (SARS) in China. No active or passive immunoprophylaxis for disease induced by SARS coronavirus is available. We investigated prophylaxis of SARS coronavirus infection with a neutralising human monoclonal antibody in ferrets, which can be readily infected with the virus. Prophylactic administration of the monoclonal antibody at 10 mg/kg reduced replication of SARS coronavirus in the lungs of infected ferrets by 3.3 logs (95% CI 2.6-4.0 logs; p<0.001), completely prevented the development of SARS coronavirus-induced macroscopic lung pathology (p=0.013), and abolished shedding of virus in pharyngeal secretions. The data generated in this animal model show that administration of a human monoclonal antibody might offer a feasible and effective prophylaxis for the control of human SARS coronavirus infection.

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Human monoclonal antibodies (MAbs) were selected from semisynthetic antibody phage display libraries by using whole irradiated severe acute respiratory syndrome (SARS) coronavirus (CoV) virions as target. We identified eight human MAbs binding to virus and infected cells, six of which could be mapped to two SARS-CoV structural proteins: the nucleocapsid (N) and spike (S) proteins. Two MAbs reacted with N protein.One of the N protein MAbs recognized a linear epitope conserved between all published human and animal SARS-CoV isolates, and the other bound to a nonlinear N epitope. These two N MAbs did not compete for binding to SARS-CoV. Four MAbs reacted with the S glycoprotein, and three of these MAbs neutralized SARS-CoV in vitro. All three neutralizing anti-S MAbs bound a recombinant S1 fragment comprising residues 318 to 510, a region previously identified as the SARS-CoV S receptor binding domain; the nonneutralizing MAb did not. Two strongly neutralizing anti-S1 MAbs blocked the binding of a recombinant S fragment (residues 1 to 565) to SARS-CoV-susceptible Vero cells completely, whereas a poorly neutralizing S1 MAb blocked binding only partially. The MAb ability to block S1-receptor binding and the level of neutralization of the two strongly neutralizing S1 MAbs correlated with the binding affinity to the S1 domain. Finally, epitope mapping, using recombinant S fragments (residues 318 to 510) containing naturally occurring mutations, revealed the importance of residue N479 for the binding of the most potent neutralizing MAb, CR3014. The complete set of SARS-CoV MAbs described here may be useful for diagnosis, chemoprophylaxis, and therapy of SARS-CoV infection and disease.Severe acute respiratory syndrome (SARS) was first identified in 2002 as a newly emerging disease in Guangdong Province, China. The disease, associated with unusual atypical pneumonia, spread in 2003 to over 30 countries worldwide with more than 8,000 reported cases and an estimated 55% mortality among the elderly (9). A virus was isolated from tissues of SARS patients (10, 21, 23, 32) and a SARS-associated coronavirus (SARS-CoV), a new member in the family of Coronaviridae, was identified as the causative agent fulfilling Koch's postulates (12).The clinical course of SARS is highly variable (31) after a relatively short 6-to 10-day incubation period (9). In ca. 20% of the patients, SARS-CoV infection progresses to a stage of respiratory failure requiring ventilation support. Overall, 10% of the patients, ca. 6.8% of patients younger and 55% of patients older than 60 years of age (9), die as a consequence of immunopathological lung damage caused by a hyperactive antiviral immune response (29).Antibodies to SARS-CoV become detectable in patient's serum between days 10 and 15 and correlate with a decline in viral loads. More than 93% of the patients were reported to have seroconverted by day 28 (31). The pattern of SARS-CoV replication and development of a neutralizing immune response observed in experimentally infected mice largely resembles...

The need to replace rabies immune globulin (RIG) as an essential component of rabies postexposure prophylaxis is widely acknowledged. We set out to discover a unique combination of human monoclonal antibodies (MAbs) able to replace RIG. Stringent criteria concerning neutralizing potency, affinity, breadth of neutralization, and coverage of natural rabies virus (RV) isolates and in vitro escape mutants were set for each individual antibody, and the complementarities of the two MAbs were defined at the onset. First, we identified and characterized one human MAb (CR57) with high in vitro and in vivo neutralizing potency and a broad neutralization spectrum. The linear antibody binding site was mapped on the RV glycoprotein as antigenic site I by characterizing CR57 escape mutants. Secondly, we selected using phage display a complementing antibody (CR4098) that recognized a distinct, nonoverlapping epitope (antigenic site III), showed similar neutralizing potency and breadth as CR57, and neutralized CR57 escape mutants. Reciprocally, CR57 neutralized RV variants escaping CR4098. Analysis of glycoprotein sequences of natural RV isolates revealed that the majority of strains contain both intact epitopes, and the few remaining strains contain at least one of the two. In vitro exposure of RV to the combination of CR57 and CR4098 yielded no escape mutants. In conclusion, a novel combination of human MAbs was discovered suitable to replace RIG.Lethal rabies is prevented by postexposure prophylaxis (PEP) through the combined administration of a rabies vaccine and rabies immune globulin (RIG). Two types of RIG are used: human RIG (HRIG) and equine RIG, both derived from pooled sera of human donors or horses vaccinated against rabies, respectively. The need to replace these hyperimmune serum preparations is widely recognized (29), and monoclonal antibodies (MAbs) that neutralize rabies virus (RV) offer the opportunity to do so.Mouse MAbs, as well as human MAbs, have been shown to protect rodents from a lethal RV challenge (6,9,12,14,20,22,24). One of the most potent human MAbs, SO57, neutralizing a variety of RV strains, was described by Dietzschold et al. (6). A cocktail of three human MAbs including SO57 and SOJA and SOJB showed effective protection of mice from a lethal dose of RV (22). We reformatted these three MAbs (renamed CR57, CRJA, and CRJB) into our own expression system for production in PER.C6 cells (19). However, we showed that the CRJA and CRJB MAbs were not suitable in combination with CR57 for use in PEP (19) because of overlapping epitope recognition, lack of neutralizing potency, and shared escape mutants. Novel anti-RV MAbs were generated using phage display technology and were characterized with special emphasis on CR57 complementarity.We considered several criteria to be of crucial importance for the inclusion of human MAbs into a cocktail aimed at effectively blocking an RV infection in humans. First, the MAbs should target distinct, nonoverlapping epitopes and should not compete for binding to RV glyco...

Peripheral blood leukocytes incubated with a semisynthetic phage antibody library and fluorochromelabeled CD3 and CD20 antibodies were used to isolate human single-chain Fv antibodies specific for subsets of blood leukocytes by flow cytometry. Isolated phage antibodies showed exclusive binding to the subpopulation used for selection or displayed additional binding to a restricted population of other cells in the mixture. At least two phage antibodies appeared to display hitherto-unknown staining patterns of B-lineage cells. This approach provides a subtractive procedure to rapidly obtain human antibodies against known and novel surface antigens in their native configuration, expressed on phenotypically defined subpopulations of cells. This approach does not depend on immunization procedures or the necessity to repeatedly construct phage antibody libraries. The effectiveness of such 4Abs in biochemical and functional assays varies; typically, the procedure used to select 4Abs determines their utility (2-6).Human monoclonal antibodies that bind to native cell surface structures are expected to have broad application in therapeutic and diagnostic procedures (7). An important extension of #Ab display technology would be a strategy for the direct selection of specific antibodies against antigens expressed on the surface of subpopulations of cells present in a heterogeneous mixture. Ideally, such antibodies would be derived from a single highly diverse library containing "every" conceivable antibody specificity. We have recently constructed a library from 49 human germline immunoglobulin heavychain variable (VH) genes fused to a heavy-chain joining segment 4 (JH4) gene and partly randomized complementarity-determining region 3 (CDR3) sequences varying in length between 6 and 15 amino acids. The CDR3s were designed to contain short stretches of fully randomized amino acid residues flanked by regions of limited variability. Residues in the latter portion of CDR3 were selected on the basis of their frequent occurrence in CDRs of natural antibody molecules (8-10). We reasoned that a designed, semirandom CDR3 would result in an increased frequency of clones producing functional antigen binding sites and, in addition, in a more efficient use of the restricted sequence space within phage display libraries. The synthetic VH segments were combined with seven different light-chain variable (VL) genes and expressed as gene III-The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. 3938 single-chain Fv (scFv) fragments on the surface of phage, resulting in a library of 3.6 X 108 clones. From this library we isolated monoclonal 4Abs (m4Abs) to a variety of different structures (haptens, proteins, and polysaccharides) by selection on solid-phase bound antigen. These phage antibodies proved to be useful reagents in a variety of biochemical assays (6).In the present experim...

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