Significance: Effective therapies are urgently needed for COVID-19. We rapidly (within a week) identified a fully human monoclonal germline-like antibody (ab1) from phage-displayed libraries that potently inhibited mouse ACE2-adapted SARS-CoV-2 replication in wild-type BALB/c mice and native virus in transgenic mice expressing human ACE2 as well as in hamsters when administered before virus challenge. It was also effective when administered after virus infection of hamsters, although at lower efficacy than when used prophylactically. Ab1 was highly specific and did not bind to human cell membrane-associated proteins. It also exhibited good developability properties including complete lack of aggregation. Ab1 has potential for prophylaxis and therapy of COVID-19 alone or in combination with other agents.

Effective therapies are urgently needed for COVID-19. Here we describe the identification of a new stable human immunoglobulin G1 heavy-chain variable (VH) domain scaffold that was used for the construction of a large library, lCAT6, of engineered human VHs. This library was panned against the receptor-binding domain (RBD) of the SARS-CoV-2 spike (S) glycoprotein. Two VH domains (VH ab6 and VH m397) were selected and fused to Fc for increased half-life in circulation. The VH-Fc ab6 and m397 specifically neutralized SARS-CoV-2 with high potencies (50% neutralization at 0.35 µg/ml and 1.5 µg/ml, respectively) as measured by two independent replication-competent virus neutralization assays. Ab6 and m397 competed with ACE2 for binding to RBD, suggesting a competitive mechanism of virus neutralization. These VH domains may have potential applications for prophylaxis and therapy of COVID-19 alone or in combination, as well as for diagnosis and as tools for research.

Neutrophil elastase (NE) is a serine protease released during neutrophil maturation. High levels of NE are related to lung tissue damage and poor prognosis in cancer; thus, NE is a potential target for therapeutic immunotherapy for multiple lung diseases and cancers. Here, we isolate and characterize two high-affinity, specific, and noncompetitive anti-NE antibodies Fab 1C10 and V_H 1D1.43 from two large phage-displayed human Fab and V_H libraries. After fusion with human IgG1 Fc, both of them (V_H-Fc 1D1.43 and IgG1 1C10) inhibit NE enzymatic activity with V_H-Fc 1D1.43 showing comparable inhibitory effects to that of the small molecule NE inhibitor SPCK and IgG1 1C10 exhibiting even higher (2.6-fold) activity than SPCK. Their epitopes, as mapped by peptide arrays combined with structural modeling, indicate different mechanisms for blocking NE activity. Both V_H-Fc and IgG1 antibodies block NE uptake by cancer cells and fibroblast differentiation. V_H-Fc 1D1.43 and IgG1 1C10 are promising for the antibody-based immunotherapy of cancer and inflammatory diseases.

Effective therapies are urgently needed for the SARS-CoV-2/COVID19 pandemic. We identified panels of fully human monoclonal antibodies (mAbs) from eight large phage-displayed Fab, scFv and VH libraries by panning against the receptor binding domain (RBD) of the SARS-CoV-2 spike (S) glycoprotein. One high affinity mAb, IgG1 ab1, specifically neutralized replication competent SARS-CoV-2 with exceptional potency as measured by two different assays. There was no enhancement of pseudovirus infection in cells expressing Fcγ receptors at any concentration. It competed with human angiotensin-converting enzyme 2 (hACE2) for binding to RBD suggesting a competitive mechanism of virus neutralization. IgG1 ab1 potently neutralized mouse ACE2 adapted SARS-CoV-2 in wild type BALB/c mice and native virus in hACE2 expressing transgenic mice. The ab1 sequence has relatively low number of somatic mutations indicating that ab1-like antibodies could be quickly elicited during natural SARS-CoV-2 infection or by RBD-based vaccines. IgG1 ab1 does not have developability liabilities, and thus has potential for therapy and prophylaxis of SARS-CoV-2 infections. The rapid identification (within 6 days) of potent mAbs shows the value of large antibody libraries for response to public health threats from emerging microbes.

Mesothelin (MSLN) has been a validated tumor-associated antigen target for several solid tumors for over a decade, making it an attractive option for therapeutic interventions. Novel antibodies with high affinity and better therapeutic properties are needed. In the current study, we have isolated and characterized a novel heavy chain variable (VH) domain 3C9 from a large-size human immunoglobulin VH domain library. 3C9 exhibited high affinity (KD [dissociation constant] <3 nM) and binding specificity in a membrane proteome array (MPA). In a mouse xenograft model, 3C9 fused to human IgG1 Fc was detected at tumor sites as early as 8 h post-infusion and remained at the site for over 10 days. Furthermore, 3C9 fused to a human Fc domain drug conjugate effectively inhibited MSLN-positive tumor growth in a mouse xenograft model. The X-ray crystal structure of full-length MSLN in complex with 3C9 reveals interaction of the 3C9 domains with two distinctive residue patches on the MSLN surface. This newly discovered VH antibody domain has a high potential as a therapeutic candidate for MSLN-expressing cancers.