Identification of a highly conserved neutralizing epitope within the RBD region of diverse SARS-CoV-2 variants.

The constant emergence of SARS-CoV-2 variants continues to impair the efficacy of existing neutralizing antibodies, especially XBB.1.5 and EG.5, which showed exceptional immune evasion properties. Here, we identify a highly conserved neutralizing epitope targeted by a broad-spectrum neutralizing antibody BA7535, which demonstrates high neutralization potency against not only previous variants, such as Alpha, Beta, Gamma, Delta and Omicron BA.1-BA.5, but also more recently emerged Omicron subvariants, including BF.7, CH.1.1, XBB.1, XBB.1.5, XBB.1.9.1, EG.5. Structural analysis of the Omicron Spike trimer with BA7535-Fab using cryo-EM indicates that BA7535 recognizes a highly conserved cryptic receptor-binding domain (RBD) epitope, avoiding most of the mutational hot spots in RBD. Furthermore, structural simulation based on the interaction of BA7535-Fab/RBD complexes dissects the broadly neutralizing effect of BA7535 against latest variants. Therapeutic and prophylactic treatment with BA7535 alone or in combination with BA7208 protected female mice from the circulating Omicron BA.5 and XBB.1 variant infection, suggesting the highly conserved neutralizing epitope serves as a potential target for developing highly potent therapeutic antibodies and vaccines.

Biparatopic antibody BA7208/7125 effectively neutralizes SARS-CoV-2 variants including Omicron BA.1-BA.5.

SARS-CoV-2 Omicron subvariants have demonstrated extensive evasion from monoclonal antibodies (mAbs) developed for clinical use, which raises an urgent need to develop new broad-spectrum mAbs. Here, we report the isolation and analysis of two anti-RBD neutralizing antibodies BA7208 and BA7125 from mice engineered to produce human antibodies. While BA7125 showed broadly neutralizing activity against all variants except the Omicron sublineages, BA7208 was potently neutralizing against all tested SARS-CoV-2 variants (including Omicron BA.1-BA.5) except Mu. By combining BA7208 and BA7125 through the knobs-into-holes technology, we generated a biparatopic antibody BA7208/7125 that was able to neutralize all tested circulating SARS-CoV-2 variants. Cryo-electron microscopy structure of these broad-spectrum antibodies in complex with trimeric Delta and Omicron spike indicated that the contact residues are highly conserved and had minimal interactions with mutational residues in RBD of current variants. In addition, we showed that administration of BA7208/7125 via the intraperitoneal, intranasal, or aerosol inhalation route showed potent therapeutic efficacy against Omicron BA.1 and BA.2 in hACE2-transgenic and wild-type mice and, separately, effective prophylaxis. BA7208/7125 thus has the potential to be an effective candidate as an intervention against COVID-19.

Two novel human anti-CD25 antibodies with antitumor activity inversely related to their affinity and in vitro activity.

High tumor regulatory T (Treg) cell infiltration is associated with poor prognosis of many cancers. CD25 is highly expressed on tumor Treg cells and is a potential target for Treg deletion. Previously characterized anti-CD25 antibodies appear to have limited efficacy in tumor inhibition. Here we identified two human anti-CD25 antibodies, BA9 and BT942, which did not prevent the activation of IL-2R signaling pathway by IL-2. BT942 had weaker binding and cytotoxic activity to human CD25-expressing cell lines than BA9. But both demonstrated significant tumor growth inhibition in early and late-stage animal cancer models. BT942 resulted in a higher expansion of CD8+ T cells and CD4+ T cells in tumor microenvironment in mouse MC38 model compared to BA9. BT942 also demonstrated significant higher tumor growth inhibition and higher expansion of CD8+ T cells and CD4+ T cells in combination with an anti-PD1 antibody. Pharmacokinetic study of BT942 in cynomolgus monkeys demonstrated a half-life of 206.97 ± 19.03 h. Structural analysis by cryo-EM revealed that BT942 recognizes an epitope on opposite side of the CD25-IL-2 binding site, consistent with no IL-2 signaling blockade in vitro. BT942 appears to be an excellent candidate for cancer immunotherapy.

Structure and function analysis of a potent human neutralizing antibody CA521FALA against SARS-CoV-2.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the ongoing COVID-19 pandemic, which has resulted in more than two million deaths at 2021 February . There is currently no approved therapeutics for treating COVID-19. The SARS-CoV-2 Spike protein is considered a key therapeutic target by many researchers. Here we describe the identification of several monoclonal antibodies that target SARS-CoV-2 Spike protein. One human antibody, CA521FALA, demonstrated neutralization potential by immunizing human antibody transgenic mice. CA521FALA showed potent SARS-CoV-2-specific neutralization activity against SARS-CoV-2 pseudovirus and authentic SARS-CoV-2 infection in vitro. CA521FALA also demonstrated having a long half-life of 9.5 days in mice and 9.3 days in rhesus monkeys. CA521FALA inhibited SARS-CoV-2 infection in SARS-CoV-2 susceptible mice at a therapeutic setting with virus titer of the lung reduced by 4.5 logs. Structural analysis by cryo-EM revealed that CA521FALA recognizes an epitope overlapping with angiotensin converting enzyme 2 (ACE2)-binding sites in SARS-CoV-2 RBD in the Spike protein. CA521FALA blocks the interaction by binding all three RBDs of one SARS-CoV-2 spike trimer simultaneously. These results demonstrate the importance for antibody-based therapeutic interventions against COVID-19 and identifies CA521FALA a promising antibody that reacts with SARS-CoV-2 Spike protein to strongly neutralize its activity.

Colon-Targeted Delivery of IgY Against Clostridium difficile Toxin A and B by Encapsulation in Chitosan-Ca Pectinate Microbeads.

This study investigated the use of a newly developed chitosan-Ca pectinate microbead formulation for the colon-targeted delivery of anti-A/B toxin immunoglobulin of egg yolk (IgY) to inhibit toxin binding to colon mucosa cells. The effect of the three components (pectinate, calcium chloride, and chitosan) used for the microbead production was examined with the aim of identifying the optimal levels to improve drug encapsulation efficiency, swelling ratio, and cumulative IgY release rate. The optimized IgY-loaded bead component was pectin 5% (w/v), CaCl2 3% (w/v), and chitosan 0.5% (w/v). Formulated beads were spherical with 1.2-mm diameter, and the drug loading was 45%. An in vitro release study revealed that chitosan-Ca pectinate microbeads inhibited IgY release in the upper gastrointestinal tract and significantly improved the site-specific release of IgY in the colon. An in vivo rat study demonstrated that 72.6% of biologically active IgY was released specifically in the colon. These results demonstrated that anti-A/B toxin IgY-loaded chitosan-Ca pectinate oral microbeads improved IgY release behavior in vivo, which could be used as an effective oral delivery platform for the biological treatment of Clostridium difficile infection (CDI).

Development of microbeads of chicken yolk antibodies against Clostridium difficile toxin A for colonic-specific delivery.

The incidence of Clostridium difficile infection has increased in Western world in the past 10 years, similar infection rates are also reported in developing countries such as China. Current antibiotics treatments have recurrence rates between 15% and 30%. IgY antibodies against toxin A of C. difficile could protect animal models from the challenge of lethal dose of C. difficile spores. However, IgY is sensitive to the low pH environment of the stomach and proteinases in the intestine. The objective of this study was to prepare colonic-specific delivery system of toxin A antigen-specific IgY to block the recognition of toxin A to the colon mucosa cells. Egg-laying hens were immunized with purified C. difficile toxin A C-terminal domain for 3 times, then egg IgY against the recombinant ToxA-C protein was purified from immunized egg yolk and frozen dried. IgY-loaded microbeads were prepared using mini fluid bed system; the loading efficiency was 21%. The pH and temperature stabilities of the microbeads were assayed. The IgY-loaded microbeads coated with 35% Eudragit S100 had colonic-specific IgY release specificity both in vitro and in vivo, the colonic-specific release of biological active IgY was 87.5% in the rat. Our study provides a new option for the biological treatment C. difficile infection.

Crystal structure of the N-terminal domain of linker L(R) and the assembly of cyanobacterial phycobilisome rods.

Phycobilisomes are light-harvesting supramolecular complexes in cyanobacteria and red algae. Linkers play a pivotal role in the assembly and energy transfer modulation of phycobilisomes. However, how linkers function remains unclear due to the lack of structural and biochemical studies of linkers, especially the N-terminal domain of L(R) (pfam00427). Here, we report the crystal structure of the pfam00427 domain of the linker L(R) (30) from Synechocystis sp. PCC 6803 at 1.9 Å. The pfam00427 presents as a previously uncharacterized point symmetric six α-helix bundle. To elucidate the binding style of pfam00427 in the C-phycocyanin (C-PC) (αß)(6) hexamer, we fixed pfam00427 computationally into the C-PC (αß)(6) inner cavity using the program AutoDock. Combined with a conserved 'C-PC binding patch' on pfam00427 identified, we arrived at a model for the pfam00427-C-PC (αß)(6) complex. This model was further optimized and evaluated as a reasonable result by a molecular dynamics simulation. In the resulting model, the pfam00427 domain is stably positioned in the central hole of the C-PC trimer. Moreover, the L(RT) (pfam01383) was docked into our pfam00427-C-PC model to generate a complete phycobilisome rod in which the linkers join individual biliprotein hexamers.