SARS-CoV-2 is primarily transmitted through droplets and airborne aerosols, and in order to prevent infection and reduce viral spread vaccines should elicit protective immunity in the airways. The neonatal Fc receptor (FcRn) transfers IgG across epithelial barriers and can enhance mucosal delivery of antigens. Here we explore FcRn-mediated respiratory delivery of SARS-CoV-2 spike (S). A monomeric IgG Fc was fused to a stabilized spike; the resulting S-Fc bound to S-specific antibodies and FcRn. Intranasal immunization of mice with S-Fc and CpG significantly induced antibody responses compared to the vaccination with S alone or PBS. Furthermore, we intranasally immunized mice or hamsters with S-Fc. A significant reduction of virus replication in nasal turbinate, lung, and brain was observed following nasal challenges with SARS-CoV-2 and its variants. Intranasal immunization also significantly reduced viral airborne transmission in hamsters. Nasal IgA, neutralizing antibodies, lung-resident memory T cells, and bone-marrow S-specific plasma cells mediated protection. Hence, FcRn delivers an S-Fc antigen effectively into the airway and induces protection against SARS-CoV-2 infection and transmission.

The respiratory tract is constantly exposed to various airborne pathogens. Most vaccines against respiratory infections are designed for the parenteral routes of administration; consequently, they provide relatively minimal protection in the respiratory tract. A vaccination strategy that aims to induce the protective mucosal immune responses in the airway is urgently needed. The FcRn mediates IgG Ab transport across the epithelial cells lining the respiratory tract. By mimicking this natural IgG transfer, we tested whether FcRn delivers vaccine Ags to induce a protective immunity to respiratory infections. In this study, we designed a monomeric IgG Fc fused to influenza virus hemagglutinin (HA) Ag with a trimerization domain. The soluble trimeric HA-Fc were characterized by their binding with conformation-dependent HA Abs or FcRn. In wild-type, but not FcRn knockout, mice, intranasal immunization with HA-Fc plus CpG adjuvant conferred significant protection against lethal intranasal challenge with influenza A/PR/8/34 virus. Further, mice immunized with a mutant HA-Fc lacking FcRn binding sites or HA alone succumbed to lethal infection. Protection was attributed to high levels of neutralizing Abs, robust and long-lasting B and T cell responses, the presence of lung-resident memory T cells and bone marrow plasma cells, and a remarkable reduction of virus-induced lung inflammation. Our results demonstrate for the first time, to our knowledge, that FcRn can effectively deliver a trimeric viral vaccine Ag in the respiratory tract and elicit potent protection against respiratory infection. This study further supports a view that FcRn-mediated mucosal immunization is a platform for vaccine delivery against common respiratory pathogens.

SARS-CoV-2 and its variants cause COVID-19, which is primarily transmitted through droplets and airborne aerosols. To prevent viral infection and reduce viral spread, vaccine strategies must elicit protective immunity in the airways. FcRn transfers IgG across epithelial barriers; we explore FcRn-mediated respiratory delivery of SARS-CoV-2 spike (S). A monomeric IgG Fc was fused to a stabilized S protein; the resulting S-Fc bound to S-specific antibodies (Ab) and FcRn. A significant increase in Ab responses was observed following the intranasal immunization of mice with S-Fc formulated in CpG as compared to the immunization with S alone or PBS. Furthermore, we intranasally immunize adult or aged mice and hamsters with S-Fc. A significant reduction of virus replication in nasal turbinate, lung, and brain was observed following nasal challenges with SARS-CoV-2, including Delta and Omicron variants. Intranasal immunization also significantly reduced viral transmission between immunized and naive hamsters. Protection was mediated by nasal IgA, serum-neutralizing Abs, tissue-resident memory T cells, and bone marrow S-specific plasma cells. Hence FcRn delivers an S-Fc antigen effectively into the airway and induces protection against SARS-CoV-2 infection and transmission. Based on these findings, FcRn-targeted non-invasive respiratory immunizations are superior strategies for preventing highly contagious respiratory viruses from spreading.

Haemonchus contortus is a blood-feeding gastrointestinal nematode that significantly impacts the health and productivity of small ruminants. The burden of parasitism and the escalating incidence of anthelmintic resistance necessitate alternative control methods. Here, we characterize the enzymatic activities of five mammalian cell-expressed recombinant H. contortus cysteine proteases (HcCPs), which include two cathepsin B-like proteins (HcCBP1 and HcCBP2) and three cysteine protease 1 proteins (HcCP1a, HcCP1b, and HcCP1c). We hypothesize that these enzymes degrade host blood proteins, thereby facilitating the parasite’s nutrient acquisition and survival. Using synthetic cathepsin (cat) substrates, we show that HcCBP2 was the only protein that exhibited high catB/L but low catB or catK activity, which was inhibited by the cysteine protease inhibitor E-64. All mHcCPs degraded fibrinogen (Fg), which led to delayed plasma clotting, reduced clot density, and lysed plasma clots. All HcCPs partially degraded hemoglobin (Hb), except for mHcCBP2, which degraded Hb and bovine serum albumin completely and bovine IgG partially in the presence of a reducing agent. In conclusion, by sustaining blood feeding and facilitating immune evasion and nutrient acquisition, the HcCPs may play an essential role in the parasite’s survival. Thus, vaccines or cysteine protease inhibitors targeting these parasitic enzymes may mitigate or prevent infections.

Ostertagia ostertagi is the most pathogenic gastrointestinal nematode of cattle and causes major productivity losses. With rising anthelmintic resistance and significant economic impact, alternative control measures, such as vaccination, are urgently needed. This study aimed to generate functionally active recombinant cysteine proteases from O. ostertagi, enzymes involved in nutrient acquisition and immune evasion, to evaluate their potential as vaccine candidates. Two phylogenetically predicted cathepsin B (catB)-like proteases, OoCBP1 and OoCBP2, were cloned into the pcDNA3.1+ vector and expressed in FreeStyle 293-F cells. Purified proteins were assessed using fluorogenic substrates and various host protein substrates, including fibrinogen, albumin, hemoglobin, IgG, mucin, and gelatin, at pH 5.0 and 7.4 with or without a reducing agent. OoCBP1 was detected in the parasite across the life stages, while OoCBP2 was only detectable in adult worms. OoCBP1 partially degraded host proteins at pH 5.0, with reduced activity at pH 7.4, whereas OoCBP2 efficiently degraded all tested proteins at both pH levels except IgG, which was only partially degraded. OoCBP1 showed catB/L activity only at acidic pH, while OoCBP2 was active at both pH conditions. Activities were inhibited by catB, catB/L, or other cysteine protease inhibitors. OoCBP1, but not OoCBP2, required low pH-mediated autoactivation for substrate degradation. OoCBP1 had no effect on coagulation under physiological conditions, while OoCBP2 accelerated clot formation with reduced clot density. With in vitro activation, both proteases delayed coagulation. These findings indicate that OoCBP1 and OoCBP2 may contribute to nutrient acquisition and immune evasion, supporting their potential as targets for vaccines or chemotherapeutics against ostertagiosis.

Escherichia coli causes diarrhea and extraintestinal infections in humans and animals. Here, we report the draft genome sequences of Escherichia coli strains 360/16 and 646, isolated from neonatal calves.