Demonstration of various pathogenicity and orientation of SARS-CoV-2 mutants in cat models

In a recent study posted on bioRxiv* Researchers at Preprint Server, University of Cornell Veterinary Medicine and University of Illinois at Urbana Champaign have found that the infectivity, pathogenicity, and tissue of multiple severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants in adults. A cat whose orientation was compared.

With the advent of new SARS-CoV-2 mutants (VOCs), a new wave of SARS-CoV-2 breakthrough infections is emerging around the world. Therefore, a better understanding of their infectivity and etiology is important for the development of improved vaccines and treatments for new SARS-CoV-2 VOCs.

..study: The Omicron variant BA.1.1 is less pathogenic than the B.1D614G and Delta mutants in the cat model of SARS-CoV-2 infection... Image credit: NIAID

About research

In this study, researchers conducted a 14-day experiment with adult cats infected with SARS-CoV-2 at the Cornell University Animal Biosafety Level 3 (ABSL-3) facility in New York, USA. They infected susceptible cats with the SARS-CoV-2 D614G (B.1) strain and two VOCs, Delta (B.1.617.2) and Omicron BA.1.1 (B.1.1.529). I did.

Researchers first evaluated clinical parameters such as the weight of infected cats and the clinical signs of respiratory disease. They collect nasal and oropharyngeal secretions (OPS) and rectal swabs (RS) and infect with SARS-CoV-2 ribonucleic acid (RNA) via real-time reverse transcriptase-polymerase chain reaction (RT-PCR). The presence of sex virus particles was detected. .. Similarly, the team evaluated viral RNA loading and infectious virus titers in bronchoalveolar lavage fluid (BALF) in test animals.

Researchers have extracted several organs from infected test animals, including nasal turbinates, tonsils, lungs, trachea, liver, spleen, heart, kidneys, small intestines, and associated lymph nodes, and SARS- The site of CoV-2 replication was identified and tissue orientation was evaluated. Next, we performed quantitative RT-PCR targeting the envelope (E) gene to quantify subgenomic RNA (sg-RNA) in the tissues of infected animals.In addition, they used there Hybridization and immunofluorescence (IFA) staining to assess the tissue distribution of SARS-CoV-2 in the airways and associated lymphoid tissues. The team also performed histological examination of the tissues of cats collected on days 3, 5, and 14 from cats infected with D614G, Delta, and Omicron.

Finally, the team used an indirect enzyme-linked immunosorbent assay (ELISA) to evaluate the antibody response to SARS-CoV-2 infection in cats. In addition, they used a multiplex Luminex bead-based immunoassay to measure serum cytokine and chemokine levels throughout the SARS-CoV-2 infection window.

Experimental design, changes in body temperature and body weight after SARS-CoV-2 inoculation to cats. Schematic diagram of an design of infection studies in male and female adult cats (24-40 months old). Animals were assigned to three inoculation groups (n = 7 per group) and a control group (simulated inoculation) (n = 3). 1 ml (0.5 ml / nasal) virus suspension containing 5 x 105 PFU SARS-CoV-2D614G (B.1), Delta (B.1.617.2), or Omicron BA.1.1 (B) in animals. Was inoculated intranasally. 1.1.529), or 1 ml of cell culture supernatant medium (control is simulated inoculation). On the 3rd and 5th day (pi) after infection, 2 cats (1 female and 1 male) in each group were humanely euthanized and the remaining cats (1 female and 2 males). Was bred until the 14th day of pi. Respiratory secretions, feces and serum were collected on specific days as shown (A). NS = Nasal Swab; OPS = Oropharyngeal Swab; RS = Rectal Swab; F = Female; M = Male. Intranasal SARS-CoV-2D614G (B.1), Delta (B.1.617.2), or Omicron BA.1.1 (B.1.1.529) Post-inoculation temperature (B) and weight changes (C) for 14 days Experimental period. The data is expressed as mean ± standard deviation. * p <0.05; ** p <0.01; *** p <0.005; **** p <0.0001.

Survey results

Cats infected with D614G and Delta were exhausted and their temperature increased between 1 and 3 days (pi) after infection, while cats infected with Omicron remained asymptomatic. Regardless of the infected VOC, the authors detected viral RNA in the nasal discharge and OPS of infected cats between days 1 and 14.

Viral RNA release in RS was lower than in nasal secretions and OPS, and cats infected with Omicron showed lower RNA levels in feces between 3 and 10 days. Cats infected with D614G and Delta showed high viral load in the nose and OPS, with a median virus titer of 2.3-6.3 log 10 in tissue culture infection (TCID).₅₀) .Ml-¹, Cats infected with Omicron significantly reduce virus titers. Infectious virus titers in BALF in Omicron-infected cats were also lower compared to D614G and Delta-infected cats, ie in the range of 2.8 and 3.0 log10TCID.₅₀ ml ml^-1 Pi on the 5th day.

The load of sgRNA detected in the tissues of cats infected with Omicron was significantly lower than that of animals infected with D614G and Delta. In addition, sgRNA was only detectable in the turbinate and trachea of ​​cats infected with Omicron. Conversely, the authors detected sgRNA in the nasal turbinates, tonsils, trachea, and lungs of cats infected with D614G and Delta.

Cats infected with Omicron were low in SARS-CoV-2 RNA and nucleocapsid (N) protein in all tissues tested, indicating limited tissue distribution, infection, and replication sites for Omicron BA.1.1 in cats. I am. Histological examination revealed that cats infected with Omicron had epithelial necrosis in the nasal passages and fluctuated epithelial decay in the upper respiratory tract. This finding reflects the limited pathogenicity of Omicron in cats.

Cats infected with Omicron BA.1.1 also showed elevated interferon gamma (IFN-γ) levels on days 7 and 14.In addition, it is regulated by activation, usually T cells Expression and secretion (RANTES) levels were higher in cats infected with Omicron on days 3 and 5 of the pie. Increased anti-inflammatory IFN-γ secretion and decreased pro-inflammatory cytokine response explain the reduced disease severity of Omicron in cats. Further studies need to investigate and compare the expression of these cytokines in the most severely affected tissues of the upper and lower respiratory tracts of infected cats.

Conclusion

Studies have shown that mouse models of SARS-CoV-2 infection are less pathogenic to the Omicron mutant. Even in humans, Omicron infection causes mild symptoms, reduces viral load, and significantly reduces the risk of hospitalization compared to previous variants. Similarly, in this study, the Omicron BA.1.1 mutant showed limited pathogenicity in the highly susceptible cat model used in the study of SARS-CoV-2 infection than the D614G and Delta mutants. rice field. As Omicron continues to evolve into distinct sub-strains, it is imperative to carry out studies to assess its biological properties in animal models.

*Important Notices

bioRxiv Publish preliminary scientific reports that should not be considered definitive as they have not been peer-reviewed, guide clinical practice / health-related behaviors, and should not be treated as established information.