In a recent study published in the latest issue of the Journal Viruses, researchers studied the immune responses in Syrian hamsters re-infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta and B.1 variants three months after the primary infection.
SARS-CoV-2 Delta variant has evolved into multiple sub-lineages circulating globally. Among these sub-lineages, AY.1 and AY.2 possess the K417N mutation, which is also found in the B.1.351 variant, suggesting its role in exceptional immune evasion capabilities of these variants.
Unfortunately, only a few studies have studied the biological characteristics of these sub-lineages, including their transmissibility, disease severity, and immune evasion capabilities.
About the study
In the present study, researchers assessed the severity of coronavirus disease 2019 (COVID-19) infection due to the Delta AY.1 variant and compared it with the Delta and B.1 variant-induced infections in Syrian hamsters.
There were three study groups, each with 12–14 weeks old 17 female Syrian hamsters. The research team intranasally inoculated these hamsters with a Delta/Delta AY.1/B.1 variants dosage of 105 median tissue culture infectious dose (TCID50) (0.1 mL volume).
They collected their throat swabs, nasal wash, and feces samples on alternate days during the study period and observed them for 14 days for bodyweight loss. They sacrificed five hamsters from each study group on days 3, 7, and 14 post-infections (p.i.) for organ harvesting and blood samples. In particular, they harvested the animal’s heart, liver, lungs, nasal turbinates, kidney, intestine, spleen, and brain.
The research team used 16–18 weeks old 12 female hamsters for the re-infection study. They randomly divided test animals into three groups, each consisting of four animals, and evaluated their immunoglobulin G (IgG) response and neutralizing antibody (NAb) levels.
Notably, these hamsters were reinfected three months after the primary infection. They inoculated test animals with the Delta/Delta AY.1/B.1 variants with a dosage of 105 TCID50 intranasally. Next, they collected their throat swabs, nasal wash, and feces samples on days 2, 4, and 6 p.i. and monitored body weight changes for seven days. They sacrificed animals on days 3, 7, and 14 p.i. to collect lungs, nasal turbinates, and blood samples.
They performed real-time quantitative reverse transcription-polymerase chain reaction (RT-qPCR) to estimate the genomic ribonucleic acid (gRNA) and subgenomic RNA (sgRNA) load in the nasal wash, throat swabs, and organ tissue samples of the test animals. The titers in the Vero cells were determined by the Reed and Muench method using lung samples harvested on day 7 post-infection.
Further, they used an in-house developed qualitative enzyme-linked immunosorbent assay (ELISA) to test for IgG antibodies in the serum samples. They had set ELISA cutoff at an optical density (OD) value of 0.2 and a positive to negative ratio of 1.5.
They also performed a plaque reduction neutralization test (PRNT) against the B.1, Delta, AY.1, and Beta variants to compute the PRNT50 titers which denoted the serum concentration required to reduce the number of plaques by 50%. Next, they performed an ELISA-based estimation to assess the cytokine levels in hamster sera sample, including interleukin-4 (IL-4), IL-6, IL-10, interferon γ (IFN-γ), and tumor necrosis factor α (TNF-α).
The lung histological examinations helped the researchers to compare and statistically analyze the cumulative severity scores (on a scale of 0 to 4) of infected and re-infected animal groups.
They performed these comparisons day-wise among Delta, AY.1, and B.1 infected groups (the primary infection group) for viral load, virus shedding, bodyweight loss, NAb titers, and histopathological scores. They compared this data with the uninfected control hamster sera to study the cytokine responses. Lastly, they performed these comparisons for the re-infection study with each variant, using the data from the primary infection group of the corresponding SARS-CoV-2 variant.
The authors noted that the K417N mutation in the Delta AY.1 variant was critical for neutralization resistance against potent anti-SARS-CoV-2 NAbs; however, it did not confer any advantage from the immune evasion standpoint against the Delta, B.1, and Beta variants.
The B.1.351 variant has a remarkable immune evasion potential due to the mutations K417N, E484K, and N501Y in the receptor-binding domain (RBD) of its spike (S) region. Accordingly, the authors observed cross-reactive NAbs against the examined variants after infection and a boost in titers after re-infection. However, the NAb titers were significantly reduced against the Beta variant in the case of the Delta and B.1 variant-infected animal sera after primary infection.
Regarding cytokine levels, they observed increased IL-6 cytokine levels after primary infection in the hamsters with SARS-CoV-2 variants. Additionally, IL-4, IFN-γ, and TNF-α also increased during the acute phase of COVID-19 in test animals. Surprisingly, there were no elevations in the cytokine responses after re-infection.
Notably, they observed the highest average lung sgRNA loads on days 7 and 14 p.i. and the highest increase in serum IL-6 levels in animals infected with the B.1 variant.
A previous study has shown that COVID-19 reinfection reduces viral replication and decreases transmission efficiency in Syrian hamsters re-infected 29 days after the primary infection. Another study conducted 15 months after the primary infection in hamsters demonstrated protection against lung disease caused by the Delta variant and prevention of viral transmission to naive hamsters.
In the present study, the authors observed a similar reduction in viral shedding; however, the nasal turbinate viral load remained comparable most likely due to a high viral dose of 105 TCID50 for the re-infection studies. Nonetheless, this finding highlights why it is critical to adhere to COVID-19-appropriate behavior.
To conclude, the Delta AY.1 variant-infected hamsters had mild COVID-19 symptoms and did not show any neutralization resistance. However, re-infection with higher viral doses of the Delta and B.1 variant three months after B.1 variant infection reduced virus shedding, disease severity, and increased NAb titers. These observations in test animals were the same irrespective of the variant studied.
Taken together, the study data demonstrated that although the previous infection could reduce the severity of secondary infection by the Delta variant, it does not guarantee prevention from re-infection as it does not provide sterilizing immunity.