SARS-CoV-2 infection induces expression and secretion of lipocalin-2 and regulates iron in a human lung cancer xenograft model

Production of a monoclonal antibody targeting the SARS-CoV-2 Omicron spike protein and analysis of SARS-CoV-2 Omicron mutations related to monoclonal antibody resistance

SARS-CoV-2 mutations have resulted in the emergence of multiple concerning variants, with Omicron being the dominant strain presently. Therefore, we developed a monoclonal antibody (mAb) against the spike (S) protein of SARS-CoV-2 Omicron for therapeutic applications. We established the 1E3H12 mAb, recognizing the receptor binding domain (RBD) of the Omicron S protein, and found that the 1E3H12 mAb can efficiently recognize the Omicron S protein with weak affinity to the Alpha, Beta, and Mu variants, but not to the parental strain and Delta variant. Based on in vitro assays, the mAb demonstrated neutralizing activity against Omicron BA.1, BA.4/5, BQ.1.1, and XBB. A humanized antibody was further produced and proved to have neutralizing activity. To verify the potential limitations of the 1E3H12 mAb due to viral escape of SARS-CoV-2 Omicron variants, we analyzed the emergence of variants by whole genome deep sequencing after serial passage in cell culture. The results showed a few unique S protein mutations in the genome associated with resistance to the mAb. These findings suggest that this antibody not only contributes to the therapeutic arsenal against COVID-19 but also addresses the ongoing challenge of antibody resistance among the evolving subvariants of SARS-CoV-2 Omicron.

Parental Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Predominates Over Coinfected SARS-CoV-2 Delta, Producing Less Lethal Variants in a Long-Term Replication Mouse Model

The evolution of SARS-CoV-2, which limits public control and treatment, seems to have occurred through multiple mechanisms, including recombination of cocirculating strains in hosts. However, insufficient experimental data have been obtained after coinfection. Therefore, we investigated the emergence of variants after coinfection with parental SARS-CoV-2 and the SARS-CoV-2 Delta. We found that fewer (approximately 50%) mutations accumulated in Calu-3 cells than in other cells after serial passaging. Previously, we established a long-term replication mouse model by infecting Calu-3 cell-derived xenograft tumors with SARS-CoV-2. Here, we utilized our model to investigate the outcome after coinfection. More diverse viral mutations, along with multiple high-frequency simultaneous mutations, were discovered in the tumors than during cell passaging. Viral isolates from the tumors showed no cytopathic effects and formed much smaller plaques. Phylogenetic analysis suggested that the genetic makeup of the variants remained largely the same as that of parental SARS-CoV-2 rather than the SARS-CoV-2 Delta. Viral challenge revealed that the isolates were less lethal than the parental SARS-CoV-2 and SARS-CoV-2 Delta strains. These findings suggest that parental SARS-CoV-2 predominates over the SARS-CoV-2 Delta when coinfected, but the SARS-CoV-2 Delta contributes to the evolution of parental SARS-CoV-2 variants toward better host adaptation without recombination.

Analysis of SARS-CoV-2 Mutations after Nirmatrelvir Treatment in a Lung Cancer Xenograft Mouse Model

Paxlovid is the first approved oral treatment for coronavirus disease 2019 and includes nirmatrelvir, a protease inhibitor targeting the main protease (Mpro) of SARS-CoV-2, as one of the key components. While some specific mutations emerged in Mpro were revealed to significantly reduce viral susceptibility to nirmatrelvir in vitro, there is no report regarding resistance to nirmatrelvir in patients and animal models for SARS-CoV-2 infection yet. We recently developed xenograft tumors derived from Calu-3 cells in immunodeficient mice and demonstrated extended replication of SARS-CoV-2 in the tumors. In this study, we investigated the effect of nirmatrelvir administration on SARS-CoV-2 replication. Treatment with nirmatrelvir after virus infection significantly reduced the replication of the parental SARS-CoV-2 and SARS-CoV-2 Omicron at 5 days post-infection (dpi). However, the virus titers were completely recovered at the time points of 15 and 30 dpi. The virus genomes in the tumors at 30 dpi were analyzed to investigate whether nirmatrelvir-resistant mutant viruses had emerged during the extended replication of SARS-CoV-2. Various mutations in several genes including ORF1ab, ORF3a, ORF7a, ORF7b, ORF8, and N occurred in the SARS-CoV-2 genome; however, no mutations were induced in the Mpro sequence by a single round of nirmatrelvir treatment, and none were observed even after two rounds of treatment. The parental SARS-CoV-2 and its sublineage isolates showed similar IC50 values of nirmatrelvir in Vero E6 cells. Therefore, it is probable that inducing viral resistance to nirmatrelvir in vivo is challenging differently from in vitro passage.

Analysis of SARS-CoV-2 omicron mutations that emerged during long-term replication in a lung cancer xenograft mouse model

SARS-CoV-2 Omicron has the largest number of mutations among all the known SARS-CoV-2 variants. The presence of these mutations might explain why Omicron is more infectious and vaccines have lower efficacy to Omicron than other variants, despite lower virulence of Omicron. We recently established a long-term in vivo replication model by infecting Calu-3 xenograft tumors in immunodeficient mice with parental SARS-CoV-2 and found that various mutations occurred majorly in the spike protein during extended replication. To investigate whether there are differences in the spectrum and frequency of mutations between parental SARS-CoV-2 and Omicron, we here applied this model to Omicron. At 30 days after infection, we found that the virus was present at high titers in the tumor tissues and had developed several rare sporadic mutations, mainly in ORF1ab with additional minor spike protein mutations. Many of the mutant isolates had higher replicative activity in Calu-3 cells compared with the original SARS-CoV-2 Omicron virus, suggesting that the novel mutations contributed to increased viral replication. Serial propagation of SARS-CoV-2 Omicron in cultured Calu-3 cells resulted in several rare sporadic mutations in various viral proteins with no mutations in the spike protein. Therefore, the genome of SARS-CoV-2 Omicron seems largely stable compared with that of the parental SARS-CoV-2 during extended replication in Calu-3 cells and xenograft model. The sporadic mutations and modified growth properties observed in Omicron might explain the emergence of Omicron sublineages. However, we cannot exclude the possibility of some differences in natural infection.

A mouse xenograft long-term replication yields a SARS-CoV-2 Delta mutant with increased lethality

We recently established a long-term SARS-CoV-2 infection model using lung-cancer xenograft mice and identified mutations that arose in the SARS-CoV-2 genome during long-term propagation. Here, we applied our model to the SARS-CoV-2 Delta variant, which has increased transmissibility and immune escape compared with ancestral SARS-CoV-2. We observed limited mutations in SARS-CoV-2 Delta during long-term propagation, including two predominant mutations: R682W in the spike protein and L330W in the nucleocapsid protein. We analyzed two representative isolates, Delta-10 and Delta-12, with both predominant mutations and some additional mutations. Delta-10 and Delta-12 showed lower replication capacity compared with SARS-CoV-2 Delta in cultured cells; however, Delta-12 was more lethal in K18-hACE2 mice compared with SARS-CoV-2 Delta and Delta-10. Mice infected with Delta-12 had higher viral titers, more severe histopathology in the lungs, higher chemokine expression, increased astrocyte and microglia activation, and extensive neutrophil infiltration in the brain. Brain tissue hemorrhage and mild vacuolation were also observed, suggesting that the high lethality of Delta-12 was associated with lung and brain pathology. Our long-term infection model can provide mutant viruses derived from SARS-CoV-2 Delta and knowledge about the possible contributions of emergent mutations to the properties of new variants.