Study sheds light on immune factors responsible for contrasting SARS-CoV-2 outcomes in humans and white-tailed deer

In a recent study posted in the bioRxiv* preprint server, researchers explored the response of respiratory epithelial cells in deers to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Study: How Do Deer Respiratory Epithelial Cells Weather The Initial Storm of SARS-CoV-2? Image Credit: MichaelSeanOLeary/Shutterstock.com

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Background

The potential infectiousness of SARS-CoV-2 in animals threatens public health and the economy, particularly due to the high SARS-CoV-2 vulnerability of white-tailed deer (WTD).

So far, no studies have investigated the innate immune variables accountable for the disparate disease outcomes associated with SARS-CoV-2 in WTD.

About the study

In the present study, researchers used respiratory epithelial cells from WTDs (Deer-RECs) and human RECs (HRECs) in SARS-CoV-2 infection investigations.

Six viral doses and associated mock-infected controls were inoculated into Deer-RECs and HRECs. Over 120 hours post-infection (hpi), the cells were incubated and observed daily.

The sequenced total ribonucleic acid (RNA) from HREC and Deer-REC SARS-CoV-2- and mock-inoculated samples had RNA Integrity Numbers (RIN) between 9.7 and 10, while the sequencing generated approximately 5,000,000 reads per sample.

Results

Virus-specific cytopathic effect (CPE), like cell rounding, cell detachment/death, and vacuolation, was noted at 48 hpi in Deer-RECs at concentrations over 10³ plaque-forming unit (PFU)/mL, whereas in HRECs, CPE was evident at 72 hpi. Mock-infected controls did not exhibit CPE.

In Deer RECs and HRECs, the CPE was SARS-CoV-2 dose- and time-dependent. However, at viral concentrations of 102 PFU/mL and more between 48 and 120 hpi, cell detachment and cell death were significantly higher in Deer-RECs than in HRECs.

Throughout the observation period, both mock-infected HRECs and Deer-RECs remained SARS-CoV-2 negative.

Volcano plots created with differential gene expression (DEG) data from SARS-CoV-2-inoculated and associated mock-controls in HREC and Deer-REC displayed upregulated and downregulated genes. In HRECs, the team observed a gradual decline in the number of DEGs as the infection progressed, while a remarkable number of DEGs were noted at six hpi, 24 hpi, and 48 hpi.

On the other hand, the number of unique DEGs improved throughout SARS-CoV-2 infection in Deer-RECs, from 36 DEGs at six hpi to 135 at 24 hpi and 280 at 48 hp.

At six hpi, infection of Deer-HRECs and HRECs with SARS-CoV-2 induced divergent signaling events in the interleukin (IL)-17 pathway.

Activation of early signaling genes like heat shock protein 90 (Hsp90) and predicted regulation of tumor necrosis factor receptor-associated factor-3 interacting protein-2 (TRAF3IP2), TRAF2, TRAF5, and SRSF1 differed significantly between human and deer cells, resulting in differential messenger RNA (mRNA) stability.

At six hpi, the expression of IL1 increased in SARS-CoV-2-infected HRECs. However, SARS-CoV-2 infected Deer-RECs demonstrated a robustly predicted downregulation of the proinflammatory cytokine and a chemokine reaction in CXCL1, CXCL3, and CXCL8 expression.

SARS-CoV-2-inoculated Deer-RECs exhibited a substantial downregulation of the cytokine tumor necrosis factor (TNF) and chemokines CXCL3 and CXCL8 within the initial 24 hpi. At 24 hpi, the nuclear factor-kappa-B (NFκ-B) inhibitors NFκ-B-inhibitor alpha (NFKBIA) or IκB, as well as the negative feedback regulator suppressor of cytokine signaling 3 (SOCS3), were downregulated in Deer-RECs.

On the other hand, at 24 hpi, nerve growth factor receptor (NGFR), Solute Carrier Family 20 Member 1 (SLC20A1), and JUN were downregulated in HRECs. In SARS-CoV-2-infected HRECs, a group of genes related to the NFκ-B signaling pathway, namely serum amyloid A2 (SAA2), tumor necrosis factor alpha-induced protein 3 (TNFAIP3), BIRC3, and interferon regulatory factor-1 (IRF1), were all upregulated. At 48 hpi, SAA2 and TNFAIP3 were upregulated, both biomarkers of severe SARS-CoV-2 infection.

At six hpi, the apoptosis inhibitor BIRC3 was upregulated, and at 48 hpi, its expression had doubled in HRECs. Additionally, IRF1 expression was upregulated in virus-infected HRECs at 48 hpi.

Furthermore, at 48 hpi, the upregulation of a gene associated with the NFκ-B pathway coincides with increased CXCL3 expression in virus-infected HRECs. At 48 hpi, IFNAR, CXCL6, and CXCL8 expression increased in Deer-RECs.

Conclusion

The study findings suggested that IL-17 and NF-B signaling pathway dysregulation could be among the main determinants of these mammalian species' different early innate immune reactions.

These researchers believe these findings could be extended to explain the absence of clinical symptoms in WTD under real and experimental conditions, in contrast to the severity of clinical manifestations in humans infected with SARS-CoV-2.

The "omics" of deer and SARS-CoV-2 require further investigation. Using 3D cell cultures obtained from WTD as an alternative strategy can enhance the understanding of host-virus interactions, leading to the development of novel interventions.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.