Potential of a SARS-CoV-2 immunoPET approach to study COVID-19 dynamics in total body in rhesus macaques

By Colin Lightfoot, M.Sc. Infection and ImmunityMar 2 2022Reviewed by Danielle Ellis, B.Sc.

The current coronavirus disease 2019 (COVID-19) pandemic has highlighted the potential broad systemic impact caused by an infection with a respiratory virus. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (COVID-19), initially infects the respiratory system but has the potential to infect other organ systems further.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

For example, there have been reports of SARS-CoV-2 infecting the gastrointestinal (GI) tract, resulting in diarrhea with mild disease and more serious complications with severe disease, such as ischemia. SARS-CoV-2 infections have also been identified within the male genital tract (MGT).

In the non-human primate rhesus macaques, SARS-CoV-2 emulates mild to moderate human disease. The respiratory tract architecture is very similar in humans and macaques, making them the ideal candidates for SARS-CoV-2 infection research. Macaques have also been found to have infections in different organ systems, the most prevalent of which is the GI tract.

The study

In a recent study posted to the bioRxiv* preprint server, an immunoPET technique that the authors were using to analyze simian immunodeficiency virus (SIV) was adapted to evaluate the spatiotemporal dynamics of SARS-CoV-2 infection in a rhesus macaque model.

ImmunoPET is a molecular imaging solution that utilizes the in vivo imaging power of combined positron emission tomography-computed tomography (PET/CT) with the specificity of an antibody-based probe tagged with a radioisotope.

The researchers used PET/CT imaging of rhesus macaques at varying times post SARS-CoV-2 inoculation to show that the Cu-labelled CR3022-F(ab') 2 probe targeting the SARS-CoV-2 spike protein could be utilized to investigate the infection dynamics within the respiratory tract and to find novel infection sites.

Following tissue dissection and necropsy, the cell-associated PET signals persist in the tissue samples. Three distinct PET/CT scans are the basis for the PET/CT guided necropsy technique, which was used to map the probe signals from the whole animal, organ, and tissue levels.  

Results

An infected male macaque showed minimal signals associated with the lungs but a very strong signal associated with the testes and penis. The 3D reconstruction of the lungs was separated from the PET datasets, and the lung signal was projected onto the CT reconstruction of the skeleton to examine the probe signal in the lungs better. As was observed in the complete animal scan, the signal linked with the lungs was broad and homogeneous across the tissue.

This was more apparent in the lung signal projection lacking CT. When the intact respiratory system and the tongue were scanned again after the necropsy, the lung signal was more visible. In this example, the lungs were deflated within the thoracic cavity, which increased signal concentration and resulted in a significantly greater signal in the second scan. As shown by the cartilaginous structures observed in the CT, two distinct respiratory tract z-series images reveal a concentrate of signal linked with the base of the tongue, overlapping with the pharynx.

In previous studies, macaques infected with the SARS-CoV-2 WA1 isolate presented mainly with mild disease. Due to this, the authors infected this set of animals with the Delta variant (the dominant strain at the time of this study).

Two PET/CT scans were performed at 3- and 21-hours after probe injection at 1-week post-infection to acquire insight into the dynamics of the probe distribution over time. From the 3- and 21-hour scans, the side and front views indicate a rapidly changing arrangement of probe distribution, indicating the importance of the PET/CT scan timing after probe injection.

The 3-hour scan clearly shows a strong probe signal in the heart's vasculature and chambers, indicating that a significant amount of the probe is still circulating in the blood following intravenous (IV) injection. Compared to the previously discussed procedure, a much greater lung probe signal was observed in this animal, which was further distributed throughout the tissue. In the 3-hour scan, the authors observed a strong signal associated with the prostate. At the 3-hour time-point, the signal from the MGT was apparent and appeared to become stronger by the 21-hour scan.

The 3D reconstruction of the animals' lungs was separated from the PET datasets, and the lung signal was superimposed onto the CT reconstruction of the skeleton to further examine the probe signal in the lungs following Delta infection. The 3-hour and 21-hour signals are both prominent and well-defined. The lung rotation series revealed a significant signal connected with the right lung's caudodorsal region and a smaller signal linked with the left lung.

In contrast to the more CT transparent left lung tissue, this comparison demonstrates an overlap of the opaque lung signal and stronger PET signal in the right lung. The dorsal portions of the right lower lung lobe displayed explicit gross disease in this animal. The animal's right lung had a lot of lung interstitial inflammation and type II pneumocyte hyperplasia, seen via histopathology.

The PET signal projection in the data shows a robust PET signal in many lung lobes, close to the focally large accumulation sites. A significant PET signal is seen in various locations of the left lung, with the caudal portion of the upper lobe of the left lung showing the strongest signal.

Implications

The immunoPET approach presented in this paper could be a valuable tool for researching and comprehending SARS-CoV-2 pathogenesis. The availability of an immunoPET probe for SARS-CoV-2 in the clinical setting has the potential to uncover the underlying mechanism of disseminated viral infection in long COVID.

*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.