Highlights from

ATS 2019

American Thoracic Society international conference

Dallas, USA 17-22 May 2019

Human lung organoids to study foetal RSV infection

Dr Terry Harford (Cleveland Clinic, USA) used human lung organoids, which model foetal lung development, to observe molecular and cellular changes when infected with respiratory syncytial virus (RSV) [1]. The researchers showed that the organoids reacted in much the same way as a real lung does upon RSV infection.

In healthy adults, RSV feels like the common cold with a runny nose, chest congestion, and cough. However, it is the second leading cause of death in infants. In fact, nearly 40% of infants who contract this widespread virus develop severe bronchiolitis or pneumonia, with 1-3% hospitalised. Each year, there are about 64 million cases and 160,000 deaths due to RSV worldwide. Contracting RSV within the first few months of life can make a child more susceptible to developing asthma later in life.

Animal models of transplacental transmission of RSV from lungs of pregnant rats to foetuses showed that 30% of the foetuses contract the infection [2]. Furthermore, in utero exposure to RSV leads to airway hyperreactivity and altered immunity to postnatal infections in rats. Prenatal exposure to RSV also increased airway smooth muscle reactivity and contractility during early-life RSV infection compared with non-exposed controls [3]. In humans, RSV RNA has been detected in the peripheral blood of a human newborn on their first day of life, and RSV RNA has also been detected in cord blood samples [4,5]. Dr Harford concluded that we need a good model to study RSV foetal infection.

Dr Harford and colleagues turned to 3-dimensional mini organs in a lab that mimic the features of a full-sized lung. The researchers created the lung "organoids" in a lab dish with the help of human pluripotent stem cells, which can potentially produce any cell or tissue the body needs to repair. The lung organoids created by researchers are the first to include branching airway and alveolar structures similar to human lungs. They replicate substantial aspects of lung architecture, with cell types including multiciliated epithelial cells, mesenchymal cells, and mucus-producing goblet cells as well as club cells. RNA sequencing suggested robust molecular overlap with foetal human lung development. The organoids used in the current study are derived from human embryonic stem cells, grown for 50 days in a Matrigel droplet in differentiation medium. At that point 200-800 nL of viral suspension of recombinant RSV virus is microinjected into the lumen of the organoids. The organoids were permissive to RSV infection as confirmed by PCR and electron microscopy. The recombinant RSV has a red fluorescent protein tag at the end of its genome, allowing quantification of infection by fluorescent microscopy. RSV infection is dose- and time-dependent, with maximum effect observed at 72 hours post injection.

Organoids infected with RSV exhibited a decrease in FOXJ1 (marker for ciliated cells). Club cells express CC10, and a marked increase of CC10 was observed after infection, which may be an anti-inflammatory response. TRPV1 -a calcium channel associated with mucus production and cough response- was upregulated in organoid mesenchymal cells consistent with a response to viral bronchiolitis. The phosphorylated species of ß-adrenergic receptor, mediating airway constriction, was mildly upregulated in epithelial cells. Other cell-specific markers, such as E-cadherin, smooth muscle actin, vimentin, and p63 were unchanged. The authors concluded that only ciliated cells and club cells differentially populate organoids after RSV infection. In addition, F-actin was structurally remodelled.

When the organoids are further differentiated to 100 days, better defined structures become apparent, with smooth-muscle-like cells (evident contractions observed) at the periphery and airway passage development. Reminiscent of foetal lung development, the observed phasic contractility and growth factor production is a critical model for prenatal exposure.

Human lung organoids can be a transformative tool that can facilitate discoveries about host-virus interactions. They facilitate studies interrogating the molecular pathogenesis, and about cell tropisms, or the virus-specificity to certain cell types or receptors within complexed structured tissue. Human lung organoids can provide a robust in vitro system to translate information obtained in animal model to human foetal lungs. Human lung organoids can be used as an unparalleled platform to screen current and future antiviral drugs.

The content and interpretation of these conference highlights are the views and comments of the speakers/authors.