Human rhinoviruses are the viruses responsible for the common cold. In patients with chronic respiratory diseases such as chronic obstructive pulmonary disease, rhinovirus infection is a major risk factor for disease exacerbations. Indeed, it is often observed that rhinovirus infection precedes secondary bacterial infection. It has been shown that rhinoviruses can impair the phagocytosis capacity of macrophages, resulting in a defective response to secondary infections. We have previously shown that rhinovirus 16 alters the ability of macrophages to ingest bacteria. However, some bacteria are still phagocytized by macrophages. It therefore remains to be understood whether, in addition to a failure to ingest bacteria, rhinovirus 16 could cause a failure in phagosome maturation, i.e. a failure to digest bacteria. Using human macrophages derived from monocytes, we investigated the effect of rhinovirus 16 on phagosome maturation and the elimination of phagocytized bacteria.
Through bacterial survival experiments, we have shown that rhinovirus 16 blocks the ability of macrophages to eliminate phagocytized bacteria, irrespective of their species. This observation can be correlated with the fact that rhinovirus 16 alters the hydrolytic activity of macrophages as well as the production of reactive oxygen species in these cells, allowing bacteria to escape degradation.
Efficient phagocytosis requires a complex process of phagosome maturation. Exposure of macrophages to rhinovirus 16 results in a delay in the maturation of early and late phagosomes into phagolysosomes. These maturation defects are associated with increased expression of the EEA1 marker associated with early phagosomes in virus-exposed cells. In addition, CD63, a marker associated with late phagosomes, is more relocalized to the plasma membrane in virus-exposed macrophages, indicating a disruption of intracellular trafficking. Further study of the ultrastructure of intracellular compartments revealed disruption of the Golgi apparatus and the transgolgi network.
A first step towards therapeutic strategies targeting macrophage reactivation
Transcriptomic analysis identified the small GTPase ARL5b as a viral target. Depletion analyses confirmed that increased ARL5b expression was responsible for the observed defects in phagosome maturation and bacterial elimination.
Finally, in cells permissive to rhinovirus 16 viral replication, ARL5b expression is decreased by the infection. Moreover, overexpression of ARL5b results in decreased excretion of viral particles, leading to the conclusion that ARL5b is a viral restriction factor in permissive cells.
Human rhinoviruses can disrupt macrophage function, leading to defective phagocytosis. Our various studies have shown that rhinovirus 16 can inhibit bacterial phagocytosis by macrophages at several levels: ingestion and elimination.
Figure legend: Exposure to human rhinovirus upregulates ARL5b expression in macrophages, which leads to impairment of the intracellular trafficking and defect in phagosome maturation. This translates into stalled EEA1+ CD63+ LAMP1+/- phagosomes and a defective bacteria clearance. Moreover, ARL5b upregulation in permissive cells inhibits viral egress.
In the future, it will be important to understand by which mechanism(s) the virus increases ARL5b expression, which signaling pathways are activated by the virus in macrophages, and how human rhinoviruses disrupt other macrophage functions such as the production of inflammatory mediators. These studies provide a first step towards the development of therapeutic strategies aimed at reactivating macrophage functions.
Reference
ARL5b inhibits human rhinovirus 16 propagation and impairs macrophage-mediated bacterial clearance. Suzanne Faure-Dupuy, Jamil Jubrail, Manon Depierre, Kshanti Africano-Gomez, Lisa Öberg, Elisabeth Israelsson, Kristofer Thörn, Cédric Delevoye, Flavia Castellano, Floriane Herit, Thomas Guilbert, David G Russell, Gaell Mayer, Danen Cunoosamy, Nisha Kurian & Florence Niedergang. EMBO Rep (2024) https://doi.org/10.1038/s44319-024-00069-x
