It has long been known that exposure to microbes can play a protective role against the subsequent development of asthma. However, the mechanisms underlying this so-called “hygiene” hypothesis remain poorly understood. In an article published in Immunity, researchers at the University of Liège show that exposure to bacterial DNA drastically amplifies a population of pulmonary macrophages and makes them strongly immunosuppressive, which prevents and treats asthma in mice. This discovery offers promising prospects for the development of a cell therapy based on the administration of these regulatory macrophages to asthmatic patients.

The incidence of asthma is increasing steadily, especially in developed countries. One of the reasons given for this rise is the excessive level of hygiene in our environment.

Led by Professor Fabrice Bureau (Ordinary Professor and Welbio Investigator – Walloon Excellence in Life Sciences and Biotechnology) and Dr. Thomas Marichal (Research Associate of the F.R.S.-FNRS), both researchers at the GIGA-ULg, the scientific team has discovered how a non-hygienic environment, rich in bacterial DNA, helps to protect against asthma.  In this study, researchers first looked at how exposure to microbial compounds (such as bacterial wall components, or their own DNA), or whole microbes, would modify the immune environment of the lung. They found that bacterial DNA, unlike the other compounds, was able to strongly amplify a population of so-called interstitial macrophages and that this expansion persisted for several months in the individual.

Surprisingly, if these same macrophages were isolated from a mouse and re-injected into the lungs of a naive recipient mouse, such individual was not capable of developing asthma against house dust mite extracts. Similarly, if these macrophages were transferred to an asthmatic mouse, the asthmatic mouse was cured and no more symptoms of asthma were present. Based on these results, the researchers now environ “making” macrophages with similar properties in vitro from monocytes, a white blood cell type found in the human blood.

“If it is possible to create suppressive macrophage from blood monocytes of asthmatic patients, it is quite conceivable to reinject these macrophages into the lungs of these same patients, during routine bronchoscopy procedures performed by pneumologists here at the CHU of Liège, and to evaluate the therapeutic potential of these cells”, concludes Prof. Fabrice Bureau.

The researchers have just filed a patent to protect their results and invention, and are going to initiate studies in humans.