Our study shows that pollutants in our environment have a direct and detrimental effect on the human immune system, and specifically the immune organs that are associated with the respiratory tract. LNs filter impurities and coordinate the clearance of harmful antigens and pathogens, but over decades the LNs connected to the lungs become clogged with particulates, and as a result they are not able to carry out essential functions of host defense and immune surveillance. In addition, the effects of pollutants are cumulative and can in part account for the worse outcome of respiratory infections in elderly individuals compared with younger populations.

Our study raises important questions concerning the mechanisms by which particulates are contained by specific macrophage subsets in the LNs. It is unknown whether these macrophages are resident in the LN or whether they are derived from lung macrophages that migrate to the associated LN. It will be interesting to assess whether targeting certain macrophage populations may facilitate the clearance of particulate matter. Moreover, we showed that particulates impair the phagocytic capacity that is mediated by scavenger receptors that are expressed by macrophages. However, the effects of particulates on other pathways for phagocytosis of pathogens and cellular debris remain to be established.

In addition, it will be important to understand the full effect of inhaled particulates in the resident immune cell populations in the lung itself, which is an area of ongoing investigation.

Basak Burcu Ural and Donna L. Farber

Department of Microbiology and Immunology, Columbia University Irving Medical Center, New York, NY, USA.