Mast Cells Impair Host Defense During Murine Streptococcus pneumoniae Pneumonia

Background. Streptococcus pneumoniae is the most common causative pathogen in community-acquired pneumonia. Mast cells (MCs) are located mainly at the host-environment interface where they function as sentinels. Objective. Our goal was to study the role of MCs during pneumonia caused by S. pneumoniae. Methods. Lung tissue of patients who had died from pneumococcal pneumonia or a nonpulmonary cause was stained for MCs and tryptase. Wild-type (WT) and MC-deficient (KitW-sh/W-sh) mice were observed or sacrificed after induction of pneumonia by intranasal inoculation of S. pneumoniae. In separate experiments, WT mice were treated with doxantrazole or cromoglycate, which are MC stabilizing agents. Results. The constitutive presence of tryptase-positive MCs was reduced in affected lungs from pneumonia patients. KitW-sh/W-sh mice showed a prolonged survival during the first few days after median lethal dose (LD)₁₀₀ and LD₅₀ infection, while overall mortality did not differ from that in WT mice. Relative to WT mice, KitW-sh/W-sh mice showed reduced bacterial counts with less bacterial dissemination to distant organs and less inflammation. Neither doxantrazole nor cromoglycate influenced antibacterial defense or inflammatory responses after airway infection with S. pneumoniae. Conclusions. MCs exhibit an unfavorable role in host defense during pneumococcal pneumonia by a mechanism independent of degranulation.