Abstract
The decline in neutrophil antimicrobial function renders vaccinated aged hosts less protected against Streptococcus pneumoniae infection. In vaccinated hosts, activation of neutrophils via phagocytic receptors, like complement and Fcγ receptors, mediates bacterial uptake and killing. However, the mechanisms behind changes in signaling downstream of these receptors with age is not known. Mitogen-activated protein kinases (MAPK) are signaling cascades activated in response to cell surface receptors. Using bone marrow neutrophils isolated from young and aged mice, and MAPK phosphorylation array, we found differences in MAPK activation in an opsonin dependent manner (complement vs antibody). Neutrophils from aged mice failed to increase MAPK phosphorylation upon infection with pneumococci opsonized with heat-inactivated (HI) sera from hosts immunized with the pneumococcal conjugate vaccine (PCV), indicating an age-related defect in FcγR signaling. Neutrophils from aged mice had higher basal levels of MAPK phosphorylation when compared to young controls, including a 15-fold increase in ERK1/2 phosphorylation. Inhibition of ERK1/2 signaling blunted pneumococcal killing by PMNs from young mice, but improved killing by neutrophils from aged mice, only when the bacteria were opsonized with HI immune but not naïve sera. In young human participants, in vitro inhibition of ERK1/2 in neutrophils resulted in decreased pneumococcal killing, but only following PCV vaccination, suggesting that the effects of ERK1/2 inhibition are clinically relevant in vaccinated hosts. This study identifies age-related changes in ERK1/2 activation and demonstrates that balanced activation of this pathway is crucial for neutrophil antimicrobial activity against antibody opsonized bacteria following vaccination in both mice and humans. SUMMARY SENTENCE: Imbalanced ERK1/2 activation impairs neutrophil activity in aged hosts.