In RSV bronchiolitis, neutrophils, account for >80% of cells recovered from airways in bronchoalveolar lavage (BAL) fluid. We investigated neutrophil activation and toll-like receptor (TLR) expression in the blood and lungs of infants with severe RSV bronchiolitis.
Methods: BAL and (blood) samples were collected from 24 (16) preterm and 23 (15) term infants, ventilated with RSV bronchiolitis, and 12 (8) control infants. We measured protein and mRNA expression of CD11b, myeloperoxidase (MPO) and TLR 2,4,7,8,9 in neutrophils.
Results: Blood neutrophils had more CD11b in preterm and term bronchiolitic infants, than control infants (P<0.025) but similar amounts of MPO. BAL neutrophils from bronchiolitic infants had increased amounts of CD11b and MPO than blood neutrophils and BAL neutrophils from controls (P<0.01). Blood neutrophils from term RSV infants had less total TLR4 protein than preterm RSV infants (P=0.005) and both had less than controls (P<0.04). Total TLR4 for each group was greater in BAL than blood neutrophils. Blood neutrophils from preterm RSV infants had greater TLR4 mRNA expression than term RSV infants (P=0.005), which had similar expression to controls (p=0.625).
Conclusions: In infants with severe RSV bronchiolitis neutrophil activation starts in the blood and progresses as they are recruited into the airways. Total neutrophil TLR4 remains low in both compartments. TLR4 mRNA expression is unimpaired. This suggests neutrophil TLR4 expression is deficient in these infants, which may explain why they develop severe RSV bronchiolitis.