Differences in Homing Potentials ofStreptococcus pneumoniae–Specific Plasmablasts in Pneumococcal Pneumonia and After Pneumococcal Polysaccharide and Pneumococcal Conjugate Vaccinations

[[[ Background: ]]] Mucosal immune mechanisms in the upper and lower respiratory tracts may serve a critical role in preventing pneumonia due to Streptococcus pneumoniae. Streptococcus pneumoniae-specific plasmablasts presumably originating in the lower respiratory tract have recently been found in the circulation in patients with pneumonia. The localization of an immune response can be evaluated by exploring homing receptors on such plasmablasts, yet no data have thus far described homing receptors in pneumonia. [[[ Methods: ]]] The expression of α4β7, L-selectin, and cutaneous lymphocyte antigen (CLA) on S. pneumoniae-specific plasmablasts was examined in patients with pneumonia (n = 16) and healthy volunteers given pneumococcal polysaccharide vaccine (PPV; n = 14) or pneumococcal conjugate vaccine (PCV; n = 11). [[[ Results: ]]] In patients with pneumonia, the proportion of S. pneumoniae-specific plasmablasts expressing L-selectin was high, the proportion expressing α4β7 was moderate, and the proportion expressing CLA was low. The homing receptor α4β7 was expressed more frequently in the pneumonia group than in the PPV (P = .000) and PCV (P = .029) groups, L-selectin was expressed more frequently in the PPV group than in the PCV group (P = .014); and CLA was expressed more frequently in the pneumonia group than in the PPV group (P = .001). [[[ Conclusions: ]]] The homing receptor profile in patients with pneumonia was unique yet it was closer to that in PCV recipients than in PPV recipients. These data suggest greater mucosal localization for immune response in natural infection, which is clinically interesting, especially considering the shortcomings of vaccines in protecting against noninvasive pneumonia.