Secondary
bacterial infections following
influenza infection are a pressing problem facing respiratory medicine. Although
antibiotic treatment has been highly successful over recent decades, fatalities due to secondary
bacterial infections remain one of the leading causes of death associated with
influenza. We have assessed whether administration of a
bacterial extract alone is sufficient to potentiate immune responses and protect against primary
infection with
influenza, and
secondary infections with either Streptococcus pneumoniae or Klebsiella pneumoniae in mice. We show that
oral administration with the
bacterial extract, OM-85, leads to a maturation of dendritic cells and B-cells characterized by increases in MHC II, CD86, and CD40, and a reduction in ICOSL. Improved immune responsiveness against influenza virus reduced the threshold of susceptibility to secondary
bacterial infections, and thus protected the mice. The protection was associated with enhanced polyclonal B-cell activation and release of
antibodies that were effective at neutralizing the virus. Taken together, these data show that
oral administration of
bacterial extracts provides sufficient mucosal immune stimulation to protect mice against a respiratory tract
viral infection and associated sequelae.