We have devised an ex vivo culture system which generates large numbers of eosinophils at high purity (>90%) from unselected mouse bone marrow progenitors. In response to 4 days of culture with recombinant mouse FLT3-L and recombinant mouse stem cell factor followed by recombinant mouse IL-5 alone thereafter, the resulting bone marrow-derived eosinophils (bmEos) express immunoreactive major basic protein, Siglec F, IL-5R α-chain, and transcripts encoding mouse eosinophil peroxidase, CCR3, the IL-3/IL-5/GM-CSF receptor common β-chain, and the transcription factor GATA-1. BmEos are functionally competent: they undergo chemotaxis toward mouse eotaxin-1 and produce characteristic cytokines, including IFN-γ, IL-4, MIP-1α, and IL-6. The rodent pathogen pneumonia virus of mice replicates in bmEos and elevated levels of IL-6 are detected in supernatants of bmEos cultures in response to active infection. Finally, differentiating bmEos are readily transfected with lentiviral vectors, suggesting a means for rapid production of genetically manipulated cells.

The inflammatory response to respiratory virus infection can be complex and refractory to standard therapy. Lactobacilli, when targeted to the respiratory epithelium, are highly effective at suppressing virus-induced inflammation and protecting against lethal disease. Specifically, wild-type mice primed via intranasal inoculation with live or heat-inactivated Lactobacillus plantarum or Lactobacillus reuteri were completely protected against lethal infection with the virulent rodent pathogen, pneumonia virus of mice (PVM); significant protection (60% survival) persisted for at least thirteen weeks. Protection was not unique to Lactobacillus species, and was also observed in response to priming with non-pathogenic gram-positive Listeria innocua. Priming with live lactobacilli resulted in diminished granulocyte recruitment, diminished expression of multiple proinflammatory cytokines (CXCL10, CXCL1, CCL2, and TNF) and reduced virus recovery, although we have demonstrated clearly that absolute virus titer does not predict clinical outcome. Lactobacillus priming also resulted in prolonged survival and protection against the lethal sequelae of PVM infection in MyD88 gene-deleted (MyD88−/−) mice, suggesting that the protective mechanisms may be Toll-like receptor-independent. Most intriguing, virus recovery and cytokine expression patterns in Lactobacillus-primed MyD88−/− mice were indistinguishable from those observed in control-primed MyD88−/− counterparts, In summary, we have identified and characterized an effective Lactobacillus-mediated innate immune shield, which may ultimately serve as critical and long-term protection against infection in the absence of specific antiviral vaccines.

Key Points Eosinophils recruited to the airways in response to A fumigatus sensitization and challenge degranulate in response to virus infection. Activated eosinophils are antiviral and promote survival from an otherwise lethal respiratory virus infection.