Bone marrow immunosuppression in Salmonella-infected mice is prolonged following influenza virus infection

Objective It has been shown previously that infection with diverse viruses induces alterations in bone marrow lineage-specific progenitor cells. As complications arising from secondary bacterial infections can adversely affect the host, we investigated whether virally induced hematological alterations could contribute to the enhanced illness observed in such cases. Materials and Methods Mice were infected with influenza virus alone or influenza virus followed by a vaccine strain of Salmonella typhimurium. The effects on hematopoiesis were analyzed by fluorescein-activated cell sorting analysis and immunohistology. Results Systemic Salmonella typhimurium infection induces depletion of bone marrow erythroid and lymphoid cells. The depletion lasted longer in mice that had been previously infected with influenza virus, compared with mice that had been previously treated with allantoic fluid. Although an increase in splenic lymphoid cells was apparent in the spleens of Salmonella-infected mice, the majority of cells in the enlarged spleens were found to be both immature and mature erythrocytes. Conclusion These results show that bone marrow progenitor cell depletion induced by bacterial infection is prolonged following a viral infection. It is possible that hematological alterations may contribute to the enhanced clinical illness observed in consecutive viral:bacterial infections.