Evidence for Phagocytosis of Influenza Virus-Infected, Apoptotic Cells by Neutrophils and Macrophages in Mice

Abstract

Influenza virus-infected cells undergo apoptosis and become susceptible to phagocytosis by macrophages in vitro, and this leads to the propagation of the virus being inhibited. We previously showed that inhibitors of phagocytosis increased the rate of mortality among influenza virus-infected mice. However, the mode of the phagocytosis of influenza virus-infected cells in vivo has not been investigated. We, in this study, assessed this issue by histochemically analyzing bronchoalveolar lavage cells and lung tissue obtained from C57BL/6 mice infected with influenza A/WSN (H1N1) virus. Both neutrophils and macrophages accumulated in the lung soon after the viral challenge, and either type of cell was capable of phagocytosing influenza virus-infected, apoptotic cells. Changes in the level of phagocytosis and the amount of virus in lung tissue roughly correlated with each other. Furthermore, alveolar macrophages prepared from influenza virus-infected mice showed greater phagocytic activity than those from uninfected mice. The phagocytic activity of macrophages was stimulated in vitro by a heat-labile substance(s) released from influenza virus-infected cells undergoing apoptosis. These results suggested that the level of phagocytosis is augmented both quantitatively and qualitatively in the lung of influenza virus-infected animals so that infected cells are effectively eliminated. Finally, lack of TLR4 caused an increase in the rate of mortality among influenza virus-challenged mice and a decrease in the level of phagocytosis of apoptotic cells in the lung. TLR4 could thus play an important role in the host defense against influenza by positively regulating the phagocytic elimination of infected cells.

Influenza virus replication in macrophages: balancing protection and pathogenesis

“…The phagocytic capacity of mouse PEC macrophages is enhanced when they are cultured with IAVinfected epithelial cells [62]. Similarly, BAL macrophages from IAV-challenged mice contain engulfed apoptotic cells and AMs from IAV-infected mice show greater phagocytic capacity towards apoptotic thymocytes than AMs from uninfected mice [63,64]. Together, these data suggest that macrophages play a role in clearing dead infected epithelial cells during IAV infection.…”

“…Together, these data suggest that macrophages play a role in clearing dead infected epithelial cells during IAV infection. Phagocytosis of apoptotic epithelial cells is a protective host response, as increased phagocytic capacity correlates with lower IAV lung titres in mice and inhibition of phagocytosis during IAV infection in vivo enhances virus lethality [63,64].…”

Role of human Toll‐like receptors in naturally occurring influenza A infections

“…Interestingly, the “bacterial‐sensing” TLRs (2 and 4) were also reported to be suppressed 27, 28. Downregulation of these TLRs may impair phagocyte recruitment and bacterial elimination, contributing to the risk of secondary infections 9, 30, 31, 32. We found no significant difference in TLR expression pattern or magnitude between A/H3N2 and A/H1N1pdm09 infections, unlike their adaptive immune responses 20.…”

“…Broadly, viral nucleic acids (dsRNA, ssRNA, CpG oligodeoxynucleotides) are detected by the endosomal TLRs (3, 7, 8, 9), whereas components of bacteria (peptidoglycans/lipoproteins, lipopolysaccharides) are detected by the cell surface TLRs (2, 4) 3, 4. Recent in vitro and in vivo studies on influenza pathogenesis have shown that TLR activation induces expression of type I interferons and pro‐inflammatory cytokines (e.g., IL‐6, TNF‐α), limiting viral replication and dissemination, mediating tissue inflammation, and link to adaptive immunity development 3, 7, 8, 9, 10, 11, 123, 7, 10, 13.…”

Neutrophils Ameliorate Lung Injury and the Development of Severe Disease during Influenza Infection

“…Neutrophils adhere to virus-infected epithelial cells (67) and virions (68) to mediate phagocytosis and/or stimulate activation signals, including production of reactive oxygen species. Neutrophils exposed to influenza virus undergo activation of the respiratory burst, followed by depression of cell function when subsequently exposed to secondary stimuli (69), although innate defense proteins of the collectin family have been shown to protect neutrophils from these depressive effects (70).…”