Signaling pathway for phagocyte priming upon encounter with apoptotic cells

Saori Nonaka,Yuki Ando,Takuto Kanetani,Chiharu Hoshi,Yuji Nakai,Firzan Nainu,Kaz Nagaosa,Akiko Shiratsuchi,Yoshinobu Nakanishi

doi:10.1074/jbc.m116.769745

Abstract

The phagocytic elimination of cells undergoing apoptosis is an evolutionarily conserved innate immune mechanism for eliminating unnecessary cells. Previous studies showed an increase in the level of engulfment receptors in phagocytes after the phagocytosis of apoptotic cells, which leads to the enhancement of their phagocytic activity. However, precise mechanisms underlying this phenomenon require further clarification. We found that the pre-incubation of a Drosophila phagocyte cell line with the fragments of apoptotic cells enhanced the subsequent phagocytosis of apoptotic cells, accompanied by an augmented expression of the engulfment receptors Draper and integrin αPS3. The DNA-binding activity of the transcription repressor Tailless was transiently raised in those phagocytes, depending on two partially overlapping signal-transduction pathways for the induction of phagocytosis as well as the occurrence of engulfment. The RNAi knockdown of tailless in phagocytes abrogated the enhancement of both phagocytosis and engulfment receptor expression. Furthermore, the hemocyte-specific RNAi of tailless reduced apoptotic cell clearance in Drosophila embryos. Taken together, we propose the following mechanism for the activation of Drosophila phagocytes after an encounter with apoptotic cells: two partially overlapping signal-transduction pathways for phagocytosis are initiated; transcription repressor Tailless is activated; expression of engulfment receptors is stimulated; and phagocytic activity is enhanced. This phenomenon most likely ensures the phagocytic elimination of apoptotic cells by stimulated phagocytes and is thus considered as a mechanism to prime phagocytes in innate immunity.

Highlights

Weavers et al [22] demonstrated that hemocytes in the fruit fly Drosophila melanogaster, equivalent to mammalian macrophages, acquired greater migratory activity toward injured area and phagocytic activity against Escherichia coli, due to an elevated mRNA level of a gene coding for receptor named Draper, apparently after the engulfment of apoptotic cells
By conducting biochemical and genetic experiments, we found that an encounter with apoptotic cells enhances the phagocytosing activity against apoptotic cells in Drosophila phagocytes through an increase in the expression of genes coding for the engulfment receptors Draper and integrin ␣PS3
A recent study demonstrated that the phagocytic activity of hemocytes in Drosophila embryos is enhanced after the engulfment of apoptotic cells through increased expression of Draper, an engulfment receptor of Drosophila [22]

Summary

Results

Drosophila possesses three types of blood cells or hemocytes: plasmatocytes, crystal cells, and lamellocytes. We tested a possible change of their expression in phagocytes during stimulation and found that the mRNA and protein level expression of Draper and integrin ␣PS3, but not integrin ␤␯, increased in l(2)mbn cells after incubation with apoptotic cell fragments (Fig. 1C). These results suggested that the phagocytic activity of hemocytes against apoptotic cells is enhanced when they encounter apoptotic cells, which may be attributed to the elevated levels of the expression of genes coding for engulfment receptors. Transcription factors that control the transcription of the top 50 up-regulated Drosophila genes in stimulated phagocytes were bibliographically searched, and 12 factors found are listed

The abbreviations used are

Discussion

Experimental procedures