Apoptosis inhibition by intracellular bacteria and its consequence on host immunity

Molecular cloning and functional characterization of MYC transcription factor in pathogen-challenged Apostichopus japonicus

Myelocytomatosis viral oncogene (MYC), a transcription factor in the MYC family, plays vital roles in vertebrate innate immunity by regulating related immune gene expressions. In this study, we cloned and characterized an MYC gene from sea cucumber Apostichopus japonicus via RNA-seq and RACE approaches (designated as AjMYC). A 2074 bp fragment representing the full-length cDNA of AjMYC was obtained. This gene includes an open reading frame (ORF) of 1296 bp encoding a polypeptide of 432 amino acid residues with the molecular weight of 48.85 kDa and theoretical pI of 7.22. SMART analysis indicated that AjMYC shares an MYC common HLH motif (354-406 aa) at the C-terminal. Spatial expression analysis revealed that AjMYC is constitutively expressed in all detected tissues with peak expression in the tentacle. Vibrio splendidus-challenged sea cucumber could significantly boost the expression of AjMYC transcripts by a 5.58-fold increase in the first stage. Similarly, 2.75- and 3.23-fold increases were detected in LPS-exposed coelomocytes at 1 and 24 h, respectively. In this condition, coelomocyte apoptotic rate increased from 11.98% to 56.23% at 1 h and to 59.08% at 24 h. MYC inhibitor treatment could not only inhibit the expression of AjMYC and Ajcaspase3, but also depress the coelomocyte apoptosis. Furthermore, AjMYC overexpression in EPC cells for 24 h also promoted the cell apoptosis rate from 21.31% to 45.85%. Collectively, all these results suggested that AjMYC is an important immune factor in coelomocyte apoptosis toward pathogen-challenged sea cucumber.

The intracellular pathogen concept

The intracellular pathogen concept classifies pathogenic microbes on the basis of their site of replication and dependence on host cells. This concept played a fundamental role in establishing the field of cellular microbiology, founded in part by Dr. Pascale Cossart, whose seminal contributions are honored in this issue of Molecular Microbiology. The recognition that microbes can access and replicate in privileged compartments within host cells has led to many new and fruitful lines of investigation into the biology of the cell and mechanisms of cell-mediated immunity. However, like any scientific concept, the intracellular pathogen concept can become a dogma that constrains thinking and oversimplifies complex and dynamic host-pathogen interactions. Growing evidence has blurred the distinction between "intracellular" and "extracellular" pathogens and demonstrated that many pathogens can exist both within and outside of cells. Although the intracellular pathogen concept remains useful, it should not be viewed as a rigid classification of pathogenic microbes, which exhibit remarkable variation and complexity in their behavior in the host.