The Bacterial Product Violacein Exerts an Immunostimulatory Effect Via TLR8

Sci Rep. 2019 Sep 20;9(1):13661. doi: 10.1038/s41598-019-50038-x.


Violacein, an indole-derived, purple-colored natural pigment isolated from Chromobacterium violaceum has shown multiple biological activities. In this work, we studied the effect of violacein in different immune cell lines, namely THP-1, MonoMac 6, ANA-1, Raw 264.7 cells, as well as in human peripheral blood mononuclear cells (PBMCs). A stimulation of TNF-α production was observed in murine macrophages (ANA-1 and Raw 264.7), and in PBMCs, IL-6 and IL-1β secretion was detected. We obtained evidence of the molecular mechanism of activation by determining the mRNA expression pattern upon treatment with violacein in Raw 264.7 cells. Incubation with violacein caused activation of pathways related with an immune and inflammatory response. Our data utilizing TLR-transfected HEK-293 cells indicate that violacein activates the human TLR8 (hTLR8) receptor signaling pathway and not human TLR7 (hTLR7). Furthermore, we found that the immunostimulatory effect of violacein in PBMCs could be suppressed by the specific hTLR8 antagonist, CU-CPT9a. Finally, we studied the interaction of hTLR8 with violacein in silico and obtained evidence that violacein could bind to hTLR8 in a similar fashion to imidazoquinoline compounds. Therefore, our results indicate that violacein may have some potential in contributing to future immune therapy strategies.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Chromobacterium / chemistry*
  • Gene Expression Regulation / drug effects
  • HEK293 Cells
  • Humans
  • Indoles / chemistry
  • Indoles / pharmacology*
  • Interleukin-1beta / metabolism
  • Interleukin-6 / metabolism
  • Leukocytes, Mononuclear / drug effects
  • Leukocytes, Mononuclear / immunology*
  • Mice
  • Models, Molecular
  • Protein Conformation
  • RAW 264.7 Cells
  • Signal Transduction / drug effects
  • THP-1 Cells
  • Toll-Like Receptor 8 / chemistry
  • Toll-Like Receptor 8 / metabolism*


  • Indoles
  • Interleukin-1beta
  • Interleukin-6
  • TLR8 protein, human
  • TLR8 protein, mouse
  • Toll-Like Receptor 8
  • violacein

Supplementary concepts

  • Chromobacterium violaceum