Melanoma inhibits macrophage activation by suppressing toll-like receptor 4 signaling.
BACKGROUND
Activated macrophages defend against tumors by secreting cytokines to recruit secondary immune cells, presenting antigen to T cells, and by direct tumor cytotoxicity. Peritoneal macrophages harvested from melanoma-bearing mice are less cytotoxic to melanoma cells, and produce less superoxide, nitric oxide, and tumor necrosis factor-alpha (TNF-alpha) than those from nontumor-bearing mice. Similar impairment of macrophage activation occurs in vitro using media harvested from cultured melanoma cells. Stimulation of Toll-like receptor 4 (TLR-4) activates macrophages and results in the release of TNF-alpha. We hypothesized that melanoma inhibits macrophage activation by suppressing TLR-4 signaling.


METHODS
Melanoma conditioned media (MCM) was generated from B16 melanoma cells. Peritoneal macrophages from TLR-4 competent or TLR-4 incompetent mice were exposed to control or MCM for 24 hours; then stimulated with lipopolysaccharide. TNF-alpha secretion, TNF-alpha mRNA production, nuclear factor-kappaB (NF-kappaB) activation, and TLR-4 surface expression were measured.


RESULTS
Peritoneal macrophages exposed to MCM produced considerably less TNF-alpha in response to stimulus than controls (691 pg/mL versus 2,066 pg/mL, p < 0.001). TNF-alpha production by TLR-4 incompetent macrophages was not affected by MCM (454 pg/mL versus 480 pg/mL). Stimulated TNF-alpha mRNA and activated NF-kappaB were decreased in MCM treated C57BL/6 macrophages (by 38% and 33%, respectively). TLR-4 surface expression, however, was not decreased by exposure to MCM.


CONCLUSIONS
Melanoma inhibits macrophage activation by suppressing TLR-4 signaling downstream of the TLR-4 receptor.
Clarke
Jason H
JH
Department of Surgery, University of Colorado Health Sciences Center, Denver, CO, USA.
Cha
John Y
JY
Walsh
Mark D
MD
Gamboni-Robertson
Fabia
F
Banerjee
Anirban
A
Reznikov
Leonid L
LL
Dinarello
Charles A
CA
Harken
Alden H
AH
McCarter
Martin D
MD
D000818:Animals; D003470:Culture Media; D008262:Macrophage Activation; D017737:Macrophages, Peritoneal / Q000276:immunology / Q000378:metabolism; D008297:Male; D008546:Melanoma, Experimental / Q000276:immunology*; D008562:Membrane Glycoproteins / Q000276:immunology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008815:Mice, Inbred Strains; D016328:NF-kappa B / Q000494:pharmacology; D012333:RNA, Messenger / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000276:immunology*; D015398:Signal Transduction / Q000502:physiology; D051197:Toll-Like Receptor 4; D051193:Toll-Like Receptors; D014409:Tumor Necrosis Factor-alpha / Q000096:biosynthesis
D003470:Culture Media; D008562:Membrane Glycoproteins; D016328:NF-kappa B; D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D051197:Toll-Like Receptor 4; D051193:Toll-Like Receptors; D014409:Tumor Necrosis Factor-alpha