Trp-P-1, a carcinogenic heterocyclic amine, inhibits lipopolysaccharide-induced maturation and activation of human dendritic cells

Carcinogens frequently provoke immunosuppressive effects thereby allowing cancer cells to persist in the host. 3-Amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) is a carcinogenic heterocyclic amine that is abundantly produced by overcooking meat and fish. Here, we investigated the effect of Trp-P-1 on dendritic cells (DCs), which play a central role in the appropriate activation of the host immune system. When human monocyte-derived DCs were stimulated with lipopolysaccharide (LPS), the DCs became mature with an increase in the expression of co-stimulatory receptors such as CD80, CD86, and MHC molecules and a decrease in phagocytic capacity. Trp-P-1 inhibited all of these phenomena under the same conditions. In addition, Trp-P-1 inhibited production of the cytokines TNF-α and IL-12 in LPS-stimulated DCs. Furthermore, DCs that were pre-exposed to Trp-P-1 were less efficient in inducing activation and proliferation of autologous T cells than control DCs. Trp-P-1 also attenuated the ability of DCs to directly kill T-cell lymphoma Jurkat cells. Mechanism studies showed that Trp-P-1 did not inhibit LPS-binding to Toll-like receptor 4 but interfered with the signaling pathways mediated through p38 kinase. In conclusion, our results suggest that Trp-P-1 is immunosuppressive by inhibiting the functionality of DCs that play an essential role in the appropriate induction of anti-cancer immune responses.