Structural variants of Salmonella Typhimurium lipopolysaccharide induce less dimerization of TLR4/MD-2 and reduced pro-inflammatory cytokine production in human monocytes

Gustavo Aldapa-Vega,Mario Adán Moreno-Eutimio,Antonio J Berlanga-Taylor,Alexis P Jiménez-Uribe,Goreti Nieto-Velazquez,Orestes López-Ortega,Ismael Mancilla-Herrera,Enoc Mariano Cortés-Malagón,John S Gunn,Armando Isibasi,Isabel Wong-Baeza,Constantino López-Macías,Rodolfo Pastelin-Palacios

doi:10.1016/j.molimm.2019.03.003

Abstract

Salmonella enterica serovar Typhimurium (S. Typhimurium) changes the structure of its lipopolysaccharide (LPS) in response to the environment. The two main LPS variants found in S. Typhimurium correspond to LPS with a hepta-acylated lipid A (LPS 430) and LPS with modified phosphate groups on its lipid A (LPS 435). We have previously shown that these modified LPS have a lower capacity than wild type (WT) LPS to induce the production of pro-inflammatory cytokines in mice. Nevertheless, it is not know if LPS 430 and LPS 435 could also subvert the innate immune responses in human cells. In this study, we found that LPS 430 and LPS 435 were less efficient than WT LPS to induce the production of pro-inflammatory cytokines by human monocytes, in addition we found a decreased dimerization of the TLR4/MD-2 complex in response to LPS 430, suggesting that structurally modified LPS are sensed differently than WT LPS by this receptor; however, LPS 430 and 435 induced similar activation of the transcription factors NF-κB p65, IRF3, p38 and ERK1/2 than WT LPS. Microarray analysis of LPS 430- and LPS 435-activated monocytes revealed a gene transcription profile with differences only in the expression levels of microRNA genes compared to the profile induced by WT LPS, suggesting that the lipid A modifications present in LPS 430 and LPS 435 have a moderate effect on the activation of the human TLR4/MD-2 complex. Our results are relevant to understand LPS modulation of immune responses and this knowledge could be useful for the development of novel adjuvants and immunomodulators.

Highlights

Lipopolysaccharide (LPS) is the main component of the outer membrane of Gram-negative bacteria; it is a strong activator of the innate immune response and plays an important role in host-pathogen interactions (Needham and Trent, 2013)
The myeloid differentiation primary response 88 (MyD88) pathway is activated at the plasma membrane and leads to p65 phosphorylation and to the nuclear translocation of the transcription factor nuclear factor kappalight-chain-enhancer of activated B cells (NF-κB) (p65/ p50) (Medzhitov and Horng, 2009), while the toll/interleukin-1 receptor-domain-containing adapterinducing interferon-β (TRIF) pathway is activated after CD14-mediated internalization of the toll-like receptor 4 (TLR4)/MD-2/LPS dimer into endosomes, and leads to the nuclear translocation of interferon regulatory factor 3 (IRF3) (Kagan et al, 2008; Zanoni et al, 2011); the MyD88 pathway leads to the activation of MAP kinases, including p38, ERK1 and ERK2, which induce the nuclear translocation of activator protein 1 (AP-1) (Pandey et al, 2014)
Since we found that LPS 430 induced decreased dimerization of the TLR4/MD-2 complex, we investigated if this decreased dimerization was associated with reduced activation of the main transcription factors of the MyD88 and TRIF pathways: NF-κB p65 and IRF3, respectively; we focused on p65 because this is the only NF-κB family member that is associated with the transcription of the tumour necrosis factor (TNF)-α mRNA (Falvo et al, 2010)

Summary

Introduction

Lipopolysaccharide (LPS) is the main component of the outer membrane of Gram-negative bacteria; it is a strong activator of the innate immune response and plays an important role in host-pathogen interactions (Needham and Trent, 2013). Several pathogenic bacteria have the capacity to modify the number of phosphate groups or acyl chains of the lipid A portion of their LPS as part of their immune evasion mechanisms (Steimle et al, 2016). The two phosphate groups of lipid A interact with positively charged amino acid residues in TLR4, MD-2 and TLR4*. These interactions lead to the formation of a dimer that contains two TLR4/MD-2/LPS complexes (Park et al, 2009). After this receptor dimerization, two signalling pathways are activated: the MyD88 pathway and the TRIF pathway. The end result of these signalling pathways is the production of pro-inflammatory cytokines (such as TNF-α, IL-1β and IL-6) and type I interferons (such as IFN-α and IFN-β), respectively (Yang and Seki, 2012)

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Conclusion