Toll-like receptors, chemokine receptors and death receptor ligands responses in SARS coronavirus infected human monocyte derived dendritic cells

Abstract

Background: The SARS outbreak in 2003 provides a unique opportunity for the study of human responses to a novel virus. We have previously reported that dendritic cells (DCs) might be involved in the immune escape mechanisms for SARS-CoV. In this study, we focussed on the gene expression of toll-like receptors (TLRs), chemokine receptors (CCRs) and death receptor ligands in SARS-CoV infected DCs. We also compared adult and cord blood (CB) DCs to find a possible explanation for the age-dependent severity of SARS.

Results: Our results demonstrates that SARS-CoV did not modulate TLR-1 to TLR-10 gene expression but significantly induced the expression of CCR-1, CCR-3, and CCR-5. There was also strong induction of TNF-related apoptosis-inducing ligand (TRAIL), but not Fas ligand gene expression in SARS-CoV infected DCs. Interestingly, the expressions of most genes studied were higher in CB DCs than adult DCs.

Conclusion: The upregulation of chemokines and CCRs may facilitate DC migration from the infection site to the lymph nodes, whereas the increase of TRAIL may induce lymphocyte apoptosis. These findings may explain the increased lung infiltrations and lymphoid depletion in SARS patients. Further explorations of the biological significance of these findings are warranted.

Figure 1

Extracellular Toll-like receptors gene expression in SARS-CoV infected human immature DCs by quantitative RT-PCR. Toll-like receptor (TLR)-1, TLR-2, TLR-4, TLR-5, TLR-6 and TLR-10 primarily expressed on cell surface. Their mRNA concentrations in adult (a) and CB (b) immature DCs were assayed at 3 h and 9 h after infection with SARS-CoV (MOI = 1). Mock infected cells were included as negative control. The concentrations were normalised to those of β-actin mRNA in the corresponding sample. In SARS-CoV infected adult DCs, significant upregulation of TLR-1 and TLR-2 at 3 h after infection was detected. The basal levels of extracellular TLRs gene expressions in CB DCs were high and no significant upregulation was detected after infection with SARS-CoV. Data are shown as mean ± SEM (adult n = 7; CB n = 5). The expression of TLR-10 was <1 copy per 10⁴ β-actin genes (Data not shown).

Figure 2

Intracellular Toll-like receptors gene expression in SARS-CoV infected human immature DCs by quantitative RT-PCR. Toll-like receptor (TLR)-3, TLR-7, TLR-8 and TLR-9 primarily expressed intracellularly. Their mRNA concentrations in adult (a) and CB (b) immature DCs were assayed at 3 h and 9 h after infection with SARS-CoV (MOI = 1). Mock infected cells were included as negative control. The concentrations were normalised to those of β-actin mRNA in the corresponding sample. In SARS-CoV infected adult DCs, low but significant upregulation of TLR-7 was detected at 9 h after infection. The basal levels of intracellular TLR-8 gene expression in CB DCs was very high and no significant upregulation was detected after infection with SARS-CoV. Data are shown as mean ± SEM (adult n = 7; CB n = 5).

Figure 3

Chemokine receptors gene expression in SARS-CoV infected human immature DCs by quantitative RT-PCR. Chemokine receptors, CCR-1, CCR-3, CCR-5, CCR-7, mRNA concentrations in adult (a) and CB (b) immature DCs were assayed at 3 h and 9 h after infection with SARS-CoV (MOI = 1). Mock infected cells were included as negative control. The concentrations were normalised to those of β-actin mRNA in the corresponding sample. In SARS-CoV infected adult DCs, significant upregulations of CCR-1, CCR-3 and CCR-5 were detected. In the CBDCs, only upregulation of CCR-3 was detected. Data are shown as mean ± SEM (adult n = 7; CB n = 5).

Figure 4

Death Receptor Ligand gene expression in SARS-CoV infected human immature DCs by quantitative RT-PCR. Death receptor ligands, FasL and TRAIL, mRNA concentrations in adult (a) and CB (b) immature DCs were assayed at 3 h and 9 h after infection with SARS-CoV (MOI = 1). Mock infected cells were included as negative control. The concentrations were normalised to those of β-actin mRNA in the corresponding sample. In both SARS-CoV infected adult DCs and CB DCs, significant upregulation of FasL mRNA was not detected. However, there was significant upregulation of TRAIL gene expression. Data are shown as mean ± SEM (adult n = 7; CB n = 5).