Figure 1.
(A) Schematic illustration of in vitro experimental studies of LPS-induced tolerance and priming effect in macrophages. (B) IL-6 mRNA levels of murine bone marrow derived macrophages treated with various combinations of LPS. * p<0.05. (C) Abstraction of the parallel LPS associated pathways into a three-node network motif and the corresponding mathematical model based on ordinary differential equations. Refer to Materials and Methods for details.
Table 1.
Description of modeling parameters.
Figure 2.
Three priming mechanisms revealed by time-course patterns.
(A) Definition of clustering axis Δx1 and Δx2. Δx1 refers to the maximum difference between x1 during the LD priming stage and the steady state value of x1 in the absence of any stimulus. Δx2 refers to the difference between the maximum values of x2 during the HD period with and without priming pretreatment. (B) The time courses of the priming data sets naturally divide into three clusters, corresponding to three priming mechanisms. The pie chart shows the relative frequencies of the priming mechanisms among all the priming parameter sets.
Figure 3.
Details of the three priming mechanisms.
(A) Backbone motifs (topological features shared by most of the good parameter sets) of each priming mechanism (see Figure S3 and Text S1 for details). The width of a line is proportional to the mean value of the corresponding ωji among data sets under each priming mechanism. The “slow” and “fast” time scales reflect the values of γj in comparison to γ3 = 1. (B–D) Typical time courses and corresponding phase space trajectories with or without LD pretreatment. Bistable results for AI and SD are shown in Figure S5.
Figure 4.
Analysis of the robust priming topologies in the SD mechanism.
(A) 139 unique topologies under SD mechanism sorted by topology density (see Figure S6 and Text S1 for detailed discussion). (B) The highest seven density topologies and the backbone motifs. Line widths are proportional to the mean value of samples of the corresponding topology. Dashed lines denote the additional link present in the top topologies but absent in the backbone motif. (C) Combination of the two backbone motifs is common in the SD data sets. 93% of SD data sets are found to contain either Motif I or Motif II as the backbone motif. Among them, 64% contain both Motif I and Motif II.
Figure 5.
Analysis of the tolerance data sets.
(A) The unique topologies generating a tolerance effect sorted by topology density. (B) Typical time courses shown with normal (left panel) or elongated (right panel) gap period between the two doses. Solid line: time course tracking the dynamics of the system under the first HD stimulation, in gap period and under a second HD stimulation. Dashed line: time course tracking the dynamics under a single HD treatment; in this case the system is treated with no LPS during the otherwise first HD period. (C) Distribution of the change of x1 level due to the initial HD stimulation reveals two mechanisms to achieve slow relaxation dynamics in the inhibitor (left panel) and the corresponding two backbone motif (right panel).
Figure 6.
Phase diagrams for priming and tolerance in a typical network motif.
(A) Regions of dosing conditions for tolerance and priming are well separated. (B) Both priming and tolerance effects are affected by the duration of two sequential treatments (with the gap period between two doses being fixed). (C) Priming and tolerance are also affected by the duration of the gap between two doses. Very long gaps fail to exhibit either priming or tolerance.
Figure 7.
Schematic illustration of constructive (PS) and destructive (AI, SD) pathway interference leading to priming effect.
PS results from the activation of the LD-responsive pathway (x2) which cooperates with the other HD-responsive pathway (x1) to boost cytokine expression in response to the following HD stimulus. AI results from activating a LD-responsive pathway (x2), which cancels the inhibition coming from the other HD-responsive inhibitor (x1) during the HD stage. SD results from deactivating a constitutively expressed suppressor (x1) during the priming stage. Red line with arrow head: activation pathway. Blue line with bar head: inhibition pathway. Line width denotes strength of the pathway controlling the downstream cytokine expression.
Table 2.
Experimental evidence supporting the proposed tolerance mechanism.
Figure 8.
Example regulatory networks supporting the priming mechanisms.
(A) The AI mechanism is consistent with observed intra- and inter-cellular molecular mechanisms for LPS priming, based on counterbalanced IL-10 and IL-12 signaling [19]. (B) The PS mechanism inspires this predicted intracellular molecular mechanism based on the selective activation of C/EBPδ by LD LPS. (C) IFN-γ self-priming and cross-priming to LPS follows the AI and PS mechanisms. Network details are retrieved from the database IPA (@Ingenuity) as well as the experimental literature listed in Table S3. Dashed lines refer to indirect regulations involving autocrine signaling loops.