The innate immune response to ischemic injury: a multiscale modeling perspective

Abstract

Background: Cell death as a result of ischemic injury triggers powerful mechanisms regulated by germline-encoded Pattern Recognition Receptors (PRRs) with shared specificity that recognize invading pathogens and endogenous ligands released from dying cells, and as such are essential to human health. Alternatively, dysregulation of these mechanisms contributes to extreme inflammation, deleterious tissue damage and impaired healing in various diseases. The Toll-like receptors (TLRs) are a prototypical family of PRRs that may be powerful anti-inflammatory targets if agents can be designed that antagonize their harmful effects while preserving host defense functions. This requires an understanding of the complex interactions and consequences of targeting the TLR-mediated pathways as well as technologies to analyze and interpret these, which will then allow the simulation of perturbations targeting specific pathway components, predict potential outcomes and identify safe and effective therapeutic targets.

Results: We constructed a multiscale mathematical model that spans the tissue and intracellular scales, and captures the consequences of targeting various regulatory components of injury-induced TLR4 signal transduction on potential pro-inflammatory or pro-healing outcomes. We applied known interactions to simulate how inactivation of specific regulatory nodes affects dynamics in the context of injury and to predict phenotypes of potential therapeutic interventions. We propose rules to link model behavior to qualitative estimates of pro-inflammatory signal activation, macrophage infiltration, production of reactive oxygen species and resolution. We tested the validity of the model by assessing its ability to reproduce published data not used in its construction.

Conclusions: These studies will enable us to form a conceptual framework focusing on TLR4-mediated ischemic repair to assess potential molecular targets that can be utilized therapeutically to improve efficacy and safety in treating ischemic/inflammatory injury.

Keywords: Boolean network; Inflammation; Ischemic injury; Macrophages; Multiscale dynamic model; TLR4.

Fig. 1

Scheme of the innate immune response to injury. Injury triggers the production of DAMPs in the tissue that activate intracellular responses via TLR4, initially in the resident macrophages and later from recruited macrophages (large gray oval). TLR4 activation stimulates two intracellular pathways, the MyD88-dependent (blue rectangles), resulting in production and secretion of the chemoattractant CCL2 which serves to recruit additional immune cells from the circulation (right). In response to CCL2, M1 monocytes leave the circulation and enter the tissue where they differentiate into pro-inflammatory M1 macrophages that clear toxic debris and become activated to produce more CCL2, perpetuating the inflammatory response. TLR4 can also signal via a MyD88-independent endocytic pathway (center left) that is mediated by CD13, TRIF and IRF3. Increased activation of this pathway can lead to production of cell-damaging ROS and increased DAMPs. Finally, M1 macrophages convert into pro-healing M2 macrophages which dampen the pro-inflammatory response by blocking production of CCL2 and DAMPs, leading to resolution

Fig. 2

Wiring diagram of the model. Injury (orange triangle) has two possible states, 0- absent, and 1- present. The response to injury occurs at two simultaneous scales, the internal cell scale (gray oval) and the extracellular tissue scale. The tissue scale initiates with production of DAMPs (purple circle) with three states, low, medium, high, and the intracellular activation of resident macrophages via the MyD88-dependent (MyD88/IRAK/NF-κB/CCL2) and -independent (CD13/TRIF/IRF3/IFN-β) pathways, resulting in recruitment of additional immune cells from the circulation (M1) and/or production of toxic ROS. The M1 node (black circle) can take on 3 states: 0, absence of macrophage activation, including resting resident macrophages; 1 standard inflammatory response- initial activation of resident macrophages and later, of recruited macrophages; and 2 exaggerated recruitment of pro-inflammatory macrophages in exacerbated injury. As the process continues, M1 macrophages become pro-healing M2 macrophages (purple circle) and dampen the pro-inflammatory response

Fig. 3

Two-scale update scheme for the model. Top, the intracellular variables and their output variables are updated until they reach a steady state, then these values are used to compute the next state of the tissue-level variables (t = 1)

Fig. 4

Flow chart of model. The intracellular model is initiated with the intracellular inputs, M2 and DAMPs, at all possible values according to their assigned number of states and simulated until steady states are reached. These steady state values of CCL2 and ROS comprise the inputs to the tissue level model, which simulation results in 3 steady state outcomes representing resolution, overwhelming inflammation or a low percentage of improbable states as described in the text