Percolation Forces in Lung Inflammation: Determining the Path to Emphysema or Fibrosis

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

this is a very interesting, intriguing and complex review on percolation of (in my understanding ) extracellular matrix constituents as a potential pathogenic mechanism to explain differences between emphysema and pulmonary fibrosis development.

three minor observations

1  percolation for me is a method to drip coffee through a membrane/filter: which should be this membrane at pulmonary/alveolar level? please describe in more detail

2 in pulmonary fibrosis epitehlial mesenchymal transition is still considered as the pivotal event in fibrosis development; it is mostly mediated by TGF-beta and makes alveolar epithelium to switch to a fibrogenesis pathway. is there any relationship between percolation and this EMT?

3 are you aware of any pathogenic common denominator (upstream mediator/trigger) of this disregulated percolation? its blocking would restore this percolation to its "physiologic"level and would protect the lung from E or PF development. please briefly elaborate on this aspect 

outstanding elaboration flow -congratulations

Author Response

Response To Reviewer Comments

 

Reviewer 1:

 

1. The concept of percolation is described in greater detail, and a more robust mathematical formulation is included in a separate section (pp 2, 3, 5-9).

 

2. The relationship between percolation and EMT is discussed in the revised manuscript (pp 17, 18).

 

3. The common pathogenic denominator may be the development of aberrant equilibria that resist the return to a normal, healthy state. The principles of game theory may provide a novel approach to treating pulmonary emphysema and interstitial fibrosis by incorporating a therapeutic strategy that makes it more advantageous to restore normal equilibria. This hypothesis is discussed on p 21 and is accompanied by a new figure illustrating this process (Figure 12).

Reviewer 2 Report

Comments and Suggestions for Authors

This review by Jerome Cantor explores the diverse consequences of inflammatory insults in the lung focusing on inflammatory stimuli that results in predominantly lung destruction (emphysema) in contrast to those that results in predominantly lung scarring/fibrosis. The author speculate that the percolation theory/forces are underlying this diverse lung tissue response frequently caused by the same insult. The author implies that transitions in the process of extracellular matrix crosslinking my underly this duality in lung responses. The authors then use this basic concept to suggest future approaches to management or therapeutic intervention.

This is an interesting and novel concept in understanding inflammatory responses in the lung

Comments:

• This is an interesting and novel way to explain the potential mechanism/s why an inflammatory stimulus such as cigarette smoking can results in either emphysema or interstitial lung fibrosis (ILD). It also suggests a potential mechanism why these two conditions sometimes are progressive in nature notwithstanding that the original insult has been removed (percolation threshold). The forces/stimuli or enzymes that promote crosslinking and stimuli that decrease crosslinking are frequently the same.

The author does not elaborate why that is and that fit into his theory of percolation in the lung. Please comment

• Furthermore, as that author mention, one frequently sees subjects with both emphysema and ILD in the same lung. What results in this regional percolation forces? Are there evidence that crosslinking enzymes are differentially expressed in the lung?  
• This reviewer think that figure 2&3 are most like not necessary, the author explain the concept of high and low crosslinking well in the text
• Clearly the hypothesis that the percolation forces are responsible for this duality in responses apply just to certain inflammatory insults in the lung predominantly cigarette smoking (stimuli such as radiation or drug such as bleomycin give just fibrosis and not emphysema). This needs to be more clearly stated.
• The section on “what drives these differences in crosslinking” resulting in either emphysema or ILD are short and cryptic specially the section of the impact of mediators and immune cells on this process. This area is most likely the key area for therapeutic intervention. Please expand and elaborate.
• Figure 9 is confusing for this reviewer. Increase in percolating forces will cause increase clustering and fibrosis and the reverse for emphysema. The percolation theory say:“  describes the behavior of a network when nodes or links are added. This is a geometric type of phase transition, since at a critical fraction of addition the network of small, disconnected clusters merge into significantly larger clusters in a non-linear manner”. This is increase percolation/crosslinking that cause fibrosis and visa versa. Please comment or correct

Please clarify

Author Response

Response To Reviewer Comments

 

Reviewer 2:

 

1. The reasons for the dichotomous activity of stimuli such as cigarette smoke may depend on the spatial heterogeneity of the inflammatory response, related to varying concentration of injurious agents, differences in cell populations such as M1 and M2 macrophages, and localized responses to toxic agents. This explanation is discussed in the revised manuscript (pp 16, 17).

 

2. Figure 2 and 3 have been replaced by more detailed mechanistic illustrations of the percolation processes in pulmonary emphysema and interstitial fibrosis.

 

3. The predominance of fibrosis in certain types of lung injury may be related to the effects of multiscale processes that rapidly shift the balance toward percolation connectivity, including distinct temporal and spatial patterns of mechanical forces. This mechanism may reduce the role of percolation processes as the disease progresses. A fuller explanation of these limitations is provided in the revised manuscript (p 9). Also, as noted in the Introduction, cadmium-chloride induced pulmonary fibrosis, can be transformed into an emphysematous process by treatment with a crosslink inhibitor, emphasizing the importance of very early percolation processes in determining disease outcomes [ref 7]. The same phenomenon may be applicable to bleomycin and radiation induced pulmonary fibrosis.

 

4. The different inflammatory mediators that are active in either pulmonary emphysema or interstitial fibrosis are discussed in greater detail (p 17).

 

5. Figure 9 has been replaced with two new ones that better illustrate the percolation forces in pulmonary emphysema and interstitial fibrosis (Figures 2, 3).

 

Reviewer 3 Report

Comments and Suggestions for Authors

This is a concept review article that proposes that chronic lung inflammation can lead to two opposite endpoints: pulmonary emphysema vs interstitial lung fibrosis. The author argues that this divergence is due to the critical threshold (percolation) transition within the lung extracellular matrix (ECM). The percolation theory states that if a particular critical threshold is crossed, then even small changes near critical points can have disproportionate effects on system-wide properties. Emphysema is framed as “downward percolation”, in which inflammation, protease-antiprotease imbalance and oxidative stress drive ECM damage. In contrast, interstitial fibrosis is presented as a “rigidity percolation” , whereby alveolar epithelial injury promotes myofibroblast activation, TGF-β signalling, LOX/LOL2L and TG2 activity and ultimately collagen accumulation.

Overall, this is an interesting and thoughtful manuscript. However, several areas should be strengthened:

1. The manuscript reads as a narrative, hypothesis-generating perspective, but this is not clearly indicated in the title. Therefore, the article type should be specified in the title
2. Crosslinking is extensively discussed in both emphysema and fibrosis, but it should be explained and discussed separately by type: elastin crosslinks vs collagen crosslinks vs inter-network crosslinks.
3. The percolation type in fibrosis requires input from genetic and epithelial biology drivers. Key examples include the MUC5B promoter variant and telomere biology.
4. In the mechanotransduction discussion, major biology players, particularly YAP/TAZ and αvβ6 integrin–mediated activation of latent TGF-β should be discussed in more detail.
5. In emphysema, protease–antiprotease imbalance, and neutrophil elastase deserve more discussion.
6. Markers of elastin crosslink breakdown, such as desmosine and isodesmosine, as well as collagen crosslinks such as  pyridinoline types and AGE crosslinks such as pentosidine and glucosepane can be quantified. The manuscript should expand on those measurable biomarkers.
7. The literature on RAGE in COPD and emphysema deserves further development.
8. MR elastography should be discussed in greater detail.

 

 

Author Response

Response To Reviewer Comments

 

Reviewer 3:

 

1. The term “paper” to describe the manuscript has been changed to “review” (Abstract and p 3).

 

2. The different types of crosslinks in collagen and elastin are discussed on pp 9 and 10, and their synthesis is illustrated in Figures 6 and 7. Non-enzymatic crosslinks which can form random extracellular matrix crosslinks are described on pp 11 an 12.

 

3. A new section on genetic factors involved in pulmonary emphysema and interstitial fibrosis is included in the revised manuscript (p 12).

 

4. The role of YAP/TAZ and integrins in mechanotransduction is described in greater detail (p 18).

 

5. The role of proteases, specifically neutrophil elastase and metalloproteases, is discussed more fully in the revised manuscript (p 17).

 

6. The use of biomarkers to distinguish between pulmonary emphysema and interstitial fibrosis is described in further detail (pp 18, 19). Furthermore, the use of the desmosine biomarker in a clinical trial of a novel COPD treatment is included in the revised manuscript (p 19).

 

7. A discussion of the role of AGE crosslinking and RAGE in pulmonary fibrosis is included in the revised manuscript (pp 11, 12).

 

8. MR elastography is described in greater detail (pp 18, 19).

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The revised manuscript has satisfactorily addressed my major concerns.