Mast Cell Tryptase and Implications for SARS-CoV-2 Pathogenesis

Round 1

Reviewer 1 Report

Serine proteases tryptase and chymase are released from mast cells following degranulation, and are widely known for their role in allergies. Many viruses can stimulate mast cells directly or indirectly, with tryptase and chymase contributing to inflammation following infection. This review by Karimi and Morovati et al. details the biology of tryptase and chymase, as well as their impacts during viral infection. The importance of mast cell tryptase is highlighted when discussing the role of tryptase in COVID-19 infection and tryptase antagonists as therapeutics that could help manage COVID-19.

Strengths:

1. The review has a good balance of discussing the typical biological function of tryptase and chymase in enough detail before pivoting to the effects of tryptase and chymase stimulation in viral infection.
2. The “drug therapies” section is clear and thoughtful, and I appreciate the subsection headings that cleanly delineate categories of tryptase-related therapies.

Introduction:

1. On the 5^th line of page two, it is stated that a Th1 phenotype is “not ideal”, but then says it is “optimal against viral infections” in the same sentence. Based on the next sentence about Th2-skewed responses, it seems like the authors intended to say that the Th1 phenotype was ideal, otherwise the statements in those two sentences appear contradictory.

Section 2:

1. 1^st paragraph, lines 2-3: This first sentence includes “display distinct characteristics serve unique roles”, which could be missing an “and” or was intended to be one phrase or the other.
2. 2^nd paragraph, line 7: The MRGPRX2 receptor is mentioned here only by its abbreviation, but it would be helpful to use the full name of receptor here instead of later in section 5. Also, there is no citation for the end of this sentence, and it would be helpful to add ref. 13, making clear that this statement is supported.
3. 2^nd paragraph, last line: In the last sentence, is it mentioned that mast cell heterogeneity can prolong or dampen immune responses during infections. Does “infections” only refer to viral infections? Also, it may be intuitive to readers how mast cell function may prolong or stimulate immune responses through degranulation, but dampening immune responses may be less clear. Does a dampened immune response stem from reduced mast cell degranulation, or from some other mechanism? If possible, it would be helpful to add more detail on how heterogeneity in mast cells may result in heterogeneity of immune responses.
4. 3^rd paragraph, lines 4-12: Several sentences about mast cell differences based on location lack citations. These examples appear to be supported by ref. 3 in a subsequent sentence, but it would help to also include this reference in the first sentence of this paragraph to make clear that this block of text is supported by this reference.
5. 3^rd paragraph, lines 13-15: This sentence states that “increases in the numbers” of mast cells leads to promotion of degranulation and augmentation of protease levels. Does this mean that mast cell activation increases simply because more mast cells are present, or does an increase in mast cell number induce an increased rate of activation? It would be helpful to clarify this statement about increased mast cell numbers and their impact in several diseases.

Section 3:

1. Last line of section 3: The authors mention that excessive degranulation can lead to “keloid and hypertrophic scars”, but there is no citation for this statement. It may also help to briefly describe these symptoms to contextualize the impact of these scars on patients.
2. As a general comment, the discussion of tryptase and chymase in this section is informative. However, this review leaves the impression that tryptase and chymase are uniform proteins in mast cells, even though serine proteases in mast cells appear to contain several tryptase and chymase enzymes, such as ⍺-Tryptase and β-Tryptase (for example, see George H. Caughey, Immunol Rev. 2007). There is also no discussion of tetrameric vs. monomeric tryptase and how this change may play a role in typical serine protease function, which would be useful to establish even though tetrameric tryptase is referenced in section 6. Since mast cell heterogeneity is covered in section 2, an introduction on serine protease heterogeneity and different enzymes would complement that section nicely. It should also be noted if the tryptase and chymase effects described in this section refer to specific serine proteases rather than all mast cell tryptases or chymases.

Section 4:

1. 1^st paragraph, lines 1-3: DENV etc. are explicitly referred to as arboviruses in the second sentence, but only implicitly as flaviviruses in the first sentence. Is there a way to restructure or combine these sentences to make clear that both descriptions refer to these viruses?
2. 5^th paragraph, lines 1-2: The names of most viruses are listed and given abbreviations, but HCV and HIV-1 are shown just as abbreviations. It would be helpful to add the names for these viruses to keep things consistent.
3. As a general comment, breaking up the information of this section into subsections (as with section 6). Any additional structure would better orient readers in this section, which covers a variety of viruses and tryptase/chymase effects.

Section 5:

1. 2^nd paragraph, lines 1-3: This sentence mentions the potential cause of COVID-19 brain fog, but no citation is included and should be added for this important point.

Section 6:

1. The “MC Stabilizers” section is very brief, and mainly refers to a previous review published by the authors. However, if this type of drug therapy is included as a subsection and is a type of tryptase inhibitor worth mentioning, a brief description of this type is warranted. This description would improve the manuscript because readers will appreciate understanding the basic concept of MC Stabilizers and will know that they can read the previous review to learn more.
2. Canonical inhibitors subsection, 1^st paragraph, lines 3-7: APC-2059 and RWJ-56423 are briefly mentioned as inhibitors, but unlike APC-366 their formula and method of inhibition is not explained. If these inhibitors have similar chemical formulas and/or use a similar mechanism of action (i.e. binding with an arginine residue), then it should be stated. Otherwise, brief explanations of these compounds and their mechanisms are warranted if available.
3. Tryptase-specific antibodies subsection: The effects of anti-tryptase antibody binding on tryptase are clearly stated, but the mechanism of inhibition (allosteric noncompetitive) is only indicated for the 31A.v11 antibody, even though that antibody, B12, and MTPS9579A have similar effects on tetrameric tryptase. Is the structural basis of tryptase inhibition by B12 or MTPS9579A known? If so, would they also be described as “allosteric noncompetitive” like 31A.v11?
4. Figure 1: Overall, this figure could be improved by increasing the text size of all labels to make them more legible for readers. This change would be especially helpful for the text corresponding to numbers 3 and 4, which are important summaries of tryptase effects and inhibitors. If the list of tryptase effects cannot fit in the lung diagram following text size increase, the authors could shift this list to outside the lung and change the text color to black. On a related note, it would be helpful to add numbers 1-4 to corresponding sentences in the legend that describe these steps.
5. After section 6: The review abruptly ends after section 6, with no discussion or conclusions section. Adding one of these sections would improve the paper by summarizing key research and reiterating main topics, helping to orient the reader on why this research matters and where future research on this topic could or should go. It is also important to address possible future directions and important questions for this active field of research. For instance, the authors could connect their focus on tryptase inhibitors back to management of COVID-19. Have any tryptase inhibitors been tested in the treatment of COVID-19 infection? Are some inhibitor types more promising candidates for COVID-19 treatment than others?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The article presents the implications of the mast cell tryptase outside of allergies, in the context of the SARS-CoV2 infection. The theme is of interest in the current context of ongoing pandemic. I think that the manuscript is well written. No further comments.

Author Response

We would like to thank the reviewer for the careful assessment of our manuscript. We enjoy the suggestions and appreciate the comments.