Peer review of: Cosmc controls B cell homing (2020)


RR2

REVIEWERS' COMMENTS:
Reviewer #1 (Remarks to the Author):
  In the present manuscript by Zeng et al., a novel role for the T-synthase chaperone Cosmc in B cell
 development and homing is demonstrated.
 The revision of the manuscript has led to a clear improvement in quality. The authors have added
 experiments on the characterization of the B cell compartment in mice with a B cell-specific Cosmc
 deletion, along with other additional experiments, which altogether make the work more complete.
I only have a few comments:
  1. The authors state that Cosmc mainly affects B cell homing to lymph nodes. However, in Fig. 2 of
 the revised paper they clearly demonstrate that also B cell development in the bone marrow is
 affected by Cosmc deletion. This should be emphasized and carefully considered in the
 interpretation/discussion, as it may also add to the homing effect observed. Along that line, the
 authors should consider choosing a different title. The current title doesn ́t reflect important findings
 shown in Fig. 2.
 2. What is the time frame of the intravital movies? Which time span was recorded? Can the authors
 add a time stamp to the movies?
 Along that line, what is the size of the field of view in the intravital movies? The authors should add
 some kind of reference, such as a scale bar. A movie legend is missing and should be added.
 Typos:
 L 94 B cell not B cells
 L 95 block instead of blocks
 Reviewer #2 (Remarks to the Author):
The authors have followed the suggestions and significantly improved the manuscript.
Reviewer #3 (Remarks to the Author):
   After having carefully studied the revised manuscript I come to the following conclusions:
 1) Contrary to common practice, the authors do not provide a manuscript in which the revised
 passages are marked in change-track mode. It is therefore rather difficult to identify those changes
 that have been have been made to the text and to figures.
 2) One of my key points when reviewing the initial manuscript was that the study was rather
 preliminary and that further investigations should done. In particular I suggested investigating into
 more detail the mechanisms underlying impaired homing of B cells via HEVs. Unfortunately this was
 not done. In their rebuttal letter, the authors write that such studies are currently not possible and
 that new experimental procedures need to be established. This is very surprising, however, as John
 Kehrl's group used multiphoton microscopy in a manuscript recently published in iScience to study in
 detail homing of lymphocytes via HEV (doi: 10.1016/j.isci.2019.05.040). That study actually provides
 a blueprint of the experiments needed here to gain further insights at what stage of their journey
 through HEV Cosmic-deficient B cell get stuck and how their behavior differs from that of their wild

 type counterpart.
 3) Was ref 14 (Cutler et al) already mentioned in the first version of the manuscript? I do not
 remember that the authors had adequately addressed impaired homing of cosmc-/- T cells there.
 Anyway! In Cutler et, it was shown that T cell-specific deletion of cosmic leads to impaired homing of
 T cells. In the present manuscript analogous results are shown for B cells without appropriately
 elucidating the underlying mechanisms. The authors speculate that the function of chemokine
 receptors may be impaired. However, only transwell assays were performed but no further analysis is
 shown.
 In summary, I maintain my opinion that the B cell-specific cosmc-/- mice show an interesting homing
 phenotype, but the results presented here are too preliminary for possible publication in Nature
 Communications.