Murine classical monocyte subsets display distinct functions and fates
Trzebanski et al. (BioRxiv) DOI: 10.1101/2023.07.29.551083
Dendritic Cell-like Monocytes
Monocytes comprise two main subsets defined as classical Ly6chigh (CM) and non-classical Ly6clow cells (NCM) in mice. However, recent transcriptomic analyses suggest that CM themselves are heterogeneous. In this study, the authors identified surface markers CD177 and CD319 that allow segregation of murine neutrophil-like and dendritic cell-like CM (NeuMo and DCMo, respectively) in the bone marrow and blood. Furthermore, the authors show different prevalence of both subsets after exposure of microbial stimuli and confirm the functional similarity of NeuMo to neutrophils. The subset specific fates of NeuMo and DCMo in their function as tissue resident macrophages are also described in vivo in the intestine, lung and central nervous system using adoptive transfer experiments into macrophage-depleted mice. Lastly, through competitive adoptive transfer experiments, the authors also demonstrate that while NeuMo and DCMo establish equal contribution in NCM and gut macrophages, DCMo preferentially give rise to lung interstitial macrophages.
Expand a little more on what they find about their potential to repopulate these organs
The paper could benefit from more linguistical clarity. It feels at certain points hard to follow which population the authors are referring to. It would be advisable if the authors consider it for a broader audience. Additionally, double-checking for errors in the figures/legends (Example: Figure 2 legend for CD319+ and CD177+ does not have the respective gene signature color scheme) is advised.
When gating through the Ly6c high blood monocytes, the authors should be aware that Ly6c low monocytes may contribute to the signal seen in CD177+ and CD319+ subsets. To these readers, it remained unclear whether Ly6c low monocytes may be included in the gate. A gating control would be helpful.
The differential recruitment after exposure to LPS and CpG of the CM subsets was already shown previously (Yanez et al., Immunity, 2017). The authors could put that data in the supplement and/or use other microbial pathogen challenge models (i.e. viral infections such as IAV) in addition to IFNg exposure.
The authors should further elaborate or speculate on the consequences of these monocyte phenotypes (NeuMo and DCMo) in comparison to neutrophils/DC during disease, which might also be accomplished by using different in vivo disease models.
To these readers, the CD177- CD319- double negative cell population remains elusive. The manuscript would benefit from clarifying the importance and/or characterise the difference of the double negative classical monocyte population seen in the PBMCs compared to the NeuMo and DCMo populations. Is it another maturity stage or a different population? How plastic are these subsets?
As shown with the NeuMo population, the functional significance of the DCMo should also be shown. For example, are the DCMo able to engage in MHC antigen presentation? Are they able to activate T cells as efficiently as dendritic cells? Are the DCMos able to drain to the lymphoid tissue more efficiently than NeuMos?
The authors use the Ms4a3Cre:R26LSL-TdTomato:Cx3cr1gfp reporter mouse model as an innovative model for GMP or MDP progeny. While the GMP progeny mapping of TdTomato+GFP+ cells is established, the MDP progeny of TdTomato-GFP+ cells should be verified (f.e. by FACS of the corresponding BM population).
The authors show depletion of blood monocytes after DT treatment in Cx3cr1DTR mice (Fig 5B), yet proceed to show endogenous monocytes in the consequent figures (Fig 5C-D). Therefore, the authors should clarify where the endogenous monocytes come from or at least comment on the efficiency of depletion in their model.
The authors make an interesting discovery with regards to the repopulation capacities of the different classical monocyte subpopulations in different tissues, however the manuscript would benefit from a clear rationale for using the different tissues presented in the paper (intestine, lung, brain) and not others that see monocyte influx in different pathologies.
The repopulation of alveolar macrophages in the lung by monocytes (irradiation of BM-recipients leads to TR-AM depletion which are partly repopulated by monocyte derived cells) or at least the possibility of this repopulation is not mentioned at all and should at least be discussed.
Using human data (f.e. already published blood RNAseq data) would elevate the significance of the paper and demonstrate the importance of classical monocyte heterogeneity in human health.
Identification of NeuMo and DCMo specific surface markers, verification of these subsets in the murine blood and BM, as well as extensive characterization. The authors use an innovative double reporter fate mapping mouse model for their transfer experiments. Additionally, the authors show the functional in vitro characterization of NeuMos. Lastly, the study describes the resulting monocyte derived macrophages phenotypes in three different organs with different homing and differentiation properties for the NeuMo and DCMo, and provides proof of the CD62L dependent engraftment of NeuMo in the dura mater.
Generally, this paper provides further evidence for the heterogeneity of monocyte populations and their differentiation properties, which paves the way to discoveries of monocyte subset specific implications in pathogen responses and disease development.
Reviewed by Lisabeth Pimenov and Azuah Gonzalez as part of a cross-institutional journal club between the Vanderbilt University Medical Center (VUMC), the Max-Delbrück Center Berlin, the Medical University of Vienna and other life science institutes in Vienna.
The authors declare no conflict of interests in relation to their involvement in the review.