15 jan. 2025
Makris et al. (BioRxiv)
Keywords
Lymph node
Fibroblast
Inflammation
Stromal Immunology
Main Findings
In this preprint, the authors investigate the role of Podoplanin (PDPN) expressed by fibroblastic reticular cells (FRCs) in the lymph node. PDPN is widely used as a fibroblast marker and has been shown to be crucial for the development of lymph nodes, however the function of this glycoprotein that is constitutively expressed by FRCs is not well defined. Using a combination of in silico and histological approaches, as well as a PdgfracreERT2x Pdpnflox/flox mouse line, the authors demonstrate that PDPN is a key mediator of FRC transcriptional profiles. Deletion of PDPN in FRCs also resulted in the accumulation of FRCs with a pro-inflammatory phenotype and altered extracellular matrix networks, suggesting PDPN is related to FRC function and lymph node morphology. The interaction between PDPN and C-lectin type 2 (CLEC2), expressed by dendritic cells, has been implicated in mediating structural changes to the lymph node in inflammation. In vitrostimulation of immortalized FRCs demonstrated that PDPN mediates FRC transcriptional signatures in both CLEC2-dependent and independent manners. This finding suggests that the PDPN-CLEC2 axis may underpin stromal-immune crosstalk in the lymph node. Within the lymph node of PdgfracreERT2 x Pdpnflox/floxanimals, signalling from FRCs to numerous myeloid cells and dendritic cells was altered and monocyte numbers increased. PDPN-deficiency further resulted in the accumulation of immature B cells in lymph nodes. Moreover, post immunization, PdgfracreERT2 x Pdpnflox/flox animals showed decreased circulating antibody as well as smaller germinal centres compared to controls, which indicates defective B cell responses. From these findings, the authors conclude that PDPN is an important mediator of fibroblast-immune crosstalk in the lymph node.
Limitations
The cre-line used in this study demonstrated low recombination efficiency, as only ~50% of FRCs demonstrated PDPN deletion as shown in figure 1. Use of additional mouse lines with higher efficiency may allow for a stronger phenotype and greater confidence in the findings.
Platelet-derived growth factor receptor alpha (PDGFRa) marks numerous cell populations in the lymph node, including a number of non-FRC subsets – defects in ECM could be due to another fibroblast population. For example, defects in the B cell response could be related to defects in Follicular Dendritic Cells. The addition of an alternative Cre-transgenic mouse lines (e.g. Ccl19CreERT2) may help to narrow down which populations are important for each of their observed phenotype. Providing an analysis on B cell development in the bone marrow would further strengthen this analysis and support the proposal of a locally acting effect of PDPN in the lymph nodes.
Numerous changes to the T cell compartment are described in figure 5. Including a better characterization of these changes through T cell numbers, phenotypes, and/or function would be an incredibly powerful addition to their observation.
The pro-inflammatory phenotype described in figures 1 and 2 is not very descriptive, inclusion of a broader profile of alterations to cytokine and chemokine production as well as other known FRC inflammatory factors would be useful.
Loss of PDPN on FRCs resulted in clear phenotypes in both the stromal and immune compartments. This extends our understanding of PDPN beyond its current definition as a fibroblast marker. However, the method by which PDPN is important for the crosstalk between FRCs and immune cells is not well defined. Are the morphological changes critical for immune changes or could altered cytokine production by FRCs be the driving factor. Efforts to mechanistically link PDPN to immune changes would be a fantastic addition to the authors’ work, specifically by further exploring the PDPN-CLEC2 axis. In line with the previous comment, would chemical inhibition of COX-2 or, alteration in Lymphotoxin signalling restore the defects of PDPN-deficiency?
Significance/Novelty
While fibroblasts have been described as important for lymph node organization, the role of these cells in the maintenance of immune cell populations has only recently been appreciated. The authors describe PDPN, a key FRC marker, as a mediator of lymph node structure and immune function. The role of fibroblasts and their surface receptors in immunological processes is a huge gap in our current knowledge. Understanding how fibroblasts affect immunity, via their most widely expressed surface markers, provides a first critical step towards rethinking how structural cells impact immune responses and diseases.
Credit
Reviewed by Erika McCartney as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute and the University of Toronto.
The author declares no conflict of interests in relation to their involvement in the review.