Lymph node fibroblastic reticular cells in health and disease

Abstract

Over the past decade, a series of discoveries relating to fibroblastic reticular cells (FRCs) — immunologically specialized myofibroblasts found in lymphoid tissue — has promoted these cells from benign bystanders to major players in the immune response. In this Review, we focus on recent advances regarding the immunobiology of lymph node-derived FRCs, presenting an updated view of crucial checkpoints during their development and their dynamic control of lymph node expansion and contraction during infection. We highlight the robust effects of FRCs on systemic B cell and T cell responses, and we present an emerging view of FRCs as drivers of pathology following acute and chronic viral infections. Lastly, we review emerging therapeutic advances that harness the immunoregulatory properties of FRCs.

Figure 1. Molecular checkpoints in fibroblastic reticular cell (FRC) development.

FRC development is incompletely defined, but a series of crucial checkpoints have been identified. a | Lymph node anlagen specification. Retinoic acid, likely provided by adjacent neurons, is required for lymph node anlage specification of retinoic acid receptor + mesenchymal precursors at the site, which develop into CXCL13+ early LTo-committed stromal precursor cells. b | Pre-adipocyte precursors. LTβR signalling is a molecular switch that drives lymph node stromal cell development at the expense of adipocyte development. The migration of pre-adipocytes to the lymph node site is augmented by CXCR4. On arrival, these cells upregulate CXCL13 to become early LTo-committed stromal precursor cells. c | Early LTos differentiate into primed LTos when then begin to express low levels of ICAM-1 and VCAM-1. Signals involved in this differentiation step are unknown, but precursors do not require ILCs or LTβR signalling to proceed to this stage. d | The development of ICAM-1hi VCAM-1hi LTo cells requires cross talk with ILC3s via RANK and lymphotoxin signals. Rorc-/- mice, which lack ILC3s, do not develop lymph nodes. There is a partial block in lymph node development in mice lacking CXCL13, CXCR5 and IL-7R, and a complete block in CXCR5-/- CCR7-/- mice,, ,,. RANK signalling to ILC3s is a crucial checkpoint, inducing the upregulation of LTα1β2. LTβR signalling induces early lymphoid tissue organiser cells to differentiate to a mature form, and is a crucial checkpoint for lymph node development. Mice with blocks in this signalling pathway develop anlagen and early LTo cells but not lymph nodes,,. e| It is hypothesised that early FRC precursor cells develop directly from differentiated LTos, but this has not been formally demonstrated. Interactions between PDPN and CLEC2 are a crucial checkpoint for FRC development,, as Pdpn-/- and Clec1b-/- develop anlage but no lymph nodes. It is unclear exactly where this block lies, but it is prior to the development of FRC precursors. The source of CLEC2 is undefined. f | The development of mature FRCs from immature FRC precursors requires lymphotoxin signalling. Lymph nodes lacking the NF-κB2 signalling pathway (Relb^-/-, Nfkb2^-/- and Ikka^-/- lymph nodes) are present, but hypoplastic and B cell deficient,, which indicates that the canonical NF-κB signalling pathway may partly compensate for loss of LTβR. The loss of LTβR from LTo cells at the point of CCL19 upregulation allows for the development of lymph nodes and FRC precursors, but blocks late-stage FRC development, thus identifying immediate FRC precurors. In humans and some primates, but not mice, CD4⁺ T cells are a crucial source of LT ligands. Abbreviations: CLEC2: c-type lectin receptor 2. FRC: fibroblastic reticular cell. ILC3s: group 3 innate lymphoid cells. LN: lymph node. LTβR: lymphotoxin beta receptor. LTos: lymphoid tissue organisers. PDPN: podoplanin.

Figure 2. Fibroblastic reticular cells (FRCs) organize the lymph node microarchitecture.

FRCs exist in various lymph node microdomains, where they regulate different leukocyte types and aspects of their function. a | Paracortical T cell zone. T cell zone FRCs from the lymph node paracortex attract naïve T cells and DCs by their expression of the CCR7 ligands CCL19 and CCL21. T cell zone FRCs express PDPN, which activates CLEC2 on mature antigen-loaded DCs; CLEC2 signalling drives DC migration on scaffolding created by FRCs,. Paracortical FRCs also create the conduit network, which conveys small molecules from afferent lymphatics towards the paracortex and HEVs, and secrete the T cell pro-survival factor IL-7. b | Marginal reticular cells and a subset of T cell zone FRCs express (or inducibly express) CXCL13 to attract CXCR5⁺ B cells and ILC3s,,. c | FRCs in the B cell follicle produce BAFF to help naïve B cells survive d | FRCs maintain tight cell-cell junctions of HEVs. When leukocytes move into lymph nodes via HEVs, PDPN expressed by FRCs ligates CLEC2 expressed by the small number of accompanying platelets. This signal mediates S1P1 release from the platelet surface, which signals to HEVs to upregulate their VE Cadherin junctions. Abbreviations: BAFF: B cell activating factor. CLEC2: c-type lectin receptor 2. DCs: dendritic cells. FRC: fibroblastic reticular cell. HEV: high endothelial venule. ILC3s: group 3 innate lymphoid cells. LN: lymph node. LTβR: lymphotoxin beta receptor. LTos: lymphoid tissue organisers. MRCs: maginal reticular cells. PDPN: podoplanin. S1P: sphingosine-1-phosphate.

Figure 3. Dynamic response of FRCs to infection.

a | During homeostasis, PDPN⁺ FRCs form an interconnected tensile network that maintains lymph node size. b | In an acute response, in response to inflammatory cues, CLEC2 expression is upregulated by both lymph node-resident and infiltrating migratory dendritic cells,. CLEC2 inhibits PDPN-driven actomyosin contractility of FRCs resulting in FRC elongation and reduced lymph node tension, which enables rapid organ expansion in the absence of stromal proliferation,. c | Acute responses also trigger FRC proliferation and/or the recruitment of FRC precursors to restore FRC:T cell ratios in the face of T cell proliferation,,,. d | Following acute damage to the FRC network, FRCs upregulate IL-7 production to recruit ILC3s, which help to regenerate and rebuild the lymph node to repair damage,. Amid chronic infection such as HIV, resolution of damage may take years, or may not occur,. Abbreviations: CLEC2: c-type lectin receptor 2. FRC: fibroblastic reticular cell. ILC3s: group 3 innate lymphoid cells. LN: lymph node. PDPN: podoplanin.

Figure 4. Viral pathology drives fibroblastic reticular cell (FRC)-mediated immunodeficiency.

a | FRCs are directly infected by Ebolavirus, LCMV and other viruses. They present viral antigens on MHC class I molecules and are likely to propagate infection by disseminating virus. Infected FRCs slow down the onset of fatal immunopathology through effects on activated T cells. This involves PD-L1 expression by FRCs and, we speculate, may also involve other known suppressive pathways such as NO release, which is produced in response to IFNγ produced by T cells. FRCs slow viral clearance through their use of such T cell-suppressive mechanisms. Eventually however, infected FRCs are destroyed with high efficiency, which disrupts lymph node architecture and function, causing systemic lymphopenia and crippling the immune response to new antigens,,. b | During chronic infection, the expansion of T_Reg cell populations results in increased output of immunosuppressive TGFβ. This signals to FRCs to increase their collagen output, causing lymph node fibrosis. T cell access to the FRC-produced survival factor IL-7 is impeded. Loss of T cells concomitantly reduces the access of FRCs to LTα1β2, which signals through LTβR. Hence, both FRCs and T cells are lost. This causes systemic, long-term lymphopenia, and loss of lymph node architecture and the conduit system,–. Abbreviations: FRC: fibroblastic reticular cell. IFNγ: interferon gamma. LCMV: lymphocytic choriomeningitis virus. LN: lymph node. LTα1β2: lymphotoxin alpha 1 beta 2. LTβR: lymphotoxin beta receptor. TGFβ:transforming growth factor beta