. Author manuscript; available in PMC: 2024 Nov 1.

Published in final edited form as: Semin Immunol. 2023 Aug 29;70:101835. doi: 10.1016/j.smim.2023.101835

Table 1.

Heterogeneity of Fibroblastic reticular cells and their alterations during aging.

Type of reticular cells	FRC subsets	Localization	Unique transcriptomic signature	Surface phenotype	Function	Age-related changes	References
T cell zone reticular cells (TRC)	Ccl19^high reticular cells	Primarily in T cell zone	Ccl19^hi, Ccl21⁺, Il7⁺, Bst1⁺, Slc7a11⁺	CD31⁻ PDPN⁺PDGFRα⁺Bst1⁺CD21/35⁻MAdCAM1⁻ CD34⁻ (does not differentiate between Ccl19^hi and Ccl19^lo cells)	Support T cell and DCs survival and maintenance via IL-7 and homeostatic chemokines. Produce collagens and ECM components and form a conduit network.	(1) Old mice LN exhibit reduced CCL19 and CCL21 protein levels. (2). Reduction in FRC homeostatic and survival signal, LTb in old LN may affect the integrity of the PDPN⁺ reticular network in the T cell zone (various FRC subsets, like Ccl19^hi, Ccl19^lo, and Cxcl9⁺ cells, in T cell zone may get affected). (3) FRC in the paracortical T cell zone exhibit denser ECM (increased accumulation of collagen and alpha-smooth muscle actin, α-SMA), suggestive of fibrosis.	[68, 86, 117, 118]
T/B border and Interfollicular reticular cells (IFRC)	Ccl19^low reticular cells	Mainly at interfollicular (IF) region, B cell follicles, and T cell zones	Ccl19^lo, Ccl21⁺, Il7⁺, Cxcl13⁺, Baff⁺	CD31⁻ PDPN⁺PDGFRα⁺Bst1⁺CD21/35⁻MAdCAM1⁻ CD34⁻	Support T cell (IL-7 and CCL21) and B cell (CXCL13, BAFF) survival and maintenance, and may be their interaction at T/B borders and in the B cell follicles. A proportion of these cells produce cholesterol-25-hydroxylase (Ch25h), an enzyme that catalyzes the production of 7 a, 25-hydroxy cholesterol, a ligand known to bind with GPR183.	(1) Loss of CCL21 and CXCL13 protein levels, correspondingly, altered T cell and B cell localization and diffused T/B border suggest the possible alteration of these stromal cells at the T/B border and IF region. (2) FRC in cortical region (that may include Ccl19^lo cells at T/B borders and IF region) exhibit increased collagen and α-SMA accumulation.	[86, 88, 117]
	Grem1⁺ reticular cells	T cell zone, T/B border	Ccl19^hi, Ccl21⁺, Il7⁺, Bst1⁺, Grem1⁺	Currently, there is no available surface marker(s) that can distinguish Grem1⁺ cells from the rest of the TRC. Grem1-Cre^ERT2::Rosa26-LSL-EYFP reporter mice allow the isolation and characterization of Grem1-EYFP⁺ stroma.	Support the survival and homeostatic proliferation of LN-resident conventional DCs (cDCs).	?	[88]
	Cxcl9⁺ reticular cells	T cell zone, IF region, Medullary area	Cxcl9⁺, Cxcl10⁺, Ccl19⁺, Ccl21⁺, Il7⁺	?	(1) Cxcl9⁺ FRC express class II MHC genes and machinery for peptide generation (chaperons) and loading (CD74) onto MHC molecules, and reduced expression of co-stimulatory molecules, indicating their possible role in tolerance. (2) Gene signature of Cxcl9⁺ FRC (Gpb3–9, STAT1, Irgm2, Ifit3, Irf1 and 8) indicate that they likely are capable of responding to IFN-signaling (type I and IFN-γ) and might represent an activated subpopulation of TRC.	(1) Loss of CCL21 and CXCL13 protein levels, correspondingly, altered T cell and B cell localization and diffused T/B border suggest the possible alteration of stromal cells at the IF region. However, no definitive evidence exists that suggest the impairment in the structure and/or function of Cxcl9⁺ stromal cells.	[86, 117, 118]
Medullary reticular cells (MedRC)	Inmt⁺ reticular cells	Medullary cord	Inmt⁺, Nr4a1^lo, Bst1^lo, Ccl19^lo, Cxcl1^lo, Cxcl2^lo, Cxcl9^lo, Cxcl10^lo, Cxcl12⁺, Cxcl13⁺, Desmin^hi, Baff⁺, lepr⁺, CD34⁻	?	(1) Localization and gene expression data suggest that Inmt⁺ stroma may represent a population of MedRC. (2) Inmt⁺ stromal cells can be distinguished from Nr4a1⁺ MedRC by Cxcl1, 2 and 10 expression. (3) Produce BAFF, IL-6, and APRIL and help maintain macrophage and plasma cells at medullary regions.	(1) In old human LN, PDPN⁺Bst1⁻ CD34⁻ supposedly MedRC (that may contain Inmt⁺ and Nr4a1⁺ cells in addition to other unknown MedRC) have been reported to be particularly sensitive for transdifferentiation into adipocytes. (2) The loss of lymphatic network and dilation of HEV have been found within a medullary area with increased adipocyte accumulation, and correspondingly, altered localization of plasma cells and reduced Naive T cells around HEV.	[86, 87, 102, 119]
	Nr4a1⁺ reticular cells	Mostly in the medullary cord. Since Nr4a1 is also expressed by other stromal cells (TRC, CD34⁺, MRC, and PvRC), Nr4a1⁺ staining can be located in IF region, T/B cell borders, T cell zones, and SCS.	Nr4a1^hi, Inmt⁺, Bst1^lo, Ccl21^lo, Cxcl1^hi, Cxcl2^hi, Cxcl9^lo, Cxcl10^hi, Baff⁺, lepr⁺, CD34⁻	CD31⁻ PDPN⁺PDGFRa⁺Bst1⁻ CD21/35⁻MAdCAM1⁻ CD34⁻ (Nr4a1-GFP reporter mice may allow isolation and characterization of Nr4a1-GFP⁺ cells. Gating out cells that express CD21/35, MAdCAM1, and Bst1 could help distinguish Nr4a1⁺ MedRC from FDC, MRC, and TRC that express Nr4a1).	(1) Like Inmt⁺ cells, Nr4a1⁺ stroma may represent a population of MedRC. (2) Express early response genes (Junb, Egr1–2, Fos, Nr4a1, Nfkbia, Nfkbiz, and Zfp36) indicative of their possible activated state. (3) Produce BAFF, IL-6, and APRIL and help maintain macrophage and plasma cells at medullary regions.		[86, 87, 102, 119]
	CD34⁺ reticular cells	Capsular area and surrounding large blood vessels in the medullary region, In the vicinity of efferent lymphatics	CD34⁺, CD248^hi, Pdgfrβ⁺, Inmt⁺ Desmin^lo, CD31⁻, Bst1⁻, Acta2⁻	CD31⁻ PDPN⁺PDGFRα⁺CD2 1/35⁻MAdCAM1⁻ CD34⁺	(1) CD34⁺ fibroblastic cells serve as a progenitor for stromal cell development. (2) CD34⁺ stromal cells exhibit a gene signature enriched in angiogenic factors (Igf1, Igfbp3, 4, and 6), indicating their possible for in new lymph and blood vessel formation. (3) They may facilitate LN expansion during immune response via CD248.	In the medullary region of old human LN, CD31⁻ PDPN⁺BSt1⁻ CD34⁺ stromal cells have been found to transdifferentiate into adipocytes, leading to increased lipomatosis, and gradual loss of lymphatic network and dilation of HEV within an area with increased lipomatosis.	[85–88, 102]
B cell zone reticular cells (BRC)	Cxcl12⁺ reticular cells	Exclusively in the dark zone of the germinal center (GC)	CD21/35^lo, Baff⁺, Cxcl13⁻, Cxcl12⁺	CXCL12 reporter strains could be used to isolate CD31⁻ PDPN⁺Bst1⁺CD21/35⁻ MAdCAM1⁻CD34⁻ cells expressing reporter driven by Cxcl12 promoter.	(1) CXCL12⁺CD 21/35^lo stromal cells form the structure of the GC dark zone. (2) Attract CXCR4⁺ B cells to the dark zone of GC and may provide microenvironment support for the B cell somatic hypermutation process.	?	[83, 86, 130, 200]
B cell zone reticular cells (BRC)	Follicular dendritic cells (FDC)	B cell follicle and light zone of GC	CD21⁺, CD35⁺, Baff⁺, Cxcl13⁺, Cxcl12⁻	CD31⁻ PDPN⁺CD21/35⁺MAd CAM1^+/− PDGFRα⁺Bst1⁺	(1) CXCL13⁺CD 21/35^hi FDC facilitate homing of CXCR5⁺ B cells and Tfh to form B cell follicle. (2) Support B cell survival and maintenance via CXCL13 and BAFF. (3) Present antigens to B cells via immunocomp lexes bound to Fc-receptors (CD16 and CD32). (4) Maintain the microenvironment of light zone of GC where class-switching and affinity maturation occurs.	(1) FDC numbers decline with age, correspondin gly fewer and smaller GC form in old LN. (2) FDC's Fc-receptor expression reduced and their ability to capture antigens and present them to B cells also diminished. (3) The proliferative response of FDC is reduced in the draining LN of vaccinated old mice and found to be linked to mislocalization of Tfh cells to the dark zone of GC.	[64, 86, 116, 130]
	Marginal reticular cells (MRC)	Lining the subcapsularsinus (SCS)	MAdCAM1⁺, Bst1⁺, RANKL⁺, Cxcl13⁺, Enpp2⁺, CD21/CD3 5-	CD31⁻ PDPN⁺PDGFRα⁺CD2 1/35⁻MAdCAM1⁺	(1) Perform barrier defense by capturing antigens from SCS space and communicate this information to B cells. (2) Support and maintain a niche for sinusoidal macrophages via RANKL. (3) Maintain type 3 innate lymphoid cells via IL-7 signaling. (4) MRC also serve as precursors for FDC and other FRC subtypes during immune activation.	(1) The numbers of MRC reduced with age. (2) Aged MRC activate and proliferate less in response to vaccination. (3) Aged MRC need a boost of TLR4-mediated signal(s) to be able to mount similar response to their adult counterparts. (4) Poor MRC response to vaccine correspond to fewer GC B cells and reduced antigen-specific antibody production.	[12, 64, 86]
	Perivascular reticular cells (PvRC)	Ensheath HEV and other blood vessels	Itgα7⁺, Pdgfrβ⁺, Acta2⁺, Esam1⁺, Mcam⁺, Fabp4⁺, Myh11^hi, Myl9^hi, Ccl19⁻ Ccl21⁻	CD31⁻ PDPN⁺PDGFRβ⁺CD2 1/35⁻MAdCAM1⁻CD34⁻ Integrin α7⁺	(1) Secrete ECM molecules and wrap arround the HEV and other vessels to maintain endothelial structure and functon. (2) Produce integrins and cell adhesion molecules to support lymphocyte migration across HEV and other endothelium.	?	[86, 88]