Alpha-smooth muscle actin expression identifies subpopulations of mouse lymph node non-hematopoietic cells
Introduction
The lymph nodes play an essential role in the orchestration of immune responses in mammalian species, including human. Various non-hematopoietic cells structurally organize the lymph nodes and functionally provide the stromal microenvironments for coordinating the immune responses by hematopoietic cells. It has been shown that the lymph node non-hematopoietic cells can be subdivided into four distinct populations according to the expression of the podocyte marker gp38 (podoplanin) and the endothelial cell marker CD31 (PECAM1). The gp38+CD31− cells include fibroblastic reticular cells (FRCs), which form a three-dimensional network in the T cell zones and regulate the homeostasis and trafficking of T cells [1]. The gp38+CD31+ and gp38−CD31+ cells contain lymphatic endothelial cells (LECs) and blood ECs (BECs), respectively, which critically regulate the egress and entry of lymphocytes [2]. However, the molecular and cellular characterization of non-hematopoietic cells in the lymph nodes is still in its infancy.
Alpha-smooth muscle actin (αSMA) is expressed in cells of smooth muscle lineage in various organs. The αSMA expression is often associated with perivascular cells (PVCs), including pericytes surrounding microvessels and vascular smooth muscle cells surrounding large vessels [3], [4]. Although αSMA-expressing cells have been detected also in the lymph nodes [5], the molecular and cellular characteristics of αSMA+ cells in the lymph nodes are unclear, especially in the context of well-studied non-hematopoietic populations that are subdivided according to the expression of gp38 and CD31.
In the present study, we examined αSMA expression in lymph nodes using αSMA-GFP-transgenic mice. Green fluorescent protein (GFP) expression in those mice faithfully reflects the specific expression of αSMA and can identify nonvascular and vascular smooth muscle cells, including renal glomerular mesangial cells, and a small population of bone marrow stromal cells [6]. Our results show that GFP+ cells in the lymph nodes of αSMA-GFP-transgenic mice mostly comprise non-hematopoietic cells and at the same time contain a small population of hematopoietic cells, which represent macrophages that have likely acquired GFP proteins from the non-hematopoietic αSMA+ cells. Interestingly, αSMA-GFP expression in the lymph nodes made possible the identification of subpopulations of gp38+CD31− and gp38−CD31− non-hematopoietic cells. The results have enabled the identification of hitherto unknown subpopulations of non-hematopoietic cells in the lymph nodes.
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Mice
αSMA-GFP-transgenic mice in C57BL/6 (B6) background, which express GFP under the control of smooth muscle type alpha-actin (αSMA) promoter, were originally from the National Eye Institute, National Institutes of Health, USA [6]. B6 mice were obtained from SLC (Shizuoka, Japan), whereas B6-Ly5.1 mice were maintained in our laboratory. The mice were maintained under specific pathogen-free conditions in our animal facility. The experiments were conducted under the approval of the Institutional
αSMA-GFP expression in lymph nodes
It was reported that GFP expression in αSMA-GFP-transgenic mice faithfully reflects the specific expression of αSMA and can identify vascular and nonvascular smooth muscle cells as well as a small population of bone marrow stromal cells [6]. In order to characterize αSMA-expressing cells in the lymph nodes, we first examined GFP expression in paraformaldehyde-fixed sections of inguinal lymph nodes from αSMA-GFP-transgenic mice. GFP signals, which were detectable in various areas of the lymph
Discussion
The present results indicate that αSMA expression can identify hitherto unknown subpopulations of non-hematopoietic cells in the lymph nodes of αSMA-GFP-transgenic mice. The gp38+CD31− non-hematopoietic lymph node cells, previously known to contain FRCs, can be clearly subdivided into αSMA+ and αSMA− subpopulations that are similar in regard to the expression of several cell-surface molecules but different in terms of chemokine gene expression. The gp38−CD31− non-hematopoietic lymph node cells,
Acknowledgments
The authors thank Drs. Sanai Sato (National Eye Institute) and Ivo Kalajzic (University of Connecticut Health Center) for providing αSMA-GFP-transgenic mice, and Drs. Mie Sakata and Izumi Ohigashi and other members of the laboratory for support in experiments and discussion. This study was supported by Grants-in-Aid for Scientific Research from MEXT and JSPS (23249025, 24111004, and 25111507), Japan.
References (14)
- et al.
Stromal cell heterogeneity in lymphoid organs
Trends Immunol.
(2010) - et al.
Fibroblastic reticular cells in lymph nodes regulate the homeostasis of naive T cells
Nat. Immunol.
(2007) - et al.
Lymphatic endothelial cell sphingosine kinase activity is required for lymphocyte egress and lymphatic patterning
J. Exp. Med.
(2010) - et al.
Heterogeneity of microvascular pericytes for smooth muscle type alpha-actin
J. Cell Biol.
(1991) - et al.
Perivascular cells for regenerative medicine
J. Cell. Mol. Med.
(2012) - et al.
Characterization of stromal cells with myoid features in lymph nodes and spleen in normal and pathologic conditions
Am. J. Pathol.
(1987) - et al.
Bone marrow lacks a transplantable progenitor for smooth muscle type alpha-actin-expressing cells
Stem Cells
(2006)
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