Abstract
Thymocyte differentiation is dependent on the availability and transport of metabolites in the thymus niche. As expression of metabolite transporters is a rate-limiting step in nutrient utilization, cell surface transporter levels generally reflect the cell's metabolic state. The GLUT1 glucose transporter is upregulated on actively dividing thymocytes, identifying thymocytes with an increased metabolism. However, it is not clear whether transporters of essential elements such as phosphate are modulated during thymocyte differentiation. While PiT1 and PiT2 are both phosphate transporters in the SLC20 family, we show here that they exhibit distinct expression profiles on both murine and human thymocytes. PiT2 expression distinguishes thymocytes with high metabolic activity, identifying immature murine double negative (CD4−CD8−) DN3b and DN4 thymocyte blasts as well as immature single positive (ISP) CD8 thymocytes. Notably, the absence of PiT2 expression on RAG2-deficient thymocytes, blocked at the DN3a stage, strongly suggests that high PiT2 expression is restricted to thymocytes having undergone a productive TCRβ rearrangement at the DN3a/DN3b transition. Similarly, in the human thymus, PiT2 was upregulated on early post-β selection CD4+ISP and TCRαβ−CD4hiDP thymocytes co-expressing the CD71 transferrin receptor, a marker of metabolic activity. In marked contrast, expression of the PiT1 phosphate importer was detected on mature CD3+ murine and human thymocytes. Notably, PiT1 expression on CD3+DN thymocytes was identified as a biomarker of an aging thymus, increasing from 8.4 ± 1.5% to 42.4 ± 9.4% by 1 year of age (p < 0.0001). We identified these cells as TCRγδ and, most significantly, NKT, representing 77 ± 9% of PiT1+DN thymocytes by 1 year of age (p < 0.001). Thus, metabolic activity and thymic aging are associated with distinct expression profiles of the PiT1 and PiT2 phosphate transporters.
Highlights
The thymus is critical for the differentiation of T lymphocytes, promoting the generation of a pool of functionally competent T cells that provide protection against pathogens and tumors while maintaining self-tolerance
Data are representative of one of eight individual thymi and quantification of transporter expression in ISP8 and SP8 thymocytes is shown. (C) Delta geometric MFIs of GLUT1, PiT1 and PiT2 staining in double negative (DN), immature single positive (ISP), double positive (DP), SP4, and SP8 subsets are presented for four individual thymi. (D) The phenotype of ISP and SP8 thymocytes was evaluated as a function of GLUT1, PiT1, and PiT2 transporters and the CD71 transferrin receptor and the percentages of cells in the different quadrants are indicated
This is critical as it is the rapid translocation of solute carriers from intracellular stores to the cell surface that reflects the cell’s response to extracellular stimuli; this has been extensively described for the insulin-mediated induction of GLUT1/GLUT4 to the cell surface within minutes of stimulation [72, 73]
Summary
The thymus is critical for the differentiation of T lymphocytes, promoting the generation of a pool of functionally competent T cells that provide protection against pathogens and tumors while maintaining self-tolerance. Β-selection occurs at a precise stage, within CD4−CD8− double negative (DN) 3 (CD25+CD44−) thymocytes [9] whereas in humans, this step occurs in CD4+ intermediate single positive (ISP) cells as well as in double positive (DP) CD4+CD8α+CD8β+ thymocytes [11, 12] This TCR rearrangement results in a proliferative burst of murine as well as human thymocytes [13, 14], requiring an increased metabolism that is dependent on PI3K signaling downstream of Notch, IL-7, CXCR4 and the TCR [15,16,17,18,19,20,21]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
