The finding that VCAM-1+ stroma express 4–1BBL, CCL19, CXCL12, and IL-7 and that adoptively transferred CD8+ memory T cells are often found in
proximity to VCAM-1+CD45− cells in the BM demonstrates the plausibility of the VCAM-1+ stromal cell as A-769662 mw the radioresistant cell that provides 4–1BBL to memory CD8+ T cells in the BM. These data support a model in which a radioresistant VCAM-1+ stromal cell attracts the VLA-4+ CD8+ memory T cells via CCL19, where they can receive 4–1BB-4–1BBL induced survival signals. As the VCAM-1-positive stromal population is very abundant in the BM, there may be heterogeneity in the VCAM-1+ stroma with respect to 4–1BBL, cytokines, and chemokines that contribute to CD8+ T-cell memory maintenance. Further analysis will be required to definitively identify the 4–1BBL-expressing radioresistant cell that contributes to CD8+ T-cell memory. C57BL/6 WT mice were obtained from Charles River Laboratories (St. Constant, QC, Canada).
4–1BB−/− mice  extensively backcrossed to the C57BL/6 (n = 10) background were bred in our facility. These mice were previously provided to us by Dr. Byoung S. Kwon (National Cancer Center, Ilsan, Korea). 4–1BBL-deficient (4–1BBL−/−) mice were originally obtained under a materials transfer agreement from Immunex (Amgen, Thousand Oaks, CA, USA) and further backcrossed to the C57BL/6 background in our facility (total n = 9). OT-I
and CD45.1 congenic mice were obtained from Jackson Laboratories (Bar Harbor, ME, USA) and crossed to Roscovitine generate CD45.1+/+ or CD45.1+/− OT-I mice. TCRα−/– mice were kindly provided by Dr. Cynthia Guidos (Hospital for Sick Children, Toronto). FoxP3gfp knock-in mice on the C57BL/6 background were kindly provided by Dr. Mohamed Oukka (Harvard Medical School) . ACTB-DsRed transgenic mice expressing DsRed protein under control of the β-actin promoter and backcrossed to B6 mice for five generations (B6.Cg-Tg (ACTB-DsRed*MST) 1Nagy/J) were obtained from the Jackson laboratories and crossed with OT-I mice to obtain OT-I ACTB-DsRed mice (OT-I-DsRed). Mice were maintained under specific pathogen-free conditions in sterile microisolators at the University of Toronto. All mouse experiments were approved Orotidine 5′-phosphate decarboxylase by the University of Toronto animal care committee in accordance with the regulations of the Canadian Council on animal care (University of Toronto approved protocol #20007828). CD8+ T cells with a central memory phenotype were generated by culture with Ag followed by IL-15 using a variation of a previous protocol [7, 29]. In brief, OT-I splenocytes were stimulated with 0.1 μg/mL SIINFEKL peptide and 1 μg/mL of LPS for 1 day, and then the nonadherent cells were rested for 2 days in fresh media (RPMI-1640 with 10% heat-inactivated FCS, 0.03% L-glutamine, antibiotics, and 2-mercaptoethanol).