*Purpose: If and how aging affects T-cell metabolism and alloimmune response is unclear. Here, we show that T-cell metabolism changes drastically in aging. Most notably, we propose an age-specific metabolic intervention for optimal immunosuppression in old recipients.

*Methods: Naïve CD4+ T cells were collected from C57BL/6 mice (3 and 18 mths) and activated with anti-CD3 and anti-CD28 for 24 hrs. Oxidative phosphorylation (OXPHOS) and aerobic glycolysis were assessed by oxygen consumption (OCR) and extracellular acidification rate (ECAR) using a Seahorse XFe96 extracellular flux analyzer. Full thickness skin grafts (3 mth DBA/2 mice) were transplanted to young and old C57BL/6 recipients (3 and 18 mths). Immune profiling was performed at sequential time intervals using a FACS Canton II flow cytometer.

*Results: Glutaminolysis has been identified as a critical metabolic pathway in T-cell alloimmunity. Upon activation in-vitro, CD4+ T cells from old mice failed to exhibit adequate metabolic reprogramming with a compromised utilization of OXPHOS and glycolysis pathways (p<0.0001). 6-diazo-5-oxo-l-norleucine (DON) is an analog of glutamine that inhibits critical enzymes of glutaminolysis. CD4+T cells treated with DON displayed age-specifically reduced activation and proliferative capacities. Interestingly, lower doses of DON inhibited IL-2 production and CFSE-determined proliferation only in old CD4+ T cells (p<0.001). Next, we tested the immunosuppressive capacity in fully mismatched transplantation model (DBA/2  young or old C57BL/6); recipients of full-thickness skin allografts were treated with DON (1.6mg/kg every other day). DON reversed the CD4:CD8 ratio only in old recipients. Most impressive were age-specific differences in allograft survival. DON improved graft survival times in young animals from 7 to 18 days; graft survival in old recipients treated with DON had been prolonged to 43 days (n=7, p<0.001). Notably, age-specific immunosuppressive effects of DON were specific to old CD4+ T-cells as adoptively transferred young CD4+ T cells prevented the prolongation of graft survival. Of additional relevance, depletion of CD8+ T cells did not alter transplant outcomes in either young or old recipients.

*Conclusions: Taken together, our data introduce age-specific metabolic pathways of CD4+ T and provide a novel concept of an age-specific immunosuppression.

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