Introduction: The proportion of older transplant recipients has increased dramatically. Age-specific effects of established immunosuppressants are poorly understood. Here, we assessed the impact of CTLA4-Ig, a fusion protein blocking costimulatory signaling between APCs and T cells through CD28. We hypothesized that the immunosuppressive capacity of CTLA4-Ig will depend on an age-specific expression of CD28 on alloreactive T-cells.
Methods: Heart and skin grafts from young DBA2/J mice were transplanted into either young or old (2-3 and 18 mths, resp.) allogeneic C57BL/6 recipients treated with CTLA4-Ig or PBS; T-cell frequencies and subsets in addition to cytokine expression were assessed by flow cytometry and ELISA; expression of CD28 was assessed on young and old T-cell subsets. To delineate the age-specific immunosuppressive potential of CTL4-Ig, T-cells were transfected with a lentiviral vector inducing CD28 expression; transfected T-cells were subsequently transferred into young RAG2^-/- mice receiving a fully mismatched skin graft.
Results: Treatment with CTLA4-Ig prolonged skin graft survival only in young (p<0.01) but not in old recipients (p=0.33). Age-specific immunosuppressive effects of CTLA4-Ig were also confirmed in a SOT model: cardiac allografts in young mice treated with CTLA4-Ig survived indefinitely. In contrast, 80% of old recipients treated with CTLA4-Ig had lost their graft by 100 days. CTLA4-Ig reduced CD4⁺ central-memory and effector-memory T cells and diminished systemic IFN-γ levels only in young (p<0.05), but not old recipients (p=0.8). These differences correlated with a reduced expression of CD28 on antigen-experienced CD4⁺ T cells in old mice (p<0.001). Interestingly, Tregs displayed an increased expression of CD28 with age making them more prone to be targeted by CTLA4-Ig. Strikingly, adoptive transfer of old CD4⁺ T cells that were transfected with a lentiviral vector augmenting the expression of CD28 accelerated the rejection of allogeneic skin grafts in young RAG2^-/- recipient mice (p<0.01 vs. controls animals that received adoptively transferred, non-transfected old T cells) demonstrating the cardinal role of CD28 expression on alloreactive CD4⁺ T cells.
Conclusion: Immunosuppressive effects of CTLA4-Ig showed distinct age-dependent effects in both skin and cardiac transplantation linked to a reduced CD28 expression of alloreactive old T-cells in parallel to an augmented CD28 expression on old T-regs. Transfection experiments confirmed the critical role of CD28 in mounting robust alloimmune responses. These findings carry relevant clinical consequences.