The effects of short-term rapamycin treatment on immune parameters have been previously assessed and both positive and negative outcomes have been seen. For example, rapamycin was shown to enhance the efficacy of viral and dendritic cell (DC)-based vaccines (1,11) which might contribute to increased longevity. On the other hand, there were negative immune consequences, including an increase in suppressive regulatory T cells (TREGS) (9) and depression of T cell proliferative responses (27). Yet, none of these earlier studies assessed the effects of rapamycin in old, as well as young, mice. Towards this end, I will be pursuing three aims.
Aim I will focus on how enterically-delivered rapamycin will affect general immune parameters that are known to change with age such as cytokine profiles, thymic involution, and T cell subsets in both young and old C57BL/6 mice. My pilot data suggest that long-term enteric delivery affects the immune system differently than the short-term protocols previously tested. Moreover, the effects in old and young animals are not identical which provides preliminary support for the hypothesis to be tested.
Aim II will ask whether rapamycin impacts humoral responses in young and old C57BL/6 mice. Mice will be immunized with a protein antigen or challenged with live bacteria, and antibody titers will be measured. In a pilot experiment, it has been established that rapamycin does not reverse the effects of immune aging on humoral responses to a protein antigen, but may in fact facilitate humoral responses to viable bacterial pathogens in old animals. Although tantalizing, the pilot study was performed with relatively small groups of old mice. In addition to expanding this study to obtain statistical significance, immune parameters will be assessed in the same animals in order to assess how enteric rapamycin is mediating this effect including experiments to determine whether suppressive regulatory T cells (TREGS) contribute to the modulation of immunity seen. Although previous studies have shown that TREGS are increased with short-term rapamycin treatment, we have shown less of an effect in the longer-term rapamycin fed mice, suggesting that other mechanisms are likely critical.
Aim III will focus on assessing rapamycin's effects on T cell tolerance since enhanced immunity could have the undesired outcome of simultaneously increasing autoimmune pathologies. To assess whether immune tolerance will be perturbed by rapamycin, a transgenic mouse model of myasthenia gravis will be employed. Importantly, our lab recently demonstrated that T-cell tolerance in this model survives aging (19) so it will be possible to study rapamycin's effects on tolerance in old, as well as young, mice. Thus, this project will use mouse models to assess the differential effects of rapamycin on immune regulation with aging.
It has been suggested that the m-TOR inhibitor rapamycin might be efficacious in slowing the aging process. However, the potential widespread use of this immunosuppressive drug in a population of elderly individuals must be considered cautiously due to their immunosenescent state and consequent increase in susceptibility to cancer and infectious agents. The long-term goal of this proposal is to determine if rapamycin treatment can reverse certain age-associated pathologies such as inflammation, which could then directly contribute to longer and healthier life.