The mammalian target of rapamycin (mTOR), a serine-threonine kinase, plays an important role in regulation of extensive cellular activities in multiple systems, including the immune system. It mediates its effects by forming 2 distinct complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2). The demonstration of activation of mTOR activities in malignant cells led to the use of rapamycin (RAPA) and its analogs (rapalogs) in cancer therapies. However, the overall anti-cancer efficacy of rapalogs is not satisfactory with one of the major reasons being that rapalogs mainly inhibit mTORC1 activities.1,2 Thus, adenosine triphosphate-competitive mTOR inhibitors (TORKinibs), which suppress both mTORC1 and mTORC2, have been developed to overcome the shortcoming and have been shown to mediate more potent anti-tumor effects in preclinical studies.1 These findings raise the question of whether immunosuppressive effects mediated by the TORKinibs are superior to those by rapalogs. Indeed, published results do support this notion. Conditional deletion of mTOR3 in T cells or pharmacological inhibition4 of both mTORC1 and mTORC2 promoted induction of Foxp3+ antigen-induced regulatory T cells. Deletion of mTOR5 or ablation of mTORC2 activities6 in B cells were shown to impair the survival of B cells and antibody responses. These data suggest that in the absence of both mTORC1 and mTORC2 or when they are both functionally suppressed, the outcome of immune responses will favor tolerance rather immunity. However, currently, how TORKinibs impact immune cells and immune responses, especially in the setting of transplantation, has remained largely unclear.
In this issue, Fantus et al7 presented intriguing results of their study to comprehensively investigate the impacts of a TORKinib, AZD2014 that has been in clinical trial for antitumor therapy, on resting immune cell populations and on alloantigen-induced T- and B cell responses. The inclusion of RAPA as the control in this study made it possible to compare the effects of these 2 drugs side-by-side, which revealed important clinically relevant results. In vitro, AZD2014 inhibited the development of dendritic cells and CD3/CD28-induced T cell proliferation in vitro, although a higher concentration was required compared to RAPA. When given to naive mice in vivo, AZD2014 depleted thymocytes in the thymus and T and B cells in secondary lymphoid tissues while sparing the naturally occurring CD4+CD25+Foxp3+ regulatory T cells in the thymus but not in the secondary lymphoid tissues. However, it wasn’t determined whether certain subsets of T and B cells (ie naive or memory) were more prone to the effects of AZD2014 and RAPA. In addition, other immune cell populations, such as dendritic cells, natural killer, natural killer T and follicular helper T cells, were also depleted by AZD2014 and RAPA, demonstrating that mTOR activities are important for the survival of these cells in the steady state. These results call for researchers' attention to monitor the effects of TORKinibs on the normal immune cells, in addition to those on malignant cells, in clinical trials to investigate the antitumor effects of TORKinibs.
They then studied the impacts of AZD2014 on alloresponses using a heart transplant model, in which AZD2014 was transiently administered from day 3 to day 11 after transplantation. Although AZD2014 was able to prolong the survival of heart allografts, its suppressive effects were less potent than RAPA, despite the fact that AZD2014 inhibited both mTORC1 and mTORC2. These data confirmed those from their earlier study to investigate the impacts of a similar TORKinib AZD8055 on allograft survival using the same model.8 More strikingly, after AZD2014 and RAPA were withdrawn after the 9-day administration, whereas levels of donor-specific antibodies (DSA) in RAPA-treated mice were still suppressed on days 21 and 100 posttransplant, levels of DSA in AZD2014-treated mice were comparable to those in the control diluent-treated group and were significantly higher than those in the RAPA-treated group at both time points. In association with the increased DSA levels, the numbers of follicular helper T cells and B cells in the spleen of AZD2014-treated mice at day 21 posttransplant were higher than those in mice receiving RAPA. Thus, Fantus et al's work is the first to investigate the effects of TORKinibs on alloantigen-induced humoral responses and demonstrates that TORKinib AZD2014 possesses inferior ability to control DSA compared to RAPA. Their results are an extension to those from a previous study9 which demonstrated that TORKinib AZD8055, but not RAPA, facilitated the antibody class switching in vitro and in vivo using a hapten-protein conjugate vaccination model.
Collectively, Fantus et al’s work demonstrate that superior immunosuppressive effects may not be achieved simply by using compounds that inhibit both mTORC1 and mTORC2. On one hand, their work underscores the complexity of signaling mediated by mTOR in different immune cell populations and its role in alloantigen-induced immune responses. On the other hand, efforts should be made to further understand the mechanisms of failure of AZD2014 to mediate better immunosuppression. The authors have proposed that the pharmacologic property of AZD2014, which leads to lower concentrations of this compound in tissues and shorter half-life compared with RAPA, may be responsible for the lower efficacy. More studies to determine the effects of prolonged use of AZD2014 will help address this issue. It was shown that AZD8055, but not RAPA, promoted induction of anti-tumor effects in combination with agonistic anti-CD40 antibody in a murine renal carcinoma model, suggesting that TORKinibs may target more than mTORC1 and mTORC2 and mediate stimulatory effects.10 It remains to be determined whether the lack of inhibition of DSA in Fantus’ study is because certain stimulatory effects triggered by AZD2014 may have offset its suppressive effects on DSA.
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