Treg cells, cancer vaccines: timing is everything

In the current issue of Blood, Litzinger and colleagues demonstrate the critical role of proper timing of the denileukin diftitox (ONTAK) treatment for its ability to boost the immunologic activity of cancer vaccines.

Therapeutic vaccines against established cancer need to counteract tumor-induced immunosuppression and to selectively enhance the effector/memory type-1 immunity, mediated by CTLs, TH1 cells, and NK cells, without enhancing preexisting regulatory T (Treg)–cell responses.¹  Unfortunately, although tumors themselves are potent inducers of Treg activity,²  the currently used cancer vaccination schemes also can lead to enhanced frequencies of Treg cells³  characterized by surface CD4⁺CD25^high phenotype, as well as the intracellular expression of FoxP3.¹ 

The ability of denileukin diftitox (DAB₃₈₉IL-2; a conjugate of interleukin 2 [IL-2] and diphtheria toxin) to preferentially eliminate the cells expressing high levels of CD25 (IL-2 receptor) has been utilized to boost the effectiveness of cancer vaccines by eliminating CD4⁺CD25^high Treg cells. However, even if human studies have shown thatappropriate dosing of denileukin diftitox allows the selective elimination of CD4⁺CD25^high Treg cells,⁴  a paucity of mouse data on the use of denileukin diftitox (traditionally, anti-CD25 antibodies, or low-dose cyclophosphamide, have been used to eliminate mouse Treg cells) makes it difficult to establish the optimal regimens aimed at selective elimination of Treg cells⁴  with concomitant sparing of the effector T cells that also express significant levels of CD25. A recent study demonstrated that denileukin diftitox can be used as a single agent against established mouse tumors,⁵  but the current lack of systematic animal studies addressing the mechanism of its function and comparing different schemes of its application makes it difficult to design optimized regimens for use of this reagent in combination with active immunotherapy.

In the current paper, Litzinger and colleagues provide a thorough analysis of the impact of denileukin diftitox on the prevalence and function of CD25⁺ FoxP3⁺ Treg cells in the blood, spleen, and bone marrow of transgenic mice expressing human CEA, and correlate the in vitro effects of denileukin diftitox in mouse and human systems. The authors demonstrate the selectivity of depletion of Treg cells, but not other subsets of CD4⁺ and CD8⁺ T cells, and the critical role of the timing of denileukin diftitox administration for its optimal immunopotentiating effect in vivo. The demonstration that the same concentration of denileukin diftitox can either potentiate the induction of antiself, CEA-specific immune responses (when administered 1 day prior to vaccination) or have a negative impact (when administered at day 3 after vaccination) helps to reconcile some of the controversies regarding its clinical use.

While the scope of the paper does not include experiments in animals bearing established (CEA-expressing) tumors, which would allow the authors to address the ability of different denileukin diftitox regimens to improve the therapeutic activity of CEA-targeting vaccines, the current data help to address, in preclinical mouse models, the role of the tumor- and vaccination-induced Treg cells in regulating the therapeutic activity of cancer vaccines, and facilitate the design of clinical trials of combination therapies involving denileukin diftitox or other Treg-targeting factors.