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S. Copsel et al.
and improved long-term allograft survival in a corneal transplant model.41 These findings led to combination therapy with synergistic effects on Treg expansion using free LD IL-2 with sirolimus in transplant models, i.e. cornea42 and skin.43 Similar results in combination with dexamethasone (Dex) were observed in EAE.44
To increase circulating IL-2 half-life and decrease the required dose, antibody/cytokine (a-IL-2/IL-2) complexes (IL-2C) are under development. Notably, in vivo utilization of different IL-2C resulted in targeting specific cell subsets, i.e. mAbJES-1A12/IL-2 and S4B6/IL-2 preferentially expanded Tregs and Tconv, respectively.45 The IL-2C (mAbJES-1A12+IL-2) caused selective Treg expansion and suppression of allergic airway inflammation,46 contact hypersensitivity,47 and experimental myasthenia gravis.48 Furthermore, in renal and cardiac ischemia reperfusion models, IL-2C expanded Tregs attenuated acute renal damage improving renal and myocardial recovery.49,50 Using a combination therapy of IL-2C, sirolimus and islet Ag peptide, protection against spontaneous and induced T1D in NOD mice occurred following increased Treg lev- els and function.51 Recombinant adeno-associated viral vector that continuously releases IL-2 achieved persistent and sufficient levels of LD IL-2 while avoiding toxic effects.52,53 Importantly, this viral vector controlled diabetes after sustained Treg expansion without impairing immune responses to infection, vaccination and cancer. Recently, an IL-2 modification creating a fusion protein between murine IL-2 and CD2554 achieved markedly extended half- life [16 hours vs. free IL-2 (<15 minutes)] and selective Treg expansion controlling T1D. Other examples of long-lived IL-2 fusion protein (FP) include: IgG-IL2 FP55 and IL-2+IL- 33 (IL-233),56 discussed below.
IL-2/CD25 Treg manipulation in pre-clinical and clinical allo- geneic hematopoietic stem cell transplantation. Adoptive trans- fer of Tregs is a promising therapy to diminish GvHD. In humans, development of cGvHD is associated with poor Treg reconstitution post HSCT.57,58 We and others have been examining the application of donor Tregs as a pro- phylactic strategy to prevent the development of GvHD in experimental models.59-65 Experiments demonstrated that donor Tregs inhibit lethal acute GvHD (aGvHD) only at high ratios, i.e. 1:1 (Treg:Tconv).59 Because circulating Tregs account for only approximately 5-10% of CD4+ T cells, a practical obstacle is collecting sufficient numbers of Tregs from donors or recipients for use in allogeneic hematopoietic stem cell transplantation (aHSCT) to sup- press activation of anti-host reactive Tconv in T-cell- replete grafts, thus in vivo expansion is attractive.
A. Treatment of recipients: pre-clinical and clinical models. In the early 1990s, using a fully MHC-mismatched bone marrow transplant (BMT) model, studies demonstrated that short-term human recombinant IL-2 administration (50,000 U twice daily) starting the day of the BMT signif- icantly reduced GvHD mortality.66 Moreover, IL-2 treat- ment did not prevent allogeneic engraftment and pre- served graft-versus-leukemia (GvL) effect. The IL-2 effect on GvHD but not on GvL was explained by selective inhi- bition of CD4-mediated activity.66 Clinical and immuno- logical effects of IL-2 administration in patients following TCD allogeneic and autologous BMT were evaluated. Strikingly, patients who received TCD-BM and LD IL-2 (2- 6x105 U/m2 daily for 90 days) to enhance GvL exhibited
high circulating NK cells.67,68 Since CD25+ Tregs had not yet been identified, the success of these studies was not immediately attributed to Treg expansion. Later, discov- ery of this suppressive population69 and the recognition that their infusion inhibited immune responses opened a new era of GvHD prophylaxis and treatment.59-62,64,65 Indeed, the addition of IL-2 (6x105 IU/m2 daily) with donor CD4+ T cells resulted in expansion of Tregs in patients post transplant.70 A seminal phase I dose-escalation study demonstrated that therapy of LD IL-2 daily for eight weeks in patients with steroid refractory active cGvHD was well tolerated and induced significant Treg expansion (Online Supplementary Table S1).71 This treatment dimin- ished cGvHD manifestations, including decreased cuta- neous sclerosis in a considerable number of patients.71 Notably, this IL-2 regimen induced selective activation of pStat5 in Tregs versus Tconv, which was associated with increased thymopoiesis and production of Tregs.72 These observations demonstrated that more nTreg homeostasis appeared to be restored following IL-2 therapy in patients with cGvHD.73
Ultra LD IL-2 (0.1-0.2x106 IU/m2 thrice weekly) was administered as aGvHD prophylaxis in pediatric patients after aHSCT starting <day 30 and continuing 6-12 weeks. Treg levels increased in recipients of matched related donor and the Treg functional suppressor activity was restored. IL-2 treated recipients had diminished humoral but not virus-specific cellular immune responses. Importantly, treated patients showed GvHD inhibition with no increase in leukemia/lymphoma relapse rates ver- sus controls, suggesting that ultra LD IL-2 increased Tregs without impairing GvL.74 In contrast, when IL-2 (1x106 IU/m2 daily) was administered in a patient seven years post-GvHD onset, therapeutic effects were decreased.75 The patient experienced partial improvement of GvHD symptoms, Treg levels were increased after one week but declined despite continual IL-2 administration for approx- imately two months. A clinical trial randomized 90 sub- jects to determine whether LD IL-2 administration post transplant could reduce the incidence of both leukemia relapse and cGvHD. Patients were treated with LD IL-2 post-HSCT (day 60) for two weeks (followed by a 2-week hiatus) and compared with untreated controls. The treated group had a lower incidence of cGvHD, accompanied by a significant increase in GvHD-free and GvHD progres- sion-free survival at three years, but administration did not decrease the incidence of leukemia relapse. Correlative studies demonstrated that circulating Treg and natural killer (NK) cells were increased in the IL-2 cohort during the treatment periods.76
In pre-clinical studies (Figure 2), treatment with LD IL- 2C in MHC-matched HSCT recipients transplanted fol- lowing reduced-intensity conditioning (RIC) regiments, administered either prior or following aHSCT, promoted expansion and activation of host Tregs within the first 7- 10 days post transplant. Notably, host-versus-graft responses (HvG) were inhibited, resulting in enhanced donor chimerism and long-term hematopoietic cell engraftment.77 This work indicated for the first time that in vivo administration of IL-2C following BMT was a viable approach to expand host Tregs and consequently regulate alloimmunity following transplantation.78 In con- trast, using an MHC-haploidentical aHSCT model, IL-2 administration post transplant for ten days did not prevent GvHD nor improve survival; comparable results were
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haematologica | 2019; 104(7)