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MyD88/IRAK4 in donor T cells potentiates GVHD
ence of inflammatory milieu or potent co-stimulation induced robust T-cell proliferation even without T-cell MyD88 signaling. Although we detected only TLR2, TLR7, IL-1R, IL-18R, IL-33R but not TLR4, TLR5, or TLR9 in resting T cells, a diverse combination of expression of the TLR/IL-1R superfamily in T cells including TLR4 and TLR5 has been demonstrated, indicating that the range of TLR/IL-1R expression could be context dependent, such as differentiation and activation status and subtype of T cells, as well as environmental milieu, such as type of inflammation and microbiota.
Although we demonstrated the T-cell co-stimulatory functions of both TLR2 and TLR7 signaling in vitro, absence of TLR2 or TLR7 in donor T cells did not change the severity of GvHD. Our results are consistent with pre- vious studies showing that neither TLR2 or TLR7 deficien- cy in donor cells nor pharmacological inhibition of TLR2 altered morbidity and mortality of GvHD.32,33 These results suggest that TLR have no role in GvHD. IL-18R and IL- 33R signaling in donor T cells has also conflicting roles in GvHD, depending on the experimental models used and the timing of administration of their ligands.34-37 Given this redundancy and conflicting role of each of IL-1R/TLR superfamily in T cells, we took advantage of MyD88-/- T cells that enabled us to block most of the signals from receptors belonging to the TLR/IL-1R superfamily. We found that MyD88-/-donor T cells showed significantly impaired survival and differentiation toward Th1, Tc1, and Th17 cells, while preserving Foxp3+ Treg expansion after allo-SCT, resulting in significant improvement of GvHD. This impairment of MyD88-/- T-cell functions after allo-SCT was confirmed in in vitro culture. It has been shown that impairment of Th1, Tc1, and Th17 results in mitigated GvHD.38 A recent study by Lim et al. showed that MyD88 deficiency in donor T cells did not ameliorate GvHD, but decreased GvL using a similar model of BMT, even though they also showed impaired Th1 differentia- tion of MyD88-/- T cells after BMT.39 It should be noted that GvHD of allogeneic controls was less severe with 30% mortality in that study compared to severe GvHD with 80% mortality in our study. Thus, the mild experi- mental condition in that study may compromise our see- ing a reduction in GvHD. GvL activity determined by leukemia mortality and in vivo BLI was preserved in the absence of T-cell MyD88, suggesting that T-cell MyD88 signaling plays a more important role on GvHD than on GvL effects in our model. However, its contribution to GvHD and GvL may change in allo-SCT following reduced intensity conditioning, as shown in the Lim et al. study.39
Pharmacological inhibition of TLR/IL-1R has been stud- ied in murine models and clinical studies. Pharmacological blockade of IL-1/IL-1R or IL-33/IL-33R interaction amelio- rates experimental GvHD;40 however, a randomized trial
failed to show any protective effect of the IL-1R antago- nist Anakinra against clinical GvHD.41-43 MyD88 is an adopter molecule to recruit IRAK to most of the receptors belonging to the IL-1R/TLR superfamily to relay signals to downstream proinflammatory pathways.44,45 Given the redundant roles of a wide range of TLR/IL-1R, inhibition of multiple TLR/IL-1R signals by MyD88 or IRAK4 inhibitors would be more effective than a single pathway blockade. We showed that pharmacological inhibition of MyD88/IRAK4 pathway using IRAK4 inhibitor ameliorat- ed experimental GvHD. IRAK4 also plays a critical role in T-cell activation46 and IRAK4-deficient T cells showed sig- nificantly delayed allogeneic skin graft rejection.46 The IRAK4 inhibitor could not only affect donor T cells, but also donor accessory cells and recipient cells; however, it has been reported that MyD88 signaling in the donor BM cells or recipient cells did not enhance GvHD, thus it is likely that IRAK4 inhibitor ameliorated GvHD by acting on donor T cells.4,47 There are several MyD88 or IRAK4 inhibitors that demonstrated anti-inflammatory effects in pre-clinical models. PF-06650833 is one of IRAK4 inhibitors in clinical development, that was previously tested in a phase I trial for patients with systemic lupus erythematosus and phase II clinical study for patients with rheumatoid arthritis is ongoing (https://clinicaltrials.gov/ ct2/show/NCT02996500?term=06650833&rank=3).48,49
In conclusion, our results demonstrate a previously unrevealed role of T-cell MyD88 signaling in the develop- ment of GvHD. Because GvL effects were at least pre- served in the absence of donor MyD88 signaling, the IRAK-4 inhibitor PF-06650833 is an ideal agent to amelio- rate GvHD, likely by inhibiting MyD88/IRAK4 signaling in donor T cells with sparing. Furthermore, the IRAK4 inhibitor is also under development as a therapeutic reagent against B-cell neoplasms with MyD88 mutation, indicating that therapeutic or prophylactic administration of IRAK4 inhibitor after allo-SCT could ameliorate GvHD with enhancing tumor control of B-cell neoplasms.50 Signaling molecules such as JAK, MEK, AURKA, and many other molecules that integrate the signals from mul- tiple receptors of cytokines or growth factors, are promis- ing therapeutic targets of GvHD.51,52 Given the critical role of IRAK4 in activation of human T cells, MyD88 and IRAK4 inhibitors that could inhibit signaling from more than ten receptors belonging to the TLR/IL-1R superfami- ly should be tested in clinical studies to explore their pro- phylactic and therapeutic potentials against GvHD.53
Funding
This study was supported by JSPS KAKENHI (21390295 and 17H04206 to TT, 17K09945 to DH), Japan Society of Hematology Research Fund (TT), the Center of Innovation Program from JST (TT), and research grants from the Mochida Memorial Foundation for Medical and Pharmaceutical Research (DH).
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