Allogeneic stem cell transplantation in the treatment of acute myeloid leukemia: An overview of obstacles and opportunities

Table 3 Strategies to separate graft-versus-host disease and graft-versus-leukemia

Separation strategies	Approaches	Brief description	Ref.
GVHD risk prediction	GVHD biomarker testing	Contributes to GVHD diagnosis and provides evidence for the early use of anti-GVHD drugs	[123]
	Cytokine gene polymorphism testing	Helps to identify patients with a high risk of severe GVHD and take preventive measures	[124]
Modification of donor graft cells	Donor T cell depletion	Donor T cell depletion reduces GVHD while increasing the risk of infections, graft rejection, and disease relapse	[109]
	Graft-specific cell population depletion	Removing specific cell populations such as naïve T cells in the graft that consistently cause severe GVHD	[118]
	DLI to treat relapse	DLI is very effective in the treatment of relapsed slow-growing hematopoietic malignancies such as CML; however, the mechanism is unknown	[122]
	Application of CAR T cell	The combination of scFv that identifies leukemia-specific antigens and the activating domain of T cells enhances specific identification and killing of leukemia cells	[125,126]
	Suicide gene transduced donor lymphocyte infusion	A genetically modified suicide gene is introduced. Donor lymphocytes expressing this gene are sensitive to prodrugs, a feature that can be used when needed to regulate GVHD through the drug clearance of transduced cells	[127]
	Selecting memory T cells	Memory T cells cause mild or no GVHD and have critical graft-versus-tumor functions	[118]
	Enhancing activated γδ T cells	γδ T cells have the ability to kill leukemic blasts, and allogeneic TCR γδ T cells are not alloreactive and do not cause GVHD	[113]
	Selecting Tregs	Tregs suppress the activation and proliferation of effector T cells and downregulate the body’s response to foreign antigens or autoantigens	[86]
	Modifying/selecting other cells in the grafts	Selecting mesenchymal cells, NK cells, and manipulating dendritic cells and dendritic cell subsets	[79,122,129]
Drug intervention	Application of immunosuppressants	Various immunosuppressants suppress T cells and reduce GVHD via different mechanisms	[130]
	Application of HDACis	HDACis, such as vorinostat, downregulate inflammatory cytokines and increase the number of Tregs, thereby reducing the occurrence of GVHD, without effecting the GVL effect of donor CTLs	[131,132]
	Suppression of cytokines related to the occurrence of GVHD	Th1 cytokines such as TNF-α, IFN-γ, and IL-6 are related to aGVHD; Th2 cytokines such as IL-4, IL-5, and IL-10 are related to cGVHD. Appropriate regulation of these cytokines facilitates GVHD management	[122]
	Enhancing cytokines that suppress GVHD	Various cytokines such as IL-11 and keratinocyte growth factor reduce GVHD while preserving the GVL effect	[122]
	Targeting MiHAs on hematopoietic cells	CTLs targeting MiHAs such as HA-1 and HA-2 (expressed on hematopoietic cells only) promote the GVL effect	[121]
	Development and application of tumor vaccines	Vaccines targeting MiHAs on hematopoietic cells and leukemia-specific antigens improve GVL specificity	[133]

aGVHD: Acute GVHD; CAR: Chimeric antigen receptor; cGVHD: chronic GVHD; CML: Chronic myeloid leukemia; CTLs: Cytotoxic T lymphocytes; DLI: Donor lymphocyte infusion; GVHD: Graft-versus-host disease; GVL: Graft versus leukemia; HA-1: Histocompatibility antigens 1; HA-2: Histocompatibility antigens 2; HDACis: Histone deacetylase inhibitors; IFN-γ: Interferon-γ; IL-10: Interleukin 10; IL-4: Interleukin 4; IL-5: Interleukin 5; IL-6: Interleukin 6; MiHAs: Minor histocompatibility antigens; NK: Natural killer; Ref.: Reference; scFv: Single-chain variable fragment; TCR: T cell receptor; Th2: T-helper 2; TNF-α: Tumor necrosis factor α; Tregs: Regulatory T cells.