Biology of chronic graft-vs-host disease: Immune mechanisms and progress in biomarker discovery

Table 1 Signs, symptoms and prevalence of chronic graft-vs-host disease in selected organs and tissues

Organ or tissue	Prevalence, %1	Diagnostic features2	Distinctive features3
Skin	75%	Poikiloderma Lichen planus-like features Sclerosis Morphea-like features	Depigmentation Papulosquamous lesions
Mouth	51%-63%	Lichen planus-like features	Xerostomia Mucoceles Mucosal atrophy Ulcers Pseudomembranes
Liver	29%-51%	None4	None4
Eye	22%-33%		Dry, gritty or painful eyes Cicatricial conjunctivitis Keratoconjunctivitis-Sicca syndrome
GI tract and esophagus	7%-45%	Esophageal web Strictures or stenosis in upper esophagus
Lung	4%-19%	Bronchiolitis obliterans	Air trapping and bronchiectasis on chest CT scan
Muscles, fascia and joints	6%	Fascitis Sclerosis Joint stiffness or contractures	Myositis or polymyositis
Genitalia	1%	Lichen planus-like features Lichen sclerosus-like features	Erosions, Fissures, Ulcers

¹Frequency of tissue involvement at initial cGVHD diagnosis (from Lee et al[4]);

²Clinical symptoms that are sufficient for cGVHD diagnosis. Information adapted from references 8 and 13;

³Clinical symptoms that are frequently seen in cGVHD, but insufficient for cGVHD diagnosis. Information adapted from references 8 and 13;

⁴While no diagnostic or distinctive features have been identified for liver cGVHD, hepatitis is often seen (and also sometimes in acute GVHD) with elevated serum levels of bilirubin, alkaline phosphatase and alanine aminotransferase (ALT)[13]. cGVHD: Chronic graft-vs-host disease; CT: Computed tomography.

Table 2 Immune cell types and their function in chronic graft-vs-host disease

Cell type	Subtypes	Key cytokines or markers	Brief summary of disease involvement
CD4+ T cells	Th1	IFN-γ, TNF-α	Pro-inflammatory. Important in acute GVHD, but role in cGVHD unclear
	Th2	IL-4, IL-13	Stimulate antibody production. Role in clinical cGVHD poorly defined
	Th17	IL-17; also IL-21, IL-22, TNF-α	Pro-inflammatory. IL-17 levels correlate with disease severity; IL-17 induces scleroderma of skin and lung
	Tregs	TGF-β, required for Treg proliferation and (differentiation)	Produced mostly in thymus. Suppress autoreactive T cells. Lower levels of Tregs present in cGVHD patients, associated with thymic damage and loss of self-tolerance in cGVHD
	T follicular helper cells1	Express CCR5, PD-1 and ICOS	Promote abnormal B cell maturation into long-lived active plasma cells, and IgG secretion
CD8+ T cells		CXCL9, CXCL10	Mediate graft-vs-tumor effect of transplant. Serum CXCL9 levels elevated in cGVHD patients
B cells (total)		Increased BAFF/B-cell ratio, elevated serum BAFF levels	Decreased in active cGVHD. Remaining B cells are resistant to apoptosis
Naïve and transitional B cells		CD19	Decreased in active cGVHD
Memory B cells (total)		CD19, CD27	Decreased in active cGVHD. Cells essential for a normal immune response to bacterial pathogens or opportunistic infections
Regulatory B cells		IL-10	Decreased in active cGVHD. Function to maintain tolerance and help prevent autoimmune disease
Plasma cells		CD27, CD38	Increased in active cGVHD. Cells secrete immunoglobulins including IgGs and are resistant to apoptosis

¹Mainly classified into Th2 and Th17 subtypes[44]. cGVHD: Chronic graft-vs-host disease; BAFF: B-cell activating factor; CD3: Cluster of Differentiation molecule 13, 19, 27 and 38; CCR5: Chemokine (C-C Motif) receptor 5; CXCL: Chemokine (C-X-C motif) ligand; ICOS: Inducible T-Cell Co-Stimulator; IFN-γ: interferon gamma; IL: Interleukin; PD-1: Programmed cell death 1; Th: T helper cell; TGF-β: Transforming growth factor beta; TNF-α: Tumor necrosis factor alpha.

Table 3 Candidate biomarkers of chronic graft-vs-host disease¹

Gene/protein	Function	Biofluid3	Ref.
BAFF, soluble; BAFF/B cell ratio	Growth factor, promotes B cell expansion and activation	Blood	[59,60,77]
CXCL92	Chemokine produced by activated T cells	Blood	[51,79]
CD-13, soluble	Antigen presentation	Blood	[59]
C-reactive protein3	Acute phase protein	Blood	[12]
Cystatin B	Inhibitor of cathepsin proteases	Saliva	[80]
IL-1ra	Inhibitor of IL-1 receptor signaling	Saliva	[80]
IL-2R, soluble	IL-2 receptor, marker of activated T cells	Blood	[59,76]
IL-6	Pro-inflammatory Th2 cytokine	Blood	[42,75]
IL-10	Th2 cytokine	Blood	[73]
IL-15	Enhances anti-tumor function of CD8+ T cells	Blood	[78]
Lactoperoxidase	Anti-microbial enzyme	Saliva	[81]
Lactoferrin	Iron-binding glycoprotein	Saliva	[81]
MICA, soluble	Stimulates T cell activity via NKG2D receptor	Blood	[30]
TGF-β	Anti-inflammatory cytokine; stimulates activity of Tregs	Blood	[33]
TNF-α	Pro-inflammatory Th1 cytokine	Blood	[73-75]

¹This table only includes proteins identified in human biofluids. Antibodies are discussed in the text;

²Blood markers were measured in either plasma or serum isolated from peripheral blood, depending on the study. For saliva, whole unstimulated saliva collected from oral cGVHD patients was used;

³Increased CRP levels were especially associated with joint/fascia and skin involvement, compared to the non-cGVHD control group. cGVHD: Chronic graft-vs-host disease; BAFF: B-cell activating factor; CXCL: Chemokine (C-X-C motif) ligand; CD-13: Cluster of Differentiation molecule 13 (or aminopeptidase N); IL-1ra: Interleukin 1 receptor antagonist; IL-2R: Interleukin 2 receptor; MICA: MHC class I-related chain A; TGF-β: Transforming growth factor beta; TNF-α: Tumor necrosis factor alpha; CRP: C-reactive protein.