As in the case of acute GHVD, chronic cutaneous GVHD exhibits an interface dermatitis, lymphocyte satellitosis, and vacuolar changes at the basal cell layer. Apoptosis within the basilar or lower spinosum layers has been proposed to be the minimal histologic criteria for active GVHD. Acanthosis and wedge-shaped hypergranulosis resembling lichen planus may be seen. In many cases it may not be possible to distinguish acute and chronic epidermal involvement histologically [53, 98].

Sclerotic involvement of the papillary dermis may resemble lichen sclerosus with atrophy, hyperkeratosis, follicular plugging, and a pale homogenized appearance of the papillary dermis collagen. If epidermal changes of GVHD are not present, dermal fibrosis with thickened collagen bundles and loss of periadnexal fat involvement may be indistinguishable from idiopathic morphea/scleroderma. Subcutaneous and fascial involvement accordingly reveal changes in the fat septae and fascia, including thickening, edema, and fibrosis, with variable infiltration of lymphocytes, histiocytes, and eosinophils [53, 95, 98].

The first step of acute GVHD begins before donor cells are infused. Tissue damage from the recipient’s underlying disease, infection, and/or the conditioning chemotherapy/radiotherapy leads to inflammatory responses through inflammatory cytokine production and host APC activation. Inflammatory cytokines (TNF-α and IL-1β) have been implicated in the pathogenesis of acute GVHD [43, 48, 49, 88, 115]. Total body irradiation (TBI) is particularly important because it activates host tissues to secrete inflammatory cytokines [43, 115] and induces damage to the gastrointestinal (GI) tract [43, 44]. Increasing evidence suggests that damage to the GI tract during acute GVHD plays a major pathophysiologic role in the amplification of systemic disease. It increases the translocation of inflammatory stimuli such as microbial products including LPS .

Clinical studies first suggested that a correlation between GVHD severity and radiation dose exists [18, 81] and that more severe GVHD occurs after conditioning regimens including radiation therapy than when only chemotherapy is used [19]. Large doses of TBI increase GVHD severity by amplifying the dysregulation of inflammatory cytokines [43]. TBI and allogeneic immune cells synergize to damage the GI tract, thereby permitting increased translocation of LPS into the systemic circulation. This damage, together with increased production of TNF-α by host cells after TBI that promotes further inflammation and additional GI tract damage [44] leads to an increase in the morbidity and mortality in patients with GVHD The GI tract is therefore critical to the propagation of the “cytokine storm ” characteristic of acute GVHD.

LPS is a potent stimulator of inflammatory cytokine production, such as TNF-α and IL-1 , which are important mediators of clinical [1, 48] and experimental GVHD [43, 45, 115]. The production of TNF-α by monocytes and macrophages is transduced by two signals. The first is a priming signal that may be provided by IFN-γ [82] and radiation [43]. The second is a triggering signal provided by bacterial products such as LPS [82]. Translocation of LPS across a damaged gut mucosa provides access to the systemic circulation where LPS triggers monocytes and macrophages primed by the effects of IFN-γ to release cytopathic amounts of inflammatory cytokines [82]. Together with natural killer (NK) and T cells, these cytokines mediate GHVD target organ damage. GVHD is associated with elevated serum levels of both TNF-α and LPS on day 7 after BMT [43]. The importance of LPS in GVHD has further been supported by a mouse study using strains that differ in their sensitivity to LPS as donors in an experimental BM transplant (BMT) system showing that donor resistance to LPS reduces the development of acute GVHD [22]. In murine GVHD, it has been demonstrated that the administration of anti-TNF-α reduces cutaneous and intestinal lesions and mortality [88]. Increased levels of TNF-α have also been found in the serum of allogeneic bone marrow transplant recipients and have been correlated with the severity of GVHD [49]. Administration of recombinant human IL-1 receptor antagonist has been shown to improve acute GVHD and the response to therapy was associated with a reduction of TNF-α mRNA levels in blood mononuclear cells [1].

This phase represents the core of the GVH reaction, where donor T cells proliferate and differentiate in response to host APCs. Inflammatory cytokines and microbial products generated in Phase I augment this activation at least in part by increasing the expression of costimulatory molecules [29]. Donor T cells are critical in the induction of acute GVHD because depletion of T cells from the bone marrow graft effectively prevents GVHD [58] but at the expense of an increase in marrow graft rejection and leukemic relapse [37]. Recent clinical studies confirm experimental data demonstrating that the severity of GVHD correlates with the number of donor T cells transfused [57].

MHC class II differences between donors and recipients stimulate CD4 T cells, whereas MHC class I differences stimulate CD8 T cells [59, 99]. In the majority of HLA-identical HCTs, both CD4 and CD8 subsets respond to minor histocompatibility antigens and can cause GVHD. Thus both donor CD4 and CD8 T cells have crucial roles in the pathogenesis of GVHD.

Acute GVHD appears to be primarily a Th1 -driven process with massive release of IFN-γ, IL-2, and TNF-α and these cytokines have been implicated in the pathophysiology of acute GVHD [7, 29].

IL-2 production by donor T cells remains the principal target of many current clinical therapeutic and prophylactic approaches to GVHD. Monoclonal antibodies (mAb) against IL-2 or its receptor prevent GVHD when administered shortly after the infusion of T cells [30, 42]. Cyclosporine and FK506 dramatically reduce IL-2 production and effectively prevent GVHD [91]. But emerging data indicate an important role for IL-2 in the generation and maintenance of CD4⁺CD25⁺ regulatory T cell s (Tregs), suggesting that prolonged interference with IL-2 may have an unintended consequence of preventing the development of long-term tolerance after allogeneic HCT [119].

IL-15 is another critical cytokine in initiating allogeneic T-cell division in vivo, which has similar biological activities to IL-2, and has been identified as an indispensable costimulator in an experimental mouse model of skin GVHD [77, 78]. In humans, elevated serum levels of IL-15 are associated with acute GVHD [61].

IFN-γ has multiple functions and can either amplify [79] or reduce GVHD [9, 116] depending on the timing of its production. IFN-γ can have immunosuppressive effects at early time points after HCT but can exacerbate disease via its pro-inflammatory properties at later stages [7, 29]. IFN-γ may amplify GVHD by increasing the expression of molecules including adhesion molecules, MHC proteins, and chemokine receptors [29]. It also primes macrophages to produce pro-inflammatory cytokines and nitric oxide (NO) in response to LPS [82]. By contrast, IFN-γ may suppress GVHD by hastening the apoptosis of activated donor T cells [92].

Th2 -type cytokines can reduce acute GVHD [112] as mice receiving donor Th2-type cells are protected from GVHD [33]. Moreover, donor T cells that lack the four classical Th2 cytokines (IL-4, 5, 9, and 13) enhance T-cell proliferative responses and aggravate GVHD [102].

Th17 cells have been shown to have a direct role in GVHD pathogenesis. It has been shown that IL-17^−/− donor T cells augmented Th1 differentiation and exacerbated acute GVHD [117] and adoptive transfer of in vitro differentiated Th17 cells induced lethal acute GVHD [13], although other studies have shown that an absence of IL-17 production by donor cells markedly impairs the development of CD4-mediated acute GVHD [56].

Genetic polymorphisms in TNF-α, IL-10, and IFN-γ have been linked to increased risk and severity of GVHD [14, 65, 71].

Subpopulations of regulatory cells can prevent GVHD. CD4⁺CD25⁺ Treg cells can suppress the proliferation of conventional T cells and prevent acute GVHD in animal models when added to the donor inoculum containing alloreactive T cells [20, 46, 103]. Donor NK cell s [2] and NKT cells of both the host and donor have been shown to suppress acute GVHD [120]. As well, CD3⁺CD4⁻CD8⁻ double-negative regulatory T cell s, which specifically suppress CD8 and CD4 T cells that are primed against the same alloantigen, inhibit GVHD [76, 118].

The final effector phase of acute GVHD is characterized by cell damage via cellular mediators including cytotoxic T lymphocytes (CTLs) and NK cells and soluble inflammatory mediators including TNF-α, IFN-γ, IL-1, and NO [29, 31]. Three cytolytic pathways have been identified as important to GVHD: the perforin/granzyme B pathway, the Fas/FasL pathway, and direct cytokine-mediated injury [8, 44]. The soluble and cellular mediators synergize to amplify local tissue injury and further promote inflammation and target tissue destruction.