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Cytokines In GVHD

Cytokines play an important role in HSCT, as they are mediators that aid in the coordination of interactions between immune cells. The aberrant production of cytokines, and the subsequent effects of which, have been implicated in being causative or at least contributory in producing some of the unfavourable outcomes following transplant, most notably GVHD. The manifestations of aGVHD are thought to arise following monocyte and T cell activation that is consequential of a ‘cytokine storm' (Hill etal., 1997a). The cytokine storm and its resultant effects can be envisaged by a 3-step process (Figure 1.8), the first phase of which is the induction of inflammatory cytokines by the conditioning regimen inflicted on the host. Ionising radiation has been found to activate host cells, inducing them to secrete pro-inflammatory cytokines such as TNF, IL-1 and IL-6. Remberger et al. (1995) found that patients who develop post-transplant complications had significantly higher levels of TNF. High levels of the inflammatory cytokine IL-1p, TNF and IL-6 have also been associated with the severity of cGVHD (Barak et al., 1995). Furthermore, increased mRNA expression of IL-ip, IL-6 and TNF has been seen in PBMCs during

the development of aGVHD/cGVHD. Moreover, the degree of increase of the mRNA was dependent on the severity of disease (Tanaka et al.,

1995).

Irrad iation

Figure 1.8 The three postulated pathophysiological phases of GVHD

(Modified from Hill at a!., 1997a).

Blockade of the cytokines that are released due to the conditioning may be an approach for GVHD prophylaxis. However, some of these cytokines and the resultant responses to them may also be necessary for the GVL effect. Hill at ai. (1999) found in a murine model that TNF is critical to the GVL effect mediated by the donor T cells. Neutralisation of IL-1 caused a decrease in GVHD mortality without affecting CTL activity, whilst neutralisation of TNF did not prevent GVHD but did impair CTL activity.

Moreover, relapse was found to be faster with inhibition of TNF compared to IL-1 when leukaemia cells were added to the bone marrow inocula (Hill

Gta/., 1999).

The pro-inflammatory cytokines released due to the conditioning regimen facilitate donor T cell activation. This may be direct by stimulating T cells or indirectly via increased antigen presentation. In addition, irradiation also causes damage to the gastro-intestinal mucosa, resulting in the release of bacterial products such as LPS (Hill et aL, 1997b). As shown by Nestel et aL (1992) this may trigger the release of TNF from primed macrophages. It has been noted that an early increase in the levels of TNF may be predictive for the onset of GVHD (Holler et aL, 1990).

In the second phase, the activated donor T cells proliferate and secrete the type 1 cytokines IL-2 and IFNy. These may originate either from CD4+ or CD8+ T cells (i.e. Thi or Tel). These cytokines amplify the immune response of the donor cells to alloantigens through CTLs and NK cells. The importance of IL-2 is recognised by the use of cyclosporine, a drug that inhibits its production. This drug is an effective prophylactic agent for GVHD. Experimentally blockade of IL-2 inhibits the disease but confusingly high doses of IL-2 may also attenuate the syndrome if it is given early (Wang et aL, 1995). The increased secretion of IFNy that is noted in GVHD primes macrophages to produce inflammatory cytokines which can then be activated by the released LPS (Nestel ef a/.J992) and contribute to the pathology of the skin and gut. Blockade of IFN

attenuates GVHD. It has also been speculated that the decreased incidence seen with the use of cord blood stem cells for transplantation could be related to the decreased production of IFNy, TNF, IL-2 and IL-4 that has been documented from these cells (Chalmers etal., 1998).

Generally, it seems that a Thi response by the donor cells leads to aGVHD, whilst a deviation to Th2 leads to cGVHD. In a murine model, mice undergoing a chronic graft-versus host reaction produced the Th2 cytokines IL-4 and IL-10. The GVH cells exhibited defective IL-2 and IFNy production but elevated IL-4 (De Wit et a!., 1993). In addition, TGFpi that can be produced by Th2 cells has been shown to increase independently of platelet counts with the presence of cGVHD (Liem et a!., 1999).

The cachexia and target cell destruction that occurs in the gut, liver and skin is not explained by the systemic presence of cytokines alone. Apart from the direct damage caused by cytokines, host tissue is attacked by the action of recruited CTLs and NK cells (phase 3). The cytotoxic mechanisms involved in the GVHD process have been reported to be via the TNF/TNFR, the Fas/FasL and the perforin/granzyme pathways. Recent evidence indicates that the blockade of the Fas/FasL interaction prevents lethal aGVHD without impairing the GVL effect. The GVL effect may be mediated principally by the use perforin (Tsukada etal., 1999).

Another cytokine that may be important in GVHD is IL-10. It has been reported that following unrelated donor BMT, higher levels of IL-10 are

associated with lower overall occurrence of transplant related complications and early death (Baker et aL, 1999). However, Takatsuka

at al. (1999) found an association between marked elevation of plasma IL-

10 in the recovery phase of BMT and severe aGVHD, where it was suggested that large amounts of IL-10 are released in an attempt to suppress the inflammatory cytokines. Other cytokines may be important in HSCT such as IL-11. For instance, in a murine model, IL-11 treatment significantly reduced GVHD without impairing the cytolytic function or subsequent GVL activity of CD8+ T cells (Teshima etal., 1999).

Therapeutic use of Cytokines foilowing BMT

Apart from cytokines being important in the pathophysiology of GVHD, some of them are also useful in treating some of the complications of HSCT. They have been used to accelerate haematopoietic recovery, treat graft failure and as a therapy for infectious complications. The first commercially available cytokine for post-transplant stimulation of myelopoiesis was granulocyte and macrophage colony stimulating factor (GM-CSF) (Weinthal et a!., 1996). Multiple studies in adults and children have confirmed the role of GM-CSF in accelerating neutrophil recovery post-transplant. Its administration for graft failure has also been shown to improve one-year survival. G-CSF also significantly improves neutrophil recovery.

G-CSF is used to mobilise PBSCs for allogeneic transplantation. Despite the large increase (10-20 fold) in donor T cells transplanted, there is no

increase in GVHD. This may be due to G-CSF causing a decrease in the amount of TNF and IL-12 produced by dendritic cells (DCs) in response to LPS (Reddy et al., 2000). Recombinant erythropoietin may also be used in patients to stimulate erythropoiesis and reduce the need for red blood cell transfusions (Link at a!., 1994). In addition, IL-11 reportedly hastens the recovery of platelets after BMT (Hawley at a!., 1996). A cytokine used to aid therapy in post-transplant infectious complications is macrophage colony stimulating factor (M-CSF). A phase I trial in patients with invasive fungal infection following BMT showed a better 100 day survival after receiving M-CSF, which stimulated the survival, proliferation and function of monocytes and macrophages (Nemunaitis at a!., 1991).