PRE ACTUADORES

Current strategies o f GVHD prophylaxis rely on non-specific immunosuppressants such as Cyclosporin A, FK 506 and rapamycin; antimetabolites such as m ycophenolate and azathioprine and cell cycle inhibitors such as methotrexate (Reviewed in (Vander Woude, 1997)). The main problem with all o f these agents is the lack o f specificity of their actions. Such global immunosuppression increases the transplant recipient’s susceptibility to infections and increases the risk o f relapse of their primary malignancy. The realisation that T -cells were crucial for the initiation o f GVHD led to the developm ent o f T -cell depletion strategies as alternative GVHD prophylaxis, demonstrating that the risk o f

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developing GVHD could be significantly reduced by T -cell depletion. Furthermore, it was realised that appropriate T -cell depletion {i.e. >2 log depletion o f T -cells from the graft) in animal models circumvented the need for non-specific post-transplant immunosuppression (Antin et al., 1991; Lowenberg et al., 1986; Young et al., 1 9 9 2 ). However, recipients o f T-cell depleted grafts experienced a higher incidence o f graft failure, leukaemic relapse and delayed immune reconstitution compared with recipients o f T -cell replete grafts (Apperley et al., 1988; Daley et al., 1987; Goldman et al., 1988; Maraninchi et al., 1987; Small et al., 1997). Therefore, T -cell depletion suffers from som e o f the same problems as non-specific immunosuppressants.

T -cell depletion strategies represent perhaps the earliest attempt to engineer haematopoietic stem cell grafts. The problems associated with depletion o f all T -cells prompted investigation o f alternative methods of graft engineering and G VHD prevention. The problem o f graft failure results from the rejection o f d onor haematopoietic cells by residual host T-cells that have survived pre-transplant conditioning (Bunjes et al., 1987; Keman et al., 1987). This problem has been circumvented by administration o f Campath antibodies in vivo prior to transplant, thereby removing residual host T-cells and alleviating the problem o f graft rejection (H ale et al., 1998). The increased relapse rates in recipients o f T -cell depleted grafts led to the developm ent o f donor leukocyte infusion (DLI) protocols (Drobyski et al., 1999; Kolb et al., 1990; Kolb et al., 1995; Mackinnon et al., 1995). T -cell depletion follow ed by DLI has been successful in treating or preventing relapse in CML patients with responses also noted in patients with acute leukaemia, CLL, myeloma and lymphom a (M andigers et al., 1998). Although DLI protocols limit the risk o f GVHD, they do not abolish the risk com pletely and GVHD remains a significant hazard associated with administration o f DLIs (Collins et al., 1997; Drobyski et al., 1993; Kolb et al., 1990; Kolb et al., 1995; M ackinnon et al., 1995; Porter et al., 1994).

More recently, a number o f groups have investigated the possibility o f ex vivo g en e therapy to insert dm g-inducible suicide genes into donor lym phocytes (Bonini et al., 1997; Bordignon et al., 1995; Drobyski et al., 2001; Thom is et al., 2001; Tiberghien et al., 1997). The herpes simplex vim s thymidine kinase (HSV-r/:)/ganciclovir (G C V )-based suicide strategy has been most widely adopted. In this system, cells are engineered to express HSW-tk such that administration o f GCV leads to cell death through r/:-catalysed m etabolism o f the dm g to a lethal metabolite. Because the YlSY-tk gene is only active in cycling cells, the administration of GCV to patients at strategic time-points after transplant eliminates only proliferating cells (probably alloreactive, but potentially cells involved in hom eostatic proliferation). This system has been used effectively to prevent G VH D although problems remain with the immunogenicity o f the viral transduced T -cells (Thom is et al., 2001).

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A more sim plified graft-engineering protocol was developed by Koh et al. (Koh et at., 1999). These investigators incubated donor and patient lym phocytes in an MLR, then depleted the cultures o f CD69"^, and hence alloreactive, T-cells. In 2° MLRs, the allospecific proliferation was -10% o f the original value while S"' party allospecific proliferation was maintained (-77% o f the original value). Furthermore, CM V antigen- specific responses were maintained, raising hopes that GVHD would be prevented while maintaining the GVL potential o f the grafts. Intelligent ex vivo graft engineering m ay therefore represent the most feasible short-term prospect for GVHD prevention. N evertheless, numerous other strategies are currently under investigation.

Blocking antibodies against CD3 (Yu et al., 2001), CD28 (Yu et al., 2000), and C D 40L (Blazar et al., 1997; Durham et al., 2000; Durie et al., 1994; Ito et al., 2001; W ekerle et al., 2 00 0) as well as CT L A 4-lg (Wekerle et al., 2000; Wekerle et al., 1998) are all proposed to prevent GVHD and in som e cases lead to the induction o f peripheral tolerance. The prevention o f GVHD by manipulating cytokine networks has also been attempted but with limited success and conflicting results. It has been suggested that acute GVHD is a T hl-m ediated process whereas chronic GVHD has been suggested to be T h2- mediated (Allen, 1993; De Wit et al., 1993; Fowler et al., 1994; Garlisi et al., 1993; Krenger and Ferrara, 1996). There is some evidence that acute GVHD can be inhibited by Th2 cells (Krenger and Ferrara, 1996). However, IL-12 (thought to represent a T hl type cytokine) can protect against (Sykes et al., 1995) or augment (Via et al., 1994; W illiam son et al., 1996) GVHD. A recent publication by Liu et al. using a suicide g en e- based T -cell depletion method demonstrated that acute GVHD can be prevented to a certain extent by removing either type 1 or type 2 cells (Liu et al., 2001).