Transfusion Associated Graft-vs-Host Disease and Irradiation of Blood Components (Part I).

Graft-vs-host-disease (GVHD), results from the engraftment of immunocompetent donor T-lymphocytes into a recipient whose immune system is unable to reject them. It is a common sequela of allogeneic bone marrow transplantation (BMT), but is also recognized as a rare risk associated with blood transfusion. Early reports of transfusion -associated (TA) GVHD were recognized in immunocompromised hosts. However, recent cases have been documented in immunocompetent transfusion recipients. While TA-GVHD appears to be a rare event, it has become a major concern in current transfusion practices for immunodeficient and immunosuppressed patients due to the associated high mortality rate. Since there is no effective therapy, the treatment goal has been and remains oriented towards prevention. This issue of Lab News will review the immunologic and pathogenic mechanisms of TA-GVHD. In Part II (to appear in the next issue), we will discuss the clinical aspects of TA-GVHD as well as risk factors and methods for prevention.

IMMUNE AND PATHOPHYSIOLOGIC MECHANISMS
There are several factors that play a role in the pathogenesis of TA-GVHD. It has been well recognized that immunocompetent donor T-lymphocytes, capable of a proliferative response, must be present in the initiating transfusion. There must be sufficient disparity between donor and host histocompatibility (HLA) antigens so the host appears foreign to the donor. As shown by Shlomchik et al., these different HLA antigens are presented to donor T-cells by host macrophages, with subsequent donor T-cell activation and blast transformation. The host, on the other hand, must be incapable of, or have an impaired ability to mount an immunologic reaction against the graft either as a result of damaged, immature, or defective cellular immunity or host tolerance of the foreign cells. The latter scenario may occur when there exists donor homozygosity for an HLA haplotype, for which the recipient is haploidentical. In this setting the recipient's lymphocytes see only self-antigens on the donor's cells. However, the homozygous donor cells see non-self antigens on recipient's cells, stimulating an alloreaction that can initiate TA-GVHD.

The dosage of immunocompetent cells transfused is also believed to be important in the development of GVHD. Based on animal studies, a minimum dose of 1x10⁷ cells per kg body weight is necessary to induce a "runting syndrome" and case studies suggest that a similar threshold is necessary to produce GVHD in man. There have been reports of fatal TA-GVHD, however, occurring in children with severe combined immunodeficiency in which a dose of only 8x104 lymphocytes per kg body weight appeared to be transfused. The threshold number of viable cells necessary to produce a graft-versus-host reaction, therefore, may vary depending upon the immune status of the host as well as the antigenic similarity, or disparity between the donor and the recipient.

The immune response in GVHD is somewhat complex and still not completely understood. There are two basic aspects in GVHD: the afferent phase, in which donor T-lymphocytes are stimulated by the recipient target tissues and undergo clonal proliferation and differentiation, and the efferent phase, in which donor effector cells damage recipient target tissues. The immunologic target has been felt to be the major histocompatibility complex (MHC) antigens possessed by the host. In the setting of BMT, minor histocompatibility antigens have also been found to play a role in the development of GVHD. In TA-GVHD, however, the recipient's B-cells, T-cells, epithelial cells and bone marrow stem cells become the main focus of attack. The current theory is that the inappropriate production of cytokines or lymphokines with subsequent activation of the donor T-cells, is an important componet of the afferent phase. In this model, inflammatory cytokines, such as tumor necrosis factor- (TNF-) and interleukin-1 (IL-1), released by the damaged host tissue after chemotherapy, radiotherapy or infection, increase the expression of MHC and other adhesion molecules (ICAM-1,VCAM-1). This upregulation results in enhanced recognition of recipient MHC and/or minor histocompatibility antigens by alloreactive donor T-cells present in the transfused blood component. The activated donor T-cells will then proliferate and secrete cytokines. It is felt that Type 1 T-cells (Th1), with the secretion of IL-2 and IFN-, initiate the cell-mediated immune response and inflammatory cascades. Type 2 T-cells (TH2), known to secrete IL-4 and IL-10, responsible for humoral immunity and delayed GVHD, are not believed to play a major role in acute GVHD. IL-2 and IFN- induce further T-cell expansion, and activate various effector cell populations, including cytotoxic T-lymphocytes (CTLs), NK cells, which will differentiate to lymphokine-activated killer (LAK) cells, and macrophages. CTLs, and LAK cells have the ability to kill the host target cells, which possess different MHC antigens. Activated macrophages will secrete additional cytokines, such as TNF- and IL-2. In this manner, a self-amplifying positive feedback loop is created and eventually produces and maintains the clinical manifestations of GVHD.

In addition to the cellular responses as mentioned above, there are several cytokines produced by activated macrophages, which are also involved in the efferent phase. As reported by Ferrara, both TNF- and IL-1 are not only important mediators in the afferent phase, but are also responsible for some clinical manifestations of GVHD. TNF- can cause weight loss and anorexia. In addition, TNF- serum levels were found to be increased in patients with acute GVHD. A phase I/II trial using monoclonal ab which blocks the TNF- receptor was conducted. Use of a TNF- receptor antibody during the conditioning regimen as prophylaxis in patients at high risk for severe acute GVHD, showed reduction in the lesions of the skin, liver and intestine. As a mechanism, it was postulated that TNF- could induce tissue destruction by activation of the TNF-/Fas antigen pathway with subsequent apoptosis. Blockade of the TNF- receptor prevents this action. IL-1 shares similar biological activities with TNF-. Hosts receiving IL-1 displayed a wasting syndrome. Animal model studies with GVHD showed increased IL-1 mRNA in the spleen and skin, where the main production of IL-1 during GVHD occurred. Activated macrophages also produce excessive nitric oxide (NO) that may, in part, contribute to their deleterious effects on GVHD target tissues. NO is found to have a direct cytotoxic effect on liver, and is thought to play a role in the intestinal pathology of GVHD. It might also suppress lymphocyte function as seen in in-vitro studies. Animal studies with the NO-synthase inhibitor (L-mono-methyl-arginine) showed reduced damage to the intestinal mucosa and diminished lymphocytic infiltration of the spleen. Other researchers have investigated the role of T-cell subsets in TA-GVHD. From the blood of a patient with TA-GVHD, Nishimura and colleagues were able to clone three types of CD8+ and CD4+ T-cells, all of which may be involved in the pathogenesis of TA-GVHD. This could be mediated through direct cell-mediated cytotoxicity, via Fas/Fas-ligand, or through cytokine-mediated cellular responses.

References:

1. Shlomchik WD, Couzens MS, Tang CB, et al. Prevention of graft-vs-host-disease by inactivation of host-antigen-presenting cells. Science 1999;285:412-5.

2. Ferrara JL, Krenger W. Graft-vs- host-disease: the influence of type 1 and type 2 T cell cytokines. Transfusion Medicine Reviews 1998;12:1-17.

3. Nishimura M, Uchida S, Mitsunaga S, et al. Characterization of T-cell cones derived from peripheral blood lymphocytes of a patient with TA-GVHD: Blood 1997;89:1440-5.

Li Chai, MD
Gregory Pomper, MD
Edward L. Snyder, MD