Alloreactive t cells and cytokines in murine graft versus host disease 1

_ Literature Survey CHAPTER LITERATURE SURVEY 1.1 Bone Marrow Transplantation Early in the 1951, Jacobson et al (1951) found that the lethal effects of total body irradiation (TBI) to mice could be prevented by intraperitoneal injection of spleen cells In the same year, it was reported that mice and guinea pigs could be protected from lethal doses of TBI when given syngeneic marrow intravenously (Lorenz et al., 1951) The results of the experiment were soon confirmed in a variety of mammalian species (Van Putten et al., 1967) The first long-term bone marrow engraftment was reported in a 26-year-old patient with acute leukemia following one dose of TBI (Mathe et al., 1963) Bone marrow transplantation (BMT) for the treatment of anemia associated with leukemia of parasitic infections date back about one century ago (Santos, 1983) Currently, it is commonly used to treat a number of diseases including congenital disorders such as severe combined immunodeficiency (SCID), aplastic anemia, lymphohematopoietic malignancies, solid tumours and age-related diseases like osteoporosis and other autoimmune disease (Marmont, 1994) However, because the donor and recipient tissues usually differ genetically, graft-vs-host disease (GVHD) in which donor T cells react against their new host and attack host organs such as the skin, liver, and the intestines is very common after BMT _ Literature Survey 1.2 Graft-vs-host disease 1.2.1 Introduction Graft-vs-host disease (GVHD) is a complex condition that can occur after allogeneic BMT Despite advances in histocompatibility matching and immunosuppressive drugs, GVHD has continued to be a common and often lethal complication of marrow transplantation GVHD occurs when donor immunocompetent T cells found in the bone marrow react against the major and minor histocompatibility antigens of recipient As early as 1966, Billingham defined the three pre-requisites for the development of GVHD which includes the presence of immmunologically competent cells in the graft, the host appearing “foreign” to the graft, and the inability of the host to react sufficiently against the graft The interaction of donor cells and host target tissues in GVHD has proven to be complex, involving multiple target organs and shifting patterns of cytokines and effector cells 1.2.2 Classification of GVHD GVHD can be divided into two forms - acute and chronic GVHD Acute GVHD (aGVHD) results in an immediate multi-organ inflammatory syndrome affecting the skin, liver and gastrointestinal tract within the first 100 days post allogeneic BMT Acute GVHD can also produce a long-term immune deficiency and an increased frequency of Chronic GVHD (cGVHD) Chronic GVHD develops usually after day 100 and describes an autoimmune-like syndrome consisting of impairment of multiple organs or organ-systems (Sullivan, 1994) Both forms of GVHD, however, evolve from a common starting point of donor CD4+ T cell recognition of host alloantigen and IL-2 production _ Literature Survey Acute GVHD is a classic Th1-type response as donor T cells activated by host alloantigens secretes mainly Th1 cytokines (IL-2 and IFN-γ) in response to host antigens Three elements found to be involved in the generation of aGVHD includes donor T cell, donor (and host) macrophages and NK cells as the main effectors, and inflammatory stimulation from environment pathogens or preparatory regimens (Ferrara, 1993) Donor T cells responding to host alloantigens release IL-2 and, most importantly, IFN-γ These cytokines enhances cell-mediated response by activating cytotoxic T ells, macrophages and NK cells Activated macrophages and NK cells release large quantities of inflammatory cytokines and active nitrogen intermediates, resulting in a “cytokine storm” (Ferrara, 1993; Antin et al., 1992) that produces systemic effects in inducing damage in target organs, mainly the gut, liver and skin Chronic GVHD is characterized by autoantibody production, particularly serum IgG and IgM (Graze et al., 1994; Storek et al., 1992), and increased collagen deposition, resulting in clinical symptoms similar to those of autoimmune disease Even when the same organs are affected, the pathology may be distinct in aGVHD and cGVHD, with necrosis dominating in aGVHD and fibrosis characteristic of cGVHD Another differentiating characteristic between aGVHD and cGVHD is their marked difference in the cytokine production pattern The predominant cytokines produced in aGVHD are Th1 cytokines while those in cGVHD are Th2 cytokines The skewing of cytokine profile towards Th2 cytokines is evident within the first week of cGVHD In the first days after induction of cGVHD, a marked increase of IL-2 mRNA synthesis and cytokine content in supernatants has been observed in cGVHD mice (Via, 1991; Garlisi et al., 1993) Early IL-2 production is necessary for the development of Th2 cells (Ben-Sasson et al., 1990) as well as the stimulation of their activity in vivo (Fowler et al 1994) By day 7, IL-2 mRNA had returned to normal levels while IL-4 _ Literature Survey and in particular IL-10 mRNA predominated thereafter (Garlisi et al., 1993), even up to – weeks (Allen et al., 1993) These Th2 cytokines are central to both the pathology and the immune dysfunction of cGVHD 1.2.3 Target organs and histopathological manifestation of GVHD Murphy found that lymphoid tissue transplanted into the chick embryo causes splenomegaly and "pocks" on the chorioallantoic membrane in 1916 Some 40 years later, Simonsen rediscovered the phenomenon in the chick embryo just as Billingham and Brent were discovering an analogous phenomenon in the mouse; it was given the generic name graft-versus-host disease (GVHD), whereas in the mouse it was known for a time as the "runt disease" (Silverstein, 2001) The classic manifestations of GVHD are found in three organs: skin, liver, and gastrointestinal tract (Chao and Schlegel, 1995; Hakim et al., 1998) The first clinical manifestation of GVHD is a skin rash usually occurring at or near the time of white blood cell engraftment In the early states this rash maybe pruritic, involving the nape of the neck, the ears and shoulders as well as the palms of the hands and sole of the feet From these initial areas of presentation, the rash may spread to the whole integument and become confluent In severe GVHD, the maculopapular rash forms bullous lesions with epidermal necrolysis Chronic GVHD skin lesions are different and characterized by dermal fibrosis and immunoglobulin deposits The second organ most commonly involved by aGVHD is the liver The earliest and most common abnormality is a rise in conjugated bilirubin and alkaline phosphatase This reflects the pathology associated with liver GVHD, that is, damage to the bile canaliculi leading to cholestasis Primary histological findings are bile duct _ Literature Survey atypia and degeneration leading occasionally to severe cholestasis Other histological observations include lymphocytic infiltrates in the peribiliary areas, intraepithelial infiltrates in the bile ducts and ductules, and degeneration of hepatocytes and biliary epithelial cells The third main organ system affected by GVHD is the gastrointestinal tract, which is often characterized by diarrhea and abdominal cramps Diarrhea can be so voluminous that it becomes difficult to maintain an adequate fluid balance in some patients In severe gut GVHD, the whole intestine may be denuded, with total loss of epithelium Lesions may also consist of shortening of crypts and degeneration of crypt epithelial cells, infiltration of lymphocytes in the lamina propria, and erosion of mucosal epithelia 1.2.4 Effector cells involved in GVHD 1.2.4.1 T Cells The T lymphocyte is undoubtedly the prime mediator of specific responses in GVHD Donor T cells rapidly expand in number in early aGVHD In patients who went on to develop GVHD, increased numbers of anti-host T cells were seen in the first 30 days post-BMT After several months, 75% of the CD4 and 50% of the CD8 T cell population was derived from the donor (Hakim et al., 1994) This expanding T cell population was found to be composed primarily of those cells specifically reactive to host antigens Depletion studies further determined that different T-cell subsets might be involved In donor/host strain combinations matched at class I MHC and mismatched at class II MHC antigens, CD4+ T cells are the dominant effectors (Korngold et al., 1987; Sprent, 1994); in contrast, in those mismatched only at class I MHC antigens, CD8+ T cells are necessary and sufficient and CD4+ depletion has little _ Literature Survey effect on mortality (Sprent et al., 1990; Serody et al., 1999) CD8+ cells are similarly required for the development of GVHD in several MHC-matched, minor antigenmismatched strain combinations (Korngold et al., 1987) 1.2.4.2 NK Cells Non-T cells also play a major role in both the tissue damage of GVHD and in the cascade of inflammatory cytokines that produce systemic effects NK cell is one of the key players NK cells (CD56+) are actually noted to decrease in the peripheral blood of patients destined to develop GVHD (Soiffer et al., 1993) The use of NK-deficient mutant mice as donor did not induce histopathological lesions in the recipient mice (Ghayur et al., 1987) 1.2.4.3 Macrophages Donor and host macrophages increase in number during the first weeks of GVHD Macrophages are ubiquitous in GVHD-affected tissues They play a dual role in GVHD Macrophages serve as antigen presenting cells to activate T-cell response Besides, activated macrophages can differentiate into a pro-inflammatory state When triggered by LPS, these macrophages then secrete high titers of TNF-α, IL-1, IL-6 and nitric oxide These factors both increase APC stimulation of T cells and mediate the wasting and tissue damage of GVHD (Smith et al., 1991) 1.2.5 Predicting GVHD Both the extent of histoincompatibility between donor and recipient and, up to a certain threshold, the number of (residual) T cells in the marrow graft (even after ex vivo T-cell depletion) have a profound impact on the incidence and severity of GVHD 10 _ Literature Survey (Anasetti et al., 1994) Predicting the risk of GVHD before performing allogeneic BMT is certainly an attractive approach to eventually minimize its incidence and severity by prospective donor selection and individualized immunosuppressive therapy in patients with low risk acute leukemia (AL) However, in order for this to be routinely applied in BMT for patients with CML and more advanced AL, an immunotherapeutic strategy that meets the apparent paradox of preventing GVHD while preserving a GVL response would be required 1.2.5.1 Genotypically HLA-identical sibling BMT Alloreactive host-specific CD8+ and CD4+ donor T cells that cause GVHD in genotypically HLA identical sibling BMT are directed against immunodominant minor histocompatibility peptide antigens (mH Ags) expressed by the recipient and presented by class I and II MHC molecules (Theobald, 1995 & 1993; Wettstein, 1995; Sugita et al., 1994) These mH Ag reactive T cells are clonally amplified during aGVHD, and their presence in vivo precedes the clinical onset of GVHD (Dietrich et al., 1994; Nierle et al., 1993; Soiffer et al., 1993) The general idea is that mH Ags are derived from normal, yet polymorphic, cellular proteins (Theobald, 1995; Wettstein 1995) Also, retroviral gene products, including those that give rise to endogenous superantigens, may provide mH Ags (Jones et al., 1994) A significant correlation between the occurrence of moderate to severe (grade II to IV) acute and chronic GVHD, and the pooled mismatches for HA1, 2, or was observed in adult patients and also, although less apparent, in children HLA-A2.1 is the most frequently expressed human class I allele among different ethnic populations, and also that HA1 represents an immunodominant mH Ag with a phenotype frequency of 69% in the HLA-A2.1+ subpopulation (Van Els et al., 1992), this mH Ag provides a prototype 11 _ Literature Survey candidate for prospective cellular typing in order to predict GVHD among genotypically HLA-A2.1 matched donor-patient pairs Recipient-specific alloreactivity of donor T cells before genotypically HLAidentical sibling BMT is virtually undetectable by both the standard mixed lymphocyte culture (MLC) and limiting dilution analysis (LDA) of CTL precursors (Theobald, 1995; Kaminski et al., 1988 & 1989) By modifying both the responder-cell read out and the type of antigen-presenting cells used as stimulator cell, however, primary in vitro assays that could predict the occurrence of aGVHD in genotypically HLAidentical BMT had been developed These assays include the inhibition of colony forming units-granulocyte macrophage (CFUGM) based on secondary donor-host cocultures (Delmonte et al., 1982), the skin explant model (Vogelsang et al., 1985; Sviland et al., 1990), the mixed epidermal cell-lymphocyte reaction (Bagot et al., 1986 & 1988), and the measurement of donor autoreactivity against a pool of MHC disparate stimulator cells (Johnsen et al., 1992a, 1992b, 1992c) LDA has been utilized as a more clinically useful predicative assay to determine the frequency of host-specific IL-2-secreting Th precursors (IL-2 Thps) in the peripheral blood of genotypically HLA-identical sibling marrow donors (Theobald et al., 1993& 1992; Schwarer et al., 1993) High frequencies of host-specific IL-2 Thps (about >1 per 100,000) have been detected before BMT in donors whose siblings later had moderate to severe aGVHD Among the donors for patients with mild (grade I) or no acute GVHD, low frequencies of IL-2 Thps (about