Disease susceptibility, transplantation and the MHC

Diseasesusceptibility,transplantation and the MHC Andrew J.T. George, Mary A. Ritter and Robert I. Lechler

In recent years, great advances have been made in the fields of transplantation, tolerance, the major histocompatibility complex (MHC) and disease susceptibility. An important historical perspective on such studies was provided by L. Brent (London), who described the situation as it was in the early 1950s. At that time, inbred strains of mice were becoming available, and the immunological basis for transplant rejection had been accepted. However, although the H-2 locus was being defined, its function and products were the subject for speculation, and the function of lymphocytes was still debatable. Over the next ten years, advances in immunology, particularly in such areas as the MHC and tolerance induction, provided an important impetus for transplantation in humans. Transplantation There has been a phenomenal increase in the success of organ transplants since the 19.50s, and transplantation has become the treatment of choice for end-stage renal and cardiac failure. The development of drugs such as cyclosporin A and FK506 has led to substantial improvements in preventing graft rejection, especially over the first year (R. Calne, Cambridge). Both of these drugs bind to a cytoplasmic protein (cyclophilin or FK-binding protein), which then interacts with calcineurin A. This interaction interferes with the activation of the NF-KB and AP-1 transcription factors, by preventing dephosphorylation of important components of these pathways (M. Schreier, Basle). Although these agents are highly effective, they have side-effects, and there is considerable interest in the development of novel immuno-

ing more than 200 000 cases) shows that the 5.6% of patients who receive a cadaveric allograft with no mismatches at the HLA-A, B or DR loci (termed OABDR) have a 65% ten-year graft survival, compared with 38% for other cadaveric grafts. If all organs were offered to the best match within the USA, then approximately 20% of patients would receive a OABDR graft. Furthermore, analysis suggests both that ztrategies. ,\nti-CD.3 certain mismatches are ‘permissible’ suppressive antibodies arc now routlnel\: used (and so have a lower probability of to treat renal, hepatic and cardiac rejection) and that good results can be obtained by ‘residue matching’, allograft rejection, although their role in prophylaxis is 11kel\ to be which involves matching seven residues thought to be important in limited I R. Cosimi, Boston 1. Other antibodies, including anti-adhesion allorecognition. If transplant centres molecules, have pro\ ed suc~essf~11 in the USA were to implement fully in animal models and arc now a policy of OABDR grafting wherentering clinical trials i K. (.osimi ever possible, followed by ‘intelliand l? Russell, Boston). Interestingly, gent mismatching in all other cases, in a rat model of experlmcntal the ten-year survival of cadaveric renal allografts may increase from allergic enccphalornyelitls. t1011depleting anti-CD4 antibc)dlt!\ not 44% to 55%, with no alteration in only reduce the T-cell response and the treatment protocols (P. Terasaki, prevent the de~~elopment of disease, Los Angeles). The selection of but also change the c!,tokine profile donors can also be improved in towards ‘1 ‘I‘ helper 2 I.rh_‘i-like the case of bone marrow transplantation, by functional crossresponse. Thus, administration of antibodies may be useful both for matching. Thus, quantitation of the inducing immunosuppressioll and precursor frequency of alloreactive cytotoxic and helper T cells is predicfor switching the response to ‘1 nontive of the incidence of severe graftpathogenic one (I). Mason, O&rdi. It is possible that clinical results lferstrs-host disease (J. Goldman, might he improved using agents London), allowing better selection that are currently available. In reoi donor-recipient combinations. nal transplantation, analysis of IJS Tolerance transplantation databases icontaInDespite all of the considerations mentioned above, the two major \vas held .lt rhe :)A symposium problems that remain in clinical Royal I’osrgradu,ite Lledical khool, transplantation are: (1) chronic reHammersmith Hospltdl. on 31-11 jection and (2) a critical shortage of September 19Y4, to ni,lrk The rermdonor organs. Chronic rejection ment of Professor J.K. Batchelor. The takes the form of ‘transplant arteriomeeting was made possthle 11~generous sclerotis’, leading to approximately support from Sandoz and the Krmsh .3-4X of long-term renal allografts

allogenric organs in order to prevent being rejected per year, and impro\;echronic reiection (J. Fabre, London I. ments in the immunosuppressive regimes have had little impact on MHC and disease these figures. Several mechanisms The MHC molecules were origare probably responsible for this inally described as the major barchronic rejection; for instance, antirier to transplantation. Subsequent bodies have been shown to be recognition of their role in antigen important in murine cardiac alloprocessing Jnd presentation has rmgrafts (I? Russell). The ultimate phasized their importance in infecsolution to this problem is to intious diseases, autoimmunity and duce a state of tolerance in the host neoplastic diseases [where loss of to the donor organ. MHC expression may allow tllTolerance to allografts can be inmours to escape immune survcilduced in animals in a number of lance (W. Kodmer, Lmxionij. While ways, including: depletion of passenger antigen-presenting cells (APCs) the occurrence of disease associated with certain MHC alleles has been from kidneys (R. Lechler, London) recognized for a long time, little or pancreatic islets (K. Lafferty, progress has been made in deterCanberra); pretreatment of recipient mining the reasons for such associanimals with cells bearing specific alloantigen (2 anti-CD4 antibody 1 ations. One possibility is that the selective nature of the peptide-bind(l? Morris, Oxford); transplantation ing groove of the MHC molecule of liver to pigs (R. Calne) or by proensures that only certain alleles drt‘ ducing mixed chimeric animals concapable of presenting pathogenic taining APCs and responding T cells peptides. Good evidence for this from the donor animal (D. Sachs, is probided by coeliac disease, in Boston). In humans, there is tantawhich the antigen (gliadin] IS lizing evidence of tolerance inducknown, and where a strong astion in patients whose grafts have sociation with HLA-DQAI “0.50 I survived up to 24 years with no and HLA-DQBl”O201 has heen obimmunosuppression (R. Calne). served (E. Thorsby, Oslo). GliadinTolerance may be produced by dereactive ‘T cells have been isolated letion or by induction of anergy in from biopsy material and have T-cell clones, both in the thymus been shown to be restricted and in the periphery, as well as by to HLA-DQA l”O.501 and HI.?immunoregulatory pathways. InDQBl”O20 I. triguingly, Lechler provided evidence Not surprisingly, the picture can that anergic cells themselves can supbe more complex. A. McMichael press the response of normally re(Oxford) described a situation in active T cells, by competing both which soexpression of HLA-RX for ligand and for growth factors with HLA-B”2702 presuch as interleukin 2 (IL-2). Such a together vented presentation to T cells b! mechanism may be responsible for influenza-infected cells carrving a the transfer of suppression by T cells particular peptide in assoiiation from tolerant animals. with HLX-BX. This was due to competition occurring between the Xenotransplantation two HLA molecules for the pepOne solution to the shortage of tide, and suggests that some of the organs suitable for transplantation immune response gene phenomena is to use organs from other species. linked to MHC alleles could be Such xenotransplantation would redue to competition for peptide b! quire genetic manipulation of the different MHC alleles. donor organ to prevent hyperacute While the natural assumption is rejection of the xenogeneic organ. that disease linkage is related to the An important target for such an peptide-binding function of the approach will be the endothelial MHC molecules, there arc other cells, whose activation following possibilities. Juvenile chronic arbinding of pre-formed xenoreactive thritis shows some association with antibodies can lead to haemorpolymorphisms of the DQA prorhagic oedema, infiltration, inflammoter region, in particular with mation and coagulation (F. Bach, point mutations in the Y box. Boston). In addition, such an apWhile the functional significance of proach might be useful to modif,-

these alterations is unknown, it is tempting to suppose that aberrant cypression of genes within the DQA region is related to disease pathogenesis (I?. Albert, Munich). It was h!,pothesized that differential expression of HLA-DR and DQ molecules on APCs could influence the ThliTh2 balance of responding T cells, and the ensuing patterns of cytokine expression may be related to the induction or pathogenesis of some diseases (A. Mitchison, Berlin). The importance of MHC class III gene products in disease is frequently overlooked. However, one of the strongest disease associations In this region is that between deficiencies of the complement component C4 and systemic lupus rrythematosus (SLE). Deficiencies \vith other components of the classical pathway are also associated \vith SLE; 29 out of 30 individuals described with homozygous Clq deficiency have SLE or discoid lupus, and approximately a third of known CZ-deficient patients have SLE. Clearance of radiolabelled immune complexes in C2-deficient patients is abnormal; there is no uptake of the immune complexes in the spleen, and uptake in the liver is faster than in normal individuals. In addition, immune complexes localized in the liver slowly leach out mto the circulation. These differences are abolished by administration of fresh frozen plasma. Therefore, it is likely that the major role of the complement deficiencies is to alter the clearance and fate of immune complexes (M. Walport, London). Research in the area of MHC associations with disease is complicated by the effects of the environment (both positive and negative) on the etiology or pathogenesis of the disease; however, the ability to understand the disease associations at a molecular level is likely to be of growing importance. Even when the study of disease associations has been of little direct use (such as in multiple sclerosis), it has pro\.ided an important impetus to the understanding of the basic cellular mechanisms causing the disease (r\. Compston, Cambridge). Conclusion The need to prevent the rejection response following transplantation

has provided an enormous stimulus for basic immunological research in areas such as tolerance, immunoregulation and the MHC, as well as in clinical areas such as autoimmunity and disease association.

During Richard Batch&r’s career, great advances have been made in these areas, and this meeting pro vided an opportunity to highlight his substantial contributions to this progress.

rItrilrt~

C;ror~c,

ill~v~ Rittev and

Robert Lechler are at the Dept of In?nrunologv, Royal Postgraduate Medical

School,

Hospital, Du (he

Hammersmith Road, kondon,

UK WI2 Oh’N.

A new era for carnitine? Giuseppe Famularo and Claudio De Simone

(3-hydroxy-4-methylL-carnitine ammoniobutanoate) and its congeners regulate the substrate flux and energy balance across cell membranes by modulating both the transport of long-chain fatty acids into mitochondria and their subsequent p-oxidation. Foods of animal origin contain substantial amounts of carnitine, but the body also makes carnitine using the amino acids lysine and methionine, along with vitamin C and other substances. Carnitine is principally stored in the skeletal muscle, and is present both in its free form and as esters with acyl groups of various chain length in most biological materials. The serum levels of carnitine vary according both to age and sex, since lower concentrations are measured in the first weeks after birth and in females’. L-carnitine is found in high concentrations in leukocytes, including peripheral blood mononuclear cells (PBMCs)‘, and it is envisaged that L-carnitine and its congeners play a role in regulating the immune response. Indeed, early experiments have confirmed the ability of L-carnitine and L-acetylcarnitine to enhance mitogen- and antigen-driven lymphocyte proliferation under in vitro conditions2. Supplementation of hydrocortisone-treated mixed lymphocyte cultures with L-acetylcarnitine has been shown to prevent cell lysis’, and recent reports indicate an anti-apoptotic activity of carnitine

Early in vitro experiments demonstrated thr imml~nomodztlating and anti-apoptotic properties of L-carnitine. More-recent studies have shown that the depletion of this compound plays a role in the pathogenesis of iml?zzlne-lrreiiiated disorders, such as AIDS. septic shock and cbrouic fi~tigue syndrome. These data, combined with the safety of L-canritine, hatIe highlighted the potential for using this compound as an addrtional therapy to treat such disorders. and L-acetylcarnitine. For example, I.-acetylcarnitine not only significantly increases the survival time of rat sensory neurons’. but also enhances the survival of I’1 9 teratoma cells”, retarding DNA fragmentation and nuclear condensation, which are markers of apoptosis. L-carnitine is currently utilized for the treatment of ischaemic heart disease. However, many of the immunological properties of L-carnitine and its congeners have been underestimated, and their therapeutic potential in immune-mediated diseases is largely unknown. Recent studies have begun to explore the role of these compounds in the pathogenesis and treatment of disorders with unregulated immune responses, such as AIDS, septic shock and chronic fatigue syndrome (CFS). AIDS AIDS is a high-risk condition for developing carnitine deficiencv due to several factors: (1) nutritional problems associated with malabsorption due to the presence of multiple opportunistic infections and 0 ,Wi,, I\CI,.\il1,‘1‘J:L’G \V”?I

malignancies; (2) human immunodeficiency virus (HIV)-related enteropathy, which also results in a considerable degree of malabsorption; (3) HIV-related kidnev abnomalities, which can lead to an increased urinarv excretion of carnitines; (4) therapeutic regimens prescribed for HIV-infected patients (e.g. antibiotics and, possibly, antiretroviral drugs), which may result in a deficiency of carnitines, as previously described for pivampicillin’, sulfadiazine6 and pyrimethamineh; and (5) the breakdown of fats to form fatty acids, and their subsequent rebuilding, which utilizes energy7 and could lead to unregulated carnitine consumption. Decreased serum levels of carnitines are present in most AIDS patients but, in a minority, the concentrations are in the normal or even the high range%. In both groups, the intracellular (i.e. in PBMCs) content of carnitines is low’. Therefore, in HIV-infected patients, the carnitine defect may be subtle and serum measurement’is consequently a fallacious index.

Carnitine, HIV and apoptosis The supplementation of mitogendriven PBMCs from HIV-infected patients with L-acetylcarnitine results in an anti-apoptotic effect”‘. This finding was associated with a striking reduction in the spontaneous release of tumor necrosis factor c? (TNF-(Y) by HIV-infected PBMCs (Ref. 101.