Improvements in the prevention or control of rejection of the kidney and liver have been largely interchangeable (1,2) and then applicable, with very little modification, to thoracic and other organs. However, the mechanism by which anti rejection treatment permits any of these grafts to be "accepted" has been an immunological enigma (3,4). We have proposed recently that the exchange of migratory leukocytes between the transplant and the recipient with consequent long-term cellular chimerism in both is the basis for acceptance of all whole-organ allografts and xenografts (5). Although such chimerism was demonstrated only a few months ago, the observations have increased our insight into transplantation immunology and have encouraged the development of alternative therapeutic strategies (6). DISCOVERY OF GRAFT CHIMERISM After Liver TransplantationSuccessful transplants were long envisioned as an alien patch in a homogeneous host (Fig. 1, left). The first unequivocal evidence that whole-organ grafts in human beings become genetic composites (chimeras) was obtained in 1969 with karyotyping studies in female recipients of livers obtained from male cadaveric donors. Postoperatively, the hepatocytes and the endothelium of the major blood vessels of the grafts retained their donor sex, whereas the entire macrophage system, including the Kupffer cells, was replaced with recipient female cells (identified by their characteristic Barr bodies) within 100 days (7,8) (Fig. 1, middle). These observations attracted considerable attention at the time, primarily because of their implication that liver-based inborn errors of metabolism could be corrected permanently by liver replacement (9,10). This prediction has been met since then in nearly two dozen such heritable diseases (11). Each report of another liver-based metabolic disorder that was corrected by liver replacement added to the illusion that the composite (chimeric) structure of the hepatic allograft was a special feature of this organ.Address reprint requests to: Thomas E. Starzl, M.D., Ph.D., Department of Surgery, 3601 Fifth Avenue, 5C Falk Clinic, University of Pittsburgh, Pittsburgh, PA, 15213. NIH Public Access Author ManuscriptHepatology. Author manuscript; available in PMC 2010 October 26. Published in final edited form as:Hepatology. 1993 June ; 17(6): 1127-1152. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript After Intestinal TransplantationThe illusion of uniqueness of the hepatic graft was dispelled in 1991 with the demonstration, first in rat models (12) and then in human beings (13), that all successfully transplanted intestines also were chimeric. The epithelium of the bowel remained that of the donor, but lymphoid, dendritic and other leukocytes of recipient phenotype quickly became the dominant cells in the lamina propria, Peyer's patches and mesenteric nodes. The transformation in experimental animals and in human beings (Fig, 2) was the same whether the bowel was transplanted alone or as a part of a multivisceral gra...

We have previously reported data from clinical and laboratory animal observations which suggest that organ tolerance after transplantation depends on a state of balanced lymphodendritic cell chimerism between the host and donor graft. We have sought further evidence to support this hypothesis by investigating HLA-mismatched liver allograft recipients. 9 of 9 female recipients of livers from male donors had chimerism in their allografts and extrahepatic tissues, according to in-situ hybridisation and molecular techniques 10 to 19 years posttransplantation. In 8 women with good graft function, evidence of the Y chromosome was found in the blood (6/8), skin (8/8), and lymph nodes (7/8). A ninth patient whose transplant failed after 12 years from recurrent chronic viral hepatitis had chimerism in her lymph nodes, skin, jejunum, and aorta at the time of retransplantation. Although cell migration is thought to take place after all types of transplantation, the large population of migratory cells in, and the extent of their seeding from, hepatic grafts may explain the privileged tolerogenicity of the liver compared with other organs.

Chimerism was demonstrated with immunocytochemical and/or polymerase chain reaction techniques in kidney allografts and in the native skin, lymph nodes, or blood of 5 of 5 patients who received continuously functioning renal transplants from 1 or 2 haplotype HLA mismatched consanguineous donors (4 parents, 1 aunt) 27-29 years ago. In the 4 cases where the kidney donor still was alive to provide stimulator lymphocytes for testing, these provoked no (n = 2) or modest (n = 2) MLR in contrast to vigorous MLR to third party lymphocytes. In all 4 cases, the donor cells failed to generate in vitro cytotoxic effector cells (cell-mediated lymphocytotoxicity). These findings are in accord with the hypothesis that cell migration, repopulation, and chimerism are seminal events that define graft acceptance and ultimately can lead to acquired donor-specific nonresponsiveness (tolerance).

The clinicopathologic features of liver allograft dysfunction occurring in 51 symptomatic recipients after more than 5 years' survival (mean 7.1 years) with the same hepatic allograft were compared with those of a similar group of 14 asymptomatic patients (mean survival, 9.9 years) who underwent a nonclinically indicated protocol liver biopsy evaluation. Predictably, patients who had clinically indicated biopsies more frequently showed histopathologic alterations (76% versus 36%, p < 0.002). After detailed clinicopathologic correlation, the changes in the symptomatic patients were attributed primarily to definite or presumed viral hepatitis in 17 of 51 (33%) patients, 11 of whom had recurrent viral disease; seven of 51 (14%) had nonviral recurrent original disease, three (6%) had obstructive cholangiopathy, and 11 (22%) had acute and/or chronic rejection. In 13 of 51 (25%) of the symptomatic patients, the clinical and pathologic abnormalities were minimal. Long-term liver allograft survival in nine of 14 (64%) of the asymptomatic patients was associated with minimally abnormal histologic alterations. Two of the asymptomatic patients had obstructive cholangiopathy; two others has recurrence of the original disease and one has possible viral hepatitis. Viral hepatitis types B and C, alcoholic liver disease, autoimmune hepatitis, granulomatous hepatitis (not otherwise specified), and probably primary biliary cirrhosis and primary sclerosing cholangitis were shown to recur after hepatic transplantation. The histopathologic changes associated with acute and chronic rejection frequently overlapped with other syndromes causing late dysfunction, such as chronic viral or autoimmune hepatitis, primary biliary cirrhosis, or primary sclerosing cholangitis; more than one insult could be identified in 15 cases, which made the differential diagnosis of causes of late liver allograft dysfunction much more difficult than early after hepatic transplantation. It is important to correlate the biopsy findings with the liver injury tests, the results of viral and autoimmune antibody serologic studies, and review of previous biopsies and to be aware of the original disease, recent changes in immunosuppression, and results of therapeutic intervention(s) to identify correctly the causes of liver allograft dysfunction in this patient population.

Seventy-two long-surviving liver transplant recipients were evaluated prospectively, including a baseline allograft biopsy for weaning off of immunosuppression. Thirteen were removed from candidacy because of chronic rejection (n=4), hepatitis (n=2), patient anxiety (n=5), or lack of cooperation by the local physician (n=2). The other 59, aged 12-68 years, had stepwise drug weaning with weekly or biweekly monitoring of liver function tests. Their original diagnoses were PBC (n=9), HCC (n=1), Wilson's disease (n=4), hepatitides (n=15), Laennec's cirrhosis (n=1), biliary atresia (n=16), cystic fibrosis (n=1), hemochromatosis (n=1), hepatic trauma (n=1), alpha-1-antitrypsin deficiency (n=9), and secondary biliary cirrhosis (n=1). Most of the patients had complications of long-term immunosuppression, of which the most significant were renal dysfunction (n=8), squamous cell carcinoma (n=2) or verruca vulgaris of skin (n=9), osteoporosis and/or arthritis (n=12), obesity (n=3), hypertension (n=11), and opportunistic infections (n=2). When azathioprine was a third drug, it was stopped first. Otherwise, weaning began with prednisone, using the results of corticotropin stimulation testing as a guide. If adrenal insufficiency was diagnosed, patients reduced to <5 mg/day prednisone were considered off of steroids. The baseline agents (azathioprine, cyclosporine, or FK506) were then gradually reduced in monthly decrements. Complete weaning was accomplished in 16 patients (27.1%) with 3-19 months drug-free follow-up, is progressing in 28 (47.4%), and failed in 15 (25.4%) without graft losses or demonstrable loss of graft function from the rejections. This and our previous experience with self-weaned and other patients off of immunosuppression indicate that a significant percentage of appropriately selected long-surviving liver recipients can unknowingly achieve drug-free graft acceptance. Such attempts should not be contemplated until 5-10 years posttransplantation and then only with careful case selection, close monitoring, and prompt reinstitution of immunosuppression when necessary.Lifetime immunosuppression has been a presumed necessity after clinical whole-organ transplantation. However, we have suggested elsewhere that liver allograft acceptance without a need for maintenance immunosuppression may have been accidentally achieved more often than realized (1). The recently proposed concept that donor leukocyte migration and long-term microchimerism is the basis of allograft acceptance (2) would account for the