Despite success of early islet allograft engraftment and survival in humans, late islet allograft loss has emerged as an important clinical problem. CD8+ T cells that are independent of CD4 + T cell help can damage allograft tissues and are resistant to conventional immunosuppressive therapies. Previous work demonstrates that islet allografts do not primarily initiate rejection by the (CD4-independent) CD8-dependent pathway. This study was performed to determine if activation of alloreactive CD4-independent, CD8 + T cells, by exogenous stimuli, can precipitate late loss of islet allografts. Recipients were induced to accept intrahepatic islet allografts (islet 'acceptors') by short-term immunotherapy with donor-specific transfusion (DST) and anti-CD154 mAb. Following the establishment of stable long-term islet allograft function for 60-90 days, recipients were challenged with donor-matched hepatocellular allografts, which are known to activate (CD4-independent) CD8 + T cells. Allogeneic islets engrafted long-term were vulnerable to damage when challenged locally with donor-matched hepatocytes. Islet allograft loss was due to allospecific immune damage, which was CD8-but not CD4-dependent. Selection of specific immunotherapy to suppress both CD4-and CD8-dependent immune pathways at the time of transplant protects islet allografts from both early and late immune damage. Key words: CD40 ligand, CD4+ T cells, CD8 + T lymphocytes, cell-mediated cytotoxicity, cellular transplantation, costimulation blockade, cytotoxic T cells, delayed-type hypersensitivity, donor-specific transfusion, effector mechanisms, hepatocyte transplantation, islet transplantation Abbreviations: ABTS, 2,2 -azino-bis(3-ethylbenzphiazoline-6-sulfonic acid; CFSE, carboxyfluorescein diacetate succinimidyl ester; DTH, delayed type hypersensitivity; FBS, fetal bovine serum; hA1AT, human alpha-1-antitrypsin; HBSS, Hank's balanced salt solution; HRP, horseradish peroxidase; i.p., intraperitoneal; i.v., intravenous; KO, knockout; LFA-1, lymphocyte function-associated antigen-1 (CD11a/CD18); mAb, monoclonal antibody; MHC, major histocompatibility complex; MST, median survival time; PBS, phosphate buffered saline; SCID, severe combined immunodeficiency disorder; STZ, streptozotocin.