The sensitization of rats by allografts transplanted to alymphatic pedicles of skin.

Pedicles of skin which lacked a lymphatic drainage were raised on the backs of rats in order to study the importance of afferent lymphatics in sensitization by skin allografts. Although allografts transplanted to the alymphatic pedicles enjoyed a prolonged survival, they contracted progressively from about 3 wk after transplantation and were reduced eventually to small scars. In contrast, autografts survived unchanged in size for the life-span of the pedicles which carried them. The slow contracture of the allografts was associated with sensitization of the host because test allografts applied orthotopically were destroyed with a second-set tempo. No regeneration of lymphatics from the long-standing pedicles could be demonstrated, and it was concluded that sensitization had occurred eventually through the blood, presumably by the process of peripheral sensitization. Allografts on skin pedicles could be destroyed rapidly by active or adoptive immunization, so it is probable that the level of sensitization to which they themselves gave rise was a low one. Although it is not disputed that afferent lymphatics are essential for the rapid destruction of skin allografts, it is clear that the absence of a lymphatic supply does not permanently exempt them from immunological attack in the rat.

Anti-interleukin 2 receptor monoclonal antibodies spare phenotypically distinct T suppressor cells in vivo and exert synergistic biological effects.

The therapeutic efficacies of ART-18, ART-65, and OX-39, mouse antibodies of IgG1 isotype recognizing distinct epitopes of the p55 beta chain of the rat IL-2-R molecule, were probed in LEW rat recipients of (LEW X BN)F1 heterotopic cardiac allografts (acute rejection in untreated hosts occurs within 8 d). A 10-d course with ART-18 prolongs graft survival to approximately 21 d (p less than 0.001). Therapy with ART-65, but not with OX-39, was effective (graft survival approximately 16 and 8 d, respectively). Anti-IL-2-R mAb treatment selectively spared T cells with donor-specific suppressor functions; the CD8+ (OX8+ W3/25-) fraction from ART-18-modified recipients, and primarily the CD4+ (W3/25+ OX8-) subset from ART-65-treated hosts conferred unresponsiveness to naive syngeneic rats after adoptive transfer, increasing test graft survival to approximately 16 and 45 d, respectively. Concomitant administration of ART-18 and ART-65 to recipient animals in relatively low doses exerted a strikingly synergistic effect, with 30% of the transplants surviving indefinitely and 50% undergoing late rejection over 50 d. These studies provide evidence that anti-IL-2-R mAbs selectively spare phenotypically distinct T cells with suppressor functions. The data also suggest that in vivo targeting of functionally different IL-2-R epitopes may produce synergistic biological effects.

Transfer of specific unresponsiveness to organ allografts by thymocytes. Specific unresponsiveness by thymocyte transfer.

Prolonged survival of vascularized organ allografts has been produced in unmodified inbred rats by transfer of thymocytes from enhanced, engrafted, syngeneic animals. For these thymocytes to increase significantly the survival of test allografts they must be harvested 6-9 d after transplantation. Thymectomy of the enhanced, engrafted animals during the same critical period causes acute rejection of othewise long surviving grafts. For optimal effect, the enhanced thymocyte donor must be actively and passively immunized and receive a cardiac allograft. The necessity for erythrocytes in the initial active immunization regimen is noted. Additionally, the antigenic specificity of the suppressor effect has been established with two histoincompatible donor rat strains. Cellular and humoral host responses mounted by test graft recipients after thymocyte transfer from enhanced, engrafted donors are different from those mounted either by unmodifed animals acutely rejecting their grafts or by enhanced rats bearing well-functioning grafts. Numbers of T lymphocytes are reduced in the grafted hearts and in the spleens of test graft recipients, a finding paralleled by the complete absence of specific direct lymphocyte-mediated cytotoxicity. In contrast, cytotoxic antibody production, although delayed, is increased in magnitude, peaking around the time of graft rejection. These studies provide evidence that different biological manipulations can modify separate pathways in the complex cellular and humoral responses towards organ allografts. They demonstrate that cellular immunity is critically involved in immunological enhancement of vascularized organ allografts, a phenomenon hitherto considered primarily humoral. It seems clear that cells with suppressor activity are present within the thymus during the early phases of immunological enhancement.

The role of the B7 costimulatory pathway in experimental cold ischemia/reperfusion injury.

Ischemia/reperfusion injury associated with organ retrieval and storage influences the development of chronic graft dysfunction, the major clinical problem in solid organ transplantation. The potential role of mononuclear cells (T cells and monocyte/macrophages) in this type of injury is unknown. Inbred male Lewis rats were uninephrectomized and the left kidney perfused in situ with 10 ml of iced University of Wisconsin solution. Immunohistological studies showed mononuclear cell infiltration of the ischemic organs associated with the upregulation of MHC class II antigen expression. Reverse transcriptase-PCR indicated that T cell associated cytokines and monocyte/macrophage activation markers/products are upregulated early after the ischemic insult. B7 expression occurred within 24 h and peaked at 3 d. Plasma creatinine levels rose transiently with complete recovery of renal function by 5 d. Animals began to develop progressive proteinuria after 8-12 wk, indicative of the long-term functional consequences of early ischemia/reperfusion injury. Blockade of T cell CD28-B7 costimulation with CTLA4Ig resulted in significant inhibition of T cell and macrophage infiltration and activation in situ. Treated animals did not exhibit transient renal dysfunction, nor developed proteinuria over time. This is the first demonstration that blocking T cell costimulatory activation in the absence of alloantigen can prevent the early and late consequences of ischemia/reperfusion injury.

Interferon-gamma deficiency prevents coronary arteriosclerosis but not myocardial rejection in transplanted mouse hearts.

We have hypothesized that T cell cytokines participate in the pathogenesis of graft arterial disease (GAD). This study tested the consequences of IFN-gamma deficiency on arterial and parenchymal pathology in murine cardiac allografts. Hearts from C-H-2(bm12)KhEg (bm12, H-2(bm12)) were transplanted into C57/B6 (B6, H-2(b)), wild-type, or B6 IFN-gamma-deficient (GKO) recipients after immunosuppression by treatment with anti-CD4 and anti-CD8 mAbs. In wild-type recipients, myocardial rejection peaked at 4 wk, (grade 2. 1+/-0.3 out of 4, mean+/-SEM, n = 9), and by 8-12 wk evolved coronary arteriopathy. At 12 wk, the GAD score was 1.4+/-0.3, and the parenchymal rejection grade was 1.2+/-0.3 (n = 8). In GKO recipients of bm12 allografts, myocardial rejection persisted at 12 wk (grade 2.5+/-0.3, n = 6), but no GAD developed (score: 0.0+/-0.0, n = 6, P < 0.01 vs. wild-type). Mice treated with anti-IFN-gamma mAbs showed similar results. Isografts generally showed no arterial changes. In wild-type recipients, arterial and parenchymal cells showed increased MHC class II molecules, intercellular adhesion molecule-1, and vascular cell adhesion molecule-1 compared to normal or isografted hearts. The allografts in GKO recipients showed attenuated expression of these molecules (n = 6). Thus, development of GAD, but not parenchymal rejection, requires IFN-gamma. Reduced expression of MHC antigens and leukocyte adhesion molecules may contribute to the lack of coronary arteriopathy in hearts allografted into GKO mice.

The cytokine-adhesion molecule cascade in ischemia/reperfusion injury of the rat kidney. Inhibition by a soluble P-selectin ligand.

Ischemia/reperfusion (I/R) injury associated with renal transplantation may influence both early graft function and late changes. The initial (

Late blockade of T cell costimulation interrupts progression of experimental chronic allograft rejection.

Early blockade of T cell-costimulatory activation pathways prevents development of experimental chronic allograft rejection. Ongoing T cell recognition of alloantigen and activation may also play an important role in progression of chronic rejection, but definitive evidence is lacking. We used the fusion protein CTLA4Ig to block CD28-B7 T cell costimulation late after the onset of initial graft injury. Using the F334 into LEW rat model of chronic renal allograft rejection, transplant recipients were treated with a 10-d course of cyclosporine, and a subgroup received a single injection of CTLA4Ig at 8 wk after transplant. Functionally, CTLA4Ig administration prevented development of progressive proteinuria (14.3+/-4.1 mg/24 h versus 41.0+/-12.0 mg/24 h at 24 wk after transplant, P < 0.05). Histologically, graft mononuclear cell infiltration, glomerular hypertrophy, focal and segmental glomerulosclerosis, and intimal vascular hyperplasia were all attenuated in CTLA4Ig-treated animals. Lastly, reverse transcriptase-PCR and immunohistologic studies showed a significant reduction in the intragraft expression of key products of T cell and macrophage activation, and upregulation of what have recently been termed as "protective" genes, including the bcl family members, Bcl-2 and Bcl-xL, and hemoxygenase. Our data are the first to demonstrate that blocking T cell-costimulatory activation late after transplantation, after initial graft injury, prevents progression of chronic allograft rejection supporting the hypothesis that ongoing T cell recognition of alloantigen and activation are key mediators of ongoing chronic allograft rejection.

Nephron supply is a major determinant of long-term renal allograft outcome in rats.

The effects of augmenting the nephron supply on indices of allograft injury were assessed in a rat model of "chronic rejection." Orthotopic renal allotransplantation into unine-phrectomized rats was followed by excision (allograft-alone group) or preservation of the remaining native kidney (allograft+native kidney group) such that the total kidney complement was either the allograft alone, or the allograft plus one retained native kidney. After 18 wk, values for GFR (1.85 +/- 0.3 ml/min) and kidney weights (2.3 +/- 0.2 g) in allograft-alone rats were far in excess of corresponding values in the allograft of allograft+native kidney rats (0.88 +/- 0.1 ml/min and 1.1 +/- 0.5 g, respectively). Proteinuria (35 +/- 2 mg/d) and allograft glomerulosclerosis (24 +/- 8%) also characterized allograft-alone but not allograft+native kidney rats, in whom glomerular structure (allograft glomerulosclerosis, 4 +/- 1%; native kidney glomerulosclerosis, 0%) and glomerular functional integrity (proteinuria 7 +/- 0.7 mg/d) were well preserved. Thus, the observed allograft protection derived from the presence of a retained recipient native kidney supports the hypothesis that a single renal allograft contains insufficient nephrons to prevent progressive renal injury, implicating nephron supply as a major determinant of long-term allograft outcome.

Chronic graft rejection.

Although chronic rejection remains the most crucial cause of organ graft loss over the long term, its etiology is not well defined. Early injury to graft endothelial cells caused by alloantigen-independent factors, such as ischemia or reperfusion, as well as alloantigen-dependent events, such as acute rejection, have been implicated. Macrophages and their products, peptide growth factors and adhesion molecules are all thought to play an important role in this process via the cytokine-adhesion molecule cascade. Although new immunosuppressive agents, including RS61443 or rapamycin, may be effective in preventing antigen-driven components of this condition, risk factors for initial non-immune injury must also be considered and, if possible, countered.

Mechanisms of chronic rejection.

Chronic rejection remains the major obstacle to long-term allograft survival. Detailed understanding of putative etiologic risk factors, both antigen-dependent and -independent, is important for designing effective therapeutic strategies to ameliorate this process. Cell senescence may be an important factor in chronic rejection.

Activation of the heart by donor brain death accelerates acute rejection after transplantation.

BACKGROUND: Donor brain death upregulates expression of inflammatory mediators in the heart. It is hypothesized that these nonspecific changes trigger and amplify acute rejection in unmodified recipients compared with hearts from normal living donors. We examined the inflammatory and immunological consequences of gradual-onset donor brain death on cardiac allografts after transplantation. METHODS AND RESULTS: Functioning hearts were engrafted from normotensive donors after 6 hours of ventilatory support. Hearts from brain-dead rats (Fisher, F344) were rejected significantly earlier (mean+/-SD, 9. 3+/-0.6 days) by their (Lewis) recipients than hearts from living donor controls (11.6+/-0.7 days, P=0.03). The inflammatory response of such organs was accelerated, with rapid expression of cytokines, chemokines, and adhesion molecules and brisk infiltration of associated leukocyte populations. Upregulation of major histocompatibility class II antigens increased organ immunogenicity. Acute rejection evolved in hearts from brain-dead donors more intensely and at a significantly faster rate than in controls. CONCLUSIONS: Donor brain death is deleterious to transplanted hearts. The resultant upregulation of inflammatory factors provokes host immune mechanisms and accelerates the acute rejection process in unmodified hosts.

Prolonged survival of alymphatic skin allografts in the rat. A humoral component.

Immunological mechanisms of increased graft survival in "immunologically privileged" sites were defined by comparing host responses against orthotopic and alymphatic skin allografts in rats. The conventional skin grafts reject by day 8; grafts placed on alymphatic skin pedicles heal in normally, but begin by day 16 to 18 to contract inexorably until only a scar remains by day 35 to 40. Lymphocyte-mediated cytotoxicity rose significantly in spleen and draining lymph nodes 8 to 10 days after orthotopic grafting, but was absent as long as 35 days after skin transplantation to alymphatic pedicles. No significant activity in antibody-dependent lymphocyte-mediated cytotoxicity was noted in either recipient group, while complement-dependent cytotoxicity was slightly elevated 8 to 10 days postoperatively in both groups. Passive transfer of serum from recipients of alymphatic skin grafts, taken 8 and 12 days after grafting, prolonged survival of test cardiac allografts significantly, although neither control serum from recipients of orthotopic skin allografts, or serum taken 28 days after alymphatic skin grafting increased test heart survival. We conclude that prolonged survival of skin grafts on alymphatic sites may be based, at least partially, on the development of host humoral factors.

Sequential cytokine expression in renal allografts in rats immunosuppressed with maintenance cyclosporine or mycophenolate mofetil.

Although the immunosuppressive agents used clinically modulate acute rejection of organ allografts, their ability to prevent chronic rejection has been less clear. To ascertain the effects of prolonged maintenance treatment with cyclosporine (CsA) and mycophenolate mofetil, we examined sequential patterns of cytokine regulation by reverse transcriptase polymerase chain reaction in long-surviving renal allografts in treated recipients. In renal allografts in animals on long-term CsA therapy, there is important up-regulation of transforming growth factor-beta, Hsp70, and endothelin as compared with control animals. Conversely, interleukin-2 receptor, interferon-gamma, and tumor necrosis factor-alpha in kidney grafts in this group were expressed at lower levels compared with those noted in chronically rejecting grafts in control animals that had received only CsA for 10 days after transplantation. Morphologically, the long-term CsA-treated kidneys had more extensive arterial obliterative changes and glomerulosclerosis after 24 weeks than control organs; these changes can presumably be attributed to the nephrotoxic effects of this drug combined with the progressive changes of chronic rejection. In contrast, mycophenolate mofetil inhibited the production of all lymphocyte and macrophage-derived cytokines throughout the entire follow-up period. Allograft kidneys in these latter recipients showed no late morphological abnormalities. This agent may be important clinically in preventing chronic rejection.

Retransplantation reverses mononuclear infiltration but not myointimal proliferation in a rat model of chronic cardiac allograft rejection.

Episodes of acute rejection seem to play an important role in the development of chronic allograft failure. Whereas acute rejection appears to be fully reversible, at least at early stages, reversibility of chronic graft alterations is still unclear. Male Fisher F344 rat hearts were heterotopically transplanted into Lewis recipients (n=8/group). Minimal immunosuppression (rapamycin 0.5 mg/kg for 14 days) guaranteed allograft survival during the observation period (group 1). Allografted hearts were retransplanted into syngeneic recipients after 14 days (group 2) and 50 days (group 3) and compared with F344 isografts undergoing retransplantation after 4 days (group 4) and with F344 isografts without a second procedure (group 5). All organs were removed after 100 days and morphologically and immunohistologically assessed. Allografts of group 1 developed concentric myointimal proliferation with dense intramural and perivascular mononuclear infiltration and intravascular thrombosis in 59 +/- 7% of coronary arteries. Retransplantation into syngeneic recipients almost completely abolished mononuclear infiltration, but did not affect the development of myointimal proliferation (groups 2/3: 46 +/- 7%/31 +/- 24%, NS). Isograft retransplantation resulted in a similar incidence of coronary lotions (group 4: 37 +/- 9%, NS), whereas coronary arteries of isografts without a second transplant procedure (group 5) remained normal (0%, P<0.001). In conclusion, syngeneic retransplantation of allografts reverses mononuclear infiltration but not myointimal proliferation. The development of coronary lesions in retransplanted isografts underlines the participation of antigen-independent stimuli in the development of myointimal proliferation. These experiments further support the hypothesis of an interaction of antigen-dependent and antigen-independent factors for the development of coronary myointimal proliferation.

Factors contributing to the development of chronic rejection in heterotopic rat heart transplantation.

BACKGROUND: The present study was devised to elucidate the influence of immunogenicity, immunosuppression, and ischemia on the development of transplant vasculopathy (TVP) as well as to investigate myointimal proliferation in syngeneic transplantation. METHODS: Fischer 344 and Brown Norway rat heart allografts and Lewis isografts were treated with rapamycin or cyclosporine, exposed to 4 hr of cold ischemia, and observed for 100 to 300 days before the incidence and degree of TVP and perivascular infiltration were assessed. RESULTS: The incidence of TVP in Fischer 344-->Lewis allografts (rapamycin, 0.5 mg/kg for 14 days) rose steadily, with dense mononuclear infiltration present in coronary lesions at all times (from 10+/-2% at 50 days to 85+/-15% at 150 days). Increased immunogenicity (Brown Norway-->Lewis) intensified TVP (62+/-13%) as compared with its control (25+/-15%, P<0.005). Enhanced immunosuppression (rapamycin, 0.5 mg/kg daily) decreased the incidence of TVP (22+/-11%, P<0.005), and additional low-dose cyclosporine was ineffective (1.5 mg/kg daily, 40+/-14%, NS). Four hours of cold ischemia before transplantation failed to have any effect on allografts, but promoted TVP in isografts (0 vs. 11+/-8%). CONCLUSIONS: The antigenic stimulus has probably the most important impact on the development of TVP, but is not necessarily essential. In most allografts, TVP probably reflects ongoing sublethal acute rejection, whereas myointimal proliferation in isografts presumably results from a perioperative antigen-independent response-to-injury mechanism.

Rapamycin inhibits transplant vasculopathy in long-surviving rat heart allografts.

We have examined the effects of rapamycin (RPM) on transplant vasculopathy in long-surviving F344 rat heart allografts transplanted heterotopically into Lewis recipients. RPM was administered intraperitoneally for the first 14 days in groups 1 and 2 (0.5 and 2 mg/kg/day), and daily throughout the follow-up period in groups 3 (0.5 mg/kg/day) and 4 (5 mg/kg for 14 days, followed by a maintenance dose of 2.5 mg/kg/day). Treatment with low dose cyclosporine (CsA; 1.5 mg/kg/day) in combination with RPM (0.5 mg/kg/day for 14 days) (group 5) and immunosuppression with CsA only (5 mg/kg for 14 days, followed by 1.5 mg/kg/day) (group 6) were also examined. F344 isograft recipients treated with RPM (0.5 mg/kg/day for 14 days) (group 7), those that were untreated (group 8), and hearts in naive F344 animals (group 9) served as controls. Grafts of group 1 were removed at 50, 75, 100, 150, and 200 days and infiltrating cell populations and surface molecules were compared with those of the other groups at 100 days. All allografts in treated hosts functioned > 100 days; in contrast, grafts in untreated recipients were rejected acutely by 8 +/- 1 days (MST +/- SD). The incidence of transplant vasculopathy in group 1 increased progressively (MST +/- SD = 10 +/- 2%, 59 +/- 7%, 85 +/- 15%, and 80 +/- 12% at 50, 100, 150, and 200 days, respectively), as manifested by myointimal proliferation with dense mononuclear infiltration (predominantly ED1+ macrophages). Numbers of MHC class II+ infiltrating cells were prominent, as was expression of adhesion molecules and cytokines. The incidence of graft disease and extent of cellular infiltration at 100 days was significantly lower in animals receiving increased maintenance doses of RPM (for groups 2, 3, and 4: 25 +/- 15%, 22 +/- 11%, and 10 +/- 3%, respectively; P < 0.005). CsA treatment either in combination with RPM or alone (groups 5 and 6) failed to improve transplant vasculopathy, but reduced mononuclear cell infiltration. Isografts (groups 7 and 8) and naive hearts (group 9) developed no structural abnormalities throughout the follow-up period, regardless of RPM treatment. We conclude that the extent of transplant vasculopathy can be reduced markedly in this rat cardiac transplant model with maintenance RPM. Addition of CsA modifies the morphological picture but does not improve myointimal proliferation.

Maturing thymocytes in accelerated rejection of cardiac allografts in presensitized rats.

LBNF1 cardiac allografts are rejected within 36 hr in LEW rats sensitized with BN skin grafts 7 days earlier (acute rejection in unmodified hosts = 8 days). We have studied and compared the function and migration patterns of thymocytes one day after engraftment in sensitized recipients, unmodified hosts, and normal naive rats. Thymocytes from animals experiencing accelerated rejection were more mature and functionally active, as shown by a significant elevation in percentage of OX-44+ (CD37+) cells, increased alloreactivity to BN and WF antigens, and proliferative responses to Con A and exogenous IL-2. However, the cells could neither lyse BN targets in vitro nor trigger rejection of otherwise indefinitely functioning test cardiac allografts in immunologically unresponsive T cell-deficient (B) rats after adoptive transfer. The traffic of 111In-labeled thymocytes was then evaluated. The migration index increased significantly during accelerated graft rejection, with thymocytes preferentially circulating in the blood, penetrating peripheral lymph nodes--and, interestingly, migrating back to the thymus. Thus, immunoresponsive and functionally active thymocytes, which lack the ability to recognize primed specific antigen, appear during accelerated rejection of cardiac allografts in sensitized rats. These cells migrate to the periphery, and then return in large numbers to their site of origin, the thymus. Hence, this study describes a novel behavior of thymocytes in the state of host alloreactivity that is distinct from the physiological one in otherwise normal thymus.

Interleukin 2 receptor-targeted therapy--rationale and applications in organ transplantation.

Despite major advances in immunopharmacology for use in clinical organ transplantation, graft rejection and drug-induced side effects remain the major problems with currently available immunosuppressive modalities. Recent advances in hybridoma technology have produced relatively effective and reproducible biological immunosuppression with monoclonal antibodies; indeed, OKT3 and anti-T12 mAbs have been employed with considerable success as adjuncts to chemical suppression in treating rejection. Nonetheless, the use of such antibodies broadly reactive to differentiation antigens on T lymphocytes does not solve the problems of side effects caused by general immunosuppression. An ideal therapeutic agent should target only lymphocytes that participate in rejection of foreign tissue without affecting physiological host immune surveillance and normal defense mechanisms. Theoretically, this goal could be achieved by "antigenic suicide," or by using the appropriate antiidiotypic antibodies or mAbs against the antigen combining site of T cell receptors. However, because of the intense polymorphism of transplantation antigens and the vast genetic repertoire encoding for the T cell antigen receptor, success of such forms of specific immunosuppression, at least at this time, is highly improbable.

Prevention of functional, structural, and molecular changes of chronic rejection of rat renal allografts by a specific macrophage inhibitor.

Chronic rejection is the primary cause of long-term allograft loss. Macrophages and their products have been shown to be critical in the development of this process in an established kidney allograft rat model. A new synthetic agent, Gamma lactone, is a specific inhibitor of macrophages and monocytes that inhibits the generation of these populations in vitro and their activities in the effector phase of host alloresponsiveness. We tested its effects on the development of chronic changes in the model. Untreated control allograft recipients developed increasing proteinuria after 12 weeks; progressive glomerulosclerosis, interstitial fibrosis, and arterial obliteration developed thereafter. Infiltrating ED1+ macrophages as noted by immunohistology increased dramatically between 12 and 16 weeks, localizing preferentially in glomeruli and perivascular areas. The presence of these cells was associated with dense expression of their products. Reverse transcription polymerase chain reaction confirmed and expanded the immunohistological findings, showing significant gene expression of macrophage-derived mediators. In contrast, recipients treated with G-Lac daily for 32 weeks never developed proteinuria; macrophage infiltration was dramatically reduced, and expression of their products was virtually absent. At 32 weeks, most glomeruli and arteries remained histologically normal. In another group in which treatment was stopped at 24 weeks, however, proteinuria began to develop by 32 weeks; macrophages infiltrated the organs and expression of their products became manifest. These results confirm the importance of macrophages and macrophage-derived factors in chronic rejection and suggest that a specific inhibitor of macrophage activation may be useful in the prevention of the process over the long term.

Influence of selective splenectomy on survival of cardiac and skin allografts in enhanced rats.

Pretreatment of LEW rats with donor cells and antidonor antiserum 11 and 12 days before transplantation, respectively, extends survival of (LEW x FN)F1 heart grafts from 1 to about 4 weeks. Previous investigations have shown that the spleen responds dynamically to the immunization regimen and to the graft itself by transient trapping of antigen-sensitive cells and massive proliferation of antibody-producing cells. In the present study we have noted that splenectomy performed before recipient pretreatment decreases graft survival. When splenectomy is performed during the periods in which the spleen is responding substantially after immunization and after transplantation, graft survival is increased remarkably (MST greater than 10 weeks, with the majority of grafts surviving indefinitely). Removal of antigen-sensitive cells trapped transiently in the spleen does not fully explain this striking effect, as an 11-day interval between pretreatment and transplantation is required for optimal graft survival in splenectomized hosts. Additionally, splenectomy is ineffective in increasing survival of skin grafts in enhanced animals, reconfirming the primary importance of the regional draining lymph nodes in rejection of these grafts.