CD4-positive helper T lymphocytes mediate mouse cardiac allograft rejection independent of donor alloantigen specific cytotoxic T lymphocytes

Recognizing Complexity of CD8 T Cells in Transplantation

The major role of CD8+ T cells in clinical and experimental transplantation is well documented and acknowledged. Nevertheless, the precise impact of CD8+ T cells on graft tissue injury is not completely understood, thus impeding the development of specific treatment strategies. The goal of this overview is to consider the biology and functions of CD8+ T cells in the context of experimental and clinical allotransplantation, with special emphasis on how this cell subset is affected by currently available and emerging therapies.

Endogenous memory CD8 T cells directly mediate cardiac allograft rejection

Differences in levels of environmentally induced memory T cells that cross-react with donor MHC molecules are postulated to account for the efficacy of allograft tolerance-inducing strategies in rodents versus their failure in nonhuman primates and human transplant patients. Strategies to study the impact of donor-reactive memory T cells on allografts in rodents have relied on the pretransplant induction of memory T cells cross-reactive with donor allogeneic MHC molecules through recipient viral infection, priming directly with donor antigen or adoptive transfer of donor antigen primed memory T cells. Each approach accelerates allograft rejection and confers resistance to tolerance induction, but also biases the T cell repertoire to strong donor reactivity. The ability of endogenous memory T cells within unprimed mice to directly reject an allograft is unknown. Here, we show a direct association between increased duration of cold ischemic allograft storage and numbers and enhanced functions of early graft infiltrating endogenous CD8 memory T cells. These T cells directly mediate rejection of allografts subjected to prolonged ischemia and this rejection is resistant to costimulatory blockade. These findings recapitulate the clinically significant impact of endogenous memory T cells with donor reactivity in a mouse transplant model in the absence of prior recipient priming.

IL-6 promotes cardiac graft rejection mediated by CD4+ cells

IL-6 mediates numerous immunologic effects relevant to transplant rejection; however, its specific contributions to these processes are not fully understood. To this end, we neutralized IL-6 in settings of acute cardiac allograft rejection associated with either CD8(+) or CD4(+) cell-dominant responses. In a setting of CD8(+) cell-dominant graft rejection, IL-6 neutralization delayed the onset of acute rejection while decreasing graft infiltrate and inverting anti-graft Th1/Th2 priming dominance in recipients. IL-6 neutralization markedly prolonged graft survival in the setting of CD4(+) cell-mediated acute rejection and was associated with decreased graft infiltrate, altered Th1 responses, and reduced serum alloantibody. Furthermore, in CD4(+) cell-dominated rejection, IL-6 neutralization was effective when anti-IL-6 administration was delayed by as many as 6 d posttransplant. Finally, IL-6-deficient graft recipients were protected from CD4(+) cell-dominant responses, suggesting that IL-6 production by graft recipients, rather than grafts, is necessary for this type of rejection. Collectively, these observations define IL-6 as a critical promoter of graft infiltration and a shaper of T cell lineage development in cardiac graft rejection. In light of these findings, the utility of therapeutics targeting IL-6 should be considered for preventing cardiac allograft rejection.

Acute cardiac allograft rejection by directly cytotoxic CD4 T cells: parallel requirements for Fas and perforin

BACKGROUND

CD4 T cells can suffice as effector cells to mediate primary acute cardiac allograft rejection. Although CD4 T cells can readily kill appropriate target cells in vitro, the corresponding role of such cytolytic activity for mediating allograft rejection in vivo is unknown. Therefore, we determined whether the cytolytic effector molecules perforin (PFP) and/or FasL (CD95L) were necessary for CD4 T cell-mediated rejection in vivo.

METHODS

Wild-type C3H(H-2) or Fas (CD95)-deficient C3Hlpr (H-2) hearts were transplanted into immune-deficient C57B6rag (H-2) mice. Then, recipients were reconstituted with naïve purified CD4 T cells from wild-type, PFP-deficient, or FasL (gld)-deficient T-cell donors.

RESULTS

In vitro, alloreactive CD4 T cells were competent to lyse donor major histocompatibility complex class II+ target cells, largely by a Fas-dependent mechanism. In vivo, the individual disruption of donor Fas expression (lpr) or CD4 T-cell-derived PFP had no significant impact on acute rejection. However, FasL-deficient (gld) CD4 T cells demonstrated delayed allograft rejection. Importantly, the simultaneous removal of both donor Fas expression and CD4 T-cell PFP completely abrogated acute rejection, despite the persistence of CD4 T cells within the graft.

CONCLUSIONS

Results demonstrate that the direct rejection of cardiac allografts by CD4 effector T cells requires the alternative contribution of graft Fas expression and T cell PFP expression. To our knowledge, this is the first demonstration that cytolytic activity by CD4 T cells can play an obligate role for primary acute allograft rejection in vivo.

OX40 costimulation prevents allograft acceptance induced by CD40-CD40L blockade

Disrupting the CD40-CD40L costimulation pathway promotes allograft acceptance in many settings. Herein, we demonstrate that stimulating OX40 overrides cardiac allograft acceptance induced by disrupting CD40-CD40L interactions. This effect of OX40 stimulation was dependent on CD4(+) T cells, which in turn provided help for CD8(+) T cells and B cells. Allograft rejection was associated with donor-reactive Th1 and Th2 responses and an unconventional granulocytic infiltrate and thrombosis of the arteries. Interestingly, OX40 stimulation induced a donor-reactive IgG class switch in the absence of CD40-CD40L interactions, and the timing of OX40 stimulation relative to transplantation affected the isotype of donor-reactive Ab produced. Inductive OX40 stimulation induced acute graft rejection, which correlated with both IgG1 and IgG2a deposition within the graft. Once graft acceptance was established following CD40-CD40L blockade, delayed OX40 stimulation did not induce acute allograft rejection despite priming of graft-reactive Th1 and Th2. Rather, chronic rejection was induced, which was characterized by IgG1 but not IgG2a deposition within the graft. These studies reveal both redundancy and key differences in function among costimulatory molecules that manifest in distinct pathologies of allograft rejection. These findings may help guide development of therapeutics aimed at promoting graft acceptance in transplant recipients.

CD4 T cell-mediated rejection of cardiac allografts in B cell-deficient mice

CD4 T cell-dependent mechanisms promoting allograft rejection include expression of inflammatory functions within the graft and the provision of help for donor-reactive CD8 T cell and Ab responses. These studies tested CD4 T cell-mediated rejection of MHC-mismatched cardiac allografts in the absence of both CD8 T and B lymphocytes. Whereas wild-type C57BL/6 recipients depleted of CD8 T cells rejected A/J cardiac grafts within 10 days, allografts were not rejected in B cell-deficient B6.muMT(-/-) recipients depleted of CD8 T cells. Isolated wild-type C57BL/6 and B6.muMT(-/-) CD4 T cells had nearly equivalent in vivo alloreactive proliferative responses. CD4 T cell numbers in B6.muMT(-/-) spleens were 10% of that in wild-type mice but were only slightly decreased in peripheral lymph nodes. CD8 T cell depletion did not abrogate B6.muMT(-/-) mice rejection of A/J skin allografts and this rejection rendered these recipients able to reject A/J cardiac allografts. Redirection of the alloimmune response to the lymph nodes by splenectomy conferred the ability of B6.muMT(-/-) CD4 T cells to reject cardiac allografts. These results indicate that the low number of splenic CD4 T cells in B6.muMT(-/-) mice underlies the inability to reject cardiac allografts and this inability is overcome by diverting the CD4 T cell response to the peripheral lymph nodes.

Regulation of alloimmune Th1 responses by the cyclin-dependent kinase inhibitor p21 following transplantation

BACKGROUND

The cyclin-dependent kinase (cdk) inhibitor p21 inhibits cellular proliferation of many cell types, including T cells. Autoimmune models, however, have yielded conflicting results regarding the role of cdk inhibitors and T-cell function. The role of p21 in T-cell function after transplantation has not been investigated directly. We hypothesized that p21 plays an important role in alloantigen-driven responses in vitro in mixed lymphocyte cultures (MLC) and in vivo using the heterotopic murine cardiac allograft model.

METHODS

Wild type (WT) and p21-deficient (p21-/-) mice were used as recipients, and the effects of p21 overexpression were assessed by transplanting p21 adenoviral-transfected cardiac allografts. Enzyme-linked immunospot (ELISPOT) and 3H-thymidine incorporation were used to evaluate for T-cell priming and proliferation in vitro, whereas graft histology was evaluated for rejection.

RESULTS

When stimulated with alloantigens in vitro, splenocytes from p21-/- mice mounted enhanced proliferative responses and decreased Th2 responses relative to their WT counterparts. No differences in Th1 responses were noted when p21-/- cells were stimulated with alloantigens in vitro; however, after cardiac transplantation, Th1 responses were enhanced in p21-/- recipients relative to WT mice. This enhanced in vivo Th1 response was associated with exacerbated graft rejection in p21-/- recipients. Interestingly, p21 transfection of WT allografts inhibited graft rejection and Th1 priming.

CONCLUSIONS

p21 controls the intensity of the immune response posttransplantation, with overexpression inhibiting allograft rejection. Our data demonstrate that p21 controls T-cell priming and suggest that p21 and other cdk inhibitors may serve as potential targets for therapeutic manipulation of alloimmune responses.

Pathways of helper CD4 T cell allorecognition in generating alloantibody and CD8 T cell alloimmunity

BACKGROUND

The relative contributions of the "direct" and "indirect" pathways of CD4 T cell allorecognition in providing help for generating effective humoral and CD8 T cell alloimmunity remain unclear. Here, the generation of alloantibody and cytotoxic CD8 T cell responses to a vascularized allograft were examined in a murine adoptive-transfer model in which help could only be provided by transferred CD4 T cells recognizing alloantigen exclusively through the direct pathway.

METHODS

Rejection kinetics and the development of alloantibody and cytotoxic CD8 T cell responses to MHC-mismatched H-2d heart grafts were compared when CD4 T cell help was present (wild-type H-2d recipients), or absent (CD4 T cell deficient, MHC class II-/- H-2b recipients [B6CII-/-]), or available only through the direct pathway (B6CII-/- mice reconstituted with wild-type CD4 T cells).

RESULTS

BALB/c allografts were rejected by B6 mice rapidly (median survival time [MST] 7 days) with strong CD8 T cell effector and alloantibody responses, but were rejected by B6CII-/- mice more slowly (MST 23 days), with markedly reduced CD8 T cell responses and no detectable alloantibody. CD4 T cell reconstitution of B6CII-/- recipients accelerated heart graft rejection to near that of wild-type recipients (MST 13 days), with complete restoration of cytotoxic CD8 T cell responses but without detectable IgM or IgG alloantibody.

CONCLUSIONS

Different pathways of helper T cell allorecognition are responsible for generating humoral and CD8 T cell alloimmunity. CD4 T cell help provided exclusively through the direct pathway generates strong cytotoxic CD8 T cell responses that effect rapid heart graft rejection.

The Effect of a Novel Immunosuppressant, FTY720, in Mice Without Secondary Lymphoid Organs

PURPOSE

FTY720 is a novel immunosuppressive agent that is thought to reduce the number of peripheral blood lymphocytes (PBL) by directing them toward secondary lymphoid organs such as the lymph nodes and Peyer's patches. We studied the effects of FTY720 on aly/aly mice that do not have either lymph nodes or Peyer's patches, as well as on splenectomized aly/aly mice.

METHODS

FTY720 was orally administered by gavage (1 mg/kg) to aly/aly mice as well as to aly/+ mice with and without a splenectomy on 14 consecutive days. The number of lymphocytes was then counted using True Cell beads and flow cytometry. The number of B220-, CD3-, and CD4-positive cells was also determined. In addition, skin grafts from C3H donor mice were performed on these mice.

RESULTS

FTY720 was effective in significantly reducing the total lymphocyte count as well as the B220-, CD3-, and CD4-positive subtypes in the peripheral blood of aly/+ mice as well as in aly/aly mice with and without a splenectomy. While we did observe allograft skin graft rejection in both the aly/+ mice as well as the aly/aly mice recipients and splenectomized aly/aly mice, the graft survival was prolonged in all groups. The skin allografts treated by FTY720 thus demonstrated fewer lymphocytic cells and less infiltration of CD4-positive cells.

CONCLUSIONS

The administration of FTY720 to mice without lymph nodes, Peyer's patches, or spleens still results in peripheral lymphopenia. In all groups, FTY720 was found to prevent the infiltration of CD4-positive cells in skin allografts while also prolonging skin allograft survival. The fate of these lymphocytes, however, is unclear.

Role of CD4+ and CD8+ T cells in murine skin and heart allograft rejection across different antigenic desparities

The factors that influence the relative contribution of the T cell subsets to allograft rejection remain unclear. We compared skin and heart rejection in CD4 Knockout (KO), and CD8 KO mice across full-, minor-, and class II histocompatibility antigen (HA) mismatches. Skin allografts were rejected by either CD4+ or CD8+ T cells alone at any degree of antigenic mismatch. However, either the absence of CD4+ cells or a lesser degree of HA mismatch resulted in prolongation of graft survival. In contrast, fully allogeneic heart grafts were accepted in CD4 KO recipients, and minor HA mismatched heart grafts were accepted by both CD4 KO and CD8 KO mice. Thus, the T cell subsets required for allograft rejection are determined by the immunogenicity of the tissue transplanted. In the absence of CD8+ T cells, perforin and Fas ligand (FasL) but not granzyme B mRNA were detected in rejecting grafts. Thus, granzyme B is a CD8+ cytotoxic T lymphocyte (CTL)-specific effector molecule.

Graft rejection mediated by CD4+ T cells via indirect recognition of alloantigen is associated with a dominant Th2 response

CD4(+) T cells that respond to indirectly presented alloantigen have been shown to mediate chronic rejection, however, the role of the indirect pathway in acute rejection has yet to be completely elucidated. To this end, BALB/c or C57BL/6 mice were depleted of CD8(+) T cells and transplanted with class II transactivator (CIITA)-deficient cardiac allografts, which cannot directly present class II alloantigens to CD4(+) T cells. In this manner, the rejection response by CD4(+) cells was forced to rely upon the indirect recognition pathway. When not depleted of CD8(+) cells, both BALB/c and C57BL/6 mice rejected CIITA-/- allografts and a polarized Th1 response was observed. In contrast, when BALB/c recipients of CIITA-/- allografts were depleted of CD8(+) T cells, the grafts were acutely rejected and a strong Th2 response characterized by eosinophil influx into the graft was observed. Interestingly, CD8-depleted C57BL/6 recipients of CIITA-/- allografts did not acutely reject their transplants and a Th2 response was not mounted. These findings indicate that CD4(+) T cells responding to indirectly presented alloantigens mediate graft rejection in a Th2-dominant manner, and provide further evidence for the role of Th2 responses in acute graft rejection.

CD70 Signaling Is Critical for CD28-Independent CD8+T Cell-Mediated Alloimmune Responses In Vivo

The inability to reproducibly induce robust and durable transplant tolerance using CD28-B7 pathway blockade is in part related to the persistence of alloreactive effector/memory CD8(+) T cells that are less dependent on this pathway for their cellular activation. We studied the role of the novel T cell costimulatory pathway, CD27-CD70, in alloimmunity in the presence and absence of CD28-B7 signaling. CD70 blockade prolonged survival of fully mismatched vascularized cardiac allografts in wild-type murine recipients, and in CD28-deficient mice induced long-term survival while significantly preventing the development of chronic allograft vasculopathy. CD70 blockade had little effect on CD4(+) T cell function but prevented CD8(+) T cell-mediated rejection, inhibited the proliferation and activation of effector CD8(+) T cells, and diminished the expansion of effector and memory CD8(+) T cells in vivo. Thus, the CD27-CD70 pathway is critical for CD28-independent effector/memory CD8(+) alloreactive T cell activation in vivo. These novel findings have important implications for the development of transplantation tolerance-inducing strategies in primates and humans, in which CD8(+) T cell depletion is currently mandatory.

Requirement for Donor and Recipient CD40 Expression in Cardiac Allograft Rejection: Induction of Th1 Responses and Influence of Donor-Derived Dendritic Cells

Costimulation through the CD40-CD40 ligand (CD40L) pathway is critical to allograft rejection, in that anti-CD40L mAb therapy prolongs allograft survival. However, the majority of studies exploring CD40-CD40L interactions have targeted CD40L. Less is known about the requirement for donor- and/or host-derived CD40 during rejection. This study assessed the relative contributions of donor and recipient CD40 expression to the rejection process. As the effectiveness of costimulatory blockade may be mouse strain dependent, this study explored the requirement for donor and recipient CD40 expression in BALB/c and C57BL/6 mice. Wild-type (WT) and CD40(-/-) BALB/c recipients readily rejected WT and CD40(-/-) C57BL/6 allografts, and rejection was associated with a prominent Th1 response. In contrast, CD40(-/-) C57BL/6 recipients failed to reject WT or CD40(-/-) BALB/c allografts and did not mount Th1 or Th2 responses. However, injection of donor CD40(-/-) dendritic cells induced both Th1 and Th2 responses and allograft rejection in CD40(-/-) C57BL/6 recipients. Finally, WT C57BL/6 mice rejected CD40(-/-) allografts, but this rejection response was associated with muted Th1 responses. These findings demonstrate that 1) CD40 expression by the recipient or the graft may impact on the immune response following transplantation; 2) the requirement for CD40 is influenced by the mouse strain; and 3) the requirement for CD40 in rejection may be bypassed by donor DC. Further, as CD40 is not required for rejection in BALB/c recipients, but anti-CD40L mAb prolongs graft survival in these mice, these results suggest that anti-CD40L therapy functions at a level beyond disruption of CD40-CD40L interactions.

Effects of T cell frequency and graft size on transplant outcome in mice

The features that determine whether graft-reactive T lymphocytes develop into effector cells capable of mediating organ destruction are not well understood. To investigate potential factors involved in this process, we first confirmed that female recipient mice acutely rejected minor Ag-disparate male skin, but not heart transplants. Despite this difference in outcome, heart and skin transplantation induced antidonor T cell responses of similar magnitude, specificity, and cytokine profile. The heart-graft-primed T cells transiently infiltrated the graft and ultimately induced the development of chronic transplant vasculopathy. Increasing the frequency of donor-reactive T cells by presensitization or by using TCR (CD8+ antimale)-transgenic recipients did not mediate acute rejection but accelerated the pace and severity of the vasculopathy. Surprisingly, decreasing the tissue mass of the donor heart by 50% resulted in acute rejection of these smaller grafts without increasing the frequency of antidonor effector T cells in the recipients. In complementary studies, placement of one or two male skin grafts on a single recipient did not affect the frequency or cytokine profile of the induced antimale T cell repertoire. Nonetheless, the recipients of single grafts acutely rejected the transplanted skin while the recipients of two skin grafts did not. These results provide new insight into the pathogenesis of transplant vasculopathy and provide an explanation for the difference in outcome between murine skin and heart transplants by highlighting the novel concept that the efficiency of transplant-reactive T cell immunity is heavily influenced by the tissue burden it encounters at the effector stage.

Vulnerability of allografts to rejection by MHC class II-restricted T-cell receptor transgenic mice

BACKGROUND

Examination of the in vivo activation and function of CD4+ T cells in response to allografts may advance our understanding of the rejection process. We analyzed the capacity of transgenic class II-restricted CD4 T cells to reject skin, cardiac, and islet transplants.

METHODS

TS1 mice possess a high frequency of CD4+ T cells specific for the immunodominant epitope of the viral hemagglutinin (HA) protein. We analyzed the kinetics of rejection of skin, heart, and islet grafts by naïve and sensitized TS1 mice and by adoptively transferred TS1 lymphocytes.

RESULTS

Rejection of heart transplants was more rapid than skin grafts (mean survival time, 12.9 vs. 26.6 days), and islet grafts survived indefinitely in TS1 mice. These findings may be partly attributable to the supranormal frequency of HA-reactive cells in TS1 mice. In support of this, we found that adoptive transfer of 5 x 10(5) TS1 lymphocytes to Balb/c hosts effected consistent rejection of HA-bearing skin transplants, whereas a significantly greater number (3 x 10(6)) was required for heart transplant rejection. The in vivo proliferative response of HA-specific T cells to heart and skin was found to be robust and predominantly localized to the draining lymph nodes.

CONCLUSION

We developed a model of allograft rejection in which the responding T cells and relevant graft antigen are specifically defined. Adoptive transfer of carboxy-fluorescein succinimidyl ester-labeled transgenic T cells allowed us to visualize a robust proliferative response in vivo to heart and skin allografts, which in both cases was localized to regional lymph nodes.

Allograft rejection in a new allospecific CD4+ TCR transgenic mouse

The application of TCR transgenic mice to transplantation immunology is hampered by the limited lines available. Recently, we reported CD4+ T cell receptor (TCR) transgenic mice specific for I-Abm12 expressed on B6.C.H-2bm12 mice. Here, we characterized rejection of skin and vascularized cardiac allografts in these mice, which we term ABM (for anti-bm12). In vivo proliferative experiments reveal that all CD4 T cells in ABM mice react to bm12 antigens. Surprisingly, while ABM mice have accelerated (compared to B6 recipients) rejection of bm12 skin allografts, they, like B6 recipients, fail to acutely reject bm12 cardiac allografts. This is not due to lack of immunogenicity of bm12 hearts, as these grafts are acutely rejected by primed ABM recipients, although not by primed B6 recipients. Lastly, long-term surviving bm12 grafts in ABM recipients are relatively free from chronic rejection (compared with B6 recipients), which may be due to skewing of the CD4 repertoire towards direct alloreactivity, and consequent lack of CD4 mediated indirect allorecognition as evidenced by the lack of IgG deposition in those grafts. The results indicate that a complex interplay between responder frequency, priming and repertoire dictates the occurrence, or lack thereof, of acute and chronic rejection.

Contrasting alloreactive CD4+ and CD8+ T cells: there's more to it than MHC restriction

Surface expression of CD4 or CD8 is commonly used to identify T-cell subsets that recognize antigen presented by class II MHC or class I MHC, respectively. This holds true for T cells that respond to allogeneic MHC molecules that are directly recognized as foreign, as well as peptides from allogeneic MHC molecules that are indirectly presented by self MHC molecules. CD4 or CD8 expression was initially believed to define cytokine secreting helper T cells or cytotoxic cells, respectively. However, this association of phenotype and function is not absolute, in that CD4+ cells may possess lytic activity and CD8+ cells secrete cytokines, notably IFNgamma. Recently, additional fundamental differences in the immunobiology of these T-cell subsets have been identified. These include differences in costimulatory requirements, cytokine responsiveness, cytokine production, cell survival, and the maintenance of memory. This review will survey these differences, emphasizing alloreactive T-cell responses as well as relevant observations that have been made in other systems.

Further analysis of the T-cell subsets and pathways of murine cardiac allograft rejection

The present study examined the role of CD4+ and CD8+ T cells in cardiac allograft rejection when either the direct or indirect pathway was eliminated for the CD4+ portion of the response. To study the pathways in vivo, we used genetically altered mouse strains that lack class II antigens as either the donors or recipients for cardiac transplantation. In contrast to earlier published studies, which used different strain combinations, we found that either CD4- or CD8-depletion prolonged cardiac allograft survival moderately, but not indefinitely, in an MHC-mismatched, minor-matched combination. When the CD4+ indirect pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present and when either CD4+ or CD8+ T cells were depleted. When the CD4+ direct pathway was eliminated, rapid graft rejection occurred when both T-cell subsets were present, there was slow rejection when CD4+ T cells were eliminated, and no rejection was seen for more than 100 days when CD8+ T cells were eliminated. However, the long-surviving allografts on the recipients with only CD4+ cells and an indirect pathway did show evidence of chronic vasculopathy. Thus, either CD4+ or CD8+ T cells can mediate acute cardiac allograft rejection in these experiments when both pathways are available. In addition, CD4+ T cells can provide help for acute rejection through either the direct or indirect pathway. Finally, recipients who have only CD4+ cells and an indirect pathway do not demonstrate acute rejection, but do show evidence of chronic rejection.

Allograft acceptance despite differential strain-specific induction of TGF-beta/IL-10-mediated immunoregulation

We examined the immune approaches that C57BI/6 and BALB/c mice take when treated to accept cardiac allografts. C57BI/6 mice accept DBA/2 cardiac allografts when treated with gallium nitrate (GN) or anti-CD40L mAb (MR1). These allograft acceptor mice fail to mount donor-reactive delayed type hypersensitivity (DTH) responses, and develop a donor-induced immunoregulatory mechanism that inhibits DTH responses. In contrast, BALB/c mice accept C57BI/6 cardiac allografts when treated with MR1 but not with GN. These allograft acceptor mice display modest donor-reactive DTH responses, and do not develop donor-induced immune regulation of DTH responses. Real-time PCR analysis of rejecting graft tissues demonstrated no strain-related skewing in the production of cytokines mRNAs. In related studies, C57BI/6 recipients of cytokine and alloantigen educated syngeneic peritoneal exudate cells (PECs) failed to mount DTH responses to the alloantigens unless neutralizing antibodies to transforming growth factor-beta (TGF-p were present at the DTH site demonstrating regulation of cell-mediated alloimmune responses. In contrast, BALB/c recipients of cytokine-and alloantigen-educated PECs expressed strong DTH responses to alloantigens demonstrating a lack of regulated alloimmunity. In conclusion, C57BI/6 mice respond to immunosuppression by accepting cardiac allografts and generating TGF-beta-related regulation of donor-reactive T cell responses, unlike BALB/c mice that do not generate these regulatory responses yet still can accept cardiac allografts.

MHC-identical heart and hepatocyte allografts evoke opposite immune responses within the same host

BACKGROUND

Purified allogeneic hepatocytes are highly antigenic and elicit immune responses that are not easily controlled. However, it is not clear whether hepatocytes are not capable of protective immune mechanisms or whether they are not to protection by immune mechanisms that permit long-term survival of other allografts. The purpose of the current study was to determine whether donor-matched allogeneic hepatocytes are protected from rejection in mice that have been induced to accept heart allografts.

METHODS

Transient treatment with anti-CD4 monoclonal antibody (mAb) or gallium nitrate (GN) was used to induce acceptance of heterotopic FVB/N (H-2(q)) heart allografts by C57BL/6 (H-2(b)) mice. Transgenic hA1AT-FVB/N hepatocytes were sequentially transplanted into C57BL/6 mice that had accepted FVB/N heart allografts more than 60 days (heart acceptor mice), CD8 depleted C57BL/6 heart acceptor mice, or B-cell knockout (BCKO, H-2(b)) heart acceptor mice. Hepatocyte survival was determined by the detection of secreted transgenic product hA1AT by enzyme-linked immunosorbent assay (ELISA).

RESULTS

FVB/N hepatocytes were rejected by day 10-14 posttransplant, while FVB/N heart allografts continued to function in C57BL/6, BCKO, and CD8 depleted heart acceptor mice. When FVB/N hepatocytes and heart allografts were transplanted into C57BL/6 or BCKO mice under short-term cover of anti-CD4 mAb or GN, hepatocyte rejection occurred by day 10 posttransplant, while most heart allografts survived for more than 60 days.

CONCLUSIONS

Hepatocyte rejection does not appear to interfere with the of mechanisms that permit heart allograft acceptance. However, immune responses to allogeneic hepatocytes are not to regulation by mechanisms induced in heart acceptor mice. The simultaneous rejection of FVB/N allogeneic hepatocytes and continued acceptance of FVB/N-matched heart allografts is independent of host CD8+ T cells and humoral immunity.

Preventing NK cell activation by donor dendritic cells enhances allospecific CD4 T cell priming and promotes Th type 2 responses to transplantation antigens

Although much progress has been made in understanding the role of NK cells in bone marrow transplantation, little is known about their function in CD4 T cell-mediated allograft rejection. We have previously shown that in the absence of CD8 T lymphocyte priming, the in vivo default development pathway of alloreactive CD4 T cells was strongly biased toward Th2 phenotype acquisition. In this study, we investigate the impact of NK cells on the activation and differentiation of alloreactive CD4 T cells in various donor/recipient combinations. Our data demonstrate that defective inhibition of host NK cells by donor APCs including dendritic cells (DCs) results in diminished allospecific Th cell responses associated with the development of effector Th cells producing IFN-gamma rather than type 2 cytokines. Turning host NK cells off was sufficient to restore strong alloreactive CD4 T cell priming and Th2 cell development. Similar results were obtained by analyzing the effect of NK cell activation on CD4 T cell responses to skin allografts. However, despite the dramatic effect of NK cells on alloreactive Th1/Th2 cell development, the kinetics of skin graft rejection were not affected. Thus, Th2 differentiation is a major pathway of alloreactive CD4 T cell development during solid organ transplant rejection, as long as host NK and CD8 T cells are not activated. We propose the hypothesis that MHC class I-driven interactions between donor DCs and host NK cells or CD8 T cells might result in DC-carried signals controlling the dynamics of alloreactive CD4 T cell priming and polarization.

CD28-independent costimulation of T cells in alloimmune responses

T cell costimulation by B7 molecules plays an important role in the regulation of alloimmune responses. Although both B7-1 and B7-2 bind CD28 and CTLA-4 on T cells, the role of B7-1 and B7-2 signaling through CTLA-4 in regulating alloimmune responses is incompletely understood. To address this question, we transplanted CD28-deficient mice with fully allogeneic vascularized cardiac allografts and studied the effect of selective blockade of B7-1 or B7-2. These mice reject their grafts by a mechanism that involves both CD4(+) and CD8(+) T cells. Blockade of CTLA-4 or B7-1 significantly accelerated graft rejection. In contrast, B7-2 blockade significantly prolonged allograft survival and, unexpectedly, reversed the acceleration of graft rejection caused by CTLA-4 blockade. Furthermore, B7-2 blockade prolonged graft survival in recipients that were both CD28 and CTLA-4 deficient. Our data indicate that B7-1 is the dominant ligand for CTLA-4-mediated down-regulation of alloimmune responses in vivo and suggest that B7-2 has an additional receptor other than CD28 and CTLA-4 to provide a positive costimulatory signal for T cells.

A serine protease inhibitor, N-alpha-tosyl-l-lysine chloromethyl ketone, prolongs rat hepatic allograft survival

BACKGROUND

Serine protease inhibitors have profound suppressive effects on cellular and humoral immune responses. We investigated the effect of a serine protease inhibitor, N-alpha-tosyl-l-lysine chloromethyl ketone (TLCK), on hepatic allograft survival in rats. Methods. Orthotopic hepatic transplantation was performed in an ACI (RT1(a))-to-LEW (RT1(1)) rat combination. TLCK was administered continuously at a dose of 4.4 mg/kg/day using an osmotic subcutaneous infusion minipump.

RESULTS

TLCK prolonged hepatic allograft survival. Histologic staging of acute rejection based on Banff criteria in TLCK-treated hepatic allografts was significantly lower than in untreated allografts. TLCK significantly reduced serum concentrations of interferon (IFN)-gamma and tumor necrosis factor (TNF) alpha in allograft recipients. TNF-alpha mRNA levels in TLCK-treated allografts were significantly lower than in untreated allografts. TLCK also decreased perforin mRNA levels in hepatic allografts. Hepatic infiltrates eluted from TLCK-treated allografts showed significantly lower cell-mediated lympholytic activity against donor Con A blast cervical lymph node cells than those from untreated allografts. In vitro, TLCK suppressed interleukin-2 production and [(3)H]thymidine incorporation into an allogeneic mixed lymphocyte reaction.

CONCLUSION

TLCK suppressed acute allograft rejection, suggesting a novel immunosuppressive strategy for therapy of acute organ rejection.

Immunobiology of allograft rejection in the absence of IFN-gamma: CD8+ effector cells develop independently of CD4+ cells and CD40-CD40 ligand interactions

Both wild-type (WT) and IFN-gamma-deficient (IFN-gamma(-/-)) C57BL/6 mice can rapidly reject BALB/c cardiac allografts. When depleted of CD8(+) cells, both WT and IFN-gamma(-/-) mice rejected their allografts, indicating that these mice share a common CD4-mediated, CD8-independent mechanism of rejection. However, when depleted of CD4(+) cells, WT mice accepted their allografts, while IFN-gamma(-/-) recipients rapidly rejected them. Hence, IFN-gamma(-/-), but not WT mice developed an unusual CD8-mediated, CD4-independent, mechanism of allograft rejection. Allograft rejection in IFN-gamma(-/-) mice was associated with intragraft accumulation of IL-4-producing cells, polymorphonuclear leukocytes, and eosinophils. Furthermore, this form of rejection was resistant to treatment with anti-CD40 ligand (CD40L) mAb, which markedly prolonged graft survival in WT mice. T cell depletion studies verified that anti-CD40L treatment failed to prevent CD8-mediated allograft rejection in IFN-gamma(-/-) mice. However, anti-CD40L treatment did prevent CD4-mediated rejection in IFN-gamma(-/-) mice, although grafts were eventually rejected when CD8(+) T cells repopulated the periphery. The IL-4 production and eosinophil influx into the graft that occurred during CD8-mediated rejection were apparently epiphenomenal, since treatment with anti-IL-4 mAb blocked intragraft accumulation of eosinophils, but did not interfere with allograft rejection. These studies demonstrate that a novel, CD8-mediated mechanism of allograft rejection, which is resistant to experimental immunosuppression, can develop when IFN-gamma is limiting. An understanding of this mechanism is confounded by its association with Th2-like immune events, which contribute unique histopathologic features to the graft but are apparently unnecessary for the process of allograft rejection.

Rejection responses to allogeneic hepatocytes by reconstituted SCID mice, CD4, KO, and CD8 KO mice

INTRODUCTION

The purpose of the current study was to investigate the capacity of CD4+, CD8+, or non-T cells to independently initiate acute rejection of allogeneic hepatocytes using reconstituted SCID, CD4 or CD8 knockout (KO) recipient mice.

METHODS

Allogeneic hepatocytes (FVB/N, H-2q) were transplanted into C57BL/6.SCID (H-2b), CD4 KO (H-2b), CD8 KO (H-2b), or beige/beige (H-2b) mice. SCID mice with functioning hepatocellular allografts subsequently received purified non-T cells (NTC), CD4+, or CD8+ splenocytes. Some mice were treated with anti-CD4, anti-CD8, and/or anti-nkl.1 mAb. Recipient mice were also assessed for donor-reactive delayed-type hypersensitivity (DTH) responses and donor-reactive alloantibody production.

RESULTS

Median hepatocellular allograft survival time (MST) was 28 days in CD4+ reconstituted SCID mice and 14 days in CD8+ reconstituted SCID mice. SCID hosts reconstituted with NTC demonstrated indefinite hepatocellular allograft survival (>120 days). MST was 10 days in untreated beige/beige (NK cell deficient) mice. MST was 14 days in untreated, 35 days in anti-CD4 mAb treated, and 10 days in anti-nkl.1 mAb treated CD8 KO mice. MST was 10 days in untreated, 35 days in anti-CD8 mAb treated, and 7 days in anti-nk1.1 mAb treated CD4 KO mice. Donor-reactive DTH responses were not detected in reconstituted SCID mice, were minimal in CD4 KO mice, and were prominent in CD8 KO mice after rejection of allogeneic hepatocytes. Similarly, donor-reactive alloantibody, was not detected in CD4 KO hosts, but was readily detected in CD8 KO hosts.

CONCLUSIONS

These studies show that both CD4+ and CD8+ T cells (but not host NTC) can independently initiate the rejection of allogeneic hepatocytes. While hepatocyte rejection by isolated CD4+ T cells is not surprising, rejection by CD8+ T cells (in the absence of CD4+ T cells) was unusual, and may explain the failure of "standard" immunosuppressive regimens to suppress acute rejection of allogeneic hepatocytes, as noted in prior studies. Furthermore, NK cells do not appear to be required for either CD4+ T cell or CD8+ T cell initiated hepatocyte rejection.

DNA-liposome versus adenoviral mediated gene transfer of transforming growth factor beta1 in vascularized cardiac allografts: differential sensitivity of CD4+ and CD8+ T cells to transforming growth factor beta1

We have developed a model of transforming growth factor (TGF)beta1 gene transfer into mouse vascularized cardiac allografts to study the use of gene transfer as an immunosuppressive therapy in transplantation. Donor hearts were perfused with either DNA-liposome complexes or adenoviral vectors that encode the active form of human TGFbeta1. DNA-liposome mediated transfection prolonged allograft survival in approximately two-thirds of transplant recipients, while adenoviral delivery of TGFbeta1 was not protective. Protective TGFbeta1 gene transfer was associated with reduced Th1 responses and an inhibition of the alloantibody isotype switch. The protective effects of TGFbeta1 gene transfer were overridden by exogenous interleukin-12 administration. Interestingly, alloreactive CD4+ and CD8+ cells exhibited distinct sensitivities to TGFbeta1 gene transfer: CD4+ Th1 function was abrogated by this modality, although CD8+ Th1 function was not. Transient depletion of recipient CD8+ cells markedly prolonged the survival of grafts transfected with either DNA-liposome complexes or adenoviral vectors. Transgene expression persisted for at least 60 days, and Th1 responses were not detectable until CD8+ T cells repopulated the periphery. However, long-term transfected allografts appeared to exhibit exacerbated fibrosis and neointimal development. These manifestations of chronic rejection were absent in long-term transfected isografts, suggesting that long-term expression of active TGFbeta1 alone is not sufficient to induce fibrosis of the grafts. Collectively, these data illustrate the utility of immunosuppressive gene therapy as a treatment for transplantation when combined with additional conditioning regimens. Further, they illustrate that alloreactive CD4+ and CD8+ cells may be differentially influenced by cytokine manipulation strategies.

Different costimulation signals used by CD4(+) and CD8(+) cells that independently initiate rejection of allogenic hepatocytes in mice

The current study evaluated the role of CD40/CD40 ligand (CD40L) and CD28/B7 costimulation signals during alloimmune responses independently mediated by CD4(+) or CD8(+) T cells. Allogeneic hepatocytes were transplanted into CD8 or CD4 knock out (KO) mice under cover of costimulatory blockade. Rejection of FVB/N (H-2(q)) hepatocytes occurred by day 10 posttransplant in untreated CD8 or CD4 KO (H-2(b)) mice. Treatment of CD8 or CD4 KO mice with anti-CD40L monoclonal antibody (mAb; MR1) resulted in significant prolongation of hepatocyte survival indicating that CD40/CD40L interactions were critical in both CD4(+) and CD8(+) T-cell initiated hepatocyte rejection. Anti-CD40L mAb also prolonged hepatocyte survival in B-cell KO (H-2(b)) mice, indicating that the efficacy of CD40/CD40L blockade in preventing hepatocyte rejection was B-cell (and antibody) independent. In contrast, treatment with CTLA4 fusion protein (CTLA4Ig), prolonged hepatocyte survival in CD8 KO but not CD4 KO mice, showing that CD28/B7 interactions were important in CD4(+) but not CD8(+) T-cell initiated hepatocyte rejection. Under selected circumstances, such as in CD40 KO mice, both CD4(+) and CD8(+) T cells mediate hepatocyte rejection in the absence of CD40/CD40L costimulation and without a significant contribution from CD28/B7 costimulation signals. These results highlight the disparate roles of CD40/CD40L and CD28/B7 costimulation signals in CD4(+) versus CD8(+) T-cell mediated immune responses to allogeneic hepatocytes. The CD4(+) T-cell independent, CD40L-sensitive, CD28/B7-independent pathway of CD8(+) T-cell activation in response to transplantation antigens is novel.

CD4 T cell-mediated cardiac allograft rejection requires donor but not host MHC class II

Numerous studies indicate that CD4 T cells are required for acute cardiac allograft rejection. However, the precise role for CD4 T cells in this response has remained ambiguous owing to the multipotential properties of this T-cell subpopulation. In the current study, we demonstrate the capacity of CD4 T cells to serve as direct effector cells of cardiac allograft rejection. We show that CD4 T cells are both necessary and sufficient for acute graft rejection, as indicated by adoptive transfer experiments in immune-deficient SCID and rag1(-/-) recipients. We have analyzed the contribution of direct (donor MHC class II restricted) and indirect (host MHC class II restricted) antigen recognition in CD4-mediated rejection. Acute CD4 T cell-mediated rejection required MHC class II expression by the allograft, indicating the importance of direct graft recognition. In contrast, reciprocal experiments indicate that CD4 T cells can acutely reject allogeneic cardiac allografts established in rag1(-/-) hosts that were also MHC class II deficient. This latter result indicates that indirect presentation of donor antigens by host MHC class II is not required for acute CD4-mediated rejection. Taken together, these results indicate that CD4 T cells can serve as effector cells for primary acute cardiac allograft rejection, predominantly via direct donor antigen recognition and independent of indirect reactivity.

Cutting Edge: Blockade of the CD28/B7 Costimulatory Pathway Inhibits Intestinal Allograft Rejection Mediated by CD4+ But Not CD8+ T Cells

The effect of blocking the CD28/B7 costimulatory pathway on intestinal allograft rejection was examined in mice. Murine CTLA4Ig failed to prevent the rejection of allografts transplanted into wild-type or CD4 knockout (KO) mice but did inhibit allograft rejection by CD8 KO recipients. This effect was associated with decreased intragraft mRNA for IFN-γ and TNF-α and increased mRNA for IL-4 and IL-5. This altered pattern of cytokine production was not observed in allografts from murine CTLA4Ig-treated CD4 KO mice. These data demonstrate that blockade of the CD28/B7 pathway has different effects on intestinal allograft rejection mediated by CD4+ and CD8+ T cells and suggest that these T cell subsets have different costimulatory requirements in vivo. The results also suggest that the inhibition of CD4+ T cell-mediated allograft rejection by CTLA4Ig may be related to down-regulation of Th1 cytokines and/or up-regulation of Th2 cytokines.

CD40 ligation induced phenotypic and functional expression of CD80 by human cardiac microvascular endothelial cells

BACKGROUND

CD40/CD40L (gp39) interactions are known to play a central role in the function of the immune system (1). CD40 is constitutively expressed on professional antigen presenting cells, such as macrophages and dendritic cells, as well as at low levels on other cell lineages, including human umbilical vein endothelial cells (HUVECs). On antigen-presenting cells, ligation of CD40 causes expression of the costimulatory molecule CD80 (B7-1). Similar ligation of CD40 on HUVECs, however, leads to up-regulation of the adhesion molecules VCAM-1, ICAM-1, and E-selectin, but not CD80.

METHODS

In efforts to provide evidence that microvascular endothelial cells (MECs) are distinct from HUVECs and that the distinguishing features play a role in allograft rejection, MEC cultures were prepared from the explanted hearts of human heart transplant recipients and primary cell lines were established. These MECs were induced to express higher levels of CD40 with interferon-gamma pretreatment, co-cultured with CD40L-transfected HeLa cells, and fluorescence-activated cell sorter-assisted phenotypic studies, in addition to functional allogeneic mixed lymphocyte reaction and accessory-cell dependent mitogen induced proliferation assays were performed.

RESULTS

CD40-CD40L interactions induced the expression of the adhesion molecules VCAM-1 and E-selectin and the costimulatory molecule CD80 but not CD86 (B7-2) on the MECs. The expressed CD80 proved functional in both allo-MLR assays and accessory-cell dependent mitogen proliferation assays.

CONCLUSIONS

MECs are distinct from HUVECs by their potential to express VCAM-1 after interferon-gamma pretreatment and CD80 after CD40 ligation, properties which enable this cell lineage to play a central role in initiating and maintaining allograft rejection in human cardiac transplants.

T-cell activation, proliferation, and memory after cardiac transplantation in vivo

OBJECTIVE

To study the response of alloantigen (H2Kb)-specific T cells to a H2b+ cardiac allograft in vivo.

SUMMARY BACKGROUND DATA

The response of T cells to alloantigen has been well characterized in vitro but has proved more difficult to assess in vivo. The aim of these experiments was to develop a model of T-cell-mediated rejection where the response of T cells after transplantation of a cardiac allograft could be followed in vivo.

METHODS

Purified CD8+ T cells from H2Kb-specific TCR transgenic mice (BM3; H2k) were adoptively transferred into thymectomized, T-cell-depleted CBA/Ca (H2k) mice. These mice were then transplanted with a H2Kb+ cardiac allograft. Using four-color flow cytometry, the proliferative response, modulation of activation markers, and potential cytokine production of the H2Kb-specific T cells was assessed after transplantation.

RESULTS

Consistent rejection of H2Kb+ cardiac allografts required the transfer of at least 6 x 10(6) CD8+ H2Kb-specific T cells. Short-term analyses revealed that the transgenic-TCR+/ CD8+ T cells proliferated and became activated after transplantation of an H2Kb+ cardiac allograft. Fifty days after transplantation, the transgenic-TCR+/CD8+ T cells remained readily detectable, bore a predominantly memory phenotype (CD44hi), and rapidly produced interleukin 2 and interferon-gamma on in vitro restimulation.

CONCLUSIONS

These data show that the activation of alloantigen-specific T cells can be followed in vivo in short-term and long-term experiments, thereby providing a unique opportunity to study the mechanisms by which T cells respond to allografts in vivo.

The role of CD8 and CD4 T cells in intestinal allograft rejection: a comparison of monoclonal antibody-treated and knockout mice

BACKGROUND

The relative contribution of CD8 and CD4 T cells to allograft rejection remains an unresolved issue. Experimental results suggest that the relative importance of these T-cell subsets may vary depending on the model used and the organ studied. We have previously shown that treatment of murine recipients of intestinal allografts with a depleting anti-CD8 or a depleting anti-CD4 monoclonal antibody (mAb) significantly inhibited allograft rejection. This study was undertaken to further examine the contribution of CD8 and CD4 T cells to the rejection of intestinal allografts.

METHODS

Intestinal allografts from B6C3F1/J (C57BL/6 x C3H/HeJ) mice were transplanted into C57BL/6 recipients. Recipient groups included mice with an acquired deficiency in CD8 or CD4 T cells caused by treatment with depleting mAb or mice genetically deficient in CD8 or CD4 T cells as a result of disruption of the genes encoding major histocompatibility complex (MHC) class I, MHC class II, CD8, or CD4. In all cases, rejection was assessed histologically at predetermined time points. In some recipient groups, graft function was also assessed using a maltose absorption assay.

RESULTS

Rejection, assessed between days 10 and 28 after transplantation, was significantly inhibited in mice deficient in CD8 or CD4 T cells after treatment with depleting mAb. In contrast, mice genetically deficient in either CD8 T cells (MHC class I or CD8 knockouts) or CD4 T cells (MHC class II or CD4 knockouts) rejected intestinal allografts promptly. Both histologic and functional evaluation of anti-CD8 mAb-treated mice on day 60 showed that the inhibition of rejection persisted even after the return of a substantial number of CD8 T cells. Although intestinal allografts from anti-CD8 mAb-treated mice displayed little to no evidence of rejection on day 60 after transplantation, these mice were able to reject both donor and third-party skin grafts.

CONCLUSIONS

These results demonstrate that the inhibition of intestinal allograft rejection associated with mAb treatment is not attributable solely to depletion of CD8 or CD4 T cells. Furthermore, anti-CD8 mAb administration did not induce donor-specific tolerance or cause nonspecific immune suppression, as indicated by the skin-grafting experiments. Our findings suggest that at least some depleting mAbs mediate their protective effect on allograft rejection via an alternative mechanism such as the induction of a regulatory cell population(s).

CD4 T cells can reject major histocompatibility complex class I-incompatible skin grafts

We have re-investigated the roles of CD4 and CD8 T cell subsets in skin graft rejection across a single class I MHC disparity. Recipient mice were transplanted with skin from donors transgenic for the class I MHC molecule Kb. As expected, CD8 T cells were sufficient for rapid injection; but surprisingly, CD4 T cells were also competent to do the same. Rejection was dependent on one or the other subset, since elimination of both resulted in indefinite graft survival. The possibility that alloantibody was the downstream effector of CD4 mediated rejection was excluded because CD8-depleted mice rendered B cell deficient still rejected rapidly, but T cell-depleted recipients with pre-existing high titers of alloantibody were unable to do so. In addition, if CD4 cells act to reject by recruiting and/or activating macrophages then this was not dependent on CR3, IFN-gamma or TNF-alpha. Transplantation of skin grafts where the MHC class I disparity was at the level of passenger leukocytes only, demonstrated that transient bystander damage could occur, but that this was insufficient to result in full rejection. We surmise that for CD4 T cells to reject an MHC class I-incompatible graft it is necessary that an appropriate allogeneic peptide is processed and presented in the context of recipient MHC class II. CD4 T cells from B6 mice may fail to reject skin from MHC class I mutants because of the lack of such MHC class II-restricted presentation.

Relative importance of cytotoxic T lymphocytes and nitric oxide-dependent cytotoxicity in contractile dysfunction of rejecting murine cardiac allografts

BACKGROUND

Previous in vitro studies have suggested that both cytotoxic T lymphocyte (CTL)-mediated and non-CTL-mediated myocyte lysis occur during murine cardiac heterotopic allograft rejection, but the relative importance of these injury mechanisms on myocardial function is not established. We therefore compared the in vivo effects of depletion of CTL and inhibition of nitric oxide synthase (NOS) on contractility of the rejecting heart.

METHODS

Syngeneic (BALB/c into BALB/c) and allogeneic (BALB/c into C57/B16) heterotopic abdominal cardiac transplants were performed. In some of the allogeneic transplants, CD8+ lymphocytes were depleted by intraperitoneal injection of anti-CD8 monoclonal antibody. NOS inhibition was accomplished by continuous infusion of NG-monomethyl-L-arginine via a subcutaneous osmotic pump. Five days after transplantation, the abdominal cavity was opened and the transplanted heart exposed. Base to apex developed force was measured during spontaneous beating at a diastolic stretch of 4 g by placing a suture through the apex of the heart and attaching it to a strain gauge. Effects of interventions on graft survival were determined by recording the days required for loss of palpable graft contractions.

RESULTS

Allogeneic hearts showed a significant reduction in systolic force compared to non-rejecting syngeneic hearts. Depletion of CD8+ cells improved contractility significantly relative to non-depleted allogeneic hearts, but contractility remained significantly reduced relative to syngeneic hearts. Developed force in allogeneic hearts was also improved by NOS inhibition (P<0.01), and NG-monomethyl-L-arginine infusion slightly prolonged graft survival.

CONCLUSION

Both CTL-mediated and NOS-dependent (possibly macrophage-mediated) mechanisms contribute to contractile dysfunction during early cardiac allograft rejection in this model. However, NOS inhibition combined with CTL depletion only slightly prolongs graft survival in this model.

Adoptive transfer: the role of perforin in mouse cytotoxic T lymphocyte rejection of human tumor xenografts in vivo

The popliteal lymph node cells of immunocompetent mice generated a strong in vitro cytotoxic response to footpad injection of several human tumor cell lines and the resulting mouse effector cells predominantly used a perforin-mediated cytotoxic mechanism. A relatively minor FasL-dependent cytotoxic response to CEM-CCRF and Jurkat leukemias, but not colon carcinoma COLO 205 cells, was also detected in immunized perforin-deficient mice. In vitro depletion of CD3+ CD8+ T cells, but not CD4+ T or NK1.1+ cells, completely inhibited lysis of human tumor cells, suggesting that CD3+ CD8+ T cells were effectors of perforin-mediated xenospecific cytotoxicity. Xenospecific cytotoxic T cells from wild-type mice were extremely efficient at rejecting tumor when adoptively transferred into scid mice bearing established COLO 205, CEM-CCRF, or Jurkat tumor xenografts. By contrast, cytotoxic T lymphocytes of perforin-deficient mice had no effect on the growth of established tumor xenografts. These data indicate that perforin, and hence direct cytotoxicity, plays a key role in the ability of adoptively transferred CD8+ cytotoxic T lymphocytes to eradicate established xenografts.

Vaccination against persistent viral infection exacerbates CD4+ T-cell-mediated immunopathological disease

Lymphocytic choriomeningitis virus (LCMV) infection of normal mice results in a fatal immunopathologic meningitis mediated by CD8+ cytotoxic T lymphocytes (CTL). We have previously shown that female beta2-microglobulin-deficient (beta2m-/-) mice, which are also deficient in CD8+ T cells, are susceptible to LCMV-induced immune-mediated meningitis, characterized by significant weight loss and mortality. This LCMV disease in beta2m-/- mice is mediated by CD4+ T lymphocytes. Our previous studies have also demonstrated that male beta2m-/- mice are less susceptible than female beta2m-/- mice to LCMV-induced, immune-mediated mortality and weight loss. In this report, we show that vaccination of male beta2m-/- mice enhances immunopathology following intracranial infection with LCMV. We observed increased production of gamma interferon (IFN-gamma), an increase in CD4+ CTL precursor frequency, and an increased frequency of IFN-gamma-producing cells from spleen cells of vaccinated male beta2m-/- mice. Vaccinated male beta2m-/- mice also had significantly increased inflammation in the cerebrospinal fluid (CSF), characterized by a large CD4+ T-cell infiltrate. CSF cells from vaccinated mice showed increased production of IFN-gamma on day 7 postchallenge. Neither vaccinated nor control beta2m-/- mice were able to clear virus, and the two groups had similarly high levels of virus early after infection. These results suggest that the magnitude of the early immune response is more important than the level of virus in the brain in determining the outcome of immunopathology in beta2m-/- mice. We show here that vaccination can increase CD4+ T-cell-dependent immunopathology to a persistent viral infection.

Treatment with either anti-CD4 or anti-CD8 monoclonal antibodies blocks alphabeta T cell-mediated rejection of intestinal allografts in mice

BACKGROUND

Rejection is the major barrier preventing the more widespread application of intestinal transplantation as treatment for intestinal failure. For this study, a one-way host-versus-graft murine model was used to investigate the contribution of T cell subsets to the rejection of allogeneic intestinal allografts.

METHODS

Intestinal grafts consisting of the donor jejunum and ileum were procured from C57BL/6J (syngeneic group) and B6C3F1/J (C57BL/6 x C3H/HeJ, allogeneic group) mice. These grafts were then transplanted into (1) normal, (2) antibody-treated, or (3) genetically mutated C57BL/6 mice. Mice were killed at predetermined intervals and the grafts assessed for rejection by a blinded pathologist.

RESULTS

No syngeneic mice demonstrated any evidence of rejection. In contrast, the recipients of allografts experienced progressive rejection. Recipient mice treated with tacrolimus developed significantly less severe allograft rejection. None of the alphabeta T cell-deficient recipient mice (T cell receptor beta chain knockout mice) experienced allograft rejection with follow-up ranging from 8 to 28 days. However, mice deficient in gammadelta T cells (T cell receptor delta chain knockout mice) rejected intestinal allografts in a manner indistinguishable from normal recipients. In order to investigate the role of CD4+ and CD8+ T cells, recipient mice were treated 2 days before transplantation with depleting monoclonal antibodies specific for either CD4+ cells or CD8+ cells. Depletion of either population of cells significantly inhibited allograft rejection.

CONCLUSIONS

These data demonstrate that rejection of intestinal allografts in the murine model was absolutely dependent on alphabeta but not gammadelta T cells. Furthermore, both CD4+ and CD8+ T cells were necessary for small bowel allograft rejection. Additional studies will be required to determine whether the effects of monoclonal antibody treatment were due solely to depletion of T cells or were mediated at least in part through an active process that altered the functional properties of the targeted T cell subset.

Prolongation of murine vascularized heart allograft survival by recipient-specific anti-major histocompatibility complex class II antibody

BACKGROUND

Antibodies targeting recipient major histocompatibility complex (MHC) class II molecules have been demonstrated to be effective at prolonging allograft survival. However, antigen-presenting cell depletion would explain this effect and has not been definitively excluded as the mechanism of action of such antibodies. We have studied an anti-MHC class II antibody (OX6) proven to be noncytotoxic in the recipient strain used.

METHODS

Antibody was administered the day before, 2 hr before, and the day after grafting.

RESULTS

Antibody administration on the day before, 2 hr before, and the day after grafting significantly prolonged vascularized cardiac allograft survival. Importantly, treatment recognizing recipient MHC was effective, whereas a similar regimen recognizing donor MHC was not.

CONCLUSIONS

Noncytotoxic recipient MHC class II-specific antibodies modify allograft rejection. Possible mechanisms for this therapeutic effect are discussed.

Prolonged murine cardiac allograft acceptance: characteristics of persistent active alloimmunity after treatment with gallium nitrate versus anti-CD4 monoclonal antibody

We have treated DBA/2-->C57BL/6 murine cardiac allograft recipients with anti-CD4 monoclonal antibody or with gallium nitrate to promote long-term (>60 days) allograft survival. Within this period, all grafts developed histologic evidence of ongoing vascular and parenchymal tissue remodeling, including interstitial fibrosis and neointimal hyperplasia, which are characteristic of chronic allograft rejection. To evaluate residual alloimmunity associated with the pharmacologic avoidance of acute graft rejection and the development of chronic tissue remodeling, we subjected these graft recipients to a battery of histologic and immunologic tests. Similar test results were obtained for graft recipients treated with either of the two immunosuppressive agents. All long-surviving allografts displayed histologic evidence of ongoing microvascular endothelial activation and interstitial leukocytic infiltration. Reverse transcriptase-polymerase chain reaction analyses demonstrated intragraft expression of mRNAs for interleukin (IL)-1, IL-2, IL-4, IL-6, tumor necrosis factor, interferon-gamma, and transforming growth factor-beta. All recipients had limiting dilution analysis-detectable, graft-reactive cytolytic T lymphocytes and helper T lymphocytes in their spleens and grafts, and all produced high titers of graft-reactive alloantibodies. In general, these observations indicate that (1) a similar immune status is achieved in long-surviving allografts and their recipients when either anti-CD4 monoclonal antibody or gallium nitrate was used for antirejection therapy, (2) this immune status is characterized by continuous, long-term inflammatory and immune processes that are qualitatively similar to those observed during acute allograft rejection, and (3) no specific immune responses developed selectively in long-term graft recipients to account for the avoidance of acute graft rejection or the development of chronic tissue remodeling in the graft.

Selective increase in CD4-positive graft-infiltrating mononuclear cells among the infiltrates in class I disparate kidney grafts undergoing rejection

Long-term tolerance to kidney allografts across a two-haplotype class I disparity is uniformly induced in miniature swine with a short course of cyclosporine (CsA). In the absence of CsA, all recipients acutely reject kidney allografts within 2 weeks. Previous experiments have shown that graft-infiltrating mononuclear cells (GIC) migrate to the allograft in both CsA-treated and untreated animals. To evaluate the correlation between GIC phenotype and the clinical status, infiltrating cells were examined by flow cytometry, using selective gating to distinguish them from other renal cells. GIC from tolerant and rejector animals were mostly mature T cells, with 84% CD8+ cells, which consisted of 68% CD8+/CD4- and 16% CD8+/ CD4+ cells. This cellular phenotype was, however, markedly different from that of peripheral blood lymphocytes, suggesting a selective migration of cells into the graft. This selective process counterselected the CD3+/CD2- subset of GIC, which was never found in the graft. The distribution of GIC subsets was initially comparable in tolerated and rejected kidneys, but the CD4 single-positive subset then increased specifically in the allograft destined to rejection. The absence of CD4 single-positive cells in tolerated grafts was unlikely to be due to a direct effect of the CsA, because long-term tolerant animals, which received a second kidney without further immunosuppression, also showed no increase in CD4 single-positive cells. The fact that CD4 single-positive cells appeared only within the rejected kidneys, strongly suggests that this cell subset may be important in mediating immune rejection and supports the hypothesis that the development of tolerance in this model depends on a relative deficit of T-cell help.