Analysis of T-cell subsets in rejection of Kb mutant skin allografts differing at class I MHC

Humanization of Immunodeficient Animals for the Modeling of Transplantation, Graft Versus Host Disease, and Regenerative Medicine

The humanization of animals is a powerful tool for the exploration of human disease pathogenesis in biomedical research, as well as for the development of therapeutic interventions with enhanced translational potential. Humanized models enable us to overcome biologic differences that exist between humans and other species, while giving us a platform to study human processes in vivo. To become humanized, an immune-deficient recipient is engrafted with cells, tissues, or organoids. The mouse is the most well studied of these hosts, with a variety of immunodeficient strains available for various specific uses. More recently, efforts have turned to the humanization of other animal species such as the rat, which offers some technical and immunologic advantages over mice. These advances, together with ongoing developments in the incorporation of human transgenes and additional mutations in humanized mouse models, have expanded our opportunities to replicate aspects of human allotransplantation and to assist in the development of immunotherapies. In this review, the immune and tissue humanization of various species is presented with an emphasis on their potential for use as models for allotransplantation, graft versus host disease, and regenerative medicine.

In Vivo Depletion of T Lymphocytes

In vivo depletion of T lymphocytes is a means of studying the role of specific T cell populations during defined phases of in vivo immune responses. In this unit, a protocol is provided for injecting monoclonal antibodies (mAbs) into wild-type adult mice. Depletion of the appropriate subset of cells is verified by flow cytometry analysis of lymph node and spleen cell suspensions in pilot experiments. Once conditions have been established, depleted mice can be used to study the impact of T cell subsets on a variety of in vivo immune responses. The depleted condition may be maintained by repeated injections of the monoclonal antibody, or reversed by normal thymopoiesis following discontinuation of antibody administration.

Skin allograft rejection

Skin allograft rejection is a test of the competence of T lymphocytes to mediate in vivo tissue destruction, which in turn reflects their role in critical functions such as anti-viral and tumor immunity. The tail-skin graft procedure described in this unit is useful predominantly because of the ease of preparation and resistance to ischemic (nonspecific) necrosis. Additionally, it is not necessary to sacrifice the donor mouse. However, rejection of tail-skin grafts should not be used to test for genetic homogeneity in breeding experiments or to detect minor histocompatibility (minor-H) antigens because tail skin is less sensitive than trunk skin in detecting such differences.

In vivo depletion of CD4- and CD8-specific T cells

In vivo depletion of CD4- and CD8-specific T cells is a means of studying the role of these subpopulations in the initiation and effector phases of particular in vivo immune responses. In this unit, a protocol is provided for harvesting anti-CD4 or anti-CD8 monoclonal antibody- producing ascites fluid or tissue culture supernatant from rat or mouse T cell hybridomas. The antibody (preferably IgG) is then purified and injected intraperitoneally into adult mice. Depletion of the appropriate subset of T cells is verified by flow cytometry analysis of lymph node and spleen cell suspensions in pilot experiments. Once conditions have been established, depleted mice can be used to study the impact of T cell subsets on in vivo immune responses. The depleted condition is maintained by repeated injections of the monoclonal antibody.

Differentiation of subsets of CD4+ and CD8+ T cells

Our knowledge of the cytokine secretion patterns of T cells and other cells is clearly becoming more complex. The T helper 1 (Th1) and Th2 patterns may represent the extremes of a spectrum of cytokine regulatory patterns controlled by several cell types. CD8+ T cells can also secrete either Th1-like or Th2-like cytokine patterns, and they can contribute to bystander B cell activation. Interactions occur between immune cytokine regulatory networks and other systems, and pregnancy and responses against infection can profoundly influence each other.

IL-4 deficiency prevents eosinophilic rejection and uncovers a role for neutrophils in the rejection of MHC class II disparate skin grafts

BACKGROUND

Acute rejection of MHC class II-disparate bm12 skin grafts by C57BL/6 recipient mice is characterized by massive graft infiltration by eosinophils, together with increased intragraft amounts of IL-4 and IL-5 mRNA. IL-5 blockade prevents the intragraft eosinophil infiltration and prolongs the survival of skin allografts. As the differentiation of T cell precursors into Th2 cells is largely driven by IL-4, we investigated the role of IL-4 in MHC class II-disparate allograft rejection.

METHODS

We performed skin grafts from MHC class II incompatible bm12 mice into wild-type C57BL/6 mice (IL-4) or C57BL/6 IL-4 deficient mice (IL-4). Graft survival, in vitro T cell reactivity, and histology were compared.

RESULTS

We observed that 50% of IL-4 mice rapidly rejected their bm12 allograft, whereas the other 50% retained their graft 60 days after transplantation. Histological examination of bm12 allografts retained by IL-4 mice showed a normal appearance with no inflammatory infiltrate and no eosinophils. Among IL-4 mice that acutely rejected their bm12 skin graft, we observed a dense polymorphonuclear infiltrate. The depletion of neutrophils significantly prolonged bm12 graft survival.

CONCLUSIONS

Eosinophil infiltrates, typical of MHC class II disparate acute skin graft rejection, are critically dependent on the availability of IL-4. IL-4 mice reject MHC class II disparate skin grafts by a pathway of rejection where neutrophils play a direct causal role.

Specific tolerance induction of allo-K(b)-skin grafts by FK506 in the CD8-depleted H-2(k) recipients required low amounts of K(b)-antigen

MHC class I allo-grafts can be directly rejected by recipient CD8 T cells and be indirectly rejected by recipient CD4 T cells. Although the experimental results using the bm mutant and C57BL/6 mice indicated that CD4-mediated rejection of class I-disparate grafts is a relatively weak process and is expected to be more sensitive to additional exogenous immunosuppression, it is unclear that whether this mechanism can be used for inducing a specific tolerance of class I disparate grafts. In this study, we hypothesize that a short course of FK506 may induce a specific tolerance of class I-disparate skin grafts in the CD8-depleted recipients. K(b)-transgenic C3H mice, Tg.H-2 K(b)-1 and Tg.H-2 K(b)-2 mice that express high copies and low copies of K(b)-antigen respectively were used as donors. Wild type C3H mice (H-2(k)) in which either CD4 or CD8 T cells were depleted by administration of anti-CD4 or CD8 monoclonal antibody (mAb) were used as recipients. Results showed that FK506 promoted longer survival of allo-K(b) skin grafts in CD8-depleted C3H mice than in CD4-depleted C3H mice. Graft survival from Tg.H-2 K(b)-2 mice was significantly longer than Tg.H-2 K(b)-1 mice. A short course of FK506 induced long-term survival of skin grafts from Tg.H-2K(b)-2 mice, but not from Tg.H-2K(b)-1 mice in CD8-depleted C3H recipients, even after FK506 was stopped. These mice also accepted grafts of Tg.H-2K(b)-1 mice when challenged with skin grafts from Tg.H-2K(b)-1 mice, but promptly rejected third party skin grafts from BALB/c (H-2(d)) mice. T cells from K(b)-tolerant C3H mice did not respond to allo-K(b)-antigen in in vitro assays of mixed lymphocyte culture and cell-mediated cytotoxicity. In conclusion we found that a short course of FK506 treatment and low amounts of K(b)-antigen induced a K(b)-specific tolerance in the CD8-depleted recipients, and this tolerance maintained even after withdrawing the anti-CD8 mAb treatment.

High numbers of IL-2-producing CD8+ T cells during viral infection: correlation with stable memory development

Using infections with lymphocytic choriomeningitis virus (LCMV) and vesicular stomatitis virus in mice as model systems, we have investigated the ability of antigen-primed CD8+ T cells generated in the context of viral infections to produce IL-2. Our results indicate that acute immunizing infection normally leads to generation of high numbers of IL-2-producing antigen-specific CD8+ T cells. By costaining for IL-2 and IFN-gamma intracellularly, we found that IL-2-producing cells predominantly constitute a subset of cells also producing IFN-gamma. Comparison of the kinetics of generation revealed that IL-2-producing cells appear slightly delayed compared with the majority of IFN-gamma producing cells, and the relative frequency of the IL-2-producing subset increases with transition into the memory phase. In contrast to acute immunizing infection, few IL-2-producing cells are generated during chronic LCMV infection. Furthermore, in MHC class II-deficient mice, which only transiently control LCMV infection, IL-2-producing CD8+ T cells are initially generated, but by 4 weeks after infection this subset has nearly disappeared. Eventually the capacity to produce IFN-gamma also becomes impaired, while cell numbers are maintained at a level similar to those in wild-type mice controlling the infection. Taken together, these findings indicate that phenotyping of T cell populations based on capacity to produce cytokines, and especially IL-2, can provide important information as to the functional status of the analysed cell subset. Specifically, combined analysis of the capacity to produce IL-2 and IFN-gamma can be used as a predictor for loss of function within the CD8+ T cell compartment.

CD8(+), alphabeta-TCR(+), and gammadelta-TCR(+) cells in the recipient hematopoietic environment mediate resistance to engraftment of allogeneic donor bone marrow

Historically, conditioning for engraftment of hematopoietic stem cells has been nonspecific. In the present study, we characterized which cells in the recipient hematopoietic microenvironment prevent allogeneic marrow engraftment. Mice defective in production of alphabeta-TCR(+), gammadelta-TCR(+), alphabeta- plus gammadelta-TCR(+), CD8(+), or CD4(+) cells were transplanted with MHC-disparate allogeneic bone marrow. Conditioning with 500 cGy total body irradiation (TBI) plus a single dose of cyclophosphamide (CyP) on day +2 establishes chimerism in normal recipients. When mice were conditioned with 300 cGy TBI plus a single dose of CyP on day +2, all engrafted, except wild-type controls and those defective in production of CD4(+) T cells. Mice lacking both alphabeta- and gammadelta-TCR(+) cells engrafted without conditioning, suggesting that both alphabeta- and gammadelta-TCR T cells in the host play critical and nonredundant roles in preventing engraftment of allogeneic bone marrow. CD8 knockout (KO) mice engrafted without TBI, but only if they received CyP on day +2 relative to the marrow infusion, showing that a CD8(-) cell was targeted by the CyP conditioning. The CD8(+) cell effector function is mechanistically different from that for conventional T cells, and independent of CD4(+) T helper cells because CD4 KO mice require substantially higher levels of conditioning than the other KO phenotypes. These results suggest that a number of cell populations with different mechanisms of action mediate resistance to engraftment of allogeneic marrow. Targeting of specific recipient cellular populations may permit conditioning approaches to allow mixed chimerism with minimal morbidity and could potentially avoid the requirement for myelotoxic agents altogether.

Effect of localized cytokine dysregulation: accelerated rejection of IL-2-expressing skin grafts

Transgenic mice were created in which a sheep keratin promoter directed the expression of IL-2 into the dermis. These KIL-2 transgenic mice were used to investigate the effects of localized IL-2 dysregulation on immune responses. Peripheral tolerance to skin antigens was not broken by in situ IL-2 expression because syngeneic KIL-2 skin grafts were not rejected. However, MHC Class I-disparate skin grafts from KIL-2 donors were rejected faster (median survival time (MST) 12 days) than grafts of non-transgenic littermate skin (MST 18 days). In contrast, the kinetics of KIL-2 H-Y-disparate skin graft rejection (MST 14 days) did not differ significantly from controls (MST 16 days), suggesting that upregulation of IL-2 at the effector site could affect CD4+ T cell- independent, but not CD4+ T cell-dependent, responses. No effect on rejection kinetics was observed when wild type allogeneic skin was grafted onto transgenic mice that expressed bcl2 constitutively in their lymphocytes (MST of 14 days, both sets), indicating that this was not simply due to increased longevity of T cells within the IL-2 expressing graft. We therefore suggest that aberrant expression of IL-2 can accelerate helper-independent CD8+ T cell responses by increasing proliferation and/or differentiation of cytolytic T cells at the effector site.

Replacement of graft-resident donor-type antigen presenting cells alters the tempo and pathogenesis of murine cardiac allograft rejection

BACKGROUND

Graft-resident antigen presenting cells (APCs) are potent stimulators of the alloresponse. To test whether replacement of graft-resident donor-type APCs with those of recipient-type alters allorecognition and the pathogenesis of both acute and chronic rejection, we created chimeric hearts for transplantation into naive recipients.

METHODS

To replace donor-type APCs with those of recipient-type, chimeric animals were created by bone marrow transplantation (BMT) in fully allogeneic mouse and rat strain combinations. The degree of APC replacement in chimeric organs was assessed phenotypically and functionally. Chimeric hearts were transplanted heterotopically into untreated recipients.

RESULTS

Flow cytometric and immunohistochemical analysis did not detect residual bone marrow recipient-type APCs in mouse BMT chimeras. Although semi-quantitative reverse transcription polymerase chain reaction detected 0.001-0.01% residual cells, APCs isolated from chimeric organs were functionally unable to stimulate donor-type cells. When transplanted into naive recipients, chimeric mouse hearts had significantly prolonged survival but were nevertheless rejected acutely. Similar results were obtained in the ACI --> LEW rat strain combination. However, in the PVG --> DA rat model, the majority of chimeric hearts survived >100 days and all long-surviving hearts developed cardiac allograft vasculopathy.

CONCLUSIONS

BMT leads to near complete replacement of organ-resident APCs. The virtual absence of donor-type APCs in chimeric hearts delays or prevents acute rejection in a strain-dependent manner. In contrast, this type of graft modification does not prevent cardiac allograft vasculopathy. This suggests that, although the CD4+ direct pathway may play a role in acute rejection, it is not essential for the development of chronic rejection in rodent cardiac allografts.

Anti-CD4 induced rat heart tolerance: no presence of primed T cells and regulatory mechanisms for cytotoxic T cells

Treatment with anti-CD4 monoclonal antibody (mAb) (OX38) induces heart, but not skin graft tolerance in WF (RT1u) to Lewis (RT1l) rat strain combinations. We examined differences in cellular responses between heart-bearing and skin-rejected hosts that were both treated with anti-CD4 mAb. In the tolerant LEW rats bearing WF heart transplants, the secondary WF heart but not skin grafts were accepted. On the other hand, in anti-CD4 treated WF skin-rejected hosts, both secondary WF heart and skin grafts were rapidly rejected. Spleen cells from anti-CD4 treated WF skin-rejected LEW rats but not from WF heart-bearing LEW rats received the same treatment generated CTL after in vitro stimulation with paraformaldehyde (PFA) treated donor WF stimulator spleen cells. Adoptive transfer of spleen cells from WF skin-rejected LEW rats with or without anti-CD4 therapy into the tolerant LEW rats at the secondary WF heart transplantation blocked the secondary heart graft acceptance. However, transfer of spleen cells from WF heart-rejected rats without immunosuppression failed to block acceptance of the secondary heart graft. Our results indicated the lack of primed T cells and presence of regulatory mechanisms for tissue specific T cells in anti-CD4 treated heart bearing hosts.

The role of peptides in T cell alloreactivity is determined by self-major histocompatibility complex molecules

By analyzing T cell responses against foreign major histocompatibility complex (MHC) molecules loaded with peptide libraries and defined self- and viral peptides, we demonstrate a profound influence of self-MHC molecules on the repertoire of alloreactive T cells: the closer the foreign MHC molecule is related to the T cell's MHC, the higher is the proportion of peptide-specific, alloreactive ("allorestricted") T cells versus T cells recognizing the foreign MHC molecule without regard to the peptide in the groove. Thus, the peptide repertoire of alloreactive T cells must be influenced by self-MHC molecules during positive or negative thymic selection or peripheral survival, much like the repertoire of the self-restricted T cells. In consequence, allorestricted, peptide-specific T cells (that are of interest for clinical applications) are easier to obtain if T cells and target cells express related MHC molecules.

Transplantation tolerance-where do we stand?

Our understanding of tolerance mechanisms has progressed to the point that tolerance-induction protocols are being tested in humans for organ transplantation. However, a range of scientific, ethical, logistic and commercial issues have arisen, and must be resolved before tolerance induction for human allograft patients can become a reality.

Gene targeting: applications in transplantation research

Gene targeting, the manipulation of gene in the mouse genome using homologous recombination in embryonic stem cells, is a powerful experimental tool that has been widely utilized in a number of disciplines. The ability to precisely alter genes in this way provides an avenue for investigating the role of a gene product in normal and pathological processes in the intact animal, with a precision and efficacy not possible using pharmacological agents, antibodies or engineered proteins. In transplant research, gene targeting provides a unique tool for discriminating the contributions of gene expression in donor versus recipient tissues. This review focuses on several areas in transplantation research where gene targeting has made useful contributions. These include studies of the role of donor and recipient multiple histocompatibility complex antigens in regulating rejection responses, the role of CD4+ T cell in mediating acute rejection, and the functions of cytokines during rejection and tolerance induction. These studies highlight the unique advantages of gene targeting in studies of complex processes in whole animals and illustrate the contributions of this technique to understanding the pathogenesis of allograft rejection.

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.

Extreme Leukoreduction of Major Histocompatibility Complex Class II Positive B Cells Enhances Allogeneic Platelet Immunity

In a murine model of platelet alloimmunization, we examined the definitive role that mononuclear cells (MC) have in modulating platelet immunity by using platelets from severe combined immunodeficient (SCID) mice. CB.17 (H-2d) SCID or BALB/c (H-2d) mouse platelets were transfused weekly into fully allogeneic CBA (H-2k) mice and antidonor antibodies measured by flow cytometry. MC levels in BALB/c platelets were 1.1 ± 0.6/μL and SCID mouse platelets could be prepared to have significantly lower (<0.05/μL) MC numbers. Transfusions with 108 BALB/c platelets (containing ≈100 MC/transfusion) stimulated IgG antidonor antibodies in 100% of the recipients by the fifth transfusion, whereas 108 SCID mouse platelets (containing ≈5 MC/transfusion) stimulated higher-titered IgG alloantibodies by the second transfusion. When titrations of BALB/c peripheral blood MC were added to the SCID mouse platelets, levels approaching 1 MC/μL reduced SCID platelet immunity to levels similar to BALB/c platelets. Characterization of the alloantibodies showed that the low levels of MC significantly influenced the isotype of the antidonor IgG; the presence of 1 MC/μL was associated with induction of noncomplement fixing IgG1 antidonor antibodies, whereas platelet transfusions, devoid of MC (<0.05/μL), were responsible for complement-fixing IgG2a production. When magnetically sorted defined subpopulations of MC were added to the SCID platelets, major histocompatability complex (MHC) class II positive populations, particularly B cells, were found to be primarily responsible for the reduced SCID mouse platelet immunity. The presence of low numbers of MC within the platelets was also associated with an age-dependent reduction in platelet immunogenicity; this relationship however, was not observed with SCID mouse platelets devoid of MC. The results suggest that a residual number of MHC class II positive B cells within allogeneic platelets are required for maximally reducing alloimmunization.

Extreme Leukoreduction of Major Histocompatibility Complex Class II Positive B Cells Enhances Allogeneic Platelet Immunity

In a murine model of platelet alloimmunization, we examined the definitive role that mononuclear cells (MC) have in modulating platelet immunity by using platelets from severe combined immunodeficient (SCID) mice. CB.17 (H-2d) SCID or BALB/c (H-2d) mouse platelets were transfused weekly into fully allogeneic CBA (H-2k) mice and antidonor antibodies measured by flow cytometry. MC levels in BALB/c platelets were 1.1 ± 0.6/μL and SCID mouse platelets could be prepared to have significantly lower (&lt;0.05/μL) MC numbers. Transfusions with 108 BALB/c platelets (containing ≈100 MC/transfusion) stimulated IgG antidonor antibodies in 100% of the recipients by the fifth transfusion, whereas 108 SCID mouse platelets (containing ≈5 MC/transfusion) stimulated higher-titered IgG alloantibodies by the second transfusion. When titrations of BALB/c peripheral blood MC were added to the SCID mouse platelets, levels approaching 1 MC/μL reduced SCID platelet immunity to levels similar to BALB/c platelets. Characterization of the alloantibodies showed that the low levels of MC significantly influenced the isotype of the antidonor IgG; the presence of 1 MC/μL was associated with induction of noncomplement fixing IgG1 antidonor antibodies, whereas platelet transfusions, devoid of MC (&lt;0.05/μL), were responsible for complement-fixing IgG2a production. When magnetically sorted defined subpopulations of MC were added to the SCID platelets, major histocompatability complex (MHC) class II positive populations, particularly B cells, were found to be primarily responsible for the reduced SCID mouse platelet immunity. The presence of low numbers of MC within the platelets was also associated with an age-dependent reduction in platelet immunogenicity; this relationship however, was not observed with SCID mouse platelets devoid of MC. The results suggest that a residual number of MHC class II positive B cells within allogeneic platelets are required for maximally reducing alloimmunization.

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.

The Role of T Cells in Allografted Tumor Rejection: IFN-γ Released from T Cells Is Essential for Induction of Effector Macrophages in the Rejection Site

Allografted Meth A tumor rejection is T cell dependent, but T cells are inactive toward the allograft; rather, the main effector cells are allograft-induced macrophages (AIM) with MHC haplotype specificity. Here, we examined the role of T cells in the induction of AIM in the rejection site. On day 4.5 after i.p. transplantation of Meth A fibrosarcoma cells to C57BL/6 (B6) mice, we obtained a kind of precursor of AIM (pro-AIM) from the transplantation site by an enrichment technique involving adherence to serum-coated dishes. The noncytotoxic pro-AIM-rich population put into a diffusion chamber became cytotoxic against Meth A cells after 2 days in the peritoneal cavity of an untreated B6 mouse. Similar activation of the chambered B6 pro-AIM-rich population occurred in IFN-γ −/− B6 mice, whereas there was no activation when chambers containing an IFN-γ −/− mouse-derived pro-AIM-rich population were placed in normal or IFN-γ −/− mice, suggesting that IFN-γ is involved in the activation. RT-PCR experiments demonstrated that among bulk infiltrates T cells were the major producer of IFN-γ; and most of the cells in a T cell-eliminated pro-AIM population in a diffusion chamber kept for 2 days in a B6 mouse did not become AIM. Furthermore, IFN-γ −/− B6 mice could not reject allografted Meth A tumor cells, whereas the grafts were rejected by i.p. injections of IFN-γ into the mutant mice. These results indicate that IFN-γ released from allograft-induced T cells is essential for both the activation of a kind of pro-AIM to AIM in the transplantation site and the rejection of an allografted tumor.

Downregulation of T cell receptor expression by CD8(+) lymphocytes in kidney allografts

Allospecific CD8(+) T lymphocytes are an important component of the cellular response in allograft rejection. These cells recognize and engage MHC class I antigens, leading to allospecific cytolytic responses and graft rejection. In mouse kidney allografts that survive to 3 wk after transplantation, we noted that the majority of CD8(+) cells do not express surface alpha/beta T cell receptor alpha/beta(TCR), gamma/deltaTCR, or CD3. However, these CD8(+)TCR- cells did express surface markers characteristic of T cells, including Thy1.2, CD2, and CD5. In addition, the CD8(+)TCR- cells expressed mRNA for TCR Vbeta gene families, and nearly half stained positive for cytoplasmic Vbeta8 protein, suggesting that they are T cells that have downregulated alpha/betaTCR protein expression from their cell surfaces. When these surface TCR- cells were isolated from kidney allografts by flow cytometry and cultured in the presence of either allogeneic or syngeneic stimulators, nearly 100% of cells reacquired normal levels of alpha/betaTCR expression with disproportionate usage of Vbeta8 chains. After recovery of their surface TCR expression, the CD8(+)TCR- population demonstrated strong alloreactivity in culture. These results suggest that the substantial number of CD8(+)TCR- cells found in long-term surviving mouse kidney allografts are alpha/beta-T cells that have downregulated their cell surface expression of TCR. While in other systems this phenotype may identify cells that have engaged antigen, our results indicate that loss of TCR expression by CD8(+) kidney graft-infiltrating cells may not depend on antigen engagement and that elements in the microenvironment of the kidney graft play a key role in this process. Factors that modulate expression of TCR by graft-infiltrating lymphocytes may have an important role in regulating rejection responses.

CD4+ T cells are able to reject class I disparate allografts

The ability of CD4+ T cells to reject class I mismatched skin allografts remains controversial. In this study, we compare the ability of CD4+ T cells to reject class I disparate skin grafts differing by either a single class I allelic disparity or only 3 amino acids encoded by the H-2K locus. We demonstrate that skin grafts across a full H-2K allelic disparity, but not across a disparity of only three amino acids are efficiently rejected by CD4+ T cells. This observation is consistent with the possibility that peptides derived from allogeneic class I molecules generated through the major histocompatibility complex (MHC) class II antigen processing pathway can be recognized by host CD4 T cells and lead to rejection of class I mismatched skin grafts. The availability of peptides derived from allogeneic MHC class I molecules for presentation by host MHC class II may determine the efficiency of rejection of class I mismatched allografts by CD4+ T cells. Thus, class I mismatched allografts can be rejected by CD4+ T cells provided that host and donor MHC class I molecules are sufficiently disparate to activate CD4+ effectors.

Interferon-gamma rapidly increases peptide transporter (TAP) subunit expression and peptide transport capacity in endothelial cells

Human cytotoxic T lymphocytes (CTL) recognize specific complexes of HLA class I molecules and peptides, which assemble when nascent class I molecules bind peptides transported from the cytoplasm into the endoplasmic reticulum by the heterodimeric transporter associated with antigen processing (TAP). Increased class I molecule expression on the cell surface increases the efficiency of CTL lysis. The kinetics of interferon (IFN)-gamma induction of TAP, peptide transport capacity, and HLA class I molecule expression was determined in endothelial cells, which are targets of CTL following transplantation or viral infection. TAP mRNAs are induced rapidly, increasing 20-fold (TAP1) or 10-fold (TAP2) by 12 h, whereas HLA class I mRNA is induced more slowly, increasing 10-fold in 24 h. TAP1 and TAP2 proteins are also induced rapidly, increasing 10-fold in 24 h, whereas HLA class I heavy chain proteins and surface expression increase more slowly. Peptide transport capacity in endothelial and HeLa cells increases within 6 h of IFN-gamma treatment, suggesting that the IFN-gamma-induced TAP heterodimers are functional. Therefore, the IFN-gamma-induced increase in TAP proteins is accompanied by an increased peptide transport capacity, which may be important in supporting the subsequent rise in HLA class I protein expression.

The mechanism of specific prolongation of class I-mismatched skin grafts induced by retroviral gene therapy

In the present study, we examine the mechanism of specific hyporesponsiveness to major histocompatibility complex (MHC) class I-mismatched skin allografts induced by retrovirus-mediated gene transfer of an allogeneic class I gene into syngeneic bone marrow (BM). Using appropriate congenic recombinant mouse strains, we have mapped MHC determinants that are capable of restoring rapid rejection of Kb-bearing skin grafts. Our results indicate that either a single class I or a single class II alloantigen expressed on skin in association with Kb is able to restore the rapid rejection of Kb-mismatched skin grafts. These data suggest that third-party alloantigens expressed on skin in association with Kb abrogate hyporesponsiveness by providing T cell help. Consistent with this interpretation, spleen cells from mice reconstituted with Kb-transduced BM were unable to elicit a significant anti-Kb cytotoxic T lymphocyte response in vitro unless interleukin-2 was added to the culture medium. Skin graft survival was also analyzed on B10. AKM mice thymectomized 3-4 weeks post-reconstitution with Kb-transduced BM. Thymectomy did not result in significantly prolonged survival of B10. MBR skin grafts compared to euthymic controls, suggesting that even early after reconstitution, intrathymic deletion of Kb-reactive T cells must have been incomplete. Taken together, these data suggest that prolongation of skin allograft survival in this model is controlled at the level of T cell help.

An immunocytochemical study of MHC class I expression on human Langerhans cells and melanocytes

Classical MHC class I glycoproteins (HLA-A, B, and C) present endogenous cytosolic peptide antigen fragments to CD8-positive T-cells. CD8-positive T-cell recognition and destruction of virus-infected cells are dependent on adequate cellular MHC class I expression. Constitutive MHC class I expression is ubiquitous, but known to be deficient on specific differentiated cell types which include hepatocytes, neurones, chondrocytes and myocytes. Although enabling assessment of MHC class I expression on individual cells, limitations of immunocytochemistry were encountered with this assessment on Langerhans cells and melanocytes. These dispersed intraepidermal cells were obscured by adjacent keratinocytes in sections immunostained for MHC class I glycoproteins. Initiatives designed to resolve the issue have included immunoelectron microscopy, cell culture techniques, and animal bone marrow chimera models. Despite the elegance of these techniques, the issue of MHC class I expression on Langerhans cells and melanocytes remains unresolved. In this immunocytochemical study, an alternative strategy was based upon the recognized deficiency of epithelial MHC class I expression within pilosebaceous adnexal units. Langerhans cells and melanocytes were therefore studied within this microenvironment of deficient MHC class I expression, using monomorphic and polymorphic MHC markers. Langerhans cells and melanocytes were demonstrated within pilosebaceous units of scalp skin by immunocytochemistry. Differentiation markers OKT6 (CD1a) and TMH1 defined Langerhans cells and melanocytes, respectively. Monomorphic MHC markers W6/32 and TAL IB5 defined invariant epitopes of HLA class I and II, respectively. Polymorphic MHC class I markers defined the HLA-Bw4 and HLA-Bw6 supertypic determinants. Constitutive MHC class I expression was shown to be deficient on Langerhans cells and melanocytes.

Identification of an HLA-DQ6-derived peptide recognized by mouse MHC class I H-2Db-restricted CD8+ T cells in HLA-DQ6 transgenic mice

CD8+ T cells from C57BL/6(B6) mice show cytotoxicity to B cell blasts prepared from syngeneic transgenic mice expressing HLA-DQ6 molecules in a mouse MHC class I H-2Db restricted manner. Although these results suggest that CD8+ T cells recognize peptides derived from DQ6 molecule bound to H-2Db on target cells, no direct evidence so far has been obtained. To clarify this, we synthesized 23 peptides corresponding to DQ6 alpha or beta chain and carrying the motifs of Db-binding peptides, and examined their capacity to induce cytotoxicity in the CD8+ T cell line. We show here that DQA1-2, one of these peptides, induced cytotoxicity of the CD8+ T cells when this peptide was pulsed to H-2Db expressing target cells, as efficiently as HLA-DQ6 expressing target cells did. Thus, our results suggest that DQA1-2 can be naturally processed from DQ6 molecules and recognized by the CD8+ T cells in the context of H-2Db molecules. These results suggest that allogeneic HLA class II molecules are involved in the rejection not only as the ligand for T cell receptor of alloreactive CD4+ T cells but also as self-peptides bound to HLA class I molecules recognized by CD8+ T cells.

The role of the perforin and Fas pathways of cytotoxicity in skin graft rejection

Cell-mediated cytotoxicity is believed to play an important role in the mechanisms of rejection following organ and tissue transplantation. We have investigated the role of the perforin and Fas pathways of cell-mediated cytotoxicity on the survival of xenogeneic, allogeneic and hemiallogeneic skin grafts. Wild type C57BL/6 (B6), perforin deficient C57BL/6 (B6 PKO) and Fas ligand deficient (B6 gld/gld) mice were grafted with xenogeneic Lewis rat skin grafts, or allogeneic B10.BR, class I disparate B6 bml, class II disparate B6 bm12, and Fas deficient allogeneic C3H lpr/lpr grafts. No significant difference was observed in the timing of rejection with B6 PKO mice as recipients when compared with normal B6 controls. All grafts were also rejected by B6 gld/gld recipients, but with a significantly delayed pattern of rejection, showing, therefore, that Fas, although not critical, may participate to the mechanisms of rejection. Neither perforin nor Fas, however, seem to be essential for rejection of a skin graft, and it is likely that alternative mechanisms assume an important role in the effector phase of rejection once these cell-cytotoxic pathways are abrogated.

The role of interleukin-4 in the induction phase of allogeneic neonatal tolerance

We previously reported that prolonged graft survival in neonatally tolerant mice was associated with enhanced Th2/Th1 cytokines. To determine whether Th2 CD4 cells function in tolerance, we examined whether we could prevent tolerance by blocking Th2 CD4 maturation, using anti-interleukin (IL)-4 monoclonal antibody treatment during neonatal antigen exposure. Anti-IL-4 treatment restored the ability BALB/c of mice to reject A/J skin grafts and blocked the induction of tolerance through multiple mechanisms. Anti-IL-4 treatment blocked the development of donor microchimerism and recovered the ability of mice to proliferate and to generate appropriate delayed-type hypersensitivity (DTH) and cytotoxic T lymphocyte (CTL) responses against A/J in a dose-dependent manner. Low-dose anti-IL-4 recovered DTH responses and interferon (IFN)-gamma production, but failed to completely prevent IL-4 production or to recover the CTL activity. No A/J-reactive IFN-gamma-producing CD8 cells were detected in these mice. In contrast, mice treated with higher doses of anti-IL-4 generated normal CTL responses against A/J, and contained A/J-reactive IFN-gamma-producing CD8 cells. The recovery of CTL responses and IFN-gamma-producing CD8 cells was associated with a more complete blocking of Th2 cytokine production. Therefore, the presence of IL-4 may play an important role in the induction of neonatal tolerance by shifting maturation of CD4 cells toward Th2 cells and away from Th1 cells, and also by preventing maturation of alloreactive CD8 CTL cells.

Peptide specificity of alloreactive CD4 positive T lymphocytes directed against a major histocompatibility complex class I disparity

The mouse strains C57BL/6 (B6, H2b) and Kbm1 mutant bm1 have a defined difference of three amino acids at position 152, 155, and 156 in the MHC class I K molecule. This causes a change in the side and the bottom of the antigen presenting groove of the K molecule resulting in strong allogeneic responses in vitro and in vivo. Here we report on the peptide specificity of CD4+ T cells of B6 origin directed against the Kbm1 mutant and speculate on the peptide specificity of CD8+ bm1-specific T lymphocytes of B6 origin. Bm1-specific CD4+ T helper cells recognized a peptide derived from the Kbm1 molecule encompassing the three mutations, presented by MHC class II molecules on syngeneic cells. The ability of this peptide to bind to MHC class II resulted from amino acid mutations at positions 155 and 156. Furthermore, the recognition of the natural peptide derived from the Kbm1 molecule presented by MHC class II I-Ab molecules on cells of bml origin could be blocked by addition of an MHC class II I-Ab binding competitor peptide. Thus, due to the mutations in an MHC class I molecule, indirect presentation via MHC class II molecules and MHC class II-restricted recognition of a peptide derived from such a MHC class I molecule is demonstrable.

Xenogeneic iso-skin graft and mixed lymphocyte reaction studies using HLA-DP transgenic mice

To elucidate the cellular responses against xeno-MHC antigens, in vitro mixed lymphocyte culture (MLC) and in vivo skin grafting (SG) studies were conducted using HLA-DP transgenic mice (B6-DP mice). Xenogenic iso-(B6-DP to B6 mice) MLC showed positive but much lower responses compared to allo-MLC responses. Nevertheless, B6-DP skin grafts were rejected in a similar time course as allo-skin grafts. To examine mechanisms underlining skin graft rejection, both in vitro cytotoxic lymphocyte (CTL) responses and delayed-type hypersensitivity (DTH) reactions were tested. The studies showed that DTH but not CTL reactions were involved for the graft rejection. SG was again conducted after the administration of anti-CD4 and/or CD8 monoclonal antibody (mAb). Mice treated with both CD4 and CD8 mAb accepted B6-DP SG for as long as up to 60 days and those treated with either CD4 or CD8 mAb alone rejected skin grafts on its own most of the time (75% in anti-CD4 mAb treated mice, 88.9% in anti-CD8 mAb treated mice), which suggests that the strict T cell restrictions for xeno-DP antigens do not exist. Even in these finally rejected cases, longer median survival time and final rejection time were observed, and in the other mice (25% in anti-CD4 mAb treated, and 11.1% in anti-CD8 mAb treated mice), graft acceptance was found. Therefore, it was suggested that the immunological reactions leading to the graft rejection occurs most efficiently when both T cell subsets are present. The above results indicate that xenogeneic HLA-DP antigens could act as significant transplantation antigens equivalent to alloantigens despite their lower stimulative activity in vitro, and also support the interpretation that DP antigens act like a minor histocompatibility antigen beyond the difference of species. Monomorphic anti-HLA class II antibodies were detected in recipients' sera as early as 2 weeks and even at 6 months, indicating that xeno-MHC antigens are prone to be memorized to B cells. It was concluded that HLA transgenic mice are useful for the investigation of cellular responses across xeno-MHC barriers.

Allogeneic class I MHC requirement for alloantigen-reactive helper T-lymphocyte responses in vivo. Evidence for indirect presentation of alloantigen

The mechanisms by which host T cells recognize transplant-associated alloantigens in vivo have not been established. Two alloantigen presentation pathways may be used: (1) allogeneic class I and class II MHC molecules may be recognized directly by host CD8+ and CD4+ cells, respectively, or (2) allogeneic MHC molecules may be processed as foreign peptide and presented by host antigen-presenting cells to CD4+ cells in the context of self class II proteins. In this study, the sponge matrix allograft model was used to examine the relative contributions of these alloantigen presentation pathways to CD4+ T-cell activation in vivo. Limiting dilution analysis was used to quantify the localization of interleukin-2-producing helper T lymphocytes (HTL) following implantation of sponge allografts. Allografts either were disparate at both class I and class 11, or were derived from beta2-microglobulin knockout (beta2M-/-) mice, which express class II but are deficient in class I. Two measures of in vivo HTL function were monitored: (1) the accumulation of HTL within the allograft (a process that is dependent upon antigen-driven cytokine production), and (2) the development of cytolytic alloantibodies. After implantation of sponge allografts expressing both class I and class II, HTL were readily detectable in the allograft, and cytolytic alloantibodies were present in the serum. When mice were implanted with beta2M-/- sponge allografts, HTL failed to infiltrate these class I-deficient allografts, and alloantibodies were not detectable in the sera of recipients of beta2M-/- sponge allografts. This in vivo requirement for class I expression was not reflected by traditional in vitro measures of HTL function; cells obtained from lymphoid tissues mounted a mixed lymphocyte response and produced interleukin-2 when stimulated with beta2M-/- splenocytes in vitro. One possible interpretation of these data is that in vivo HTL functions are dependent upon the presence of class I-reactive CD8+ T cells. However, HTL readily infiltrated grafts expressing both class I and class II when recipients depleted of CD8+ T cells, and alloantibodies were produced. These observations support the idea that indirect presentation of allogeneic class I molecules plays a critical role in regulating CD4+ HTL functions associated with allograft rejection in vivo.

Cytokines and cell surface markers in prediction of cardiac allograft rejection

Endomyocardial biopsy is generally used to quantify heart allograft rejection and guide immunotherapy. Biopsy, however, is invasive, costly, and risky. Since rejection requires lymphocyte activation, the purpose of this study was to assess alternative methods to evaluate rejection dynamics by investigating serum levels of cytokines and cell surface markers after heart transplantation. Interleukin-2-receptor bearing CD4+T (IL-2R/CD4) cell levels were higher in the peripheral blood of human transplant recipients with rejection grade 2 (p < 0.02). HLA-DR/CD3 levels were somewhat higher in rejection grade 2. There was no correlation between biopsy scores and serum levels of tumor necrosis factor (TNF-alpha), IL-2, or percentage of T cell, NK cell, B cell, CD4+T cell, CD8+T cell, HLA-DR/CD4, HLA-DR/CD8, IL-2R/CD3, IL-2R/CD8. Interleukin-1 (IL-1 beta) was not detectable in all of the samples. The current studies suggest that monitoring lymphocyte IL-2R/CD4 and HLA-DR/CD3 levels is useful in predicting cardiac transplant rejection.

Dissociation of mouse cardiac transplant rejection and donor alloantigen-specific T cell responsiveness

Mouse hearts transplanted into MHC disparate donors are usually rejected 1 week after placement. It is widely accepted that alloantigen-reactive helper T lymphocytes (HTL) and cytotoxic T lymphocytes (CTL) are the key mediators of acute allograft rejection. This report demonstrates that the presence or absence of 'traditional' graft-reactive HTL and CTL is not necessarily related to allograft survival. In these studies, donor/recipient combinations disparate only at MHC or only at minor histocompatibility (mH) loci were employed. Allograft survival was monitored, donor-reactive IL-2 (interleukin-2) producing HTL and CTL were quantified by modified limiting dilution analysis, and serum levels of cytolytic alloantibody were determined. C57BL/10 hearts (H-2b) transplanted into B10.BR (H-2k) recipients (full MHC disparity) enjoyed prolonged survival despite massive infiltration of the allograft by donor-reactive HTL and CTL. IgM, but not IgG, donor-reactive alloantibody was present in the sera of these mice. Hence, traditional IL-2 producing HTL and CTL were not capable of mediating acute graft rejection, nor of providing help for alloantibody isotype switching in this MHC disparate combination. In contrast, C3H/HeN (H-2k) hearts transplanted into B10.BR (H-2k) recipients (mH disparity only) were acutely rejected. Donor-reactive HTL and CTL were rare or not detectable in these recipients, and cytolytic alloantibody was not detectable. Similar observations were made when B10.BR hearts were transplanted into C3H/HeN recipients. Interestingly, treatment of recipients with anti-CD4 monoclonal antibody prevented rejection of mH disparate allografts. Therefore, CD4+ T cells were required for rejection of mH disparate allografts, but this process was independent of detectable IL-2 production or CTL function. Hence, the significance of monitoring 'traditional' T cell functions should be questioned in certain donor/recipient combinations.

The pharmacology of immunosuppressant drugs in skin transplant rejection in mice and other rodents

1. Skin transplantation in rodents is a convenient, widely used method, particularly in mice. It is used as much as an indicator of immune responsiveness as for pharmacological studies. 2. Many differences exist in experimental protocols, both for transplantation and drug administration and in this review, the increase in graft survival time with respect to control times is used to indicate drug effects, in an attempt to account for these differences. 3. The mechanisms underlying skin graft rejection in rodents are described, emphasising the crucial role of both helper and effector T cells. 4. The pharmacology of clinically-used immunosuppressants, including CsA, FK506, rapamycin and purine or pyrimidine synthesis inhibitors, in rodent models of skin transplantation is reviewed. 5. The effects of other potential immunosuppressants and compounds modulating immune responses are described, including the effects of UV light and involvement of platelet-derived factors, prostaglandins and thromboxanes.

Acute rejection of vascular heart allografts by perforin-deficient mice

To study the role of perforin in cell-mediated graft rejection, vascularized hearts were grafted to perforin-deficient C57BL/6 and control C57BL/6 recipient mice. Fully allogeneic heart grafts (BALB/c) were acutely rejected by both recipients within 6 days. Peritoneal exudate lymphocytes from control mice but not from perforin-deficient mice exhibit a strong alloreactive cytotoxic activity in vitro. Histological analysis of the rejected tissues demonstrated extensive mononuclear cell infiltrates in both recipients. Flow cytometry analysis and immunohistology of graft-infiltrating cells showed similar proportions of lymphocyte subsets (CD8 >> CD4). Collectively, these data indicate that perforin is not essential in the cell-mediated acute rejection of a fully mismatched heart allograft. However, perforin-dependent effector mechanisms appeared to be limiting in the T cell-mediated rejection of heart allografts differing only at a single major histocompatibility complex class I antigen (bm1), because these grafts survived longer (mean 87.8 days) in perforin-deficient than in control mice (mean 31.5 days).

L3T4+ but not LYT2+ T-helper cells are required for in vitro maturation of in vivo primed T cells in the cytotoxic response to MHC class I disparate cells following footpad immunization

The induction characteristics of major histocompatibility complex (MHC) class I-specific cytotoxic T lymphocytes (CTL) after footpad immunization were studied. Primary CTL were generated in the regional lymph nodes of C57Bl/6 mice by footpad injection with 10(7) irradiated (2000 rad) spleen cells from MHC class I mutant mouse strain (bm1) followed by a short in vitro culturing without antigen. The requirement of accessory cells and L3T4+ T cells during in vitro maturation of in vivo primed CTL precursor (CTLp) was shown. Moreover, using inhibitory antibodies, the need for IL-2 and IL-4 for in vitro maturation of CTL was established. We have suggested that accessory cells act at the level of L3T4+ T cells which in turn non-specifically up-regulate the CTL response through the production of growth and differentiation factors. Thus, the T-helper population of L3T4+ but not Lyt2+ phenotype appears to be recruited in the in vitro maturation of in vivo primed CTLp in a given system. Possible mechanisms of this phenomenon are discussed.

Reduction of mortality and lymphadenopathy in MRL-lpr/lpr mice treated with nonmitogenic anti-CD3 monoclonal antibody

OBJECTIVE

To evaluate the therapeutic efficacy of nonmitogenic anti-CD3 monoclonal antibody (MAb) in a preexisting autoaggressive response, using the MRL-lpr/lpr (MRL/l) murine model of autoimmune disease.

METHODS

Female MRL/l mice, 8-10 weeks of age, were treated with nonmitogenic anti-CD3 MAb or phosphate buffered saline and effects on mortality, lymphadenopathy, T cell phenotypes, anti-DNA titers, and total IgG titers were measured.

RESULTS

Nonmitogenic anti-CD3 MAb treatment resulted in a dramatic reduction in lymphadenopathy and mortality, as well as an early reduction in alpha/beta+, CD4-, CD8-, Thy+, B220+ (double-negative) lymph node cells. No significant effects on anti-DNA or IgG titers were observed. No morbidity was observed following administration of nonmitogenic anti-CD3 MAb.

CONCLUSION

A short course of treatment with nonmitogenic anti-CD3 MAb can suppress preexisting autoimmune responses without inducing the cytokine-mediated toxicity characteristic of mitogenic forms of anti-CD3 MAb. The use of nonmitogenic anti-CD3 MAb may be efficacious in the clinical setting for the treatment of T cell-mediated autoimmune disorders.

T lymphocyte effector mechanisms in the retina in posterior uveitis

Loss of vision in posterior uveitis is often the consequence of chronic retinal oedema and immune-mediated damage to the retinal parenchyma. Research in other putative autoimmune diseases such as rheumatoid arthritis, and in animal models of autoimmune disease, has uncovered a number of mechanisms which may contribute to the development of inflammatory disease within the eye. With recent developments in specific anti-cytokine therapy an understanding of these mechanisms, most of which are cytokine-mediated, is essential in order to plan more effective therapeutic strategies. In this paper we review recent research investigating the functional characteristics of the T cells which are recruited into the retina in experimental autoimmune uveoretinitis, including activation status, antigen-specific proliferation in vitro and cytokine mRNA production in the inflamed retina.

T-cell-T-cell collaboration in allograft responses

T-cell-T-cell collaboration in allogeneic responses traditionally has been viewed as the requirement for CD4+ T helper cells in the activation of CD8+ cytotoxic T cells. In this regard, the role of the CD4+ T cell is primarily to provide growth factors, such as interleukin-2, on which the CD8+ T cell is dependent. However, expanding information concerning the function of T-cell subsets, and the roles of antigen-presenting cells and cytokines in regulating immune responses, requires that the basic tenets of T-cell interactions be re-evaluated.

Roles of CD8+ and CD4+ cells on lethal graft-versus-host disease in nude mice

Roles of CD8+ and CD4+ cells on lethal graft-versus-host disease (GVHD) were investigated. Injection of spleen cells from C57BL/6 (B6) female mice into (BALB/c x B6)F1 nu/nu female mice caused subacute lethal GVHD (survival: 10-50 days). Injection of anti-Lyt-2.2 (CD8) monoclonal antibody (mAb) on days zero, four and 14 into recipient mice prolonged their survival for at least the 200-day observation period. Injection of anti-L3T4 (CD4) mAb also prolonged survival of the mice for more than 70 days, but they eventually died by 150 days. Pretreatment of the donor B6 spleen cells with anti-Lyt-2.2 (CD8) mAb and complement (C) prevented the development of GVHD, and their pretreatment with anti-L3T4 (CD4) mAb and C markedly prolonged the survival of recipient mice. Injection of a mixture of donor spleen cells pretreated with anti-Lyt-2.2 (CD8) mAb and C and those pretreated with anti-L3T4 (CD4) mAb and C induced subacute lethal GVHD. Injection of anti-L3T4 (CD4) mAb, but not anti-Lyt-2.2 (CD8) mAb on days five, nine and 14 prolonged survival of the recipient mice. These results indicated that the collaboration of CD8+ cells and CD4+ cells was necessary for induction of subacute lethal GVHD. CD4+ cells but not CD8+ cells were involved in mediating subacute GVHD from the onset of the disease. CD8+ cells were, however, capable of inducing late-onset lethal GVHD. Direct phenotyping of T cells in the recipient mice revealed that the CD4+ cells were incapable of repopulating without CD8+ cells, but that CD8+ cells were capable of repopulating without CD4+ cells.

Cytokines and peripheral tolerance to alloantigen

The induction of peripheral tolerance to alloantigen is accompanied in many cases by a decrease in the production of cytokines such as IL-2 and IFN gamma, yet a sustained production of cytokines such as IL-10 and IL-4. Whether or not this altered pattern of cytokine production in tolerant animals is causally related to the induction and/or maintenance of the tolerant state has yet to be fully determined, although experiments blocking selectively the action of IL-2 with CD25 antibodies suggest that manipulation of cytokine production may at least be a route to tolerance. Alternative methods for directly influencing the cytokine balance are sought and recent experiments on the CD28/CTLA-4-B7 interaction suggest a possible approach.

The role of "indirect" recognition in initiating rejection of skin grafts from major histocompatibility complex class II-deficient mice

In vitro studies have revealed several pathways by which T cells can respond to alloantigens, including CD4+ direct responses to allogeneic class II antigens, CD8+ direct responses to allogeneic class I antigens, and CD4+ "indirect" responses to peptides of alloantigens presented in association with responder class II molecules. In vivo studies of skin graft rejection, however, have so far provided clear evidence for the contribution of only the two direct pathways and not for indirect recognition. We have used major histocompatibility complex class II-deficient mice as donors to test the role of indirect recognition in rejection of skin grafts. Class II-deficient skin was always rejected without delay by normal recipients. Removal of recipient CD8+ cells (to leave the animals dependent on CD4+ function) or depletion of recipient CD4+ cells revealed that CD4+ cells were usually involved and sometimes absolutely required in this rapid rejection. Since the donor grafts lacked class II antigens, the CD4+ cells must have recognized donor antigens presented in association with recipient class II molecules. These results therefore indicate that indirect recognition can initiate rapid skin graft rejection.

Local immunosuppressive therapy with monoclonal anti-T cell antibody on renal allograft survival in the rat

Considerable interest in the experimental and clinical use of MoAbs as potential therapeutic agents in allograft rejection has been generated by the recent reports of striking prolongation. In this study we investigated the efficacy of the local administration of MoAb OX-19 which is directed to the rat CD5 equivalent, through the renal artery using a rat kidney transplant model, in order to develop a potent method for modifying rejection while minimizing the systemic side effects. Untreated Lewis rats (LEW, RT-1(1)) rejected Brown-Norway rat (BN, RT-1n) kidney at 7.8 +/- 0.2 days (n = 10). Mean survival time (MST) of recipients treated with OX-19 (75 micrograms/kg per day) as single bolus injections via the dorsal penile vein for 7 days was 7.0 +/- 0.2 days (n = 5, NS). LEW hosts receiving OX-19 (75 micrograms/kg per day) continuously for 7 days via a femoral vein by using an osmotic minipump (IV-treated group) showed a slight prolongation of graft survival (MST = 8.8 +/- 0.9 days, n = 5), but this was not statistically significant. On the other hand, local continuous intrarenal arterial infusion of OX-19 (75 micrograms/kg per day) for 7 days (RA-treated group) significantly prolonged the graft survivals (MST = 16.8 +/- 1.3 days, n = 8, P < 0.01). Histological examination of MoAb-treated LEW hosts on day 6 post-grafting revealed that kidney grafts from RA-treated hosts showed a slight tubular necrosis, but reduced mononuclear cell infiltration, whereas kidney grafts from IV-treated hosts displayed a severe mononuclear cell infiltration around the artery with interstitial oedema. Moreover, the local intrarenal administration of OX-19, even when the dose is delayed until day 4 after renal grafting, has a therapeutic effect for on-going acute allograft rejection (MST = 11.4 +/- 0.8 days, n = 8) compared with administration of OX-19 intravenously from day 4 after grafting (MST = 7.6 +/- 0.2 days, n = 5, P < 0.01) or with no treatment (MST = 7.8 +/- 0.2 days, P < 0.01). The phenotype of graft infiltrating cells (GIC) was investigated on day 6 post-grafting. There was a significantly lower percentage of cells positive for OX-19, OX-8, OX-26 (transferrin receptor), and OX-39 (IL-2 receptor) in the RA group than in the IV group.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of the rejection of major histocompatibility complex class I-disparate murine skin grafts: rejection can be mediated by CD4+ cells activated by allo-class I + II antigen in CD8+ cell-depleted hosts

In the preceding article, we analyzed the immunohistochemical rejection mechanism of major histocompatibility complex (MHC) class I (H-2K)- disparate murine skin grafts, and showed that only CD8+ cells infiltrated at the site of the epithelial tissue of MHC class I- disparate graft. We also showed that perfect survival of MHC class I- disparate grafts were attained in thymectomized recipients treated with anti-Lyt-2 monoclonal antibody. In this report, we showed that these long-surviving allo-class I grafts were rejected in the absence of CD8+ cells by stimulation with allo-MHC class I + II-disparate graft as the second stimulation. Furthermore, it was immunohistochemically revealed that under that condition, a large number of CD4+ cells infiltrated into the epithelial tissue of these long-surviving class I grafts, which were going to be rejected 2-5 d after the transplantation of a second graft with MHC class I + II difference. This result directly shows that CD4+ cells are able to became effectors for the rejection of allo-MHC class I (H-2K) skin graft.