Heterogenous graft rejection pathways in class I major histocompatibility complex-disparate combinations and their differential susceptibility to immunomodulation induced by intravenous presensitization with relevant alloantigens

Indirectly Activated Treg Allow Dominant Tolerance to Murine Skin-grafts Across an MHC Class I Mismatch After a Single Donor-specific Transfusion

BACKGROUND

Tolerance induced in stringent animal transplant models using donor-specific transfusions (DST) has previously required additional immunological manipulation. Here, we demonstrate a dominant skin-allograft tolerance model induced by a single DST across an major histocompatibility class I mismatch in an unmanipulated B6 host.

METHODS

C57BL/6 (H-2) (B6) mice were injected intravenously with splenocytes from B6.C.H-2 (H-2k) (bm1) or F1 (B6 × bm1) mice before skin transplantation. Mice were transplanted 7 days postinjection with donor (bm1 or F1) and third-party B10.BR (H-2) skin grafts.

RESULTS

B6 hosts acutely rejected skin grafts from B6.C.H-2 (bm1) and F1 (B6 × bm1) mice. A single transfusion of F1 splenocytes into B6 mice without any additional immune modulation led to permanent acceptance of F1 skin grafts. This graft acceptance was associated with persistence of donor cells long-term in vivo. The more rapid removal of DST bm1 cells than F1 cells was reduced by natural killer-cell depletion. Tolerant grafts survived an in vivo challenge with naive splenocytes. Both CD4CD25 and CD4CD25 T cells from F1 DST treated B6 mice suppressed alloproliferation in vitro. Tolerance was associated with expansion of peripheral Foxp3CD4CD25 regulatory T cells (Treg) and increased forkhead box P3 (Foxp3) expression in tolerant grafts. In tolerant mice, Foxp3 Treg arises from the proliferation of indirectly activated natural Foxp3 Treg (nTreg) and depletion of Foxp3 Treg abrogates skin-graft tolerance.

CONCLUSIONS

This study demonstrates that the persistence of transfused semiallogeneic donor cells mismatched at major histocompatibility class I can enhance tolerance to subsequent skin allografts through indirectly expanded nTreg leading to dominant tolerance without additional immunological manipulation.

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.

B7.2-Ig fusion proteins enhance IL-4-dependent differentiation of tumor-sensitized CD8+ cytotoxic T lymphocyte precursors

The B7/CD28 co-stimulatory pathway plays a critical role in T cell activation and differentiation. Our previous study demonstrated that administration of B7.2-Ig fusion proteins to tumor-bearing mice elicits IL-4-dependent, CD8+ T cell-mediated tumor regression. Here, we investigated whether B7.2-Ig stimulation of tumor-sensitized CD8+ CTL precursors during in vitro antigen re-sensitization actually results in their differentiation into mature CTLs and if so, whether such a process depends on IL-4 signals. Splenocytes from tumor-sensitized (tumor-bearing or tumor-immunized) mice exhibited low levels of anti-tumor CTL responses upon culturing alone, but induced strikingly enhanced CTL responses when stimulated in vitro with B7.2-Ig fusion proteins. Because CTLs were not generated from normal splenocytes even by B7.2-Ig stimulation, the expression of the B7.2-Ig effect required the in vivo tumor sensitization of CD8+ CTL precursors. Administration of anti-CD4 or anti-CD40 ligand (CD40L) to mice before tumor sensitization resulted in almost complete inhibition of CTL responses generated in the subsequent culture containing B7.2-Ig. In contrast, anti-IL-4 did not influence in vivo tumor sensitization required for CTL induction. However, B7.2-Ig stimulation of tumor-sensitized splenocytes enhanced IL-4 production and neutralization of this IL-4 with anti-IL-4 potently down-regulated CTL responses. These results indicate that B7.2-Ig enhances IL-4-dependent differentiation of anti-tumor CD8+ CTL precursors that can be sensitized in vivo depending on collaboration with CD4+ T cells involving CD40L function.

Anti–HER-2/neu Immune Responses Are Induced before the Development of Clinical Tumors but Declined following Tumorigenesis in HER-2/neu Transgenic Mice

HER-2/neu oncogene products have been implicated as a potential target of T cell-mediated immune responses to HER-2/neu-induced tumors. Using HER-2/neu transgenic mice (oncomice), we investigated whether, and if so how, anti-HER-2/neu immune responses are induced and modulated in these oncomice from birth to tumor initiation. Female oncomice carrying the activated HER-2/neu oncogene displayed apparent hyperplasia in mammary glands at 10 weeks of age and developed mammary carcinomas around an average age of 26 weeks. Unfractionated spleen cells from 10- to 15-week-old oncomice that were cultured without any exogenous stimuli exhibited cytotoxicity against the F31 tumor cell line established from an HER-2/neu-induced mammary carcinoma mass. The final antitumor effectors were a macrophage lineage of cells. However, this effector population was activated, depending on the stimulation of oncomouse CD4(+) T cells with oncomouse-derived antigen-presenting cell (APC) alone or with wild-type mouse APC in the presence of F31 membrane fractions, suggesting the presence of HER-2/neu-primed CD4(+) T cells and HER-2/neu-presenting APC in 10- to 15-week-old oncomice. These antitumor cytotoxic responses were detected at approximately 5 weeks of age and peaked at age 10 to 15 weeks. However, the responses then declined at tumor-bearing stages in which the expression of target proteins could progressively increase. This resulted from the dysfunction of CD4(+) T cells but not of APC or effector macrophages. These results indicate that an anti-HER-2/neu CD4(+) T cell-mediated immune response was generated at the pretumorigenic stage but did not prevent tumorigenesis and declined after the development of clinical tumors.

Both T and non-T cells with proliferating potentials are effective in inducing suppression of allograft responses by alloantigen-specific intravenous presensitization combined with suboptimal doses of 15-deoxyspergualin

In an MHC class I-disparate combination of mouse strains, a single intravenous injection of donor spleen cells combined with 10 suboptimal doses of 15-deoxyspergualin (DSG) administration was effective in inducing donor-specific suppression of cytotoxic T-lymphocyte (CTL) responses and prolonged survival of the relevant skin allograft. Proliferative potentials of the donor spleen cells were requirement for the induction of suppressed allospecific responses, but both highly purified T cells and non-T cells were equally effective to induce the suppression of CTL responses by intravenous injection. These results have shown that, although working on different mechanisms, DSG is as effective as FK506 or rapamycin in inducing allograft tolerance when used at suboptimal doses along with the donor-specific intravenous presensitization, and an immune mechanism other than well-characterized veto T cells is working in this model in suppressing alloreactive CTL precursors.

Induction of Tumor Regression by Administration of B7-Ig Fusion Proteins: Mediation by Type 2 CD8+T Cells and Dependence on IL-4 Production

CD28 signals contribute to either type 1 or type 2 T cell differentiation. Here, we show that administration of B7.2-Ig fusion proteins to tumor-bearing mice induces tumor regression by promoting the differentiation of antitumor type 2 CD8(+) effector T cells along with IL-4 production. B7.2-Ig-mediated regression was not induced in IL-4(-/-) and STAT6(-/-) mice. However, it was elicited in IFN-gamma(-/-) and STAT4(-/-) mice. By contrast, IL-12-induced tumor regression occurred in IL-4(-/-) and STAT6(-/-) mice, but not in IFN-gamma(-/-) and STAT4(-/-) mice. Moreover, B7.2-Ig treatment was effective in a tumor model not responsive to IL-12. B7.2-Ig administration elicited elevated levels of IL-4 production. Tumor regression was predominantly mediated by CD8(+) T cells, although the induction of these effector cells required CD4(+) T cells. Tumor regression induced by CD8(+) T cells alone was inhibited by neutralizing the IL-4 produced during B7.2-Ig treatment. Thus, these results indicate that stimulation in vivo of CD28 with B7.2-Ig in tumor-bearing mice results in enhanced induction of antitumor type 2 CD8(+) T cells (Tc2) leading to Tc2-mediated tumor regression.

Helper T-cell type 1 or type 2 function of xeno-MHC-restricted T-cell clones in a direct xenoantigen recognition

There have been several reports that xeno-MHC-restricted T-cells have a cytotoxic function through a direct xenoantigen recognition, but yet no report that they have a helper function. Previously we showed that both xeno-MHC-restricted CD4(+) and CD8(+) T-cells recognized xenoantigens directly in a mouse anti-rat combination. In this study, we investigated whether or not xeno-MHC-restricted T-cells had a helper function. Mouse T-cell clones recognizing rat antigens directly were derived from T-cell lines using the limiting dilution method. Phenotype, cytotoxic activity and cytokine production of these clones were analyzed by flow cytometry, 51Cr release assay and ELISA, respectively. Rat-MHC class I-restricted mouse CD8(+) T-cell clones showed a specific cytotoxic activity against rat antigens. One CD4(+) clone produced IL-4 and IL-10, and the other CD4(+) clone produced not T-helper (Th) 2 cytokine but TNF-alpha. Our results suggested that xeno-MHC class I-restricted CD8(+) T-cells should have a cytotoxic function, and xeno-MHC class II-restricted CD4(+) T-cells should have either Th1 or Th2 function.

Induction of hyperacute rejection of skin allografts by CD8+ lymphocytes

BACKGROUND

Second-set rejection is generally regarded as a phenomenon mainly mediated by humoral cytotoxic antibodies, although a few discordant data have been presented. In the reported experiments, we have taken advantage of the absence of production of specific cytotoxic alloantibodies contrasting with the normal development of transplantation cellular immunity, in two murine models: chimeric mice and RAG mice.

METHODS

Chimeras (BALB/c-->CBA) were obtained by transplantation of 2x10(7) fetal liver cells from BALB/c (H-2d) mice to lethally irradiated CBA (H-2k) mice. After hyperimmunization with third-party C57/ BL6 (B6) (H-2b) skin transplants and with injections of 2x10(7) B6 spleen cells, antibody production, and skin graft survival were analyzed. To identify further the factors or cells responsible for accelerated rejection of B6 skin transplants in hyperimmunized chimeras, transfer experiments were carried out involving the injection of serum, whole spleen cells, spleen T cells, spleen CD8+ T cells or spleen CD4+ T cells from chimeras into BALB/c mice that had received 6 Gy irradiation. The recipient mice were then grafted with B6 skin. Similarly, the immunodeficient RAG mice were used to construct a model of recipient animals with anti-H-2d hyperimmunized B6 T cells in the total absence of antibody.

RESULTS

In chimeras, anti-B6 cytotoxic antibodies were not detectable in any of hyperimmunized chimeric mice, yet accelerated rejection of B6 skin transplant occurred: a graft survival of 8.6+/-0.5 days (d), comparable to 8.9+/-0.8 d survival in CBA control mice subjected to the same hyperimmunization procedure, and significantly shorter than that in nonhyperimmunized (BALB/c-->CBA) chimeras (11.6+/-0.5 d) or in non-hyperimmunized CBA control mice (12.1+/-0.6 d). High titers of anti-B6 cytotoxic antibodies were present in the serum of hyperimmunized CBA control mice. In transfer experiments, the graft survival was over 14 d in mice treated with irradiation alone, with irradiation + serum or with irradiation + CD4+ T cells. It was significantly shorter in mice treated with irradiation + whole spleen cells, with irradiation + T cells or with irradiation + CD8+ T cells (8.9+/-0.8 d). Similarly, in immunodeficient RAG mice, reconstitution of the T cell compartment with T cells from hyperimmunized B6 mice led to accelerated rejection of BALB/c skin allografts (11.4+/-1.1 d vs. 18.8+/-0.8 d when T cells were provided by nonimmunized mice). In a second transfer of cells from these reconstituted RAG mice into naive RAG mice, CD8+ T cells were shown to induce accelerated rejection of skin allografts (12.0+/-0.6 d) whereas CD4+ T cells were much less efficient (16.5+/-0.1 d).

CONCLUSION

These data indicate that T cells, and especially the CD8+ subset, can be responsible for second-set rejection in the absence of anti-donor antibodies in chimeric and RAG mouse models. These sensitized CD8+ T cells are also likely to play an important role in normal mice, in addition to that of cytotoxic antibodies.

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.

Suppression of allograft responses induced by interleukin-6, which selectively modulates interferon-gamma but not interleukin-2 production

BACKGROUND

Interferon (IFN)-gamma produced by activated T cells represents an important effector cytokine in mediating an inflammatory response.

METHODS

The present study investigated the modulation of allograft responses by inhibiting IFN-gamma production. C57BL/6 (B6) lymph node cells were stimulated with class II H2-disparate B6-C-H-2bm12 (bm12) spleen cells.

RESULTS

Addition of interleukin (IL)-6 to the primary B6 anti-bm12 mixed lymphocyte reaction (MLR) inhibited neither proliferative responses nor IL-2 production. However, IL-6 induced a dose-dependent suppression of IFN-gamma production in the same MLR cultures. B6 mice were engrafted with bm12 skin grafts, and IL-6 was given to bm12 skin graft recipients every other day. T cells from these recipient mice produced significantly less IFN-gamma in secondary B6 anti-bm12 MLR than those from bm12 skin graft recipients that had not received IL-6 injections. IFN-gamma production by these T cells was suppressed more strongly when the secondary MLR was conducted in the presence of IL-6. In addition to suppression of IFN-gamma expression, IL-6 injections resulted in prolongation of bm12 skin graft survival. The critical involvement of IFN-gamma in anti-bm12 rejection responses was substantiated by evidence that administration of anti-IFN-gamma monoclonal antibody strikingly prolonged bm12 skin graft survival. The prolongation of graft survival by in vivo treatment with either IL-6 or anti-IFN-gamma monoclonal antibody was found to be induced without blocking cellular infiltration of the grafts.

CONCLUSIONS

These results indicate that IFN-gamma acts as a key cytokine in a B6 anti-bm12 allograft response and that IL-6 may down-regulate this response by inhibiting IFN-gamma production of alloreactive T cells.

Direct recognition of rat MHC antigens on rat antigen-presenting cells by mouse CD4+ and CD8+ T cells and establishment of T cell clones exhibiting a direct recognition pathway

Alloantigens are recognized by T cells either through a direct pathway, which involves recognition of alloantigens expressed on allogeneic antigen-presenting cells (APC), or through an indirect pathway, which involves recognition of processed alloantigens presented by self APC. We investigated whether rat xenoantigens are also recognized by direct (xenogeneic APC-restricted) and/or indirect (self APC-restricted) pathways. C57BL/6 (B6) mouse anti-F344 or WKAH rat mixed lymphocyte reactions (MLRs) were partially inhibited by addition of either anti-mouse CD4 or CD8 monoclonal antibody (mAb) and almost completely blocked in the presence of both mAbs. These xenogeneic MLRs were almost completely inhibited by simultaneous depletion of both self and xeno APCs and only partially suppressed by the elimination of either type of APC, indicating that freshly prepared splenic mouse T cells can recognize rat xenoantigens through both direct and indirect pathways. Anti-F334 T cell lines were generated from B6 anti-F344 MLR cultures, and four CD4+ and four CD8+ T cell clones were isolated from these parental lines. The parental lines and those derived T cell clones were tested for their ability to proliferate depending on the presence of F344 APC. Proliferation of CD8 clones by stimulation with F344 APC was inhibited by the addition of anti-rat class I MHC mAb but not of anti-class II MHC mAbs. Conversely, proliferation of CD4 clones was reduced by addition of anti-class II MHC mAbs. Thus, these results indicate that xeno (rat)-reactive mouse T cells recognize xenoantigens via both indirect (self APC-restricted) and direct (xeno APC-restricted) pathways and that both CD4 and CD8 subsets of T cells participate in a direct pathway of xenoantigen recognition.

Evidence of direct recognition subset in xeno-reactive helper T cells

There are two functional subsets of alloreactive helper T cells: one recognizes the alloantigens on self-antigen-presenting cells (APCs), and the other recognizes the alloantigens on allo-APCs. In this study, we analyzed which specific Th cells have a possible effect on concordant-xenograft rejection responses and how Th cells recognize xeno-antigens. In the proliferative response stimulated with spleen cells obtained from rats (r-SPC), mouse T cells (m-Tc) were not inhibited by GK1.5 (CD4) or 2.43 (CD8) antibodies. In a xeno-mixed lymphocyte reaction, m-Tc that included APC (APC(+)) proliferated when stimulated with r-SPC APC(+) or APC(-). In contrast, B6 T cell APC(-) proliferated only when stimulated with r-APC(+)SPC but not when stimulated with APC(-). The m-Tc lines responded to F344 SPC APC(+) but not to SPC APC(-); however, the m-Tc line did not respond to SPC from another rat strain. We hypothesize that both CD4+ Th and CD8+ Th are included in the concordant xeno-reactive Th; there are both xeno-APC-restricted Th and self-APC-restricted Th; and xeno-APC-restricted Th recognizes xeno-major histocompatibility complexes.

CD4+ T cell-mediated rejection of major histocompatibility complex class I-disparate grafts: a role for alloantibody

Experimental studies of the T cell requirement for rejection of class I major histocompatibility complex (MHC)-disparate grafts have generated controversy over both the autonomy of CD8+ T cells and the mechanism whereby CD4+ T cells are able to independently mediate rejection. In this study of rejection of RT1Aa class I MHC-disparate rat cardiac and skin allografts by high-responder PVG RT1u recipients, we show that elimination of CD8+ T cells [by anti-CD8 monoclonal antibody (mAb) administration in vivo] fails to prolong graft survival, whereas partial depletion of CD4+ T cells (by anti-CD4 mAb treatment) markedly delays rejection of class I-disparate heart grafts, and marginally prolongs survival of skin grafts. Anti-CD4-treated PVG-RT1u athymic nude rats reconstituted with CD8+ T cells failed to reject class I-disparate skin grafts for several weeks and eventual rejection correlated with re-emergence of a small number of donor derived CD4+ T cells. Conversely, anti-CD8-treated nude rats reconstituted with CD4+ T cells alone rapidly rejected class I-disparate skin grafts. Passive transfer of anti-class I immune serum to anti-CD4-treated euthymic recipients promptly restored their ability to specifically reject a class I-disparate heart graft. Similarly, passive transfer of immune serum to PVG-RT1u nude rats bearing skin allografts caused destruction of class I-disparate but not third-party grafts. These results demonstrate that CD4+ T cells are both necessary and sufficient to cause rejection of class I-disparate heart and skin grafts in this model and that CD4+ T cell-dependent alloantibody plays a decisive role in effecting rejection.

Comparison of tolerance inducibility to class I or class II antigens between cyclophosphamide (CP)-induced tolerance and transfusion with donor cells: general effectiveness of CP-induced tolerance and difference of skin graft prolongation in each class I antigen-disparate combination

Transfusion with allogeneic cells alone was reported to prolong skin allograft survival in the MHC class I antigen alone-disparate combination of B6.C-H-2bm1 (bm1; Kbm1, IAb, IE-, Db)-->C57BL/6 CrSlc (B6; H-2b). Using 6 class I antigen-disparate and 2 class II antigen-disparate combinations, we compared the effectiveness for skin graft prolongation between transfusion with donor cells (TDC) and our system of CP-induced tolerance, which comprises intravenous (i.v.) injection of allogeneic cells followed by cyclophosphamide (CP), i.e., TDC followed by CP. TDC was effective only in the combination of bm1-->B6, but not at all in the other combinations. On the other hand, CP-induced tolerance was effective in the 5 class I antigen-disparate and 2 class II antigen-disparate combinations except for bm1-->B6 combination. These results suggest that CP-induced tolerance may be more general for the induction of unresponsiveness than TDC.

Cellular mechanisms of rejection

Studies by molecular biologists, protein chemists and cell biologists are rapidly providing new tools and information for those interested in the cellular mechanisms of graft rejection. Despite these contributions, a clear picture of the mechanisms involved in rejection has not yet evolved. However, whole new areas for research have developed, providing opportunities for new insights as well as therapeutic interventions.

Correlation between lymphocyte-induced donor-specific tolerance and donor cell recirculation

Intravenous infusion of mice with major histocompatibility complex (MHC) incompatible lymphocytes can inhibit the response of recipient T cells capable of recognizing the injected cells, and can enhance survival of grafts sharing MHC with the injected cells. However, neither T cell inactivation nor graft survival enhancement is always achieved. This is particularly true for donor cells that are fully allogeneic (as compared to semiallogeneic) to the recipient. We show here that both donor-specific induced response reduction and graft survival enhancement are directly correlated with the ability of the injected lymphoid cells to persist in the recirculating lymphocyte pool of the host. Whether donor cells persist correlates inversely with the level of natural killer cell (NK) activity in the host. Fully allogeneic cells can only persist in hosts with low NK activity and can then induce response reduction. Both persistence and response reduction are abrogated by injection of the host with poly-I:C, a treatment that boosts host NK activity. The same treatment also destroys the ability of semiallogeneic injected cells to persist, to induce response reduction, and to enhance skin graft survival.

Cell-cell interaction in graft rejection responses: induction of anti-allo-class I H-2 tolerance is prevented by immune responses against allo-class II H-2 antigens coexpressed on tolerogen

The intravenous sensitization of C57BL/6 (B6) mice with class I H-2-disparate B6-C-H-2bm1 (bm1) spleen cells results in almost complete abrogation of anti-bm1 CD8+ helper (proliferative and interleukin 2-producing) T cell (Th) activities. Although an appreciable portion of CD8+ cytotoxic T lymphocyte (CTL) precursors themselves remained after this regimen, such a residual CTL activity was eliminated after the engrafting of bm1 grafts, and these grafts exhibited prolonged survival. In contrast, the intravenous sensitization with (bm1 x B6-C-H-2bm12 [bm12])F1 cells instead of bm1 cells failed to induce the prolongation of bm1 graft survival as well as bm12 and (bm1 x bm12)F1 graft survival. In the (bm1 x bm12)F1-presensitized B6 mice before as well as after the engrafting of bm1 grafts, anti-bm1 CTL responses that were comparable to or slightly stronger than those observed in unpresensitized mice were induced in the absence of anti-bm1 Th activities. bm1 graft survival was also prolonged by intravenous presensitization with a mixture of bm1 and bm12 cells but not with a mixture of bm1 and (bm1 x bm12)F1 cells. The capacity of CD4+ T cells to reject bm12 grafts was eliminated by intravenous presensitization with antigen-presenting cell (APC)-depleted bm12 spleen cells. However, intravenous presensitization with APC-depleted (bm1 x bm12)F1 cells failed to induce the prolongation of bm1 graft survival under conditions in which appreciably prolonged bm12 graft survival was induced. More surprisingly, bm1 graft survival was not prolonged even when the (bm1 x bm12)F1 cell presensitization was performed in CD4+ T cell-depleted B6 mice. This contrasted with the fact that conventional class I-disparate grafts capable of activating self Ia-restricted CD4+ as well as allo-class I-reactive CD8+ Th exhibited prolonged survival in CD4+ T cell-depleted, class I-disparate cell-presensitized mice. These results indicate that: (a) intravenous presensitization with class I- and II-disparate cells fails to reduce anti-allo-class I rejection responses that would otherwise be eliminated using only class I-disparate cells; (b) such failure is generated according to the coexpression of both classes of alloantigens on a single cell as tolerogen; and (c) allo-class II antigens coexpressed on tolerogen function to activate CD4+ as well as non-CD4+ Th leading to the generation of anti-class I effector T cell responses.