In vivo induction of antigen-specific transplantation tolerance to Qa1a by exposure to alloantigen in the absence of T-cell help

Th cells promote CTL survival and memory via acquired pMHC-I and endogenous IL-2 and CD40L signaling and by modulating apoptosis-controlling pathways

Involvement of CD4⁺ helper T (Th) cells is crucial for CD8⁺ cytotoxic T lymphocyte (CTL)-mediated immunity. However, CD4⁺ Th’s signals that govern CTL survival and functional memory are still not completely understood. In this study, we assessed the role of CD4⁺ Th cells with acquired antigen-presenting machineries in determining CTL fates. We utilized an adoptive co-transfer into CD4⁺ T cell-sufficient or -deficient mice of OTI CTLs and OTII Th cells or Th cells with various gene deficiencies pre-stimulated in vitro by ovalbumin (OVA)-pulsed dendritic cell (DCova). CTL survival was kinetically assessed in these mice using FITC-anti-CD8 and PE-H-2K^b/OVA_257-264 tetramer staining by flow cytometry. We show that by acting via endogenous CD40L and IL-2, and acquired peptide-MHC-I (pMHC-I) complex signaling, CD4⁺ Th cells enhance survival of transferred effector CTLs and their differentiation into the functional memory CTLs capable of protecting against highly-metastasizing tumor challenge. Moreover, RT-PCR, flow cytometry and Western blot analysis demonstrate that increased survival of CD4⁺ Th cell-helped CTLs is matched with enhanced Akt1/NF-κB activation, down-regulation of TRAIL, and altered expression profiles with up-regulation of prosurvival (Bcl-2) and down-regulation of proapoptotic (Bcl-10, Casp-3, Casp-4, Casp-7) molecules. Taken together, our results reveal a previously unexplored mechanistic role for CD4⁺ Th cells in programming CTL survival and memory recall responses. This knowledge could also aid in the development of efficient adoptive CTL cancer therapy.

Human prostate tumor antigen-specific CD8+ regulatory T cells are inhibited by CTLA-4 or IL-35 blockade

Regulatory T cells play important roles in cancer development and progression by limiting the generation of innate and adaptive anti-tumor immunity. We hypothesized that in addition to natural CD4(+)CD25(+) regulatory T cells (Tregs) and myeloid-derived suppressor cells, tumor Ag-specific Tregs interfere with the detection of anti-tumor immunity after immunotherapy. Using samples from prostate cancer patients immunized with a DNA vaccine encoding prostatic acid phosphatase (PAP) and a trans-vivo delayed-type hypersensitivity (tvDTH) assay, we found that the detection of PAP-specific effector responses after immunization was prevented by the activity of PAP-specific regulatory cells. These regulatory cells were CD8(+)CTLA-4(+), and their suppression was relieved by blockade of CTLA-4, but not IL-10 or TGF-β. Moreover, Ag-specific CD8(+) Tregs were detected prior to immunization in the absence of PAP-specific effector responses. These PAP-specific CD8(+)CTLA-4(+) suppressor T cells expressed IL-35, which was decreased after blockade of CTLA-4, and inhibition of either CTLA-4 or IL-35 reversed PAP-specific suppression of tvDTH response. PAP-specific CD8(+)CTLA-4(+) T cells also suppressed T cell proliferation in an IL-35-dependent, contact-independent fashion. Taken together, these findings suggest a novel population of CD8(+)CTLA-4(+) IL-35-secreting tumor Ag-specific Tregs arise spontaneously in some prostate cancer patients, persist during immunization, and can prevent the detection of Ag-specific effector responses by an IL-35-dependent mechanism.

Requirement of CD4 help for induction of CD8 T cell response specific for virally derived h60

CD40-CD40L-mediated help from CD4 T cells is essential to induce primary CD8 T cell responses specific to the non-inflammatory cell-based antigen H60. In this study, using H60 as a model antigen, we generated recombinant vaccinia viruses (rVVs) expressing the H60 CD8 epitope and investigated whether CD4 help was required to activate the CD8 T cell response specific to the virally expressed H60. The immune response after infection with rVVs expressing H60 was similar to that after immunization with H60 congenic splenocytes, with a peak frequency of H60-specific CD8 T cells detected in the blood on day 10 post-infection. A CD8 T cell response specific for virally derived H60 was not induced in CD4-depleted mice, but was in CD40-deficient mice. These results provide insights into the characterization of the CD8 T cell response specifically for antigens originating from cellular sources compared to viral sources.

CD4+ T cell effects on CD8+ T cell location defined using bioluminescence

T lymphocytes of the CD8+ class are critical in delivering cytotoxic function and in controlling viral and intracellular infections. These cells are “helped” by T lymphocytes of the CD4+ class, which facilitate their activation, clonal expansion, full differentiation and the persistence of memory. In this study we investigated the impact of CD4+ T cells on the location of CD8+ T cells, using antibody-mediated CD4+ T cell depletion and imaging the antigen-driven redistribution of bioluminescent CD8+ T cells in living mice. We documented that CD4+ T cells influence the biodistribution of CD8+ T cells, favoring their localization to abdominal lymph nodes. Flow cytometric analysis revealed that this was associated with an increase in the expression of specific integrins. The presence of CD4+ T cells at the time of initial CD8+ T cell activation also influences their biodistribution in the memory phase. Based on these results, we propose the model that one of the functions of CD4+ T cell “help” is to program the homing potential of CD8+ T cells.

CD4 T cells are required for CD8 T cell survival during both primary and memory recall responses

The role of CD4 T cell help in primary and secondary CD8 T cell responses to infectious pathogens remains incompletely defined. The primary CD8 T response to infections was initially thought to be largely independent of CD4 T cells, but it is not clear why some primary, pathogen-specific CD8 T cell responses are CD4 T cell dependent. Furthermore, although the generation of functional memory CD8 T cells is CD4 T cell help dependent, it remains controversial when the "help" is needed. In this study, we demonstrated that CD4 T cell help was not needed for the activation and effector differentiation of CD8 T cells during the primary response to vaccinia virus infection. However, the activated CD8 T cells showed poor survival without CD4 T cell help, leading to a reduction in clonal expansion and a diminished, but stable CD8 memory pool. In addition, we observed that CD4 T cell help provided during both the primary and secondary responses was required for the survival of memory CD8 T cells during recall expansion. Our study indicates that CD4 T cells play a crucial role in multiple stages of CD8 T cell response to vaccinia virus infection and may help to design effective vaccine strategies.

Human bullous pemphigoid antigen 2 transgenic skin elicits specific IgG in wild-type mice

Bullous pemphigoid antigen 2 (BPAG2) is targeted by autoantibodies in patients with bullous pemphigoid (BP), and absent in patients with one type of epidermolysis bullosa (OMIM #226650). A keratin 14 promoter construct was used to produce transgenic (Tg) mice appropriately expressing human BPAG2 (hBPAG2) in murine epidermal basement membrane (BM). Grafts of Tg skin placed on gender-matched, syngeneic wild type (Wt) or major histocompatibility complex I (MHC I)-/- mice elicited IgG that bound human epidermal BM and BPAG2. Production of such IgG in grafted mice was prompt (detectable within 16+/-2 days), robust (titer > or = 1,280), durable (present > or = 380 days), and correlated with the involution and loss of Tg skin grafts. MHC II-/- mice grafted with Tg skin did not develop anti-hBPAG2 IgG or graft loss indicating that MHC II:CD4+ T cell interactions were crucial for these responses. Tg skin grafts on Wt mice developed neutrophil-rich infiltrates, dermal edema, subepidermal blisters, and deposits of immunoreactants in epidermal BM. This model shows fidelity to alterations seen in patients with BP, has relevance to immune responses that may arise in patients with epidermolysis bullosa following BPAG2 gene replacement, and can be used to identify interventions that may block production of IgG against proteins in epidermal BM.

Altered primary CD8+ T cell response to a modified virus Ankara(MVA)-vectored vaccine in the absence of CD4+ T cell help

T cell receptor-transgenic F5 mice were used to assess primary CD8+ T cell responses to a modified virus Ankara (MVA)-vectored vaccine in the absence of CD4+ T cell help. Naive, CD8-enriched, CFSE-labelled F5 cells were transferred into normal or CD4+ cell-depleted mice and the mice were vaccinated with MVA.HIVA-NP. At different time points during the primary response, F5 cells were re-isolated and analysed on divisional basis for a number of parameters. We demonstrated that the primary CD8+ T cell response in the absence of CD4+ T cell help differed from that in normal CD4+ cell-undepleted mice. While in the absence of CD4+ T cell help, the initial migratory progress from the local response to a systemic one was not grossly affected, the proportion of dying F5 cells during the expansion phase was markedly increased and resulted in an overall smaller expansion and significantly decreased frequency of CD8+ T cell memory after contraction. T cells primed without help displayed accelerated proliferation and activation, while expression of interferon-gamma remained similar. These phenomena were observed in the lymph nodes draining the MVA.HIVA-NP immunization site and were similar, but delayed by 2-3 days in spleen and non-draining lymph nodes.

Generation of antifungal effector CD8+ T cells in the absence of CD4+ T cells during Cryptococcus neoformans infection

Immunity to the opportunistic fungus Cryptococcus neoformans is dependent on cell-mediated immunity. Individuals with defects in cellular immunity, CD4(+) T cells in particular, are susceptible to infection with this pathogen. In host defense against a number of pathogens, CD8(+) T cell responses are dependent upon CD4(+) T cell help. The goal of these studies was to determine whether CD4(+) T cells are required for the generation of antifungal CD8(+) T cell effectors during pulmonary C. neoformans infection. Using a murine intratracheal infection model, our results demonstrated that CD4(+) T cells were not required for the expansion and trafficking of CD8(+) T cells to the site of infection. CD4(+) T cells were also not required for the generation of IFN-gamma-producing CD8(+) T cell effectors in the lungs. In CD4(-) mice, depletion of CD8(+) T cells resulted in increased intracellular infection of pulmonary macrophages by C. neoformans, increasing the pulmonary burden of the infection. Neutralization of IFN-gamma in CD4(-)CD8(+) mice similarly increased macrophage infection by C. neoformans, thereby blocking the protection provided by CD8(+) T cells. Altogether, these data support the hypothesis that effector CD8(+) T cell function is independent of CD4(+) T cells and that IFN-gamma production from CD8(+) T cells plays a role in controlling C. neoformans by limiting survival of C. neoformans within macrophages.

A New Dynamic Model of CD8+T Effector Cell Responses via CD4+T Helper-Antigen-Presenting Cells

A long-standing paradox in cellular immunology has been the conditional requirement for CD4(+) Th cells in priming of CD8(+) CTL responses. We propose a new dynamic model of CD4(+) Th cells in priming of Th-dependent CD8(+) CTL responses. We demonstrate that OT II CD4(+) T cells activated by OVA-pulsed dendritic cells (DC(OVA)) are Th1 phenotype. They acquire the immune synapse-composed MHC II/OVAII peptide complexes and costimulatory molecules (CD54 and CD80) as well as the bystander MHC class I/OVAI peptide complexes from the DC(OVA) by DC(OVA) stimulation and thus also the potential to act themselves as APCs. These CD4(+) Th-APCs stimulate naive OT I CD8(+) T cell proliferation through signal 1 (MHC I/OVAI/TCR) and signal 2 (e.g., CD54/LFA-1 and CD80/CD28) interactions and IL-2 help. In vivo, they stimulate CD8(+) T cell proliferation and differentiation into CTLs and induce effective OVA-specific antitumor immunity. Taken together, this study demonstrates that CD4(+) Th cells carrying acquired DC Ag-presenting machinery can, by themselves, efficiently stimulate CTL responses. These results have substantial implications for research in antitumor and other aspects of immunity.

Qa-1 restriction of CD8+ suppressor T cells

There is increasing evidence that the immune response can be inhibited by several T cell subsets, including NK T cells, CD25+CD4+ T cells, and a subpopulation of CD8+ T cells. Animal model studies of multiple sclerosis have suggested an important role for suppressor CD8+ T cells in protection against disease recurrence and exacerbation. The molecular lynchpin of CD8+ suppressive activity is the murine MHC molecule Qa-1, termed HLA-E in humans. Here we summarize findings from work on Qa-1 that have begun to delineate suppressor CD8+ T cells and their mechanisms of action in the context of self tolerance and autoimmune disease.

Helper Requirements for Generation of Effector CTL to Islet β Cell Antigens

We have dissected the helper requirements for converting a tolerogenic CD8 T cell response into one capable of causing destruction of the pancreatic islets. Injection of naive OVA-specific CD8 T cells into transgenic mice expressing OVA in the pancreas only resulted in islet destruction when activated CD4 Th cells were coinjected. This requirement for activated CD4 T cell help for induction of primary CD8 T cell-mediated immunity to tissue Ags contrasts recent reports suggesting that help is only important for CTL memory. Our findings show that signaling of CD40 on the dendritic cell presenting to CD8 T cells is important, but not sufficient, for induction of diabetes. Furthermore, once helpers are activated, they need not recognize Ag on the dendritic cells they license. This provides insight into the helper requirements for adoptive transfer immunotherapy of tumors and suggests key points for inhibition of CTL-mediated autoimmunity.

Cutting edge: CD4+ T cell help can be essential for primary CD8+ T cell responses in vivo

Recent studies have shown that CD4(+) T cell help is required for the generation of memory CD8(+) T cells that can proliferate and differentiate into effector cells on Ag restimulation. The importance of help for primary CD8(+) T cell responses remains controversial. It has been suggested that help is not required for the initial proliferation and differentiation of CD8(+) T cells in vivo and that classical models of helper-dependent responses describe impaired secondary responses to Ag in vitro. We have measured primary CD8(+) T cell responses to peptide-pulsed dendritic cells in mice by cytokine ELISPOT and tetramer staining. No responses were detected in the absence of help, either when normal dendritic cells were injected into MHC II-deficient mice or when MHC II-deficient dendritic cells were injected into normal mice. Thus, the primary in vivo CD8(+) T cell response depends absolutely on help from CD4(+) T cells in our experimental system.

Development of a novel transgenic mouse for the study of interactions between CD4 and CD8 T cells during graft rejection

The goal of this study was the development of a system in which the cooperative interactions between CD4 and CD8 T cells specific for defined peptides from a single minor histocompatibility antigen could be studied. A transgenic mouse strain that expresses chicken ovalbumin (Act-mOVA) on the surface of all cells in the body was produced as a source of tissues containing such an antigen. Skin grafts from Act-mOVA donors were rapidly and completely rejected by wild-type recipients, but only when both CD4 and CD8 T cells were present. CD4 T cells by themselves caused an incomplete form of rejection characterized by rapid but partial contraction of Act-mOVA grafts. CD8 T cells alone caused complete rejection of Act-mOVA skin grafts but only after a long delay. Adoptively transferred ovalbumin-specific TCR-transgenic CD4 and CD8 T cells were stimulated by Act-mOVA graft antigens and CD8 T-cell accumulation in the grafts was enhanced by specific CD4 T cells. These findings, together with the fact that the ligand for ovalbumin peptide-specific CD8 T cells can be detected in Act-mOVA tissues with an MHC-restricted antibody, make this an ideal system for the study of cooperation between CD4 and CD8 T cells.

Pancreatic NOD beta cells express MHC class II protein and the frequency of I-A(g7) mRNA-expressing beta cells strongly increases during progression to autoimmune diabetes

AIMS/HYPOTHESIS

In the NOD mouse model, attempts to show MHC class II expression by pancreatic beta cells were unsuccessful so far. We readdressed this question by analysing I-A(g7) expression in single pancreatic beta cells.

METHODS

Single-cell multiplex RT PCR and single-cell immunofluorescence were used to study MHC class II expression in NOD and NOD/SCID beta cells.

RESULTS

Pancreatic beta cells from NOD mice express the I-A(g7) protein as well as the corresponding mRNA. The frequency of MHC class II mRNA-expressing beta cells is drastically increased during the progression to overt diabetes. MHC class II protein is accumulated intracellularly, and invariant chain is co-expressed. Beta cells from 9- to 10-week-old NOD/SCID mice express MHC class II at the same low frequency as beta cells from 3-week-old NOD mice.

CONCLUSION/INTERPRETATION

NOD beta cells express I-A(g7) and could be a direct target of autoreactive CD4+ T cells. This MHC class II expression is triggered by infiltrating lymphocytes.

Defective CD8+ T cell peripheral tolerance in nonobese diabetic mice

Nonobese diabetic (NOD) mice develop spontaneous autoimmune diabetes that involves participation of both CD4+ and CD8+ T cells. Previous studies have demonstrated spontaneous reactivity to self-Ags within the CD4+ T cell compartment in this strain. Whether CD8+ T cells in NOD mice achieve and maintain tolerance to self-Ags has not previously been evaluated. To investigate this issue, we have assessed the extent of tolerance to a model pancreatic Ag, the hemagglutinin (HA) molecule of influenza virus, that is transgenically expressed by pancreatic islet beta cells in InsHA mice. Previous studies have demonstrated that BALB/c and B10.D2 mice that express this transgene exhibit tolerance of HA and retain only low-avidity CD8+ T cells specific for the dominant peptide epitope of HA. In this study, we present data that demonstrate a deficiency in peripheral tolerance within the CD8+ T cell repertoire of NOD-InsHA mice. CD8+ T cells can be obtained from NOD-InsHA mice that exhibit high avidity for HA, as measured by tetramer (K(d)HA) binding and dose titration analysis. Significantly, these autoreactive CD8+ T cells can cause diabetes very rapidly upon adoptive transfer into NOD-InsHA recipient mice. The data presented demonstrate a retention in the repertoire of CD8+ T cells with high avidity for islet Ags that could contribute to autoimmune diabetes in NOD mice.

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.

Dendritic cells need T cell help to prime cytotoxic T cell responses to strong antigens

Peptide-pulsed mouse dendritic cells (DC) primed peptide-specific CD8+ cytotoxic T cell responses very effectively if they expressed minor histocompatibility antigens, which could stimulate a CD4+ T helper cell response. These DC could also prime most syngeneic mice, although there was no deliberate immunization for help (the DC were prepared in syngeneic mouse serum, to avoid any response to fetal calf serum antigens). In contrast, DC were unable to prime MHC class II-deficient mice for cytotoxic responses to the classical helper-dependent antigens Qa1a and HY. More strikingly, Balb.B DC failed to prime B6 MHC class II-deficient mice for cytotoxic responses to Balb minor antigens, even though these two strains differ at more than 40 minor histocompatibility loci. When peptide-pulsed DC were prepared without enzymes (used to release DC from lymphoid tissues), they failed to prime the majority of normal syngeneic mice, even though they expressed high levels of B7 and ICAM-1 co-stimulatory molecules, suggesting that help was provided by responses to antigens in the enzyme cocktail. The enzyme treatment itself did not provide signals that could substitute for help, since DC prepared with enzymes could not prime MHC class II-deficient mice. The observation that highly immunogenic minor-incompatible DC failed to prime MHC class II-deficient mice suggests that in the absence of inflammatory signals, even strong antigens cannot stimulate CD8+ T cell responses without help.

CD40 activation in vivo overcomes peptide-induced peripheral cytotoxic T-lymphocyte tolerance and augments anti-tumor vaccine efficacy

The outcome of antigen recognition by naive CD8+ cytotoxic T lymphocytes (CTLs) in the periphery is orchestrated by CD4+ T-helper cells, and can either lead to priming or tolerization. The presence of T-helper cells favors the induction of CTL immunity, whereas the absence of T-helper cells can result in CTL tolerance. The action of T helper cells in CTL priming is mediated by CD40-CD40 ligand interactions. We demonstrate here that triggering of CD40 in vivo can considerably enhance the efficacy of peptide-based anti-tumor vaccines. The combination of a tolerogenic peptide vaccine containing a minimal essential CTL epitope with an activating antibody against CD40 converts tolerization into strong CTL priming. Moreover, CD40 ligation can provide an already protective tumor-specific peptide vaccine with the capacity to induce therapeutic CTL immunity in tumor-bearing mice. These findings indicate that the CD40-CD40 ligand pair can act as a 'switch', determining whether naive peripheral CTLs are primed or tolerized, and support the clinical use of CD40-stimulating agents as components of anti-cancer vaccines.

Effective depletion of alloreactive lymphocytes from peripheral blood mononuclear cell preparations

BACKGROUND

T cells present in an allogeneic bone marrow transplant may produce graft-versus-host disease but also contribute to immune reconstitution and enhance engraftment. Our aim was to separate alloreactive from nonalloreactive T lymphocytes, by performing a mixed lymphocyte culture (MLC) stimulation of donor cells, followed by selective depletion of activated cells expressing the high-affinity interleukin 2 receptor. We then characterized the resulting depleted cell fraction.

METHODS

Donor peripheral blood mononuclear cells were cocultured with irradiated peripheral blood mononuclear cells from HLA-nonidentical recipient stimulators in an MLC. After 3 days, CD25+ lymphocytes (alloreactive cells expressing the alpha chain of the interleukin 2 receptor) were removed by immunomagnetic separation. The depleted donor fraction and untreated cells were then rechallenged in a secondary MLC with the original irradiated stimulator cells or a third party to assess relative alloreactivity.

RESULTS

Inhibition of the secondary MLC and of host-specific cytotoxic activities was observed as well as a disappearance of interleukin 2 receptor-positive cells. Alloreactivity against unrelated third-party cells was preserved. Limiting dilution analysis of residual alloantigen-reactive T lymphocytes demonstrated a 1.3 log reduction of antihost reactivity. The depletion largely removed host-specific alloreactive CD4+ cells.

CONCLUSIONS

This method reduces alloreactivity while retaining reactivity against third-party targets. This approach may allow therapeutic infusion of T cells after HLA-nonidentical allografts with a reduced capacity to produce graft-versus-host disease.

Persisting viruses and chronic inflammation: understanding their relation to autoimmunity

Viral infections may induce and sustain autoimmune processes via several and overlapping mechanisms. We outline how chronic inflammation, sustained by persisting viruses, may be "the prerequisite" for initiation and maintenance of the multistep process leading to autoimmunity. Chronic inflammation may favour priming of autoreactive T cells which have escaped thymic tolerance and are able to mount a cross-reactive response to self-mimicking antigens carried by viruses in the periphery. Moreover, chronic inflammation and persisting viruses can synergistically support autoimmunity through other relevant mechanisms: unveiling of cryptic self-epitopes, determinant spreading, activation of dendritic cells, constant priming of new autoreactive T cells, and efficient generation and restimulation of memory cells. Therefore, viruses seem to play a key role among the many environmental factors which, together with the genetic background, have been implicated in the pathogenesis of autoimmune diseases. We will also discuss some hypotheses explaining why autoimmunity is a rare event.

A conditioned dendritic cell can be a temporal bridge between a CD4+ T-helper and a T-killer cell

To generate an immune response, antigen-specific T-helper and T-killer cells must find each other and, because they cannot detect each other's presence, they are brought together by an antigen-loaded dendritic cell that displays antigens to both. This three-cell interaction, however, seems nearly impossible because all three cell types are rare and migratory. Here we provide a potential solution to this conundrum. We found that the three cells need not meet simultaneously but that the helper cell can first engage and 'condition' the dendritic cell, which then becomes empowered to stimulate a killer cell. The first step (help) can be bypassed by modulation of the surface molecule CD40, or by viral infection of dendritic cells. These results may explain the long-standing paradoxical observation that responses to some viruses are helper-independent, and they evoke the possibility that dendritic cells may take on different functions in response to different conditioning signals.

Differential Requirement for CD4 Help in the Development of an Antigen-Specific CD8+ T Cell Response Depending on the Route of Immunization

Previous studies in our laboratory have shown that DBA/2 mice injected i.p. with syngeneic P815 tumor cells transfected with the HLA-CW3 gene (P815-CW3) showed a dramatic expansion of activated CD8+CD62L− T cells expressing exclusively the Vβ10 segment. We have used this model to study the regulatory mechanisms involved in the development of the CW3-specific CD8+ response, with respect to different routes of immunization. Whereas both intradermal (i.d.) and i.p. immunization of DBA/2 mice with P815-CW3 cells led to a strong expansion of CD8+CD62L−Vβ10+ cells, only the i.d. route allowed this expansion after immunization with P815 cells transfected with a minigene coding for the antigenic epitope CW3 170–179 (P815 miniCW3). Furthermore, depletion of CD4+ T cells in vivo completely abolished the specific response of CD8+CD62L−Vβ10+ cells and prevented the rejection of P815-CW3 tumor cells injected i.p, whereas it did not affect CD8+CD62L−Vβ10+ cell expansion after i.d. immunization with either P815-CW3 or P815 miniCW3. Finally, the CW3-specific CD8+ memory response was identical whether or not CD4+ T cells were depleted during the primary response. Collectively, these results suggest that the CD8+ T cell response to P815-CW3 tumor cells injected i.p. is strictly dependent upon recognition of a helper epitope by CD4+ T cells, whereas no such requirement is observed for i.d. injection.

Activation Requirements, Lytic Mechanism, and Development of a Novel Anti-CD8-Resistant CTL Population

Almost all conventional CD8+ CTL and their CD8+ precursors are inhibited by anti-CD8 mAb. This requirement for CD8 function reflects both an avidity-augmentation role and a signal-transduction role for CD8 on T cells. We have, however, previously identified and partially characterized a novel functional population of CD8+, but anti-CD8-resistant, MHC class I-allospecific CTL. These CTL have unusual activation requirements in that their efficient generation in vitro requires inhibition of the CD8 avidity contribution (but not the CD8 signaling contribution), by anti-CD8 mAb. In this study, we have investigated the relationship of anti-CD8-sensitive and anti-CD8-resistant CTL by several criteria. These CTL populations share the phenotypic markers we have tested to date, they have similar but not identical Ag-specific repertoires, and they both appear to be generated from naive unprimed T cells. However, anti-CD8-sensitive and anti-CD8-resistant CTL populations exhibit important functional differences. They differ in their kinetics of activation in vitro, their dependence on exogenous cytokines, their use of lytic effector mechanisms, and their tissue distribution during ontogeny. Based on these results, we favor the hypothesis that these CTL populations represent distinct T cell lineages or subsets, and not merely different TCR avidity ranges within a single T cell lineage or subset.

CD4+ T cell help impairs CD8+ T cell deletion induced by cross-presentation of self-antigens and favors autoimmunity

Self-antigens expressed in extrathymic tissues such as the pancreas can be transported to draining lymph nodes and presented in a class I-restricted manner by bone marrow-derived antigen-presenting cells. Such cross-presentation of self-antigens leads to CD8+ T cell tolerance induction via deletion. In this report, we investigate the influence of CD4+ T cell help on this process. Small numbers of autoreactive OVA-specific CD8+ T cells were unable to cause diabetes when adoptively transferred into mice expressing ovalbumin in the pancreatic beta cells. Coinjection of OVA-specific CD4+ helper T cells, however, led to diabetes in a large proportion of mice (68%), suggesting that provision of help favored induction of autoimmunity. Analysis of the fate of CD8+ T cells indicated that CD4(+) T cell help impaired their deletion. These data indicate that control of such help is critical for the maintenance of CD8+ T cell tolerance induced by cross-presentation.

Major histocompatibility complex class I presentation of exogenously acquired minor alloantigens initiates skin allograft rejection

Major histocompatibility complex (MHC) class I molecules present peptides from endogenous proteins. However, in some cases class I-restricted peptides can also derive from exogenous antigens. This MHC class I exogenous presentation could be involved in minor histocompatibility antigen (mHAg)-disparate allograft rejection when donor alloantigens are not expressed in graft antigen-presenting cells (APC) that initiate the rejection mechanism. Here we addressed this question by using a skin graft experimental model where donors (H-2b or H-2d Tg beta-gal mice) expressed the mHAg like beta-galactosidase (beta-gal) in keratinocytes but not in Langerhans' cells (LC) which have an APC function. Rejection of Tg beta-gal skin by a beta-gal-specific CD8 cytotoxic T lymphocyte (CTL) effector mechanism should require presentation by donor and/or recipient LC of MHC class I-restricted peptides of exogenous beta-gal shed by keratinocytes. Indeed, our results showed that 1) H-2b Tg beta-gal skin was rejected by H-2bxs and H-2bxd recipients; 2) rejection was mediated by beta-gal-specific CD8+ CTL effectors; and 3) H-2bxd mice having rejected H-2b Tg beta-gal skin generated beta-gal-specific CTL restricted by H-2b and H-2d class I molecules and rejected subsequently grafted H-2d Tg beta-gal skin in an accelerated fashion, demonstrating that recipient LC have presented exogenous beta-gal-derived MHC class I epitopes. These results lead to the conclusion that MHC class I exogenous presentation of donor mHAg can initiate allograft rejection.

Differentiation and functions of T cell subsets

The Tc1 and Tc2 subsets of CD8+ T effector cells secrete different patterns of cytokines, but have similar functions, including perforin- and Fas-dependent cytotoxicity, and induction of delayed type hypersensitivity (DTH) reactions involving oedema and granulocytic infiltration. The characteristic cytokines of Tc1 (gamma-interferon) and Tc2 (interleukins 4 and 5) are expressed in vivo during the DTH reaction. Tc1 cells that are deficient in cytokine synthesis also induce similar levels of DTH, supporting the lack of correlation between CD8+ T cell cytokine patterns and DTH. CD8+ T cells often produce lower cytokine levels than CD4 cells because the CD8 cells kill their antigen-presenting cells before full stimulation can occur. This effect can be counteracted by increasing the frequency of stimulation, or using perforin-deficient T cells. A multiparameter analysis of cytokine effects on CD8+ T cell differentiation has been initiated, on the basis of the principle that normal immune responses involve complex cytokine mixtures. All combinations of seven cytokines were tested. In some combinations, the combined effect could not have been predicted from individual cytokine functions. Conditions were identified in which each of interleukins 4, 10 and 12 could have opposite effects on CD8+ T cell differentiation.

Perforin and Fas killing by CD8+ T cells limits their cytokine synthesis and proliferation

During an immune response, effector CD8+ T cells can kill infected cells by the perforin-dependent pathway. In comparison to CD4+ T cells, which are major sources of cytokines, normal CD8+ T cells produced less interleukin 2 and interferon gamma, and proliferated less vigorously after antigenic stimulation. Killing of target cells was a major cause of these reduced responses, since perforin-deficient CD8+ T cells showed substantially increased cytokine synthesis and proliferation. Cytotoxicity by the alternate Fas pathway also resulted in self-limitation of CD8+ T cell cytokine synthesis. This relationship between cytotoxicity and cytokine synthesis may regulate CD8+ T function in different phases of an immune response.

The immune response: the afferent arm

The key to understanding afferent immunity is the mechanism of activation of T lymphocytes by specialized antigen presenting cells, which bind antigenic peptide to Class II major histocompatibility molecules, and stimulate T cells via Signal 1 (antigen) and Signal 2 (costimulation). The best studied costimulatory pathway is the interaction of B7-1 or B7-2 ligand molecules on antigen presenting cells with CD28 or CTLA-4 receptors on T cells. T cell signaling occurs through the T cell receptor-CD3 complex and is augmented by cosignaling via CD4, CD8, and CD45. The activation of T cells to alloantigen occurs by either a direct pathway of recognition of allogenic major histocompatibility molecules (with or without an associated endogenous peptide), or by an indirect pathway of recognition of processed donor alloantigens via recipient antigen presenting cells. Afferent immunity on the musculoskeletal system is of special interest because of the absence of viable donor antigen presenting cells in processed grafts that makes them susceptible to the indirect pathway of alloantigen recognition.

Long-lasting CD8 T cell memory in the absence of CD4 T cells or B cells

The cellular basis of immunological memory has been a debated issue. It is not clear whether CD8 T cell memory is maintained by long-lived cells or by specific or nonspecific restimulation. Here, we have approached the question from a different angle, asking whether the cellular interactions that are required to maintain memory are the same as those necessary to activate cytotoxic T lymphocytes. We studied the CD8 memory response to the male antigen H-Y in mice deficient in CD4 cells, or B cells and found that memory in these mice was virtually unimpaired. These results suggest that CD8 memory is CD4 independent and that there is no requirement for long term retention of immune complexes on follicular dendritic cells, nor for B cells as antigen-presenting cells.

T cell tolerance and activation to a transgene-encoded tumor antigen

Much has been learned in recent years concerning the nature of tumor antigens recognized by T cells. To apply this knowledge clinically, the nature of the host response to individual and multiple tumor antigens has to be characterized. This will help to define the efficacy of immune surveillance and the immune status of the host following exposure to tumor antigens expressed on pre-neoplastic tissue. To approach these questions, we have developed a transgenic mouse which expresses the tumor-specific antigen P91A. The single amino acid substitution in P91A results in the expression of a new MHC class I (H-2Ld)-binding peptide. In transgenic tissue, the H-2Ld/P91A complex is expressed in isolation from other tumor-associated antigens, allowing definition of the immune response to a single defined tumor antigen, a situation closely analogous to events during tumorigenesis. We show that CD8+ T cell immune surveillance of P91A is ineffective without the introduction of a helper determinant operating through stimulation of CD4+ T cells. Recognition of the isolated P91A tumor antigen on normal tissue by CD8+ T cells is a tolerogenic process. Induction of T cell tolerance suggests tumor antigen-T cell interactions occurring during tumorigenesis may elicit T cell tolerance and hence confound some immunotherapeutic approaches.

Neonatal tolerance revisited: turning on newborn T cells with dendritic cells

For some time it has been thought that antigenic challenge in neonatal life is a tolerogenic rather than immunogenic event. Reexamination of the classic neonatal tolerance experiments of Billingham, Brent, and Medawar showed that tolerance is not an intrinsic property of the newborn immune system, but that the nature of the antigen-presenting cell determines whether the outcome is neonatal tolerance or immunization.

Mechanisms of peripheral tolerance and suppression induced by monoclonal antibodies to CD4 and CD8

Over the last five years it has become increasingly clear that the peripheral immune system can maintain tolerance to both self and non-self antigens through a variety of mechanisms. Although clonal deletion may play an important part in limiting rapidly expanding responses, there are many examples where antigen reactive T cells remain. It has been proposed that tolerance is maintained in this situation either by the induction of anergy or by ongoing suppression. The phenomenon known as immune deviation, where non-inflammatory Th2 responses could suppress Th1 and positively reinforce themselves provided an attractive explanation for infectious tolerance, where tolerant T cells could guide further naive T cells also to tolerance. However, experiments to test this hypothesis in the models of CD4 and CD8 antibody-induced tolerance have given conflicting data, with no clear evidence of Th2 responses in tolerant mice. In this paper we review recent data that IL-4 plays a role in suppression, but that the source of IL-4 may not be the tolerant/suppressor T cell. We also discuss how infectious tolerance can operate on third party antigens if they are linked on the same antigen presenting cell and how CD4+ T cells can suppress CD8+ T-cell responses. Finally, we suggest a model of infectious anergy that is compatible with the available data.

A quantitative analysis of antigen-presenting cell function: activated B cells stimulate naive CD4 T cells but are inferior to dendritic cells in providing costimulation

Ligation of CD28 on CD4 Th1 clones and freshly isolated mixtures of naive and memory CD4 T cells triggered their T cell receptors (TCR) is sufficient to induce the costimulatory signals necessary for interleukin 2 (IL-2) production by these cells. CTLA-4-reactive ligands expressed on antigen-presenting cells (APC) are critical in providing costimulatory signals to these T cell populations. We demonstrate that these activation characteristics apply equally to purified naive CD4 T cells. Because B cell blasts express CTLA-4-reactive ligands and high levels of adhesion and major histocompatibility complex class II molecules, they would be expected to engage both the TCR and CD28 and consequently stimulate IL-2 production by naive CD4 T cells. Using purified populations of cells in limiting dilution cultures, we have carried out a quantitative analysis of the interaction between naive CD4 T cells and either activated B or dendritic cells. We demonstrate that B cell blasts stimulate a high frequency of naive CD4 T cells. Slight differences in TCR signaling efficiency between the two APC types were observed. Even at optimal peptide concentrations, however, the amount of IL-2 made by individual T cells was fourfold lower in response to B cell blasts than to dendritic cells. This relative deficiency of activated B cells was due to their inability to optimally costimulate naive CD4 T cells.

Engineered human vaccines

The limitations of human vaccines in use at present and the design requirements for a new generation of human vaccines are discussed. The progress in engineering of human vaccines for bacteria, viruses, parasites, and cancer is reviewed, and the data from human studies with the engineered vaccines are discussed, especially for cancer and AIDS vaccines. The final section of the review deals with the possible future developments in the field of engineered human vaccines and the requirement for effective new human adjuvants.

Modern concepts in immune recognition and lymphocyte activation. Relevance for the development of useful vaccines

Adaptive immunity requires both specific recognition of an antigen and its translation into appropriate lymphocyte responses. This paper reviews the striking differences in B- and T-lymphocyte antigen recognition, details the pathways for conversion of protein antigens into peptide-major histocompatibility complex molecule ligands for T-cell receptors, and summarizes the roles of costimulatory signals in lymphocyte activation. This information is used to suggest new approaches for the rational design of vaccines.

CD4+8- help prevents rapid deletion of CD8+ cells after a transient response to antigen

We have followed the fate of mature CD8+ T cells with a male-specific transgenic T cell receptor after antigenic stimulation with hemopoietic cells in the absence or presence of help. Our data show that mature CD8+ T cells can be deleted after a 3-week period of transient activation and that help, e.g. in the form of interleukin-2, can considerably delay the deletion. These experiments have implications for the design of protocols aiming at the establishment of specific immunological tolerance in T cells.

Induction of long-term H-Y-specific tolerance in female mice given male lymphoid cells while transiently depleted of CD4+ or CD8+ T cells

Rejection of H-Y-bearing primary skin grafts and generation of H-Y-specific cytolytic T cells by female mice requires the participation of both CD4+ and CD8+ T lymphocytes. Studies were conducted to investigate long-term tolerance of H-Y antigen induced in female mice by transiently depleting them of CD4+ and/or CD8+ T cells and, at the same time, giving them an injection of male lymphoid cells. We confirmed that after recovery of CD4+ to normal levels, female mice that had been transiently depleted of CD4+ cells and concurrently given an injection of male spleen cells were unable to generate H-Y-specific cytolytic T cells. Tolerance was also manifest by greatly extended survival (probably permanent in most cases) of male skin grafts. Further investigations revealed that female mice transiently depleted of CD8+ cells, and concurrently given an injection of male spleen cells, were similarly tolerant of H-Y antigen later when numbers of CD8+ T cells returned to normal. Moreover, small numbers of male cells were detectable in spleen and lymph nodes of tolerant females long after they had been given an injection of male cells and depleted of either CD4+ or CD8+ T cells, whereas no male cells were detected in (nontolerant) females given male cells and control antibodies. These findings show that tolerance of the relatively weak transplantation antigen, H-Y, can be achieved simply by giving male antigen-bearing spleen cells to the host while it is transiently depleted of a type of cell it needs in order to reject those cells, thus allowing the male cells to persist in the host. Furthermore, depletion of helper cells is not obligatory to achieve tolerance. It has been hypothesized that tolerance of H-Y antigen in females given male lymphoid cells while temporarily depleted of CD4+ lymphocytes results from unresponsiveness (anergy) induced in H-Y-specific CD8+ cells that are exposed to H-Y antigen in the absence of help from CD4+ cells. Interpretations of our findings are discussed in relation to this hypothesis.

A fail-safe mechanism for maintaining self-tolerance

Using cytotoxic T lymphocyte (CTL) responses to the class I histocompatibility antigen Qa1 and to the minor histocompatibility antigen H-Y, we show that the immune system maintains a peripheral screening process that is able to tolerize a wide variety of potentially autoimmune CTL. The critical factor is the presence or absence of specific T helper cells. If T help is available, CTL precursors that recognize antigen are activated. In the absence of help, they are tolerized. Thus, T helper cells are guardians of peripheral tolerance in CTL.

Cytotoxic T lymphocyte precursor cells specific for the major histocompatibility complex class I-like antigen, Qa-2, require CD4+ T cells to become primed in vivo and to differentiate into effector cells in vitro

Experiments were performed to determine whether CD4+ T cells are required for the generation of cytotoxic T lymphocytes (CTL) specific for the nonpolymorphic major histocompatibility complex (MHC) class I-like antigen, Qa-2. Splenic T cells from BALB/cBy (Qa-2b) mice that had been immunized with irradiated BALB/cJ (Qa-2a) splenocytes generated CTL following in vitro stimulation with BALB/cJ splenocytes. These CTL lysed all Qa-2+, but not Qa-2- targets, regardless of the H-2 haplotypes of target cells or their non-MHC backgrounds. This apparent MHC class I-unrestricted recognition of Qa-2 antigen was confirmed using Qa-2-specific CTL clones. The Qa-2-primed CTL precursor cells (CTLp) and CTL were found to be CD8+ T cells. Primed splenocytes depleted of CD4+ T cells prior to culture failed to generate CTL, but addition of lymphokines to the culture restored the CTL generation. Stimulation of primed splenic T cells with irradiated Qa-2+ T blast cells, instead of splenocytes or B blast cells, led to little to no CTL generation, suggesting that MHC class II molecules are involved in the presentation of Qa-2 antigen to CD4+ T cells. This was also supported by the results of experiments using Qa-2+, class II- thymoma cells of BALB/c origin. Stimulation of the thymoma-primed splenic T cells with the mitomycin C-treated thymoma cells resulted in no generation of anti-Qa-2 CTL, despite the fact that high levels of CTL specific for minor histocompatibility (H) antigens and H-2d were generated by immunizing the corresponding allogeneic hosts with the thymoma. However, the addition of lymphokines rendered thymoma-primed T cells capable of generating anti-Qa-2 CTL. Both CD4+ and CD8+ T cell populations, isolated from the BALB/cJ splenocyte-primed responder cells, proliferated in vitro in response to the Qa-2+ splenocytes, suggesting that Qa-2-reactive CD4+ T cells were present in the immunized mice. Depletion of CD4+ T cells from thymectomized BALB/cBy mice with anti-L3T4 monoclonal antibodies markedly reduced, but did not eliminate anti-Qa-2 CTL generation. In contrast, depletion of CD8+ T cells led to a complete abrogation of the CTL response. Addition of lymphokines to the culture of responder cells depleted of either T cell subset did not restore their reactivity. It is concluded that anti-Qa-2 CTLp need "help" from CD4+ T cells to become primed in vivo. Furthermore, primed CTLp also need "help" or lymphokines provided by CD4+ T cells to differentiate into effector CTL in vitro.(ABSTRACT TRUNCATED AT 400 WORDS)

Tolerance to liver-specific antigens

We have described a TG model for peripheral tolerance of alloreactive CTL. Expression of Q10/L on hepatocytes renders mice functionally tolerant, although in vitro we observe that TG animals have normal numbers of CTL.Pf directed against this antigen. The basis for the tolerance presumably resides in the fact that the TG mice are lacking a subpopulation, either through deletion or anergy, that is responsible for recognition of the antigen on hepatocytes in vivo. The data are consistent with a tolerance model where cells with high affinity receptors are silenced. The presumed low affinity antigen-specific cells remaining in TG mice cannot be primed in vivo when immunized with antigen on spleen cells. Further, these CTL generate poor lytic activity in vitro. This failure to prime TG CTL in vivo could be attributed to primed cells traveling to the liver where they become tolerized when exposed to antigen on hepatocytes. However, we show that TG cells, after transfer to non-TG recipients, cannot be primed in vivo, indicating that the presumed low-affinity cells remaining in TG mice are not readily activable in this milieu. These data also indicate that this tolerance is not readily reversible during a 10- to 17-d time interval.

Clonal deletion versus clonal anergy: the role of the thymus in inducing self tolerance

During development in the thymus, T cells are rendered tolerant to self antigens. It is now apparent that thymocytes bearing self-reactive T cell receptors can be tolerized by processes that result in physical elimination (clonal deletion) or functional inactivation (clonal anergy). As these mechanisms have important clinical implications for transplantation and autoimmunity, current investigations are focused on understanding the cellular and molecular interactions that generate these forms of tolerance.