Induction of H-2-restricted cytotoxic T cells: in vivo induction has the appearance of being unrestricted

Pathways of Antigen Recognition by T Cells in Allograft Rejection

The adaptive immune response leading to the rejection of allogeneic transplants is initiated and orchestrated by recipient T cells recognizing donor antigens. T-cell allorecognition is mediated via 3 distinct mechanisms: the direct pathway in which T cells recognize allogeneic major histocompatibility complex (MHC) molecules on donor cells, the indirect pathway through which T cells interact with donor peptides bound with self-MHC molecules on recipient antigen-presenting cells, and the recently described semidirect pathway whereby T cells recognize donor MHC proteins on recipient antigen-presenting cells. In this article, we present a description of each of these allorecognition pathways and discuss their role in acute and chronic rejection of allogeneic transplants.

The evolving biology of cross-presentation

Cross-priming was first recognized in the context of in vivo cytotoxic T lymphocyte (CTL) responses generated against minor histocompatibility antigens induced by immunization with lymphoid cells. Even though the basis for T cell antigen recognition was still largely unclear at that time, these early studies recognized the implication that such minor histocompatibility antigens were derived from the immunizing cells and were obtained exogenously by the host's antigen presenting cells (APCs) that directly prime the CTL response. As antigen recognition by the T cell receptor became understood to involve peptides derived from antigens processed by the APCs and presented by major histocompatibility molecules, the "cross-priming" phenomenon was subsequently recast as "cross-presentation" and the scope considered for examining this process gradually broadened to include many different forms of antigens, including soluble proteins, and different types of APCs that may not be involved in in vivo CTL priming. Many studies of cross-presentation have relied on in vitro cell models that were recently found to differ from in vivo APCs in particular mechanistic details. A recent trend has focused on the APCs and pathways of cross-presentation used in vivo, especially the type 1 dendritic cells. Current efforts are also being directed towards validating the in vivo role of various putative pathways and gene candidates in cross-presentation garnered from various in vitro studies and to determine the relative contributions they make to CTL responses across various forms of antigens and immunologic settings. Thus, cross-presentation appears to be carried by different pathways in various types of cells for different forms under different physiologic settings, which remain to be evaluated in an in vivo physiologic setting.

DCs at the center of help: Origins and evolution of the three-cell-type hypothesis

Last year was the 10th anniversary of Ralph Steinman's Nobel Prize awarded for his discovery of dendritic cells (DCs), while next year brings the 50th anniversary of that discovery. Current models of anti-viral and anti-tumor immunity rest solidly on Steinman's discovery of DCs, but also rely on two seemingly unrelated phenomena, also reported in the mid-1970s: the discoveries of "help" for cytolytic T cell responses by Cantor and Boyse in 1974 and "cross-priming" by Bevan in 1976. Decades of subsequent work, controversy, and conceptual changes have gradually merged these three discoveries into current models of cell-mediated immunity against viruses and tumors.

A CB2-Selective Cannabinoid Suppresses T-Cell Activities and Increases Tregs and IL-10

We have previously shown that agonists selective for the cannabinoid receptor 2 (CB2), including O-1966, inhibit the Mixed Lymphocyte Reaction (MLR), an in vitro correlate of organ graft rejection, predominantly through effects on T-cells. Current studies explored the mechanism of this immunosuppression by O-1966 using mouse spleen cells. Treatment with O-1966 dose-relatedly decreased levels of the active nuclear forms of the transcription factors NF-κB and NFAT in wild-type T-cells, but not T-cells from CB2 knockout (CB2R k/o) mice. Additionally, a gene expression profile of purified T-cells from MLR cultures generated using a PCR T-cell activation array showed that O-1966 decreased mRNA expression of CD40 ligand and CyclinD3, and increased mRNA expression of Src-like-adaptor 2 (SLA2), Suppressor of Cytokine Signaling 5 (SOCS5), and IL-10. The increase in IL-10 was confirmed by measuring IL-10 protein levels in MLR culture supernatants. Further, an increase in the percentage of regulatory T-cells (Tregs) was observed in MLR cultures. Pretreatment with anti-IL-10 resulted in a partial reversal of the inhibition of proliferation and blocked the increase of Tregs. Additionally, O-1966 treatment caused a dose-related decrease in the expression of CD4 in MLR cultures from wild-type, but not CB2R k/o, mice. These data support the potential of CB2-selective agonists as useful therapeutic agents to prolong graft survival in transplant patients, and strengthens their potential as a new class of immunosuppressive agents with broader applicability.

Heat shock protein vaccination and directed IL-2 therapy amplify tumor immunity rapidly following bone marrow transplantation in mice

Tumor relapse is the primary cause of mortality in patients with hematologic cancers following autologous hematopoietic stem cell transplantation (HSCT). Vaccination early after HSCT can exploit both the state of lymphopenia and minimal residual disease for generating antitumor immunity. Here, multiple vaccinations using lymphoma cells engineered to secrete heat shock protein fusion gp96-Ig within 2 weeks of T cell-replete syngeneic HSCT led to cross-presentation and increased survival of lymphoma-bearing mice. To enhance vaccine efficacy, interleukin (IL)-2 was directed to predominantly memory phenotype CD8(+) T lymphocytes and natural killer (NK) cells via administration bound to anti-IL-2 monoclonal antibody clone S4B6 (IL-2S4B6). Combination therapy with gp96-Ig vaccination and coordinated infusions of IL-2S4B6 resulted in marked prolongation of survival, which directly correlated with ~500% increase in effector CD8(+) T-cell numbers. Notably, this dual regimen elicited large increases in both donor CD8(+) T and NK cells, but not CD4(+) T lymphocytes; the former 2 populations are essential for both vaccine efficacy and protection against opportunistic infections after HSCT. Indeed, IL-2S4B6-treated HSCT recipients infected with Listeria monocytogenes exhibited decreased bacterial levels. These preclinical studies validate a new strategy particularly well suited to the post-HSCT environment, which may augment adaptive and innate immune function in patients with malignant disease receiving autologous HSCT.

HLA-haploidentical donor lymphocyte infusions for patients with relapsed hematologic malignancies after related HLA-haploidentical bone marrow transplantation

Treatment of relapse after related HLA-haploidentical T cell-replete bone marrow transplantation (haploBMT) with post-transplantation cyclophosphamide (PTCy) using haploidentical donor lymphocyte infusion (haploDLI) is not documented. All patients who received haploDLI after haploBMT with PTCy between June 2003 and October 2012 were identified and assessed for graft-versus-host disease (GVHD) and outcomes. Forty patients received 52 haploDLI doses. Sixteen patients had acute myeloid leukemia, 11 had lymphomas, and 34 had nonmyeloablative conditioning before haploBMT. The median time from haploBMT to relapse was 183 (range, 0 to 1399) days. The median age at haploDLI was 48 (range, 3 to 70) years. The first haploDLI doses were 1 × 10(5) CD3(+) cells/kg with subsequent escalation. The most commonly used first haploDLI dose was 1 × 10(6) CD3(+) cells/kg. The median follow-up after haploDLI was 7 (mean, 15.4; range, .5 to 96) months for the entire cohort, and 17.5 (mean, 28; range, 2.4 to 96) months for the responders. Acute GVHD developed in 10 patients (25%), 6 patients had grade 3 to 4, and 3 developed chronic GVHD. Twelve (30%) patients achieved a complete response (CR) with a median duration of 11.8 (mean, 22.5; range, .4 to 94) months. At last follow-up, 8 responders were alive in CR; 6 for over a year. HaploDLI for relapse after haploBMT is associated with acceptable toxicities and can result in durable responses.

Immune recognition and rejection of allogeneic skin grafts

The transplantation of allogeneic skin grafts is associated with a potent inflammatory immune response leading to the destruction of donor cells and the rejection of the graft. Shortly after transplantation, skin dendritic cells (DCs) migrate out of the graft through lymphatic vessels and infiltrate the recipient's draining lymph nodes where they present donor antigens via two mechanisms: the direct pathway, in which T cells recognize intact donor MHC antigens on donor DCs; and the indirect pathway, involving T-cell recognition of donor peptides bound to self-MHC molecules on recipient DCs. Some recent studies have suggested that T cells can become activated via recognition of donor MHC molecules transferred on recipient antigen-presenting cells (semidirect pathway). Activation of T cells via direct or indirect allorecognition is sufficient to trigger acute rejection of allogeneic skin grafts. In addition, allospecific antibodies contribute to the rejection process either by killing allogeneic targets in a complement-dependent fashion or by opsonizing donor cells and forming immune complexes. Finally, several studies demonstrate that NK cells, activated due to missing self-MHC class I molecules on allogeneic cells, are involved in allogeneic skin graft rejection via direct killing of donor cells and through the production of proinflammatory cytokines including IFN-γ and TNF-α.

Cell-secreted Gp96-Ig-peptide complexes induce lamina propria and intraepithelial CD8+ cytotoxic T lymphocytes in the intestinal mucosa

Induction of mucosal immunity is critical for protection from enteric pathogens. Heat shock protein gp96 is one of the primary peptide and protein chaperones located in the endoplasmic reticulum. We reported previously that a cell-secreted gp96-Ig fusion protein (gp96-Ig) mediated strong systemic, antigen-specific CD8-CTL expansion in vivo. We now evaluate the mucosal immune response to stimulation by secreted gp96 using allogeneic NIH-3T3 transfected with ovalbumin (OVA) and gp96-Ig. A single intraperitoneal NIH-3T3-OVA-gp96-Ig immunization caused significant homing of OVA-specific TCR transgenic CD8 cells (OT-I) to Peyer's patches, to the intraepithelial compartment and to the lamina propria. Intraperitoneal immunization with cells secreting gp96-Ig provided stronger mucosal immunity than the same dose instilled vaginally or rectally or injected subcutaneously or intradermally. Our results provide the first evidence that cell-based gp96-Ig-secreting vaccines may serve as a potent modality to induce mucosal immunity.

CD8 T Cells Specific for a Donor-Derived, Self-Restricted Transplant Antigen Are Nonpathogenic Bystanders after Vascularized Heart Transplantation in Mice

CD8 T cell cross-priming, an established mechanism of protective antiviral immunity, was originally discovered during studies involving minor transplantation Ags. It is unclear whether or how cross-primed CD8 T cells, reactive to donor-derived, but recipient class I MHC-restricted epitopes, could injure a fully MHC-disparate, vascularized transplant. To address this question we studied host class I MHC-restricted, male transplantation Ag-reactive T cell responses in female recipients of fully MHC-disparate, male heart transplants. Cross-priming to the immune-dominant determinant HYUtyp occurred at low frequency after heart transplantation. CD8 T cell preactivation through immunization with HYUtyp mixed in CFA did not alter the kinetics of acute rejection. Furthermore, neither HYUtyp immunization nor adoptive transfer of HYUtyp-specific TCR-transgenic T cells affected outcome in 1) a model of chronic rejection in the absence of immunosuppression or 2) a model of allograft acceptance induced by costimulatory blockade. The results support the contention that CD8 T cells reactive to host-restricted, but donor-derived, Ags are highly specific and are nonpathogenic bystanders during rejection of MHC-disparate cardiac allografts.

Loading of MHC class I and II presentation pathways by exogenous antigens: a quantitative in vivo comparison

The MHC class I pathway is usually fueled by endogenous Ags, while exogenous Ags reach the MHC class II pathway. Although exogenous epitopes may also enter the MHC class I pathway, quantification of the efficiency of the process has remained a difficult task. In an attempt of such a quantification, we directly compared the amount of exogenous virus-like particles required for induction of cytotoxic T cell responses by cross-priming with the amount of virus-like particles required for induction of Th cell responses by the conventional route of MHC class II loading as an internal standard. Surprisingly, we found that cross-presentation of peptides derived from exogenous Ags on MHC class I molecules is of only marginally lower efficiency ( approximately 1- to 10-fold) than the classical MHC class II pathway in vitro and in vivo. Thus, Ag quantities required for cross-presentation and cross-priming are similar to those required for fueling the MHC class II pathway.

The history, evolution, and clinical use of dendritic cell-based immunization strategies in the therapy of brain tumors

Despite advancements in therapeutic regimens, the prognosis remains poor for patients with malignant gliomas. Specificity has been an elusive goal for current modalities, but immunotherapy has emerged as a potential means of designing more tumor-specific treatments. Dendritic cells (DC) are the specialized antigen presenting cells of the immune system and have served now as a platform for therapeutic immunizations against such cancers as lymphoma, multiple myeloma, melanoma, prostate cancer, renal cell carcinoma, non-small cell lung carcinoma, colon cancer, and even malignant gliomas. DC-based immunizations offer a number of advantages over traditional immunotherapeutic approaches to brain tumors, approaches that have proved promising despite concerns over central nervous system immune privilege and glioma-mediated immunosuppression. The future success of clinical trials will depend on the optimization and standardizing of procedures for DC generation, loading, and administration.

T-cell allorecognition and transplant rejection: a summary and update

Transplant biologists have made significant progress over the last 20 years towards unraveling the immunologic intricacies of allograft rejection. This large body of work has resulted in an improved understanding of T-cell allorecognition at a molecular level and has provided new insight into the functional consequences resulting from the allorecognition events. The findings suggest that the survival and the histologic features of a transplanted organ are influenced not only by the T-cell recognition pathway, but also by the frequency, the induced effector functions and the specific cellular targets of the alloreactive T-cell repertoire.

Role for heat shock proteins in the immune response to measles virus infection

Heat shock proteins (HSPs) are recognized for their support of protein metabolism. Interaction with viral proteins also enhances the development of innate and adaptive immune responses against the infecting agent. At the level of the infected cell, HSPs are uniquely expressed on the cell surface, where they represent targets of lymphokine activated killer cells. Necrosis of the infected cell releases complexes of HSP and viral protein, which, in turn, binds antigen-presenting cells (APCs). One effect of binding is to stimulate APC maturation and the release of proinflammatory cytokines, an adjuvant effect that prepares the way for adaptive immune responses. A second effect of binding is to direct the antigenic cargo of the HSP into endogenous MHC presentation pathways for priming of naive cytotoxic T cells (CTL) or activation of antigen-specific CTLs. This alternate pathway of antigen presentation is essential to CTL priming following primary brain infection. Using heat shock to elevate brain levels of HSP in a mouse model of measles virus (MV) persistent infection, we provide evidence supporting a role for HSPs in promoting cell-mediated viral clearance from brain. The findings highlight the probable relevance of HSPs to anti-MV immunity, suggesting novel routes of both therapeutic intervention and preventative measures.

Principles of tumor immunosurveillance and implications for immunotherapy

Although antigen loss variants, major histocompatibility (MHC) class I down-regulation, or the expression of inhibitory molecules may explain the failure of immunosurveillance against some tumors, this seems not to apply for many other solid peripheral or lymphohematopoietic tumors. Why then is immunosurveillance so ineffective and can it be improved? This review focuses on one important aspect of tumor immunity, namely the relevance of antigen dose and localization. Immune responses in vivo are induced in organized lymphoid tissues, i.e., in lymph nodes and spleen. The antigen dose that reaches secondary lymphoid organs over time is a crucial parameter that drives antiviral and antitumoral immune responses. Tumors use various strategies to prevent efficient presentation of their antigens in lymphoid organs. A major obstacle to the induction of an endogenous tumor-specific cytotoxic T lymphocyte (CTL) response is the inefficient presentation of tumor antigen on MHC class I molecules of professional antigen-presenting cells. Peripheral solid tumors that develop outside lymphoid organs are, therefore, often ignored by the immune system. In other situations, tumors - especially of lymphohematopoietic origin - may tolerize specific CTLs. Understanding tumor immunosurveillance is key to the design of efficient antitumor vaccines. Attempts to improve immunity to tumors include vaccination strategies to (a) provide the tumor antigen to secondary lymphoid organs using recombinant viruses or dendritic cells as carriers, (b) express costimulatory signals on tumor cells, or (c) improve the efficiency of cross-priming.

Cross-primed CD8(+) T cells mediate graft rejection via a distinct effector pathway

To prevent bystander destruction of healthy host tissues, cytotoxic CD8(+) T lymphocytes are fitted with specific receptors that direct their destructive forces specifically against chosen targets. We show here, however, that anti-H-Y monospecific, H-2(b-restricted MataHari CD8(+) T cells reject H-2(k) male skin grafts, with which they cannot directly interact. Such rejection is interferon-gamma-dependent and only occurs if the recipient endothelium expresses H-2(b). The findings suggest an alternate indirect effector pathway that requires processing and presentation of the donor H-Y antigen by recipient endothelium and have implications for both transplantation and autoimmune disease.

Antigen loading of MHC class I molecules in the endocytic tract

Major histocompatibility complex (MHC) class I molecules bind antigenic peptides that are translocated from the cytosol into the endoplasmic reticulum by the transporter associated with antigen processing. MHC class I loading independent of this transporter also exists and involves peptides derived from exogenously acquired antigens. Thus far, a detailed characterization of the intracellular compartments involved in this pathway is lacking. In the present study, we have used the model system in which peptides derived from measles virus protein F are presented to cytotoxic T cells by B-lymphoblastoid cells that lack the peptide transporter. Inhibition of T cell activation by the lysosomotropic drug ammoniumchloride indicated that endocytic compartments were involved in the class I presentation of this antigen. Using immunoelectron microscopy, we demonstrate that class I molecules and virus protein F co-localized in multivesicular endosomes and lysosomes. Surprisingly, these compartments expressed high levels of class II molecules, and further characterization identified them as MHC class II compartments. In addition, we show that class I molecules co-localized with class II molecules on purified exosomes, the internal vesicles of multivesicular endosomes that are secreted upon fusion of these endosomes with the plasma membrane. Finally, dendritic cells, crucial for the induction of primary immune responses, also displayed class I in endosomes and on exosomes.

Prevention of graft versus host disease by inactivation of host antigen-presenting cells

Graft versus host disease, an alloimmune attack on host tissues mounted by donor T cells, is the most important toxicity of allogeneic bone marrow transplantation. The mechanism by which allogeneic T cells are initially stimulated is unknown. In a murine allogeneic bone marrow transplantation model it was found that, despite the presence of numerous donor antigen-presenting cells, only host-derived antigen-presenting cells initiated graft versus host disease. Thus, strategies for preventing graft versus host disease could be developed that are based on inactivating host antigen-presenting cells. Such strategies could expand the safety and application of allogeneic bone marrow transplantation in treatment of common genetic and neoplastic diseases.

Receptor mediated and fluid phase pathways for internalization of the ER Hsp90 chaperone GRP94 in murine macrophages

Immunization of mice with GRP94, the endoplasmic reticulum (ER) Hsp90, elicits cytotoxic T lymphocyte (CTL) responses to chaperone-bound, source cell-derived peptides. Elicitation of a CTL response requires that GRP94-associated peptides be transferred onto major histocompatability complex (MHC) class I molecules, a process that is postulated to accompany GRP94 internalization by antigen presenting cells, such as macrophages (Mphi) and dendritic cells (DC). In studies of GRP94 uptake in elicited Mphi, we report that Mphi display specific cell surface binding of GRP94, and that surface-bound GRP94 can be internalized via receptor mediated endocytosis. GRP94 internalized by this pathway co-localized predominately with transferrin-positive early endosomes. At time periods of up to 20 minutes, little trafficking of GRP94 to the lysosomal compartment was observed. When GRP94 was present in the medium, and thus accessible to both receptor-mediated and fluid phase internalization pathways, internalization was modestly inhibited in the presence of yeast mannan, a competitive inhibitor of mannose/fucose receptor activity, and substantially inhibited by dimethylamiloride, an inhibitor of macropinocytosis. GRP94 internalized via macropinocytosis did not display prominent co-staining with the lysosomal marker LAMP-2. These data identify multiple pathways of GRP94 internalization and indicate that receptor-dependent uptake of GRP94 is not dependent upon its high mannose oligosaccharide moiety. Most significantly, these data demonstrate the existence of cell surface receptor(s), apparently unique to antigen presenting cells, that function in the binding and internalization of the ER chaperone GRP94.

In Vivo and In Vitro Activation of T Cells After Administration of Ag-Negative Heat Shock Proteins

Heat shock proteins (HSP) Hsp70 and gp96 prime class I-restricted cytotoxic T cells against Ags present in the cells from which they were isolated. The immunization capacity of HSPs is believed to rely on their ability to bind antigenic peptides. In this study, we employed the well-established OVA and β-galactosidase (β-gal) antigenic model systems. We show that in vitro long-term established OVA and β-gal-specific CTL clones release TNF-α and IFN-γ when incubated with Ag-negative Hsp70 and gp96. In the absence of antigenic peptides, HSP-mediated secretion of TNF-α and IFN-γ requires cell contact of the APC with the T cell but is not MHC-I restricted. Moreover, Hsp70 molecules purified from Ag-negative tissue, e.g., negative for antigenic peptide, are able to activate T cells in vivo, leading to significant higher frequencies in OVA-specific CD8+ T cells. In unprimed animals, these T cells lyse OVA-transfected cell lines and produce TNF-α and IFN-γ after Ag stimulus. Taken together our data show that, besides the well-established HSP/peptide-specific CTL induction and activation, a second mechanism exists by which Hsp70 and gp96 molecules activate T cells in vivo and in vitro.

Heat shock proteins and the antitumor T cell response

Heat shock proteins (HSP) have been shown to participate in the antitumor T cell response. First, HSP play a crucial role in the intracellular pathway for antigen processing where HSP can make complexes with a broad spectrum of cellular proteins and peptides through their chaperone functions. In this pathway, macrophages are required for processing the chaperoned peptides to make stable molecules with the major histocompatibility complex (MHC) class I molecules, even when HSP-peptide complexes are exogenously administered. Through this pathway, vaccination with HSP-peptide complexes is thus able to elicit the response of CD8+ T cells specific for the chaperoned peptides. These findings suggest an essential role of HSP in 'cross-priming' and their usefulness for antitumor vaccination with tumor peptides. Second, HSP have been suggested to be expressed on the cell surface by transformation and, in addition, to function as antigen-presenting molecules for double negative T cells. Third, HSP derived from tumor cells have reportedly been recognized by T cells with either T cell receptor (TCR)-alphabeta or TCR-gammadelta. These lines of evidence therefore indicate that HSP may be potentially promising target molecules for antitumor T cell immunotherapy.

Immunodominant minor histocompatibility antigen peptides recognized by cytolytic T lymphocytes primed by indirect presentation

BACKGROUND

Indirect presentation of minor histocompatibility antigens (HA) as revealed by cross-priming of H2 heterozygous recipients effectively primes minor HA-specific cytolytic T lymphocytes (CTLs). However, it is not known if indirect priming generates CTLs specific for the same set of immunodominant minor HA recognized by CTLs primed by direct spleen cell injections.

METHODS

In order to indirectly prime minor HA-specific CTLs, we implanted (C57BL/6 x B6.C-H2d)F1 recipients with BALB.B and BALB/c splenocytes loaded into immunoisolation devices that effectively preclude direct donor:host contact. Responder spleen cells from these recipients were stimulated in vitro to expand BALB.B- and BALB/c-specific CTLs to reveal classical cross-priming.

RESULTS

Tests of CTL specificity using (1) CXB recombinant inbred strain targets that express different arrays of BALB/c minor HA and (2) high-performance liquid chromatography fractions of peptides from BALB.B Kb and Db molecules revealed that anti-BALB.B CTLs were specific for two previously identified dominant peptides, CTT-2 and CTT-5, presented by Kb molecules. Variation of responders and priming cells resulted in CTL responses to additional dominant peptides that had been identified previously with CTLs generated by direct priming with spleen cell injections. Indirect priming was not limited to this set of peptides recognized by CTLs in vitro because devices loaded with cells devoid of the CTL-detected peptides primed for accelerated skin allograft rejection.

CONCLUSIONS

Indirect presentation of minor HA in vivo stimulates the generation of CTLs specific for a subset of dominant minor HA peptides recognized by CTLs primed by direct presentation.

Regression of tumors in mice vaccinated with professional antigen-presenting cells pulsed with tumor extracts

Vaccination with tumor extracts circumvents the need to identify specific tumor rejection antigens and extends the use of active immunotherapy to the vast majority of cancers, in which specific tumor antigens have not yet been identified. In this study we examined the efficacy of tumor vaccines comprised of unfractionated tumor material presented by professional antigen-presenting cells (APC): dendritic cells (DC) or macrophages (M phi). To enhance the relevance of these studies for human patients we used 2 poorly immunogenic murine tumor models and evaluated the effectiveness of the vaccination protocols in tumor-bearing animals. APC (in particular DC) pulsed with unfractionated extracts from these "poorly immunogenic" tumors were highly effective in eliciting tumor-specific cytotoxic T lymphocytes. A measurable CTL response could be detected after even a single immunization with tumor extract-pulsed DC. DC or M phi pulsed with tumor extract were also effective vaccines in tumor-bearing animals. In the murine bladder tumor (MBT-2) model a modest extension of survival and 40% cure rate was seen in the animal groups immunized with DC or M phi pulsed with MBT-2 tumor extract. DC or M phi pulsed with B16/F10.9 tumor extract were also remarkably effective in the B16 melanoma lung metastasis model, as shown by the observation that treatment with APC caused a significant reduction in lung metastases. Cumulatively, the CTL and immunotherapy data from the two murine tumor systems suggest that APC (in particular DC) pulsed with unfractionated cell extracts as a source of tumor antigen may be equally or more effective than genetically modified tumor vaccines.

Antigen-presenting cells pulsed with unfractionated tumor-derived peptides are potent tumor vaccines

Vaccination with peptides isolated from tumor cells circumvents the need for identifying specific tumor rejection antigens and extends the use of active immunotherapy to the majority of cancers where specific tumor antigens have not yet been identified. In this study, we examined the efficacy of tumor vaccines composed of unfractionated tumor peptides presented by antigen-presenting cells (APC) to induce cytotoxic T lymphocyte (CTL) responses and tumor immunity. RMA-S cells pulsed with peptides isolated from ovalbumin (OVA)-expressing tumor cells were highly effective at inducing primary, OVA-specific CTL responses in vitro and priming CTL responses in vivo. In addition, tumor peptide-pulsed RMA-S cells induced protective immunity in mice when challenged with OVA-expressing tumor cells. To enhance the clinical relevance of these studies, cells pulsed with tumor peptides were evaluated in the poorly immunogenic, B16/F10.9 melanoma post-surgical metastasis model. Treatment of tumor-bearing mice with peptide-pulsed RMA-S cells or with adherent splenocytes (enriched for professional APC) caused a significant reduction in lung metastases. The antimetastatic effect of peptide-pulsed splenocytes could be further enhanced by pretreating the cells with antisense oligonucleotides directed against the TAP-2 gene which was previously shown to increase the density of specific peptide/MHC class I complexes and thereby improve the APC function of the treated cells (Nair et el., J. Immunol. 1996. 156: 1772). This study suggests that APC loaded with unfractionated peptides derived from poorly immunogenic, highly metastatic tumor cells may represent a potent form of tumor vaccine.

Immunology and immunity studied with viruses

Immunity to viruses is used to define important biological parameters of immunology. Specificity, tolerance and T and B cell memory were analysed with murine model infections. The key parameters of antigen kinetics, localization and patterns of T and B cell response induction in maintaining memory and in causing deletion of reactive lymphocytes were compared for self and for viral foreign antigens. Evidence is reviewed that suggests that B cells essentially recognize antigen patterns, whereas T cells react against antigens newly brought into lymphoid tissues; antigens outside lymphoid tissues are ignored, and antigens always present in, or spreading too fast throughout, lymphoid tissues exhaust and delete T cell responses. Finally, effector mechanisms of antiviral immunity are summarized, as they vary with different viruses. On this basis immunological T and B cell memory against viruses is reviewed. Memory studies suggest that increased precursor frequencies of B and T cells appear to remain in the host independent of antigen persistence. However, in order to protect against cytopathic viruses, memory B cells have to produce antibody to maintain protective elevated levels of antibody: B cell differentiation into plasma cells is driven by persisting antigen. Similarly, to protect against infection with a non-cytopathic virus, cytotoxic T cells have to recirculate through peripheral organs. Activation and capacity to emigrate into solid tissues as well as cytolytic effector function are also dependent upon, and driven by, persisting antigen. Because no convincing evidence is yet available of the existence of identifiable B or T cells with specialized memory characteristics, the phenotype of protective immunological memory correlates best with antigen-driven activation of low frequency effector T cells and plasma cells.

The preferential cytolytic T lymphocyte response to immunodominant minor histocompatibility antigen peptides

C57BL/6 mice preferentially generate cytolytic T lymphocytes (CTL) to a limited number of immunodominant minor antigens and associated immunogenic peptides when primed with H2-matched Balb.B spleen cells despite multiple minor histocompatibility (H) antigen differences. We have examined the complexity of dominant H antigens recognized by these CTLs to estimate the number of peptides associated with single antigens. Peptides eluted from Kb molecules of lymphoblasts from Balb.B and CXB recombinant inbred (RI) strains were tested for sensitization of RMA-S cells for lysis by short-term C57BL/6 CTL lines specific for Balb.B and CXB strains. Anti-Balb.B CTLs recognized four Kb-bound peptides; subsets of these peptides were recognized by anti-CXB CTLs when tested with peptides from the respective CXB strains. Single peptides segregated independently among the CXB strains, confirming that single peptides were encoded by independently segregating alleles. These peptides were expressed in diverse inbred mouse strains and were recognized preferentially by C57BL/6 CTLs stimulated by different inbred mouse strains. This set of peptides was subclassified by their capacity to sensitize targets when presented in unfractionated mixtures of Kb-bound peptides. The peptide associated with the previously classified dominant CTT-2 antigen was the only peptide to strongly sensitize RMA-S cells for lysis under these conditions. These results suggest that dominant peptides have a wide strain distribution and may have a distinct advantage over dominated peptides in binding to class I molecules and/or in presentation to CTLs.

Cross-priming of minor histocompatibility antigen-specific cytotoxic T cells upon immunization with the heat shock protein gp96

Vaccination of mice with heat shock proteins isolated from tumor cells induces immunity to subsequent challenge with those tumor cells the heat shock protein was isolated from but not with other tumor cells (Udono, H., and P.K. Srivastava. 1994. J. Immunol. 152:5398-5403). The specificity of this immune response is caused by tumor-derived peptides bound to the heat shock proteins (Udono., H., and P.K. Srivastava. 1993. J. Exp. Med. 178:1391-1396). Our experiments show that a single immunization with the heat shock protein gp96 isolated from beta-galactosidase (beta-gal) expressing P815 cells (of DBA/2 origin) induces cytotoxic T lymphocytes (CTLs) specific for beta-gal, in addition to minor H antigens expressed by these cells. CTLs can be induced in mice that are major histocompatibility complex (MHC) identical to the gp96 donor cells (H-2d) as well as in mice with a different MHC (H-2b). Thus gp96 is able to induce "cross priming" (Matzinger, P., and M.J. Bevan. 1977. Cell. Immunol. 33:92-100), indicating that gp96-associated peptides are not limited to the MHC class I ligands of the gp96 donor cell. Our data confirm the notion that samples of all cellular antigens presentable by MHC class I molecules are represented by peptides associated with gp96 molecules of that cell, even if the fitting MHC molecule is not expressed. In addition, we extend previous reports on the in vivo immunogenicity of peptides associated gp96 molecules to two new groups of antigens, minor H antigens, and proteins expressed in the cytosol.

Fibroblasts as efficient antigen-presenting cells in lymphoid organs

Only so-called "professional" antigen-presenting cells (APCs) of hematopoietic origin are believed capable of inducing T lymphocyte responses. However, fibroblasts transfected with viral proteins directly induced antiviral cytotoxic T lymphocyte responses in vivo, without involvement of host APCs. Fibroblasts induced T cells only in the milieu of lymphoid organs. Thus, antigen localization affects self-nonself discrimination and cell-based vaccine strategies.

The effect of allogeneic presensitization on Hya-isograft survival

The effect of allogeneic presensitization on Hya isograft survival was reinvestigated in C57BL, C3H and (C57BL x C3H)F1 females. Three weeks after transplantation of H-2 or non-H-2 differing female or male skin grafts, a syngeneic male skin graft (Hya isograft) was transplanted onto female recipients. C57BL and (C57BL x C3H)F1 females pretreated with a female skin graft of any origin rejected the C57BL male isograft as a first-set, while females pretreated with a male skin graft rejected the C57BL male isograft in a second-set manner. The same happened with the C3H male skin graft transplanted onto previously challenged (C57BL x C3H)F1 females. C3H females pretreated with a C3H or CBA female graft rejected the C3H male isograft as a first-set, whereas those pretreated with a C3H or CBA male graft rejected the male C3H isograft in a second-set manner. However, after pretreatment with a BALB/c male skin graft, C3H females were not sensitized to Hya. They rejected the C3H male isograft slightly faster than was expected for the first graft but this accelerated rejection was not specific, since it occurred as well in C3H females pretreated with grafts of BALB/c females. The best explanation for this unsuccessful immunization of C3H females is that an inferior, non-H-2-dependent Hya recognition system is involved in male isograft rejection.

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.

Nuclear myxovirus-resistance protein Mx is a minor histocompatibility antigen

Minor histocompatibility antigens (MiHAgs) cause slow-to-rapid organ transplant rejection by immunocompetent hosts and mild-to-severe graft-versus-host reactions in immunosuppressed hosts. MiHAgs are allelic forms of major histocompatibility complex (MHC) class I-restricted self-antigens recognized by cytotoxic T cells and usually are defined immunogenetically. Although structurally not identified as yet, it is assumed that MiHAgs are internal cell antigens that are processed and then presented by MHC class I proteins similar to viral antigens. To define a MiHAg both molecularly and functionally, we took advantage of the allelic difference of the structurally characterized intracellular myxovirus-resistance protein (Mx) and investigated its antigenicity. Skin grafts from congenic Mx+ mice carrying the functional Mx1 gene were rejected by mice lacking a functional Mx1 gene (Mx- mice). In parallel, cytotoxic MHC class I-restricted effector T cells specific for Mx protein and the H-2Kk antigen (but not for several other allelic H-2 antigens) were strongly induced in Mx- mice immunized with spleen cells from interferon-treated Mx+ mice. These data show that allelic forms of cell internal proteins presented by MHC class I may act as MiHAgs.

Antiviral T cell competence and restriction specificity of mixed allogeneic (P1 + P2----P1) irradiation chimeras

Mixed irradiation bone marrow chimeras were prepared by reconstituting lethally irradiated C57BL/10 (B10) or B10.D2 mice with T cell-depleted bone marrow cells of B10 plus B10.D2 origin. These chimeras were healthy and survived well under conventional housing conditions and after experimental laboratory infections. Of a total of 17 chimeras tested, 2 died spontaneously or from the injected virus. Twelve of fifteen chimeras mounted a measurable cytotoxic T cell response to virus. Despite approximately equal percentages of B10 and B10.D2 lymphocytes in chimeras, cytotoxic T cell responses to vaccinia virus and lymphocytic choriomeningitis virus were mediated variably by either syngeneic or allogeneic donor lymphocytes; thus the H-2 type of effector T cells frequently did not correspond to the 50:50 distribution of spleen or peripheral blood lymphocytes. Cytotoxic responses were restricted exclusively to recipient H-2 type. All mixed chimeras examined were able to mount a good IgG response to vesicular stomatitis virus. These results confirm previous data suggesting that such mixed chimeras are healthy and immunocompetent and demonstrate strict recipient-determined restriction specificity of effector T cells; they also suggest that if T help is necessary for induction of virus-specific cytotoxic T cells, it does not require host-restricted interactions between helper T cells and precursor cytotoxic T cells.

The T-cell repertoire is heavily influenced by tolerance to polymorphic self-antigens

T cells with V beta 3+ alpha beta receptors are deleted by self-tolerance in mice with particular major histocompatibility complex/self-antigen combinations. This also occurs for other V beta elements. Polymorphism in the major histocompatibility complex and/or the self-antigens that cause massive deletion of T cells using particular V beta elements may be maintained by the need to balance the advantage of a diverse T-cell repertoire against the potential involvement of those elements in autoimmune disease.

Further evidence for H-2-unrestricted induction of minor histocompatibility antigens-specific T cell immunity in vivo

Antigenic requirements for inducing minor histocompatibility antigens (MIHA)-specific T cell immunity for second set rejection (SSR) of a MIHA-allogeneic tumor were studied. An intravenous injection of surprisingly small numbers (10(4)-10(5] of live allogenetic spleen cells (SC) effectively primed mice for SSR of the allogeneic tumor, and this immunity was developed as early as 2-3 days after injection of the SC. In contrast, sonication-disrupted allogeneic SC, which should be readily processed by host antigen presenting cells (APC), were not active as immunogens, even at a dose 1000 times higher than the minimum effective dose of live SC. The possibility that host APC preferentially receive MIHA antigens shed by live allogeneic SC for T cell activation was ruled out. These results demonstrated that antigen processing via conventional pathways is very little involved in the mechanism of T cell activation. Under such restricted experimental conditions, the induction phase but not the effector phase of the MIHA-specific T cell immunity was shown to be H-2-unrestricted.

Role of antigen-presenting cells in the cytotoxic T-cell response to minor histocompatibility antigens (MIHA). I. In vitro function of cells pulsed with MIHA in vivo

Although antigen-presenting cells (APC) appear to be able to process minor histocompatibility antigens (MIHA) expressed on allogeneic cells and present them in association with intrinsic H-2 of the APC in vivo, this does not occur in vitro. This could be due to fundamentally different mechanisms of antigen handling by APC in vitro, or it could represent compromised APC function. In order to distinguish these possibilities, we designed a system in which BALB/c spleen cells are transferred into an MIHA-disparate irradiated host and allowed to reside for 2-3 days; the spleen cells of the repopulated host are removed and used as stimulator cells for the CTL response of primed BALB/c responder cells to DBA/2 MIHA. These cells are referred to as in vivo-pulsed APC (IVP-APC). Donor BALB/c cells are able to pick up DBA/2 MIHA after a passage in DBA/2 hosts and efficiently present MIHA to primed CTL precursors to generate DBA/2-specific CTL. The donor cell type able to pick up and present MIHA is present in spleen but not thymus or bone marrow, is Thy-1.2 negative, Ia+, and nylon wool-adherent. Its stimulatory capacity is as efficient on a per cell basis as that of DBA/2 spleen cells. The generation of IVP-APC requires repopulation of the irradiated host, which must express MIHA foreign to the BALB/c donor cells. When we attempted to generate IVP-APC in H-2 incompatible hosts, we found that, although the IVP-APC could present H-2 antigens, they were unable to present MIHA in association with intrinsic APC H-2 antigens. Use of intra-H-2 recombinant strains as host mice indicated that compatibility of donor and host at the KI region of H-2 was essential for the generation of IVP-APC able to present apparently unprocessed MIHA to primed BALB/c responder cells. Thus, we were unable to reproduce the antigen-processing function of APC encountering antigen in situ using an adoptive transfer method of pulsing APC with MIHA in vivo. In addition, we suggest that our results may impose constraints on the formulation of models to account for the association of MIHA with H-2 antigens.

The localized primary cytotoxic T-cell response to cells expressing minor histocompatibility differences

Cytotoxic T lymphocytes (CTL) are able to eliminate P815 (DBA/2) mastocytoma cells growing in cerebrospinal fluid of BALB/c H-2-compatible but minor histocompatibility (H) antigen-different mice and in H-2-incompatible C3H/He mice. We examined the magnitude of the primary CTL response to multiple, minor H antigens and to determinants of the major histocompatibility complex (MHC) by using a direct cytolytic assay and limiting-dilution analysis to estimate CTL frequency. By these criteria, no obvious differences emerged, and the responses appeared comparable at the site of inflammatory process, despite differences in the number of clonal progenitors. Experiments with radiation chimeras showed evidence of a strong cytotoxic T-cell response against P815 cells in [(ddd X bbb)F1----ddd] and (F1----bbd), but not in (F1----bbb) radiation chimeras. Therefore, this cytotoxic T-cell response against minor H antigens obeys the postulated rules for thymic restriction of precursors. Compatibility at the H-2 D-end of the MHC is apparently sufficient to ensure a strong response.

T helper cell lines that augment in vivo cytotoxic T-cell responses to minor alloantigens

We have investigated the ability of long-term cultured T helper (Th) cell lines to help an in vivo cytotoxic T lymphocyte (CTL) response to non-H-2 alloantigens (minor antigens). Th cell lines specific for various single or undefined minor antigens were selected by regular restimulation with antigen in vitro. They were antigen specific and H-2 restricted in proliferation assays and were found to be able to help primary CTL responses to multiple minor antigens and secondary CTL responses to single minor antigens. Although the Th were antigen specific they did not determine the specificity of the CTL. Th cells were both necessary and limiting for an effective CTL response indicating that "helper-independent" CTL are not in themselves sufficient to generate a strong in vivo response. Under conditions where a CTL response was clearly H-2 restricted, Th cells were not. Thus, the Th cells appeared to be activated by reprocessed antigen rather than antigen on the surface of the injected antigenic cells even though the CTL themselves reacted directly to the injected antigen.

Differential expression of the repertoire in in vitro and in vivo responses to minor alloantigens

The response to multiple minor alloantigens in the secondary mixed lymphocyte culture (MLC) between mouse strains that are identical at the major histocompatibility complex (MHC) generally yields effector cytotoxic T lymphocytes (CTL) which show cross-reactive killing of most or all third-party mouse strains which also share MHC haplotypes. We have investigated the clonal diversity of CTL responses in vivo versus in vitro by examination of such cross-reactions, using CTL effector cells derived from a primary response, an in vivo secondary response, and an in vitro secondary MLC. CTL from these three responses were assayed on a panel of H-2k targets. Restimulation of antigen-primed spleen cells in vitro yielded CTL which were strongly cross-reactive on all targets, whereas the in vivo responses were much less so. We conclude that the set of clones which become cytotoxic effectors in vivo is much less diverse than the set which is primed on a first encounter with antigen and that powerful constraints must therefore operate on the specificity of in vivo responses to non-MHC antigens.

Computer-assisted analysis of in vivo cytotoxic T cell responses

An approach has been devised for analysing data generated in the study of in vivo cytotoxic T cell responses. The method calculates the total number of lytic units generated in the peritoneal cavity, or in cultures of the lymph nodes draining the rear footpad, following antigenic stimulation. The assessment of group differences is facilitated, and the method lends itself to computerised and hence objective data analysis.

Induction of cytotoxic T cell precursors in vivo. Role of T helper cells

Strain AS rats respond with two populations of cytotoxic T lymphocytes to stimulation in vitro by the major histocompatibility complex (MHC)-incompatible strain HL rat tumor (HL-A2T2). One is specific for MHC alloantigens present on both HL-A2T2 and normal HL targets, the other is tumor specific. The activation of these killer cells requires helper T lymphocytes. The tumor-specific helper cells depend on syngeneic radioresistant accessory cells to present the tumor antigens in an immunogenic form. The appropriate helper-accessory cell interaction results in the production of soluble factors which then induce the maturation of precursor cells into effective killer cells. Studies with a procedure for inducing negative selection of T cells in vivo showed that short-term exposure to HL-A2T2 tumor induced selection only for TH but not cytotoxic T lymphocyte precursors (CTLp). Simultaneous injection of supernatants from concanavalin A-activated spleen cell cultures, however, did produce selection of CTLp. These and other findings suggest that under normal circumstances in vivo, both signals (recognition of antigen and acceptance of maturation factors) are provided in the vicinity of an antigen presenting macrophage-like accessory cell.

Lethal GVHD across minor histocompatibility barriers: nature of the effector cells and role of the H-2 complex

Transfer of T-cells to heavily irradiated, H-2-compatible mice frequently leads to a high incidence of lethal graft-versus-host disease (GVHD). Lymphoid cells depleted of Thy1+ cells fail to cause GVHD. Studies with a variety of different, H-2-compatible, strain-combinations suggest that minor, histocompatibility antigens (minor HA) are the main targets for eliciting lethal GVHD. Experiments in which T-cells are negatively selected to minor HA by acute blood-to-lymph recirculation through irradiated hosts have indicated that the T-cells eliciting GVHD to minor HA, are H-2-restricted. In H-2-compatible hosts, the donor T-cells recognize the minor HA of the host and become temporarily trapped in the lymphoid tissues for 1-2 days; during this stage of negative selection, the donor T-cells entering the lymph are specifically devoid of cells able to elicit GVHD against the host, minor HA on further transfer. When the selection host is H-2-different with respect to the donor T-cells, by contrast, the T-cells ignore the host, minor HA and negative selection fails to occur. The T-cells recirculate normally and are unimpaired in their capacity to elicit GVHD on further transfer. By the use of various H-2-recombinant mice as selection hosts it has been shown that, as for T-cells exerting cell-mediated lympholysis (CML) to minor HA in vitro, the T-cells which elicit lethal GVHD to minor HA comprise two distinct subsets of H-2-restricted cells. One subset recognizes minor HA in the context of H-2K (or K end) molecules whereas the other is specific for minor HA-plus-H-2D. Curiously, in marked contrast to the findings on CML responses in vitro, no evidence has been found that H-2I-restricted T-cells contribute to GVHD, either as effector cells or as helper cells. Purified populations of Lyt 1-2+ T-cells have potent GVHD activity, whereas Lyt 1+2- cells fail to cause GVHD. Studies with various types of bone-marrow chimeras suggest that in the induction phase, T-cells recognize minor HA only on lymphohematopoietic cells. In the effector phase, by contrast, non-marrow-derived cells appear to be the main targets of attack. Although the pathogenesis of GVHD is poorly understood, the lethal form of the disease probably reflects the penetration of mucosal surfaces by pathogenic organisms, perhaps as the result of direct destruction of epithelial cells by minor HA-specific cytotoxic lymphocytes. Direct support for this notion has yet to be obtained.

Immune recognition of tumor cells in vivo. I. Role of H-2 gene products in T lymphocyte activation against minor histocompatibility antigens displayed by adenocarcinoma cells

Challenges from ADK-1t adenocarcinoma cells of BALB/c (H-2d, Mlsb) mouse origin are rejected by DBA/2 (H-2d, Mlsa) mice on the basis of differences in a limited number of minor histocompatibility antigens. This T lymphocyte-dependent reaction is highly specific, and efficiently triggered only by Ia+ leukocytes infiltrating the tumor mass. ADK-1t challenges depleted of Ia+ infiltrating BALB/c leukocytes grow and kill DBA/2 mice, whereas the simultaneous injection of Ia+-inactivated BALB/c leukocytes induces tumor rejection. The expression of Mlsb-incompatible determinants on the Ia+ BALB/c leukocyte membrane is irrelevant in the induction of this efficient T lymphocyte reaction against BALB/c minor histocompatibility antigens. By contrast, a critical requirement is H-2 matching between the Ia+ leukocytes and the recipient mice at the inductive phase of the reaction.

Features of T cells causing H-2-restricted lethal graft-vs.-host disease across minor histocompatibility barriers

Evidence is presented that T cells that produce lethal graft-vs.-host disease (GVHD) to minor histocompatibility antigens (minor HA) comprise discrete subgroups of H-2K- and H-2D-restricted T cells; double negative selection of T cells in irradiated H-2 recombinant mice was used to separate these two subgroups. No evidence could be found that I-restricted T cells contributed to GVHD, either as effector cells or helper cells. The (unprimed) precursor cells for GVHD expressed the Thy-1+, Lyt-1+/-2, Ia- phenotype. Studies in which H-2-semiallogeneic bone marrow chimeras were used as hosts for negative selection suggested that presentation of minor HA to T cells during the induction phase is controlled by marrow-derived cells; indirect evidence was obtained that these latter cells can "process" minor HA presented on H-2 different cells and thereby render the antigens immunogenic. Studies in which minor HA-different, H-2-compatible chimeras were re-irradiated and then injected with donor-vs.-host T cells suggested that the effector phase of lethal GVHD involves contact of antigen on non-marrow-derived cells.