Cognate peptides induce self-destruction of CD8+ cytolytic T lymphocytes

Enhancing cellular immunotherapies in cancer by engineering selective therapeutic resistance

Adoptive cell therapies engineered to express chimeric antigen receptors (CARs) or transgenic T cell receptors (TCRs) to recognize and eliminate cancer cells have emerged as a promising approach for achieving long-term remissions in patients with cancer. To be effective, the engineered cells must persist at therapeutically relevant levels while avoiding off-tumour toxicities, which has been challenging to realize outside of B cell and plasma cell malignancies. This Review discusses concepts to enhance the efficacy, safety and accessibility of cellular immunotherapies by endowing cells with selective resistance to small-molecule drugs or antibody-based therapies to facilitate combination therapies with substances that would otherwise interfere with the functionality of the effector cells. We further explore the utility of engineering healthy haematopoietic stem cells to confer resistance to antigen-directed immunotherapies and small-molecule targeted therapies to expand the therapeutic index of said targeted anticancer agents as well as to facilitate in vivo selection of gene-edited haematopoietic stem cells for non-malignant applications. Lastly, we discuss approaches to evade immune rejection, which may be required in the setting of allogeneic cell therapies. Increasing confidence in the tools and outcomes of genetically modified cell therapy now paves the way for rational combinations that will open new therapeutic horizons.

The Biological Significance of Trogocytosis

Trogocytosis is the intercellular transfer of membrane and membrane-associated proteins between cells. Trogocytosis is an underappreciated phenomenon that has historically routinely been dismissed as an artefact. With a greater understanding of the process and the implications it has on biological systems, trogocytosis has the potential to become a paradigm changer. The presence on a cell of molecules they don't endogenously express can alter the biological activity of the cell and could also lead to the acquisition of new functions. To better appreciate this phenomenon, it is important to understand how these intercellular membrane exchanges influence the function and activity of the donor and the recipient cells. In this chapter, we will examine how the molecules acquired by trogocytosis influence the biology of a variety of systems including mammalian fertilization, treatment of hemolytic disease of the newborn, viral and parasitic infections, cancer immunotherapy, and immune modulation.

Lymphocytes and Trogocytosis-Mediated Signaling

Trogocytosis is the intercellular transfer of membrane and membrane-associated molecules. This underappreciated process has been described in a variety of biological settings including neuronal remodeling, fertilization, viral and bacterial spread, and cancer, but has been most widely studied in cells of the immune system. Trogocytosis is performed by multiple immune cell types, including basophils, macrophages, dendritic cells, neutrophils, natural killer cells, B cells, γδ T cells, and CD4+ and CD8+ αβ T cells. Although not expressed endogenously, the presence of trogocytosed molecules on cells has the potential to significantly impact an immune response and the biology of the individual trogocytosis-positive cell. Many studies have focused on the ability of the trogocytosis-positive cells to interact with other immune cells and modulate the function of responders. Less understood and arguably equally important is the impact of these molecules on the individual trogocytosis-positive cell. Molecules that have been reported to be trogocytosed by cells include cognate ligands for receptors on the individual cell, such as activating NK cell ligands and MHC:peptide. These trogocytosed molecules have been shown to interact with receptors on the trogocytosis-positive cell and mediate intracellular signaling. In this review, we discuss the impact of this trogocytosis-mediated signaling on the biology of the individual trogocytosis-positive cell by focusing on natural killer cells and CD4+ T lymphocytes.

An emerging role for Serine Protease Inhibitors in T lymphocyte immunity and beyond

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmo-regulation, tissue re-modeling and immunity to infection. T lymphocytes are required for adaptive cell mediated immunity and serine proteases are not only important for effector function but also homeostatic regulation of cell numbers. Serine Protease Inhibitors (Serpins) are the physiological regulators of serine proteases activity. In this review, I will discuss the role of serpins in controlling the recognition of antigen, effector function and homeostatic control of T lymphocytes through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This will be discussed in the context of the T lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. The potent anti-apoptotic properties of serpins can also work against adaptive cell immunity by protecting viruses and tumors from eradication by cytotoxic T cells (CTL). Recent insights from knock-out mouse models demonstrate that these serpins also are required for hematological progenitor cells and so are critical for the development of lineages other than T lymphocytes. Given the emerging role of serpins in multiple aspects of lymphocyte immunity and blood development I will review the progress to date in developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

MHC-restricted fratricide of human lymphocytes expressing survivin-specific transgenic T cell receptors

The apoptosis inhibitor protein survivin is overexpressed in many tumors, making it a candidate target molecule for various forms of immunotherapy. To explore survivin as a target antigen for adoptive T cell therapy using lymphocytes expressing survivin-specific transgenic T cell receptors (Tg-TCRs), we isolated HLA-A2-allorestricted survivin-specific T cells with high functional avidity. Lymphocytes expressing Tg-TCRs were derived from these T cells and specifically recognized HLA-A2+ survivin+ tumor cells. Surprisingly, HLA-A2+ but not HLA-A2- lymphocytes expressing Tg-TCRs underwent extensive apoptosis over time. This demise was caused by HLA-A2-restricted fratricide that occurred due to survivin expression in lymphocytes, which created ligands for Tg-TCR recognition. Therefore, survivin-specific TCR gene therapy would be limited to application in HLA-A2-mismatched stem cell transplantation. We also noted that lymphocytes that expressed survivin-specific Tg-TCRs killed T cell clones of various specificities derived from HLA-A2+ but not HLA-A2- donors. These results raise a general question regarding the development of cancer vaccines that target proteins that are also expressed in activated lymphocytes, since induction of high-avidity T cells that expand in lymph nodes following vaccination or later accumulate at tumor sites might limit themselves by self-MHC-restricted fratricide while at the same time inadvertently eliminating neighboring T cells of other specificities.

Serine protease inhibitors and cytotoxic T lymphocytes

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmoregulation, tissue remodeling, and immunity to infection. Cytotoxic T lymphocytes (CTLs) are required for adaptive cell-mediated immunity to intracellular pathogens by killing infected cells and through the development of memory T cells. Serine proteases not only allow a CTL to kill but also impose homeostatic control on CTL number. Serine protease inhibitors (serpins) are the physiological regulators of serine proteases' activity. In this review, I discuss the role of serpins in controlling the recognition of antigen, effector function, and homeostatic control of CTLs through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This view is discussed in the context of the T-lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. Given the important role serpins play in CTL immunity, I discuss the potential for developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

Can oligomeric T-cell receptor be used as a tool to detect viral peptide epitopes on infected cells?

We have utilized soluble HIV Gag-specific T-cell receptor (TCR) D3 with low affinity and TCR-like antibody 25-D1.16 recognizing its natural peptide-MHC (pMHC) ligand with high affinity to determine how affinity and off-rate of the receptor-pMHC interactions affect the sensitivity of pMHC detection on the cell surface. We found that with soluble TCR cognate pMHCs can be detected only at relatively high cell surface densities when the TCR was oligomerized using either Streptavidin or quantum dot (QD) scaffolds. While the higher affinity probe led to a greater sensitivity of pMHC detection, monomers and oligomers of the probe showed essentially the same detection limit, which is restricted by the sensitivity of standard flow cytometry technique. We have also shown that imaging of QD/TCR specifically bound to cognate pMHC on the cell surface yielded a very bright fluorescent signal that can enhance the sensitivity of viral peptide detection on infected cells.

Fluorescence-activated cell sorting and cloning of bona fide CD8+ CTL with reversible MHC-peptide and antibody Fab' conjugates

The isolation of subsets of Ag-specific T cells for in vitro and in vivo studies by FACS is compromised by the fact that the soluble MHC-peptide complexes and Abs used for staining, especially when combined, induce unwanted T cell activation and eventually apoptosis. This is especially a problem for CD8+ CTL, which are susceptible to activation-dependent cell death. In this study, we show that reversible MHC-peptide complexes (tetramers) can be prepared by conjugating MHC-peptide monomers with desthiobiotin (DTB; also called dethiobiotin) and multimerization by reaction with fluorescent streptavidin. While in the cold these reagents are stable and allow good staining, they rapidly dissociate in monomers at elevated temperatures, especially in the presence of free biotin. FACS cloning of Melan-A (MART-1)-specific CTL from a melanoma-infiltrated lymph node with reversible HLA-A2 Melan-A26-35 multimers yielded over two times more clones than when using the conventional biotin-containing multimers. CTL clones obtained by means of reversible multimers killed Melan-A-positive tumor cells more efficiently as compared with clones obtained with the stable multimers. Among the CTL obtained with the reversible multimers, but much less among those obtained with the stable multimers, a high proportion of clones exhibited high functional and physical avidity and died upon incubation with soluble MHC-peptide complexes. Finally, we show that Fab' of an anti-CD8 Ab can be converted in reversible DTB streptavidin conjugates the same way. These DTB reagents efficiently and reversibly stained murine and human CTL without affecting their viability.

Serine protease inhibitor 6 protects cytotoxic T cells from self-inflicted injury by ensuring the integrity of cytotoxic granules

How cytotoxic T lymphocytes (CTLs) kill intracellular pathogens without killing themselves has been a recurring question ever since their discovery. By using mice deficient in Serine Protease Inhibitor 6 (Spi6), we show that by inhibiting granzyme B (GrB), Spi6 protects CTLs from self-inflicted injury. Infection with either Lymphocytic Choriomeningitis virus (LCMV) or Listeria monocytogenes (LM) revealed increased apoptosis and diminished survival of Spi6 knockout (KO) CTLs, which was cell autonomous and could be corrected by GrB deficiency. Spi6 KO mice in turn were impaired in their ability to clear LCMV infection. Spi6 KO CTLs revealed a breakdown in the integrity of cytotoxic granules, increased cytoplasmic GrB, and ensuing apoptosis. We conclude that Spi6 protects CTLs from suicide caused by GrB-mediated breakdown of cytotoxic granules.

Soluble MHC-peptide complexes containing long rigid linkers abolish CTL-mediated cytotoxicity

Soluble MHC-peptide (pMHC) complexes induce intracellular calcium mobilization, diverse phosphorylation events, and death of CD8+ CTL, given that they are at least dimeric and co-engage CD8. By testing dimeric, tetrameric, and octameric pMHC complexes containing spacers of different lengths, we show that their ability to activate CTL decreases as the distance between their subunit MHC complexes increases. Remarkably, pMHC complexes containing long rigid polyproline spacers (> or =80 A) inhibit target cell killing by cloned S14 CTL in a dose- and valence-dependent manner. Long octameric pMHC complexes abolished target cell lysis, even very strong lysis, at nanomolar concentrations. By contrast, an altered peptide ligand antagonist was only weakly inhibitory and only at high concentrations. Long D(b)-gp33 complexes strongly and specifically inhibited the D(b)-restricted lymphocytic choriomeningitis virus CTL response in vitro and in vivo. We show that complications related to transfer of peptide from soluble to cell-associated MHC molecules can be circumvented by using covalent pMHC complexes. Long pMHC complexes efficiently inhibited CTL target cell conjugate formation by interfering with TCR-mediated activation of LFA-1. Such reagents provide a new and powerful means to inhibit Ag-specific CTL responses and hence should be useful to blunt autoimmune disorders such as diabetes type I.

Delivering the kiss of death

Cytotoxic T lymphocytes and natural killer cells kill their targets by secreting specialized granules that contain potent cytotoxic molecules. Through the study of rare immunodeficiency diseases in which this granule pathway of killing is impaired, proteins such as Rab27a have been identified as components of the secretory machinery of these killer cells. Recent evidence suggests that the destruction of activated lymphocytes through granule-mediated killing may be an important mechanism of immunological homeostasis. Although the process by which this occurs is not yet known, it is possible that events taking place at the immunological synapse may render the killer cell susceptible to fratricidal attack by other killer cells.

Antigen-specific targeting of CD8+ T cells with receptor-modified T lymphocytes

Chimeric receptors that link ligand recognition domains, such as antibody Fv fragments, with TCR signaling domains can redirect T lymphocytes against MHC-unrestricted targets. Such receptor-modified T lymphocytes have shown promise in the treatment of infectious diseases and cancer. We hypothesized that receptor-modified T lymphocytes may also be designed to target antigen-specific T cells. We synthesized chimeric receptors consisting of the extracellular and transmembrane domains of the class I MHC H-2K(b) molecule linked to the signaling domains of either TCR-zeta, CD28 and zeta, or CD28, zeta, and lck. T lymphocytes modified to express these receptors and pulsed with antigenic peptide specifically killed precursor CTL. Cytolysis was efficient, even at effector:target ratios of less than one, and specific, selectively killing antigen-specific precursor CTL among a mixed population of T cells. Cytolysis required activation of the receptor-modified T cells, and did not occur with a signaling-deficient chimeric receptor. In contrast to precursor CTL, differentiated CTL proved resistant to lysis by the receptor-modified T cells. These data demonstrate the feasibility of redirecting T lymphocytes against antigen-specific T cells. Receptor-modified T cells expressing chimeric MHC receptors have potential application in autoimmune and alloimmune diseases.

Class I negative CD8 T cells reveal the confounding role of peptide-transfer onto CD8 T cells stimulated with soluble H2-Kb molecules

Crosslinking of the T cell receptor has been proposed to be a prerequisite for T cell activation. Although the evidence supports this notion for CD4 T cells, the situation for CD8 T cells is less clear. Soluble class I monomers have been used to determine activation requirements in vitro with contradictory results. The possibility of transfer of peptide from soluble class I molecules onto class I molecules present on the surface of CD8 T cells, with ensuing presentation to other CD8 T cells, has been widely ignored. We show that monomeric and tetrameric class I molecules as well as free peptide can stimulate naive CD8 T cells in vitro. We generate and characterize CD8 T cells that express the OT-I T cell receptor (for K(b)/SIINFEKL) yet lack K(b) and D(b) molecules, and show that their activation requirements differ from their class I positive counterparts when stimulated with soluble K(b) molecules. By eliminating the confounding effect of peptide transfer, we unmask the true activation requirements for naive CD8 T cells and show that multivalent engagement of T cell receptors, as well as costimulation, is required for optimal stimulation.

Soluble peptide-MHC monomers cause activation of CD8+ T cells through transfer of the peptide to T cell MHC molecules

T cell receptor (TCR)-mediated activation of CD4(+) T cells is known to require multivalent engagement of the TCR by, for example, oligomeric peptide-MHC complexes. In contrast, for CD8(+) T cells, there is evidence for TCR-mediated activation by univalent engagement of the TCR. We have here compared oligomeric and monomeric L(d) and K(b) peptide-MHC complexes and free peptide as stimulators of CD8(+) T cells expressing the 2C TCR. We found that the monomers are indeed effective in activating naive and effector CD8(+) T cells, but through an unexpected mechanism that involves transfer of peptide from soluble monomers to T cell endogenous MHC (K(b)) molecules. The result is that T cells, acting as antigen-presenting cells, are able to activate other naive T cells.

Activation-induced cell death in human T cells is a suicidal process regulated by cell density but superantigen induces T cell fratricide

Repeated ligation of the TCR results in apoptosis (activation-induced cell death; AICD). Superantigens such as Staphylococcal enterotoxin B (SEB) are particularly efficient at inducing AICD in T cells. We investigated whether apoptosis in human T cell subsets was due to fratricide (killing of neighboring cells) or suicide (cell autonomous death). AICD of Th1, Th2, Tc1, and Tc2 effector cells was dramatically enhanced at low cell densities and could be observed in single cell microcultures. AICD was unaffected by adhesion molecules or neighboring cells undergoing AICD, confirming the predominance of a suicidal mechanism. However, SEB was able to induce fratricidal apoptosis of type 1, but not type 2 cells. Fratricide was also observed when unstimulated T cells were exposed to activated Tc1 effector cells. Thus, AICD is tightly regulated to allow clonal T cell expansion and memory cell generation, but superantigens may subvert this process by allowing T cell fratricide.

Surface cathepsin B protects cytotoxic lymphocytes from self-destruction after degranulation

The granule exocytosis cytotoxicity pathway is the major molecular mechanism for cytotoxic T lymphocyte (CTL) and natural killer (NK) cytotoxicity, but the question of how these cytotoxic lymphocytes avoid self-destruction after secreting perforin has remained unresolved. We show that CTL and NK cells die within a few hours if they are triggered to degranulate in the presence of nontoxic thiol cathepsin protease inhibitors. The potent activity of the impermeant, highly cathepsin B-specific membrane inhibitors CA074 and NS-196 strongly implicates extracellular cathepsin B. CTL suicide in the presence of cathepsin inhibitors requires the granule exocytosis cytotoxicity pathway, as it is normal with CTLs from gld mice, but does not occur in CTLs from perforin knockout mice. Flow cytometry shows that CTLs express low to undetectable levels of cathepsin B on their surface before degranulation, with a substantial rapid increase after T cell receptor triggering. Surface cathepsin B eluted from live CTL after degranulation by calcium chelation is the single chain processed form of active cathepsin B. Degranulated CTLs are surface biotinylated by the cathepsin B-specific affinity reagent NS-196, which exclusively labels immunoreactive cathepsin B. These experiments support a model in which granule-derived surface cathepsin B provides self-protection for degranulating cytotoxic lymphocytes.

How do cultured CD8(+) murine T cell clones survive repeated ligation of the TCR?

Many murine T cell clones grow continuously in culture despite weekly ligation of their TCR by antigen. To learn how the cultured cells avoid or minimize antigen-induced cell death (AICD), we compared Fas and tumor necrosis factor (TNF) receptors (TNFR) on several long-term cultured CD8(+) T cell clones with those on naive and activated naive cells expressing the same TCR (2C). In contrast to the naive cells, Fas was absent on the cultured clones and the TNFR-II receptor, present initially at high levels on the cultured cells, was rapidly down-modulated in response to TCR ligation and had virtually disappeared by 2 h, when only approximately 10% of the cloned cells had been induced to express TNF-alpha. The extent of AICD of the cultured clones in response to cognate peptide-MHC on the presenting cells used for routine stimulation of the cultures was also considerably less than the massive cell death of the clones following exposure to anti-CD3 antibody plate-bound at high density.

Antigen presentation of a modified tumor-derived peptide by tumor infiltrating lymphocytes

CD8+ T-lymphocytes recognize peptides in the context of major histocompatibility complex (MHC) class I antigens. Upon activation, these cells differentiate into effector cytotoxic T lymphocytes (CTL) and no longer require formal antigen presentation by professional antigen presenting cells (APC). Subsequently, any cell expressing MHC class I/cognate peptide can stimulate CTL. Using TIL specific for a melanoma antigen-derived peptide, IMDQVPFSV (g209 2M), we sought to determine whether these CTL could present peptide to each other. Our findings demonstrate that peptide presentation of the g209 2M peptide epitope by TIL is comparable to conventional methods of using T2 cells as APC. We report here that CTL are capable of self-presentation of antigenic peptide to neighboring CTL resulting in IFN-gamma secretion, proliferation, and lysis of peptide-loaded CTL. These results demonstrate that human TIL possess both APC functions as well as cytotoxic functions and that this phenomenon could influence CTL activity elicited by immunotherapy.

Peptide vaccines against hepatitis B virus: from animal model to human studies

An estimated 400 million people are chronically infected with the hepatitis B virus (HBV). Chronic viral hepatitis infection incurs serious sequelae such as liver cirrhosis and hepatocellular carcinoma. Prevention and treatment, thus, represent an important target for public health. Preventive vaccines using HBsAg alone or combined with other antigens allow for the generation of neutralizing antibodies which effectively prevent infection in immunocompetent individuals. Cell-mediated immunological mechanisms are thought to be crucial in determining viral persistence or viral elimination. Therapeutic approaches aiming to shift cellular immunity towards viral elimination have been on the research agenda for many years. This paper summarizes pre-clinical and clinical results obtained with the use of immunogenic peptides formulated as vaccines to selectively boost cellular immune responses. Such vaccines are capable of generating cellular immune responses in animal models as well as in humans and represent an important step towards the development of a therapeutic vaccine against chronic hepatitis.

Suicide induced by cytolytic activity controls the differentiation of memory CD8(+) T lymphocytes

Cytotoxic T lymphocytes (CTL) confer protection against intracellular pathogens, yet the mechanism by which some escape activation induced cell death (AICD) and give rise to long-lived memory cells is unclear. We studied the differentiation of transgenic TCR CD8(+) cells into CTL and memory cells using a novel system that allowed us to control cytolytic activity. The perforin/granzyme granules used to lyse targets induced the apoptosis of CTL in a fratricide-independent manner. After adoptive transfer to antigen-free mice, the ability of CTL to give generate memory cells was determined. We found that the extent of cytolysis by a common pool of CTL controlled the differentiation into memory cells, which were only generated under conditions of minimal cytolytic activity. Thus, the differentiation of naive CD8(+) cells into memory cells may not depend on the presence on a subset of committed CTL precursors, but rather is controlled by the extent of granule-mediated cytolysis.

Counter-regulation of cytolytic activity and cytokine production in HIV-1-specific murine CD8+ cytotoxic T lymphocytes by free antigenic peptide

We have reported previously that the cytolytic activity of murine CD8(+) cytotoxic T lymphocytes (CTL) specific for HIV-1 gp160 envelope glycoprotein was markedly inhibited by brief exposure to the free minimal antigenic peptide (I-10: 10mer peptide from gp160) by direct binding to class I MHC molecules of specific CTL in the absence of antigen-presenting cells (APC). Here, we show that treatment of such CTL with the peptide induced not only the inhibition of cytolytic activity but also IL-2Rbeta down-modulation, followed by the inhibition of IL-2-dependent growth. The peptide-mediated inhibition and restoration of expression of IL-2Rbeta were well correlated with changes in both cytolytic activity and IL-2-dependent growth of the CTL. Since enzymatic activity of granzyme B, and mRNA expression of granzyme B and perforin were significantly reduced in peptide-treated CTL, the inhibition of cytolytic activity was mainly caused by the exhaustion of cytolytic molecules. Moreover, treatment of the CTL with the epitopic peptide resulted in production of high levels of IL-2, IFN-gamma, tumor necrosis factor-alpha and MIP-1beta in the culture supernatant. Maximum amounts of cytokines were obtained in the culture supernatant when the level of cytolytic activity was the lowest. Thus, although the CTL temporarily lost their cytolytic activities, they simultaneously gained the abilities to produce cytokines for activation of various cell populations. These changes induced by free antigenic peptide in CD8(+) CTL reveal an interesting counter-regulation between their cytolytic activities and cytokine production.

Fratricide among CD8(+) T lymphocytes naturally infected with human T cell lymphotropic virus type I

Infection and gene expression by the human T lymphotropic virus type I (HTLV-I) in vivo have been thought to be confined to CD4(+) T lymphocytes. We show here that, in natural HTLV-I infection, a significant proportion of CD8(+) T lymphocytes are infected by HTLV-I. Interestingly, HTLV-I-specific but not Epstein-Barr virus-specific CD8(+) T lymphocytes were shown to be infected. Furthermore, HTLV-I protein expression in naturally infected CD8(+) T lymphocytes renders them susceptible to fratricide mediated by autologous HTLV-I-specific CD8(+) T lymphocytes. Fratricide among virus-specific CTLs could impair the immune control of HTLV-I and possibly other lymphotropic viruses.

Molecular and cellular mechanisms regulating T and B cell apoptosis through Fas/FasL interaction

Fas (CD95) and Fas ligand (FasL) are a receptor/ligand pair critically involved in lymphocyte homeostasis and peripheral tolerance such that genetic defect in either Fas or FasL results in an autoimmune lymphoproliferative syndrome. Fas is a type I transmembrane protein and a member of the tumor necrosis factor receptor (TNFR) family whereas FasL is a type II transmembrane protein and a member of TNF family. Binding of Fas by FasL induces apoptosis of the Fas-expressing cells. In the past few years, Fas/FasL interaction has been connected to a series of important phenomena previously viewed as independent immune processes. The activation-induced T cell death (AICD) and the FasL-mediated cytotoxicity by activated T cells are two critical mechanisms that can account for most of these phenomena. It is in the context of the two mechanisms that we discuss in this review the molecular and cellular events that occur during T/T and T/B interactions that account for the down-regulation of the immune response. We have also discussed recent advances in the areas of FasL gene regulation, lymphokine regulation of AICD, and regulation of B cell susceptibility to FasL. Investigation in these areas should help elucidate the role of Fas/FasL in the complex network of regulatory mechanisms that control immune response and autoimmunity.

Consequences of antigen self-presentation by tumor-specific cytotoxic T cells

CD8-positive cytotoxic T cells (CTL) recognize antigenic peptides in combination with major histocompatibility complex (MHC) class I molecules on the surface of syngeneic antigen presenting cells (APC). In the present paper we show that cells from tumor antigen-specific CTL clones present their cognate antigenic peptide to other CTL from the same clone. Inter-CTL peptide presentation resulted in activation of the cells of one CTL clone to MHC-unrestricted lysis of bystander cells. In contrast to the behaviour of this clone, another CTL clone did not lyse bystander cells after incubation with the cognate peptide, but was activated to self-destruction. The human herpes virus Epstein-Barr virus is involved in the pathogenesis of a broad spectrum of human neoplasias. Using freshly established non-clonal T cells with specificity for a peptide derived from an Epstein-Barr virus encoded antigen we found again lysis of MHC mismatched bystander cells as a consequence of inter-CTL peptide presentation, indicating that bystander lysis following antigen self-presentation is not a phenomenon restricted to long-term in vitro cultured T cell clones. The potential implications for immunosurveillance against cancer and for tumor escape mechanisms are discussed.

Homeostatic regulation of CD8+ T cells by perforin

To prevent uncontrolled expansion, the massive proliferation of T cells during an acute immune response has to be followed by controlled deletion. Here we show that similar to Fas, perforin is not only an important effector molecule of cytotoxic T lymphocytes (CTL) but also involved in down-regulating peripheral T cells. Mice deficient for both the CTL effector molecule perforin and the apoptosis-inducing Fas ligand spontaneously develop infiltration of highly activated CD8(+) T cells in kidney and liver and die between 5 and 12 weeks of age. Injection of staphylococcal enterotoxin B (SEB) into perforin-deficient mice results in dramatically increased selective expansion and prolonged persistence of CD8(+), but not CD4(+), SEB-reactive T cells. Also, secondary immunization of TCR transgenic perforin-deficient mice with the lymphocytic choriomeningitis virus glycoprotein-derived epitope peptide leads to an increased proliferation of transgenic CD8(+) T cells, that is not explained by failure to deplete professional antigen-presenting cells. These results point to a novel mechanism of T cell homeostasis in which the acquisition of perforin-dependent cytotoxic activity regulates the expansion and persistence of CD8(+) effector T cells in vivo.

Cutting Edge: N-Hydroxy Peptides: A New Class of TCR Antagonists

TCR antagonists are altered T cell epitopes that specifically inactivate T cells. Commonly, they are derived from agonists by amino acid side chain replacement at positions accessible to the TCR. In this paper we report for the first time that a main chain N-hydroxylation, which is not exposed at the surface of the MHC peptide complex, renders an agonist into an antagonist. These mimotopes are a new, yet undescribed class of TCR antagonists. The antagonists are about 100 times more potent than an unrelated peptide that competes for binding to the MHC molecule. The novel main chain modification enhances biostability and maintains side chain constitution and thus opens new prospects for the use of TCR antagonists in the treatment of pathological immune reactions.

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.

Calreticulin Displays In Vivo Peptide-Binding Activity and Can Elicit CTL Responses Against Bound Peptides

Calreticulin is an endoplasmic reticulum (ER) chaperone that displays lectin activity and contributes to the folding pathways for nascent glycoproteins. Calreticulin also participates in the reactions yielding assembly of peptides onto nascent MHC class I molecules. By chemical and immunological criteria, we identify calreticulin as a peptide-binding protein and provide data indicating that calreticulin can elicit CTL responses to components of its bound peptide pool. In an adoptive immunotherapy protocol, dendritic cells pulsed with calreticulin isolated from B16/F10.9 murine melanoma, E.G7-OVA, or EL4 thymoma tumors elicited a CTL response to as yet unknown tumor-derived Ags or the known OVA Ag. To evaluate the relative efficacy of calreticulin in eliciting CTL responses, the ER chaperones GRP94/gp96, BiP, ERp72, and protein disulfide isomerase were purified in parallel from B16/F10.9, EL4, and E.G7-OVA tumors, and the capacity of the proteins to elicit CTL responses was compared. In both the B16/F10.9 and E.G7-OVA models, calreticulin was as effective as or more effective than GRP94/gp96 in eliciting CTL responses. Little to no activity was observed for BiP, ERp72, and protein disulfide isomerase. The observed antigenic activity of calreticulin was recapitulated in in vitro experiments, in which it was observed that pulsing of bone marrow dendritic cells with E.G7-OVA-derived calreticulin elicited sensitivity to lysis by OVA-specific CD8+ T cells. These data identify calreticulin as a peptide-binding protein and indicate that calreticulin-bound peptides can be re-presented on dendritic cell class I molecules for recognition by CD8+ T cells.

Apoptosis of human naive NK cells mediated by a rat IgG2b anti CD2 mAb through a fractricidal ADCC reaction

LO-CD2a/BTI-322, a rat anti human CD2 mAb, shows in vitro and in vivo immunosuppressive properties and induces T-cell depletion resulting partially from an antibody dependent cellular cytotoxicity (ADCC) mediated by NK cells. The aim of this paper is to study the in vitro effect of LO-CD2a/BTI-322 on NK cells, the majority of them also expressing the CD2 molecule. The addition of the mAb to purified naive NK cells induces apoptosis of CD2+ cells. The apoptosis is rapid, Fas ligand independent and completely inhibited by the calcium chelator EGTA, suggesting a fractricidal ADCC reaction and implying that NK cells are not resistant to lysis when used as target cells. At the end of the reaction, the CD2 - remaining cells are still capable of natural cytotoxicity against K562 cells, but at a lower rate than untreated cells.

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.

A role for perforin in downregulating T-cell responses during chronic viral infection

Cytotoxic T cells secrete perforin to kill virus-infected cells. In this study we show that perforin also plays a role in immune regulation. Perforin-deficient (perf -/-) mice chronically infected with lymphocytic choriomeningitis virus (LCMV) contained greater numbers of antiviral T cells compared to persistently infected +/+ mice. The enhanced expansion was seen in both CD4 and CD8 T cells, but the most striking difference was in the numbers of LCMV-specific CD8 T cells present in infected perf -/- mice. Persistent LCMV infection of +/+ mice results in both deletion and anergy of antigen-specific CD8 T cells, and our results show that this peripheral "exhaustion" of activated CD8 T cells occurred less efficiently in perf -/- mice. This excessive accumulation of activated CD8 T cells resulted in immune-mediated damage in persistently infected perf -/- mice; approximately 50% of these mice died within 2 to 4 weeks, and mortality was fully reversed by in vivo depletion of CD8 T cells. This finding highlights an interesting dichotomy between the role of perforin in viral clearance and immunopathology; perforin-deficient CD8 T cells were unable to clear the LCMV infection but were capable of causing immune-mediated damage. Finally, this study shows that perforin also plays a role in regulating T-cell-mediated autoimmunity. Mice that were deficient in both perforin and Fas exhibited a striking acceleration of the spontaneous lymphoproliferative disease seen in Fas-deficient (lpr) mice. Taken together, these results show that the perforin-mediated pathway is involved in downregulating T-cell responses during chronic viral infection and autoimmunity and that perforin and Fas act independently as negative regulators of activated T cells.

Variant antigenic peptide promotes cytotoxic T lymphocyte adhesion to target cells without cytotoxicity

Timelapse video microscopy has been used to record the motility and dynamic interactions between an H-2Db-restricted murine cytotoxic T lymphocyte clone (F5) and Db-transfected L929 mouse fibroblasts (LDb) presenting normal or variant antigenic peptides from human influenza nucleoprotein. F5 cells will kill LDb target cells presenting specific antigen (peptide NP68: ASNENMDAM) after "browsing" their surfaces for between 8 min and many hours. Cell death is characterized by abrupt cellular rounding followed by zeiosis (vigorous "boiling" of the cytoplasm and blebbing of the plasma membrane) for 10-20 min, with subsequent cessation of all activity. Departure of cytotoxic T lymphocytes from unkilled target cells is rare, whereas serial killing is sometimes observed. In the absence of antigenic peptide, cytotoxic T lymphocytes browse target cells for much shorter periods, and readily leave to encounter other targets, while never causing target cell death. Two variant antigenic peptides, differing in nonamer position 7 or 8, also act as antigens, albeit with lower efficiency. A third variant peptide NP34 (ASNENMETM), which differs from NP68 in both positions and yet still binds Db, does not stimulate F5 cytotoxicity. Nevertheless, timelapse video analysis shows that NP34 leads to a significant modification of cell behavior, by up-regulating F5-LDb adhesive interactions. These data extend recent studies showing that partial agonists may elicit a subset of the T cell responses associated with full antigen stimulation, by demonstrating that TCR interaction with variant peptide antigens can trigger target cell adhesion and surface exploration without activating the signaling pathway that results in cytotoxicity.

Partial agonism and independent modulation of T cell receptor and CD8 in hapten-specific cytotoxic T cells

We recently demonstrated antagonism for hapten-reactive T cells by altered hapten ligands. Here we investigated partial peptide- or hapten-agonism and effects of antigen stimulation on the expression of TCR and the CD8 coreceptor using a set of DNP- or TNP-peptide-induced, H-2Kb-restricted mouse CTL clones. Various Kb-binding TNP- and DNP-peptides acted as partial agonists, cross-reactively stimulating individual clones for cytotoxicity and IFN-gamma secretion, but failing to induce proliferation or TNF-alpha production. Full agonism, i.e. activation of all possible functions, was usually restricted to those hapten-peptide combinations used for the induction of the respective clones. Our data imply distinctive kinetic optima for TCR antigen contacts in the induction of the various T cell effector functions. Down-regulation of TCR was efficiently induced by full, but with one exception not by partial, agonists, indicating the independence of cytotoxicity or IFN-gamma secretion from TCR modulation. On the other hand, a reduction of TCR expression induced by full agonists was usually not accompanied by synchronous down-modulation of CD8 as reported by others for human T cells. In fact, three of four full agonists and all partial agonists markedly enhanced rather than reduced the expression of CD8. Increased CD8 surface levels enhanced cytolytic potential and increased cross-reactivity patterns of individual clones. Brefeldin A blocked this CD8 induction by partial agonists, and in the case of full agonists resulted in a parallel reduction of both, TCR and CD8. Thus, antigenic stimulation of mouse T cells initially down-modulates CD8 together with TCR, but the loss of coreceptor is over-compensated by a signal for increased CD8 export.

Peptide antagonism and T cell receptor interactions with peptide-MHC complexes

We describe antagonist peptides that specifically inhibit cytolytic activity of T cell clones and lines that express the antigen-specific receptor of CD8+ T lymphocyte clone 2C, which recognizes peptides in association with syngeneic (Kb) and allogeneic (Ld) MHC proteins. Addition of an antagonist peptide that can bind to Kb on 2C cells decreased the tyrosine phosphorylation of CD3 zeta chains elicited by prior exposure of the cells to an agonist peptide-Kb complex. Contrary to previous agonist-antagonist comparisons, the 2C T cell receptor had higher affinity for an antagonist peptide-Kb complex than for a weak agonist peptide-Kb complex. This difference is considered in light of evidence that antigen-specific receptor affinity values can be substantially higher when determined with the receptor on live cells than with the receptor in cell-free systems.

Self-veto mechanism of CD8+ cytotoxic effector T cells. Peptide-induced paralysis affects the peptide-MHC-recognizing cytotoxic T lymphocytes and is independent of Fas/Fas ligand interactions

The lytic activity of CD8+ cytotoxic T lymphocyte (CTL) cell lines or clones can be inhibited by addition of the peptide recognized by these cells. The mechanisms underlying this phenomenon are not fully understood. Here we have analyzed peptide-induced CTL paralysis using in vivo generated ovalbumin (OVA)-specific CTL. Lytic activity of OVA-specific CTL was inhibited by addition of the immunodominant OVA-peptide SIINFEKL in a dose-dependent manner. Paralysis was induced rapidly and binding of the peptide to MHC class I molecules was required. Using mixing experiments with CTL populations of different peptide specificities restricted to the same MHC class I molecule we identified a veto-like mechanism: the cytotoxic activity of the peptide-recognizing CTL was inhibited while the lytic activity of the peptide-presenting CTL was unaltered. Only CD8+ CTL but not CD4+ T cells or B+ cells induced paralysis. After removal of the peptide-presenting CTL by magnetic cell sorting, paralysis was maintained and paralyzed CTL showed no signs of apoptosis. Loss of cytotoxicity could be induced in CTL populations from Fas-deficient (lpr+/lpr+) or Fas ligand-deficient (gld+/gld+) mice and mixtures thereof, implying that Fas/Fas ligand interactions are not involved during induction of paralysis. Hence, peptide-induced paralysis of CTL is due to a self-veto mechanism rather than to mutual killing of CTL. These findings may have implications for in vivo immunization with peptides, viral escape and peripheral tolerance mechanisms.

Dendritic cell/macrophage precursors capture exogenous antigen for MHC class I presentation by dendritic cells

Presentation of MHC class I antigens by professional antigen-presenting cells (APC) is an important pathway in priming cytotoxic T lymphocyte responses in vivo. This study sought to identify the nature of the professional APC responsible for indirect class I presentation by examining a special feature of professional APC, namely their ability to process exogenous forms of antigen for class I presentation. Incubation of highly purified bone marrow-derived precursor cells with chicken ovalbumin (OVA) led to the efficient presentation of the major class I-restricted OVA determinant by mature dendritic cells (DC), but not by macrophages (Mphi) derived from the precursor population. DC as well as macrophages were, however, able to mediate class II presentation of OVA, suggesting that macrophages were deficient in class I processing but not in capturing exogenous OVA. The majority of mature DC, i.e. over 80 %, generated from the precursor cells pulsed with OVA, presented the class I OVA epitope. Upon maturation, class I presentation of OVA by DC was greatly reduced, suggesting that class I processing of exogenous antigen is modulated during DC maturation in a manner similar to class II antigen processing. This study shows that bone marrow-derived DC/ME progenitors capture exogenous antigen for class I presentation, and that cells of the DC lineage can be functionally distinguished from cells of the macrophage lineage based on their ability to process exogenous antigen for class I presentation.

A mutant human beta2-microglobulin can be used to generate diverse multimeric class I peptide complexes as specific probes for T cell receptors

Antigen-specific receptors (TCR) on CD8 T lymphocytes form relatively short-lived complexes with their natural ligands: peptides in association with major histocompatibility complex (MHC) class I molecules, which consist of a polymorphic heavy chain and a conserved light chain, beta2-microglobulin (beta2-M). To produce soluble MHC-peptide complexes in a form that would bind more stably and could be used to identify, count, and isolate CD8 T cells having the appropriate TCR, we prepared multimeric MHC-peptide complexes. Our work builds on the assembly of recombinant MHC class I peptide complexes using a mutant human beta2-M chain (Tyr 67 > Cys) which can form stable heterodimers with diverse MHC heavy chains. With biotin added to the SH group, the assembled MHC-peptide monomers formed multimers with avidin linked to a fluorochrome. The specific reactivity of the multimeric reagents with human and mouse cytotoxic T cells (CTL) is described. The present approach permits the production of class I multimers, without the necessity of genetic engineering each heavy chain, a significant advantage in view of the enormous polymorphism of MHC heavy chains. Because human beta2-M forms stable heterodimers with diverse class I heavy chains from various species (human and non human primates, mouse, etc.), this procedure is a general method for producing multimers of MHC-peptide complexes as T cell receptor-specific probes.

T:T Antigen Presentation by Activated Murine CD8+ T Cells Induces Anergy and Apoptosis

Using an IL-2-secreting, noncytolytic, H-Y-specific, CD8+ T cell clone, the functional consequences of Ag presentation by T cells to T cells were investigated. Incubation of the T cells with H-Y-soluble peptide led to nonresponsiveness to Ag rechallenge. This was due to the simultaneous induction of apoptosis, involving approximately 40% of the T cells, and of anergy in the surviving cells. These effects were strictly dependent upon bidirectional T:T presentation, in that exposure of C6 cells to peptide-pulsed T cells from the same clone induced proliferation but not apoptosis or anergy. The inhibitory effects of T:T presentation were not due to a lack of costimulation, since the T cells expressed levels of CD80 and CD86 higher than those detected on cultured dendritic cells and equipped them to function as efficient APCs for primary CD8+ T cell responses. Following incubation with soluble peptide, CD80 expression increased, and high levels of CTLA-4 (CD152) expression were induced. Although addition of anti-CTLA-4 Ab augmented proliferation in response to soluble peptide, no protection from apoptosis or anergy was observed. Neither Fas nor TNF-α was expressed/produced by the C6 cells, and coligation of MHC class I molecules and TCR failed to reproduce the effects of T:T presentation. Taken together, these data suggest that T:T Ag presentation induces anergy and apoptosis in murine CD8+ T cells and may reflect the regulatory consequences of T:T interactions in the course of clonal expansion in vivo.

Isolation of processed, H-2Kb-binding ovalbumin-derived peptides associated with the stress proteins HSP70 and gp96

Stress-induced proteins or heat shock proteins (HSP) of 96 kDa mass (gp96) and 70 kDa mass (HSP70) have been shown previously to elicit specific immunity to tumors from which they are isolated. This immunity is dependent on CD8+ cytotoxic T cells which are readily primed in vivo by immunization with HSP. The immunization capacity of HSP relies on their ability to bind antigenic peptides. Here we show that HSP70 and gp96 preparations purified from the ovalbumin (OVA)-transfected cell line E.G7 are associated with processed H-2Kb-binding peptides which contain the major H-2Kb-associated epitope SIINFEKL (OVA257-264). Our data show for the first time in the well-defined OVA antigen system that not only endoplasmic reticulum-resident HSP, like gp96, are associated with processed antigenic peptides but that also the cytosolic HSP70 protein forms complexes with major finally processed MHC-binding epitopes.

Antigen presentation by T cells: T cell receptor ligation promotes antigen acquisition from professional antigen-presenting cells

The purpose of this study was to determine whether the clonotypic specificity of the T cell receptor influences the specificity of T cell-mediated antigen presentation. We have previously shown that myelin basic protein (MBP)-specific Lewis rat GP2.E5/R1 (R1) T cells cultured with antigen, irradiated syngeneic splenocytes (IrrSPL) and tolerogenic monoclonal antibody become highly effective antigen-presenting cells (APC). In the current studies, we investigated the transfer of specific (MBP) and unrelated (conalbumin) antigens from antigen-pulsed SPL to R1 T cells. R1 T cells cultured with IrrSPL that were pulsed simultaneously with both MBP and conalbumin acquired and presented both antigens to the appropriate T cell responders in a secondary assay. These results suggested a physical transfer of major histocompatibility complex (MHC)/peptide complexes from professional APC to R1 T cells. Transfer of conalbumin from professional APC to R1 T cells required specific recognition of MBP and was optimal when both conalbumin and MBP were presented on the same group of professional APC. Antigens transfer did not occur when allogeneic SPL were used as APC. The anti-I-A mAb OX6 inhibited antigen transfer but only when added during the initiation of culture. OX6 also inhibited antigen acquisition by R1-trans, a variant of the R1 T cell line which constitutively synthesizes high levels of I-A, from MBP-pulsed IrrSPL but blockade of I-A did not inhibit antigen acquisition when soluble MBP was added directly to the culture. Despite constitutive synthesis of I-A, R1-trans T cells did not acquire guinea pig MBP from pulsed allogeneic APC. These studies demonstrate that although T cells of a particular specificity can present unrelated antigens, the cognate interaction of the T cell antigen receptor with the appropriate antigen/self-MHC complex strongly promotes acquisition of these complexes from professional APC.

Heat shock fusion proteins as vehicles for antigen delivery into the major histocompatibility complex class I presentation pathway

Mice immunized with heat shock proteins (hsps) isolated from mouse tumor cells (donor cells) produce CD8 cytotoxic T lymphocytes (CTL) that recognize donor cell peptides in association with the major histocompatibility complex (MHC) class I proteins of the responding mouse. The CTL are induced apparently because peptides noncovalently associated with the isolated hsp molecules can enter the MHC class I antigen processing pathway of professional antigen-presenting cells. Using a recombinant heat shock fusion protein with a large fragment of ovalbumin covalently linked to mycobacterial hsp70, we show here that when the soluble fusion protein was injected without adjuvant into H-2b mice, CTL were produced that recognized an ovalbumin-derived peptide, SIINFEKL, in association with Kb. The peptide is known to arise from natural processing of ovalbumin in H-2b mouse cells, and CTL from the ovalbumin-hsp70-immunized mice and a highly effective CTL clone (4G3) raised against ovalbumin-expressing EL4 tumor cells (EG7-OVA) were equally effective in terms of the concentration of SIINFEKL required for half-maximal lysis in a CTL assay. The mice were also protected against lethal challenge with ovalbumin-expressing melanoma tumor cells. Because large protein fragments or whole proteins serving as fusion partners can be cleaved into short peptides in the MHC class I processing pathway, hsp fusion proteins of the type described here are promising candidates for vaccines aimed at eliciting CD8 CTL in populations of MHC-disparate individuals.

Polyclonal T cell elimination by prolonged immunostimulation in an experimental model

An experimental model of immunological deficiency obtained by treating mice for 6 months with serum of human blood drawn from different healthy individuals has been studied. The results show that an alteration of a circulating lymphocyte population with alterations of the ratio CD4+/CD8+ appeared in mice stimulated for a long period with immunogens. Mice treated for 2-4 months showed an increase in B lymphocytes and a decrease in the total number of T lymphocytes, with a decrease in CD4+ lymphocytes and an increase in CD8+ lymphocytes. After 4 months, the CD8+ lymphocyte population started to decrease, with a ratio of CD4+/CD8+ reaching almost 1. In animals treated for 2-3 months, the mean survival time (MST) following experimental infection with Salmonella typhimurium presented a decrease to 5 days, and after 5-6 months of treatment presented a decrease to 3-2.5 days. The bacteraemia was modified in comparison with controls. Prolonged exposure to antigens also induced lymphocyte apoptosis: cells of animals treated for 4-6 months presented increased levels of apoptosis with a percentage that reached 30-35%. A semiquantitative evaluation of the level of heat shock protein (hsp) in splenic lymphocytes showed an increase in the presence of hsp60 and hsp70 in the first 3 months of treatment, which then remained constant for up to 6 months.

Altered hapten ligands antagonize trinitrophenyl-specific cytotoxic T cells and block internalization of hapten-specific receptors

Low molecular chemicals (haptens) frequently cause T cell-mediated adverse immune reactions. Our previous work provided evidence that hapten-specific T cells, in analogy to those specific for nominal peptide antigens, direct their TCR towards hapten-modified, MHC-associated peptides. We now demonstrate that trinitrophenyl (TNP)-specific, class I MHC-restricted CTL from mice may exhibit exquisite specificity for subtle structural details of these hapten determinants, surpassing even the specificity of immunoglobulins. More importantly, these CTL could be antagonized by ligands altered either in their peptide sequence or in their hapten structure. The system was employed to examine the molecular basis of T cell antagonism. Whereas agonists resulted in a dose-dependent downregulation of TCR in different mouse T cell clones, antagonistic peptides totally failed to do so despite engaging the specific TCR. Moreover, simultaneous presentation of antagonist and agonist on the same antigen presenting cell prevented TCR internalization. No signs of anergy or functional receptor inactivation were observed in CTL treated with antagonist-loaded target cells. Based on a serial triggering model of T cell activation, our data favor a model in which antagonists block T cell functions by competitively engaging the specific TCR in unproductive interactions.

Peptide antigen treatment of naive and virus-immune mice: antigen-specific tolerance versus immunopathology

Peptide-specific down-regulation of T cell responses may represent a powerful tool to intervene in autoimmune diseases or graft rejections. It is therefore important to know whether peptide treatment tolerizes both naive and antigen-experienced memory T lymphocytes. Here we show that a major histocompatibility complex class I binding peptide, derived from the glycoprotein (GP33 peptide) of lymphocytic choriomeningitis virus (LCMV), specifically tolerized naive cytotoxic T lymphocytes (CTL) when administered three times intraperitoneally in incomplete Freund's adjuvants. However, in the presence of GP33-specific memory CTL in LCMV-primed mice, the same treatment had a general immunosuppressive effect on unrelated third-party antigen-specific T cell responses and caused severe immunopathological damage to the spleen.

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.

Role of antigen, CD8, and cytotoxic T lymphocyte (CTL) avidity in high dose antigen induction of apoptosis of effector CTL

Experimental data suggest that negative selection of thymocytes can occur as a result of supraoptimal antigenic stimulation. It is unknown, however, whether such mechanisms are at work in mature CD8+ T lymphocytes. Here, we show that CD8+ effector cytotoxic T lymphocytes (CTL) are susceptible to proliferative inhibition by high dose peptide antigen, leading to apoptotic death mediated by TNF-alpha release. Such inhibition is not reflected in the cytolytic potential of the CTL, since concentrations of antigen that are inhibitory for proliferation promote efficient lysis of target cells. Thus, although CTL have committed to the apoptotic pathway, the kinetics of this process are such that CTL function can occur before death of the CTL. The concentration of antigen required for inhibition is a function of the CTL avidity, in that concentrations of antigen capable of completely inhibiting high avidity CTL maximally stimulate low avidity CTL. Importantly, the inhibition can be detected in both activated and resting CTL. Blocking studies demonstrate that the CD8 molecule contributes significantly to the inhibitory signal as the addition of anti-CD8 antibody restores the proliferative response. Thus, our data support the model that mature CD8+ CTL can accommodate an activation signal of restricted intensity, which, if surpassed, results in deletion of that cell.

Inactivation of human immunodeficiency virus (HIV)-1 envelope-specific CD8+ cytotoxic T lymphocytes by free antigenic peptide: a self-veto mechanism?

Free peptide has been found to inhibit cytotoxic T lymphocyte (CTL) activity, and veto cells bearing peptide-major histocompatibility complex (MHC) complexes have been found to inactivate CTL, but the two phenomena have not been connected. Here we show that a common mechanism may apply to both. CD8+ CTL lines or clones specific for a determinant of the human immunodeficiency virus (HIV) 1 IIIB envelope protein gp160, P18IIIB, are inhibited by as little as 10 min exposure to the minimal 10-mer peptide, I-10, within P18IIIB, free in solution, in contrast to peptide already bound to antigen-presenting cells (APC), which does not inhibit. Several lines of evidence suggest that the peptide must be processed and presented by H-2Dd on the CTL itself to the specific T cell receptor (TCR) to be inhibitory. The inhibition was not killing, in that CTL did not kill 51Cr-labeled sister CTL in the presence of free peptide, and in mixing experiments with CTL lines of different specificities restricted by the same MHC molecule, Dd, the presence of free peptide recognized by one CTL line did not inhibit the activity of the other CTL line that could present the peptide. Also, partial recovery of activity could be elicited by restimulation with cell-bound peptide, supporting the conclusion that neither fratricide nor suicide (apoptosis) was involved. The classic veto phenomenon was ruled out by failure of peptide-bearing CTL to inactivate others. Using pairs of CTL lines of differing specificity but similar MHC restriction, each pulsed with the peptide for which the other is specific, we showed that the minimal requirement is simultaneous engagement of the TCR and class I MHC molecules of the same cell. This could occur in single cells or pairs of cells presenting peptide to each other. Thus, mechanistically, the inhibition is analogous to veto, and might be called self-veto. As a clue to a possible mechanism, we found that free I-10 peptide induced apparent downregulation of expression of specific TCR as well as interleukin 2 receptor, CD69, lymphocyte function-associated antigen 1, and CD8. This self-veto effect also has implications for in vivo immunization and mechanisms of viral escape from CTL immunity.