TAP-ing into TIEPPs for cancer immunotherapy

Cancer immunotherapy in which cytotoxic T cells (CTLs) target tumor-specific antigens complexed to MHC-I molecules has been used successfully for several types of cancer; however, MHC-I is frequently downregulated in tumors, resulting in CTL evasion. Recently, it has been shown that MHC-I^lo tumors produce a set of T cell epitopes associated with impaired peptide processing (TEIPP) that have potential to be exploited for immunotherapy. TEIPP-specific CTLs recognize tumors defective in antigen presentation machinery (APM) but not those with intact APM. In this issue of the JCI, Doorduljn et al. evaluated thymus selection and peripheral behavior of TEIPP-specific T cells, using a unique T cell receptor (TCR) transgenic mouse model. They demonstrated that TEIPP-specific T cells in TAP-deficient mice have largely been deleted by central tolerance, while the same T cells in WT mice are naive and sustained. The results of this study suggest that TIEPPs have potential to be successful targets for elimination of MHC-I^lo tumors.     

MAL2 drives immune evasion in breast cancer by suppressing tumor antigen presentation

Immune evasion is a pivotal event in tumor progression. To eliminate human cancer cells, current immune checkpoint therapy is set to boost CD8⁺ T cell–mediated cytotoxicity. However, this action is eventually dependent on the efficient recognition of tumor-specific antigens via T cell receptors. One primary mechanism by which tumor cells evade immune surveillance is to downregulate their antigen presentation. Little progress has been made toward harnessing potential therapeutic targets for enhancing antigen presentation on the tumor cell. Here, we identified MAL2 as a key player that determines the turnover of the antigen-loaded MHC-I complex and reduces the antigen presentation on tumor cells. MAL2 promotes the endocytosis of tumor antigens via direct interaction with the MHC-I complex and endosome-associated RAB proteins. In preclinical models, depletion of MAL2 in breast tumor cells profoundly enhanced the cytotoxicity of tumor-infiltrating CD8⁺ T cells and suppressed breast tumor growth, suggesting that MAL2 is a potential therapeutic target for breast cancer immunotherapy.     

Definition of target antigens for naturally occurring CD4(+) CD25(+) regulatory T cells

The antigenic targets recognized by naturally occurring CD4⁺ CD25⁺ regulatory T cells (T reg cells) have been elusive. We have serologically defined a series of broadly expressed self-antigens derived from chemically induced mouse sarcomas by serological identification of antigens by recombinant expression cloning (SEREX). CD4⁺ CD25⁺ T cells from mice immunized with SEREX-defined self-antigens had strong suppressive activity on peptide-specific proliferation of CD4⁺ CD25⁻ T cells and CD8⁺ T cells. The suppressive effect was observed without in vitro T cell stimulation. Foxp3 expression in these CD4⁺ CD25⁺ T cells from immunized mice was 5–10 times greater than CD4⁺ CD25⁺ T cells derived from naive mice. The suppressive effect required cellular contact and was blocked by anti-glucocorticoid–induced tumor necrosis factor receptor family–related gene antibody. In vitro suppressive activity essentially disappeared 8 wk after the last immunization. However, it was regained by in vitro restimulation with cognate self-antigen protein but not with control protein. We propose that SEREX-defined self-antigens such as those used in this study represent self-antigens that elicit naturally occurring CD4⁺ CD25⁺ T reg cells.     

CTLs are targeted to kill beta cells in patients with type 1 diabetes through recognition of a glucose-regulated preproinsulin epitope

The final pathway of β cell destruction leading to insulin deficiency, hyperglycemia, and clinical type 1 diabetes is unknown. Here we show that circulating CTLs can kill β cells via recognition of a glucose-regulated epitope. First, we identified 2 naturally processed epitopes from the human preproinsulin signal peptide by elution from HLA-A2 (specifically, the protein encoded by the A*0201 allele) molecules. Processing of these was unconventional, requiring neither the proteasome nor transporter associated with processing (TAP). However, both epitopes were major targets for circulating effector CD8⁺ T cells from HLA-A2⁺ patients with type 1 diabetes. Moreover, cloned preproinsulin signal peptide–specific CD8⁺ T cells killed human β cells in vitro. Critically, at high glucose concentration, β cell presentation of preproinsulin signal epitope increased, as did CTL killing. This study provides direct evidence that autoreactive CTLs are present in the circulation of patients with type 1 diabetes and that they can kill human β cells. These results also identify a mechanism of self-antigen presentation that is under pathophysiological regulation and could expose insulin-producing β cells to increasing cytotoxicity at the later stages of the development of clinical diabetes. Our findings suggest that autoreactive CTLs are important targets for immune-based interventions in type 1 diabetes and argue for early, aggressive insulin therapy to preserve remaining β cells.     

Atractylenolide I enhances responsiveness to immune checkpoint blockade therapy by activating tumor antigen presentation

One of the primary mechanisms of tumor cell immune evasion is the loss of antigenicity, which arises due to lack of immunogenic tumor antigens as well as dysregulation of the antigen processing machinery. In a screen for small-molecule compounds from herbal medicine that potentiate T cell–mediated cytotoxicity, we identified atractylenolide I (ATT-I), which substantially promotes tumor antigen presentation of both human and mouse colorectal cancer (CRC) cells and thereby enhances the cytotoxic response of CD8⁺ T cells. Cellular thermal shift assay (CETSA) with multiplexed quantitative mass spectrometry identified the proteasome 26S subunit non–ATPase 4 (PSMD4), an essential component of the immunoproteasome complex, as a primary target protein of ATT-I. Binding of ATT-I with PSMD4 augments the antigen-processing activity of immunoproteasome, leading to enhanced MHC-I–mediated antigen presentation on cancer cells. In syngeneic mouse CRC models and human patient–derived CRC organoid models, ATT-I treatment promotes the cytotoxicity of CD8⁺ T cells and thus profoundly enhances the efficacy of immune checkpoint blockade therapy. Collectively, we show here that targeting the function of immunoproteasome with ATT-I promotes tumor antigen presentation and empowers T cell cytotoxicity, thus elevating the tumor response to immunotherapy.     

MHC-derived allopeptide activates TCR-biased CD8+ Tregs and suppresses organ rejection

In a rat heart allograft model, preventing T cell costimulation with CD40Ig leads to indefinite allograft survival, which is mediated by the induction of CD8⁺CD45RC^lo regulatory T cells (CD8⁺CD40Ig Tregs) interacting with plasmacytoid dendritic cells (pDCs). The role of TCR-MHC-peptide interaction in regulating Treg activity remains a topic of debate. Here, we identified a donor MHC class II–derived peptide (Du51) that is recognized by TCR-biased CD8⁺CD40Ig Tregs and activating CD8⁺CD40Ig Tregs in both its phenotype and suppression of antidonor alloreactive T cell responses. We generated a labeled tetramer (MHC-I RT1.A^a/Du51) to localize and quantify Du51-specific T cells within rat cardiac allografts and spleen. RT1.A^a/Du51-specific CD8⁺CD40Ig Tregs were the most suppressive subset of the total Treg population, were essential for in vivo tolerance induction, and expressed a biased, restricted Vβ11-TCR repertoire in the spleen and the graft. Finally, we demonstrated that treatment of transplant recipients with the Du51 peptide resulted in indefinite prolongation of allograft survival. These results show that CD8⁺CD40Ig Tregs recognize a dominant donor antigen, resulting in TCR repertoire alterations in the graft and periphery. Furthermore, this allopeptide has strong therapeutic activity and highlights the importance of TCR-peptide-MHC interaction for Treg generation and function.     

CD4+ T Cell Help Impairs CD8+ T Cell Deletion Induced by Cross-presentation of Self-Antigens and Favors Autoimmunity

Self-antigens expressed in extrathymic tissues such as the pancreas can be transported to draining lymph nodes and presented in a class I–restricted manner by bone marrow-derived antigen-presenting cells. Such cross-presentation of self-antigens leads to CD8⁺ T cell tolerance induction via deletion. In this report, we investigate the influence of CD4⁺ T cell help on this process. Small numbers of autoreactive OVA-specific CD8⁺ T cells were unable to cause diabetes when adoptively transferred into mice expressing ovalbumin in the pancreatic β cells. Coinjection of OVA-specific CD4⁺ helper T cells, however, led to diabetes in a large proportion of mice (68%), suggesting that provision of help favored induction of autoimmunity. Analysis of the fate of CD8⁺ T cells indicated that CD4⁺ T cell help impaired their deletion. These data indicate that control of such help is critical for the maintenance of CD8⁺ T cell tolerance induced by cross-presentation.There is now considerable evidence that CD8⁺ T cell responses can be induced in vivo by professional APCs capable of MHC class I–restricted presentation of exogenous antigens (1–3). This mechanism is known as cross-presentation and was suggested to be instrumental in the immune response to pathogens that avoid professional APCs (2–4). However, if this pathway was only directed towards induction of immunity, cross-presentation of self-antigens to autoreactive CD8⁺ T cells would result in autoimmunity. Recently, in studies using transgenic mice that express a membrane-bound form of OVA under the control of the rat insulin promoter (RIP-mOVA), we have shown that this is not the case. RIP-mOVA mice express membrane-bound OVA in pancreatic islets, kidney proximal tubular cells, thymus and testis. In these mice, OVA was found to enter the class I presentation pathway of a bone marrow– derived cell population and then activate transgenic OVA-specific CD8⁺ T cells (OT-I cells) (3) in LNs draining the sites of antigen expression. Although this form of activation initially led to proliferation of OT-I cells, it ultimately caused their deletion (5). Thus, cross-presentation can remove autoreactive CD8⁺ T cells, and may tolerize the CD8⁺ T cell compartment to self-antigens. These studies, however, did not explain why cross-presentation of a self-antigen induced CD8⁺ T cell tolerance, whereas foreign antigens induced immunity (1–4, 6).In numerous models, CD4⁺ T cell help has been shown to be important for the induction or maintenance of immune responses by CD8⁺ T cells (7–11), but such help is not always essential (12–14). CD4⁺ T cell help has also been shown to be important for avoiding CTL tolerance induction (15–17). In these reports, however, it was not known whether CD8⁺ T cells were activated by cross-presentation or by direct recognition of antigen. Thus, whether CD4⁺ T cell help can affect tolerance induced by cross-presentation has not been addressed. Recently, we demonstrated that cross-priming by foreign antigens requires CD4⁺ T cell help for induction of CTL immunity (6). In this study, we have investigated the influence of such help on the deletion of CD8⁺ T cells induced by cross-presentation of self-antigens.     

Variable HIV peptide stability in human cytosol is critical to epitope presentation and immune escape

Induction of virus-specific CD8⁺ T cell responses is critical for the success of vaccines against chronic viral infections. Despite the large number of potential MHC-I–restricted epitopes located in viral proteins, MHC-I–restricted epitope generation is inefficient, and factors defining the production and presentation of MHC-I–restricted viral epitopes are poorly understood. Here, we have demonstrated that the half-lives of HIV-derived peptides in cytosol from primary human cells were highly variable and sequence dependent, and significantly affected the efficiency of cell recognition by CD8⁺ T cells. Furthermore, multiple clinical isolates of HLA-associated HIV epitope variants displayed reduced half-lives relative to consensus sequence. This decreased cytosolic peptide stability diminished epitope presentation and CTL recognition, illustrating a mechanism of immune escape. Chaperone complexes including Hsp90 and histone deacetylase HDAC6 enhanced peptide stability by transient protection from peptidase degradation. Based on empirical results with 166 peptides, we developed a computational approach utilizing a sequence-based algorithm to estimate the cytosolic stability of antigenic peptides. Our results identify sequence motifs able to alter the amount of peptide available for loading onto MHC-I, suggesting potential new strategies to modulate epitope production from vaccine immunogens.     

Specific T Helper Cell Requirement for Optimal Induction of Cytotoxic T Lymphocytes against Major Histocompatibility Complex Class II Negative Tumors

This study shows that induction of tumor-specific CD4⁺ T cells by vaccination with a specific viral T helper epitope, contained within a synthetic peptide, results in protective immunity against major histocompatibility complex (MHC) class II negative, virus-induced tumor cells. Protection was also induced against sarcoma induction by acutely transforming retrovirus. In contrast, no protective immunity was induced by vaccination with an unrelated T helper epitope. By cytokine pattern analysis, the induced CD4⁺ T cells were of the T helper cell 1 type. The peptide-specific CD4⁺ T cells did not directly recognize the tumor cells, indicating involvement of cross-priming by tumor-associated antigen-presenting cells. The main effector cells responsible for tumor eradication were identified as CD8⁺ cytotoxic T cells that were found to recognize a recently described immunodominant viral gag-encoded cytotoxic T lymphocyte (CTL) epitope, which is unrelated to the viral env-encoded T helper peptide sequence. Simultaneous vaccination with the tumor-specific T helper and CTL epitopes resulted in strong synergistic protection. These results indicate the crucial role of T helper cells for optimal induction of protective immunity against MHC class II negative tumor cells. Protection is dependent on tumor-specific CTLs in this model system and requires cross-priming of tumor antigens by specialized antigen-presenting cells. Thus, tumor-specific T helper epitopes have to be included in the design of epitope-based vaccines.Adequate T helper cell activation is essential in the initiation of an immune reaction. The inability to control tumor outgrowth can be due to inadequate T helper responses underlying poor tumor-specific immunity. In the cellular immune response, specialized APCs process protein and present antigenic peptide fragments in MHC class II molecules to CD4⁺ T helper lymphocytes. These provide “help” to effector cells via the production of cytokines. Although tumor cells can directly present endogenously processed antigenic peptide in surface MHC class I molecules to CD8⁺ CTL precursors, initiation of tumor-specific CTL responses is likely to involve indirect presentation of tumor antigens by specialized APCs.Evidence for a role of T helper cell–mediated immunity comes from studies with genetically modified tumor cells. CD4⁺ cells can be directly activated by transfection of MHC class II α and β chain genes in mouse tumor cells (1–4). These cells become immunogenic, lose their tumorigenicity, and even induce protection against wild-type MHC class II negative tumors, indicating that direct MHC class II presentation of tumor expressed antigens can induce efficient anti–tumor responses.A central role of CD4⁺ T cells emerged from studies of immunity against FMR (Friend, Moloney, Rauscher)¹ murine leukemia virus (MuLV) type tumors by Greenberg (5). Transfer of purified polyclonal T cells from FBL (Friend MuLV-induced erythroleukemia cell line) vaccinated mice in naive animals can protect these mice against subsequent tumor challenge. Both purified CD4⁺ and CD8⁺ T cells transfer protection to FBL tumors (6). FBL cells do not express MHC class II molecules, but CD4⁺ T cells can protect mice even in the absence of CD8⁺ T cells. In this case, macrophages seem to play an important effector role. CD8⁺ T cells can only be effective if CD4⁺ T cells are present or if exogenous IL-2 is administered. Neither B cells nor NK cells seem to exert a significant role in the FBL sytem. These data suggest involvement of APCs, presenting tumor antigens, and a crucial regulatory role of Ths, which was strongly supported by experiments performed in Friend MuLV env-transgenic mice (7). These mice were rendered tolerant for env-specific Th responses and it was not possible to protect these mice against FBL tumors by vaccination.Immune responsiveness to MuLV is classically regulated by the genes of the H-2 (MHC) complex (8). In particular, the H-2^b haplotype confers resistance, and studies using H-2 recombinant and H-2 mutant mouse strains have mapped the protective effects to the class II I-A^b locus (9, 10). This MHC class II association indicates an important role of T helper cells influencing both CTL activity as well as class switching of antiviral antibodies from IgM to IgG. The H-2 I-A^b phenotype protects against early lymphomagenesis. The identification of two Friend MuLV env-derived epitopes, presented by MHC class II, I-A^b and I-E^b/d, respectively, indicated that tumor-directed T helper immunity is virus specific (11). The few lymphomas that arise in H-2^b mice have abrogated viral antigen or (more rarely) MHC class I expression (12), indicating that CTLs also play a crucial role. CTLs have been proven to recognize viral antigens, both gag and env proteins encode CTL epitopes (13). We have identified a K^b-presented, env-derived Moloney and Rauscher CTL epitope that is subdominant in C57BL/6 mice making use of the D^b mutant BM13 mouse strain (14). The D^b-presented gag-leader (gag-L) derived immunodominant CTL epitope for the FMR type of MuLV has been identified only recently (15).Vaccination with a synthetic peptide comprising a relevant T cell epitope is a powerful method to induce highly specific T cells. Protective vaccination using CTL peptide epitopes has been achieved in pathogenic viral models (16, 17) and tumor models (18–20). Peptide vaccination in IFA led to measurable specific CTL induction and protective immunity against virulent virus or tumor cells. Importantly, peptide vaccination can also be applied succesfully for therapy of established tumors by presenting the peptide in IFA, on RMA-S cells, or on activated dendritic cells (21, 22).We now report the induction of tumor-protective immunity by a single vaccination with a tumor-specific MuLV env-encoded T helper peptide. Strong protection can be achieved against highly aggressive tumor cells that lack MHC class II expression. This indicates the requirement of cross-priming of tumor antigens by local APCs. We show that CD8⁺ T cells, recognizing the gag-L–encoded CTL epitope, are crucial effector cells that are efficiently activated with help from peptide-primed tumor-specific CD4⁺ T cells. Vaccination with a mixture of the T helper peptide and the immunodominant CTL epitope resulted in synergistic, long-term tumor protection.     

The Peripheral Deletion of Autoreactive CD8+ T Cells Induced by Cross-presentation of Self-antigens Involves Signaling through CD95 (Fas,Apo-1)

Recently, we demonstrated that major histocompatibility complex class I–restricted cross-presentation of exogenous self-antigens can induce peripheral T cell tolerance by deletion of autoreactive CD8⁺ T cells. In these studies, naive ovalbumin (OVA)-specific CD8⁺ T cells from the transgenic line OT-I were injected into transgenic mice expressing membrane-bound OVA (mOVA) under the control of the rat insulin promoter (RIP) in pancreatic islets, kidney proximal tubules, and the thymus. Cross-presentation of tissue-derived OVA in the renal and pancreatic lymph nodes resulted in activation, proliferation, and then the deletion of OT-I cells. In this report, we investigated the molecular mechanisms underlying this form of T cell deletion. OT-I mice were crossed to tumor necrosis factor receptor 2 (TNFR2) knockout mice and to CD95 (Fas, Apo-1) deficient mutant lpr mice. Wild-type and TNFR2-deficient OT-I cells were activated and then deleted when transferred into RIP-mOVA mice, whereas CD95-deficient OT-I cells were not susceptible to deletion by cross-presentation. Furthermore, cross-presentation led to upregulation of the CD95 molecule on the surface of wild-type OT-I cells in vivo, consistent with the idea that this is linked to rendering autoreactive T cells susceptible to CD95-mediated signaling. This study represents the first evidence that CD95 is involved in the deletion of autoreactive CD8⁺ T cells in the whole animal.     

Dendritic Cells Loaded With mRNA Encoding Full-length Tumor Antigens Prime CD4+ and CD8+ T Cells in Melanoma Patients

It is generally thought that dendritic cells (DCs) loaded with full-length tumor antigen could improve immunotherapy by stimulating broad T-cell responses and by allowing treatment irrespective of the patient''s human leukocyte antigen (HLA) type. To investigate this, we determined the specificity of T cells from melanoma patients treated with DCs loaded with mRNA encoding a full-length tumor antigen fused to a signal peptide and an HLA class II sorting signal, allowing presentation in HLA class I and II. In delayed-type hypersensitive (DTH)-biopsies and blood, we found functional CD8⁺ and CD4⁺ T cells recognizing novel treatment-antigen-derived epitopes, presented by several HLA types. Additionally, we identified a CD8⁺ response specific for the signal peptide incorporated to elicit presentation by HLA class II and a CD4⁺ response specific for the fusion region of the signal peptide and one of the antigens. This demonstrates that the fusion proteins contain newly created immunogenic sequences and provides evidence that ex vivo-generated mRNA-modified DCs can induce effector CD8⁺ and CD4⁺ T cells from the naive T-cell repertoire of melanoma patients. Thus, this work provides definitive proof that DCs presenting the full antigenic spectrum of tumor antigens can induce T cells specific for novel epitopes and can be administered to patients irrespective of their HLA type.     

Human transporters associated with antigen processing (TAPs) select epitope precursor peptides for processing in the endoplasmic reticulum and presentation to T cells.

Antigen presentation by major histocompatibility complex (MHC) class I molecules requires peptide supply by the transporters associated with antigen processing (TAPs), which select substrates in a species- and, in the rat, allele-specific manner. Conflicts between TAPs and MHC preferences for COOH-terminal peptide residues in rodent cells strongly reduce the efficiency of MHC class I antigen presentation. Although human TAP is relatively permissive, some peptide ligands for human histocompatibility leukocyte antigen class I molecules are known to possess very low TAP affinities; the significance of these in vitro findings for cellular antigen presentation is not known. We studied two naturally immunodominant viral epitopes presented by HLA-A2 that display very low affinities for human TAP. Low TAP affinities preclude minimal epitope access to the endoplasmic reticulum (ER) and assembly with HLA-A2 in vitro, as well as presentation by minigene-expressing cells to cytotoxic T lymphocytes. However, NH(2)-terminally but not COOH-terminally extended epitope variants with higher TAP affinities assemble in vitro and are presented to cytotoxic T lymphocytes with high efficiency. Thus, human TAP can influence epitope selection and restrict access to the ER to epitope precursors. Analysis of TAP affinities of a panel of viral epitopes suggests that TAP selection of precursors may be a common phenomenon for HLA-A2-presented epitopes. We also analyzed HLA-A2-eluted peptides from minigene-expressing cells and show that an NH(2)-terminally extended variant with low A2 binding affinity undergoes ER processing, whereas another with high affinity is presented unmodified. Therefore, the previously reported aminopeptidase activity in the ER can also act on TAP-translocated peptides.     

Superior antigen cross-presentation and XCR1 expression define human CD11c+CD141+ cells as homologues of mouse CD8+ dendritic cells

In recent years, human dendritic cells (DCs) could be subdivided into CD304⁺ plasmacytoid DCs (pDCs) and conventional DCs (cDCs), the latter encompassing the CD1c⁺, CD16⁺, and CD141⁺ DC subsets. To date, the low frequency of these DCs in human blood has essentially prevented functional studies defining their specific contribution to antigen presentation. We have established a protocol for an effective isolation of pDC and cDC subsets to high purity. Using this approach, we show that CD141⁺ DCs are the only cells in human blood that express the chemokine receptor XCR1 and respond to the specific ligand XCL1 by Ca²⁺ mobilization and potent chemotaxis. More importantly, we demonstrate that CD141⁺ DCs excel in cross-presentation of soluble or cell-associated antigen to CD8⁺ T cells when directly compared with CD1c⁺ DCs, CD16⁺ DCs, and pDCs from the same donors. Both in their functional XCR1 expression and their effective processing and presentation of exogenous antigen in the context of major histocompatibility complex class I, human CD141⁺ DCs correspond to mouse CD8⁺ DCs, a subset known for superior antigen cross-presentation in vivo. These data define CD141⁺ DCs as professional antigen cross-presenting DCs in the human.The adaptive immune response is initiated through presentation of antigen to T cells by DCs. In the mouse, DCs can be broadly grouped into plasmacytoid DCs (pDCs) and conventional DCs (cDCs; earlier termed myeloid DCs). Mouse cDCs can be further subdivided into several DC types, which are apparently specialized for optimal antigen uptake, processing, and presentation to T cells in different body compartments (Steinman and Banchereau, 2007; Heath and Carbone, 2009; Segura and Villadangos, 2009). One particular type of antigen presentation is cross-presentation: in this case, extracellular antigen is not classically presented in the context of MHC-II but is instead shunted into the MHC-I presentation pathway (Bevan, 2006; Shen and Rock, 2006; Villadangos et al., 2007). CD8⁺ T cells can thus be activated by antigens taken up from the extracellular space and then differentiate into cytotoxic T cells. This mechanism is thought to be of major importance for the recognition of viral or bacterial antigens when DCs are not directly infected. In these instances, debris of cells that were infected and have subsequently undergone apoptosis as part of a cellular stress reaction is taken up and cross-presented by specialized DCs. Through this type of processing, the antigenic composition of the pathogen can become visible to the CD8⁺ T cell immune system. In the mouse, extensive experimentation has demonstrated that within cDCs, CD8⁺ DCs are the most effective in antigen cross-presentation (den Haan et al., 2000; Iyoda et al., 2002; Schulz and Reis e Sousa, 2002; Heath et al., 2004). Whether mouse pDCs play a significant role in antigen presentation and more so in antigen cross-presentation is controversial (Colonna et al., 2004; Liu, 2005; Villadangos and Young, 2008).We have recently shown in the mouse system that splenic CD8⁺ DCs (and their counterparts in other organs) are the only cells in the body expressing XCR1, a chemokine receptor with a unique ligand, XCL1 (Dorner et al., 2009). In vitro, XCL1 induces potent chemotaxis of XCR1⁺ CD8⁺ DCs. In vivo, XCL1 secreted by activated CD8⁺ T cells augments their expansion and differentiation into cytotoxic T cells when the antigen is cross-presented by CD8⁺ DCs in the context of MHC-I (Dorner et al., 2009). Collectively, these observations indicate that the XCL1–XCR1 communication axis optimizes the cooperation of antigen-specific CD8⁺ T cells with XCR1⁺ DCs, which cross-present antigen to them.Based on our studies in the mouse, we were interested to determine whether human DCs express XCR1. Human DCs have been extensively phenotyped in the past and subdivided again into pDC and into CD1c⁺ (BDCA-1⁺), CD16⁺, and CD141⁺ (BDCA-3⁺) cDC subsets (Dzionek et al., 2000; MacDonald et al., 2002; Piccioli et al., 2007; for review see Ju et al., 2010). Meticulous gene expression analyses of all human and mouse DCs have recently revealed a large gene expression program shared by human and mouse pDCs, and also led to the suggestion that human CD141⁺ DCs correspond to mouse CD8⁺ DCs (Robbins et al., 2008). In spite of this groundbreaking work on the subdivision of human DCs into subsets, information on the function of human primary DCs remained very scarce, apparently because of the limitations imposed by the very low frequencies of DCs in human blood (CD1c⁺ DCs, 0.31 ± 0.14% SD; CD16⁺ DCs, 0.75 ± 0.41%; CD141⁺ DCs, 0.04 ± 0.03%; pDCs, 0.29 ± 0.08%; n = 8; not depicted). Instead, antigen cross-presentation in the human system was essentially analyzed with DCs derived from monocytes in culture (Fonteneau et al., 2003), a system that may not reflect all of the functional properties of primary DCs.In the present study, we demonstrate that CD141⁺ DCs are the only population in human blood that expresses the chemokine receptor XCR1. Human CD141⁺ DCs react to the chemokine XCL1 by mobilization of intracellular Ca²⁺ ([Ca²⁺]_i) and by strong chemotaxis in vitro. More importantly, our experiments demonstrate that primary CD141⁺ DCs excel in cross-presentation of antigen when directly compared with CD1c⁺ DCs, CD16⁺ DCs, and pDCs from the same donors. Collectively, these functional data strongly indicate that human CD141⁺ DCs are the homologue of mouse CD8⁺ DCs. At the same time, the professional capacity of human CD141⁺ DCs to cross-present antigen is of major interest in the ongoing quest to develop vaccines capable of inducing antiviral or antitumor cytotoxicity in the human.     

Antagonist Peptide Selects Thymocytes Expressing a Class II Major Histocompatibility Complex–restricted T Cell Receptor into the CD8 Lineage

CD4/CD8 lineage decision is an important event during T cell maturation in the thymus. CD8 T cell differentiation usually requires corecognition of major histocompatibility complex (MHC) class I by the T cell receptor (TCR) and CD8, whereas CD4 T cells differentiate as a consequence of MHC class II recognition by the TCR and CD4. The involvement of specific peptides in the selection of T cells expressing a particular TCR could be demonstrated so far for the CD8 lineage only. We used mice transgenic for an MHC class II-restricted TCR to investigate the role of antagonistic peptides in CD4 T cell differentiation. Interestingly, antagonists blocked the development of CD4⁺ cells that normally differentiate in thymus organ culture from those mice, and they induced the generation of CD8⁺ cells in thymus organ culture from mice impaired in CD4⁺ cell development (invariant chain–deficient mice). These results are in line with recent observations that antagonistic signals direct differentiation into the CD8 lineage, regardless of MHC specificity.     

Liver Damage Preferentially Results from CD8+ T Cells Triggered by High Affinity Peptide Antigens

Little is understood of the anatomical fate of activated T lymphocytes and the consequences they have on the tissues into which they migrate. Previous work has suggested that damaged lymphocytes migrate to the liver. This study compares class I versus class II major histocompatibility complex (MHC)–restricted ovalbumin-specific T cell antigen receptor (TCR) transgenic mice to demonstrate that after in vivo activation with antigen the emergence of CD4⁻CD8⁻B220⁺ T cells occurs more frequently from a CD8⁺ precursor than from CD4⁺ T cells. Furthermore, this change in phenotype is conferred only by the high affinity native peptide antigen and not by lower affinity peptide variants. After activation of CD8⁺ cells with only the high affinity peptide, there is also a dramatically increased number of liver lymphocytes with accompanying extensive hepatocyte damage and elevation of serum aspartate transaminase. This was not observed in mice bearing a class II MHC–restricted TCR. The findings show that CD4⁻CD8⁻B220⁺ T cells preferentially derive from a CD8⁺ precursor after a high intensity TCR signal. After activation, T cells can migrate to the liver and induce hepatocyte damage, and thereby serve as a model of autoimmune hepatitis.     

Immune MAL2-practice: breast cancer immunoevasion via MHC class I degradation

The success of tumor immunotherapy, while partial, confirms the existence and importance of tumor immunosurveillance. CD8⁺ T cell recognition of tumor-specific peptides bound to MHC class I (MHC-I) molecules is central to this process. In this issue of the JCI, Fang, Wang, et al. describe a unique tumor immunoevasion strategy based on endocytosis and degradation of MHC-I complexes mediated by the trafficking factor MAL2. Notably, MAL2 expression was associated with poor prognosis of breast cancer, and its downregulation enhanced CD8⁺ T cell recognition of breast cancer in various experimental models. This work demonstrates that a deeper understanding of tumor interference with MHC-I stability and trafficking has considerable potential for enhancing immunotherapies.     

Transporters Associated with Antigen Processing (TAP)-independent Presentation of Soluble Insulin to α/β T Cells by the Class Ib Gene Product,Qa-1b

T cell hybridomas isolated from nonresponder H-2^b mice immunized with pork insulin were stimulated by insulin in the presence of major histocompatibility complex (MHC)-unmatched antigen presenting cells. The restriction element used by these CD4⁻ T cells was mapped to an oligomorphic MHC class Ib protein encoded in the T region and identified as Qa-1^b using transfectants. The antigenic determinant was localized to the insulin B chain, and experiments with truncated peptides suggested that it is unexpectedly long, comprising most or all of the 30 amino acid B chain. The antigen processing pathway used to present insulin to the Qa-1^b– restricted T cells does not require transporters associated with antigen processing (TAP), and it is inhibited by chloroquine. A wide variety of cell lines from different tissues efficiently present soluble insulin to Qa-1^b–restricted T cells, and insulin presentation is not enhanced by phagocytic stimuli. Our results demonstrate that Qa-1^b can function to present exogenous protein to T cells in a manner similar to MHC class II molecules. Therefore, this class Ib protein may have access to a novel antigen processing pathway that is not available to class Ia molecules.     

Arming Cytokine-induced Killer Cells With Chimeric Antigen Receptors: CD28 Outperforms Combined CD28–OX40 “Super-stimulation”

Cytokine-induced killer (CIK) cells raised interest for use in cellular antitumor therapy due to their capability to recognize and destroy autologous tumor cells in a HLA-independent fashion. The antitumor attack of CIK cells, predominantly consisting of terminally differentiated CD8⁺CD56⁺ cells, can be improved by redirecting by a chimeric antigen receptor (CAR) that recognizes the tumor cell and triggers CIK cell activation. The requirements for CIK cell activation were, however, so far less explored and are likely to be different from those of “younger” T cells. We revealed that CD28 and OX40 CARs produced higher interferon- secretion as compared with the first-generation ζ-CAR; CD28-ζ and the third-generation CD28-ζ–OX40 CAR, however, performed similar in modulating most CIK cell effector functions. Compared with the CD28-ζ CAR, however, the CD28-ζ–OX40 CAR accelerated terminal maturation of CD56⁺ CIK cells producing high frequencies in activation-induced cell death (AICD) and reduced antitumor efficiency in vivo. Consequently, CD28-ζ CAR CIK cells of CD56⁻ phenotype were superior in redirected tumor cell elimination. CAR-mediated CIK cell activation also increased antigen-independent target cell lysis; the CD28-ζ CAR was more efficient than the CD28-ζ–OX40 CAR. Translated into therapeutic strategies, CAR-redirected CIK cells benefit from CD28 costimulation; “super-costimulation” by the CD28-ζ–OX40 CAR, however, performed less in antitumor efficacy due to increased AICD.     

Immature Dendritic Cells Phagocytose Apoptotic Cells via αvβ5 and CD36, and Cross-present Antigens to Cytotoxic T Lymphocytes

Dendritic cells, but not macrophages, efficiently phagocytose apoptotic cells and cross-present viral, tumor, and self-antigens to CD8⁺ T cells. This in vitro pathway corresponds to the in vivo phenomena of cross-priming and cross-tolerance. Here, we demonstrate that phagocytosis of apoptotic cells is restricted to the immature stage of dendritic cell (DC) development, and that this process is accompanied by the expression of a unique profile of receptors, in particular the α_vβ₅ integrin and CD36. Upon maturation, these receptors and, in turn, the phagocytic capacity of DCs, are downmodulated. Macrophages engulf apoptotic cells more efficiently than DCs, and although they express many receptors that mediate this uptake, they lack the α_vβ₅ integrin. Furthermore, in contrast to DCs, macrophages fail to cross-present antigenic material contained within the engulfed apoptotic cells. Thus, DCs use unique pathways for the phagocytosis, processing, and presentation of antigen derived from apoptotic cells on class I major histocompatibility complex. We suggest that the α_vβ₅ integrin plays a critical role in the trafficking of exogenous antigen by immature DCs in this cross-priming pathway.