An in vitro model of peptide-mediated immunomodulation of the human T cell response to Dermatophagoides spp (house dust mite)

Allergic sensitivity of Dermatophagoides spp (house dust mites) is mediated by specific IgE antibody, the production of which requires the presence of CD4⁺ helper T cells. Attempts to hyposensitize this response in allergic individuals have depended on the administration of extracts of specific allergen. However, the ability of peptides derived from unrelated antigens to inhibit specific immune responses offers an alternative approach to therapy. We have addressed this question by examining the ability of a nonstimulatory peptide analogue derived from influenza virus hemagglutinin to modulate T cell recognition of house dust mite. The peptide inhibited the response of mite-specific CD4⁺ T cell clones restricted by either the HLA-DRABI or DRAB3 gene products. Furthermore, mite-induced polyclonal T cell responses were negatively modulated by the peptide, whereas recognition of common recall antigens remained intact. The inhibitory effects were mediated at the level of the antigen-presenting cell, since no inhibition of mitogen or anti-CD3 antibody-driven T cell proliferation was observed. In direct binding assays, the peptide analogue bound to selected HLA-DR molecules expressed on the membrane of antigen-presenting cells, with specificity predominantly for those class II proteins capable of restricting house dust mite-allergen T cell recognition.     

Characterization of the specificity and duration of T cell tolerance to intranasally administered peptides in mice: a role for intramolecular epitope suppression

Mucosal administration of antigens in experimental animals leads to the induction of peripheral T cell tolerance. We have previously reported that in H-2b mice, intranasal (i.n.) or oral administration of a peptide containing the immunodominant T cell epitope will down-regulate the function of CD4+ T cells reactive with Der P 1, a major target antigen in both B and T cell responses to house dust mite. In the present study we have investigated the tolerogenicity of peptides containing both dominant and subdominant determinants when given i.n. to nalve mice. Induction of tolerance by the nasally administered immunodominant peptide leads to a diminution in all T cell-derived cytokines and modulation of delayed-type hypersensitivity responses, but IgE production did not seem to be affected, furthermore the induction of T cell tolerance was stable, lasting beyond 6 months. We have also examined the specificity of intramolecular epitope suppression which is a feature of mucosal tolerance induced by nasally administered peptides and demonstrate that regulatory CD4+ T cells may exert their suppressive effect by linked recognition of epitopes on the same or neighbouring antigen-presenting cells.      

Clonal analysis of CD4 mediated accessory function on the effector activity of human CD4+ T cell subsets

Background: It has been reported for the peripheral T cell repertoire that CD4 molecules may enhance adhesion between T cells and antigen presenting cells and, through their physical association with T cell antigen receptors, contribute to signal transduction. Objective: The aims of this study were to determine if the modulation of CD4 molecules had differential effects on T cell recognition, antigen induced cytokine (IL-4 and IFNγ), release and the induction of specific anergy for human TH-0, TH-1 and TH-2 cells. Methods: A panel of anti-CD4 antibodies was examined for its ability to modulate T cell proliferation, cytokine production and tolerance induction in house dust mite (TH-0 and TH-2) and influenza haemagglutinin (TH-1) specific human CD4+ T cell clones all restricted by DRB1^*1101 and isolated from dust mite allergic individuals. Results: We observed that anti-CD4 antibodies may inhibit or enhance antigen mediated T cell proliferation, which may reflect the differential requirements of T cells for selective functions of CD4. Furthermore, IFNγ and IL-4 production was differentially modulated depending on the specificity of the anti-CD4 antibody and the clone of T cells. However, pretreatment of T cells with anti-CD4 antibody alone neither induced nor enhanced the susceptibility of T cells to peptide mediated anergy. Conclusion: Antigen recognition by different subsets of human CD4+ T cells has differential requirements on CD4, whereas the induction of specific anergy appeared to be independent of the functions of CD4 molecules. Antigen induced IFNγ production was more susceptible than IL-4 to the inhibitory effects of anti-CD4 antibodies. Furthermore, it appeared that certain anti-CD4 antibodies can dissociate antigen induced IFNγ and IL-4 production, and may downregulate IFNγ synthesis without inhibiting antigen dependent proliferation.     

Multispecific responses by T cells expanded by endogenous self-peptide/MHC complexes

The paradox of autoreactivity to self-peptides in physiological as opposed to pathological immune responses is not well understood. Here, we directly examined the human T cell response to endogenous self-peptides in a series of healthy subjects. CFSE-labeled T cells were stimulated with unmanipulated antigen-presenting cells containing endogenous self-antigen, and the resulting CD4+ populations entering into cell cycle (CFSE(low)) or non-proliferating CD4+ cells (CFSE(high)) were single-cell sorted, cloned and screened against a panel of self-antigens and microbial recall antigens to interrogate their antigen reactivity. The percentage of CD4+ T cells entering cell cycle in response to self-peptide/MHC was calculated to be 0.04%, and entry into cell cycle was dependent upon CD28 costimulation. Clones derived from CFSE(low) T cells exhibited significantly greater cross-reactivity to multiple antigens than CFSE(high) clones or other CD4+ clones generated after microbial antigen stimulation. Sequencing the TCRbeta chains indicated that CFSE(low) clones were indeed clonal. These data demonstrate that T cell clones generated on stimulation by endogenous self-peptides exhibit a high degree of multispecificity, and we speculate that their multispecificity is based upon recognition of shared-backbone MHC determinants.     

Highly efficient antigen targeting to M-DC8+ dendritic cells via FcgammaRIII/CD16-specific antibody conjugates

Conjugates of peptide antigens with antibodies specifically recognizing surface molecules on dendritic cells (DC) represent an attractive approach to target antigens to antigen-presenting cells (APC) for the induction of specific T cell responses. The present study evaluates the potential of M-DC8(+) DC, a sub-population of professional APC in the blood, for an antibody-based vaccination strategy. We prepared, by chemical cross-linking, conjugates of peptide model antigens with antibodies directed against different cell surface molecules of DC. Antigen-peptide conjugates using an anti-CD16 (FcgammaRIII) antibody were most potent in inducing in vitro activation of a specific CD4(+) T cell response. They were at least 300 times more efficient than two other antibody-antigen conjugates and approximately 500 times more efficient than unconjugated antigen peptides. Our data demonstrate that specific antigen targeting via CD16 on M-DC8(+) DC is a promising vaccination approach for the efficient induction of specific CD4(+) T cell responses ex vivo, and perhaps in vivo.     

Dendritic cells lose ability to present protein antigen after stimulating antigen-specific T cell responses, despite upregulation of MHC class II expression

Immature dendritic cells (DC) take up, process and present protein antigens; mature DC are specialized for stimulating primary T cell responses with increased expression of MHC class II and co-stimulatory molecules, but are incapable of processing and presenting soluble protein. The current study examined whether maturation of DC is triggered by T cell recognition of antigens presented by immature DC. Human DC derived from CD34+ progenitor cells by culture with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-6 (IL-6) in serum-free medium could prime naive CD4+ T cells to keyhole limpet hemocyanin (KLH) and ovalbumin (OVA). The cultured DC retained the ability to prime T cells to native protein for at least 15 days. To test for changes in DC function after participation in an immune response, DC were co-cultured with either allogeneic or autologous CD4+ T cells. DC co-cultured with autologous T cells retained the ability to prime T cells to intact protein antigens. By contrast, DC which had previously stimulated an allogeneic T cell response lost ability to prime T cells to soluble proteins. However, such induced a MLR and stimulated peptide-specific primary CD4+ T cell responses. This indicated that did not die or lose the ability to prime, but lost the ability to process and present subsequent antigens. Following participation in T cell activation, DC increased surface expression of MHC class II, co-stimulatory molecules CD40 and B7.2, and the intercellular adhesion molecule-1 (ICAM-1). In addition, our data suggest that interferon gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) are involved in this T cell-mediated DC maturation.     

T cell recognition of donor major histocompatibility complex class I peptides during allograft rejection.

LEW (RTI1) recipients of DA (RTIav1) skin and kidney allografts were tested for the capacity of their T lymphocytes to proliferate to three 22-24-amino acid peptides from the hypervariable regions of the RTI-Aav1 classical MHC class I molecule. Ten days after rejecting second-set DA kidney allografts, spleen cells (but interestingly not lymph node cells) from LEW recipients showed strong, LEW antigen-presenting cell-dependent, CD4+ T cell proliferation to peptide 1 (from the alpha helical region of the alpha 1 domain). CD8+ T cells showed no response to peptide 1. There was no response by the spleen cells to peptide 2 (from the beta sheet of the alpha 2 domain) or peptide 3 (from the alpha helical region of the alpha 2 domain). Immunization of LEW rats with pure RTI-Aav1 class I H chain in Freund's adjuvant gave responses identical to that seen after grafting, i.e. good CD4+ T cell proliferation to peptide 1, but none to peptides 2 and 3. However, immunization of LEW rats with peptides 1, 2 and 3 in Freund's adjuvant resulted in good CD4+ T cell proliferation responses to each of the peptides. These data demonstrate that indirect allorecognition can be stimulated by allograft rejection, and emphasize that the physiological processing of donor antigens will influence which peptides will be important in indirect allorecognition in transplantation.     

The alloresponse

The alloresponse can be divided into two components. The first of these is allorecognition, which refers to the recognition of antigens, expressed on the surface of cells of non-self origin, by the host's lymphocytes. The second part is the immune effector mechanisms generated by this recognition process. The molecules recognised have been termed histocompatibility antigens and fall into two categories. The strongest responses are provoked by allogeneic major histocompatibility complex (MHC) antigens. T cells recognise these antigens either directly or after being processed like conventional antigens by antigen-presenting cells, in what has been termed indirect presentation. In the context of MHC identity, responses are observed against the second category of antigens, namely minor histocompatibility antigens (mHAgs). Although weaker, these responses are of clinical importance, particularly in bone marrow transplant recipients. CD4+ T cells play a central role in orchestrating the immune response to alloantigens. They secrete cytokines to attract effector cells, such as macrophages and CD8+ T cells, into the graft and are able to interact with B cells that will secrete highly specific alloreactive antibodies. In clinical terms, the result of the immune response to transplanted allografts can be classified as hyperacute rejection, acute and chronic rejection. The immunological effector mechanisms involved in each of these processes are discussed.     

Optimal induction of antigen-specific CD8+ T cell responses requires bystander cell participation

Efficient activation of specific immune responses requires a concerted interaction between T cells and antigen-presenting cells. A requirement for bystander participation of CD4+ T cells for expansion and maintenance of memory CD8+ T cells has been noted in several models, but a role with regard to effector CD8+ T responses has not been well-defined. In this report, the requirement of bystander participation for optimal induction of antigen-specific CD8+ T cell effector function was determined by directly quantitating antigen-specific interferon-gamma (IFN-gamma) CD8+ T cell responses by enzyme-linked immunospot assays, and by indirectly evaluating induction of the chemokine monokine induced by IFN-gamma as a marker for IFN-gamma-mediated effector function. Our results demonstrate that bystander cell participation, mediated by CD4+ T cell and natural killer (NK) cells, is required for optimal induction of antigen-specific CD8+ T cell effector responses. Our data further establish a novel role for NK cells in the activation of antigen-specific immune responses.     

Considerations on clinical use of T cell immunotherapy for cancer

The recognition by effector T lymphocytes of novel antigenic targets on tumor cells is the premise of specific, targeted immunotherapy of cancer. With the molecular characterization of peptide epitopes from melanoma antigens and, more recently, broadly expressed tumor antigens, there has been considerable enthusiasm for clinical evaluation of peptide tumor vaccines. Immunologic monitoring of vaccinated patients has demonstrated an expansion of CD8+ T cells that react with the relevant peptide and, more importantly, with native tumor. In most instances, however, vaccine-induced CD8+ T cell responses alone have not been sufficiently robust or sustained to translate into a high percentage of durable clinical responses. Vaccine strategies have also utilized dendritic cells (DCs) that have been modified to present tumor antigens. The superior antigen-processing capacity and co-stimulatory function of DCs convey a powerful stimulatory signal to both CD4+ and CD8+ T cells. Several strategies are attempting to broaden the immune response beyond single antigens by introducing the entire complement of tumor antigens into DCs. Adoptive immunotherapy is a promising strategy to recover tumor-reactive precursor T cells from patients, stimulate them to induce numerical expansion, and then re-infuse them. Ex vivo manipulation of the tumor-reactive T cells also permits cytotoxic therapy to be administered to the patient without damaging the effector cells. Recently, host lymphodepletion prior to adoptive transfer of effector T cells has resulted in an extremely high and sustained frequency of effectors that has achieved therapeutic efficacy against bulky metastatic disease in a substantial fraction of treated patients.     

The role of B cell antigen presentation in the initiation of CD4+ T cell response

B cells have been known for their ability to present antigens to T cells for almost 40 years. However, the precise roles of B cell antigen presentation in various immune responses are not completely understood. The term “professional” antigen-presenting cells (APCs) was proposed to distinguish APCs that are required for initiating the immune responses from those use antigen presentation to enhance their own effector functions. Unlike dendritic cells, which are defined as professional APCs for their well-established functions in activating naive T cells, B cells have been shown in the past to mostly present antigens to activated CD4+ T cells mainly to seek help from T helper cells. However, recent evidence suggested that B cells can act as professional APCs under infectious conditions or conditions mimicking viral infections. B cell antigen receptors (BCRs) and the innate receptor Toll-like receptors are activated synergistically in response to pathogens or virus-like particles, under which conditions B cells are not only potent but also the predominant APCs to turn naive CD4+ T cells into T follicular helper cells. The discovery of B cells as professional APCs to initiate CD4+ T cell response provides a new insight for both autoimmune diseases and vaccine development.     

General and specific immunosuppression caused by antiviral T-cell responses

Summary: Immunosuppression caused by the non-cytopathic lymphocytic choriomeningltis virus (LCMV) (an RNA virus) is mediated by anti-viral cytotoxic T cells that destroy LCMV-infected cells, also of the immune system. While this immunopathological destruction of antigen-presenting cells, macrophages and follicular dendritic cells and of some CD4⁺ T cells causes general immunosuppression and impairs immune response to third party antigens, it also enhances exhaustion/deletion of LCMV-specific CD8⁺ T-cell responses. LCMV seems in addition to infect neutralizing antibody-producing B cells via the specific receptor; immunopathological LCMV-specific CD8T T-cell-mediated elimination of these infected B cells (but not of uninfected internal virus antigen-specific B cells) causes a highly specific immunosuppression that delays neutralizing antibody responses and thereby enhances virus persistence. Both generalized and specific immunosuppression by CD8⁺ T-cell-mediated immunopathology may be involved in human infections with HIV; hepatitis B virus or hepatitis C virus.     

Tails of wonder: endocytic-sorting motifs key for exogenous antigen presentation

Antigen-presenting molecules, including MHC I, II and CD1, have central roles in the induction of T cell-mediated immunity against pathogens and tumors and also in the maintenance of tolerance towards self-antigens. The presentation of exogenously derived peptide and lipid antigens to specific T cells by professional antigen-presenting cells (pAPCs) is an essential part of both processes. Exogenous antigen loading takes place mostly within specialized endocytic and phagocytic compartments of pAPCs and targeting of antigen-presenting molecules to these intracellular compartments is mediated by highly conserved cytoplasmic sorting motifs. Recent data have revealed that the cytoplasmic tails of antigen-presenting molecules, by controlling the access of these molecules to exogenously derived antigens, have a crucially important and largely underappreciated role in the generation of tolerance and T-cell mediated immunity.     

Use of Liposomes as an Immunopotentiating Delivery System: in Perspective of Vaccine Development

Liposomes have been widely used to deliver antigens to the antigen-presenting cells (APCs) and also to modify their immunological behaviour in model animals. We recently demonstrated the potential of yeast lipid liposomes to undergo membrane-membrane fusion with cytoplasmic membrane of the target cells. Interestingly, studies in the present report revealed that antigen encapsulated in yeast lipid liposomes could be successfully delivered simultaneously into the cytosolic as well as endosomal processing pathways of APCs, leading to the generation of both CD4+ T helper and CD8+ cytotoxic T cells. In contrast, encapsulation of same antigen in egg phosphatidyl-choline (PC) liposomes, just like its free form, has inefficient access to the cytosolic pathway of major histocompatibility complex (MHC) I dependent antigen presentation and failed to generate antigen specific CD8+ cytotoxic T-cell response. However, both egg PC as well as yeast lipid liposomes have elicited strong antigen specific antibody responses in immunized animals. These results imply usage of liposome encapsulated antigen as potential candidate vaccine capable of eliciting both cell mediated as well as humoral immune responses.     

A Human Melanoma Cell Line, Recognized by Both HLA Class I and Class II Restricted T Cells, is Capable of Initiating both Primary and Secondary Immune Responses

We have characterized a melanoma cell line, FM3, established from a metastasis of a 75 year old female patient (HLA–A2, HLA–DQ7) with malignant melanoma. This cell line expresses both HLA class I and class II antigens, as well as several important accessory molecules at high levels. FM3 cells were shown to function as a stimulator of both allogeneic as well as autologous mixed lymphocyte tumour cell culture (MLTC). From these autologous MLTC we were able to generate cytotoxic T cell clones indicating that FM3 is capable of processing and presenting endogenous antigens.
We have used this cell line in a model system to investigate whether these cells were able to initiate and support an immune response with specificity for selected peptide antigens. The FM3 cell line was capable of presenting a HLA–DQ7 restricted ras derived peptide (5–21, 13Gly–Asp) to a previously established T cell clone, RM70. The ability of FM3 to function as an antigen presenting cell (APC) was comparable to that of an autologous Epstein Barr virus (EBV) transformed B cell line. The CD4⁺ T cell clone RM70 showed a peptide–specific anti–proliferative effect on FM3 cells. This growth inhibition was not due to cytotoxicity as measured in a standard 4h chromium release assay. The FM3 cell line also presented a HLA–A2 restricted nonapeptide derived from the influenza matrix protein. M 1 (58–66) to a CD8⁺ T cell line specific for this peptide. This resulted in an effective killing of the melanoma cells.
Together, these data suggest that some melanomas may initiate an immune response by presenting their own specific antigens in an immunogenic context, and subsequently serve as targets for T cells of both the CD4⁺ and CD8⁺ phenotype.     

The role of CD40 in peripheral T cell tolerance and immunity

CD40 and CD40 ligand (CD40L) have been implicated as important molecules for the transformation of nonactivated antigen-presenting cells (APC) into cells that are potent inducers of cytotoxic T lymphocyte (CTL) immunity. The onset of a successful immune response lies within the control of the CD4+ T helper cells which, after specific antigen recognition, can up-regulate CD40L and subsequently activate APC through CD40 signaling. Triggering of CD40 with antibodies in vivo can replace the need for CD40L-expressing CD4+ T helper cells for cross-priming of CTL. Blocking of CD40-CD40L interactions can also have profound effects on the generation of T cell immunity. Interestingly, differential involvement of CD40/CD40L in immune responses can be observed between various immunological sites in the body. In most sites of the periphery interruption of CD40-CD40L interactions can lead to the induction of T cell tolerance whereas in mucosal tissues this interruption can lead to abrogation of T cell tolerance. Furthermore, in vivo CD40 activation can convert specific T cell tolerance following peptide vaccination into efficient T cell priming. Thus intervention of CD40-CD40L interactions can result in enhancement or down-modulation of T cell reactivity and therefore modulation of these interactions may form the foundation of new treatment modalities directed against malignancies, allergies, organ rejections and autoimmunity.     

Functional inactivation of Dermatophagoides spp. (house dust mite) reactive human T-cell clones

Staphylococcal enterotoxins are able both to stimulate powerful polyclonal proliferative responses and to induce non-responsiveness of T lymphocytes expressing the appropriate T-cell antigen receptor V beta gene products. T-cell clones representative of the human response to house dust mite were identified that express either V beta 3 or V beta 6 gene products. The specificity of the latter was confirmed by serology. Pre-treatment of cloned V beta 3+ T cells with the Staphylococcus aureus enterotoxins B or C1 rendered them non-responsive to immunogenic challenge with their natural ligand, while retaining responsiveness to exogenous IL-2. Similarly, exposure of the V beta 6+ dust mite reactive T cells to the staphylococcal enterotoxin of the appropriate specificity, SEE, induced specific anergy. The development of non-responsiveness was associated with changes in the T-cell phenotypes. Downregulation of the T-cell receptor, Ti-CD3, was paralleled by enhanced expression of both CD2 and the IL-2 receptor, CD25. Differential co-modulation of CD4 and Ti-CD3 suggested that for some T cells CD4 may form part of the specific antigen recognition structure. Toxicity of the staphylococcal enterotoxins may be removed by chemical modification, thus their ability functionally to inactivate subpopulations of T cells expressing antigen-specific receptors with shared characteristics may be of potential value in the regulation of allergic diseases if the diversity of the T-cell repertoire proves to be limited.     

Oligopeptide Induction of a Secondary Cytotoxic T-cell Response to Epstein-Barr Virus In Vitro

Three Epstein-Barr virus (EBV) nuclear antigen (EBNA)-encoded oligopeptide epitopes have been mapped, each capable of acting as a recognition determinant for class I-restricted lysis by CD8+ cytotoxic T lymphocytes (CTL). This report shows that each peptide, when presented on an appropriate autologous antigen-presenting cell (APC), also stimulates EBV-specific memory T cells present in peripheral blood mononuclear cell (PBMC) populations to develop in vitro into peptide-specific CTL. These CTL specifically lysed autologous EBV-infected lymphoblastoid cell lines (LCL) and peptide-sensitized uninfected targets. Identical viral oligopeptides could therefore function as recognition determinants for both the induction and commission of class I-restricted specific cytotoxicity. A model system is described in which autologous phytohaemagglutinin (PHA) blasts present exogenous peptide during the stimulation phase. The magnitude of the peptide-specific CTL response was dependent on the concentration of peptide added to the APC and specific lysis was inhibited by anti-class I monoclonal antibody (MoAb) but not anti-class II MoAb. Cultures depleted of CD8+ T cells by cell separation with immunomagnetic beads prior to stimulation invariably failed to generate a peptide-specific CTL response. However, the effect of CD4 depletion on CTL activity was equivocal and indicated that a need for CD4+ T cells as accessory helper cells may depend on the efficiency of the APC to elaborate their own help. This model has advantages in the analysis of events involved in the development of CTL activity in vitro.     

Eosinophils can function as antigen-presenting cells to induce primary and secondary immune responses to Strongyloides stercoralis

Several studies have demonstrated roles for eosinophils during innate and adaptive immune responses to helminth infections. However, evidence that eosinophils are capable of initiating an immune response to parasite antigens is lacking. The goal of the present in vitro study was to investigate the potential of eosinophils to serve as antigen-presenting cells (APC) and initiate an immune response to parasite antigens. Purified eosinophils were exposed to soluble Strongyloides stercoralis antigens, and the expression of various surface markers involved in cell activation was examined. Antigen-exposed eosinophils showed a sixfold increase in expression levels of CD69 and major histocompatibility complex (MHC) class II, a fourfold increase in levels of T-cell costimulatory molecule CD86, and a twofold decrease in levels of CD62L compared to eosinophils cultured in medium containing granulocyte-macrophage colony-stimulating factor. The ability of eosinophils to present antigen to T cells was determined by culturing them with T cells in vitro. Eosinophils pulsed with antigen stimulated antigen-specific primed T cells and CD4+ T cells to increase interleukin-5 (IL-5) production. The blocking of MHC class II expression on eosinophils inhibited their ability to induce IL-5 production by CD4+ T cells in culture. Antigen-pulsed eosinophils were able to prime na?ve T cells and CD4+ T cells in culture and polarized them into Th2 cells producing IL-5 similar to that induced by antigen-loaded dendritic cells. These results demonstrate that eosinophils are capable of activating antigen-specific Th2 cells inducing the release of cytokines and assist in the priming of na?ve T cells to initiate Th2 responses against infection. This study highlights the potential of eosinophils to actively induce immune responses against infection by amplifying antigen-specific Th2-cell responses.     

Ito cells are liver-resident antigen-presenting cells for activating T cell responses

Here we identified Ito cells (hepatic stellate cells, HSC), known for storage of vitamin A and participation in hepatic fibrosis, as professional liver-resident antigen-presenting cells (APC). Ito cells efficiently presented antigens to CD1-, major histocompatibility complex (MHC)-I-, and MHC-II-restricted T cells. Ito cells presented lipid antigens to CD1-restricted T lymphocytes such as natural killer T (NKT) cells and promoted homeostatic proliferation of liver NKT cells through interleukin-15. Moreover, Ito cells presented antigenic peptides to CD8(+) and CD4(+) T cells and mediated crosspriming of CD8(+) T cells. Peptide-specific T cells were activated by transgenic Ito cells presenting endogenous neoantigen. Upon bacterial infection, Ito cells elicited antigen-specific T cells and mediated protection. In contrast to other liver cell types that have been implicated in induction of immunological tolerance, our data identify Ito cells as professional intrahepatic APCs activating T cells and eliciting a multitude of T cell responses specific for protein and lipid antigens.