Toxoplasma gondii HLA-B*0702-restricted GRA7(20-28) peptide with adjuvants and a universal helper T cell epitope elicits CD8(+) T cells producing interferon-γ and reduces parasite burden in HLA-B*0702 mice

The ability of CD8⁺ T cells to act as cytolytic effectors and produce interferon-γ (IFN-γ) was demonstrated to mediate resistance to Toxoplasma gondii in murine models because of the recognition of peptides restricted by murine major histocompatibility complex (MHC) class I molecules. However, no T gondii-specific HLA-B07-restricted peptides were proven protective against T gondii. Recently, 2 T gondii-specific HLA-B*0702-restricted T cell epitopes, GRA7_20-28 (LPQFATAAT) and GRA3_27-35 (VPFVVFLVA), displayed high-affinity binding to HLA-B*0702 and elicited IFN-γ from peripheral blood mononuclear cells of seropositive HLA-B*07 persons. Herein, these peptides were evaluated to determine whether they could elicit IFN-γ in splenocytes of HLA-B*0702 transgenic mice when administered with adjuvants and protect against subsequent challenge. Peptide-specific IFN-γ-producing T cells were identified by enzyme-linked immunosorbent spot and proliferation assays utilizing splenic T lymphocytes from human lymphocyte antigen (HLA) transgenic mice. When HLA-B*0702 mice were immunized with one of the identified epitopes, GRA7_20-28 in conjunction with a universal CD4⁺ T cell epitope (PADRE) and adjuvants (CD4⁺ T cell adjuvant, GLA-SE, and TLR2 stimulatory Pam₂Cys for CD8⁺ T cells), this immunization induced CD8⁺ T cells to produce IFN-γ and protected mice against high parasite burden when challenged with T gondii. This work demonstrates the feasibility of bioinformatics followed by an empiric approach based on HLA binding to test this biologic activity for identifying protective HLA-B*0702-restricted T gondii peptides and adjuvants that elicit protective immune responses in HLA-B*0702 mice.     

Sustained cross‐presentation capacity of murine splenic dendritic cell subsets in vivo

An exclusive feature of dendritic cells (DCs) is their ability to cross‐present exogenous antigens in MHC class I molecules. We analyzed the fate of protein antigen in antigen presenting cell (APC) subsets after uptake of naturally formed antigen‐antibody complexes in vivo. We observed that murine splenic DC subsets were able to present antigen in vivo for at least a week. After ex vivo isolation of four APC subsets, the presence of antigen in the storage compartments was visualized by confocal microscopy. Although all APC subsets stored antigen for many days, their ability and kinetics in antigen presentation was remarkably different. CD8α⁺ DCs showed sustained MHC class I‐peptide specific CD8⁺ T‐cell activation for more than 4 days. CD8α^? DCs also presented antigenic peptides in MHC class I but presentation decreased after 48 h. In contrast, only the CD8α^? DCs were able to present antigen in MHC class II to specific CD4⁺ T cells. Plasmacytoid DCs and macrophages were unable to activate any of the two T‐cell types despite detectable antigen uptake. These results indicate that naturally occurring DC subsets have functional antigen storage capacity for prolonged T‐cell activation and have distinct roles in antigen presentation to specific T cells in vivo.     

CD8+ T cells of Listeria monocytogenes‐infected mice recognize both linear and spliced proteasome products

CD8⁺ T cells responding to infection recognize pathogen‐derived epitopes presented by MHC class‐I molecules. While most of such epitopes are generated by proteasome‐mediated antigen cleavage, analysis of tumor antigen processing has revealed that epitopes may also derive from proteasome‐catalyzed peptide splicing (PCPS). To determine whether PCPS contributes to epitope processing during infection, we analyzed the fragments produced by purified proteasomes from a Listeria monocytogenes polypeptide. Mass spectrometry identified a known H‐2K^b‐presented linear epitope (LLO_296‐304) in the digests, as well as four spliced peptides that were trimmed by ERAP into peptides with in silico predicted H‐2K^b binding affinity. These spliced peptides, which displayed sequence similarity with LLO_296‐304, bound to H‐2K^b molecules in cellular assays and one of the peptides was recognized by CD8⁺ T cells of infected mice. This spliced epitope differed by one amino acid from LLO_296‐304 and double staining with LLO_296‐304‐ and spliced peptide‐folded MHC multimers showed that LLO_296‐304 and its spliced variant were recognized by the same CD8⁺ T cells. Thus, PCPS multiplies the variety of peptides that is processed from an antigen and leads to the production of epitope variants that can be recognized by cross‐reacting pathogen‐specific CD8⁺ T cells. Such mechanism may reduce the chances for pathogen immune evasion.     

Extensive major histocompatibility complex class I binding promiscuity for Mycobacterium tuberculosis TB10.4 peptides and immune dominance of human leucocyte antigen (HLA)‐B*0702 and HLA‐B*0801 alleles in TB10.4 CD8+ T‐cell responses

The molecular definition of major histocompatibility complex (MHC) class I‐presented CD8⁺ T‐cell epitopes from clinically relevant Mycobacterium tuberculosis (Mtb) target proteins will aid in the rational design of T‐cell‐based diagnostics of tuberculosis (TB) and the measurement of TB vaccine‐take. We used an epitope discovery system, based on recombinant MHC class I molecules that cover the most frequent Caucasian alleles [human leucocyte antigen (HLA)‐A*0101, A*0201, A*0301, A*1101, A*2402, B*0702, B*0801 and B*1501], to identify MHC class I‐binding peptides from overlapping 9‐mer peptides representing the Mtb protein TB10.4. A total of 33 MHC class I‐binding epitopes were identified, spread across the entire amino acid sequence, with some clustering at the N‐ and C‐termini of the protein. Binding of individual peptides or closely related peptide species to different MHC class I alleles was frequently observed. For instance, the common motif of xIMYNYPAMx bound to six of eight alleles. Affinity (50% effective dose) and off‐rate (half life) analysis of candidate Mtb peptides will help to define the conditions for CD8⁺ T‐cell interaction with their nominal MHC class I‐peptide ligands. Subsequent construction of tetramers allowed us to confirm the recognition of some of the epitopes by CD8⁺ T cells from patients with active pulmonary TB. HLA‐B alleles served as the dominant MHC class I restricting molecules for anti‐Mtb TB10.4‐specific CD8⁺ T‐cell responses measured in CD8⁺ T cells from patients with pulmonary TB.     

Epithelial expression of human papillomavirus type 16 E7 protein results in peripheral CD8 T‐cell suppression mediated by CD4+CD25+ T cells

The role of thymic versus peripheral epithelium in the regulation of the antigen‐specific CD8 T‐cell repertoire is still largely unresolved. We generated TCR‐β chain transgenic mice in which an increased frequency of peripheral CD8 T cells recognizes an epitope from a viral oncoprotein (HPV16E7) in the context of H‐2D^b MHC class I. When T cells from these mice developed through the thymus of mice expressing functional E7 protein from a keratin 14 promoter, no major perturbation to transgenic T‐cell development in the thymus was observed in these double‐transgenic mice. In contrast, peripheral CD8 T‐cell responses in the single‐transgenic, K14E7 mice, including those unrelated to E7 antigen, are reduced whereas CD4 T‐cell responses and antibody production are unchanged in these mice. Peripheral non‐responsiveness among CD8 T cells was mediated largely by CD4⁺CD25⁺ T cells. This suggested that epithelium expressing HPV16E7 protein induces Treg that specifically down‐regulate CD8 T‐cell responses in the periphery. This may have important consequences for the treatment of cervical pre‐cancers and provides a model for understanding differential suppression of T and B lymphocyte subsets by Treg.     

The impact of HLA class I and EBV latency‐II antigen‐specific CD8+ T cells on the pathogenesis of EBV+ Hodgkin lymphoma

In 40% of cases of classical Hodgkin lymphoma (cHL), Epstein–Barr virus (EBV) latency‐II antigens [EBV nuclear antigen 1 (EBNA1)/latent membrane protein (LMP)1/LMP2A] are present (EBV⁺cHL) in the malignant cells and antigen presentation is intact. Previous studies have shown consistently that HLA‐A*02 is protective in EBV⁺cHL, yet its role in disease pathogenesis is unknown. To explore the basis for this observation, gene expression was assessed in 33 cHL nodes. Interestingly, CD8 and LMP2A expression were correlated strongly and, for a given LMP2A level, CD8 was elevated markedly in HLA‐A*02^– versus HLA‐A*02⁺ EBV⁺cHL patients, suggesting that LMP2A‐specific CD8⁺ T cell anti‐tumoral immunity may be relatively ineffective in HLA‐A*02^– EBV⁺cHL. To ascertain the impact of HLA class I on EBV latency antigen‐specific immunodominance, we used a stepwise functional T cell approach. In newly diagnosed EBV⁺cHL, the magnitude of ex‐vivo LMP1/2A‐specific CD8⁺ T cell responses was elevated in HLA‐A*02⁺ patients. Furthermore, in a controlled in‐vitro assay, LMP2A‐specific CD8⁺ T cells from healthy HLA‐A*02 heterozygotes expanded to a greater extent with HLA‐A*02‐restricted compared to non‐HLA‐A*02‐restricted cell lines. In an extensive analysis of HLA class I‐restricted immunity, immunodominant EBNA3A/3B/3C‐specific CD8⁺ T cell responses were stimulated by numerous HLA class I molecules, whereas the subdominant LMP1/2A‐specific responses were confined largely to HLA‐A*02. Our results demonstrate that HLA‐A*02 mediates a modest, but none the less stronger, EBV‐specific CD8⁺ T cell response than non‐HLA‐A*02 alleles, an effect confined to EBV latency‐II antigens. Thus, the protective effect of HLA‐A*02 against EBV⁺cHL is not a surrogate association, but reflects the impact of HLA class I on EBV latency‐II antigen‐specific CD8⁺ T cell hierarchies.     

IL‐15 is critical for the maintenance and innate functions of self‐specific CD8+ T cells

IL‐15 is a pleiotropic cytokine involved in host defense as well as autoimmunity. IL‐15‐deficient mice show a decrease of memory phenotype (MP) CD8⁺ T cells, which develop naturally in naïve mice and whose origin is unclear. It has been shown that self‐specific CD8⁺ T cells developed in male H‐Y antigen‐specific TCR transgenic mice share many similarities with naturally occurring MP CD8⁺ T cells in normal mice. In this study, we found that H‐Y antigen‐specific CD8⁺ T cells in male but not female mice decreased when they were crossed with IL‐15‐deficient mice, mainly due to impaired peripheral maintenance. The self‐specific TCR transgenic CD8⁺ T cells developed in IL‐15‐deficient mice showed altered surface phenotypes and reduced effector functions ex vivo. Bystander activation of the self‐specific CD8⁺ T cells was induced in vivo during infection with Listeria monocytogenes, in which proliferation but not IFN‐γ production was IL‐15‐dependent. These results indicated important roles for IL‐15 in the maintenance and functions of self‐specific CD8⁺ T cells, which may be included in the naturally occurring MP CD8⁺ T‐cell population in naïve normal mice and participate in innate host defense responses.     

DC‐induced CD8+ T‐cell response is inhibited by MHC class II‐dependent DX5+CD4+ Treg

CD4⁺ T cells are important for CD8⁺ T‐cell priming by providing cognate signals for DC maturation. We analyzed the capacity of CD4⁺ T cells to influence CD8⁺ T‐cell responses induced by activated DC. Surprisingly, mice depleted for CD4⁺ cells were able to generate stronger antigen‐specific CD8⁺ T‐cell responses after DC vaccination than non‐depleted mice. The same observation was made when mice were vaccinated with MHC class II^?/? DC, indicating the presence of a MHC class II‐dependent CD4⁺ T‐cell population inhibiting CD8⁺ T‐cell responses. Recently we described the expansion of DX5⁺CD4⁺ T cells, a T‐cell population displaying immune regulatory properties, upon vaccination with DC. Intriguingly, we now observe an inverse correlation between CD8⁺ T‐cell induction and expansion of DX5⁺CD4⁺ T cells as the latter cells did not expand after vaccination with MHC class II^?/? DC. In vitro, DX5⁺CD4⁺ T cells were able to limit proliferation, modulate cytokine production and induce Foxp3⁺ expression in OVA‐specific CD8⁺ T cells. Together, our data show an inhibitory role of CD4⁺ T cells on the induction of CD8⁺ T‐cell responses by activated DC and indicate the involvement of DX5⁺CD4⁺, but not CD4⁺CD25⁺, T cells in this process.     

Murine CD8+ T suppressors against mycobacterial 65-kDa antigen compete for IL-2 and show lack of major histocompatibility complex-imposed restriction specificity in antigen recognition

The mechanism of antigen-specific suppression and reasons for aberrant major histocompatibility complex (MHC) class II restriction mediated by CD8⁺ T cells was investigated in a previously reported murine model of immunosuppression, generated by intraperitoneal priming with Mycobacterium vaccae. Both the CD4⁺ T helper cells (T_h) and CD8⁺ T supressor cell (T_s) of M.vaccae-primed mice recognized the 65-kDa antigen of the bacillus, presented by I-A and I-E, respectively. The CD8⁺ T_s could inhibit non-antigen-specific proliferation of primed CD4⁺ T cells induced by the exogenously added interleukin (IL)-2 (concanvalin A-stimulated culture supernatant). For inhibition, the T_s had to be activated by the 65-kDa antigen. The degree of inhibition was dependent upon the amount of added IL-2 and the relative numbers of primed CD8⁺ and CD4⁺ T cells. On incubation with antigen-presenting cells, and the 65-kDa antigen, the primed CD8⁺ T cells absorbed IL-2 as efficiently as primed CD4⁺ T cells. Based on this, it was concluded that the primed CD8⁺ T cells induced suppression by competition for IL-2. Employing the same model, the MHC restriction of recognition of the suppressor epitope of the 65-kDa antigen by the CD8⁺ T_s was investigated. The epitopes presented by diverse MHC class II molecules, such as self I-A, I-E and even allogeneic I-E were similar, because they were recognized by the same population of primed CD8⁺ T_s. Further, immunization of C57BL/6 mice with Ltk-cells expressing H-2 D^kK^k alloantigens, stimulated CD8⁺ T cells capable of recognizing M.vaccae 65-kDa antigen. Based on these data, it was proposed that recognition of the suppressor epitope of the 65-kDa antigen by the primed CD8⁺ T_s exhibits lack of restriction specificity imposed by MHC diversity.     

Parenchymal cells critically curtail cytotoxic T‐cell responses by inducing Bim‐mediated apoptosis

To develop cytolytic effector functions, CD8⁺ T lymphocytes need to recognize specific Ag/MHC class I complexes in the context of costimuli on Ag‐presenting DC. Thereafter they differentiate into effector and memory CTL able to confer protection against pathogen infection. Using transgenic mice with DC‐selective MHC class I expression and DC‐specific versus ubiquitous vaccination regimen, we found that DC are sufficient to prime CTL responses. However, Ag recognition on parenchymal non‐professional APC negatively affected CD8⁺ T‐cell responses in mice by inducing expression of the pro‐apoptotic bcl2‐family member bim in CTL. This unexpected induction of apoptosis in the early phase of effector CTL accumulation lead to suboptimal clonal burst size and diminished long‐term memory. Thus, our data demonstrate that effector CTL differentiation and apoptosis are regulated independently. Moreover, Ag distribution on cells other than DC critically reduces CTL responses.     

Generation, specificity, and function of CD8+ T cells in Trypanosoma cruzi infection

Summary: CD8⁺ T cells are crucial to the control of Trypanosoma cruzi infection and probably act via multiple mechanisms, the most important being the production of interferon‐γ (IFN‐γ). In the absence of CD8⁺ T cells, mice quickly succumb to the infection or develop a more severe chronic disease. Reduced production of IFN‐γ by CD8⁺ T cells is also associated with increased severity of chagasic disease in humans. CD8⁺ T cells in chronic T. cruzi infection are maintained as effector memory cells, undergo rapid expansion, and demonstrate effector functions following re‐exposure to antigen. However, the initial generation of T. cruzi‐specific CD8⁺ T‐cell responses appears to be relatively slow to develop. In addition, the expression of the effector function of the CD8⁺ T cells is compromised in some tissues, particularly in muscle. The targets of effective CD8⁺ T‐cell responses in T. cruzi infection are multiple and varied, and they represent some of the best vaccine candidates described to date. Further analysis of CD8⁺ T cells will provide insight into the disease process in T. cruzi infection and should identify methods to assess and enhance immunity to T. cruzi infection and protection from the symptoms of Chagas' disease.     

Activated monocytes prime naïve T cells against autologous cancer: vigorous cancer destruction in vitro and in vivo

It has not been considered so far that antigen‐presenting cells (APC) may phagocytose immunogenic material from autologous cancer cells. Assuming the presence of cancer‐immunogenic material in APC, we developed a novel autologous priming method that does not require tumour cells or identified peptides. Cancer‐immunogenic information came from CD14⁺ monocytes. When stimulated with CD3‐activated T cells, monocytes primed CD3⁺CD4⁺ and CD3⁺CD8⁺ resting/naïve T cells to become powerful effector cells within 24 h. During priming, depletion of CD14⁺ monocytes but not CD1a⁺ CD83⁺ dendritic cells prevented T cell priming. During cancer cell destruction, dendritic cells, but not monocytes, enhanced cancer cell lysis. The cascade‐primed (CAPRI) immune cell quartet comprising monocytes, dendritic cells, CD4⁺ T and CD8⁺ T cells induced a significant decrease in the number of suppressive CD25^highFoxP3⁺CD4⁺ T cells. CAPRI cells induced a marked upregulation of MHC class I and class II expression in cancer cells, which is crucial for autoimmune‐like lysis. We show in vivo evidence of the CAPRI cell concept in nude mice. In humans, we present circumstantial clinical evidence showing the efficacy of CAPRI cells in an adjuvant treatment attempt for breast cancer patients with metastasis (N = 42). Compared to patients at the Munich Tumor Center (no CAPRI treatment N = 428), almost double the expected number of patients survived 5 years (P = 1.36 × 10^?14). The CAPRI method is a safe procedure without negative side effects. High numbers of cancer‐specific CAPRI cells can be obtained within a week against different cancer types for efficient adoptive cell therapy.     

Co‐expression of HLA‐B7 and HLA‐B27 alleles is associated with B7‐restricted immunodominant responses following influenza infection

It is recognized that host response following viral infection is characterized by immunodominance, but deciphering the different factors contributing to immunodominance has proved a challenge due to concurrent expression of multiple MHC class I alleles. To address this, we generated H2‐K^?/?/D^?/? double‐knockout transgenic mice expressing either one or two human MHC‐I alleles. We hypothesized that co‐expression of different allele combinations figures critically in immunodominance and examined this in influenza‐infected, double Tg MHC‐I mice. In A2/B7 or A2/B27 mice, using ELISpot assays with the A2‐restricted matrix I.58–66, the B7‐restricted NP418–426 or the B27‐restricted NP383–391 influenza A (flu) epitopes, we observed the expected recognition of both peptides for both alleles. In contrast, in flu‐infected B7/B27 mice, a significantly reduced level of B27/NP383‐restricted CTL response was detected while there was no change in the B7/NP418‐restricted CTL response. Flu‐specific tetramer studies revealed a partial deletion of Vβ8.1⁺ NP383/B27‐restricted CD8⁺ T cells, and a diminished Vβ12⁺ CD8⁺ T‐cell expansion in B7/B27 Tg mice. Using HLA Tg chimeric mice, we confirmed these findings. These findings shed light on the immune consequences of co‐dominant expression of MHC‐I alleles for host immune response to pathogens.     

Antigen processing and immune regulation in the response to tumours

The MHC class I and II antigen processing and presentation pathways display peptides to circulating CD8⁺ cytotoxic and CD4⁺ helper T cells respectively to enable pathogens and transformed cells to be identified. Once detected, T cells become activated and either directly kill the infected / transformed cells (CD8⁺ cytotoxic T lymphocytes) or orchestrate the activation of the adaptive immune response (CD4⁺ T cells). The immune surveillance of transformed/tumour cells drives alteration of the antigen processing and presentation pathways to evade detection and hence the immune response. Evasion of the immune response is a significant event tumour development and considered one of the hallmarks of cancer. To avoid immune recognition, tumours employ a multitude of strategies with most resulting in a down‐regulation of the MHC class I expression at the cell surface, significantly impairing the ability of CD8⁺ cytotoxic T lymphocytes to recognize the tumour. Alteration of the expression of key players in antigen processing not only affects MHC class I expression but also significantly alters the repertoire of peptides being presented. These modified peptide repertoires may serve to further reduce the presentation of tumour‐specific/associated antigenic epitopes to aid immune evasion and tumour progression. Here we review the modifications to the antigen processing and presentation pathway in tumours and how it affects the anti‐tumour immune response, considering the role of tumour‐infiltrating cell populations and highlighting possible future therapeutic targets.     

Biphasic role of 4‐1BB in the regulation of mouse cytomegalovirus‐specific CD8+ T cells

The initial requirement for the emergence of CMV‐specific CD8⁺ T cells is poorly understood. Mice deficient in the cosignaling TNF superfamily member, 4‐1BB, surprisingly developed exaggerated early CD8⁺ T‐cell responses to mouse CMV (MCMV). CD8⁺ T cells directed against acute MCMV epitopes were enhanced, demonstrating that 4‐1BB naturally antagonizes these primary populations. Paradoxically, 4‐1BB‐deficient mice displayed reduced accumulation of memory CD8⁺ T cells that expand during chronic/latent infection. Importantly, the canonical TNF‐related ligand, 4‐1BBL, promoted the accumulation of these memory CD8⁺ T cells, whereas suppression of acute CD8⁺ T cells was independent of 4‐1BBL. These data highlight the dual nature of the 4‐1BB/4‐1BBL system in mediating both stimulatory and inhibitory cosignaling activities during the generation of anti‐MCMV immunity.     

T cell Suppression In Vitro During Toxoplasma gondii Infection is the Result of IL‐2 Competition Between Tregs and T cells Leading to Death of Proliferating T cells

A reduced proliferation to T cell mitogens is observed in vitro in murine cells isolated during the acute phase of Toxoplasma gondii infection. Foxp3⁺ regulatory T cells (Tregs) mediate this suppression, which is interleukin (IL)‐2 dependent. In this work, we analysed the mechanism of this Treg‐mediated suppression. We found that removal of antigen‐presenting cells (APC) from spleen cells from infected mice did not modify suppression but further elimination of Tregs led to a restored proliferation, demonstrating that Tregs mediate suppression in the absence of APC. Production of IL‐2 by T cells from infected animals was abolished but partially restored when Tregs were removed. However, IL‐2 levels and T cell proliferation were restored when Tregs and T cells were separated by transwells, indicating that Tregs require close proximity with T cells to induce suppression. Tregs from infected mice were able to reduce proliferation of CTLL‐2 cells in the classical IL‐2 bioassay, strongly suggesting that Tregs compete with T cells for IL‐2. We found that T cells from infected mice died after a few rounds of division in vitro, but addition of recombinant IL‐2 or removal of Tregs abolished this effect. Our results showed that suppression of T cell proliferation during acute Toxoplasma gondii infection is the result of death of proliferating T cells by Treg‐mediated IL‐2 competition. Thus, immunosuppression is due to death of proliferating T cells as a consequence of low IL‐2 availability.     

Characterization of the human CD4+ T‐cell repertoire specific for major histocompatibility class I‐restricted antigens

While CD4⁺ T lymphocytes usually recognize antigens in the context of major histocompatibility (MHC) class II alleles, occurrence of MHC class‐I restricted CD4⁺ T cells has been reported sporadically. Taking advantage of a highly sensitive MHC tetramer‐based enrichment approach allowing detection and isolation of scarce Ag‐specific T cells, we performed a systematic comparative analysis of HLA‐A*0201‐restricted CD4⁺ and CD8⁺ T‐cell lines directed against several immunodominant viral or tumoral antigens. CD4⁺ T cells directed against every peptide‐MHC class I complexes tested were detected in all donors. These cells yielded strong cytotoxic and T helper 1 cytokine responses when incubated with HLA‐A2⁺ target cells carrying the relevant epitopes. HLA‐A2‐restricted CD4⁺ T cells were seldom expanded in immune HLA‐A2⁺ donors, suggesting that they are not usually engaged in in vivo immune responses against the corresponding peptide‐MHC class I complexes. However, these T cells expressed TCR of very high affinity and were expanded following ex vivo stimulation by relevant tumor cells. Therefore, we describe a versatile and efficient strategy for generation of MHC class‐I restricted T helper cells and high affinity TCR that could be used for adoptive T‐cell transfer‐ or TCR gene transfer‐based immunotherapies.     

Enhanced MHC class II-restricted presentation of measles virus (MV) hemagglutinin in transgenic mice expressing human MV receptor CD46

This study analyzes the role of the measles virus (MV) receptor, i.e. the human CD46 molecule, in the MHC class II-restricted presentation of MV hemagglutinin (H). We generated transgenic mice ubiquitously expressing CD46, with a similar level of transgene expression on the surface of antigen-presenting cells (APC), i.e. B cells, dendritic cells (DC) and macrophages. APC isolated from transgenic mice and nontransgenic controls were tested for their ability to present MV H to H-specific CD4⁺ I-E ^d -restricted T cell hybridomas. All three populations of APC were capable of presenting MV to T cell hybridomas, DC being the most efficient. Expression of CD46 on B lymphocytes increased MHC class II-dependent presentation of MV H up to 100-fold, while CD46-transgenic DC stimulated H-specific T cell hybridomas up to 10-fold better than nontransgenic DC. Interestingly, expression of CD46 did not change the presentation efficiency of transgenic macrophages, indicating that CD46-dependent enhancement of antigen presentation depends on the nature of the APC. Furthermore, a single injection of UV-inactivated MV particles into CD46-transgenic mice, but not nontransgenic controls, induced generation of MV-specific T lymphocytes and production of anti-H antibodies, suggesting a role for CD46 in the efficient capture of MV in vivo. These results show for the first time that one ubiquitously expressed cell surface receptor, like CD46, could function in receptor-mediated antigen presentation both in vitro and in vivo and its performance depends on the type of APC which expresses it.