Neuropilin-1 cooperates with PD-1 in CD8+ T-cells predicting outcomes in melanoma patients treated with anti-PD1

Targeting immune checkpoints, such as Programmed cell Death 1 (PD1), has improved survival in cancer patients by restoring antitumor immune responses. Most patients, however, relapse or are refractory to immune checkpoint blocking therapies. Neuropilin-1 (NRP1) is a transmembrane glycoprotein required for nervous system and angiogenesis embryonic development, also expressed in immune cells. We hypothesized that NRP1 could be an immune checkpoint co-receptor modulating CD8⁺ T cells activity in the context of the antitumor immune response. Here, we show that NRP1 is recruited in the cytolytic synapse of PD1⁺CD8⁺ T cells, cooperates and enhances PD-1 activity. In mice, CD8⁺ T cells specific deletion of Nrp1 improves anti-PD1 antibody antitumor immune responses. Likewise, in human metastatic melanoma, the expression of NRP1 in tumor infiltrating CD8⁺ T cells predicts poor outcome of patients treated with anti-PD1. NRP1 is a promising target to overcome resistance to anti-PD1 therapies.

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NRP1 modulates PD1 activity secondary to complexes formation on CD8⁺ T cells

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Anti-PD1 therapy is synergistic with NRP1 specific deletion on CD8⁺ T cells in mouse

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NRP1 expression on CD8⁺ TILs predicts poor outcome in patients treated with anti-PD1

Role of Regulatory T Cell and Effector T Cell Exhaustion in Liver-Mediated Transgene Tolerance in Muscle

The pro-tolerogenic environment of the liver makes this tissue an ideal target for gene replacement strategies. In other peripheral tissues such as the skeletal muscle, anti-transgene immune response can result in partial or complete clearance of the transduced fibers. Here, we characterized liver-induced transgene tolerance after simultaneous transduction of liver and muscle. A clinically relevant transgene, α-sarcoglycan, mutated in limb-girdle muscular dystrophy type 2D, was fused with the SIINFEKL epitope (hSGCA-SIIN) and expressed with adeno-associated virus vectors (AAV-hSGCA-SIIN). Intramuscular delivery of AAV-hSGCA-SIIN resulted in a strong inflammatory response, which could be prevented and reversed by concomitant liver expression of the same antigen. Regulatory T cells and upregulation of checkpoint inhibitor receptors were required to establish and maintain liver-mediated peripheral tolerance. This study identifies the fundamental role of the synergy between Tregs and upregulation of checkpoint inhibitor receptors in the liver-mediated control of anti-transgene immunity triggered by muscle-directed gene transfer.

Dual muscle-liver transduction imposes immune tolerance for muscle transgene engraftment despite preexisting immunity

Immune responses to therapeutic transgenes are a potential hurdle to treat monogenic muscle disorders. These responses result from the neutralizing activity of transgene-specific B cells and cytotoxic T cells recruited upon gene transfer. We explored here how dual muscle-liver expression of a foreign transgene allows muscle transgene engraftment after adenoassociated viral vector delivery. We found in particular that induction of transgene-specific tolerance is imposed by concurrent muscle and liver targeting, resulting in the absence of CD8+ T cell responses to the transgene. This tolerance can be temporally decoupled, because transgene engraftment can be achieved in muscle weeks after liver transduction. Importantly, transgene-specific CD8+ T cell tolerance can be established despite preexisting immunity to the transgene. Whenever preexisting, transgene-specific CD4+ and CD8+ memory T cell responses are present, dual muscle-liver transduction turns polyclonal, transgene-specific CD8+ T cells into typically exhausted T cells with high programmed cell death 1 (PD-1) expression and lack of IFN-γ production. Our results demonstrate that successful transduction of muscle tissue can be achieved through liver-mediated control of humoral and cytotoxic T cell responses, even in the presence of preexisting immunity to the muscle-associated transgene.

Induction of tumor-specific CTL responses using the C-terminal fragment of Viral protein R as cell penetrating peptide

The discovery of tumor-associated antigens recognized by T lymphocytes opens the possibility of vaccinating cancer patients with defined antigens. However, one of the major limitation of peptide-based vaccines is the low immunogenicity of antigenic peptides. Interestingly, if these epitopes are directly delivered into the cytoplasm of antigen presenting cells, they can be efficiently presented via the direct MHC class I presentation pathway. To improve antigen entry, one promising approach is the use of cell penetrating peptides (CPPs). However, most studies use a covalent binding of the CPP with the antigen. In the present study, we focused on the C-terminal domain of Vpr which was previously demonstrated to efficiently deliver plasmid DNA into cells. We provide evidence that the peptides Vpr55-91 and Vpr55-82 possess the capacity of delivering proteins and epitopes into cell lines as well as into human primary dendritic cells, without the necessicity for a chemical linkage. Moreover, immunization of HLA-A2 transgenic mice with Vpr55-91 as the sole adjuvant is able to induce antigen-specific cytotoxic T lymphocytes against multiple tumor epitopes.

Mast Cell Degranulation Exacerbates Skin Rejection by Enhancing Neutrophil Recruitment

Recent evidences indicate an important role of tissue inflammatory responses by innate immune cells in allograft acceptance and survival. Here we investigated the role of mast cells (MC) in an acute male to female skin allograft rejection model using red MC and basophil (RMB) mice enabling conditional MC depletion. Kinetic analysis showed that MCs markedly accelerate skin rejection. They induced an early inflammatory response through degranulation and boosted local synthesis of KC, MIP-2, and TNF. This enhanced early neutrophil infiltration compared to a female-female graft-associated repair response. The uncontrolled neutrophil influx accelerated rejection as antibody-mediated depletion of neutrophils delayed skin rejection. Administration of cromolyn, a MC stabilizer and to a lesser extent ketotifen, a histamine type I receptor antagonist, and absence of MCPT4 chymase also delayed graft rejection. Together our data indicate that mediators contained in secretory granules of MC promote an inflammatory response with enhanced neutrophil infiltration that accelerate graft rejection.

Intradermal Immunization with rAAV1 Vector Induces Robust Memory CD8+ T Cell Responses Independently of Transgene Expression in DCs

Recombinant adeno-associated viral (rAAV) vectors exhibit interesting properties as vaccine carriers for their ability to induce long-lasting antibody responses. However, rAAV-based vaccines have been suggested to trigger functionally impaired long-term memory CD8⁺ T cell responses, in part due to poor dendritic cell (DC) transduction. Such results, albeit limited to intramuscular immunization, undermined the use of rAAV as vaccine vehicles against intracellular pathogens. We report here that intradermal immunization with a model rAAV2/1-based vaccine drives the development of bona fide long-term memory CD8⁺ T cell responses. The intradermal route of immunization and the presence of potent major histocompatibility complex (MHC) class II responses showed synergistic effects on the overall quantity and quality of systemic long-term effector memory transgene-specific CD8⁺ T cells being generated against the transgene. Of key interest, we found that the induction of memory cytotoxic T lymphocytes (CTLs) following intradermal immunization was solely dependent on the cross-presentation of skin-expressed transgene products, which appeared highly enhanced as compared to muscle-expressed transgene products. Overall our results highlight key tissue-specific differences in transgene presentation pathway requirements of importance for the design of rAAV-based T cell-inducing vaccines.

Mast cells aggravate sepsis by inhibiting peritoneal macrophage phagocytosis

Controlling the overwhelming inflammatory reaction associated with polymicrobial sepsis remains a prevalent clinical challenge with few treatment options. In septic peritonitis, blood neutrophils and monocytes are rapidly recruited into the peritoneal cavity to control infection, but the role of resident sentinel cells during the early phase of infection is less clear. In particular, the influence of mast cells on other tissue-resident cells remains poorly understood. Here, we developed a mouse model that allows both visualization and conditional ablation of mast cells and basophils to investigate the role of mast cells in severe septic peritonitis. Specific depletion of mast cells led to increased survival rates in mice with acute sepsis. Furthermore, we determined that mast cells impair the phagocytic action of resident macrophages, thereby allowing local and systemic bacterial proliferation. Mast cells did not influence local recruitment of neutrophils and monocytes or the release of inflammatory cytokines. Phagocytosis inhibition by mast cells involved their ability to release prestored IL-4 within 15 minutes after bacterial encounter, and treatment with an IL-4-neutralizing antibody prevented this inhibitory effect and improved survival of septic mice. Our study uncovers a local crosstalk between mast cells and macrophages during the early phase of sepsis development that aggravates the outcome of severe bacterial infection.

Vaccine activation of the nutrient sensor GCN2 in dendritic cells enhances antigen presentation

The yellow fever vaccine YF-17D is one of the most successful vaccines ever developed in humans. Despite its efficacy and widespread use in more than 600 million people, the mechanisms by which it stimulates protective immunity remain poorly understood. Recent studies using systems biology approaches in humans have revealed that YF-17D-induced early expression of general control nonderepressible 2 kinase (GCN2) in the blood strongly correlates with the magnitude of the later CD8(+) T cell response. We demonstrate a key role for virus-induced GCN2 activation in programming dendritic cells to initiate autophagy and enhanced antigen presentation to both CD4(+) and CD8(+) T cells. These results reveal an unappreciated link between virus-induced integrated stress response in dendritic cells and the adaptive immune response.

Extrathymic induction of Foxp3⁺ regulatory T cells declines with age in a T-cell intrinsic manner

Extrathymically induced Foxp3⁺ regulatory T (Treg) cells contribute to the pool of Treg cells and are implicated in the maintenance of immune tolerance at environmental interfaces. The impact of T-cell senescence on their generation and function is, however, poorly characterized. We report here that steady-state induction of Foxp3 is impaired in aged T cells in vivo. In vitro assays further revealed that this defective generation of Treg cells was independent from the strength of TCR stimulation and arose before T-cell proliferation. Importantly, they also revealed that this impairment of Foxp3 induction is unrelated to known age-related T-cell defects, such as IL-2 secretion impairment, accumulation of activated T-cell populations, or narrowing of the T-cell repertoire. Finally, a loss of extrathymic induction of Foxp3 and tolerance to minor-mismatched skin graft were observed in aged mice treated by nondepleting anti-CD4 antibody. The T-cell intrinsic impairment of Treg-cell generation revealed here highlights age as a key factor to be considered in immune tolerance induction.

IRAP identifies an endosomal compartment required for MHC class I cross-presentation

Major histocompatibility complex (MHC) class I molecules present peptides, produced through cytosolic proteasomal degradation of cellular proteins, to cytotoxic T lymphocytes. In dendritic cells, the peptides can also be derived from internalized antigens through a process known as cross-presentation. The cellular compartments involved in cross-presentation remain poorly defined. We found a role for peptide trimming by insulin-regulated aminopeptidase (IRAP) in cross-presentation. In human dendritic cells, IRAP was localized to a Rab14+ endosomal storage compartment in which it interacted with MHC class I molecules. IRAP deficiency compromised cross-presentation in vitro and in vivo but did not affect endogenous presentation. We propose the existence of two pathways for proteasome-dependent cross-presentation in which final peptide trimming involves IRAP in endosomes and involves the related aminopeptidases in the endoplasmic reticulum.

Highly efficient transduction of human plasmacytoid dendritic cells without phenotypic and functional maturation

BACKGROUND

Gene modified dendritic cells (DC) are able to modulate DC functions and induce therapeutic immunity or tolerance in an antigen-specific manner. Among the different DC subsets, plasmacytoid DC (pDC) are well known for their ability to recognize and respond to a variety of viruses by secreting high levels of type I interferon.

METHODS

We analyzed here, the transduction efficiency of a pDC cell line, GEN2.2, and of pDC derived from CD34+ progenitors, using lentiviral vectors (LV) pseudotyped with different envelope glycoproteins such as the vesicular stomatitis virus envelope (VSVG), the gibbon ape leukaemia virus envelope (GaLV) or the feline endogenous virus envelope (RD114). At the same time, we evaluated transgene expression (E-GFP reporter gene) under the control of different promoters.

RESULTS

We found that efficient gene transfer into pDC can be achieved with VSVG-pseudotyped lentiviral vectors (LV) under the control of phoshoglycerate kinase (PGK) and elongation factor-1 (EF1alpha) promoters (28% to 90% of E-GFP+ cells, respectively) in the absence of phenotypic and functional maturation. Surprisingly, promoters (desmin or synthetic C5-12) described as muscle-specific and which drive gene expression in single strand AAV vectors in gene therapy protocols were very highly active in pDC using VSVG-LV.

CONCLUSION

Taken together, our results indicate that LV vectors can serve to design pDC-based vaccines in humans, and they are also useful in vitro to evaluate the immunogenicity of the vector preparations, and the specificity and safety of given promoters used in gene therapy protocols.

Enhanced Lentiviral Transduction of Monocyte-Derived Dendritic Cells in The Presence of Conditioned Medium from Dying Monocytes

Lentiviral vectors (LVs) are attractive vehicles for the transduction of human dendritic cells (DCs) in order to mobilize their endogenous antigen presentation pathways. We analyzed here how to improve the efficiency of LV transduction, which we performed at the initial stages of the differentiation of purified monocytes into dendritic cells (Mo-DCs). Using LVs pseudotyped with the vesicular stomatitis virus envelope G glycoprotein (VSV-G), we found that a conditioned medium derived from dying monocytes (MCM) improved by 2- to 10- fold the proportion of transduced Mo-DCs. This enhanced transduction efficiency requires the presence of MCM during the initial stage of LV transduction and does not affect the phenotype and antigen presentation function of terminally differentiated Mo-DCs. Importantly, we found that MCM derived from a human acute monocytic leukemia cell line, THP-1, was equally effective. The MCM activity was heat stable (56 degrees C) and was present in the soluble fraction after high-speed centrifugation. Altogether our results show that a soluble factor present in dying monocyte cultures can replace advantageously facilitating agents such as Polybrene, to achieve high LV transductions levels. This protocol can be performed with autologous monocytes and is therefore applicable in clinical settings.

Simple conditioning with monospecific CD4+CD25+ regulatory T cells for bone marrow engraftment and tolerance to multiple gene products

A major impediment to gene replacement therapy is immune elimination of genetically modified cells. In principle, this can be dealt with by inducing a strong, specific, and enduring tolerance through engraftment of transgene-modified autologous bone marrow (BM). Because usual myeloablation and/or immunosuppression are risk factors in most pathologies, we assessed the potential of monospecific CD4(+)CD25(+) regulatory T cells (Tregs) to engraft minor-mismatched BM without preconditioning. We found that as few as 5 x 10(4) Tregs directed to the male DBY protein promote the engraftment of foreign male BM into sex-mismatched female hosts, establishing sustained chimerism in all hematopoeitic compartments. We achieved concomitantly strong tolerance to all foreign antigens expressed in the BM, likely occurring through induction of anergy and/or deletion of antidonor T cells. Chimerism was obtained in thymectomized mice too, underlining the major role of peripheral tolerance mechanisms in our system. This allowed us to engraft gene-modified tissues while preserving full immunocompetence to third-party antigens. Our results demonstrate that very few donor-specific Tregs are effective as the sole conditioning to induce mixed molecular chimerism and long-term tolerance to multiple foreign antigens.

Hyperthermia enhances CTL cross-priming

Dendritic cells (DCs) loaded with killed allogeneic melanoma cells can cross-prime naive CD8(+) T cells to differentiate into melanoma-specific CTLs in 3-wk cultures. In this study we show that DCs loaded with killed melanoma cells that were heated to 42 degrees C before killing are more efficient in cross-priming of naive CD8(+) T cells than DCs loaded with unheated killed melanoma cells. The enhanced cross-priming was demonstrated by several parameters: 1) induction of naive CD8(+) T cell differentiation in 2-wk cultures, 2) enhanced killing of melanoma peptide-pulsed T2 cells, 3) enhanced killing of HLA-A*0201(+) melanoma cells in a standard 4-h chromium release assay, and 4) enhanced capacity to prevent tumor growth in vitro in a tumor regression assay. Two mechanisms might explain the hyperthermia-induced enhanced cross-priming. First, heat-treated melanoma cells expressed increased levels of 70-kDa heat shock protein (HSP70), and enhanced cross-priming could be reproduced by overexpression of HSP70 in melanoma cells transduced with HSP70 encoding lentiviral vector. Second, hyperthermia resulted in the increased transcription of several tumor Ag-associated Ags, including MAGE-B3, -B4, -A8, and -A10. Thus, heat treatment of tumor cells permits enhanced cross-priming, possibly via up-regulation of both HSPs and tumor Ag expression.

Efficient transduction of monocyte- and CD34+- derived Langerhans cells with lentiviral vectors in the absence of phenotypic and functional maturation

BACKGROUND

Gene delivery in dendritic cells (DC) has raised considerable interest to modulate DC functions and induce therapeutic immunity or tolerance in an antigen-specific fashion. Among immature DC, Langerhans cells (LC) are attractive candidates for antigen delivery using lentiviral vectors (LV).

METHODS

LC derived from monocytes (Mo-LC), or derived from CD34+ cells (CD34-LC) in the presence of cytokine cocktail, were transduced with LV expressing enhanced green fluorescent protein (E-GFP) under the control of the ubiquitous phosphoglycerate kinase (PGK) promoter at a multiplicity of infection of 18, at days 0 to 3 for Mo-LC, or at days 0 to 12 for CD34-LC. We assessed gene transfer levels from the percentage of E-GFP+ cells in the final cultures, and examined the morphology, immunophenotype, state of differentiation and function of transduced LC.

RESULTS

Day 0 transduction of monocytes or CD34+ progenitors before cytokine pre-activation and LC differentiation resulted in stable gene expression in 7.8% of Mo-LC and 24% of CD34-LC. Monocyte-derived DC (Mo-DC) differentiated in serum-free medium were also efficiently transduced up to 13.2%. Interestingly, Mo-LC cells committed towards LC phenotype were permissive for transduction up to day 3. Transduction levels of CD34-LC peaked at day 6 to 44% and decreased thereafter. LV transduction did not perturb viability, phenotype and function of E-GFP-expressing LC.

CONCLUSIONS

LC generated ex vivo can serve as vaccine vehicles in humans through efficient transduction by LV. These LC will be helpful to assess in vitro the immunogenicity of gene therapy vectors, from the characterization of their phenotypic and functional maturation.

Efficient migration of dendritic cells toward lymph node chemokines and induction of TH1 responses require maturation stimulus and apoptotic cell interaction

Dendritic cells (DCs) have the unique ability to initiate primary immune responses, and they can be conditioned for vaccinal purposes to present antigens after the engulfment of apoptotic cells. To recruit the rare antigen-specific naive T cells, DCs require a maturation step and subsequent transport toward lymph node (LN). To date, prostaglandin E2 (PGE2) is the best-characterized compound inducing this LN-directed migration in vitro, but PGE2 may skew the immune responses in a TH2 direction. We demonstrate here that on incubation with apoptotic tumor cells and tumor necrosis factor-α (TNF-α) or lipopolysaccharide (LPS), human monocyte-derived DCs become fully mature and acquire high migratory capacities toward LN-directing chemokines. The migration of TNF-α-treated DCs occurs only after cotreatment with apoptotic cells but not with necrotic cells. DC migration requires CD36 expression and incubation with apoptotic cells in the presence of heat-labile serum components. Moreover, on treatment with apoptotic cells and LPS, the migrating DCs are able to recruit naive T cells to generate TH1 immune responses. Our results show that the cotreatment of DCs with apoptotic tumor cells and inflammatory signals is promising for the design of an antitumoral DC-based vaccine. (Blood. 2005;106:1734-1741)

Dynamics and function of Langerhans cells in vivo: dermal dendritic cells colonize lymph node areas distinct from slower migrating Langerhans cells

Langerhans cells (LCs) are prominent dendritic cells (DCs) in epithelia, but their role in immunity is poorly defined. To track and discriminate LCs from dermal DCs in vivo, we developed knockin mice expressing enhanced green fluorescent protein (EGFP) under the control of the langerin (CD207) gene. By using vital imaging, we showed that most EGFP(+) LCs were sessile under steady-state conditions, whereas skin inflammation induced LC motility and emigration to lymph nodes (LNs). After skin immunization, dermal DCs arrived in LNs first and colonized areas distinct from slower migrating LCs. LCs reaching LNs under steady-state or inflammatory conditions expressed similar levels of costimulatory molecules. Langerin and EGFP were also expressed on thymic DCs and on blood-derived, CD8alpha(+) DCs from all secondary lymphoid organs. By using a similar knockin strategy involving a diphtheria toxin receptor (DTR) fused to EGFP, we demonstrated that LCs were dispensable for triggering hapten-specific T cell effectors through skin immunization.

Imaging calpain protease activity by multiphoton FRET in living mice

Constant efforts are ongoing for the development of new imaging methods that allow the investigation of molecular processes in vivo. Protein-protein interactions, enzymatic activities and intracellular Ca2+ fluxes, have been resolved in cultured cells using a variety of fluorescence resonance energy transfer (FRET) detection methods. However, FRET has not been used so far in conjunction with 3D intravital imaging. We evaluated here a combination of multiphoton microscopy (MPM), method of choice for non-destructive living tissue investigation, and FRET imaging to monitor calpain proteolytic activity in living mice muscle. We show that kinetics of ubiquitous calpains activation can be efficiently and quantitatively monitored in living mouse tissues at cellular level with a FRET-based indicator upon calcium influx. The ability to visualize calpain activity in living tissue offers a unique opportunity to challenge remaining questions on the biological functions of calpains and to evaluate the therapeutic potential of calpain inhibitors in many degenerative conditions.

HLA-A*0201-restricted cytolytic responses to the rtTA transactivator dominant and cryptic epitopes compromise transgene expression induced by the tetracycline on system

The tetracycline-controlled transcription system (Tet-on) is widely used to regulate gene expression in mammalian cells. In gene therapy applications, immune responses to Tet-on proteins such as the rtTA transcription factor have been reported, raising concerns about their occurrence in humans. To monitor the HLA class I cytolytic responses against Tet-on regulators, we characterized the immunogenic CD8+ epitopes within rtTA and tTS regulators using HLA-A*0201 class I transgenic mice. Epitope prediction programs, HLA-A*0201 binding assays, and peptide immunization were used to select a set of immunogenic peptides within rtTA and tTS sequences. To identify further the rejection epitopes, we expressed Tet-on protein components in vivo and found a single dominant rtTA186 CTL epitope in the rtTA tetracycline repressor domain. Target cells expressing rtTA were susceptible to CTL lysis, and rtTA expression compromised muscle transgene engraftment. To reduce the occurrence of immune responses to rtTA protein, we mutated the dominant rtTA186 epitope and found that this leads to the appearance of subdominant epitopes. As a result, we think that an epitope modification strategy is not applicable to blunt the immune response in this model. Moreover, the identification of HLA-A*0201 rtTA epitopes allowed us to demonstrate here that the delivery of the Tet-on system with weakly immunogenic rAAV vectors does not trigger primary CTL responses in mice, in contrast to DNA transfer. Altogether, the existence of HLA-A*0201 rtTA epitopes may lead to the occurrence of immune responses depending on vectors and local inflammation in gene therapy applications involving rtTA-based regulatory systems.

High vaccination efficiency of low-affinity epitopes in antitumor immunotherapy

Most of the human tumor-associated antigens (TAAs) characterized thus far are derived from nonmutated "self"-proteins. Numerous strategies have been developed to break tolerance to TAAs, combining various forms of antigens with different vectors and adjuvants. However, no study has yet determined how to select epitopes within a given TAA to induce the highest antitumor effector response. We addressed this question by evaluating in HLA-A*0201-transgenic HHD mice the antitumor vaccination efficacy of high- and low-affinity epitopes from the naturally expressed murine telomerase reverse transcriptase (mTERT). Immunity against low-affinity epitopes was induced with heteroclitical variants. We show here that the CTL repertoire against high-affinity epitopes is partially tolerized, while that against low-affinity epitopes is composed of frequent CTLs with high avidity. The high-affinity p797 and p545 mTERT epitopes are not able to protect mice from a lethal challenge with the mTERT-expressing EL4-HHD tumor. In contrast, mice developing CTL responses against the p572 and p988 low-affinity epitopes exhibit potent antitumor immunity and no sign of autoimmune reactivity against TERT-expressing normal tissues. Our results strongly argue for new TAA epitope selection and modification strategies in antitumor immunotherapy applications in humans.

High vaccination efficiency of low-affinity epitopes in antitumor immunotherapy

Most of the human tumor-associated antigens (TAAs) characterized thus far are derived from nonmutated "self"-proteins. Numerous strategies have been developed to break tolerance to TAAs, combining various forms of antigens with different vectors and adjuvants. However, no study has yet determined how to select epitopes within a given TAA to induce the highest antitumor effector response. We addressed this question by evaluating in HLA-A*0201-transgenic HHD mice the antitumor vaccination efficacy of high- and low-affinity epitopes from the naturally expressed murine telomerase reverse transcriptase (mTERT). Immunity against low-affinity epitopes was induced with heteroclitical variants. We show here that the CTL repertoire against high-affinity epitopes is partially tolerized, while that against low-affinity epitopes is composed of frequent CTLs with high avidity. The high-affinity p797 and p545 mTERT epitopes are not able to protect mice from a lethal challenge with the mTERT-expressing EL4-HHD tumor. In contrast, mice developing CTL responses against the p572 and p988 low-affinity epitopes exhibit potent antitumor immunity and no sign of autoimmune reactivity against TERT-expressing normal tissues. Our results strongly argue for new TAA epitope selection and modification strategies in antitumor immunotherapy applications in humans.

Selective accumulation of mature DC-Lamp+ dendritic cells in tumor sites is associated with efficient T-cell-mediated antitumor response and control of metastatic dissemination in melanoma

The clinical relevance of dendritic cells (DCs) at the tumor site remains a matter of debate concerning their role in the generation of effective antitumor immunity in human cancers. We performed a comprehensive immunohistochemical analysis using a panel of DC-specific antibodies on regressing tumor lesions and sentinel lymph nodes (SLNs) in melanoma patients. Here we show in a case report involving spontaneous regression of metastatic melanoma that the accumulation of DC-Lamp+ DCs, clustered with tumor cells and lymphocytes, is associated with local expansion of antigen-specific memory effector CTLs. These findings were extended in a series of 19 melanoma-positive SLNs and demonstrated a significant correlation between the density of DC-Lamp+ DC infiltrates in SLNs with the absence of metastasis in downstream lymph nodes. This study, albeit performed in a limited series of patients, points to a pivotal role of mature DCs in the local expansion of efficient antitumor T-cell-mediated immune responses at the initial sites of metastasis and may have important implications regarding the prognosis, staging, and immunotherapy of melanoma patients.

CD208/dendritic cell-lysosomal associated membrane protein is a marker of normal and transformed type II pneumocytes

Dendritic cell-lysosomal associated membrane protein (DC-LAMP)/CD208, a member of the lysosomal associated membrane protein (LAMP) family, is specifically expressed by human DCs on activation. However, its mouse counterpart could not be detected in mature DCs. The present study demonstrates that DC-LAMP is constitutively expressed by mouse, sheep, and human type II pneumocytes. Confocal and immunoelectron microscopy showed that mouse DC-LAMP protein co-localizes with lbm180, a specific marker for the limiting membrane of lamellar bodies that contain surfactant protein B, as well as with intracellular MHC class II molecules that accumulate in the same organelles. Expression of DC-LAMP was also occasionally detected at the cell surface of type II pneumocytes. Interestingly, human bronchioloalveolar carcinoma tumor cells, which correspond to transformed type II pneumocytes, express DC-LAMP. Similar observations were made in the Jaagsiekte sheep retrovirus-associated ovine pulmonary adenocarcinoma, a model of human bronchioloalveolar carcinoma. This study establishes that DC-LAMP is constitutively expressed in normal type II pneumocytes. Furthermore, DC-LAMP appears to be a marker of transformed type II pneumocytes as well, an observation that may help the study and the classification of human lung adenocarcinomas.

CD4+CD25+ regulatory T cells inhibit immune-mediated transgene rejection

Like cellular transplantation, gene therapy is often limited by immune rejection of the newly expressed antigen. In a model of gene transfer in muscle, delivery of the influenza hemagglutinin (HA) membrane protein by adeno-associated virus (AAV) is impaired by a strong immune response that leads to a rapid rejection of the transduced fibers. We show here that injection of HA-specific CD4+CD25+ T cells from T-cell receptor (TCR)-transgenic animals, concomitant with gene transfer, down-regulates the anti-HA cytotoxic and B-lymphocyte responses and enables persistent HA expression in muscle. This demonstrates for the first time that adoptive transfer of antigen-specific CD4+CD25+ regulatory T cells can be used to induce sustained transgene engraftment in solid tissues.

ER-phagosome fusion defines an MHC class I cross-presentation compartment in dendritic cells

Induction of cytotoxic T-cell immunity requires the phagocytosis of pathogens, virus-infected or dead tumour cells by dendritic cells. Peptides derived from phagocytosed antigens are then presented to CD8+ T lymphocytes on major histocompatibility complex (MHC) class I molecules, a process called "cross-presentation". After phagocytosis, antigens are exported into the cytosol and degraded by the proteasome. The resulting peptides are thought to be translocated into the lumen of the endoplasmic reticulum (ER) by specific transporters associated with antigen presentation (TAP), and loaded onto MHC class I molecules by a complex "loading machinery" (which includes tapasin, calreticulin and Erp57). Here we show that soon after or during formation, phagosomes fuse with the ER. After antigen export to the cytosol and degradation by the proteasome, peptides are translocated by TAP into the lumen of the same phagosomes, before loading on phagosomal MHC class I molecules. Therefore, cross-presentation in dendritic cells occurs in a specialized, self-sufficient, ER-phagosome mix compartment.

Identification of an HLA-A*0201-restricted epitopic peptide from human dystrophin: application in duchenne muscular dystrophy gene therapy

Dystrophin-based gene therapy treatments aimed at correcting the Duchenne muscular dystrophy phenotype require stable expression of normal dystrophin (DYST) protein in myocytes without immune responses, which would compromise long-term expression. To predict cytotoxic T-cell-mediated responses elicited by transgenes, we used here H-2-negative HLA-A*0201 transgenic mice and identified human DYST epitopes, which elicit HLA-A*0201-restricted cytotoxic T cell activities. Among a series of eight peptides predicted from the human DYST sequence, not shared with the endogenous mouse DYST sequence, four of them were able to bind to HLA-A*0201 molecules and to induce cytotoxic T lymphocyte (CTL) responses. After human DYST DNA transfer in muscle of HLA-A*0201 mice, only the human DYST1281 epitope, located in the spectrin-like repeat 9 domain, induced strong CD8(+) CTL responses. Using the corresponding human DYST1281 peptide/HLA-A*0201 tetramer, we detected human DYST1281-specific CD8(+) T cells in peripheral lymphoid organs and blood of HLA-A*0201 mice injected with human DYST DNA. Our results demonstrate that muscle injection with human DYST DNA systematically triggers CTL responses against this HLA-A*0201-restricted human DYST1281 peptide, which is present in long human DYST isoforms. Identification of such immunodominant human DYST epitopes and use of peptide/HLA tetramers will allow the immunomonitoring of CTL responses in HLA-phenotyped Duchenne muscular dystrophy patients undergoing gene therapy. Finally, the knowledge of HLA-A*0201-restricted human DYST peptides will be of importance to test, in mouse models, new immunomodulatory interventions allowing long-term engraftment of human dystrophin.

Human germinal center B cells differ from naive and memory B cells by their aggregated MHC class II-rich compartments lacking HLA-DO

To generate memory B cells bearing high-affinity antibodies, naive B cells first encounter antigen in the T cell-rich areas of secondary lymphoid organs. There, they are activated by antigen-specific T cells and become germinal center (GC) founder B cells. GC founders enter the GC to become centroblasts that proliferate and mutate their BCR. Centroblasts differentiate into centrocytes that undergo selection, which requires both the recognition/capture of antigen on follicular dendritic cells and the presentation of processed antigen to GC T cells. Because at each stage of differentiation B cells act as antigen-presenting cells, we analyzed their content of HLA-DR(+)-rich compartments (MIIC), as well as their expression of HLA-DM, which catalyzes peptide loading of class II molecules, and HLA-DO, which interacts with HLA-DM and focuses MHC class II peptide loading on antigens internalized by the BCR. Naive and memory B cells concentrate HLA-DR, -DM and -DO into compartments dispersed under the cell surface, which are identified by their expression of lysosome-associated membrane protein (Lamp)-1 as late endosomes/lysosomes. GC founders and GC B cells express larger Lamp-1(+)DR(+) compartments that are concentrated in the juxta-nuclear region. These compartments express lower levels of HLA-DM and virtually no HLA-DO. Upon induction of a GC founder phenotype through the prolonged (days) co-ligation of BCR and CD40, the naive B cell's peripheral DR(+)DM(+)Lamp-1(+) compartments aggregate in a polar fashion close to the nucleus. Furthermore, HLA-DO expression virtually disappears, whereas low levels of HLA-DM remain co-localized with HLA-DR. Anti-kappa/lambda antibodies, used as surrogate antigens, are promptly (minutes) endocytosed in naive, memory and GC B cells. Then, naive and memory B cells target the surrogate antigen to their peripheral HLA-DO(+) MIIC, while GC B cells target it to their HLA-DO(-) MIIC aggregates. Taken together, our results show that human GC B cells differ from naive and memory B cells by their aggregated MIIC that lack HLA-DO.

The melanocytic protein Melan-A/MART-1 has a subcellular localization distinct from typical melanosomal proteins

To delineate the role of the melanocyte lineage-specific protein Melan-A/MART-1 in melanogenic functions, a set of biochemical and microscopical studies was performed. Biochemical analysis revealed that Melan-A/MART-1 is post-translationally acylated and undergoes a rapid turnover in a pigmented melanoma cell line. Immunofluorescence and immunoelectron microscopy analyses indicated that Melan-A/MART-1 is mainly located in the Golgi area and only partially colocalizes with melanosomal proteins. Quantitative immunoelectron microscopy showed that the highest proportion of the cellular content of Melan-A/MART-1 was found in small vesicles and tubules throughout the cell, whereas the concentration was maximal in the Golgi region, particularly the trans-Golgi network. Substantial labeling was also present on melanosomes, endosomes, ER, nuclear envelope, and plasma membrane. In early endosomes, Melan-A was enriched in areas of the limiting membrane covered by a bi-layered coat, a structural characteristic of melanosomal precursor compartments. Upon melanosome maturation, Melan-A concentration decreased and its predominant localization shifted from the limiting membrane to internal vesicle membranes. In conjunction with its acylation, the high expression levels of Melan-A in the trans-Golgi network, in dispersed vesicles, and on the limiting membrane of premelanosomes indicate that the protein may play a role during the early stage of melanosome biogenesis.

Accumulation of immature Langerhans cells in human lymph nodes draining chronically inflamed skin

The coordinated migration and maturation of dendritic cells (DCs) such as intraepithelial Langerhans cells (LCs) is considered critical for T cell priming in response to inflammation in the periphery. However, little is known about the role of inflammatory mediators for LC maturation and recruitment to lymph nodes in vivo. Here we show in human dermatopathic lymphadenitis (DL), which features an expanded population of LCs in one draining lymph node associated with inflammatory lesions in its tributary skin area, that the Langerin/CD207(+) LCs constitute a predominant population of immature DCs, which express CD1a, and CD68, but not CD83, CD86, and DC-lysosomal-associated membrane protein (LAMP)/CD208. Using LC-type cells generated in vitro in the presence of transforming growth factor (TGF)-beta1, we further found that tumor necrosis factor (TNF)-alpha, as a prototype proinflammatory factor, and a variety of inflammatory stimuli and bacterial products, increase Langerin expression and Langerin dependent Birbeck granules formation in cell which nevertheless lack costimulatory molecules, DC-LAMP/CD208 and potent T cell stimulatory activity but express CCR7 and respond to the lymph node homing chemokines CCL19 and CCL21. This indicates that LC migration and maturation can be independently regulated events. We suggest that during DL, inflammatory stimuli in the skin increase the migration of LCs to the lymph node but without associated maturation. Immature LCs might regulate immune responses during chronic inflammation.

Lateral Mobility of the Cell Adhesion Molecule L1 Within the Surface Membrane of Morphologically Undifferentiated and Differentiated Neuroblastoma Cells

Lateral mobility and localization in the surface membrane of the adhesion molecule L1 was studied in morphologically undifferentiated and differentiated neuroblastoma cells to gain insight into its possible association with the different molecular forms of N-CAM. In undifferentiated cells, the fraction of mobile L1 molecules is high and similar to that of N-CAM 140. Upon long-term morphological differentiation, the fraction of mobile L1 molecules is reduced by a factor of three and is similar to that of N-CAM 180, the predominant molecular form of N-CAM in differentiated neuroblastoma cells. Comparable to N-CAM 180, L1 is also preferentially accumulated at contact sites between these cells as seen by indirect immunofluorescence. These observations raise the question of whether at least part of the L1 molecules may be directly or indirectly (e.g. via N-CAM 180) linked to the cytoskeleton, thus stabilizing cell contacts between differentiated cells.

Interleukin 15 skews monocyte differentiation into dendritic cells with features of Langerhans cells

Langerhans cells (LCs) represent a subset of immature dendritic cells (DCs) specifically localized in the epidermis and other mucosal epithelia. As surrounding keratinocytes can produce interleukin (IL)-15, a cytokine that utilizes IL-2Rgamma chain, we analyzed whether IL-15 could skew monocyte differentiation into LCs. Monocytes cultured for 6 d with granulocyte/macrophage colony-stimulating factor (GM-CSF) and IL-15 differentiate into CD1a(+)HLA-DR(+)CD14(-)DCs (IL15-DCs). Agents such as lipopolysaccharide (LPS), tumor necrosis factor (TNF)alpha, and CD40L induce maturation of IL15-DCs to CD83(+), DC-LAMP(+) cells. IL15-DCs are potent antigen-presenting cells able to induce the primary (mixed lymphocyte reaction [MLR]) and secondary (recall responses to flu-matrix peptide) immune responses. As opposed to cultures made with GM-CSF/IL-4 (IL4-DCs), a proportion of IL15-DCs expresses LC markers: E-Cadherin, Langerin, and CC chemokine receptor (CCR)6. Accordingly, IL15-DCs, but not IL4-DCs, migrate in response to macrophage inflammatory protein (MIP)-3alpha/CCL20. However, IL15-DCs cannot be qualified as "genuine" Langerhans cells because, despite the presence of the 43-kD Langerin, they do not express bona fide Birbeck granules. Thus, our results demonstrate a novel pathway in monocyte differentiation into dendritic cells.

Cross-priming of naive CD8 T cells against melanoma antigens using dendritic cells loaded with killed allogeneic melanoma cells

The goal of tumor immunotherapy is to elicit immune responses against autologous tumors. It would be highly desirable that such responses include multiple T cell clones against multiple tumor antigens. This could be obtained using the antigen presenting capacity of dendritic cells (DCs) and cross-priming. That is, one could load the DC with tumor lines of any human histocompatibility leukocyte antigen (HLA) type to elicit T cell responses against the autologous tumor. In this study, we show that human DCs derived from monocytes and loaded with killed melanoma cells prime naive CD45RA(+)CD27(+)CD8(+) T cells against the four shared melanoma antigens: MAGE-3, gp100, tyrosinase, and MART-1. HLA-A201(+) naive T cells primed by DCs loaded with HLA-A201(-) melanoma cells are able to kill several HLA-A201(+) melanoma targets. Cytotoxic T lymphocyte priming towards melanoma antigens is also obtained with cells from metastatic melanoma patients. This demonstration of cross-priming against shared tumor antigens builds the basis for using allogeneic tumor cell lines to deliver tumor antigens to DCs for vaccination protocols.

IL-6 switches the differentiation of monocytes from dendritic cells to macrophages

Monocytes can give rise to either antigen presenting dendritic cells (DCs) or scavenging macrophages. This differentiation is initiated when monocytes cross the endothelium. But the regulation of DC and macrophage differentiation in tissues remains elusive. When stimulated with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4), monocytes yield DCs. However, we show here that the addition of fibroblasts switches differentiation to macrophages. On contact with monocytes, fibroblasts release IL-6, which up-regulates the expression of functional M-CSF receptors on monocytes. This allows the monocytes to consume their autocrine M-CSF. Thus, the interplay between IL-6 and M-CSF switches monocyte differentiation to macrophages rather than DCs, and IL-6 is an essential factor in the molecular control of antigen presenting cell development.

Dendritic cells capture killed tumor cells and present their antigens to elicit tumor-specific immune responses

Due to their capacity to induce primary immune responses, dendritic cells (DC) are attractive vectors for immunotherapy of cancer. Yet the targeting of tumor Ags to DC remains a challenge. Here we show that immature human monocyte-derived DC capture various killed tumor cells, including Jurkat T cell lymphoma, malignant melanoma, and prostate carcinoma. DC loaded with killed tumor cells induce MHC class I- and class II-restricted proliferation of autologous CD8+ and CD4+ T cells, demonstrating cross-presentation of tumor cell-derived Ags. Furthermore, tumor-loaded DC elicit expansion of CTL with cytotoxic activity against the tumor cells used for immunization. CTL elicited by DC loaded with the PC3 prostate carcinoma cell bodies kill another prostate carcinoma cell line, DU145, suggesting recognition of shared Ags. Finally, CTL elicited by DC loaded with killed LNCap prostate carcinoma cells, which express prostate specific Ag (PSA), are able to kill PSA peptide-pulsed T2 cells. This demonstrates that induced CTL activity is not only due to alloantigens, and that alloantigens do not prevent the activation of T cells specific for tumor-associated Ags. This approach opens the possibility of using allogeneic tumor cells as a source of tumor Ag for antitumor therapies.

Immunobiology of dendritic cells

Dendritic cells (DCs) are antigen-presenting cells with a unique ability to induce primary immune responses. DCs capture and transfer information from the outside world to the cells of the adaptive immune system. DCs are not only critical for the induction of primary immune responses, but may also be important for the induction of immunological tolerance, as well as for the regulation of the type of T cell-mediated immune response. Although our understanding of DC biology is still in its infancy, we are now beginning to use DC-based immunotherapy protocols to elicit immunity against cancer and infectious diseases.

Flt3-ligand and granulocyte colony-stimulating factor mobilize distinct human dendritic cell subsets in vivo

Dendritic cells (DCs) have a unique ability to stimulate naive T cells. Recent evidence suggests that distinct DC subsets direct different classes of immune responses in vitro and in vivo. In humans, the monocyte-derived CD11c+ DCs induce T cells to produce Th1 cytokines in vitro, whereas the CD11c- plasmacytoid T cell-derived DCs elicit the production of Th2 cytokines. In this paper we report that administration of either Flt3-ligand (FL) or G-CSF to healthy human volunteers dramatically increases distinct DC subsets, or DC precursors, in the blood. FL increases both the CD11c+ DC subset (48-fold) and the CD11c- IL-3R+ DC precursors (13-fold). In contrast, G-CSF only increases the CD11c- precursors (>7-fold). Freshly sorted CD11c+ but not CD11c- cells stimulate CD4+ T cells in an allogeneic MLR, whereas only the CD11c- cells can be induced to secrete high levels of IFN-alpha, in response to influenza virus. CD11c+ and CD11c- cells can mature in vitro with GM-CSF + TNF-alpha or with IL-3 + CD40 ligand, respectively. These two subsets up-regulate MHC class II costimulatory molecules as well as the DC maturation marker DC-lysosome-associated membrane protein, and they stimulate naive, allogeneic CD4+ T cells efficiently. These two DC subsets elicit distinct cytokine profiles in CD4+ T cells, with the CD11c- subset inducing higher levels of the Th2 cytokine IL-10. The differential mobilization of distinct DC subsets or DC precursors by in vivo administration of FL and G-CSF offers a novel strategy to manipulate immune responses in humans.

Naked antigen-presenting molecules on dendritic cells

Antigen-presenting cells work to present peptides derived from exogenous and endogenous antigens to circulating T cells, sparking off an immune response. Dendritic cells are unique amongst antigen-presenting cells, not least for their newly described ability to circumvent the need to internalize exogenous antigens before presenting them.

Langerin, a novel C-type lectin specific to Langerhans cells, is an endocytic receptor that induces the formation of Birbeck granules

We have identified a type II Ca2+-dependent lectin displaying mannose-binding specificity, exclusively expressed by Langerhans cells (LC), and named Langerin. LC are uniquely characterized by Birbeck granules (BG), which are organelles consisting of superimposed and zippered membranes. Here, we have shown that Langerin is constitutively associated with BG and that antibody to Langerin is internalized into these structures. Remarkably, transfection of Langerin cDNA into fibroblasts created a compact network of membrane structures with typical features of BG. Langerin is thus a potent inducer of membrane superimposition and zippering leading to BG formation. Our data suggest that induction of BG is a consequence of the antigen-capture function of Langerin, allowing routing into these organelles and providing access to a nonclassical antigen-processing pathway.

In breast carcinoma tissue, immature dendritic cells reside within the tumor, whereas mature dendritic cells are located in peritumoral areas

We have analyzed the presence of immature and mature dendritic cells (DCs) within adenocarcinoma of the breast using immunohistochemistry. Immature DCs were defined by expression of CD1a-, Langerin-, and intracellular major histocompatibility complex class II-rich vesicles. Mature DCs were defined by expression of CD83 and DC-Lamp. Breast carcinoma cells were defined by morphology and/or cytokeratin expression. We demonstrate two levels of heterogeneity of DCs infiltrating breast carcinoma tissue: (a) immature CD1a(+) DCs, mostly of the Langerhans cell type (Langerin(+)), were retained within the tumor bed in 32/32 samples and (b) mature DCs, CD83(+)DC-Lamp(+), present in 20/32 samples, are confined to peritumoral areas. The high numbers of immature DCs found in the tumor may be best explained by high levels of macrophage inflammatory protein 3alpha expression by virtually all tumor cells. Confirming the immature/mature DC compartmentalization pattern, in vitro-generated immature DCs adhere to the tumor cells, whereas mature DCs adhere selectively to peritumoral areas. In some cases, T cells are clustering around the mature DCs in peritumoral areas, thus resembling the DC-T cell clusters of secondary lymphoid organs, which are characteristic of ongoing immune reactions.

Evidence and a novel hypothesis for the role of dendritic cells and Porphyromonas gingivalis in adult periodontitis

We have proposed a novel overall hypothesis and approach to understanding the pathophysiology of adult periodontitis, one of the most common diseases that afflicts the US population. While mortality of the dentition is the most familiar outcome of adult periodontitis, its links with other more severe diseases, including coronary artery disease, respiratory diseases and pre-term labor, cannot be ignored. We have called attention to the many intriguing parallels between adult periodontitis and contact hypersensitivity (CHS). CHS is among the most common of dermatoses that afflicts mankind and one of the most intensively studied of in vivo immune responses. Both adult periodontitis and CHS target the host integument (gingiva or skin) and appear to involve the activation and sensitization of similar subsets of antigen capture and presenting cells, the dendritic cells (DCs), as well as similar T cell subsets. DCs have been termed "nature's adjuvant", being more efficient at antigen-presentation than macrophages or B cells and the only antigen-presenting cells that can stimulate naïve T cells to proliferate. This immunostimulatory capacity can also have detrimental effects for the host, as typified by graft-vs.-host disease and CHS responses. Both AP and CHS involve a predominantly destructive T cell response mediated by both regulatory and effector T cells. In the present paper, we show intriguing evidence that Porphyromonas gingivalis is a unique pathogen in this regard, able to infect, sensitize and activate DCs in vitro and, probably, in situ. Many questions about the role of P. gingivalis-sensitized DCs in adult periodontitis, and of the parallels between adult periodontitis and CHS, however, remain to be answered.

The monoclonal antibody DCGM4 recognizes Langerin, a protein specific of Langerhans cells, and is rapidly internalized from the cell surface

We generated monoclonal antibody (mAb) DCGM4 by immunization with human dendritic cells (DC) from CD34+ progenitors cultured with granulocyte-macrophage colony-stimulating factor and TNF-alpha. mAb DCGM4 was selected for its reactivity with a cell surface epitope present only on a subset of DC. Reactivity was strongly enhanced by the Langerhans cell (LC) differentiation factor TGF-beta and down-regulated by CD40 ligation. mAb DCGM4 selectively stained LC, hence we propose that the antigen be termed Langerin. mAb DCGM4 also stained intracytoplasmically, but neither colocalized with MHC class II nor with lysosomal LAMP-1 markers. Notably, mAb DCGM4 was rapidly internalized at 37 degrees C, but did not gain access to MHC class II compartments. Finally, Langerin was immunoprecipitated as a 40-kDa protein with a pI of 5.2 - 5.5. mAb DCGM4 will be useful to further characterize Langerin, an LC-restricted molecule involved in routing of cell surface material in immature DC.

Trafficking of Shigella lipopolysaccharide in polarized intestinal epithelial cells

Bacterial lipopolysaccharide (LPS) at the apical surface of polarized intestinal epithelial cells was previously shown to be transported from the apical to the basolateral pole of the epithelium (Beatty, W.L., and P.J. Sansonetti. 1997. Infect. Immun. 65:4395-4404). The present study was designed to elucidate the transcytotic pathway of LPS and to characterize the endocytic compartments involved in this process. Confocal and electron microscopic analyses revealed that LPS internalized at the apical surface became rapidly distributed within endosomal compartments accessible to basolaterally internalized transferrin. This compartment largely excluded fluid-phase markers added at either pole. Access to the basolateral side of the epithelium subsequent to trafficking to basolateral endosomes occurred via exocytosis into the paracellular space beneath the intercellular tight junctions. LPS appeared to exploit other endocytic routes with much of the internalized LPS recycled to the original apical membrane. In addition, analysis of LPS in association with markers of the endocytic network revealed that some LPS was sent to late endosomal and lysosomal compartments.

Presentation of Antigens Internalized Through the B Cell Receptor Requires Newly Synthesized MHC Class II Molecules

Exogenous Ags taken up from the fluid phase can be presented by both newly synthesized and recycling MHC class II molecules. However, the presentation of Ags internalized through the B cell receptor (BCR) has not been characterized with respect to whether the class II molecules with which they become associated are newly synthesized or recycling. We show that the presentation of Ag taken up by the BCR requires protein synthesis in splenic B cells and in B lymphoma cells. Using B cells transfected with full-length I-Ak molecules or molecules truncated in cytoplasmic domains of their α- or β-chains, we further show that when an Ag is internalized by the BCR, the cytoplasmic tails of class II molecules differentially control the presentation of antigenic peptides to specific T cells depending upon the importance of proteolytic processing in the production of that peptide. Integrity of the cytoplasmic tail of the I-Ak β-chain is required for the presentation of the hen egg lysozyme determinant (46–61) following BCR internalization, but that dependence is not seen for the (34–45) determinant derived from the same protein. The tail of the β-chain is also of importance for the dissociation of invariant chain fragments from class II molecules. Our results demonstrate that Ags internalized through the BCR are targeted to compartments containing newly synthesized class II molecules and that the tails of class II β-chains control the loading of determinants produced after extensive Ag processing.

Presentation of antigens internalized through the B cell receptor requires newly synthesized MHC class II molecules

Exogenous Ags taken up from the fluid phase can be presented by both newly synthesized and recycling MHC class II molecules. However, the presentation of Ags internalized through the B cell receptor (BCR) has not been characterized with respect to whether the class II molecules with which they become associated are newly synthesized or recycling. We show that the presentation of Ag taken up by the BCR requires protein synthesis in splenic B cells and in B lymphoma cells. Using B cells transfected with full-length I-Ak molecules or molecules truncated in cytoplasmic domains of their alpha- or beta-chains, we further show that when an Ag is internalized by the BCR, the cytoplasmic tails of class II molecules differentially control the presentation of antigenic peptides to specific T cells depending upon the importance of proteolytic processing in the production of that peptide. Integrity of the cytoplasmic tail of the I-Ak beta-chain is required for the presentation of the hen egg lysozyme determinant (46-61) following BCR internalization, but that dependence is not seen for the (34-45) determinant derived from the same protein. The tail of the beta-chain is also of importance for the dissociation of invariant chain fragments from class II molecules. Our results demonstrate that Ags internalized through the BCR are targeted to compartments containing newly synthesized class II molecules and that the tails of class II beta-chains control the loading of determinants produced after extensive Ag processing.

Engagement of B cell receptor regulates the invariant chain-dependent MHC class II presentation pathway

The intracellular sites in which Ags delivered by the B cell receptor (BCR) are degraded and loaded onto class II molecules remain poorly defined. To address this issue, we generated wild-type and invariant chain (Ii)-deficient H-2k mice bearing BCR specific for hen egg lysozyme. Our results show that, 1) unlike Ags taken up from the fluid phase, Ii is required for presentation of hen egg lysozyme internalized through the BCR in a manner independent of the peptide analyzed; 2) BCR ligation induces intracellular accumulation of MHC class II molecules only in Ii-positive B cells; and 3) these class II molecules reach intracellular compartments where BCR targets exogenous Ag. No differences in expression of adhesion and costimulatory molecules or in the presentation of soluble peptides were detectable between Ii-positive and -negative B cells. Therefore, the BCR delivers its ligand to compartments containing MHC class II-Ii complexes and bypasses the Ii-independent presentation pathway. The linked roles of Ag internalization and B cell activation of the BCR leads to potent Ii-dependent presentation in splenic B cells.

Tagging the human immunodeficiency virus gag protein with green fluorescent protein. Minimal evidence for colocalisation with actin

The assembly and budding of human immunodeficiency virus type 1, encoded solely in the Gag protein precursor Pr55Gag, occur at the plasma membrane of infected cells. However, little is known about the routing of the Gag molecule from its site of synthesis in the cytoplasm to the site of budding, with past studies suggesting that the cytoskeleton, particularly actin, may be involved in the translocation. We have constructed a T7 promoter-driven gag gene fusion with green fluorescent protein (GFP) that expresses Gag-GFP in both cells and supernatant. The distribution of Gag-GFP was the same as Gag only, suggesting that cellular routing was not affected by fusion to GFP, and using colabelling techniques, Gag-GFP was shown to have no particular colocalisation with actin. After detergent extraction of expressing cells, Gag and Gag-GFP remained cell associated, whereas GFP only was wholly released. These data suggest that Gag may associate with other cytoskeletal components or, perhaps more likely, that a partial assembly to a large-molecular-weight intermediate occurs before localisation at the plasma membrane.

Engagement of B Cell Receptor Regulates the Invariant Chain-Dependent MHC Class II Presentation Pathway

The intracellular sites in which Ags delivered by the B cell receptor (BCR) are degraded and loaded onto class II molecules remain poorly defined. To address this issue, we generated wild-type and invariant chain (Ii)-deficient H-2k mice bearing BCR specific for hen egg lysozyme. Our results show that, 1) unlike Ags taken up from the fluid phase, Ii is required for presentation of hen egg lysozyme internalized through the BCR in a manner independent of the peptide analyzed; 2) BCR ligation induces intracellular accumulation of MHC class II molecules only in Ii-positive B cells; and 3) these class II molecules reach intracellular compartments where BCR targets exogenous Ag. No differences in expression of adhesion and costimulatory molecules or in the presentation of soluble peptides were detectable between Ii-positive and -negative B cells. Therefore, the BCR delivers its ligand to compartments containing MHC class II-Ii complexes and bypasses the Ii-independent presentation pathway. The linked roles of Ag internalization and B cell activation of the BCR leads to potent Ii-dependent presentation in splenic B cells.