CD36 is not involved in scavenger receptor-mediated endocytic uptake of glycolaldehyde- and methylglyoxal-modified proteins by liver endothelial cells

Circulating proteins modified by advanced glycation end-products (AGE) are mainly taken up by liver endothelial cells (LECs) via scavenger receptor-mediated endocytosis. Endocytic uptake of chemically modified proteins by macrophages and macrophage-derived cells is mediated by class A scavenger receptor (SR-A) and CD36. In a previous study using SR-A knockout mice, we demonstrated that SR-A is not involved in endocytic uptake of AGE proteins by LECs [Matsumoto et al. (2000) Biochem. J. 352, 233-240]. The present study was conducted to determine the contribution of CD36 to this process. Glycolaldehyde-modified BSA (GA-BSA) and methylglyoxal-modified BSA (MG-BSA) were used as AGE proteins. 125I-GA-BSA and 125I-MG-BSA underwent endocytic degradation by these cells at 37 degrees C, and this process was inhibited by several ligands for the scavenger receptors. However, this endocytic uptake of 125I-GA-BSA by LECs was not inhibited by a neutralizing anti-CD36 antibody. Similarly, hepatic uptake of (111)In-GA-BSA after its intravenous injection was not significantly attenuated by co-administration of the anti-CD36 antibody. These results clarify that CD36 does not play a significant role in elimination of GA-BSA and MG-BSA from the circulation, suggesting that the receptor involved in endocytic uptake of circulating AGE proteins by LEC is not SR-A or CD36.     

Class B scavenger receptors CD36 and SR-BI are receptors for hypochlorite-modified low density lipoprotein

The presence of HOCl-modified epitopes inside and outside monocytes/macrophages and the presence of HOCl-modified apolipoprotein B in atherosclerotic lesions has initiated the present study to identify scavenger receptors that bind and internalize HOCl-low density lipoprotein (LDL). The uptake of HOCl-LDL by THP-1 macrophages was not saturable and led to cholesterol/cholesteryl ester accumulation. HOCl-LDL is not aggregated in culture medium, as measured by dynamic light scattering experiments, but internalization of HOCl-LDL could be inhibited in part by cytochalasin D, a microfilament disrupting agent. This indicates that HOCl-LDL is partially internalized by a pathway resembling phagocytosis-like internalization (in part by fluid-phase endocytosis) as measured with [14C]sucrose uptake. In contrast to uptake studies, binding of HOCl-LDL to THP-1 cells at 4 degrees C was specific and saturable, indicating that binding proteins and/or receptors are involved. Competition studies on THP-1 macrophages showed that HOCl-LDL does not compete for the uptake of acetylated LDL (a ligand to scavenger receptor class A) but strongly inhibits the uptake of copper-oxidized LDL (a ligand to CD36 and SR-BI). The binding specificity of HOCl-LDL to class B scavenger receptors could be demonstrated by Chinese hamster ovary cells overexpressing CD36 and SR-BI and specific blocking antibodies. The lipid moiety isolated from the HOCl-LDL particle did not compete for cell association of labeled HOCl-LDL to CD36 or SR-BI, suggesting that the protein moiety of HOCl-LDL is responsible for receptor recognition. Experiments with Chinese hamster ovary cells overexpressing scavenger receptor class A, type I, confirmed that LDL modified at physiologically relevant HOCl concentrations is not recognized by this receptor.     

Hypochlorous acid enhances immunogenicity and uptake of allogeneic ovarian tumor cells by dendritic cells to cross-prime tumor-specific T cells

Background: Ovarian cancer commonly relapses after remission and new strategies to target microscopic residual diseases are required. One approach is to activate tumor-specific cytotoxic T cells with dendritic cells loaded with tumor cells. In order to enhance their immunogenicity, ovarian tumor cells (SK-OV-3, which express two well-characterized antigens HER-2/neu and MUC-1) were killed by oxidation with hypochlorous acid (HOCl). Results: Treatment for 1 h with 60 μM HOCl was found to induce necrosis in all SK-OV-3 cells. Oxidized, but not live, SK-OV-3 was rapidly taken up by monocyte-derived dendritic cells, and induced partial dendritic cell maturation. Dendritic cells cultured from HLA-A2 healthy volunteers were loaded with oxidized SK-OV-3 (HLA-A2⁻) and co-cultured with autologous T cells. Responding T cells were tested for specificity after a further round of antigen stimulation. In ELISPOT assays, T cells produced interferon-gamma (IFN-γ) in response to the immunizing cellular antigen, and also to peptides coding for MUC-1 and HER-2/neu HLA-A2 restricted epitopes, demonstrating efficient cross-presentation of cell-associated antigens. In contrast, no responses were seen after priming with heat-killed or HCl-killed SK-OV-3, indicating that HOCl oxidation and not cell death/necrosis per se enhanced the immunogenicity of SK-OV-3. Finally, T cells stimulated with oxidized SK-OV-3 showed no cross-reaction to oxidized melanoma cells, nor vice versa, demonstrating that the response was tumor-type specific. Conclusions: Immunization with oxidized ovarian tumor cell lines may represent an improved therapeutic strategy to stimulate a polyclonal anti-tumor cellular immune response and hence extend remission in ovarian cancer.     

Scavenger receptor-A mediates gp96/GRP94 and calreticulin internalization by antigen-presenting cells

gp96 (GRP94) elicits antigen-presenting cell (APC) activation and can direct peptides into the cross- presentation pathways of APC. These responses arise through interactions of gp96 with Toll-like (APC activation) and endocytic (cross-presentation) receptors of APC. Previously, CD91, the alpha2-macroglobulin receptor, was identified as the heat shock/chaperone protein receptor of APC. Recent data indicates, however, that inhibition of CD91 ligand binding does not alter gp96 recognition and uptake. Furthermore, CD91 expression is not itself sufficient for gp96 binding and internalization. We now report that scavenger receptor class-A (SR-A), a prominent scavenger receptor of macrophages and dendritic cells, serves a primary role in gp96 and calreticulin recognition and internalization. gp96 internalization and peptide re-presentation are inhibited by the SR-A inhibitory ligand fucoidin, although fucoidin was without effect on alpha2-macroglobulin binding or uptake. Ectopic expression of SR-A in HEK 293 cells yielded gp96 recognition and uptake activity. In addition, macrophages derived from SR-A-/- mice were substantially impaired in gp96 binding and uptake. These data identify new roles for SR-A in the regulation of cellular responses to heat shock proteins.     

RASSF1A suppresses the activated K-Ras-induced oxidative DNA damage

Dendritic cells are the most potent antigen presenting cells responsible for the development of immune responses in cancer. However, it is known that the function of dendritic cells in tumor-bearing hosts is severely compromised. Our previous studies demonstrated that the defects in dendritic cell function are due, to a large extent, to the accumulation of high amounts of lipids – predominantly triglycerides – in a substantial proportion of dendritic cells in tumor-bearing mice and patients with cancer. The dendritic cells accumulation of lipids is likely associated with their up-regulation of a scavenger receptor A. This receptor is primarily responsible for uptake of modified lipids. Here, by using different versions of liquid chromatography–mass spectrometry, we identified several molecular species of oxygenated lipids in plasma of tumor-bearing animals that may be responsible for their uptake and accumulation by dendritic cells via scavenger receptor A-dependent pathway – the effect that may be associated with the loss of dendritic cell’s immune surveillance function in cancer.     

Scavenger receptors class A-I/II and CD36 are the principal receptors responsible for the uptake of modified low density lipoprotein leading to lipid loading in macrophages

Modification of low density lipoprotein (LDL) can result in the avid uptake of these lipoproteins via a family of macrophage transmembrane proteins referred to as scavenger receptors (SRs). The genetic inactivation of either of two SR family members, SR-A or CD36, has been shown previously to reduce oxidized LDL uptake in vitro and atherosclerotic lesions in mice. Several other SRs are reported to bind modified LDL, but their contribution to macrophage lipid accumulation is uncertain. We generated mice lacking both SR-A and CD36 to determine their combined impact on macrophage lipid uptake and to assess the contribution of other SRs to this process. We show that SR-A and CD36 account for 75-90% of degradation of LDL modified by acetylation or oxidation. Cholesteryl ester derived from modified lipoproteins fails to accumulate in macrophages taken from the double null mice, as assessed by histochemistry and gas chromatography-mass spectrometry. These results demonstrate that SR-A and CD36 are responsible for the preponderance of modified LDL uptake in macrophages and that other scavenger receptors do not compensate for their absence.     

Mannosylation of Virus-Like Particles Enhances Internalization by Antigen Presenting Cells

Internalization of peptides by antigen presenting cells is crucial for the initiation of the adaptive immune response. Mannosylation has been demonstrated to enhance antigen uptake through mannose receptors, leading to improved immune responses. In this study we test the effect of surface mannosylation of protein-based virus-like particles (VLP) derived from Rabbit hemorrhagic disease virus (RHDV) on uptake by murine and human antigen presenting cells. A monomannoside and a novel dimannoside were synthesized and successfully conjugated to RHDV VLP capsid protein, providing approximately 270 mannose groups on the surface of each virus particle. VLP conjugated to the mannoside or dimannoside exhibited significantly enhanced binding and internalization by murine dendritic cells, macrophages and B cells as well as human dendritic cells and macrophages. This uptake was inhibited by the inclusion of mannan as a specific inhibitor of mannose specific uptake, demonstrating that mannosylation of VLP targets mannose receptor-based uptake. Consistent with mannose receptor-based uptake, partial retargeting of the intracellular processing of RHDV VLP was observed, confirming that mannosylation of VLP provides both enhanced uptake and modified processing of associated antigens.     

Multiple intracellular routes in the cross-presentation of a soluble protein by murine dendritic cells

Soluble heat shock fusion proteins (Hsfp) stimulate mice to produce CD8+ CTL, indicating that these proteins are cross-presented by dendritic cells (DC) to naive CD8 T cells. We report that cross-presentation of these proteins depends upon their binding to DC receptors, likely belonging to the scavenger receptor superfamily. Hsfp entered DC by receptor-mediated endocytosis that was either inhibitable by cytochalasin D or not inhibitable, depending upon aggregation state and time. Most endocytosed Hsfp was transported to lysosomes, but not the small cross-presented fraction that exited early from the endocytic pathway and required access to proteasomes and TAP. Naive CD8 T cell (2C and OT-I) responses to DC incubated with Hsfp at 1 microM were matched by incubating DC with cognate octapeptides at 1-10 pM, indicating that display of very few class I MHC-peptide complexes per DC can be sufficient for cross-presentation. With an Hsfp (heat shock protein-OVA) having peptide sequences for both CD4+ (OT-II) and CD8+ (OT-I) cells, the CD4 cells responded far more vigorously than the CD8 cells and many more class II MHC-peptide than class I MHC-peptide complexes were displayed.     

T-cell recognition of glycolipids presented by CD1 proteins

The most well-known molecular paradigm of antigen recognition by T cells involves partial digestion of proteins to generate small peptides, which bind to major histocompatibility complex (MHC) proteins. Recent studies of CD1, an MHC class I homolog encoded outside the MHC, have revealed that it presents diverse glycolipids to T cells. The molecular mechanism for lipid antigen recognition involves insertion of the lipid portion of antigens into a hydrophobic groove to form CD1-lipid complexes, which contact T-cell receptors (TCRs). Here, we examine the known antigen structures presented by CD1, the majority of which have sugar moieties that are capable of interacting with TCRs. Recognition of carbohydrate epitopes is precise, and lipid-reactive T cells alter systemic immune responses in models of infectious and autoimmune disease. These findings provide a previously unrecognized mechanism by which the cellular immune system can recognize alterations in many types of carbohydrate structures.     

Tumors and the danger model

This article reviews the evidence for the danger model in the context of immune response to tumors and the insufficiency of the immune system to eliminate tumor growth. Despite their potential immunogenicity tumors do not induce significant immune responses which could destroy malignant cells. According to the danger model, the immune surveillance system fails to detect tumor antigens because transformed cells do not send any danger signals which could activate dendritic cells and initiate an immune response. Instead, tumor cells or antigen presenting cells turn off the responding T cells and induce tolerance. The studies reviewed herein based on model tumor antigens, recombinant viral vectors and detection of tumor specific T cells by MHC/peptide tetramers underscore the critical role of tumor antigen presentation and the context in which it occurs. They indicate that antigen presentation only by activated but not by cancer or resting dendritic cells is necessary for the induction of immune responses to tumor antigens. It becomes apparent that the inability of dendritic cells to become activated provides a biological niche for tumor escape from immune destruction and seems to be a principal mechanism for the failure of tumor immune surveillance.     

The heat shock protein gp96: a receptor-targeted cross-priming carrier and activator of dendritic cells

Heat shock proteins like gp96 (grp94) are able to induce specific cytotoxic T-cell (CTL) responses against cells from which they originate and are currently studied in clinical trials for use in immunotherapy of tumors. We have recently demonstrated that gp96 binds to at least one yet unidentified receptor restricted to antigen-presenting cells (APCs) like dendritic cells (DCs) but not to T cells. Moreover we have shown, that for CTL activation by gp96-chaperoned peptides receptor-mediated uptake of gp96 by APCs is required. Lately, we have discovered a second function of gp96 when interacting with professional APCs. Gp96 is able to mediate maturation of DCs as determined by upregulation of MHC class II, CD86 and CD83 molecules, secretion of pro-inflammatory cytokines IL-12 and TNF-alpha and enhanced T-cell simulatory capacity. Furthermore, the gp96 receptor(s) are down-regulated on mature DCs, suggesting that the gp96 receptor(s) behave similar to other endocytic receptors like CD36, mannose receptor etc. Our findings now provide additional evidence for the remarkable immunogenicity of gp96: first, the existence of specific gp96 receptors on APCs and second, the capacity to activate dendritic cells which is strictly required to enable these highly sophisticated APCs to prime CTL responses.     

Cd36, a member of the class b scavenger receptor family, as a receptor for advanced glycation end products

Interaction of advanced glycation end products (AGE) with AGE receptors induces several cellular phenomena potentially relating to diabetic complications. Five AGE receptors identified so far are RAGE (receptor for AGE), galectin-3, 80K-H, OST-48, and SRA (macrophage scavenger receptor class A types I and II). Since SRA is known to belong to the class A scavenger receptor family, and the scavenger receptor collectively represents a family of multiligand lipoprotein receptors, it is possible that CD36, although belonging to the class B scavenger receptor family, can recognize AGE proteins as ligands. This was tested at the cellular level in this study using Chinese hamster ovary (CHO) cells overexpressing human CD36 (CD36-CHO cells). Cellular expression of CD36 was confirmed by immunoblotting and immunofluorescent microscopy using anti-CD36 antibody. Upon incubation at 37 degrees C, (125)I-AGE-bovine serum albumin (AGE-BSA) and (125)I-oxidized low density lipoprotein (LDL), an authentic ligand for CD36, were endocytosed in a dose-dependent fashion and underwent lysosomal degradation by CD36-CHO cells, but not wild-type CHO cells. In binding experiments at 4 degrees C, (125)I-AGE-BSA exhibited specific and saturable binding to CD36-CHO cells (K(d) = 5.6 microg/ml). The endocytic uptake of (125)I-AGE-BSA by these cells was inhibited by 50% by oxidized LDL and by 60% by FA6-152, an anti-CD36 antibody inhibiting cellular binding of oxidized LDL. Our results indicate that CD36 expressed by these cells mediates the endocytic uptake and subsequent intracellular degradation of AGE proteins. Since CD36 is one of the major oxidized LDL receptors and is up-regulated in macrophage- and smooth muscle cell-derived foam cells in human atherosclerotic lesions, these results suggest that, like oxidized LDL, AGE proteins generated in situ are recognized by CD36, which might contribute to the pathogenesis of diabetic macrovascular complications.     

The role of heat shock proteins and their receptors in the activation of the immune system

Heat shock proteins (HSPs) have been described as potent tumor vaccines in animal models and are currently studied in clinical trials. The underlying immune response relies on immunogenic peptides that the HSPs have acquired intracellularly by interfering with the classical antigen processing pathways. There have been numerous reports shedding light on how HSPs are able to gain this function and a number of important requirements for HSP-mediated specific immunity have been described: first, the ability of HSPs to bind immunogenic peptides. Second, the acquisition of HSPs by specialized antigen presenting cells with efficient antigen processing pathways capable of inducing cellular immune responses. Third, the existence of specific receptors on the surfaces of antigen presenting cells, allowing efficient and rapid uptake of HSP-peptide complexes from the extracellular fluid. And fourth, the ability of heat shock proteins to activate antigen presenting cells, enabling the latter to prime cytotoxic T cell responses against the peptides associated to HSPs.     

Human immature dendritic cells efficiently bind and take up secretory IgA without the induction of maturation.

Immature dendritic cells (DC) reside in peripheral tissues, where they pick up and process incoming pathogens via scavenger receptors or FcR such as FcgammaR and FcepsilonR. At mucosal surfaces, IgA is the main Ig to protect the body from incoming pathogens. In addition, DC are present in high numbers at these sites. We detected expression of FcalphaR (CD89) on the CD14+ population of CD34+ progenitor-derived DC and on monocyte-derived DC (MoDC). However, CD89 expression was strongly decreased upon differentiation from monocyte to DC. We found only minimal binding of serum IgA to MoDC but strong binding of secretory IgA (SIgA). The SIgA binding to MoDC could not be blocked by anti-CD89 blocking Abs. DC efficiently internalized SIgA, but not serum IgA, and uptake of SIgA could be blocked by specific sugars or partially by Ab reactive with mannose receptor. Importantly, binding and uptake of SIgA was not accompanied by signs of DC maturation, such as increased expression of CD86 and CD83 or induction of cytokine secretion. These data indicate that SIgA can interact with DC not via CD89, but via carbohydrate-recognizing receptors like mannose receptor and suggest that uptake of SIgA-containing immune complexes by immature DC may be a mechanism to modulate mucosal immune responses.     

Innate immunity: structure and function of TLRs

The innate immune system provides the first line of defence against infection. Through a limited number of germline-encoded receptors called pattern recognition receptors (PRRs), innate cells recognize and are activated by highly conserved structures expressed by large group of microorganisms called pathogen-associated molecular patterns (PAMPs). PRRs are involved either in recognition (scavenger receptors, C-type lectins) or in cell activation (Toll-like receptors or TLR, helicases and NOD molecules). TLRs play a pivotal role in cell activation in response to PAMPs. TLR are type I transmembrane proteins characterized by an intracellular Toll/IL 1 receptor homology domain that are expressed by innate immune cells (dendritic cells, macrophages, NK cells), cells of the adaptive immunity (T and B lymphocytes) and non immune cells (epithelial and endothelial cells, fibroblasts). In all the cell types analyzed, TLR agonists, alone or in combination with costimulatory molecules, induce cell activation. The crucial role played by TLR in immune cell activation has been detailed in dendritic cells. A TLR-dependent activation of dendritic cells is required to induce their maturation and migration to regional lymph nodes and to activate na?ve T cells. The ability of different cell types to respond to TLR agonists is related to the pattern of expression of the TLRs and its regulation as well as their intracellular localization. Recent studies suggest that the nature of the endocytic and signaling receptors engaged by PAMPs may determine the nature of the immune response generated against the microbial molecules, highlighting the role of TLRs as molecular interfaces between innate and adaptive immunity. In this review are summarized the main biological properties of the TLR molecules.     

Effects of incorporation of immunoglobulin G and complement component C1q on uptake and degradation of Alzheimer's disease amyloid fibrils by microglia

Microglia are macrophage-like immune system cells found in the brain. They are associated with Alzheimer's Disease plaques, which contain fibrillar beta-amyloid (fAbeta) and other components such as complement proteins. We have shown previously that murine microglia bind and internalize fAbeta microaggregates via the type A scavenger receptor, but degradation of internalized fAbeta is significantly slower than normal degradation. In this study, we compared internalization by microglia of fAbeta microaggregates to that of anti-Abeta-antibody-coated fAbeta (IgG-fAbeta) microaggregates and found that the uptake of the latter is increased by about 1.5-fold versus unmodified fAbeta. The endocytic trafficking of IgG-fAbeta is similar to that of fAbeta microaggregates, following an endosomal/lysosomal pathway. We also compared the internalization of fAbeta microaggregates to that of complement protein, C1q-coated fAbeta microaggregates, and found that the levels of uptake are also increased by about 1.5-fold. Rates of degradation of both types of modified fAbeta microaggregates are unchanged compared with unmodified fAbeta microaggregates. We demonstrated by blocking studies that internalization of IgG-fAbeta is mediated by Fc receptors. These data suggest that, in vivo, several different microglial receptors may play a part in internalizing fAbeta, but the involvement of other receptors may not increase the degradation of fAbeta.     

Immunostimulation of dendritic cells by cationic liposomes

A nano-aggregate liposome-polycation-DNA (LPD), composed of a cationic lipid, protamine and plasmid DNA was found to effectively deliver a human papillomavirus (HPV)-E7 epitope antigen to the antigen presenting cells of the immune system, eliciting enhanced anti-tumor immune responses in mouse models of cervical carcinoma. Both the cationic liposome and plasmid DNA were essential for the full immunostimulation activity of LPD. Interestingly, cationic liposomes alone could stimulate the antigen presenting dendritic cells (DC) leading to the expression of co-stimulatory molecules, CD80 and CD86. However, cationic lipids could not stimulate DC for the expression of pro-inflammatory cytokines. Moreover, they were unable to enhance the expression of NF-κB, suggesting that dendritic cells stimulation by cationic lipids is signaled through an NF-κB independent mechanism. DC stimulation was specific to cationic lipids, the zwitterionic and anionic lipids showed little or no activity. The ability of different cationic lipids to stimulate the expression of co-stimulatory molecules on DC varied significantly. In general, the cationic lipids bearing ethyl phosphocholine head groups were better stimulants than their trimethylammonium counterparts. In case of the cationic lipids bearing trimethyl ammonium head groups, the ones bearing unsaturated or shorter saturated hydrophobic chains exhibited enhanced immunostimulatory activity. The LPS-induced TNF-α expression by dendritic cells was inhibited by active cationic lipids but not the inactive ones, suggesting the possible involvement of lipopolysaccharide binding protein (LBP) in cationic lipid mediated DC stimulation. Based on the structure-specific activation of dendritic cells by cationic lipids, a model for the immunostimulation of DC by such lipids is proposed.     

Immunostimulation of dendritic cells by cationic liposomes

A nano-aggregate liposome-polycation-DNA (LPD), composed of a cationic lipid, protamine and plasmid DNA was found to effectively deliver a human papillomavirus (HPV)-E7 epitope antigen to the antigen presenting cells of the immune system, eliciting enhanced anti-tumor immune responses in mouse models of cervical carcinoma. Both the cationic liposome and plasmid DNA were essential for the full immunostimulation activity of LPD. Interestingly, cationic liposomes alone could stimulate the antigen presenting dendritic cells (DC) leading to the expression of co-stimulatory molecules, CD80 and CD86. However, cationic lipids could not stimulate DC for the expression of pro-inflammatory cytokines. Moreover, they were unable to enhance the expression of NF-kappaB, suggesting that dendritic cells stimulation by cationic lipids is signaled through an NF-kappaB independent mechanism. DC stimulation was specific to cationic lipids, the zwitterionic and anionic lipids showed little or no activity. The ability of different cationic lipids to stimulate the expression of co-stimulatory molecules on DC varied significantly. In general, the cationic lipids bearing ethyl phosphocholine head groups were better stimulants than their trimethylammonium counterparts. In case of the cationic lipids bearing trimethyl ammonium head groups, the ones bearing unsaturated or shorter saturated hydrophobic chains exhibited enhanced immunostimulatory activity. The LPS-induced TNF-alpha expression by dendritic cells was inhibited by active cationic lipids but not the inactive ones, suggesting the possible involvement of lipopolysaccharide binding protein (LBP) in cationic lipid mediated DC stimulation. Based on the structure-specific activation of dendritic cells by cationic lipids, a model for the immunostimulation of DC by such lipids is proposed.     

The Third Way: Progress on pathways of antigen processing and presentation by CD1

CD1 proteins are a third family of antigen presenting molecules that bind bacterial and autologous lipid antigens for presentation to T cells. With the solution of the crystal structures of several complexes of CD1 molecules with lipids, a greater appreciation has been gained of the adaptability of CD1 in binding lipid antigens with diverse structural features. Biochemical studies of the interactions between the TCR and CD1-lipid complexes have revealed striking contrasts with TCR that bind to peptides presented by MHC-encoded class I and class II molecules. The sphingolipid activating proteins (SAP) have recently been found to facilitate the transfer of lipid antigens onto CD1 molecules. This helps to provide an explanation as to how the thermodynamic barrier, caused by loading hydrophobic lipid antigens in a hydrophilic environment, can be overcome. Mechanisms of CD1 endosomal trafficking are being delineated, including the means by which adaptor proteins induce the localization of some types of CD1 molecules to lysosomes, where they bind antigens. Unlike MHC class I and class II proteins, specialized molecules that function solely in chaperoning CD1 molecules, or in facilitating their antigen loading, have not been found. This suggests that the CD1 antigen presenting system, which diverged early in vertebrate evolution from MHC antigen presenting molecules, is a simpler system with a character closer to the primordial antigen presenting function.     

The autophagy machinery restrains iNKT cell activation through CD1D1 internalization

Invariant natural killer T (iNKT) cells are innate T cells with powerful immune regulatory functions that recognize glycolipid antigens presented by the CD1D protein. While iNKT cell-activating glycolipids are currently being explored for their efficacy to improve immunotherapy against infectious diseases and cancer, little is known about the mechanisms that control CD1D antigen presentation and iNKT cell activation in vivo. CD1D molecules survey endocytic pathways to bind lipid antigens in MHC class II-containing compartments (MIICs) before recycling to the plasma membrane. Autophagosomes intersect with MIICs and autophagy-related proteins are known to support antigen loading for increased CD4⁺ T cell immunity. Here, we report that mice with dendritic cell (DC)-specific deletion of the essential autophagy gene Atg5 showed better CD1D1-restricted glycolipid presentation in vivo. These effects led to enhanced iNKT cell cytokine production upon antigen recognition and lower bacterial loads during Sphingomonas paucimobilis infection. Enhanced iNKT cell activation was independent of receptor-mediated glycolipid uptake or costimulatory signals. Instead, loss of Atg5 in DCs impaired clathrin-dependent internalization of CD1D1 molecules via the adaptor protein complex 2 (AP2) and, thus, increased surface expression of stimulatory CD1D1-glycolipid complexes. These findings indicate that the autophagic machinery assists in the recruitment of AP2 to CD1D1 molecules resulting in attenuated iNKT cell activation, in contrast to the supporting role of macroautophagy in CD4⁺ T cell stimulation.