Trafficking of major histocompatibility complex class II molecules in human B-lymphoblasts deficient in the AP-3 adaptor complex

Discerning clinicopathological features of congenital neutropenia syndromes: an approach to diagnostically challenging differential diagnoses

The congenital neutropenia syndromes are rare haematological conditions defined by impaired myeloid precursor differentiation or function. Patients are prone to severe infections with high mortality rates in early life. While some patients benefit from granulocyte colony-stimulating factor treatment, they may still face an increased risk of bone marrow failure, myelodysplastic syndrome and acute leukaemia. Accurate diagnosis is crucial for improved outcomes; however, diagnosis depends on familiarity with a heterogeneous group of rare disorders that remain incompletely characterised. The clinical and pathological overlap between reactive conditions, primary and congenital neutropenias, bone marrow failure, and myelodysplastic syndromes further clouds diagnostic clarity.We review the diagnostically useful clinicopathological and morphological features of reactive causes of neutropenia and the most common primary neutropenia disorders: constitutional/benign ethnic neutropenia, chronic idiopathic neutropenia, cyclic neutropenia, severe congenital neutropenia (due to mutations in ELANE, GFI1, HAX1, G6PC3, VPS45, JAGN1, CSF3R, SRP54, CLPB and WAS), GATA2 deficiency, Warts, hypogammaglobulinaemia, infections and myelokathexis syndrome, Shwachman-Diamond Syndrome, the lysosomal storage disorders with neutropenia: Chediak-Higashi, Hermansky-Pudlak, and Griscelli syndromes, Cohen, and Barth syndromes. We also detail characteristic cytogenetic and molecular factors at diagnosis and in progression to myelodysplastic syndrome/leukaemia.

Invariant chain regulates endosomal fusion and maturation through an interaction with the SNARE Vti1b

The invariant chain (Ii, also known as CD74) is a multifunctional regulator of adaptive immune responses and is responsible for sorting major histocompatibility complex class I and class II (MHCI and MHCII, respectively) molecules, as well as other Ii-associated molecules, to a specific endosomal pathway. When Ii is expressed, endosomal maturation and proteolytic degradation of proteins are delayed and, in non-antigen presenting cells, the endosomal size increases, but the molecular mechanisms underlying this are not known. We identified that a SNARE, Vti1b, is essential for regulating these Ii-induced effects. Vti1b binds to Ii and is localized at the contact sites of fusing Ii-positive endosomes. Furthermore, truncated Ii lacking the cytoplasmic tail, which is not internalized from the plasma membrane, relocates Vti1b to the plasma membrane. Knockout of Ii in an antigen-presenting cell line was found to speed up endosomal maturation, whereas silencing of Vti1b inhibits the Ii-induced maturation delay. Our results suggest that Ii, by interacting with the SNARE Vti1b in antigen-presenting cells, directs specific Ii-associated SNARE-mediated fusion in the early part of the endosomal pathway that leads to a slower endosomal maturation for efficient antigen processing and MHC antigen loading.

The Role of Adaptor Proteins in the Biology of Natural Killer T (NKT) Cells

Adaptor proteins contribute to the selection, differentiation and activation of natural killer T (NKT) cells, an innate(-like) lymphocyte population endowed with powerful immunomodulatory properties. Distinct from conventional T lymphocytes NKT cells preferentially home to the liver, undergo a thymic maturation and differentiation process and recognize glycolipid antigens presented by the MHC class I-like molecule CD1d on antigen presenting cells. NKT cells express a semi-invariant T cell receptor (TCR), which combines the Vα14-Jα18 chain with a Vβ2, Vβ7, or Vβ8 chain in mice and the Vα24 chain with the Vβ11 chain in humans. The avidity of interactions between their TCR, the presented glycolipid antigen and CD1d govern the selection and differentiation of NKT cells. Compared to TCR ligation on conventional T cells engagement of the NKT cell TCR delivers substantially stronger signals, which trigger the unique NKT cell developmental program. Furthermore, NKT cells express a panoply of primarily inhibitory NK cell receptors (NKRs) that control their self-reactivity and avoid autoimmune activation. Adaptor proteins influence NKT cell biology through the integration of TCR, NKR and/or SLAM (signaling lymphocyte-activation molecule) receptor signals or the variation of CD1d-restricted antigen presentation. TCR and NKR ligation engage the SH2 domain-containing leukocyte protein of 76kDa slp-76 whereas the SLAM associated protein SAP serves as adaptor for the SLAM receptor family. Indeed, the selection and differentiation of NKT cells selectively requires co-stimulation via SLAM receptors. Furthermore, SAP deficiency causes X-linked lymphoproliferative disease with multiple immune defects including a lack of circulating NKT cells. While a deletion of slp-76 leads to a complete loss of all peripheral T cell populations, mutations in the SH2 domain of slp-76 selectively affect NKT cell biology. Furthermore, adaptor proteins influence the expression and trafficking of CD1d in antigen presenting cells and subsequently selection and activation of NKT cells. Adaptor protein complex 3 (AP-3), for example, is required for the efficient presentation of glycolipid antigens which require internalization and processing. Thus, our review will focus on the complex contribution of adaptor proteins to the delivery of TCR, NKR and SLAM receptor signals in the unique biology of NKT cells and CD1d-restricted antigen presentation.

Hermansky-Pudlak syndrome: Report of two patients with updated genetic classification and management recommendations

Hermansky-Pudlak syndrome (HPS) is a rare autosomal recessive disorder caused by mutations in one of nine genes involved in the packaging and formation of specialized lysosomes, including melanosomes and platelet-dense granules. The cardinal features are pigmentary dilution, bleeding diathesis, and accumulation of ceroid-like material in reticuloendothelial cells. Pulmonary fibrosis induced by tissue damage is seen in the most severe forms, and one subtype is characterized by immunodeficiency. We describe two patients with HPS type 1 and review the updated gene-based classification, clinical features, and recommendations for evaluation and follow-up.

Hemophagocytic lymphohistiocytosis and primary immune deficiency disorders

Hemophagocytic lymphohistiocytosis (HLH) is characterized by uncontrolled immune activation and is traditionally associated with inherited gene defects or acquired causes. In addition to abnormalities in cytotoxic granules and lysosomes, various primary immune deficiency disorders (PID) have been identified among patients suffering from HLH. Our purpose was twofold: to better characterize and detail the association between PID and HLH. We found that HLH occurs infrequently among patients with PID, particularly those suffering from abnormalities that impair T cell function. The prognosis of patients suffering from PID and HLH is poor, emphasizing the need for rapid clinical and genetic diagnosis of the PID as well as initiation of appropriate management of the HLH, including allogeneic hematopoietic stem cell transplantations. The association of HLH and PID implicates abnormal T cell function as an important factor in HLH development. It also suggests that the partition of HLH into genetic versus acquired forms might be misleading.

Clinical, laboratory and molecular signs of immunodeficiency in patients with partial oculo-cutaneous albinism

Hypopigmentation disorders that are associated with immunodeficiency feature both partial albinism of hair, skin and eyes together with leukocyte defects. These disorders include Chediak Higashi (CHS), Griscelli (GS), Hermansky-Pudlak (HPS) and MAPBP-interacting protein deficiency syndromes. These are heterogeneous autosomal recessive conditions in which the causal genes encode proteins with specific roles in the biogenesis, function and trafficking of secretory lysosomes. In certain specialized cells, these organelles serve as a storage compartment. Impaired secretion of specific effector proteins from that intracellular compartment affects biological activities. In particular, these intracellular granules are essential constituents of melanocytes, platelets, granulocytes, cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Thus, abnormalities affect pigmentation, primary hemostasis, blood cell counts and lymphocyte cytotoxic activity against microbial pathogens. Among eight genetically distinct types of HPS, only type 2 is characterized by immunodeficiency. Recently, a new subtype, HPS9, was defined in patients presenting with immunodeficiency and oculocutaneous albinism, associated with mutations in the pallidin-encoding gene, PLDN.Hypopigmentation together with recurrent childhood bacterial or viral infections suggests syndromic albinism. T and NK cell cytotoxicity are generally impaired in patients with these disorders. Specific clinical and biochemical phenotypes can allow differential diagnoses among these disorders before molecular testing. Ocular symptoms, including nystagmus, that are usually evident at birth, are common in patients with HPS2 or CHS. Albinism with short stature is unique to MAPBP-interacting protein (MAPBPIP) deficiency, while hemophagocytic lymphohistiocytosis (HLH) mainly suggests a diagnosis of CHS or GS type 2 (GS2). Neurological disease is a long-term complication of CHS, but is uncommon in other syndromic albinism. Chronic neutropenia is a feature of HPS2 and MAPBPIP-deficiency syndrome, whereas it is usually transient in CHS and GS2. In every patient, an accurate diagnosis is required for prompt and appropriate treatment, particularly in patients who develop HLH or in whom bone marrow transplant is required. This review describes the molecular and pathogenetic mechanisms of these diseases, focusing on clinical and biochemical aspects that allow early differential diagnosis.

Presentation of phagocytosed antigens by MHC class I and II

Phagocytosis provides innate immune cells with a mechanism to take up and destroy pathogenic bacteria, apoptotic cells and other large particles. In some cases, however, peptide antigens from these particles are preserved for presentation in association with major histocompatibility complex (MHC) class I or class II molecules in order to stimulate antigen-specific T cells. Processing and presentation of antigens from phagosomes presents a number of distinct challenges relative to antigens internalized by other means; while bacterial antigens were among the first discovered to be presented to T cells, analyses of the cellular mechanisms by which peptides from phagocytosed antigens assemble with MHC molecules and by which these complexes are then expressed at the plasma membrane have lagged behind those of conventional model soluble antigens. In this review, we cover recent advances in our understanding of these processes, including the unique cross-presentation of phagocytosed antigens by MHC class I molecules, and in their control by signaling modalities in phagocytic cells.

Adaptor protein-3 in dendritic cells facilitates phagosomal toll-like receptor signaling and antigen presentation to CD4(+) T cells

Effective major histocompatibility complex-II (MHC-II) antigen presentation from phagocytosed particles requires phagosome-intrinsic Toll-like receptor (TLR) signaling, but the molecular mechanisms underlying TLR delivery to phagosomes and how signaling regulates antigen presentation are incompletely understood. We show a requirement in dendritic cells (DCs) for adaptor protein-3 (AP-3) in efficient TLR recruitment to phagosomes and MHC-II presentation of antigens internalized by phagocytosis but not receptor-mediated endocytosis. DCs from AP-3-deficient pearl mice elicited impaired CD4(+) T cell activation and Th1 effector cell function to particulate antigen in vitro and to recombinant Listeria monocytogenes infection in vivo. Whereas phagolysosome maturation and peptide:MHC-II complex assembly proceeded normally in pearl DCs, peptide:MHC-II export to the cell surface was impeded. This correlated with reduced TLR4 recruitment and proinflammatory signaling from phagosomes by particulate TLR ligands. We propose that AP-3-dependent TLR delivery from endosomes to phagosomes and subsequent signaling mobilize peptide:MHC-II export from intracellular stores.

Competition model for upregulation of the major histocompatibility complex class II-associated invariant chain by human immunodeficiency virus type 1 Nef

The human immunodeficiency virus type 1 (HIV-1) Nef protein upregulates the expression of the invariant chain (Ii)/major histocompatibility complex class II (MHC-II) complex at the cell surface. This complex appears to reach the antigen-loading endosomal compartment at least in part via an indirect pathway in which it is internalized from the cell surface via the adaptor protein 2 (AP-2) complex. Here we provide evidence for a competition model to explain how Nef upregulates the expression of Ii at the cell surface. In this model, Nef and Ii compete for binding to AP-2. In support of this model, Nef decreased the rate of internalization of Ii from the cell surface. The AP-binding dileucine motif in Nef, ENTSLL(165), was necessary and sufficient for the upregulation of Ii. In addition, two leucine-based AP-binding motifs in the Ii cytoplasmic tail, DDQRDLI(8) and EQLPML(17), were critical for the efficient upregulation of Ii by Nef. Experiments using Nef variants in which the native dileucine-based sorting motif was replaced with similar motifs from cellular transmembrane proteins allowed modulation of AP-binding specificity. Analysis of these variants suggested that the binding of Nef to AP-2 is sufficient to upregulate Ii at the plasma membrane. Finally, interference with the expression of AP-2 caused an upregulation of Ii at the plasma membrane, and this decreased the effect of Nef. These data indicate that Nef usurps AP-2 complexes to dysregulate Ii trafficking and potentially interfere with antigen presentation in the context of MHC-II.

Pathways of CD1 and lipid antigen delivery, trafficking, processing, loading, and presentation

Specific T cell responses to a variety of self and microbial lipids depend on proper assembly and intracellular trafficking of CD 1 molecules that intersect with and load processed lipid antigens. These pathways involve unique membrane trafficking and chaperones that are distinct from those utilized for major histocompatibility complex (MHC)-mediated presentation of peptide antigens, and thus define unique lipid antigen presentation pathways. Furthermore, recent studies have identified components of lipid metabolism that participate in lipid delivery, uptake, processing and loading onto CD1 molecules. Defects in these pathways result in impaired T cell development and function, underscoring their critical role in the lipid-specific T cell immune responses.

Involvement of clathrin and AP-2 in the trafficking of MHC class II molecules to antigen-processing compartments

Major histocompatibility complex class II (MHC-II) molecules are composed of two polymorphic chains, alpha and beta, which assemble with an invariant chain, Ii, in the endoplasmic reticulum. The assembled MHC-II complexes are transported to the Golgi complex and then to late endosomes/lysosomes, where Ii is degraded and alphabeta dimers bind peptides derived from exogenous antigens. Targeting of MHC-II molecules to these compartments is mediated by two dileucine-based signals in the cytoplasmic domain of Ii. These signals bind in vitro to two adaptor protein (AP) complexes, AP-1 and AP-2, which are components of clathrin coats involved in vesicle formation and cargo sorting. The physiological roles of these proteins in MHC-II molecule trafficking, however, remain to be addressed. Here, we report the use of RNA interference to examine the involvement of clathrin and four AP complexes (AP-1, AP-2, AP-3, and AP-4) in MHC-II molecule trafficking in vivo. We found that depletion of clathrin or AP-2 caused >10-fold increases in Ii expression on the cell surface and a concomitant decrease in Ii localization to endosomal/lysosomal vesicles. In addition, depletion of clathrin or AP-2 delayed the degradation of Ii and reduced the surface expression of peptide-loaded alphabeta dimers. In contrast, depletion of AP-1, AP-3, or AP-4 had little or no effect. These findings demonstrate that clathrin and AP-2 participate in MHC-II molecule trafficking in vivo. Because AP-2 is only associated with the plasma membrane, these results also indicate that a significant pool of MHC-II molecules traffic to the endosomal-lysosomal system by means of the cell surface.

AP2 clathrin adaptor complex, but not AP1, controls the access of the major histocompatibility complex (MHC) class II to endosomes

Newly synthesized MHC II alpha- and beta-chains associated with the invariant chain chaperone (Ii) enter the endocytic pathway for Ii degradation and loading with peptides before transport to the cell surface. It is unclear how alphabetaIi complexes are sorted from the Golgi apparatus and directed to endosomes. However, indirect evidence tends to support direct transport involving the AP1 clathrin adaptor complex. Surprisingly, we show here that knocking down the production of AP1 by RNA interference did not affect the trafficking of alphabetaIi complexes. In contrast, AP2 depletion led to a large increase in surface levels of alphabetaIi complexes, inhibited their rapid internalization, and strongly delayed the appearance of mature MHC II in intracellular compartments. Thus, in the cell systems studied here, rapid internalization of alphabetaIi complexes via an AP2-dependent pathway represents a key step for MHC II delivery to endosomes and lysosomes.

New insights into pathways for CD1-mediated antigen presentation

Recent studies of CD1 structure and intracellular trafficking have demonstrated significant differences among the CD1 isoforms (CD1a, CD1b, CD1c and CD1d). The molecular and structural basis for the differential trafficking of CD1 molecules has also been delineated. These observations broaden our understanding of why the immune system has evolved multiple CD1 isoforms to survey different cellular compartments for lipid antigen presentation, to provide host defense against the microbial world and to offer immunoregulation with relevance to tumor immunity and autoimmunity.

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.

Endocytic recycling

After endocytosis, most membrane proteins and lipids return to the cell surface, but some membrane components are delivered to late endosomes or the Golgi. We now understand that the pathways taken by internalized molecules that eventually recycle to the cell surface can be surprisingly complex and can involve a series of sorting events that occur in several organelles. The molecular basis for many of these sorting processes is only partly understood.

Lysosomal localization of murine CD1d mediated by AP-3 is necessary for NK T cell development

The presentation of lipid and glycolipid Ags to T cells is mediated through CD1 molecules. In the mouse and rat only a single isoform, CD1d, performs these functions, while humans and all other mammals studied have members of both group I (CD1a, -b, and -c) and group II (CD1d) isoforms. Murine CD1d contains a cytoplasmic tyrosine-based sorting motif that is similar to motifs recognized by adaptor protein complexes that sort transmembrane proteins. Here we show that the adaptor protein complex, AP-3, directly interacts with murine CD1d and controls its targeting to lysosomes. AP-3 deficiency results in a redistribution of CD1d from lysosomes to the cell surface of thymocytes, B cell-depleted splenocytes, and dendritic cells. The altered trafficking of CD1d in AP-3-deficient mice results in a significant reduction of NK1.1(+)TCR-beta(+) and CD1d tetramer-positive cells, consistent with a defect in CD1d self-Ag presentation and thymocyte-positive selection. The AP-3 complex has recently been shown to associate with the human CD1b isoform, which has an intracellular distribution pattern similar to that of murine CD1d. We propose that lysosomal sampling may be so critical for efficient host defense that mice have evolved mechanisms to target their single CD1 isoform to lysosomes for sampling lipid Ags. Here we show the dominant mechanism for this trafficking is mediated by AP-3.

Signals for sorting of transmembrane proteins to endosomes and lysosomes

Sorting of transmembrane proteins to endosomes and lysosomes is mediated by signals present within the cytosolic domains of the proteins. Most signals consist of short, linear sequences of amino acid residues. Some signals are referred to as tyrosine-based sorting signals and conform to the NPXY or YXXO consensus motifs. Other signals known as dileucine-based signals fit [DE]XXXL[LI] or DXXLL consensus motifs. All of these signals are recognized by components of protein coats peripherally associated with the cytosolic face of membranes. YXXO and [DE]XXXL[LI] signals are recognized with characteristic fine specificity by the adaptor protein (AP) complexes AP-1, AP-2, AP-3, and AP-4, whereas DXXLL signals are recognized by another family of adaptors known as GGAs. Several proteins, including clathrin, AP-2, and Dab2, have been proposed to function as recognition proteins for NPXY signals. YXXO and DXXLL signals bind in an extended conformation to the mu2 subunit of AP-2 and the VHS domain of the GGAs, respectively. Phosphorylation events regulate signal recognition. In addition to peptide motifs, ubiquitination of cytosolic lysine residues also serves as a signal for sorting at various stages of the endosomal-lysosomal system. Conjugated ubiquitin is recognized by UIM, UBA, or UBC domains present within many components of the internalization and lysosomal targeting machinery. This complex array of signals and recognition proteins ensures the dynamic but accurate distribution of transmembrane proteins to different compartments of the endosomal-lysosomal system.

The adaptor protein AP-3 is required for CD1d-mediated antigen presentation of glycosphingolipids and development of Valpha14i NKT cells

Relatively little is known about the pathway leading to the presentation of glycolipids by CD1 molecules. Here we show that the adaptor protein complex 3 (AP-3) is required for the efficient presentation of glycolipid antigens that require internalization and processing. AP-3 interacts with mouse CD1d, and cells from mice deficient for AP-3 have increased cell surface levels of CD1d and decreased expression in late endosomes. Spleen cells from AP-3-deficient mice have a reduced ability to present glycolipids to natural killer T (NKT) cells. Furthermore, AP-3-deficient mice have a significantly reduced NKT cell population, although this is not caused by self-tolerance that might result from increased CD1d surface levels. These data suggest that the generation of the endogenous ligand that selects NKT cells may also be AP-3 dependent. However, the function of MHC class II-reactive CD4+ T lymphocytes is not altered by AP-3 deficiency. Consistent with this divergence from the class II pathway, NKT cell development and antigen presentation by CD1d are not reduced by invariant chain deficiency. These data demonstrate that the AP-3 requirement is a particular attribute of the CD1d pathway in mice and that, although MHC class II molecules and CD1d are both found in late endosomes or lysosomes, different pathways mediate their intracellular trafficking.

Lysosome-related organelles: a view from immunity and pigmentation

Lysosomes are ubiquitous organelles that carry out essential household functions. Certain cell types, however, contain lysosome-related organelles with specialized functions. Their specialized functions are usually reflected by specific morphological and compositional features. A number of diseases that develop due to genetic mutations, pathogen exposure or cell transformation are characterized by dysfunctional lysosomes and/or lysosome-related organelles. In this review we highlight adaptations and malfunction of the endosomal/lysosomal system in normal and pathological situations with special focus on MHC class II compartments in antigen presenting cells and melanosomes in pigment cells.

A tubular EHD1-containing compartment involved in the recycling of major histocompatibility complex class I molecules to the plasma membrane

The Eps15 homology (EH) domain-containing protein, EHD1, has recently been ascribed a role in the recycling of receptors internalized by clathrin-mediated endocytosis. A subset of plasma membrane proteins can undergo internalization by a clathrin-independent pathway regulated by the small GTP-binding protein ADP-ribosylation factor 6 (Arf6). Here, we report that endogenous EHD proteins, as well as transgenic tagged EHD1, are associated with long, membrane-bound tubules containing Arf6. EHD1 appears to induce tubule formation, which requires nucleotide cycling on Arf6 and intact microtubules. Mutations in the N-terminal P-loop domain or deletion of the C-terminal EH domain of EHD1 prevent association of EHD1 with tubules or induction of tubule formation. The EHD1 tubules contain internalized major histocompatibility complex class I (MHC-I) molecules that normally traffic through the Arf6 pathway. Recycling assays show that overexpression of EHD1 enhances MHC-I recycling. These observations suggest an additional function of EHD1 as a tubule-inducing factor in the Arf6 pathway for recycling of plasma membrane proteins internalized by clathrin-independent endocytosis.

Failure of trafficking and antigen presentation by CD1 in AP-3-deficient cells

Endocytosed microbial antigens are primarily delivered to lysosomal compartments where antigen binding to MHC and CD1 molecules occurs in an acidic and proteolytically active environment. Signal-dependent delivery to lysosomes has been suggested for these antigen-presenting molecules, but molecular interactions with vesicular coat proteins and adaptors that direct their lysosomal sorting are poorly understood. Here CD1b but not other CD1 isoforms bound the AP-3 adaptor protein complex. In AP-3-deficient cells derived from patients with Hermansky-Pudlak syndrome type 2 (HPS-2), CD1b failed to efficiently gain access to lysosomes, resulting in a profound defect in antigen presentation. Since MHC class II traffics normally in AP-3-deficient cells, defects in CD1b antigen presentation may account for recurrent bacterial infections in HPS-2 patients.

Genetic analyses of adaptin function from yeast to mammals

Adaptor protein (AP) complexes are heterotetrameric assemblies of subunits named adaptins. Four AP complexes, termed AP-1, AP-2, AP-3, and AP-4, have been described in various eukaryotic organisms. Biochemical and morphological evidence indicates that AP complexes play roles in the formation of vesicular transport intermediates and the selection of cargo molecules for inclusion into these intermediates. This understanding is being expanded by the application of genetic interference procedures. Here, we review recent progress in the genetic analysis of the function of AP complexes, focusing on studies that make use of targeted interference or naturally-occurring mutations in various model organisms.

Transient surface delivery of invariant chain-MHC II complexes via endosomes: a quantitative study

Newly synthesized major histocompatibility complex class II needs to be directed to late endocytic compartments to combine with peptide antigens. Efficient transport requires complexes of major histocompatibility complex class II and invariant chain (alphabetaIi). Since such complexes have been detected on the plasma membrane in human cells, this compartment was proposed as the primary destination for alphabetaIi exiting the trans-Golgi network. Here, I have used density gradient electrophoresis and selective biotinylation to investigate the trafficking route of alphabetaIi quantitatively. Density gradient electrophoresis analysis showed that alphabetaIi was transported from the trans-Golgi network to endosomes at approximately 1.7% min-1. Surface delivery of alphabetaIi was delayed relative to endosome transport by approximately 10 min and showed slower kinetics ( approximately 0.4% min-1), suggesting that alphabetaIi reached the plasma membrane only after arrival in endosomes. A biotinylation assay revealed that 20-40% of endosomal alphabetaIi was delivered to the plasma membrane at steady state, suggesting that surface alphabetaIi was entirely derived from endosomes. Surface alphabetaIi was rapidly re-internalized and either returned to the cell surface or accessed degradative compartments. Peptide loading commenced approximately 30 min after delivery to endosomes. Thus alphabetaIi directly traffics from trans-Golgi network to endosomes and enters an endosome-plasma membrane 'carousel' until transport to peptide-loading compartments ensues.

Trafficking of MHC class II molecules in the late secretory pathway

Antigen presenting cells (APCs) alert the immune system to attack by extracellular organisms; APCs achieve this via internalization, degradation, and display of antigenic fragments on the cell surface by MHC class II molecules. These class II molecules bind to an accessory protein, termed the invariant chain, that ensures proper folding of the molecules. Invariant-chain binding also directs class II molecules to lysosomes, which are probably the most important sites for antigen loading. Endosomes are intermediates in the transport of class-II-invariant chain complexes to antigen-processing compartments, whereas trafficking of class II-peptide complexes to the membrane (and beyond) is less-well understood. Unlike other APCs, dendritic cells alter their capacity to present peptides via MHC class II molecules during differentiation, revealing a complex level of regulated antigen-presentation by this APC subtype.

Human Vam6p promotes lysosome clustering and fusion in vivo

Regulated fusion of mammalian lysosomes is critical to their ability to acquire both internalized and biosynthetic materials. Here, we report the identification of a novel human protein, hVam6p, that promotes lysosome clustering and fusion in vivo. Although hVam6p exhibits homology to the Saccharomyces cerevisiae vacuolar protein sorting gene product Vam6p/Vps39p, the presence of a citron homology (CNH) domain at the NH(2) terminus is unique to the human protein. Overexpression of hVam6p results in massive clustering and fusion of lysosomes and late endosomes into large (2-3 microm) juxtanuclear structures. This effect is reminiscent of that caused by expression of a constitutively activated Rab7. However, hVam6p exerts its effect even in the presence of a dominant-negative Rab7, suggesting that it functions either downstream of, or in parallel to, Rab7. Data from gradient fractionation, two-hybrid, and coimmunoprecipitation analyses suggest that hVam6p is a homooligomer, and that its self-assembly is mediated by a clathrin heavy chain repeat domain in the middle of the protein. Both the CNH and clathrin heavy chain repeat domains are required for induction of lysosome clustering and fusion. This study implicates hVam6p as a mammalian tethering/docking factor characterized with intrinsic ability to promote lysosome fusion in vivo.

Intracellular transport of MHC class II and associated invariant chain in antigen presenting cells from AP-3-deficient mocha mice

MHC class II-restricted antigen presentation requires trafficking of newly synthesized class II-invariant chain complexes from the trans-Golgi network to endosomal, peptide-loading compartments. This transport is mediated by dileucine-like motifs within the cytosolic tail of the invariant chain. Although these signals have been well characterized, the cytosolic proteins that interact with these dileucine signals and mediate Golgi sorting and endosomal transport have not been identified. Recently, an adaptor complex, AP-3, has been identified that interacts with dileucine motifs and mediates endosomal/lysosomal transport in yeast, Drosophila, and mammals. In this report, we have assessed class II-invariant chain trafficking in a strain of mice (mocha) which lacks expression of AP-3. Our studies demonstrate that the lack of AP-3 does not affect the kinetics of invariant chain degradation, the route of class II-invariant chain transport, or the rate and extent of class II-peptide binding as assessed by the generation of SDS-stable dimers. The possible role of other known or unknown adaptor complexes in class II-invariant chain transport is discussed.

Hermansky-Pudlak syndrome and related disorders of organelle formation

Hermansky-Pudlak syndrome (HPS) consists of a group of genetically heterogeneous disorders which share the clinical findings of oculocutaneous albinism, a platelet storage pool deficiency, and some degree of ceroid lipofuscinosis. Related diseases share some of these findings and may exhibit other symptoms and signs but the underlying defect in the entire group of disorders involves defective intracellular vesicle formation, transport or fusion. Two HPS-causing genes, HPS1 and ADTB3A, have been isolated but the function of only the latter has been determined. ADTB3A codes for the beta 3A subunit of adaptor complex-3, responsible for vesicle formation from the trans-Golgi network (TGN). The many HPS patients who do not have HPS1 or ADTB3A mutations have their disease because of mutations in other genes. Candidates for these HPS-causing genes include those responsible for mouse models of HPS or for the 'granule' group of eye color genes in Drosophila. Each gene responsible for a subset of HPS or a related disorder codes for a protein which almost certainly plays a pivotal role in vesicular trafficking, inextricably linking clinical and cell biological interests in this group of diseases.