Birbeck granule-like "organized smooth endoplasmic reticulum" resulting from the expression of a cytoplasmic YFP-tagged langerin

Langerin is required for the biogenesis of Birbeck granules (BGs), the characteristic organelles of Langerhans cells. We previously used a Langerin-YFP fusion protein having a C-terminal luminal YFP tag to dynamically decipher the molecular and cellular processes which accompany the traffic of Langerin. In order to elucidate the interactions of Langerin with its trafficking effectors and their structural impact on the biogenesis of BGs, we generated a YFP-Langerin chimera with an N-terminal, cytosolic YFP tag. This latter fusion protein induced the formation of YFP-positive large puncta. Live cell imaging coupled to a fluorescence recovery after photobleaching approach showed that this coalescence of proteins in newly formed compartments was static. In contrast, the YFP-positive structures present in the pericentriolar region of cells expressing Langerin-YFP chimera, displayed fluorescent recovery characteristics compatible with active membrane exchanges. Using correlative light-electron microscopy we showed that the coalescent structures represented highly organized stacks of membranes with a pentalaminar architecture typical of BGs. Continuities between these organelles and the rough endoplasmic reticulum allowed us to identify the stacks of membranes as a form of “Organized Smooth Endoplasmic Reticulum” (OSER), with distinct molecular and physiological properties. The involvement of homotypic interactions between cytoplasmic YFP molecules was demonstrated using an A206K variant of YFP, which restored most of the Langerin traffic and BG characteristics observed in Langerhans cells. Mutation of the carbohydrate recognition domain also blocked the formation of OSER. Hence, a “double-lock” mechanism governs the behavior of YFP-Langerin, where asymmetric homodimerization of the YFP tag and homotypic interactions between the lectin domains of Langerin molecules participate in its retention and the subsequent formation of BG-like OSER. These observations confirm that BG-like structures appear wherever Langerin accumulates and confirm that membrane trafficking effectors dictate their physiology and, illustrate the importance of molecular interactions in the architecture of intracellular membranes.

HLA-DQA2 and HLA-DQB2 genes are specifically expressed in human Langerhans cells and encode a new HLA class II molecule

The precise role of human epidermal Langerhans cells (LCs) in immune response is highly controversial. While studying the gene expression profile of these cells, we were intrigued to identify the HLA-DQB2 gene as potentially expressed in LCs. Despite a strong evolutionary conservation of their sequences, the concomitant expression of the poorly polymorphic HLA-DQA2/HLA-DQB2 genes, paralogous to the HLA-DQA1/HLA-DQB1 genes, has never been detected in any cell type. We confirmed by RT-PCR that the HLA-DQA2 and -DQB2 genes are both expressed in LCs, but not in monocyte-derived dendritic cells, or in blood CD1c(+) or plasmacytoid dendritic cells. The presence of the HLA-DQβ2 chain in LCs could be demonstrated by Western blotting, whereas immunofluorescence revealed its localization in early endosomes. As in the case of other HLA class II molecules, the HLA-DQα2 and -DQβ2 chains formed heterodimers that had to associate with the invariant chain to reach endosomal compartments. HLA-DQα2/β2 heterodimers were expressed at the cell surface, where they could mediate staphylococcal superantigen stimulation of T cells. Interestingly, HLA-DQα2 and HLA-DQβ1 chains formed mixed heterodimers which efficiently left the endoplasmic reticulum. These observations strongly suggest that the poorly polymorphic HLA-DQA2 and -DQB2 genes should be considered to be of immunological importance. The HLA-DQα2/β2 molecules could influence the complexity of the repertoire of Ags presented by LCs.

Phenotypic studies of natural killer cell subsets in human transporter associated with antigen processing deficiency

Peripheral blood natural killer (NK) cells from patients with transporter associated with antigen processing (TAP) deficiency are hyporesponsive. The mechanism of this defect is unknown, but the phenotype of TAP-deficient NK cells is almost normal. However, we noticed a high percentage of CD56^bright cells among total NK cells from two patients. We further investigated TAP-deficient NK cells in these patients and compared them to NK cells from two other TAP-deficient patients with no clinical symptoms and to individuals with chronic inflammatory diseases other than TAP deficiency (chronic lung diseases or vasculitis). Peripheral blood mononuclear cells isolated from venous blood were stained with fluorochrome-conjugated antibodies and the phenotype of NK cells was analyzed by flow cytometry. In addition, ⁵¹Chromium release assays were performed to assess the cytotoxic activity of NK cells. In the symptomatic patients, CD56^bright NK cells represented 28% and 45%, respectively, of all NK cells (higher than in healthy donors). The patients also displayed a higher percentage of CD56^dimCD16⁻ NK cells than controls. Interestingly, this unusual NK cell subtype distribution was not found in the two asymptomatic TAP-deficient cases, but was instead present in several of the other patients. Over-expression of the inhibitory receptor CD94/NKG2A by TAP-deficient NK cells was confirmed and extended to the inhibitory receptor ILT2 (CD85j). These inhibitory receptors were not involved in regulating the cytotoxicity of TAP-deficient NK cells. We conclude that expansion of the CD56^bright NK cell subtype in peripheral blood is not a hallmark of TAP deficiency, but can be found in other diseases as well. This might reflect a reaction of the immune system to pathologic conditions. It could be interesting to investigate the relative distribution of NK cell subsets in various respiratory and autoimmune diseases.

Rab11A controls the biogenesis of Birbeck granules by regulating Langerin recycling and stability

The extent to which Rab GTPases, Rab-interacting proteins, and cargo molecules cooperate in the dynamic organization of membrane architecture remains to be clarified. Langerin, a recycling protein accumulating in the Rab11-positive compartments of Langerhans cells, induces the formation of Birbeck granules (BGs), which are membrane subdomains of the endosomal recycling network. We investigated the role of Rab11A and two members of the Rab11 family of interacting proteins, Rip11 and RCP, in Langerin traffic and the biogenesis of BGs. The overexpression of a dominant-negative Rab11A mutant or Rab11A depletion strongly influenced Langerin traffic and stability and the formation of BGs, whereas modulation of other Rab proteins involved in dynamic regulation of the endocytic-recycling pathway had no effect. Impairment of Rab11A function led to a missorting of Langerin to lysosomal compartments, but inhibition of Langerin degradation by chloroquine did not restore the formation of BGs. Loss of RCP, but not of Rip11, also had a modest, but reproducible effect on Langerin stability and BG biogenesis, pointing to a role for Rab11A-RCP complexes in these events. Our results show that Rab11A and Langerin are required for BG biogenesis, and they illustrate the role played by a Rab GTPase in the formation of a specialized subcompartment within the endocytic-recycling system.

Functional CD1a is stabilized by exogenous lipids

Self-glycosphingolipids bind to surface CD1 molecules and are readily displaced by other CD1 ligands. This capacity to exchange antigens at the cell surface is not common to other antigen-presenting molecules and its physiological importance is unclear. Here we show that a large pool of cell-surface CD1a, but not CD1b molecules, is stabilized by exogenous lipids present in serum. Under serum deprivation CD1a molecules are altered and functionally inactive, as they are unable to present lipid antigens to T cells. Glycosphingolipids and phospholipids bind to, and restore functionality to CD1a without the contribution of newly synthesized and recycling CD1a molecules. The dependence of CD1a stability on exogenous lipids is not related to its intracellular traffic and rather to its antigen-binding pockets. These results indicate a functional dichotomy between CD1a and CD1b molecules and provide new information on how the lipid antigenic repertoire is immunologically sampled.

Human Epidermal Langerhans Cells Express the Immunoregulatory Enzyme Indoleamine 2,3-Dioxygenase

Langerhans cells (LC) are a special subset of dendritic cells integrating cutaneous immunity. The study of LC function is of major interest not only for efforts of vaccine design and immunotherapy but also for gaining an insight into the pathogenesis of immune-mediated cutaneous diseases and neoplasias. Recently, defined antigen-presenting cells were described that express indoleamine 2,3-dioxygenase (IDO) and inhibit T cell proliferation in vitro and in vivo. Here, we show that stimulation with interferon-gamma (IFN-gamma) induces the expression of functionally active IDO in highly purified human epidermal LC. The induction of IDO after stimulation of LC with IFN-gamma seems to follow a defined kinetic with rapid upregulation followed by a downregulation after about 24 h of culture. Accordingly, proliferation of T cells induced by anti-CD3 antibodies was modulated by supernatants of IFN-gamma-activated human epidermal LC. Importantly, downregulation of T cell proliferation by supernatants of 24 h IFN-gamma-activated LC was prevented by inhibition of IDO. These results indicate that LC not only have the capacity to stimulate but also to inhibit T cells, and suggest that LC possess an immunoregulatory function in promoting T cell tolerance by production of IDO.

Reproduction of Langerin/CD207 traffic and Birbeck granule formation in a human cell line model

Birbeck granules (BG) are organelles specific to Langerhans cells (LCs), which form where the C-type lectin Langerin accumulates. Their function remains obscure due to morphologic and dynamic alterations induced by maturation of isolated LC. In this study, we attempted to reconstitute Langerin traffic and BG formation in the endosomal pathway of a human melanoma cell line. In the selected Langerin-transfected cell line, M10-22E, Langerin is distributed between the early recycling endosomal compartment and the plasma membrane, as in LC. Whereas mainly concentrated in membranes related to the Rab11(+) endosomal recycling compartment at the steady state, Langerin also recycles in M10-22E cells and drives BG biogenesis in the endosomal recycling compartment. Interruption of endocytosis or recycling induces redistribution of intracellular Langerin with an associated alteration in BG location and morphology. We have, therefore, generated a stable, Langerin-transfected cell line in which Langerin traffic and distribution and BG morphology replicate that seen in freshly isolated LC. This practical model can now be used to further delineate the nature and function of BG.

Endotoxin-free heat-shock protein 70 fails to induce APC activation

Previous work has suggested that the peptide-carrier, heat-shock protein (hsp)70, could directly activate APC. Here we show that this ability is related to endotoxin contamination of the human rhsp70 produced in Escherichia coli. Hence, the ability of 1-3 microg/ml of rhsp70 to induce the maturation of human monocyte-derived DC is abrogated in the presence of the LPS-antagonist polymyxin B or when the rhsp70 contains less than 60 IU/mg endotoxin. Such a level of contamination of the rhsp70 is, however, sufficient - in the presence of soluble rCD14, the LPS co-receptor - to induce cytokine secretion from monocytes and DC, despite the presence of polymyxin B. However, when endotoxin contamination is below 10 IU/mg, rhsp70 does not induce cytokine secretion - even in the presence of soluble rCD14 - or activate p38 mitogen-activated protein kinase signaling pathways, thus showing that an "endotoxin free" hsp70 does not activate APC.

Evidence of a trimolecular complex involving LPS, LPS binding protein and soluble CD14 as an effector of LPS response

The kinetics of the interaction of lipopolysaccharide (LPS), lipopolysaccharide binding protein (LBP) and CD14 was studied using surface plasmon resonance. The association and dissociation rate constants for the binding of LPS and rsCD14 were 2.9 x 10(4) M(-1) s(-1) and 0.07 s(-1) respectively, yielding a binding constant of 4.2 x 10(5) M(-1). Significantly, the presence of LBP increased not only the association rate but also the association constant for the interaction between LPS and CD14 by three orders of magnitude. Our experimental results suggest that LBP interacts with LPS and CD14 to form a stable trimolecular complex that has significant functional implications as it allows monocytes to detect the presence of LPS at a concentration as low as 10 pg/ml or 2 pM, and to respond by secreting interleukin-6. Thus, LBP is not merely transferring LPS to CD14 but it forms an integral part of the LPS-rLBP-rsCD14 complex.

Heat shock proteins 70 and 60 share common receptors which are expressed on human monocyte-derived but not epidermal dendritic cells

Priming of CTL by means of heat shock proteins (hsp) is dependent on antigen-presenting cells (APC), which present the hsp-associated peptides, via their cell surface MHC class I molecules, toCD8(+) T cells. It has not yet been established how human (hu) hsp70 interacts with the major (hu)APC, the dendritic cells (DC). Here we show that (hu)hsp70 is specifically internalized intoCD14(-), Toll-like receptor 4(-) monocyte-derived (hu)DC by receptor-mediated endocytosis. We further demonstrate that (hu)hsp70 and (hu)hsp60 share the same receptors on (hu)monocyte-derived DC. Both molecules as well as MHC class I molecules are spontaneously internalized and reach the MHC class II-enriched compartments. Finally, freshly isolated (hu) epidermal Langerhans cells (LC), the DC of the skin, as well as CD34(+)-derived LC do not bind hsp60 or hsp70. Given the likely importance of the internalization of hsp70 by APC in the induction of the immune responses, the finding that hsp60 and hsp70 are internalized through the same receptor(s) may explain why microbial hsp60 represents a major T cell antigen. This may rationalize the use of microbial hsp60 to prime immune responses against microbes. The lack of hsp60/70 receptors on epidermal LC raises the crucial question as to whether absence of priming of the skin and mucosal immune systems by hsp-polypeptide complexes could account for some tissue-specific diseases. This work also points to a potential advantage of using monocyte-derived DC in human immunotherapeutic applications of hsp60/70.

Birbeck granules are subdomains of endosomal recycling compartment in human epidermal Langerhans cells, which form where Langerin accumulates

Birbeck granules are unusual rod-shaped structures specific to epidermal Langerhans cells, whose origin and function remain undetermined. We investigated the intracellular location and fate of Langerin, a protein implicated in Birbeck granule biogenesis, in human epidermal Langerhans cells. In the steady state, Langerin is predominantly found in the endosomal recycling compartment and in Birbeck granules. Langerin internalizes by classical receptor-mediated endocytosis and the first Birbeck granules accessible to endocytosed Langerin are those connected to recycling endosomes in the pericentriolar area, where Langerin accumulates. Drug-induced inhibition of endocytosis results in the appearance of abundant open-ended Birbeck granule-like structures appended to the plasma membrane, whereas inhibition of recycling induces Birbeck granules to merge with a tubular endosomal network. In mature Langerhans cells, Langerin traffic is abolished and the loss of internal Langerin is associated with a concomitant depletion of Birbeck granules. Our results demonstrate an exchange of Langerin between early endosomal compartments and the plasma membrane, with dynamic retention in the endosomal recycling compartment. They show that Birbeck granules are not endocytotic structures, rather they are subdomains of the endosomal recycling compartment that form where Langerin accumulates. Finally, our results implicate ADP-ribosylation factor proteins in Langerin trafficking and the exchange between Birbeck granules and other endosomal membranes.

Gene induction during differentiation of human monocytes into dendritic cells: an integrated study at the RNA and protein levels

Changes in gene expression occurring during differentiation of human monocytes into dendritic cells were studied at the RNA and protein levels. These studies showed the induction of several gene classes corresponding to various biological functions. These functions encompass antigen processing and presentation, cytoskeleton, cell signalling and signal transduction, but also an increase in mitochondrial function and in the protein synthesis machinery, including some, but not all, chaperones. These changes put in perspective the events occurring during this differentiation process. On a more technical point, it appears that the studies carried out at the RNA and protein levels are highly complementary.

CD1a molecules traffic through the early recycling endosomal pathway in human Langerhans cells

In this work, we studied the localization and traffic of CD1a molecules in human epidermal Langerhans cells and the ability of these cells to stimulate CD1a-restricted T cell clones. We found that CD1a was spontaneously internalized into freshly isolated Langerhans cells, where it was rapidly distributed to the early/sorting endosomes and then to the early/recycling endosomes. In the latter compartments, CD1a colocalized with Rab11, a small GTPase known to be involved in the recycling of transmembrane proteins from early endosomes to the cell surface. In the steady state, intracellular CD1a was mainly located in Rab11+ recycling endosomal compartments. When endocytosis was blocked, intracellular CD1a moved rapidly from the early/recycling endosomes to the cell surface where it accumulated. The resultant increase in the cell surface expression of CD1a enhanced the capacity of Langerhans cells to stimulate a CD1a-restricted T cell clone. These findings are consistent with a dynamic exchange of CD1a between recycling compartments and the plasma membrane and suggest that the antigen-presenting function of CD1a depends on its traffic through the early/recycling endosomal pathway.

Modulation of MHC class II transport and lysosome distribution by macrophage-colony stimulating factor in human dendritic cells derived from monocytes

The macrophage-colony stimulating factor (M-CSF) has been already shown to affect the function of dendritic cells (DC). Therefore, the differentiation of dendritic cells into macrophages (M(PHI)) might represent a pathway which could inhibit the immune response initiated by DC. Because Major Histocompatibility Complex class II molecules (MHC-II) are crucial for DC function, we asked whether M-CSF may influence the intracellular transport of MHC-II in monocyte derived DC. We found that, at early stages, M-CSF induced first a rapid redistribution of MHC-II from the MHC-II containing compartments (MIIC) to the plasma membrane and second an increase in MHC-II synthesis as observed with LPS or TNF-(alpha). These processes were associated with the sorting of MHC-II from lysosomal membranes which underwent a drastic structural reorganization. However, in contrast to tumor necrosis factor (TNF)-(alpha) or lipopolysaccharide (LPS), M-CSF neither potentiated the allostimulatory function of DC nor allowed the stabilization of MHC-II at the cell surface, but rather increased MHC-II turnover. We conclude that the rapid modulation of MHC-II transport and distribution may participate in the inhibitory effect of M-CSF on DC function and differentiation.

Measles virus induces abnormal differentiation of CD40 ligand-activated human dendritic cells

Measles virus (MV) infection induces a profound immunosuppression responsible for a high rate of mortality in malnourished children. MV can encounter human dendritic cells (DCs) in the respiratory mucosa or in the secondary lymphoid organs. The purpose of this study was to investigate the consequences of DC infection by MV, particularly concerning their maturation and their ability to generate CD8+ T cell proliferation. We first show that MV-infected Langerhans cells or monocyte-derived DCs undergo a maturation process similarly to the one induced by TNF-alpha or LPS, respectively. CD40 ligand (CD40L) expressed on activated T cells is shown to induce terminal differentiation of DCs into mature effector DCs. In contrast, the CD40L-dependent maturation of DCs is inhibited by MV infection, as demonstrated by CD25, CD69, CD71, CD40, CD80, CD86, and CD83 expression down-regulation. Moreover, the CD40L-induced cytokine pattern in DCs is modified by MV infection with inhibition of IL-12 and IL-1alpha/beta and induction of IL-10 mRNAs synthesis. Using peripheral blood lymphocytes from CD40L-deficient patients, we demonstrate that MV infection of DCs prevents the CD40L-dependent CD8+ T cell proliferation. In such DC-PBL cocultures, inhibition of CD80 and CD86 expression on DCs was shown to require both MV replication and CD40 triggering. Finally, for the first time, MV was shown to inhibit tyrosine-phosphorylation level induced by CD40 activation in DCs. Our data demonstrate that MV replication modifies CD40 signaling in DCs, thus leading to impaired maturation. This phenomenon could play a pivotal role in MV-induced immunosuppression.

Functions of Fc receptors on human dendritic Langerhans cells

Immature dendritic cells are antigen presenting cells highly specialized for capturing and processing foreign protein antigens. These cells express Fc gamma RII and Fc epsilon RI which, by their ability to internalize and use the endocytic pathway, increase their capacity to process antigens. Immature dendritic cells, such as epidermal Langerhans cells, also release soluble forms of Fc gamma RII. These latter molecules are likely to compete with the membrane-associated Fc gamma R to diminish or abrogate the capacity of dendritic cells to present immune complexes, as suggested by our in vitro experiments using both human and mouse epidermal Langerhans cells. However, when dendritic cells mature in vitro and become efficient stimulators of resting T cells, they rapidly down-regulate and sometimes completely abolish the expression of their membrane-associated Fc gamma R and Fc epsilon RI. Consequently, they lose or at least strongly diminish their capacity to capture immune complexes. At this stage, the release of soluble Fc gamma R by dendritic cells is also markedly diminished. One can hypothesize that the membrane-associated Fc gamma RII and the soluble Fc gamma RII are molecules expressed when dendritic cells are potent capturing and processing cells, the soluble Fc gamma RII molecule acting by competition as a negative regulatory element on the Fc gamma RII-mediated internalization of IgG-containing immune complexes. Thus, the expression of membrane-associated Fc gamma R and Fc epsilon RI, as well as the release of soluble Fc gamma R, would seem to characterize the immature stage of dendritic cells.

An intronic mutation responsible for a low level of expression of an HLA-A*24 allele

HLA class I typing performed in parallel by molecular biology and serology has revealed cases where an HLA class I allele was identified but the corresponding antigen on the cell surface was not detected. In the present report, we describe three members of a family in whom an HLA-A24 allele identified at the molecular level was typed as A "blank" by lymphocytotoxicity. This serologically blank antigen was nevertheless faintly detectable by isoelectric focusing (IEF) and FACS analyses. Sequencing of the HLA-A*24 allele from the promoter region to the eighth exonic region revealed a point mutation in the acceptor site of the second intron as compared to the normal HLA-A*24 allele. This mutation could lead to incorrect processing of mRNA through a cryptic acceptor site located at the beginning of the third exon and hence to alternative splicing with a frame shift introducing an early stop codon into the fourth exon.

Human peptide transporter deficiency: importance of HLA-B in the presentation of TAP-independent EBV antigens

Two siblings with a peptide TAP deficiency were recently described. Despite poor cell surface expression of HLA class I molecules, these patients were not unusually susceptible to viral infections. The majority of the cell surface-expressed class I molecules were HLA-B products as assessed by cytofluorometry and biochemical analysis. Analysis of two peptides eluted from the class I molecules expressed by TAP-deficient EBV B lymphoblastoid cell lines indicated that both were derived from cytosolic proteins and presented by HLA-B molecules. Peripheral alphabeta CD8+ T cells were present and their TCR repertoire was polyclonal. Most of the alphabeta CD8+ T cell clones studied (21 of 22) were nonreactive against cells expressing normal levels of the same HLA alleles as those of the TAP-deficient patients. However, it was possible to isolate one cytotoxic CD8+ alphabeta T cell clone recognizing the EBV protein LMP2 presented by HLA-B molecules on TAP-deficient cells. These observations suggest that in the TAP-deficient patients, CD8+ alphabeta T cells could mature and be recruited in immune responses to mediate HLA class I-restricted cytotoxic defense against viral infections. They also strengthen the physiologic importance of a TAP-independent processing pathway of the LMP2 protein, which was previously shown to contain several other TAP-independent epitopes.

Human peptide transporter deficiency: importance of HLA-B in the presentation of TAP-independent EBV antigens

Two siblings with a peptide TAP deficiency were recently described. Despite poor cell surface expression of HLA class I molecules, these patients were not unusually susceptible to viral infections. The majority of the cell surface-expressed class I molecules were HLA-B products as assessed by cytofluorometry and biochemical analysis. Analysis of two peptides eluted from the class I molecules expressed by TAP-deficient EBV B lymphoblastoid cell lines indicated that both were derived from cytosolic proteins and presented by HLA-B molecules. Peripheral alphabeta CD8+ T cells were present and their TCR repertoire was polyclonal. Most of the alphabeta CD8+ T cell clones studied (21 of 22) were nonreactive against cells expressing normal levels of the same HLA alleles as those of the TAP-deficient patients. However, it was possible to isolate one cytotoxic CD8+ alphabeta T cell clone recognizing the EBV protein LMP2 presented by HLA-B molecules on TAP-deficient cells. These observations suggest that in the TAP-deficient patients, CD8+ alphabeta T cells could mature and be recruited in immune responses to mediate HLA class I-restricted cytotoxic defense against viral infections. They also strengthen the physiologic importance of a TAP-independent processing pathway of the LMP2 protein, which was previously shown to contain several other TAP-independent epitopes.

Soluble CD16/Fc gamma RIII induces maturation of dendritic cells and production of several cytokines including IL-12

Fc gamma RIII (CD16), a low affinity FcR which binds IgG-containing immune-complexes, exists under membrane-associated forms and under a soluble form (sFc gamma RIII). The latter, present in biological fluids (serum, saliva), is generated by proteolytic cleavage of the two membrane-associated Fc gamma RIII isoforms, Fc gamma RIII-A (expressed by macrophages and NK cells) and Fc gamma RIII-B (expressed exclusively by neutrophils). Herein we demonstrate that dendritic cells (DCs), generated by culturing monocytes with GM-CSF and IL-4, bind biotinylated recombinant sFc gamma RIII. This binding is specific and involves the complement receptor CR3 (CD11b/CD18) and CR4 (CD11c/CD18). Indeed, preincubation of DCs with anti-CD11b and anti-CD11c mAbs decreased by 52% and 62% respectively the binding with sFc gamma RIII. Moreover, electron microscopy showed that binding of gold-labeled sFc gamma RIII to DCs maintained at 4 degrees C occurred within clathrin-coated pits. Once internalized, at 37 degrees C, sFc gamma RIII entered the endocytic pathway and reached the MHC class II compartments. Furthermore, DCs incubated for 48 h with multivalent sFc gamma RIII expressed increased levels of CD40, CD80, CD86, CD54, CD58, HLA class I and class II molecules and decreased levels of CD23 and CD32. These effects result in an increased capacity of DCs to trigger proliferative responses by CD4+ CD45RA+ allogeneic T cells. RT-PCR amplification demonstrated that incubation of DCs for 20 h in the presence of multivalent sFc gamma RIII induced the appearance of GM-CSF and IL-12 p40 mRNA. Among the cytokines constitutively expressed, IL-1 beta and IL-8 were strongly up-regulated whereas IL-6 and IL-12 p35 mRNA were increased to a lesser extent and the expression of MIP-1 alpha mRNA remained constant. Finally, ELISA tests demonstrated that DCs incubated with multivalent sFc gamma RIII secreted the cytokines IL-1 beta, IL-6, IL-8, GM-CSF and IL-12 p75. Thus, while becoming internalized sFc gamma RIII could affect the capacity of DCs to present antigens and, via the induction of accessory molecules and the release of the IL-12 p75 protein, could initiate Th1 type immune response.

CD1 expression is not affected by human peptide transporter deficiency

Conventional major histocompatibility complex class I molecules are highly polymorphic and present peptides to cytotoxic T cells. These peptides derive from the proteolytic degradation of endogenous proteins in the cytosol and are translocated into the endoplasmic reticulum by a peptide transporter consisting of two transporter associated with antigen processing (TAP) molecules. Absence of this transporter leads to the synthesis of unstable peptide free class I molecules that are weakly expressed on the cell surface. Mouse nonconventional class I molecules (class Ib) may also present TAP-dependent peptides. In humans, CD1 antigens are nonconventional class I molecules. Recently, we characterized a human HLA class I deficiency resulting from a homozygous TAP deficiency. We show here that CD1a and -c are normally expressed on epidermal Langerhans cells of the TAP-deficient patients, as are CD1a, -b, and -c on dendritic cells differentiated in vitro from monocytes. Moreover, the CD1a antigens present on the surface of the dendritic cells are functional, since they internalize by receptor-mediated endocytosis gold-labeled F(ab')2 fragments of an anti-CD1a mAb. This suggests either that CD1 molecules are empty molecules, that they are more stable than empty conventional class I proteins, or that CD1 molecules present TAP-independent peptides.

Homozygous human TAP peptide transporter mutation in HLA class I deficiency

Human lymphocyte antigen (HLA) class I proteins of the major histocompatibility complex are largely dependent for expression on small peptides supplied to them by transporter associated with antigen processing (TAP) protein. An inherited human deficiency in the TAP transporter was identified in two siblings suffering from recurrent respiratory bacterial infections. The expression on the cell surface of class I proteins was very low, whereas that of CD1a was normal, and the cytotoxicity of natural killer cells was affected. In addition, CD8+ alpha beta T cells were present in low but significant numbers and were cytotoxic in the most severely affected sibling, who also showed an increase in CD4+CD8+ T cells and gamma delta T cells.