Stability of surface H-2K(b), H-2D(b), and peptide-receptive H-2K(b) on splenocytes

CXCL14 suppresses human papillomavirus-associated head and neck cancer through antigen-specific CD8+ T-cell responses by upregulating MHC-I expression

Evasion of the host immune responses is critical for both persistent human papillomavirus (HPV) infection and associated cancer progression. We have previously shown that expression of the homeostatic chemokine CXCL14 is significantly downregulated by the HPV oncoprotein E7 during cancer progression. Restoration of CXCL14 expression in HPV-positive head and neck cancer (HNC) cells dramatically suppresses tumor growth and increases survival through an immune-dependent mechanism in mice. While CXCL14 recruits natural killer (NK) and T cells to the tumor microenvironment, the mechanism by which CXCL14 mediates tumor suppression through NK and/or T cells remained undefined. Here, we report that CD8⁺ T cells are required for CXCL14-mediated tumor suppression. Using a CD8⁺ T cell receptor transgenic model, we show that the CXCL14-mediated antitumor CD8⁺ T cell responses require antigen specificity. Interestingly, CXCL14 expression restores major histocompatibility complex class I (MHC-I) expression on HPV-positive HNC cells downregulated by HPV, and knockdown of MHC-I expression in HNC cells results in loss of tumor suppression even with CXCL14 expression. These results suggest that CXCL14 enacts antitumor immunity through restoration of MHC-I expression on tumor cells and promoting antigen-specific CD8⁺ T cell responses to suppress HPV-positive HNC.

Endocytic Recycling of MHC Class I Molecules in Non-professional Antigen Presenting and Dendritic Cells

Major histocompatibility complex class I (MHC I) molecules are glycoproteins that display peptide epitopes at the cell surface of nucleated cells for recognition by CD8⁺ T cells. Like other cell surface receptors, MHC class I molecules are continuously removed from the surface followed by intracellular degradation or recycling to the cell surface, in a process likely involving active quality control the mechanism of which remains unknown. The molecular players and pathways involved in internalization and recycling have previously been studied in model cell lines such as HeLa. However, dendritic cells (DCs), which rely on a specialized endocytic machinery that confers them the unique ability to “cross”-present antigens acquired by internalization, may use distinct MHC I recycling pathways and quality control mechanisms. By providing MHC I molecules cross-presenting antigens, these pathways may play an important role in one of the key functions of DCs, priming of T cell responses against pathogens and tumors. In this review, we will focus on endocytic recycling of MHC I molecules in various experimental conditions and cell types. We discuss the organization of the recycling pathway in model cell lines compared to DCs, highlighting the differences in the recycling rates and pathways of MHC I molecules between various cell types, and their putative functional consequences. Reviewing the literature, we find that conclusive evidence for significant recycling of MHC I molecules in primary DCs has yet to be demonstrated. We conclude that endocytic trafficking of MHC class I in DCs remains poorly understood and should be further studied because of its likely role in antigen cross-presentation.

Analysis of the CD8+ T cell anti-HIV activity in heterologous cell co-cultures reveals the benefit of multiple HLA class I matches

CD8⁺ T lymphocytes can reduce the production of human immunodeficiency virus 1 (HIV-1) by CD4⁺ T cells by cytotoxic and non-cytotoxic mechanisms. To investigate the involvement of human leukocyte antigen (HLA) class I compatibility in anti-HIV responses, we co-cultured primary CD8⁺ T cells, isolated from the peripheral blood of HIV-1-infected individuals, with panels of autologous and heterologous acutely HIV-1-infected primary CD4⁺ T cells. Altogether, CD8⁺ T cell anti-HIV activity was evaluated in more than 200 co-cultures. Marked heterogeneity in HIV-1 replication levels was observed among the co-cultures sharing a common CD8⁺ T cell source. The co-cultures that exhibited greater than 50% reduction in HIV production were found to have significantly increased numbers of matching HLA class I alleles (Yates chi-square = 54.21; p < 0.001). With CD8⁺ T cells from HIV controllers and asymptomatic viremic individuals, matching HLA-B and/or HLA-C alleles were more predictive of strong anti-HIV activity than matching HLA-A alleles. Overall, HLA class I genotype matches were more closely associated with CD8⁺ T cell anti-HIV activity than supertype pairings. Antibodies against HLA class I and CD3 reduced the CD8⁺ T cell anti-HIV activity. Stimulated CD8⁺ T cells exhibited increased anti-HIV activity and reduced dependency on HLA compatibility. These findings provide evidence that the maximal suppression of HIV replication by CD8⁺ T cells requires the recognition of multiple epitopes. These studies provide insight for HIV vaccine development, and the analytic approach can be useful for the functional characterization of HLA class I alleles and tentative HLA class I supertypes.

Intracellular recycling and cross-presentation by MHC class I molecules

Cross-presentation of internalized antigens by dendritic cells requires efficient delivery of Major Histocompatibility Complex (MHC) class I molecules to peptide-loading compartments. Strong evidence suggests that such loading can occur outside of the endoplasmic reticulum; however, the trafficking pathways and sources of class I molecules involved are poorly understood. Examination of non-professional, non-phagocytic cells has revealed a clathrin-independent, Arf6-dependent recycling pathway likely traveled by internalized optimally loaded (closed) class I molecules. Some closed and all open MHC class I molecules travel to late endosomes to be degraded but might also partly be re-loaded with peptides and recycled. Studies of viral interference revealed pathways in which class I molecules are directed to degradation in lysosomes upon ubiquitination at the surface, or upon AP-1 and HIV-nef-dependent misrouting from the Golgi network to lysosomes. While many observations made in non-professional cells remain to be re-examined in dendritic cells, available evidence suggests that both recycling and neo-synthesized class I molecules can be loaded with cross-presented peptides. Recycling molecules can be recruited to phagosomes triggered by innate signals such as TLR4 ligands, and may therefore specialize in loading with phagocytosed antigens. In contrast, AP-1-dependent accumulation at, or trafficking through, a Golgi compartment of newly synthesized molecules appears to be important for cross-presentation of soluble proteins and possibly of long peptides that are processed in the so-called vacuolar pathway. However, significant cell biological work will be required to confirm this or any other model and to integrate knowledge on MHC class I biochemistry and trafficking in models of CD8(+) T-cell priming by dendritic cells.

The Optimization of Bioorthogonal Epitope Ligation within MHC-I Complexes

Antigen recognition followed by the activation of cytotoxic T-cells (CTLs) is a key step in adaptive immunity, resulting in clearance of viruses and cancers. The repertoire of peptides that have the ability to bind to the major histocompatibility type-I (MHC-I) is enormous, but the approaches available for studying the diversity of the peptide repertoire on a cell are limited. Here, we explore the use of bioorthogonal chemistry to quantify specific peptide-MHC-I complexes (pMHC-I) on cells. We show that modifying epitope peptides with bioorthogonal groups in surface accessible positions allows wild-type-like MHC-I binding and bioorthogonal ligation using fluorogenic chromophores in combination with a Cu(I)-catalyzed Huisgen cycloaddition reaction. We expect that this approach will make a powerful addition to the antigen presentation toolkit as for the first time it allows quantification of antigenic peptides for which no detection tools exist.

Leishmania spp. Proteome Data Sets: A Comprehensive Resource for Vaccine Development to Target Visceral Leishmaniasis

Visceral leishmaniasis is a neglected infectious disease caused primarily by Leishmania donovani and Leishmania infantum protozoan parasites. A significant number of infections take a fatal course. Drug therapy is available but still costly and parasites resistant to first line drugs are observed. Despite many years of trial no commercial vaccine is available to date. However, development of a cost effective, needle-independent vaccine remains a high priority. Reverse vaccinology has attracted much attention since the term has been coined and the approach tested by Rappuoli and colleagues. This in silico selection of antigens from genomic and proteomic data sets was also adapted to aim at developing an anti-Leishmania vaccine. Here, an analysis of the efforts is attempted and the challenges to be overcome by these endeavors are discussed. Strategies that led to successful identification of antigens will be illustrated. Furthermore, these efforts are viewed in the context of anticipated modes of action of effective anti-Leishmania immune responses to highlight possible advantages and shortcomings.

Seminal plasma peptides may determine maternal immune response that alters success or failure of pregnancy in the abortion-prone CBAxDBA/2 model

Spontaneous abortion (resorption) in the DBA/2-mated CBA/J mouse involves a deficiency in Treg cell activity against paternal antigens at the time of mating. Preimmunization of female CBA/J by BALB/c splenocytes, but not DBA/2 splenocytes, protects against subsequent abortions after a CBAxDBA/2 mating. Previous immunogenetic studies with BALB/cxDBA/2 recombinants have indicated that H-2(d)-restricted presentation of a single minor non-H-2(d) peptide might be responsible for protection, while the product of a second independent allele might promote abortions. Using brefeldin-treated BALB/c and DBA/2 splenocytes, we found that incubation in BALB/c seminal plasma rendered DBA/2 splenocytes protective and DBA/2 seminal plasma eliminated protection. The active protective moiety was <10 kD consistent with a peptide. DBA/2 seminal plasma contained a <10-kD peptide that boosted the abortion rate. Maternal H-2(k) CBA/J splenocytes were unable to present the protective activity. Amicon fractionation also unmasked a <10-kD activity in DBA/2 seminal plasma that could boost abortion rates when presented by BALB/c splenocytes. SELDI-TOF mass spectrometry proteomic analysis of <10-kD filtrates reproducibly detected 1416, 1468, 1774 D peptides in BALB/c that were reduced or absent in DBA/2, and the presence of 2662, 4559 and 5320 D molecules in DBA/2, the latter two definitely not present in BALB/c. Direct antigen presentation of paternal H-2(d)-restricted paternal peptides (600-1800 D) may prevent the rejection of the CBAxDBA/2 embryos, and larger sized peptides may bind to immunizing splenocytes and augment abortion mechanisms.

Promiscuous binding of extracellular peptides to cell surface class I MHC protein

Algorithms derived from measurements of short-peptide (8-10 mers) binding to class I MHC proteins suggest that the binding groove of a class I MHC protein, such as K(b), can bind well over 1 million different peptides with significant affinity (<500 nM), a level of ligand-binding promiscuity approaching the level of heat shock protein binding of unfolded proteins. MHC proteins can, nevertheless, discriminate between similar peptides and bind many of them with high (nanomolar) affinity. Some insights into this high-promiscuity/high-affinity behavior and its impact on immunodominant peptides in T-cell responses to some infections and vaccination are suggested by results obtained here from testing a model developed to predict the number of cell surface peptide-MHC complexes that form on cells exposed to extracellular (exogenous) peptides.

Cutting edge: dendritic epidermal γδ T cell ligands are rapidly and locally expressed by keratinocytes following cutaneous wounding

TCR-specific activation is pivotal to dendritic epidermal T cell (DETC) function during cutaneous wound repair. However, DETC TCR ligands are uncharacterized, and little is known about their expression patterns and kinetics. Using soluble DETC TCR tetramers, we demonstrate that DETC TCR ligands are not constitutively expressed in healthy tissue but are rapidly upregulated following wounding on keratinocytes bordering wound edges. Ligand expression is tightly regulated, with downmodulation following DETC activation. Early inhibition of TCR-ligand interactions using DETC TCR tetramers delays wound repair in vivo, highlighting DETC as rapid responders to injury. To our knowledge, this is the first visualization of DETC TCR ligand expression, which provides novel information about how ligand expression impacts early stages of DETC activation and wound repair.

HIV-1 Nef binds a subpopulation of MHC-I throughout its trafficking itinerary and down-regulates MHC-I by perturbing both anterograde and retrograde trafficking

The HIV protein Nef is thought to mediate immune evasion and promote viral persistence in part by down-regulating major histocompatibility complex class I protein (MHC-I or HLA-I) from the cell surface. Two different models have been proposed to explain this phenomenon as follows: 1) stimulation of MHC-I retrograde trafficking from and aberrant recycling to the plasma membrane, and 2) inhibition of anterograde trafficking of newly synthesized HLA-I from the endoplasmic reticulum to the plasma membrane. We show here that Nef simultaneously uses both mechanisms to down-regulate HLA-I in peripheral blood mononuclear cells or HeLa cells. Consistent with this, we found by using fluorescence correlation spectroscopy that a third of diffusing HLA-I at the endoplasmic reticulum, Golgi/trans-Golgi network, and the plasma membrane (PM) was associated with Nef. The binding of Nef was similarly avid for native HLA-I and recombinant HLA-I A2 at the PM. Nef binding to HLA-I at the PM was sensitive to specific inhibition of endocytosis. It was also attenuated by cyclodextrin disruption of PM lipid micro-domain architecture, a change that also retarded lateral diffusion and induced large clusters of HLA-I. In all, our data support a model for Nef down-regulation of HLA-I that involves both major trafficking itineraries and persistent protein-protein interactions throughout the cell.

MHC I stabilizing potential of computer-designed octapeptides

Experimental results are presented for 180 in silico designed octapeptide sequences and their stabilizing effects on the major histocompatibility class I molecule H-2K^b. Peptide sequence design was accomplished by a combination of an ant colony optimization algorithm with artificial neural network classifiers. Experimental tests yielded nine H-2K^b stabilizing and 171 nonstabilizing peptides. 28 among the nonstabilizing octapeptides contain canonical motif residues known to be favorable for MHC I stabilization. For characterization of the area covered by stabilizing and non-stabilizing octapeptides in sequence space, we visualized the distribution of 100,603 octapeptides using a self-organizing map. The experimental results present evidence that the canonical sequence motives of the SYFPEITHI database on their own are insufficient for predicting MHC I protein stabilization.

Generation of in silico predicted coxsackievirus B3-derived MHC class I epitopes by proteasomes

Proteasomes are known to be the main suppliers of MHC class I (MHC-I) ligands. In an attempt to identify coxsackievirus B3 (CVB3)-MHC-I epitopes, a combined approach of in silico MHC-I/transporters associated with antigen processing (TAP)-binding and proteasomal cleavage prediction was applied. Accordingly, 13 potential epitopes originating from the structural and non-structural protein region of CVB3 were selected for further in vitro processing analysis by proteasomes. Mass spectrometry demonstrated the generation of seven of the 13 predicted MHC-I ligands or respective ligand precursors by proteasomes. Detailed processing analysis of three adjacent MHC-I ligands with partially overlapping sequences, i.e. VP2(273-281), VP2(284-292) and VP2(285-293), revealed the preferential generation predominantly of the VP2(285-293) epitope by immunoproteasomes due to altered cleavage site preferences. The VP2(285-293) peptide was identified to be a high affinity binder, rendering VP2(285-293) a likely candidate for CD8 T cell immunity in CVB3 infection. In conclusion, the concerted usage of different in silico prediction methods and in vitro epitope processing/presentation studies was supportive in the identification of CVB3 MHC-I epitopes.

Feline herpesvirus-1 down-regulates MHC class I expression in an homologous cell system

Cytotoxic T lymphocytes (CTLs) are an essential component of the immune defense against many virus infections. CTLs recognize viral peptides in the context of the major histocompatibility complex (MHC) class I molecules on the surface of infected cells. Many viruses have evolved mechanisms to interfere with MHC class I expression as a means of evading the host immune response. In the present research we have studied the effect of in vitro Feline Herpesvirus 1 (FeHV-1) infection on MHC class I expression. The results of this study demonstrate that FeHV-1 down regulates surface expression of MHC class I molecules on infected cells, presumably to evade cytotoxic T-cell recognition and, perhaps, attenuate induction of immunity. Sensitivity to UV irradiation and insensitivity to a viral DNA synthesis inhibitor, like phosphonacetic acid, revealed that immediate early or early viral gene(s) are responsible. Use of the protein translation inhibitor cycloheximide confirmed that an early gene is primarily responsible.

Plasmid DNA and IL-4 modulate expression of mHC class I and costimulatory molecules in B lymphocytes

B lymphocytes are capable of spontaneous internalization of plasmid (p)DNA, an event that set in motion the antigen-presenting function in this class of hemopoietic cells. Previously, we showed that priming of CD8 T lymphocytes by spontaneously transgenic B lymphocytes requires T-cell help, and that this can be replaced by soluble IL-4. To better understand this phenomenon we studied the relative role of pDNA and IL-4 on the expression of MHC-I and a panel of critical costimulatory molecules (CD40, CD80, CD86, OX40L, and LAG-3). Whereas upregulation of MHC-I is contributed by pDNA, IL-4 mainly upregulates CD86 and to a lesser degree CD40. The two effects appear to be independent. In addition, however, it was found that IL-4 stabilizes MHC-I transcription in lymphocytes after spontaneous transgenesis with pDNA. These results further our understanding of events that take place in specialized mammalian cells after exposure to pDNA. They also point to the fact after pDNA internalization that the antigen-presenting function of B lymphocytes can be complemented by IL-4, a cytokine normally produced by activated CD4 T cells.

Bap31 enhances the endoplasmic reticulum export and quality control of human class I MHC molecules

The assembly of class I MHC molecules and their export from the endoplasmic reticulum (ER) is governed by chaperones and accessory proteins. We present evidence that the putative cargo receptor protein Bap31 participates in the transport and the quality control of human class I molecules. Transfection of the human adenocarcinoma cell line HeLa with yellow fluorescent protein-Bap31 chimeras increased surface levels of class I in a dose-dependent manner, by as much as 3.7-fold. The increase in surface class I resulted from an increase in the rate of export of newly synthesized class I molecules to the cell surface and from an increase in the stability of the exported molecules. We propose that Bap31 performs quality control on class I molecules in two distinct phases: first, by exporting peptide-loaded class I molecules to the ER/Golgi intermediate compartment, and second, by retrieving class I molecules that have lost peptides in the acidic post-ER environment. This function of Bap31 is conditional or redundant, because we find that Bap31 deficiency does not reduce surface class I levels. Overexpression of the Bap31 homolog, Bap29, decreases surface class levels in HeLa, indicating that it does not substitute for Bap31.

CD4+ T Cells Can Protect APC from CTL-Mediated Elimination

Professional APC play a central role in generating antiviral CD8(+) CTL immunity. However, the fate of such APC following interaction with these same CTL remains poorly understood. We have shown previously that prolonged Ag presentation persists in the presence of a strong CTL response following HSV infection. In this study, we examined the mechanism of survival of APC in vivo when presenting an immunodominant determinant from HSV. We show that transferred peptide-labeled dendritic cells were eliminated from draining lymph nodes in the presence of HSV-specific CTL. Maturation of dendritic cells with LPS or anti-CD40 before injection protected against CTL lysis in vivo. Furthermore, endogenous APC could be eliminated from draining lymph nodes early after HSV infection by adoptive transfer of HSV-specific CTL, yet the cotransfer of significant virus-specific CD4(+) T cell help promoted prolonged Ag presentation. This suggests that Th cells may assist in prolonging class I-restricted Ag presentation, potentially enhancing CTL recruitment and allowing more efficient T cell priming.

Inclusion of Brefeldin A during dendritic cell isolation allows in vitro detection of cross-presented self-antigens

The mechanism of cross-presentation enables dendritic cells (DC) to induce immunity against intracellular pathogens and to tolerize autoreactive CD8 T cells. The antigen-presenting cells (APCs) responsible for cross-presentation of self-antigens have been identified as CD8alpha(+) CD11c(+) DC. Isolation of these cells has been notoriously difficult, and the resulting responses of T cell hybridomas were too low to permit further studies. Here, we demonstrate that inclusion of Brefeldin A (BfA), an agent reported to block MHC class I-peptide complex turnover on the cell surface, during DC isolation from transgenic RIP-mOVA mice facilitated activation and proliferation of naïve OVA-specific CD8(+) T cells in vitro. CD8alpha(+) DC were more efficient than CD8alpha(-) CD11c(+). BfA also reversibly preserved expression of costimulatory molecules by DC, as evidenced by their expression of costimulatory markers and by an increased stimulatory capacity of DC matured in vivo by LPS. We conclude that the use of BfA notably improves sensitivity of detection of cross-presented self-antigens.

Structural basis of the differential stability and receptor specificity of H-2Db in complex with murine versus human beta2-microglobulin

beta(2)-Microglobulin (beta(2)m) is non-covalently linked to the major histocompatibility complex (MHC) class I heavy chain and interacts with CD8 and Ly49 receptors. Murine MHC class I heavy chains can bind human beta(2)m (hbeta(2)m) and peptide, and such hybrid molecules are often used in structural and functional studies. The replacement of mouse beta(2)m (mbeta(2)m) with hbeta(2)m has several functional consequences for MHC class I complex stability and specificity, but the structural basis for this is presently unknown. To investigate the impact of species-specific beta(2)m subunits on MHC class I conformation, we provide a crystallographic comparison of H-2D(b) in complex with LCMV-derived gp33 peptide and either hbeta(2)m or mbeta(2)m. The conformation of the gp33 peptide is not affected by the beta(2)m species. Comparison of the interface between beta(2)m and the alpha(1)alpha(2) domains of the heavy chain in these two crystal structures reveals a marked increase in both polarity and number of hydrogen bonds between hbeta(2)m and the alpha(1)alpha(2) domains of H-2D(b). We propose that the positioning of two hydrogen bond rich regions at the hbeta(2)m/alpha(1)alpha(2) interface plays a central role in the increased overall stability and peptide exchange capacity in the H-2D(b)/hbeta(2)m complex. These two regions act as bridges, holding and stabilizing the underside of the alpha(1) and alpha(2) helices, enabling a prolonged peptide-receptive conformation of the peptide binding cleft. Furthermore, analysis of H-2D(b) in complex with either mbeta(2)m or hbeta(2)m provides a structural explanation for the differential binding of H-2D(b)/hbeta(2)m to both Ly49A and Ly49C. Our comparative structural study emphasizes the importance of beta(2)m residues at positions 3, 6 and 29 for binding to Ly49A and suggests that sterical hindrance by residue K6 on hbeta(2)m impairs the recognition of Ly49C by H-2D(b)/gp33/hbeta(2)m. Finally, comparison of the two H-2D(b) crystal structures implies that the beta(2)m species may affect the strength of TCR recognition by affecting CD8 binding.

Influence of xenogeneic beta2-microglobulin on functional recognition of H-2Kb by the NK cell inhibitory receptor Ly49C

NK cells maintain self-tolerance through expression of inhibitory receptors that bind MHC class I (MHC-I) molecules. MHC-I can exist on the cell surface in several different forms, including "peptide-receptive" or PR-MHC-I that can bind exogenous peptide. PR-MHC-I molecules are short lived and, for H-2K(b), comprise approximately 10% of total MHC-I. In the present study, we confirm that signaling through the mouse NK inhibitory receptor Ly49C requires the presence of PR-K(b) and that this signaling is prevented when PR-K(b) is ablated by pulsing with a peptide that can bind to it with high affinity. Although crystallographic data indicate that Ly49C can engage H-2K(b) loaded with high-affinity peptide, our data suggest that this interaction does not generate an inhibitory signal. We also show that no signaling occurs when the PR-K(b) complex has mouse beta(2)-microglobulin (beta(2)m) replaced with human beta(2)m, although replacement with bovine beta(2)m has no effect. Furthermore, we show that beta(2)m exchange occurs preferentially in the PR-K(b) component of total H-2K(b). These conclusions were reached in studies modulating the sensitivity to lysis of both NK-resistant syngeneic lymphoblasts and NK-sensitive RMA-S tumor cells. We also show, using an in vivo model of lymphocyte recirculation, that engrafted lymphocytes are unable to survive NK attack when otherwise syngeneic lymphocytes express human beta(2)m. These findings suggest a qualitative extension of the "missing self" hypothesis to include NK inhibitory receptors that are restricted to the recognition of unstable forms of MHC-I, thus enabling NK cells to respond more quickly to events that decrease MHC-I synthesis.

The nonclassical MHC class I molecule Qa-1 forms unstable peptide complexes

The MHC class Ib molecule Qa-1 is the primary ligand for mouse CD94/NKG2A inhibitory receptors expressed on NK cells, in addition to presenting Ags to a subpopulation of T cells. CD94/NKG2A receptors specifically recognize Qa-1 bound to the MHC class Ia leader sequence-derived peptide Qdm. Qdm is the dominant peptide loaded onto Qa-1 under physiological conditions and this peptide has an optimal sequence for binding to Qa-1. Peptide dissociation experiments demonstrated that Qdm dissociates from soluble or cell surface Qa-1(b) molecules with a t(1/2) of approximately 1.5 h at 37 degrees C. In comparison, complexes of an optimal peptide (SIINFEKL) bound to the MHC class Ia molecule H-2K(b) dissociated with a t(1/2) in the range from 11 to 31 h. In contrast to K(b), the stability of cell surface Qa-1(b) molecules was independent of bound peptides, and several observations suggested that empty cell surface Qa-1(b) molecules might be unusually stable. Consistent with the rapid dissociation rate of Qdm from Qa-1(b), cells become susceptible to lysis by CD94/NKG2A(+) NK cells under conditions in which new Qa-1(b)/Qdm complexes cannot be continuously generated at the cell surface. These results support the hypothesis that Qa-1 has been selected as a specialized MHC molecule that is unable to form highly stable peptide complexes. We propose that the CD94/NKG2A-Qa-1/Qdm recognition system has evolved as a rapid sensor of the integrity of the MHC class I biosynthesis and Ag presentation pathway.

Analysis of dendritic cell trafficking using EGFP-transgenic mice

Dendritic cells (DCs) are professional antigen presenting cells, well equipped to initiate an immune response. For effective induction of an immune response, DC should migrate from the periphery to the lymph node to present the antigen to T lymphocytes. Currently, tumor-antigen loaded DCs are used in clinical vaccination trials in cancer patients. To investigate the migratory capacity of DC in vivo, a variety of fluorescent and radioactive labels have been used. Here we introduce a novel tool to study DC migration in vivo: DCs generated from enhanced green fluorescent protein (EGFP)-transgenic mice. DC from EGFP-transgenic mice display typical DC behavior and can be matured without affecting their autofluorescence in vitro. In addition, the continuously produced cytoplasmic EGFP in living cells functions as a viability marker, since EGFP released from dying cells does not stain DC from C57Bl/6 mice upon coculture. In vivo migration studies using EGFP-DC and indium-111-labeled DC were performed to determine the efficiency of i.d. versus s.c. administered DC to reach the draining lymph node. The analysis demonstrates that i.d. injection increases the amount of EGFP-DC/indium-111-labeled DC in the lymph node compared to s.c. injection. Subsequent quantitative, phenotypical and ultrastuctural analysis demonstrate that DC generated from EGFP-transgenic mice are well suited to study the migratory behavior, distribution and phenotype of DC in vivo.

Pathways accessory to proteasomal proteolysis are less efficient in major histocompatibility complex class I antigen production

Degradation of cytosolic proteins depends largely on the proteasome, and a fraction of the cleavage products are presented as major histocompatibility complex (MHC) class I-bound ligands at the cell surface of antigen presenting cells. Proteolytic pathways accessory to the proteasome contribute to protein turnover, and their up-regulation may complement the proteasome when proteasomal proteolysis is impaired. Here we show that reduced reliance on proteasomal proteolysis allowed a reduced efficiency of MHC class I ligand production, whereas protein turnover and cellular proliferation were maintained. Using the proteasomal inhibitor adamantane-acetyl-(6-aminohexanoyl)3-(leucinyl)3-vinyl-(methyl)-sulphone, we show that covalent inhibition of all three types of proteasomal beta-subunits (beta(1), beta(2), and beta(5)) was compatible with continued growth in cells that up-regulate accessory proteolytic pathways, which include cytosolic proteases as well as deubiquitinating enzymes. However, under these conditions, we observed poor assembly of H-2D(b) molecules and inhibited presentation of endogenous tumor antigens. Thus, the tight link between protein turnover and production of MHC class I ligands can be broken by enforcing the substitution of the proteasome with alternative proteolytic pathways.

Down-regulation of MHC class I expression by equine herpesvirus-1

There is good evidence that cytotoxic T lymphocytes play an important role in the clearance of equine herpesvirus-1 (EHV1) in horses. We have demonstrated that, in common with other alphaherpesviruses, EHV1 infection can lead to dramatic down-regulation of MHC class I expression at the cell surface, a common strategy for pathogen evasion of the host immune response. This down-regulation is specific for MHC class I and does not reflect a general shut-off of host-cell protein synthesis. The use of monoclonal antibodies that recognize different MHC class I epitopes has demonstrated that the effect may be allele- or locus-specific. Use of the viral DNA synthesis inhibitor phosphonoacetic acid, which prevents late viral gene expression, showed that the effect is mediated by an immediate-early or early viral gene, and use of the protein translation inhibitor cycloheximide confirmed that an early gene is primarily responsible. The data indicate that EHV1 infection results in enhanced endocytosis of MHC class I from the cell surface; the only other herpesvirus reported to use this mechanism is human herpesvirus-8. Elucidation of the precise mechanisms used by EHV1 in this process and identification of the genes responsible may lead to improved vaccination strategies.

Macrophages and dendritic cells use the cytosolic pathway to rapidly cross-present antigen from live, vaccinia-infected cells

Professional APCs (pAPC) can process and present on their own MHC class I molecules Ags acquired from Ag donor cells (ADC). This phenomenon of cross-presentation is essential in the induction of CD8(+) T cell responses to viruses that do not infect pAPC and possibly contributes to the induction of CD8(+) responses to many other viruses. However, little is known about the mechanisms underlying this process. In this study, we show that dendritic cells and macrophages cross-present a model Ag supplied by vaccinia virus-infected ADC via the cytosolic route. Strikingly, we also found that cross-presentation of Ags provided by vaccinia-infected cells occurs within a couple of hours of pAPC/ADC interaction, that the duration of cross-presentation lasts for only 16 h, and that cross-presentation can occur at early times of infection when the ADC are still alive.