Association of the human invariant chain with H-2 Db class I molecules

Development of a Molecular Adjuvant to Enhance Antigen-Specific CD8+ T Cell Responses

Despite promising progress in malaria vaccine development, an efficacious subunit vaccine against P. falciparum remains to be licensed and deployed. This study aimed to improve on the immunogenicity of the leading liver-stage vaccine candidate (ChAd63-MVA ME-TRAP), known to confer protection by eliciting high levels of antigen-specific CD8⁺ T cells. We previously showed fusion of ME-TRAP to the human MHC class II invariant chain (Ii) could enhance CD8⁺ T cell responses in non-human primates, but did not progress to clinical testing due to potential risk of auto-immunity by vaccination of humans with a self-antigen. Initial immunogenicity analyses of ME-TRAP fused to subdomains of the Ii showed that the Ii transmembrane domain alone can enhance CD8⁺ T cell responses. Subsequently, truncated Ii sequences with low homology to human Ii were developed and shown to enhance CD8⁺ T cell responses. By systematically mutating the TM domain sequence, multimerization of the Ii chain was shown to be important for immune enhancement. We subsequently identified several proteins from a variety of microbial pathogens with similar characteristics, that also enhance the CD8⁺ T cell response and could therefore be used in viral vector vaccines when potent cell mediated immunity is required.

Unusual antigen presentation offers new insight into HIV vaccine design

Recent findings with a rhesus monkey cytomegalovirus based simian immunodeficiency virus vaccine have identified strong CD8+ T cell responses that are restricted by MHC-E. Also mycobacteria specific CD8+ T cells, that are MHC-E restricted, have been identified. MHC-E therefore can present a wide range of epitope peptides to CD8+ T cells, alongside its well defined role in presenting a conserved MHC-class I signal peptide to the NKG2A/C-CD94 receptor on natural killer cells. Here we explore the antigen processing pathways involved in these atypical T cell responses.

Endosomal compartment: Also a dock for MHC class I peptide loading

The endosomal compartment, which contains all the components required for loading peptides onto MHC class II molecules, is classically considered to be dedicated to the loading of MHC class II but not MHC class I molecules. However, a report in this issue of the European Journal of Immunology [Eur. J. Immunol. 2014. 44: 774-784], together with other recent studies, shows that the endosomal compartment also supports efficient loading of MHC class I molecules. These results bring a new perspective on the crosstalk between the MHC class II and MHC class I antigen-processing pathways, and may inspire new ideas for the design of vaccines against viruses and tumors.

Intracellular localization and association of MHC class I with porcine invariant chain

The objective was to investigate the intracellular localization and association of pig major histocompatibility complex (MHC) class I subunits with invariant chain (Ii). Pig MHC class I subunit cDNAs were cloned by RT-PCR and eukaryotic expression plasmids of α and β2m were constructed with fusions to red or enhanced green fluorescent protein (pDsRed2-N1-α, pEGFP-N1-α, pDsRed2-N1-β2m, and pEGFP-N1-β2m). A pig Ii mutant with a deleted CLIP region (DCLIP-Ii) was constructed by overlap extension PCR. Wild-type Ii and mutant Ii were cloned into pEGFP-C1 (pEGFP-C1-Ii, pEGFP-C1-DCLIP-Ii). The recombinant plasmids of MHC I subunits and pEGFP-C1-Ii (pEGFP-C1-ΔCLIP-Ii) were transiently cotransfected into COS-7 cells with Lipofectamine 2000. Immunofluorescence microscopy was performed to detect expression and intracellular localization of Ii and MHC I subunits, and immunoprecipitation was used to analyze their association. Our results indicated that pig Ii associates with integrated MHC I subunits to form oligomers, but cannot associate with single MHC I subunits. Furthermore, deletion of the Ii CLIP sequence blocks association with integrated MHC I subunits. Thus, pig Ii cannot associate with a single MHC I molecule, the α or β2m chain, but Ii and the integrated MHC I molecule can form complexes that colocalize in the endomembranes of COS-7 cells. The Ii of CLIP plays a key role in assembly of Ii and MHC I.

Promiscuous binding of invariant chain-derived CLIP peptide to distinct HLA-I molecules revealed in leukemic cells

Antigen presentation by HLA class I (HLA-I) and HLA class II (HLA-II) complexes is achieved by proteins that are specific for their respective processing pathway. The invariant chain (Ii)-derived peptide CLIP is required for HLA-II-mediated antigen presentation by stabilizing HLA-II molecules before antigen loading through transient and promiscuous binding to different HLA-II peptide grooves. Here, we demonstrate alternative binding of CLIP to surface HLA-I molecules on leukemic cells. In HLA-II-negative AML cells, we found plasma membrane display of the CLIP peptide. Silencing Ii in AML cells resulted in reduced HLA-I cell surface display, which indicated a direct role of CLIP in the HLA-I antigen presentation pathway. In HLA-I-specific peptide eluates from B-LCLs, five Ii-derived peptides were identified, of which two were from the CLIP region. In vitro peptide binding assays strikingly revealed that the eluted CLIP peptide RMATPLLMQALPM efficiently bound to four distinct HLA-I supertypes (-A2, -B7, -A3, -B40). Furthermore, shorter length variants of this CLIP peptide also bound to these four supertypes, although in silico algorithms only predicted binding to HLA-A2 or -B7. Immunization of HLA-A2 transgenic mice with these peptides did not induce CTL responses. Together these data show a remarkable promiscuity of CLIP for binding to a wide variety of HLA-I molecules. The found participation of CLIP in the HLA-I antigen presentation pathway could reflect an aberrant mechanism in leukemic cells, but might also lead to elucidation of novel processing pathways or immune escape mechanisms.

A CD74-dependent MHC class I endolysosomal cross-presentation pathway

Immune responses are initiated and primed by dendritic cells (DCs) that cross-present exogenous antigen. The chaperone CD74 (invariant chain) is thought to promote DC priming exclusively in the context of major histocompatibility complex (MHC) class II. However, we demonstrate here a CD74-dependent MHC class I cross-presentation pathway in DCs that had a major role in the generation of MHC class I-restricted, cytolytic T lymphocyte (CTL) responses to viral protein- and cell-associated antigens. CD74 associated with MHC class I in the endoplasmic reticulum of DCs and mediated the trafficking of MHC class I to endolysosomal compartments for loading with exogenous peptides. We conclude that CD74 has a previously undiscovered physiological function in endolysosomal DC cross-presentation for priming MHC class I-mediated CTL responses.

Effect of invariant chain on major histocompatibility complex class I molecule expression and stability on human breast tumor cell lines

Invariant chain (Ii) binds to the human leukocyte antigen (HLA) class II molecule and assists it in the process of peptide acquisition. In addition, Ii binds to the HLA class I molecule, although there has been little study of its effects on the HLA class I molecule. In addition to its normal expression on antigen-presenting cells, Ii expression is up regulated in a variety of tumors. By flow cytometric analysis, we found that expression of Ii resulted in an increase in the number of cell surface HLA class I molecules and in the proportion of unstable HLA class I molecules at the surface of breast tumor cell lines. These data suggest that the expression of Ii by tumor cells may quantitatively and qualitatively alter the presentation of antigens on those cells.

Regulation of CD1a surface expression and antigen presentation by invariant chain and lipid rafts

In immature dendritic cells (DCs), CD1a is almost exclusively expressed at the cell surface and its membrane organization is poorly understood. In this study, we report that MHC class II, invariant chain (Ii), and CD9 molecules are coimmunoprecipitated with CD1a in immature DCs, and that CD1a/Ii colocalization is dependent on lipid raft integrity. In HeLa-CIITA cells CD1a expression leads to increased Ii trafficking to the cell surface, confirming the relevance of this association. Furthermore, silencing of Ii in DCs induces significant CD1a accumulation on the plasma membrane whereas the total CD1a expression remains similar to that of control cells. These data suggest that CD1a recycling is facilitated by the association with the Ii. The CD1a localization in lipid rafts has functional relevance as demonstrated by inhibition of CD1a-restricted presentation following raft disruption. Overall, these findings identify Ii and lipid rafts as key regulators of CD1a organization on the surface of immature DCs and of its immunological function as Ag-presenting molecule.

Macrophage migration inhibitory factor has a MHC class I-like motif and function

Macrophage migration inhibitory factor (MIF) is found in immune-privileged sites and inhibits cytotoxicity mediated by CD3-ve lymphokine-activated killer cells (LAK). The mechanism by which MIF attenuates LAK cytotoxicity is unknown. We provide evidence that MIF has a major histocompatibility complex (MHC) class I-like motif. A monoclonal antibody (OX18) that binds a conserved region of rat MHC class I proteins binds native MIF. Anti-MIF polyclonal antibodies bind MHC class I. Epitope mapping suggests OX18 binds a loop of MHC class I bound by several receptors for MHC class I. A sequence (PRPEG) within the proposed OX18-binding site on MHC class I exists with a short insertion in MIF. OX18 does not bind MIF that is denatured by SDS-PAGE. This suggests the OX18 epitope is dependent on higher order structure in MIF. Interestingly, MIF inhibits binding of tetramers of MHC class I (H2D(b)) to LAK cells, suggesting it may bind to receptors for MHC class I. MIF may be an example where small regions of MHC class I are used by endogenous and viral proteins to control cytotoxicity mediated by immune cells.

CLIP-region mediated interaction of Invariant chain with MHC class I molecules

An association between the MHC class II chaperone molecule Invariant chain (Ii) and MHC class I molecules is known to occur, but the basis of the interaction is undetermined. Evidence is presented here that the CLIP region of Ii is involved in this interaction. A peptide encoding residues 91-99 of CLIP (MRMATPLLM) stabilised multiple MHC class I alleles, with the methionine residue at position 99 having a crucial role in binding to H2-K(b), whereas methionine at position 91 also appeared important in binding to RT1-A(a). Ii can also be detected in the class I MHC peptide loading complex. These data provide an explanation for the association of Ii and MHC class I molecules.

Mobilization of MHC class I molecules from late endosomes to the cell surface following activation of CD34-derived human Langerhans cells

Langerhans cells are a subset of dendritic cells (DCs) found in the human epidermis with unique morphological and molecular properties that enable their function as "sentinels" of the immune system. DCs are pivotal in the initiation and regulation of primary MHC class I restricted T lymphocyte immune responses and are able to present both endogenous and exogenous antigen onto class I molecules. Here, we study the MHC class I presentation pathway following activation of immature, CD34-derived human Langerhans cells by lipopolysaccharide (LPS). LPS induces an increase in all components of the MHC class I pathway including the transporter for antigen presentation (TAP), tapasin and ERp57, and the immunoproteasome subunits LMP2 and LMP7. Moreover, in CD34-derived Langerhans cells, the rapid increase in expression of MHC class I molecules seen at the cell surface following LPS activation is because of mobilization of MHC class I molecules from HLA-DM positive endosomal compartments, a pathway not seen in monocyte-derived DCs. Mobilization of class I from this compartment is primaquine sensitive and brefeldin A insensitive. These data demonstrate the regulation of the class I pathway in concert with the maturation of the CD34-derived Langerhans cells and suggest potential sites for antigen loading of class I proteins.

A soluble major histocompatibility complex class I peptide-binding platform undergoes a conformational change in response to peptide epitopes

Class I major histocompatibility complexes (MHC) are heterotrimeric structures comprising heavy chains (HC), beta2-microglobulin (beta2-m), and short antigenic peptides of 8-10 amino acids. These components assemble in the endoplasmic reticulum and are released to the cell surface only when a peptide of the appropriate length and sequence is incorporated into the structure. The binding of beta2-m and peptide to HC is cooperative, and there is indirect evidence that the formation of a stable heterotrimer from an unstable HC:beta2-m heterodimer involves a peptide-induced conformational change in the HC. Such a conformational change could ensure both a strong interaction between the three components and also signal the release of stably assembled class I MHC molecules from the endoplasmic reticulum. A peptide-induced conformational change in HC has been demonstrated in cell lysates lacking beta2-m to which synthetic peptides were added. Many features of this conformational change suggest that it may be physiologically relevant. In an attempt to study the peptide-induced conformational change in detail we have expressed a soluble, truncated form of the mouse H-2Db HC that contains only the peptide binding domains of the class I molecule. We have shown that this peptide-binding "platform" is relatively stable in physiological buffers and undergoes a conformational change that is detectable with antibodies, in response to synthetic peptides. We also show that the structural features of peptides that induce this conformational change in the platform are the same as those required to observe the conformational change in full-length HC. In this respect, therefore, the HC alpha1 and alpha2 domains, which together form the peptide binding site of class I MHC, are able to act independently of the rest of the molecule.

Processing of exogenous hepatitis B surface antigen particles for Ld-restricted epitope presentation depends on exogenous beta2-microglobulin

Processing of exogenous hepatitis B surface antigen (HBsAg) particles in an endolysosomal compartment generates peptides that bind to the major histocompatibility complex (MHC) class I molecule Ld and are presented to CD8+ cytotoxic T lymphocytes. Surface-associated 'empty' MHC class I molecules associated neither with peptide, nor with beta2-microglobulin (beta2m) are involved in this alternative processing pathway of exogenous antigen for MHC class I-restricted peptide presentation. Here, we demonstrate that internalization of exogenous beta2m is required for endolysosomal generation of presentation-competent, trimeric Ld molecules in cells pulsed with exogenous HBsAg. These data point to a role of endocytosed exogenous beta2m in the endolysosomal assembly of MHC class I molecules that present peptides from endosomally processed, exogenous antigen.

Recognition of out-of-frame major histocompatibility complex class I-restricted epitopes in vivo

In the course of constructing a recombinant vaccinia virus encoding the influenza A nucleoprotein (NP) gene preceded by the hemagglutinin leader sequence, we isolated a single base-pair deletion mutant which gave rise to L+NP(1-159) in which only the first 159 amino acids were in frame. Despite this, when we infected target cells, we found that the point mutant was able to sensitize them for lysis not only by cytotoxic T cells recognizing residues 50-58 (the in-frame portion), but also by CTL to epitopes which are downstream of the mutation (366-374 and 378-386). Furthermore, normal C57BL/6 mice can be primed with the frameshift NP to recognize the immunodominant Db-restricted epitope 366-374 (which is out of frame). Experiments in which the mutant gene product was processed in the endoplasmic reticulum of target cells suggested that the apparent suppression occurred during polypeptide extension.

Biosynthesis of major histocompatibility complex molecules and generation of T cells in Ii TAP1 double-mutant mice

Major histocompatibility complex (MHC) class I and II molecules are loaded with peptides in distinct subcellular compartments. The transporter associated with antigen processing (TAP) is responsible for delivering peptides derived from cytosolic proteins to the endoplasmic reticulum, where they bind to class I molecules, while the invariant chain (Ii) directs class II molecules to endosomal compartments, where they bind peptides originating mostly from exogenous sources. Mice carrying null mutations of the TAP1 or Ii genes (TAP10) or Ii0, respectively) have been useful tools for elucidating the two MHC/peptide loading pathways. To evaluate to what extent these pathways functionally intersect, we have studied the biosynthesis of MHC molecules and the generation of T cells in Ii0TAP10 double-mutant mice. We find that the assembly and expression of class II molecules in Ii0 and Ii0TAP10 animals are indistinguishable and that formation and display of class I molecules is the same in TAP10 and Ii0TAP10 animals. Thymic selection in the double mutants is as expected, with reduced numbers of both CD4+ CD8- and CD4- CD8+ thymocyte compartments. Surprisingly, lymph node T-cell populations look almost normal; we propose that population expansion of peripheral T cells normalizes the numbers of CD4+ and CD8+ cells in Ii0TAP10 mice.

Allelic differences affecting invariant chain dependency of MHC class II subunit assembly

The conserved invariant chain associates with highly polymorphic alpha and beta subunits guiding class II transport through the secretory pathway. Early associations of these three polypeptides inside antigen-presenting cells are poorly understood. The present experiments provide a detailed picture of the structure and fate of class II alpha and beta subunits in invariant chain mutants possessing different MHC haplotypes. In the absence of invariant chain, A alpha bA beta b is predominantly expressed as free A alpha b and A beta b chains by both splenocytes and activated LPS/IL-4 blasts, confirming that A alpha bA beta b assembly is strongly dependent on invariant chain coexpression. A quite different situation exists with respect to other allelic products. In the absence of invariant chain, A alpha kA beta k, E alpha kE beta k, and A alpha dA beta d molecules assemble efficiently and are conformationally similar to mature wild-type heterodimers. The contribution of invariant chain to subunit assembly thus differs for allelic variants, suggesting that sequential associations of alpha, beta, and invariant chain may be affected by polymorphic differences.

Association of the invariant chain with major histocompatibility complex class I molecules directs trafficking to endocytic compartments

Major histocompatibility complex (MHC) class I and class II molecules have been shown to present peptides of different origin to alpha beta T cells. Most peptides presented by class I molecules are derived from endogenously synthesized proteins, whereas most peptides presented by class II molecules are from exogenous sources. This functional dichotomy can largely be achieved by the preferential intracellular association of the invariant chain (Ii) with MHC class II molecules, which may inhibit binding of endogenous peptides to class II molecules and direct them to endocytic compartments where extracellularly derived peptides can be sampled. Here, we show that Ii also can associate with a subset of MHC class I molecules and direct them to endocytic compartments. Ii was coprecipitated with class I molecules after lysis of human lymphocytes in mild detergent such as 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonic acid or digitonin, and the association was more clearly visualized by the use of dithiobis[succinimidyl-propionate], a homobifunctional chemical cross-linker. The class I.Ii complex was reconstituted in Ii negative cells by transfection of corresponding cDNA clones and was found to be transported through the Golgi to acidic endocytic compartments. These observations may explain how some exogenous antigens can be presented by MHC class I molecules and how MHC class II molecules can bind self peptides derived from MHC class I molecules in endocytic compartments.

A polymorphic residue in the amino terminal alpha 1 hemi-domain of the mouse Ld class I molecule affects its assembly and surface expression

In comparison to Dd and most other mouse major histocompatibility complex class I molecules, the Ld molecule is poorly expressed on the cell surface, has a lower affinity for beta 2-microglobulin and is trafficked more slowly to the cell surface. Previous studies using Ld-Dd exon-shuffled constructs and the chimeric Ddm1 molecules suggested that the Ld alpha 1 domain was responsible for this phenotype. Two constructs, one containing an Ld-Dd hemi-exon-shuffled alpha 1 exon and the other containing a Dd-Ld hemi-exon-shuffled alpha 1 exon, were inserted into either Ld or Dd to replace the intact alpha 1 exon. These constructs were transfected into mouse L cells. Flow cytometric analyses of the resulting transfectants indicate that the Dd-Ld alpha 1/Ld molecules, similar to the Dd alpha 1/Dd alpha 2/Ld molecules, were expressed at a higher level on the cell surface than either the Ld-Dd alpha 1/Ld molecules or intact Ld molecules. Analyses of the molecules in lysates suggested that a higher proportion of the Dd-Ld alpha 1/Ld molecules, like the Dd alpha 1/Dd alpha 2/Ld molecules, as compared to the Ld-Dd alpha 1/Ld and intact Ld molecules were assembled as detected by alpha 2 domain-reactive monoclonal antibodies. Pulse-chase and lysate stability studies suggested that the lower steady state levels of assembled Ld-Dd alpha 1 molecules resulted from a slower assembly rate rather than instability. Collectively, these studies suggest that residues in the amino terminal half of the Ld alpha 1 domain are responsible for its inefficient assembly, probably leading to its low cell surface expression. To determine which polymorphic residues in the amino terminal alpha 1 hemi-domain might influence this phenotype, several Ld point mutants, in which a Dd amino terminal alpha 1 hemi-domain residue was substituted into the corresponding position of Ld, were analysed. These analyses suggested that, while the residue at position 9 has only a slight effect on beta 2-microglobulin association, it has a striking effect on assembly and cell surface expression.

Interaction of MHC class I molecules with the transporter associated with antigen processing

The transporter associated with antigen processing (TAP) delivers cytosolic peptides into the endoplasmic reticulum (ER) where they bind to nascent class 1 histocompatibility molecules. Class 1-peptide complexes are then displayed at the cell surface for recognition by cytotoxic T lymphocytes. Immunoprecipitation of either TAP or class 1 molecules revealed an association between the transporter and diverse class 1 products. TAP bound preferentially to heterodimers of the class 1 heavy chain and beta 2-microglobulin, and the complex subsequently dissociated in parallel with transport of class 1 molecules from the ER to the Golgi apparatus. The TAP-class 1 complexes could also be dissociated in vitro by the addition of class 1-binding peptides. The association of class 1 molecules with TAP likely promotes efficient capture of peptides before their exposure to the lumen of the ER.

Cell biology of antigen presentation

MHC class I molecules present degradation products derived from intracellular proteins, whereas MHC class II molecules generally present peptides derived from extracellular or surface proteins. Recent insights into the cell biology of MHC class I and II molecules explain this difference.