Stable surface expression of invariant chain prevents peptide presentation by HLA-DR

Dynamic range of Nef-mediated evasion of HLA class II-restricted immune responses in early HIV-1 infection

HLA class II-restricted CD4(+) T lymphocytes play an important role in controlling HIV-1 replication, especially in the acute/early infection stage. But, HIV-1 Nef counteracts this immune response by down-regulating HLA-DR and up-regulating the invariant chain associated with immature HLA-II (Ii). Although functional heterogeneity of various Nef activities, including down-regulation of HLA class I (HLA-I), is well documented, our understanding of Nef-mediated evasion of HLA-II-restricted immune responses during acute/early infection remains limited. Here, we examined the ability of Nef clones from 47 subjects with acute/early progressive infection and 46 subjects with chronic progressive infection to up-regulate Ii and down-regulate HLA-DR and HLA-I from the surface of HIV-infected cells. HLA-I down-regulation function was preserved among acute/early Nef clones, whereas both HLA-DR down-regulation and Ii up-regulation functions displayed relatively broad dynamic ranges. Nef's ability to down-regulate HLA-DR and up-regulate Ii correlated positively at this stage, suggesting they are functionally linked in vivo. Acute/early Nef clones also exhibited higher HLA-DR down-regulation and lower Ii up-regulation functions compared to chronic Nef clones. Taken together, our results support enhanced Nef-mediated HLA class II immune evasion activities in acute/early compared to chronic infection, highlighting the potential importance of these functions following transmission.

Misdirection of membrane trafficking by HIV-1 Vpu and Nef: Keys to viral virulence and persistence

The HIV-1 accessory protein Nef is well known for its manipulation of host cell endosomal trafficking. By linking transmembrane proteins to endosomal coats, Nef removes them from the surface of infected cells. Modulation of MHC proteins leads to viral evasion of cellular adaptive immunity, whereas modulation of receptors for the HIV envelope glycoprotein, including CD4, enhances viral infectivity. The other HIV-1 accessory proteins, Vif, Vpr and Vpu, share a mechanism of action distinct from Nef in that each interacts with a multi-subunit ubiquitin ligase complex to target cellular proteins for proteosomal degradation. However, newly uncovered functions and mechanistic aspects of Vpu likely involve endosomal trafficking: these include counteraction of the innate antiviral activity of the cellular transmembrane protein BST-2 (tetherin), as well as the removal of the lipid-antigen presenting protein CD1d and the natural killer cell ligand NTB-A from the cell surface. This review focuses on how Nef and Vpu interfere with normal intracellular membrane trafficking to facilitate the spread and virulence of HIV-1.

Invariant chain increases the half-life of MHC II by delaying endosomal maturation

Mounting adaptive immune responses requires the cell surface expression of major histocompatibility class II molecules (MHC II) loaded with antigenic peptide. However, in the absence of antigenic stimuli, the surface population of MHC II is highly dynamic and exhibits a high turnover. Several studies have focused on the regulation of MHC II, and it is now recognized that ubiquitination is one key mechanism operating in the turnover of MHC II in B cells and dendritic cells. Here, we describe how the invariant chain (Ii) can prolong the half-life of MHC II through its action on the endocytic pathway. We find that in cells expressing intermediate-to-high levels of Ii, the half-life of MHC II is increased, with MHC II accumulating in slowly-maturing endosomes. The accumulation in endosomes is not due to retention of new MHC II directed from the endoplasmatic reticulum, as also mature, not Ii associated, MHC II is preserved. We suggest that this alternative endocytic pathway induced by Ii would serve to enhance the rate, quantity and diversity of MHC II antigen presentation by concentrating MHC II into specialized compartments and reducing the need for new MHC II synthesis upon antigen encounter.

Immune evasion and counteraction of restriction factors by HIV-1 and other primate lentiviruses

Retroviruses have evolved effective strategies to evade the host immune response, such as high variability and latent infection. In addition, primate lentiviruses, such as HIV-1, have acquired several "accessory" genes that antagonize antiviral host restriction factors and facilitate viral immune evasion, thereby allowing continuous and efficient viral replication despite apparently strong innate and acquired immune responses. Here, I summarize some of our current knowledge on the acquisition and function of the viral vif, vpr, vpu, and nef genes, with a particular focus on the evolution and specific properties of pandemic HIV-1 strains that may contribute to their efficient spread and high virulence.

Implications of Nef: host cell interactions in viral persistence and progression to AIDS

The HIV and SIV Nef accessory proteins are potent enhancers of viral persistence and accelerate progression to AIDS in HIV-1-infected patients and non-human primate models. Although relatively small (27-35 kD), Nef can interact with a multitude of cellular factors and induce complex changes in trafficking, signal transduction, and gene expression that together converge to promote viral replication and immune evasion. In particular, Nef recruits several immunologically relevant cellular receptors to the endocytic machinery to reduce the recognition and elimination of virally infected cells by the host immune system, while simultaneously interacting with various kinases to promote T cell activation and viral replication. This review provides an overview on selected Nef interactions with host cell proteins, and discusses their possible relevance for viral spread and pathogenicity.

Major histocompatibility complex class II-peptide complexes internalize using a clathrin- and dynamin-independent endocytosis pathway

Major histocompatibility complex (MHC) class II molecules (MHC-II) function by binding antigenic peptides and displaying these peptides on the surface of antigen presenting cells (APCs) for recognition by peptide-MHC-II (pMHC-II)-specific CD4 T cells. It is known that cell surface MHC-II can internalize, exchange antigenic peptides in endosomes, and rapidly recycle back to the plasma membrane; however, the molecular machinery and trafficking pathways utilized by internalizing/recycling MHC-II have not been identified. We now demonstrate that unlike newly synthesized invariant chain-associated MHC-II, mature cell surface pMHC-II complexes internalize following clathrin-, AP-2-, and dynamin-independent endocytosis pathways. Immunofluorescence microscopy of MHC-II expressing HeLa-CIITA cells, human B cells, and human DCs revealed that pMHC enters Arf6(+)Rab35(+)EHD1(+) tubular endosomes following endocytosis. These data contrast the internalization pathways followed by newly synthesized and peptide-loaded MHC-II molecules and demonstrates that cell surface pMHC-II internalize and rapidly recycle from early endocytic compartments in tubular endosomes.

Invariant chain modulates HLA class II protein recycling and peptide presentation in nonprofessional antigen presenting cells

The expression of MHC class II molecules and the invariant chain (Ii) chaperone, is coordinately regulated in professional antigen presenting cells (APC). Ii facilitates class II subunit folding as well as transit and retention in mature endosomal compartments rich in antigenic peptides in these APC. Yet, in nonprofessional APC such as tumors, fibroblasts and endocrine tissues, the expression of class II subunits and Ii may be uncoupled. Studies of nonprofessional APC indicate class II molecules access antigenic peptides by distinct, but poorly defined pathways in the absence of Ii. Here, investigations demonstrate that nonprofessional APC such as human fibroblasts lacking Ii internalize antigenic peptides prior to the binding of these ligands to recycling class II molecules. By contrast, fibroblast lines expressing Ii favor exogenous peptides binding directly to cell surface class II molecules without a need for ligand internalization. Endocytosis of class II molecules was enhanced in cells lacking Ii compared with Ii-expressing APC. These results suggest enhanced reliance on the endocytic recycling pathway for functional class II presentation in nonprofessional APC.

Nef alleles from children with non-progressive HIV-1 infection modulate MHC-II expression more efficiently than those from rapid progressors

BACKGROUND

It has been established that defective nef genes and differences in the Nef-mediated downmodulation of CD4 and MHC-I cell surface expression can be associated with different rates of HIV-1 disease progression.

OBJECTIVE

To evaluate whether nef alleles derived from perinatally HIV-1-infected children showing no, slow or rapid disease progression differ in their abilities to downmodulate mature MHC-II or to upregulate the invariant chain (Ii) associated with immature MHC-II complexes.

METHODS

Nef alleles derived from HIV-1-infected children were cloned into expression vectors and proviral HIV-1 constructs co-expressing Nef and enhanced green fluorescence protein via an internal ribosomal entry site. Nef-mediated modulation of CD4, MHC-I, MHC-II or Ii surface expression was analysed by flow cytometric analysis of Jurkat T cells, monocytic THP-1 cells, CD4 T cells and macrophages transduced with vesicular stomatitis virus G-pseudotyped HIV-1 nef variants or transiently transfected HeLa class II transactivator cells.

RESULTS

: Nef alleles derived from HIV-1-infected children with non-progressive infection were significantly more active in the upregulation of Ii and downregulation of MHC-II than those derived from rapid progressors.

CONCLUSION

Nef alleles particularly active in interfering with MHC-II antigen presentation are more frequently found in perinatally HIV-1-infected non-progressors than rapid progressors. Possibly in the context of an immature host immune system, strongly impaired MHC-II function might contribute to lower levels of immune activation and a decelerated loss of CD4 T cells.

Replicating Ad-recombinants encoding non-myristoylated rather than wild-type HIV Nef elicit enhanced cellular immunity

OBJECTIVE

To determine if immunization with non-myristoylated nef would elicit enhanced cellular immune responses resulting from improved presentation of Nef peptides by MHC-I on the cell surface, and enhanced T-cell help.

DESIGN

The myristoylation site of HIV and SIV Nef is required for several Nef functions that modulate the immune response in an infected host, including downregulation of MHC-I, MHC-II, and CD4, and increased expression of the invariant chain on the cell surface. We constructed replication-competent Ad5- and Ad7-HIV recombinants encoding wild-type nef (nefWT) or a nef mutant (nefNM) lacking 19 amino-terminal amino acids, including the myristoylation site, and sequentially immunized chimpanzees mucosally, first with Ad5-HIVnef recombinants and subsequently with Ad7-HIVnef recombinants.

METHODS

Peripheral blood lymphocytes were evaluated over the immunization course for Nef-specific cellular immune responses by interferon (IFN)-gamma ELISPOT and T-cell proliferation assays. Nef-specific CD4 and CD8 memory T cells that produced intracellular IFN-gamma, interleukin-2, and tumor necrosis factor (TNF)-alpha were assessed by flow cytometry.

RESULTS

In comparison to immunization with Ad-HIVnefWT, Ad-HIVnefNM elicited statistically significant increases in numbers of IFN-gamma-secreting cells after the Ad7-HIVnefNM immunization and increased T-cell proliferative responses following both Ad5- and Ad7-HIVnefNM immunizations. Nef-specific CD4 and CD8 memory T-cell populations secreting TNF-alpha were also significantly increased in the Ad-HIVnefNM immunization group.

CONCLUSIONS

The results support the hypothesis that immunization with Ad-recombinants encoding HIVnefNM rather than HIVnefWT elicits enhanced cellular immunity resulting from improved antigen presentation and greater T-cell help.

Clostridium difficilelacks detectable superantigen activity

Clostridium difficile colitis causes striking leukocytosis. We examined the possibility that toxins A or B, or other nontoxin products of C. difficile, act as superantigens, thereby stimulating leukocytosis. Our results failed to show major histocompatibility complex class II-dependent T lymphocyte proliferation, the hallmark of superantigen activity. Elevated white blood cell counts in C. difficile colitis are probably due to increased generation of cytokines such as interleukin-6 (IL-6) or IL-8.

A three-amino-acid-long HLA-DRbeta cytoplasmic tail is sufficient to overcome ER retention of invariant-chain p35

The p35 isoform of the human invariant chain (Iip35) contains an N-terminal RXR endoplasmic-reticulum (ER) retention signal that becomes nonfunctional only after assembly with MHC-class-II molecules. We have previously shown that the MHC-class-II beta-chain cytoplasmic tail is crucial for the maturation of class-II/Iip35 complexes. In order to shed some light on the molecular determinants involved in shielding the RXR motif, we performed site-directed mutagenesis of the DRbeta chain and Ii cytoplasmic domains. Chimeric beta chains with irrelevant cytoplasmic tails allowed the efficient transport of Iip35 out of the ER in transiently transfected HEK 293T cells. An alanine scan of the cytoplasmic tail of HLA-DRbeta confirmed that no specific motif is required to overcome ER retention. Surprisingly, a beta chain with a three-amino-acid-long cytoplasmic tail (Tyr-Phe-Arg) was sufficient to overcome the Iip35 RXR motif. Moreover, replacement of residues F231 and R232 with alanines created a cytoplasmic tail (Tyr-Ala-Ala) that allowed ER egress. Given the limited length of this tail, steric hindrance would only be possible if the Ii ER retention motif was close to the membrane in the first place. However, this is not likely because an Ii molecule with an internal cytoplasmic deletion bringing the RXR motif closer to the membrane is not retained in the ER, even in the absence of class-II molecules. These results suggest that MHC-class-II molecules overcome ER retention and prevent COPI binding to the Iip35 RXR motif through a mechanism distinct from steric hindrance by its beta chain.

A defective viral superantigen-presenting phenotype in HLA-DR transfectants is corrected by CIITA

Activation of T lymphocytes by mouse mammary tumor virus superantigen (vSAg) requires binding to MHC class II molecules. The subcellular location where functional interactions occur between MHC class II molecules and vSAgs is still a matter of debate. To gain further insight into this issue, we have used human epithelial HeLa cells expressing HLA-DR1. Surprisingly, the human cells were unable to present transfected vSAg7 or vSAg9 to a series of murine T cell hybridomas. The defect is not related to a lack of vSAg processing, because these cells can indirectly activate T cells after coculture in the presence of B lymphocytes. However, after IFN-gamma treatment, the HeLa DR1(+) cells became apt at directly presenting the vSAg. Furthermore, transfection of CIITA was sufficient to restore presentation. Reconstitution experiments demonstrated the necessity of coexpressing HLA-DM and invariant chain (Ii) for efficient vSAg presentation. Interestingly, inclusion of a dileucine motif in the DRbeta cytoplasmic tail bypassed the need for HLA-DM expression and allowed the efficient presentation of vSAg7 in the presence of Ii. A similar trafficking signal was included in vSAg7 by replacing its cytoplasmic tail with the one of Ii. However, sorting of this chimeric Ii/vSAg molecule to the endocytic pathway completely abolished both its indirect and direct presentation. Together, our results suggest that functional vSAgs-DR complexes form after the very late stages of class II maturation, most probably at the cell surface.

Comprehensive analysis of nef functions selected in simian immunodeficiency virus-infected macaques

A variety of simian immunodeficiency virus (SIVmac) nef mutants have been investigated to clarify which in vitro Nef functions contribute to efficient viral replication and pathogenicity in rhesus macaques. Most of these nef alleles, however, were only functionally characterized for their ability to down-modulate CD4 and class I major histocompatibility complex (MHC-I) cell surface expression and to enhance SIV replication and infectivity. To obtain information on the in vivo relevance of more recently established Nef functions, we examined the ability of a large panel of constructed SIVmac Nef mutants and of variants that emerged in infected macaques to down-regulate CD3, CD28, and MHC-II and to up-regulate the MHC-II-associated invariant chain (Ii). We found that all these four Nef functions were restored in SIV-infected macaques. In most cases, however, the initial mutations and the changes selected in vivo affected several in vitro Nef functions. For example, truncated Nef proteins that emerged in animals infected with SIVmac239 containing a 152-bp deletion in nef efficiently modulated both CD3 and Ii surface expression. Overall, our results suggest that the effect of Nef on each of the six cellular receptors investigated contributes to viral fitness in the infected host but also indicate that modulation of CD3, MHC-I, MHC-II, or Ii surface expression alone is insufficient for SIV virulence.

Nef proteins from simian immunodeficiency virus-infected chimpanzees interact with p21-activated kinase 2 and modulate cell surface expression of various human receptors

The accessory Nef protein allows human immunodeficiency virus type 1 (HIV-1) to persist at high levels and to cause AIDS in infected humans. The function of HIV-1 group M subtype B nef alleles has been extensively studied, and a variety of in vitro activities believed to be important for viral pathogenesis have been established. However, the function of nef alleles derived from naturally simian immunodeficiency virus (SIV)-infected chimpanzees, the original host of HIV-1, or from the HIV-1 N and O groups resulting from independent zoonotic transmissions remains to be investigated. In the present study we demonstrate that SIVcpz and HIV-1 group N or O nef alleles down-modulate CD4, CD28, and class I or II MHC molecules and up-regulate surface expression of the invariant chain (Ii) associated with immature major histocompatibility complex (MHC) class II. Furthermore, the ability of Nef to interact with the p21-activated kinase 2 was generally conserved. The functional activity of HIV-1 group N and O nef genes did not differ significantly from group M nef alleles. However, SIVcpz nef genes as a group showed a 1.8- and 2.0-fold-higher activity in modulating CD28 (P = 0.0002) and Ii (P = 0.016) surface expression, respectively, but were 1.7-fold less active in down-regulating MHC class II molecules (P = 0.006) compared to HIV-1 M nef genes. Our finding that primary SIVcpz nef alleles derived from naturally infected chimpanzees modulate the surface expression of various human cellular receptors involved in T-cell activation and antigen presentation suggests that functional nef genes helped the chimpanzee virus to persist efficiently in infected humans immediately after zoonotic transmission.

Down-modulation of mature major histocompatibility complex class II and up-regulation of invariant chain cell surface expression are well-conserved functions of human and simian immunodeficiency virus nef alleles

Recently, it has been demonstrated that the human immunodeficiency virus type 1 (HIV-1) Nef from laboratory strains down-modulates cell surface expression of mature major histocompatibility complex class II (MHC-II) molecules, while up-regulating surface expression of the invariant chain (Ii) associated with immature MHC-II (P. Stumptner-Cuvelette, S. Morchoisne, M. Dugast, S. Le Gall, G. Raposo, O. Schwartz, and P. Benaroch, Proc. Natl. Acad. Sci. USA 98:12144-12149, 2001). These Nef functions could contribute to impaired CD4(+)-T-helper-cell responses found in HIV-1-infected patients with progressive disease. However, it is currently unknown whether nef alleles derived from HIV-1-infected individuals or from other primate lentiviruses also modulate MHC-II and Ii. In the present study, we demonstrate that both activities are conserved among primary HIV-1 nef alleles, as well as among HIV-2 and simian immunodeficiency virus (SIV) nef alleles. Down-modulation of mature MHC-II required high levels of Nef expression. In contrast, surface expression of Ii was already strongly increased at low to medium levels of Nef expression. Notably, nef genes derived from two of four HIV-1-infected long-term nonprogressors did not up-regulate Ii, whereas nef alleles derived from 10 individuals with progressive disease were active in this assay. Unlike other in vitro Nef functions, the average activity of Nef in modulating MHC-II and Ii surface expression did not change significantly during the course of infection. Mutational analysis confirmed that MHC-II down- and Ii up-regulation are functionally separable from each other and from other Nef functions and identified acidic residues, located at the base of the flexible C-proximal loop of Nef, that are critical for increased Ii expression. Overall, our results suggest that the ability of Nef to interfere with MHC-II antigen presentation might play a role in AIDS pathogenesis.

HIV-1 Nef impairs MHC class II antigen presentation and surface expression

HIV-1-infected cells can avoid cytotoxic T lymphocyte killing by Nef-mediated down-regulation of surface MHC I. Here, we show that HIV-1 Nef inhibits MHC II restricted peptide presentation to specific T cells and thus may affect the induction of antiviral immune responses. Nef mediates this effect by reducing the surface level of mature (i.e., peptide-loaded) MHC II while increasing levels of immature MHC II, which are functionally incompetent because of their association with the invariant chain. Nef was the only HIV-1 gene product to possess this capacity, which was also observed in the context of the whole HIV-1 genome. Other proteins of the endocytic pathway were not affected by Nef expression, suggesting that Nef effects on MHC II did not result from a general alteration of the endocytic pathway. Response patterns to previously characterized mutations of Nef differed for Nef-induced modulation of mature and immature MHC II. Furthermore, the doses of Nef required to observe each of the two effects were clearly different, suggesting that Nef could affect MHC II peptide presentation through distinct mechanisms. Cooperation between those mechanisms may enable Nef to efficiently inhibit MHC II function.

Intracellular transport and peptide loading of MHC class II molecules: regulation by chaperones and motors

MHC class II molecules are important in the onset and modulation of cellular immune responses. Studies on the intracellular transport of these molecules has provided insight into the way pathogens are processed and presented at the cell surface and may result in future immunological intervention strategies. Recent reviews have extensively described structural properties and early events in the biosynthesis of MHC class II (1-3). In this review, the focus will be on the function of the dedicated chaperone proteins Ii, DM and DO in the class II assembly, transport and peptide loading as well on proteins involved in transport steps late in the intracellular transport of MHC class II.

The role of H2-O and HLA-DO in major histocompatibility complex class II-restricted antigen processing and presentation

The function of major histocompatibility complex (MHC) class II molecules is to sample exogenous antigens for presentation to CD4+ T helper cells. After synthesis in the endoplasmic reticulum, class II molecules are directed into the endosomal system by association with the invariant chain (Ii), which is sequentially cleaved, generating class II dimers loaded with Ii-derived peptides (CLIP). These class II-peptide complexes are physiological substrates for H2-M/HLA-DM, a resident of the endosomal/lysosomal system which facilitates the removal of CLIP from newly synthesised class II alpha beta dimers. Exchange of CLIP for antigenic class II-binding peptides is also promoted by the action of H2-M/HLA-DM, resulting in stable peptide-class II complexes that are transported to the cell surface for presentation to CD4+ T cells. Recent evidence suggests that this H2-M/HLA-DM-mediated 'peptide editing' is influenced by another MHC class II-encoded molecule, H2-O/HLA-DO. This non-polymorphic alpha beta heterodimer is associated with H2-M/HLA-DM during intracellular transport and within the endosomal system of B cells. H2-O/HLA-DO alters the peptide exchange function of H2-M/HLA-DM in a pH-dependent manner, so that H2-M/HLA-DM activity is limited to more acidic conditions, corresponding to lysosomal compartments. Indeed, H2-O/HLA-DO may serve to limit the presentation of antigens after fluid phase uptake by B cells, while augmenting presentation of antigens internalised via membrane Ig receptors. Such a mechanism may maintain the fidelity of the B-cell-CD4+ T-cell interaction, counteracting self reactivity arising from less stringent lymphocyte activation. Here, data evaluating the role of H2-O/HLA-DO shall be reviewed and its putative function discussed.

HLA-DM can partially replace the invariant chain for HLA-DR transport and surface expression in transfected endocrine epithelial cells

The function of HLA class II molecules as peptide presenters to CD4+ T cells depends on the expression of associated molecules such as the invariant chain (Ii) and DM responsible for the correct transport of and high-stability peptide binding to the class II dimers. In organs affected by autoimmune diseases, endocrine epithelial cells express class II molecules, which presumably are involved in the presentation of self-peptides to autoreactive T cells. We have transfected the rat insulinoma cell line RINm5F with different combinations of HLA-DR, Ii and HLA-DM cDNAs and have studied how Ii and DM affect the transport and stability of class II molecules expressed by the different transfectants. Immunofluorescence and biochemical analysis showed that cells transfected with DR and DM in the absence of Ii expressed mostly stable molecules in their surface, and showed a lower accumulation of DR molecules in the endoplasmic reticulum (ER) than cells expressing only DR. This suggests that, in the absence of invariant chain, DM molecules can not only exchange peptides other than class II-associated invariant chain peptide (CLIP) but may also be involved in the transport of class II molecules out of the ER towards the endosomal route. In addition, these data confirm that expression of DR alone or DR+Ii do not allow the formation of sodium dodecyl sulphate (SDS)-stable complexes, that cells expressing DR+Ii have most DR molecules occupied by CLIP and that Ii and DM molecules allow regular routing and peptide loading of class II molecules.

A cytoplasmic sequence in human tyrosinase defines a second class of di-leucine-based sorting signals for late endosomal and lysosomal delivery

Distinct cytoplasmic sorting signals target integral membrane proteins to late endosomal compartments, but it is not known whether different signals direct targeting by different pathways. The availability of multiple pathways may permit some cell types to divert proteins to specialized compartments, such as the melanosome of pigmented cells. To address this issue, we characterized sorting determinants of tyrosinase, a tissue-specific resident protein of the melanosome. The cytoplasmic domain of tyrosinase was both necessary and sufficient for internalization and steady state localization to late endosomes and lysosomes in HeLa cells. Mutagenesis of two leucine residues within a conventional di-leucine motif ablated late endosomal localization. However, the properties of this di-leucine-based signal were distinguished from that of CD3gamma by overexpression studies; overexpression of the tyrosinase signal, but not the well characterized CD3gamma signal, induced a 4-fold enlargement of late endosomes and lysosomes and interfered with endosomal sorting mediated by both tyrosine- and other di-leucine-based signals. These properties suggest that the tyrosinase and CD3gamma di-leucine signals are distinctly recognized and sorted by distinct pathways to late endosomes in non-pigmented cells. We speculate that melanocytic cells utilize the second pathway to divert proteins to the melanosome.

A dominant-negative clathrin mutant differentially affects trafficking of molecules with distinct sorting motifs in the class II major histocompatibility complex (MHC) pathway

The role of clathrin in intracellular sorting was investigated by expression of a dominant-negative mutant form of clathrin, termed the hub fragment. Hub inhibition of clathrin-mediated membrane transport was established by demonstrating a block of transferrin internalization and an alteration in the intracellular distribution of the cation-independent mannose-6-phosphate receptor. Hubs had no effect on uptake of FITC-dextran, adaptor distribution, organelle integrity in the secretory pathway, or cell surface expression of constitutively secreted molecules. Hub expression blocked lysosomal delivery of chimeric molecules containing either the tyrosine-based sorting signal of H2M or the dileucine-based sorting signal of CD3gamma, confirming a role for clathrin-coated vesicles (CCVs) in recognizing these signals and sorting them to the endocytic pathway. Hub expression was then used to probe the role of CCVs in targeting native molecules bearing these sorting signals in the context of HLA-DM and the invariant chain (I chain) complexed to HLA-DR. The distribution of these molecules was differentially affected. Accumulation of hubs before expression of the DM dimer blocked DM export from the TGN, whereas hubs had no effect on direct targeting of the DR-I chain complex from the TGN to the endocytic pathway. However, concurrent expression of hubs, such that hubs were building to inhibitory concentrations during DM or DR-I chain expression, caused cell surface accumulation of both complexes. These observations suggest that both DM and DR-I chain are directly transported to the endocytic pathway from the TGN, DM in CCVs, and DR-I chain independent of CCVs. Subsequently, both complexes can appear at the cell surface from where they are both internalized by CCVs. Differential packaging in CCVs in the TGN, mediated by tyrosine- and dileucine-based sorting signals, could be a mechanism for functional segregation of DM from DR-I chain until their intended rendezvous in late endocytic compartments.

Protein sorting within the MHC class II antigen-processing pathway

Major histocompatibility complex (MHC) class II molecules are required for the presentation of antigenic peptides that are derived predominantly from internalized proteins. The assembly of MHC class II/peptide complexes occurs within endosomal compartments of antigen-presenting cells (APCs). Therefore, for assembly to occur, MHC class II molecules, foreign proteins, and accessory molecules must be sorted to appropriate intracellular sites. My laboratory is trying to understand how proteins are sorted to various antigen-processing compartments as well as to conventional endosomal organelles. Using chimeric marker proteins and a variety of biochemical and genetic approaches, we are addressing the specificity of protein sorting and the mechanisms by which sorting signals are deciphered. By using a similar chimeric protein approach to target endogenous proteins to distinct compartments, we hope to address the role of processing events in each compartment in the generation of MHC class II ligands.

MHC Class II Expression in Double Mutant Mice Lacking Invariant Chain and DM Functions

Invariant (Ii) chain and DM functions are required at distinct stages during class II maturation to promote occupancy by diverse peptide ligands. The class II molecules expressed by mutant mouse strains lacking Ii chain or DM activities display discrete structural and functional abnormalities. The present report describes the cellular and biochemical characteristics of Ii−DM− doubly deficient mice. As for Ii chain mutants, their mature AαbAβb dimers similarly exhibit reduced mobilities in SDS-PAGE, and in functional assays these molecules behave as if empty or occupied by an easily displaced peptide. Additionally, the present experiments demonstrate that the production of floppy AαbAβb dimers is TAP independent. In comparison with Ii chain mutants, Ii−DM− doubly deficient cell populations exhibit increased peptide binding activities and consistently greater presentation abilities in T cell stimulation assays. These functional differences appear to reflect higher class II surface expression associated with their increased representation of B lymphocytes. We also observe defective B cell maturation in mice lacking Ii chain or DM expression, and interestingly, B cell development appears more severely compromised in Ii−DM− double mutants. These mutant mice lacking both Ii chain and DM activities should prove useful for analyzing nonconventional class II Ag presentation under normal physiological conditions in the intact animal.

MHC class II-associated invariant chain-induced enlarged endosomal structures: a morphological study

The major histocompatibility complex class II-associated invariant chain is believed to direct newly synthesized class II to endocytic compartments. Invariant chain synthesized at high levels in transiently transfected cells induces formation of large vesicular structures. We have examined the effect of stable expression of invariant chain in human fibroblasts by light and electron microscopy. Invariant chain expression dramatically modified endocytic compartments and induced the formation of greatly enlarged structures. These modifications were not lethal. Ultrastructurally, at least three morphologically distinct enlarged compartments could be discerned in the cells. These three compartments may represent early and late endosomes and lysosomes. Internalization of anti-invariant chain antibodies shows that invariant chain may reach the large endosomes via rapid internalization from the plasma membrane. Internalized protein remained in the enlarged vesicles for 4-6 h, indicating an invariant chain-induced delay in the pathway to lysosomes. Although the large invariant chain-induced vesicles have not yet been seen in professional antigen-presenting cells, the invariant chain-induced effects may play a role in regulating the endocytic pathway, creating a special environment for MHC class II to bind antigen.

Decreased endosomal delivery of major histocompatibility complex class II-invariant chain complexes in dynamin-deficient cells

Major histocompatibility complex class II molecules are heterodimeric cell surface molecules which acquire antigenic peptides in the endosomal/lysosomal system. Invariant chain (Ii), a third chain which is associated with class II molecules intracellularly mediates the endosomal targeting, but it is debated whether class II molecules reach the endosomal system mainly from the trans-Golgi network or via the cell surface. Dynamin is a cytosolic GTPase which is necessary for the formation of clathrin-coated vesicles from the plasma membrane, but which is not required for vesicle formation from the trans-Golgi network. Here we have used HeLa cells expressing a dominant negative form of dynamin to show that inhibition of clathrin-mediated uptake from the plasma membrane leads to accumulation of transfected Ii-class II complexes at the cell surface, while delivery of such complexes to endosomes/lysosomes is decreased. Our data therefore suggest that in this experimental system the majority of Ii-class II complexes traverse the cell surface before they reach the endosomal system.

MHC class II antigen-associated invariant chain on renal cell cancer may contribute to the anti-tumor immune response of the host

To investigate the association between renal cell cancer (RCC) and the host immune system, we examined the expression of invariant chain (Ii) and HLA-DR on renal cancer. Immunohistochemically, Ii was detected in 53 of the 60 cases of RCC. Significant correlation was found between the expression of Ii and the degree of lymphocyte infiltration. Flow cytometric analysis for HLA-DR and Ii on RCC cell line (ACHN) showed no positive cells, whereas interferon (IFN)-gamma treatment induced HLA-DR. Immunoprecipitation showed the presence of cytoplasmic Ii in ACHN cells. In addition, IFN-gamma-treated ACHN cells showed more intense signals than untreated cells. These results suggest that Ii associated with class II antigens on RCC may contribute to the anti-tumor immune response of the host and that IFN-gamma, which is administered for the treatment of cancer, may increase the immunogenicity of RCC.

Biochemical analysis of plasma-soluble invariant chains and their complex formation with soluble HLA-DR

The invariant chain (CD74) is preferentially localized in the cytoplasm and regulates the loading of exogenous derived peptides into HLA class II heterodimers. In addition, a small proportion of CD74:class II complexes is also expressed on the cell surface. We identified and quantified soluble CD74 (sCD74) molecules in the plasma and sCD74:sHLA-DR complexes by ELISA. EDTA plasma samples from 86 healthy probands were analyzed. sCD74 could be detected in all samples with a mean concentration of 1.14 relative units +/- 1.04 SD (range 0.17-4.31). Approximately 10% of the samples had increased amounts of sCD74 (>3.0 relative units). Complexes of sCD74 and sHLA-DR were detected in all samples and their quantities were positively correlated (r=0.83, p<0.001) with the sCD74 concentrations. SDS-PAGE analysis of plasma samples with high sCD74 concentrations (>3.0 relative units) revealed four isoforms of sCD74 with molecular weights of 45, 43, 35, 31 kDa corresponding to known sizes of intracellular CD74. However, only molecular weights of the 45 and 43 kDa isoforms of sCD74 are found complexed with sHLA-DR. Our data demonstrate, that CD74 molecules are present in their soluble form in the plasma of healthy probands and form complexes with soluble HLA-DR molecules.

Selective modulation of the major histocompatibility complex class II antigen presentation pathway following B cell receptor ligation and protein kinase C activation

We noticed that B cell receptor ligation or phorbol 12-myristate 13-acetate treatment induced intracellular vesicles containing major histocompatibility complex (MHC) class II and invariant chain (Ii), and increased the amount of transmembrane p12 Ii fragments coimmunoprecipitated with class II molecules. To determine the influence of protein kinase C activation on the MHC class II presentation pathway, we analyzed the subcellular distribution of Ii, the induction of SDS-stable forms of class II molecules, and their ability to present different antigens. Ii chains visualized with luminal and cytoplasmic directed antibodies appeared in early endosomal compartments accessible to transferrin in response to phorbol 12-myristate 13-acetate treatment, whereas transmembrane Ii degradation products equivalent to the p12 Ii fragments were colocalized with the B cell receptors internalized after cross-linking. Protein kinase C activation delayed in parallel the formation of SDS-stable forms of class II molecules and reduced the presentation of antigenic determinants requiring newly synthesized class II alphabeta-Ii complexes. These data indicate that B cell activation affects Ii processing and MHC class II peptide loading in endosomal compartments intersecting the biosynthetic pathway.

CD21 augments antigen presentation in immune individuals

CD21 (complement receptor 2, CR2) binds the C3 degradation products, iC3b and C3d, which are covalently linked to antigen or immune complexes in the process of complement activation. The ability of antigen-nonspecific B cells to present immune complexes containing high titers of acquired antibodies was tested. Influenza virus was incubated with serum from immune donors to create complement-containing complexes. These bound specifically to CD21 on transfected fibroblasts and B cell lines, as measured by microcytofluorimetry. Binding of immune complexes was ablated by inactivation of serum complement. In addition, the immunoglobulin in immune human serum blocked influenza binding to cells in the absence of complement, implying a minimal role for immunoglobulin-Fc receptor interactions in this system. Significant immune complex binding required a threshold level of CD21 expression, suggesting that only those cells with the highest levels of CD21 are likely to participate in the processing of macromolecular antigens. B cells pulsed with complement-influenza complexes elicited an augmented response from a panel of influenza-specific, class II-restricted T cell clones, as compared with those which had bound immunoglobulin-influenza complexes lacking complement. This enhanced response did not require CD35. In addition, B cell lines expressing higher levels of CD21 were more efficient in processing antigen than those with lower levels. These findings suggest that presentation of antigen by B cells in immune individuals is dependent on the binding of complement-antigen immune complexes to CD21.

MHC class II antigen-associated invariant chain on renal cell cancer may contribute to the anti-tumor immune response of the host

To investigate the association between renal cell cancer (RCC) and the host immune system, we examined the expression of invariant chain (Ii) and HLA-DR on renal cancer. Immunohistochemically, Ii was detected in 53 of the 60 cases of RCC. Significant correlation was found between the expression of Ii and the degree of lymphocyte infiltration. Flow cytometric analysis for HLA-DR and Ii on RCC cell line (ACHN) showed no positive cells, whereas interferon (IFN)-K treatment induced HLA-DR. Immunoprecipitation showed the presence of cytoplasmic Ii in ACHN cells. In addition, IFN-K-treated ACHN cells showed more intense signals than untreated cells. These results suggest that Ii associated with class II antigens on RCC may contribute to the anti-tumor immune response of the host and that IFN-K, which is administered for the treatment of cancer, may increase the immunogenicity of RCC.

Trimeric interactions of the invariant chain and its association with major histocompatibility complex class II alpha beta dimers

The invariant chain (I chain) associates with major histocompatibility complex class II alphabeta heterodimers upon synthesis, preventing them from binding peptides and unfolded proteins in the endoplasmic reticulum and directing class II transport to post-Golgi endosomal compartments. To assess which regions of the I chain are involved in binding class II molecules, we have studied proteolytic fragments of the I chain generated both by natural proteolytic degradation of alphabeta dimer-invariant chain complexes (alphabeta.I) within human B cells and by in vitro digestion of purified alphabeta middle dotI complexes with proteinase K. The 18-kDa luminal I chain fragment generated by proteinase K, called K3, remains associated with alphabeta dimers and retains the complex (alphabeta.K3) in a high molecular mass nonameric configuration. The N terminus of the K3 fragment was identified as glycine 110. This indicates that the K3 fragment lies outside of the class II-associated invariant chain peptide region (amino acids 81-104) of the I chain, shown to be important for initial alphabeta.I assembly. An N-terminal 12-kDa I chain fragment called p12, generated intracellularly, was also analyzed and was found to remain associated with alphabeta dimers in a high molecular mass form analogous to the nonameric alphabeta.I complex. These results demonstrate that at least two class II contact points exist along the length of the I chain and that different regions of the I chain can stabilize the alphabeta.I nonamer. Additional evidence suggests that the O-linked glycan(s) characteristic of the I chain is added to the short C-terminal region absent from the K3 fragment.

Invariant chain protects class II histocompatibility antigens from binding intact polypeptides in the endoplasmic reticulum

Unlike class I histocompatibility (MHC) antigens, most newly synthesized MHC class II molecules fail to be loaded with peptides in the endoplasmic reticulum (ER), binding instead to the invariant chain glycoprotein (Ii). Ii blocks the class II peptide binding groove until the class II:Ii complexes are transported to endosomes where Ii is removed by proteolysis, thus permitting loading with endosomal short peptides (approximately 12-25 amino acids). Ligands from which the groove is protected by Ii have not yet been identified; theoretically they could be short peptides or longer polypeptides (or both), because the class II groove is open at both ends. Here we show that in Ii- deficient cells, but not in cells expressing large amounts of Ii, a substantial fraction of class II alpha beta dimers forms specific, SDS-resistant 1:1 complexes with a variety of polypeptides. Different sets of polypeptides bound to H-2Ak, Ek, Ed and HLA-DR1 class II molecules; for Ak, a major species of Mr 50 kDa (p50) and further distinct 20 and 130 kDa polypeptides were detectable. Class II binding of p50 was characterized in detail. Point mutations within the Ak antigen binding groove destabilized the p50:class II complexes; a mutation outside the groove had no effect. A short segment of p50 was sufficient for association with Ak. The p50 polypeptide was synthesized endogenously, bound to Ak in a pre-Golgi compartment, and was transported to the cell surface in association with Ak. Thus, Ii protects the class II groove from binding endogenous, possibly misfolded polypeptides in the ER. The possibility is discussed that polypeptide binding is an ancestral function of the MHC antigen binding domain.

Association between HLA-DM and HLA-DR in vivo

The exchange of HLA class II-associated invariant chain peptides (CLIP) for cognate peptide is catalyzed by HLA-DM under acidic conditions in vitro by an unknown mechanism. Here, we show an association between HLA-DM and HLA-DR in vivo by coprecipitation of the two heterodimers. The association is favored at low pH and in the nonionic detergent digitonin. Most DM-DR complexes are isolated from dense subcellular fractions. Recovery of HLA-DM by the conformation-dependent DR3 monoclonal antibody 16.23 suggests an association with HLA-DR heterodimers beyond the stage at which CLIP is released. The additional N-linked glycan on mutant DR3 molecules isolated from the 10.24.6 cell line, which interferes with DM-enhanced CLIP release from DR3 in vitro, also affects the DM-DR interaction.

The MHC class II molecule H2-M is targeted to an endosomal compartment by a tyrosine-based targeting motif

The nonpolymorphic human class II molecule HLA-DM (DM) has been found to play a key role in antigen presentation by MHC class II molecules. HLA-DM and its murine equivalent H2-M are located intracellularly and are absent from the cell surface. In transfected HeLa cells, H2-M was transported to an endosomal compartment in the absence of invariant chain. A tyrosine-based targeting motif in the cytoplasmic tail of H2-M beta was responsible for the endosomal location and, if this tyrosine was mutated, H2-M accumulated at the cell surface. In the presence of invariant chain the mutated H2-M was redistributed to endosomes. The targeting motif of H2-M appeared not to be crucial for efficient peptide loading of class II, but if the invariant chain targeting motif also was removed, peptide loading decreased drastically. Thus, the targeting motif of H2-M appears to be supplementary, rather than essential for class II-peptide association.

Major histocompatibility class II peptide occupancy, antigen presentation, and CD4+ T cell function in mice lacking the p41 isoform of invariant chain

We used a "hit and run" gene targeting strategy to generate mice expressing only the p31 isoform of the conserved invariant (Ii) chain associated with major histocompatibility complex (MHC) class II molecules. Spleen cells from these mice appear indistinguishable from wild type with respect to class II subunit assembly, transport, peptide acquisition, surface expression, and the ability to present intact protein antigens. Moreover, these mutant mice have normal numbers of thymic and peripheral CD4+ T cells, and intact CD4+ T-dependent proliferative responses towards a soluble antigen. In short, MHC class II expression and function are surprisingly unaffected in mice lacking p41 invariant chain, implying that the p31 and p41 isoforms may be functionally redundant in the intact animal.

DM enhances peptide binding to class II MHC by release of invariant chain-derived peptide

Major histocompatibility complex (MHC) class II molecules bind antigenic peptides rapidly after biosynthesis in antigen-presenting cells (APCs). By contrast, the rate of peptide binding to purified class II molecules is remarkably slow. We find that purified HLA-DR molecules bind peptides rapidly in the presence but not the absence of HLA-DM, a recently identified heterodimer required for efficient antigen processing. The same effect is seen with immunoprecipitated DM, suggesting that DM interacts directly with DR. Class II-associated invariant chain peptides (CLIP) are selectively and rapidly released from DR during incubation with DM at pH 5. We conclude that DM is a cofactor that enhances peptide binding to DR molecules through a mechanism involving peptide exchange.

Invariant chain peptides enhancing or inhibiting the presentation of antigenic peptides by major histocompatibility complex class II molecules

Two soluble invariant chain (Ii) peptides with overlapping sequences had contrasting effects on the presentation of antigenic peptides by murine Ad, Ak, Ed, and Ek major histocompatibility complex (MHC) class II molecules. Naturally produced class II-associated invariant chain peptides human (h)Ii81-104/murine (m)Ii80-103 inhibited antigen presentation on these MHC class II alleles in a manner consistent with competitive inhibition. The Ii-4 peptides hIi77-92/mIi76-91 enhanced presentation of antigenic peptides on I-E class II alleles by promoting the exchange of peptides at the cell surface. Treatment of antigen-presenting cells (APC) with Ii-4 before the addition of antigenic peptide greatly enhanced subsequent T cell responses, while treatment of APC with Ii-4 after antigenic peptide binding decreased subsequent T cell responses. The hIi81-104 and mIi80-103 peptides inhibited T cell responses in both types of assays. The binding of biotinylated antigenic peptide to MHC class II-transfected L cells, as measured by flow cytometry, was inhibited by mIi80-103 and enhanced by mIi-4. Segments of Ii fragments remaining associated with MHC class II, or released Ii peptides, appear to regulate the formation of stable antigenic peptide/MHC class II complexes either positively or negatively through interactions at or near the antigenic peptide binding site. These findings open a pathway for the design of novel therapeutics based on the structure and function of natural and rationally designed fragments of Ii.

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.

Invariant chain induces a delayed transport from early to late endosomes

Invariant chain associated with class II molecules is proteolytically processed in several distinct intermediates during its transport through the endocytic pathway. Using subcellular fractionation, early and late endosomal compartments were separated in human fibroblasts transfected with HLA-DR (4N5 cells) and supertransfected with invariant chain (4N5Ii cells) or invariant chain lacking most of the cytoplasmic tail (4N5 delta 20Ii cells). Early and late endosome membrane fractions were characterized by morphology and by analyzing the presence of the Rab5 and Rab7 GTPases as markers of early and late endosomes, respectively. The transfer of endocytosed horseradish peroxidase from early to late endosomes proceeded relatively rapid both in 4N5 and 4N5 delta 20Ii cells (t1/2 = 25 min), whereas this transfer was significantly delayed (t1/2 = 2 h) in 4N5Ii cells. Pulse-chase experiments showed that invariant chain and its degradation products were first observed in early endosomes and thereafter in late endosomes. Our results strongly suggest that invariant chain induces a retention mechanism in the endocytic pathway.

Surface expression of the invariant chain (CD74) is independent of concomitant expression of major histocompatibility complex class II antigens

Whether or not intracellular transport and surface expression of the invariant chain (Ii; CD74) occurs independent of the presence of major histocompatibility complex (MHC) class II molecules was examined by comparing the class II-negative mutant lymphoblastoid cell line 174 x CEM.T2 (T2) and its class II-positive parental cell line 174 x CEM.T1 (T1). We found a similar proportion of Ii being transported to the Golgi complex in T1 and T2, as monitored by the degree of sialic acid addition to glycan side chains of Ii. In agreement with this result, T1 and T2 expressed comparable amounts of Ii at the cell surface, as measured by flow cytometry. This indicates that, although not associated with class II molecules, a proportion of Ii is transported to the plasma membrane. Both in T1 and T2, surface Ii (sIi) was rapidly internalized with a half-life of 3-4 min, suggesting that some Ii enters the endocytic route via the cell surface after being internalized. Our data demonstrate transport of Ii on a route alternative to the endocytic pathway. This alternative route could also account for delivery of newly synthesized class II-Ii complexes to processing compartments in antigen-presenting cells.

Poor loading of major histocompatibility complex class II molecules with endogenously synthesized short peptides in the absence of invariant chain

In normal antigen-presenting cells, newly synthesized major histocompatibility complex (MHC) class II molecules associate with the invariant chain (Ii) glycoprotein in the endoplasmic reticulum (ER). They are loaded with peptides only after proteolytic removal of the Ii in post-Golgi endocytic vesicles. Since the Ii inhibits peptide binding to MHC class II molecules, this association could protect MHC class II molecules from being loaded with endogenous peptides early after biosynthesis. If this were an important function of the Ii in vivo, MHC class II molecules synthesized in cells lacking the Ii should be loaded efficiently with short endogenous peptides in the ER; such peptides are known to be present there due to TAP-mediated import from the cytosol. To examine this possibility, we have studied peptide loading in HeLa transfectants expressing murine H-2Ak MHC class II molecules either alone or together with an excess of Ii. Endogenous peptides could readily be extracted from conformationally intact Ak alpha beta dimers of biosynthetically labeled Ii+ cells, whereas peptide loading was greatly (> 95%) diminished in the absence of Ii. Significant amounts of sodium dodecyl sulfate-(SDS) stable 55-kDa peptide: Ak complexes were only found in the Ii+ transfectants. In the absence of Ii, the MHC class II molecules instead formed stable complexes with long (20 and 50 kDa) polypeptides. Known Ak-binding peptides bound stably to Ak molecules on Ii- cells, could be co-purified with them, and were resistant to release in SDS, suggesting that poor recovery of endogenous peptides was not due to decreased stability of Ak:peptide complexes in the absence of Ii. We conclude that protection of MHC class II molecules from endogenous short peptides does not appear to be a quantitatively important function of the Ii molecule, because peptide loading is inefficient in its absence.

The bio-logical role of invariant chain (Ii) in MHC class II antigen presentation

Foreign antigens are internalized by antigen presenting cells by endocytosis and processed to peptides. To enable presentation of antigenic peptides by MHC class II molecules, these molecules have to be sorted to endosomal compartments where they can meet and bind the peptides. Invariant chain is complexed with MHC class II molecules and contains sorting signals responsible for MHC class II accumulation in endosomes. Invariant chain also has several other features contributing to the immune system's specific combat against invaders.

Retinoblastoma protein regulation of surface CD74 (invariant chain) expression in breast carcinoma cells

The HLA class II genes encode heterodimeric cell surface proteins which bind peptide antigen recognized by T-cell receptors on CD4+ T-cells. The class II proteins are inducible by IFN-gamma, and this induction requires, or is strongly enhanced, by retinoblastoma protein (RB) in a series of breast carcinoma cell lines. Loading of peptide onto the class II protein appears to be regulated by CD74, which associates with class II during their transition to the endosomal compartment, where class II binds peptide. Class II proteins and CD74 are largely regulated in concert, provoking the question, is CD74 induction by IFN-gamma affected by RB? Results described here indicate that IFN-gamma induction of CD74 surface expression in a series of breast carcinoma lines is enhanced by RB, while RB has no effect on CD74 mRNA induction. Also, neither the class II nor the CD74 promoter regions are activated by RB in cotransfection experiments where RB activates the SV40 promoter.

Endosomal localization of MHC class II-invariant chain complexes

CD4-positive T cells recognize foreign antigens displayed on the surface of antigen-presenting cells as small peptides bound to MHC class II molecules. Thus, the ability of antigen-presenting cells to generate these class II-peptide complexes is central to the initiation and regulation of immune responses. Class II predominantly associates with peptides derived from soluble protein antigens that are internalized and degraded within endosomal compartments. It is within these endosomal compartments that class II encounters and binds antigenic peptides. A number of signals have been implicated in directing the intracellular transport of class II to endosomes. These include sequences within class II itself and within the class II-associated invariant chain (Ii)

Antigenic peptide binding to MHC class II molecules at increased peptide concentrations

Affinity-purified major histocompatibility complex (MHC) class II molecules are known to bind antigenic peptides in vitro. The percentage of MHC class II molecules occupied with such peptides is usually very low and varies significantly depending upon the sequence and size of a given antigenic peptide. The present study describes a method by which complete saturation of affinity-purified MHC class II with antigenic peptide can be achieved by simply incubating purified MHC class II molecules at neutral pH in the presence of several 100-fold molar excess of antigenic peptide. Complexes of human HLA-DR2 and a peptide analog from human myelin basic protein MBP (83-102)Y83 were selected for this study. The on-rate kinetic results showed saturation of MHC class II occupancy at 300-500-fold molar excess peptide concentrations. The specificity of the MBP (83-102)Y83 peptide binding to HLA-DR2 at higher peptide concentration was demonstrated by incubating an equivalent amount of another epitope from myelin basic protein [MBP (1-14) peptide] as well as by competitive binding assays. The quantitation of bound peptide was carried out using biotinylated-MBP (83-102)Y83 peptide which showed 100-125% occupancy of HLA-DR2 with a recovery of 100%. The presence of a single peptide entity in purified complexes was confirmed by reverse-phase narrowbore HPLC analysis of the acid extracted supernatant and by mass spectrometry analysis. Two-dimensional gel electrophoresis (IEF/SDS) of purified HLA-DR2 and DR2.MBP (83-102)Y83 complexes showed the absence of various endogenous polypeptides in 100% loaded complexes. These results demonstrate that higher peptide concentrations can be useful in generating MHC class II-peptide complexes of defined composition. Such complexes of MHC class II occupied with a single peptide may have significant clinical relevance for antigen-specific therapy of various autoimmune diseases and may provide better understanding of MHC-peptide-TCR interactions.

An LI and ML motif in the cytoplasmic tail of the MHC-associated invariant chain mediate rapid internalization

Invariant chain (Ii) is a transmembrane protein that associates with the MHC class II molecules in the endoplasmic reticulum. Two regions of the 30 residue cytoplasmic tail of Ii contain sorting information able to direct Ii to the endocytic pathway. The full-length cytoplasmic tail of Ii and the two tail regions were fused to neuraminidase (NA) forming chimeric proteins (INA). Ii is known to form trimers and when INA was transfected into COS cells it assembled as a tetramer like NA. The INA molecules were targeted to the endosomal pathway and cotransfection with Ii showed that both molecules appeared in the same vesicles. By labelling the INA fusion proteins with iodinated antibody it was found that molecules with either endocytosis signal were expressed at the plasma membrane and internalized rapidly. Point mutations revealed that an LI motif within the first region of the cytoplasmic tail and an ML motif in the second region were essential for efficient internalization. The region containing the LI motif is required for Ii to induce large endosomes but a functional LI internalization motif was not fundamental for this property. The cytoplasmic tail of Ii is essential for efficient targeting of the class II molecules to endosomes and the dual LI and ML motif may thus be responsible for directing these molecules to the endosomal pathway, possibly via the plasma membrane.