Cell type-specific processing of the I-Ed-restricted hen egg lysozyme determinant 107–116

Class II major histocompatibility complex heterodimers present to T cells determinants as sets of antigen fragments with ragged N and C termini. It is not yet elucidated whether different types of antigen-presenting cells generate identical sets of peptides containing the same determinant. Taking advantage of recombinant I-E^d molecules produced by insect cells as empty heterodimers, a sensitive T cell stimulation assay was developed to analyze naturally processed hen egg lysozyme (HEL) peptides. I-E^d preparations were isolated from antigen-presenting cells cultured with HEL. Following acid treatment, peptides eluted from I-E^d were chromatographed and the fractions incubated at acidic pH with purified recombinant I-E^d molecules, conditions which favor peptide binding. The stimulatory capacity of the reconstituted peptide-I-E^d complexes adsorbed on the well surface of cell culture plates was then evaluated by measuring interleukin-2 secreted by an HEL 107–116-specific, I-E^d-restricted T cell hybridoma. We found that the B lymphoma A20 and an I-E^d-transfected fibroblast cell line generated distinct sets of peptides containing the HEL sequence 107–116. Our results suggest the possibility that presentation of one determinant by different types of antigen-presenting cells stimulates populations of T cells with distinct fine antigen specificities.     

Assembly of an abundant endogenous major histocompatibility complex class II/peptide complex in class II compartments

To identify the intracellular site(s) of formation of an endogenous class II/peptide complex in a human B cell line, we employed kinetic pulse-chase labeling experiments followed by subcellular fractionation by Percoll density gradient centrifugation and immunogold labeling on ultrathin cryosections. For direct demonstration of assembly of such complexes, we used the monoclonal antibody YAe, which detects an endogenous complex of the mouse class II molecule I-A^b with a 17-amino acid peptide derived from the α chain of HLA-DR (DRα52–68). We show that in human B lymphocytes, these class II/peptide complexes assemble and transiently accumulate in major histocompatibility complex class II-enriched compartments before reaching the cell surface.     

Targeting major histocompatibility complex class II molecules to the cell surface by invariant chain allows antigen presentation upon recycling

We studied the functional consequences of targeting class II molecules to either the cell surface or to endocytic structures by expressing HLA-DR1 in human kidney cells in the presence or absence of different forms of the invariant chain (Ii). Transfectants expressing class II molecules in the absence of Ii present influenza virus efficiently and co-expression of full length Ii does not further increase antigen presentation. Chimeric Ii containing the cytoplasmic domain of the transferrin receptor (Tfr-Ii) delivers class II molecules associated with Tfr-Ii to endosomal compartments, but this does not result in efficient antigen presentation. When class II molecules are targeted to the cell surface by Ii lacking either 15 (Δ15Ii) or 23 (Δ23Ii) amino acids from the cytoplasmic domain, a fraction of free class II molecules is also observed. Whereas Δ15Ii did not affect antigen presentation by class II molecules, Δ23Ii inhibited, but did not abrogate, the response. We show that class II molecules expressed in the presence of Δ23Ii can be internalized, followed by degradation of Δ23Ii and return of free class II αβ heterodimers to the cell surface. A fraction of the resulting free class II molecules is sodium dodecyl sulfate stable, indicating that internalization and reappearance of class II molecules at the cell surface can be an alternative route for antigen presentation. In all transfectants, class II molecules were found in endocytic compartments that labeled for CD63 and resembled the multilaminar MIIC compartments found in B cell lines. Ii is not required for endosomal targeting of class II molecules. The number of class II molecules observed in the multilaminar compartments correlates with the efficiency of antigen presentation.     

Soluble mouse major histocompatibility complex class II molecules produced in Drosophila cells

We have exploited Drosophila melanogaster. Schneider cells and compatible inducible expression vectors to produce large amounts of secreted major histocompatibility complex (MHC) class II molecules (I-E^d). A simple two-step purification protocol was developed. In the first step, recombinant molecules were enriched using a monoclonal anti-class II antibody column followed by a nickel chelate column which further purified and concentrated the recombinant protein to several mg/ml. Characterization of the purified material indicates that the molecules are correctly assembled into αβ heterodimers. Further analysis shows that the recombinant MHC class II molecules are devoid of endogenous peptides and, therefore, homogenous peptide/MHC complexes could be prepared by adding exogenous I-E^d-specific peptides at slightly acidic pH. Upon peptide addition, molecules underwent a conformational change into a more compact form revealed by gel filtration analysis. In addition, the peptide/MHC complexes were biologically active. As little as 10 ng of these complexes coated on plastic from a 100 ng/ml solution were sufficient to trigger antigen-specific T cell hybridomas. These MHC class II molecules, together with various forms of soluble T cell receptor (TcR) proteins, provide valuable tools to analyze the molecular details of TcR/antigen recognition.     

Excessive degradation of intracellular protein in macrophages prevents presentation in the context of major histocompatibility complex class II molecules

The endogenous major histocompatibility complex (MHC) class II presentation pathway allows biosynthesized, intracellular antigens access for presentation to MHC class II-restricted T cells. This pathway has been well documented in B cells and fibroblasts, but may not be universally available in all antigen-presenting cell types. This study compares the ability of different antigen-presenting cells, expressing endogenous C5 protein (fifth component of mouse complement) as a result of transfection, to present their biosynthesized C5 to MHC class II-restricted T cells. B cells and fibroblasts expressing C5 were able to present several epitopes of this protein with MHC class II molecules, whereas macrophages were unable to do so, but readily presented C5 from an extracellular source. However, macrophage presentation of endogenous C5 could be achieved when they were treated with low doses of the lysosomotropic agent ammonium chloride. In the presence of an inhibitor of autophagy, presentation of endogenous C5 was abrogated, indicating that biosynthesized C5 is shuttled into lysosomal compartments for degradation before making contact with MHC class II molecules. Taken together, this suggests that proteolytic activity in lysosomes of macrophages may be excessive, compared with fibroblasts and B cells, and destroys epitopes of the C5 protein before they can gain access to MHC class II molecules. Thus, there are inherent differences in presentation pathways between antigen-presenting cell types; this could reflect their specialized functions within the immune system with macrophages focussing preferentially on internalization, degradation, and presentation of extracellular material.     

Inhibition of natural killer cell-mediated bone marrow graft rejection by allogeneic major histocompatibility complex class I,but not class II molecules

The role of major histocompatibility complex (MHC) class I and class II molecules in natural killer (NK) cell-mediated rejection of allogeneic, semi-syngeneic and MHC-matched bone marrow grafts was investigated. The use of β₂-microglobulin (β₂m) -/- and β₂m +/- mice as bone marrow donors to MHC-mismatched recipients allowed an analysis of whether the presence of semi-syngeneic and allogeneic MHC class I gene products would be triggering, protective or neutral, in relation to NK cell-mediated rejection. Loss of β₂m did not allow H-2^b bone marrow cells to escape from NK cell-mediated rejection in allogeneic (BALB/c) or semi-allogeneic (H-2D^d transgenic C57BL/6) mice. On the contrary, it led to stronger rejection, as reflected by the inability of a larger bone marrow cell inoculum to overcome rejection by the H-2-mismatched recipients. In H-2-matched recipients, loss of β₂m in the graft led to a switch from engraftment to rejection. At the recipient level, loss of β₂m led to loss of the capability to reject H-2-matched β₂m-deficient as well as allogeneic grafts. When MHC class II-deficient mice were used as donors, the response was the same as that against donors of normal MHC phenotype: allogeneic and semi-syngeneic grafts were rejected by NK cells, while syngeneic grafts were accepted. These data suggest a model in which allogeneic class I molecules on the target cell offer partial protection, while certain syngeneic class I molecules give full protection from NK cell-mediated rejection of bone marrow cells. There was no evidence for a role of MHC class II molecules in this system.     

Induction of class II major histocompatibility complex blockade as well as T cell tolerance by peptides administered in soluble form

Peptides binding to a particular class II major histocompatibility complex (MHC) molecule can inhibit the activation of T cells by other peptides binding to the same molecule, a phenomenon termed class II MHC blockade. All class II-binding peptides exert MHC blockade in vivo in depot form with adjuvant, and some also retain their blocking properties in soluble form. We demonstrate here that soluble peptides, when used at doses causing short-term MHC blockade, can also induce long-term antigen-specific T cell tolerance to themselves. The tolerogenicity of soluble peptides correlates with their antigenicity in adjuvant, but it is not necessarily related to their capacity to act as class II blockers in vivo. The tolerant state is manifested in a decreased production of both T helper cell 1 (Th1)-type and Th2-type lymphokines, and it cannot be reversed by interleukin-2. Once T cells are primed with a peptide in complete Freund's adjuvant, they are resistant to tolerization with the same peptide applied in soluble form. Tolerance induction is partially impaired in B cell-deficient μMT^−/− mice, suggesting a role for B cell antigen presentation in this process. The results suggest that the potential immunogenicity of class II MHC blockers could be circumvented by choosing a tolerogenic mode of application.     

Presentation of exogenous antigens by macrophages: analysis of major histocompatibility complex class I and II presentation and regulation by cytokines

There is an antigen presenting cell (APC) in the lymphoid organs capable of presenting exogenous antigen (Ag) with major histocompatibility complex (MHC) class I molecules. This study was initiated to isolate clones of these APC to definitively establish their phenotype and to further study their properties. Murine bone marrow macrophages (BM MΦ) were immortalized by overexpressing myc and raf oncogenes. Five BM MΦ cell lines were generated that are phagocytic and expressed at their surface MΦ differentiation Ag. All five cell lines processed and presented exogenous ovalbumin (OVA) with MHC class I molecules. They all presented OVA-linked to a phagocytic substrate 10²–10⁴-fold more efficiently than soluble Ag. Clonal isolates of two of the MΦ cell lines had an identical phenotype and functional properties as the uncloned lines. These results definitively establish that MΦ are APC with the capacity of presenting exogenous Ag with MHC class I molecules. Interferon (IFN)-γ interleukin-4, granulocyte-macrophage colony stimulating factor and lipopolysaccharide either alone or in combination induced little or no augmentation and in some cases decreased presentation of exogenous OVA with MHC class I. In contrast, all of MΦ activating factors increased MHC class I expression. Moreover, IFN-γ increased the presentation of cytosolic OVA, demonstrating differences between the presentation of cytosolic Ag versus exogenous Ag with MHC class I. Finally, some lines constitutively processed and presented exogenous OVA with MHC class II while others only presented after stimulation with IFN-γ. These results demonstrate that the pathways involved in the presentation of exogenous Ag with MHC class I and class II are independently regulated and that a cloned cell is capable of presenting exogenous Ag through both pathways.     

A study of complexes of class II invariant chain peptide: Major histocompatibility complex class II molecules using a new complex-specific monoclonal antibody

Complexes of major histocompatibility complex (MHC) class II molecules containing invariant chain (Ii)-derived peptides, known as class II-associated invariant chain peptides (CLIP), are expressed at high levels in presentation-deficient mutant cells. Expression of these complexes in mutant and wild-type antigen-presenting cells suggests that they represent an essential intermediate in the MHC class II antigen-presenting pathway. We have generated a monoclonal antibody, 30-2, which is specific for these complexes. Using this antibody, we have found quantitative differences in CLIP: MHC class II surface expression in mutant and wild-type cells. Our experiments also show that CLIP: MHC class II complexes are preferentially expressed on the cell surface similar to total mature MHC class II molecules. These complexes are found to accumulate in the endosomal compartment in the process of endosomal Ii degradation. Analysis of the fine specificity of the antibody indicates that these complexes have Ii peptide bound to the peptide-binding groove.     

Characterizing immunodominant and protective influenza hemagglutinin epitopes by functional activity and relative binding to major histocompatibility complex class II sites

In the present study the analysis of functional activity and major histocompatibility complex (MHC) binding of two adjacent MHC class II-restricted epitopes, located in the C-terminal 306–329 region of human influenza A virus hemagglutinin 1 subunit (HA1) conserved with subtype sequences and not affected by antigenic drift, was undertaken to explore the hierarchy of local immnodominance. The functional activity of two T cell hybridomas of the memory/effector Th1 phenotype in combination with in vivo immunization studies provided a good tool for investigating the functional characteristics of the T cell resonse. The in vitro binding assays performed with a series of overlapping, N-terminal biotinylated peptides covering the 306–341 sequence enabled us to compare the relative binding efficiency of peptides, comprising two distinct epitopes of this region, to I-E^d expressed on living antigen-presenting cells. Our studies revealed that (i) immunization of BALB/c mice with the 306–329 H1 or H2 peptides resulted in the activation and proliferation of T cells recognizing both the 306–318 and the 317–329 epitopes, while the 306–329 H3 peptide elicits predomonantly 306–318-specific T cells, (ii) the 317–329 HA1 epitope of the H1 and H2 but not the H3 sequence is recognized by T cells and is available for recognition not only in the 317–329 peptide but also in the extended 306–329 or 306–341 peptides, (iii) the 306–318 and the 317–329 hemagglutinin peptides encompassing the H1, H2 but not the H3 sequence bind with an apparently similar affinity to and therefore compete for I-E^d binding sites, and (iv) the 317–341, the 317–329 peptides and their truncated analogs show subtype-dependent differences in MHC binding and those with lower binding capacity represent the H3 subtype sequences. These results demonstrate that differences in the binding capacity of peptides comprising two non-overlapping epitopes located in the C-terminal 306–329 region of HA1 of all three subtype-specific sequences to MHC class II provide a rationale for the local and also for the previously observed in vivo immunodominance of the 306–318 region over the 317–329 epitope in the H3 but not in the H1 or H2 sequences. In good correlation with the results of the binding and functional inhibition assays, these data demonstrate that in the H1 and H2 subtypes both regions are available for T cell recognition, they compete for the same restriction element with an appearently similar binding efficiency and, therefore, function as co-dominant epitopes. Due to the stabilizing effect of the fusion peptide, peptides comprising the 306–341 or 317–341 H1 sequences are highly immunogenic and elicit a protective immune response which involves the production of antibodies and interleukin-2 and tumor necrosis factor producing effector Th1 cells both directed against the 317–329 region. Based on the similarity of the I-E^d and HLA-DR1 peptide binding grooves and motifs, these results suggest that amino acid substitutions inserted to the H3 subtype sequence during viral evolution can modify the relative MHC binding capacity and invert the local hierarchy of immunodominance of two closely situated epitopes that are able to bind to the same MHC class II molecule.     

Endogenous and exogenous forms of the same antigen are processed from different pools to bind MHC class II molecules in endocytic compartments

The current studies were carried out to examine the basis for the differences in the antigenic peptides generated from exogenous and endogenous forms of hen egg white lysozyme (HEL). The role of different intracellular compartments in the generation and binding of HEL peptides derived from two endogenous forms of HEL, either secreted (sHEL) or retained in the endoplasmic reticulum (ER, KDEL HEL), presented by MHC class II molecules was examined and compared to exogenous HEL. Initially it was found that antigen-presenting cells bearing both intracellular forms of HEL generated and presented a number of IA^k-restricted HEL epitopes to T cell hybridomas, although s HEL was processed more efficiently than KDEL HEL. There were differences, however, for some determinants between endogeneous and exogenous HEL. At equivalent antigen-presenting efficiencies, endogenous HEL-bearing cells displayed a lower surface density of IA^k-bound HEL-52-61-related peptides than cells pulsed with exogenous HEL, as detected by a specific monoclonal antibody. Neither endogenous HEL degradation nor peptide binding to MHC class II molecules occurred in the ER. Processing of sHEL and KDEL HEL appears to take place either in a post-trans-Golgi network acidic compartment or in the cytosol, whereas peptide binding to MHC class II molecules occurs in endocytic compartments . Furthermore, the peptides generated were derived from an endogenous source rather than from secreted and re-endocytosed HEL. Thus, processing of endogenous HEL is from a different pool than exogenous HEL and occurs in different compartments.     

Hepatitis B virus small surface antigen particles are processed in a novel endosomal pathway for major histocompatibility complex class I-restricted epitope presentation

We investigated the major histocompatibility complex (MHC) class I-restricted presentation of an epitope of the hepatitis B virus small surface (S) antigen particle to cloned murine cytotoxic T lymphocytes (CTL). Efficient L^d-restricted presentation of the S_28–39 epitope to CTL is observed in cells of different tissue origin pulsed in vitro, either with the antigenic S_28–39 12-mer S-peptide, or with particulate S-antigen. The kinetics of epitope presentation differ in S-peptide-pulsed and in S-particle-pulsed cells: while a 15-min pulse with the antigenic peptide sensitizes targets for class I-restricted CTL lysis, presentation of S-particles requires 30–60 min to sensitize cells for CTL lysis. Uptake of antigenic material and active metabolism of the presenting cell are required for processing of S-particles, but not for sensitizing targets with S-peptides. Intracellular processing and presentation of S-particles is blocked in cells treated with chloroquine, NH₄Cl, primaquine, or leupeptin, but not by treatment with cycloheximide or brefeldin A. This processing pathway operates efficiently in peptide-transporter-deficient, L^d-transfected T2 cells, revealing a novel endosomal/lysosomal processing pathway for class I-restricted presentation of peptides derived from exogenous S-particles.     

Epitope cleavage by Leishmania endopeptidase(s) limits the efficiency of the exogenous pathway of major histocompatibility complex class I-associated antigen presentation

The activation of CD8⁺ T cell responses is commonplace during infection with a number of nonviral pathogens. Consequently, there has been much interest in the pathways of presentation of such exogenous antigens for major histocompatibility complex class I-restricted recognition. We had previously shown that Leishmania promastigotes transfected with the ovalbumin (OVA) gene could efficiently target OVA to the parasitophorous vacuole (PV), with subsequent recognition by class II-restricted T cells. We now report the results of studies aimed at evaluating the PV as a route of entry into the exogenous class I pathway. Bone marrow-derived macrophages can present soluble OVA (albeit at high concentrations) to the OVA_257–264-specific T cell hybridoma 13.13. In contrast, infection with OVA-transfected Leishmania promastigotes failed to result in the stimulation of this hybridoma. This appeared unrelated to variables such as antigen concentration, parasite survival, and macrophage activation status. These results prompted an analysis of the effects of promastigotes on class I peptide binding using RMA-S cells and OVA_257–264. Our data indicate that the major surface protease of Leishmania, gp63, inhibits this interaction by virtue of its endopeptidase activity against the OVA_257–264 peptide. The data suggest that this activity, if maintained within the PV, would result in loss of the OVA_257–264 epitope. Although we can therefore draw no conclusions from these studies regarding the efficiency of the PV as a site of entry of antigen into the exogenous class I pathway, we have identified a further means by which parasites may manipulate the immune repertoire of their host.     

Trypanosoma cruzi-infected macrophages are defective in major histocompatibility complex class II antigen presentation

Trypanosoma cruzi, the intracellular protozoan parasite that causes Chagas' disease, interferes with the host immune response to establish a persistent infection. In this report, we demonstrate that macrophages infected with T. cruzi are unable to effectively present antigens to CD4 T cells. The interference is due to defective antigen-presenting cell (APC) function, as antigen-independent stimulation of the T cell in the presence of infected macrophages is not affected. The defect is distal to antigen processing and is not due to decreased major histocompatibility complex (MHC) class II expression, decreased viability, defective peptide loading in the infected macrophages, nor absence of CD28 co-stimulation. There was a role for gp39:CD40 co-stimulation during antigen presentation to the T cells we studied, but the expression of CD40 on T. cruzi-infected macrophages was not decreased. Antigen-specific adhesion between macrophages and T cells was reduced by infection. Equivalent levels of the adhesion molecules lymphocyte function-associated antigen-1, intercellular adhesion molecule-1, vascular cell adhesion molecule-1 or very late antigen-4 are found on infected and uninfected APC, suggesting that reduced expression of these adhesion molecules was not responsible for the defect in antigen-specific adhesion. The defective T cell:macrophage adhesion may be due to the reduced expression of other adhesion molecules or other changes in the cell induced by infection. Interfering with MHC class II antigen presentation in infected macrophages may help T. cruzi to blunt the immune response by the host.     

Endogenous antigen presentation by MHC class II molecules

T cell recognition of antigen requires that a complex form between peptides derived from the protein antigen and cell surface glycoproteins encoded by genes within the major histocompatibility complex (MHC). MHC class II molecules present both extracellular (exogenous) and internally synthesized (endogenous) antigens to the CD4 T cell subset of lymphocytes. The mechanisms of endogenous antigen presentation are the subject of this review. Isolation and amino acid sequencing of peptides bound to the class II molecule indicate that a very high proportion (70–90%) of the total peptides presented by the class II molecule are in fact derived from the pool of proteins that are synthetized within the antigen-presenting cell (APC). This type of sequence information as well as the study of model antigens has indicated that proteins expressed in a diversity of intracellular sites, including the cell surface, endoplasmic reticulum and cytosol can gain access to the class II molecule, albeit with different efficiencies. The main questions that remain to be answered are the intracellular trafficking patterns that allow colocalization of internally synthesized antigens with the class II molecule, the site(s) within the cell where peptide: class II molecule complex formation can take place and whether presentation of ‘foreign’ as well as ‘self’ antigens takes place by mechanisms that vary from one cell type to another or that vary with the metabolic state of the APC. If such variability exists, is would imply that the array of peptides displayed by class II molecules at the cell surface has similar variability, a possibility that would impact on self tolerance and autoimmunity.     

A fungal metabolite which inhibits the interaction of CD4 with major histocompatibility complex-encoded class II molecules

CD4, a cell-surface glycoprotein expressed on a subpopulation of T cells, is the receptor for class II molecules of the major histocompatibility complex (MHC II) and a receptor for the envelope glycoprotein (gp 120) of human immunodeficiency virus-1 (HIV-1). Screening of microbial metabolites for CD4-binding activity using an enzyme-linked immunosorbent assay based on the binding of the CD4-specific monoclonal antibody (mAb), anti-Leu3a, identified a family of compounds comprising several novel polyketides. The parent compound (411F, Vinaxanthone) is a C₂₈ molecule probably arising from a dimerization of two C₁₄ polyketide units. It strongly inhibited the interaction of anti-Leu 3a and that of several other D1/D2 epitope-specific mAb with CD4, but only weakly inhibited the binding of HIV-1 gp120. Binding of a representative MHC class II molecule, HLA-DRB*0401, was also inhibited by 411F with a comparable inhibitory concentration (IC₅₀ = 1 μM ). In functional assays 411F inhibited antigen-induced CD4-dependent T cell proliferative responses of peripheral blood mononuclear cells. At the clonal level 411F exhibited selectivity in that the compound inhibited peptide-induced CD4⁺ T cell proliferative responses but not alloantigen-induced CD8⁺ T cell proliferation. It is hypothesized that 411F, a polyanionic compound in aqueous solution at neutral pH, inhibits CD4-dependent functions by binding over a broad area of the positively charged amino-terminal D1 and D2 domains implicated in the interaction with MHC II molecules. 411F has the potential for development as an immunosuppressive agent with a novel mechanism of action.     

Recognition of altered self major histocompatibility complex molecules modulated by specific peptide interactions

Antigen-specific and major histocompatibility complex (MHC)-restricted recognition by the T cell receptor involves multiple structural contacts over a large molecular surface area. Using a human T cell clone specific for a rubella viral peptide restricted by subsets of HLA DR4 molecules, we identified structurally diverse combinations of peptide-MHC complexes which were functionally equivalent for T cell recognition. Presentation of the rubella-derived peptide on DR4 molecules with an E-74 polymorphism triggered T cell recognition, as did presentation of a single amino acid-substituted peptide in the context of the DR4 molecule which lacked the E-74 site. Peptide binding and molecular modeling analysis indicates the structural and functional complementarity of T cell recognition for a specific amino acid side chain, whether contributed by the peptide or by the MHC molecule.     

Invriant 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 A^d, A^k, E^d, and E^k 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 antigenpresenting 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.     

Soluble major histocompatibility complex molecules in immune regulation: highlighting class II antigens

The involvement of major histocompatibility complex (MHC) antigens in the development and regulation of immune response has been well defined over the years, starting from maturation, antigenic peptide loading, migration to the cell membrane for recognition by the T‐cell receptor and recycling for immune response cessation. During this intracellular trafficking, MHC antigens find a way to be excreted by the cells, because they can be found as soluble MHC class I (sMHC‐I) and class II (sMHC‐II) molecules in all body fluids. Although secretion mechanisms have not been sufficiently studied, sMHC molecules have been shown to display important immunoregulatory properties. Their levels in the serum have been shown to be altered in a variety of diseases, including viral infections, inflammation, autoimmunities and cancer, etc. while they seem to be involved in a number of physiological reactions, including maintenance of tolerance, reproduction, as well as mate choice vis‐à‐vis species evolution. The present review aims to present the thus far existing literature on sMHC molecules and point out the importance of these molecules in the maintenance of immune homeostasis.