The cell biology of MHC class I antigen presentation

MHC class I antigen presentation refers to the co-ordinated activities of many intracellular pathways that promote the cell surface appearance of MHC class I/beta2m heterodimers loaded with a spectrum of self or foreign peptides. These MHC class I peptide complexes form ligands for CD8 positive T cells and NK cells. MHC class I heterodimers are loaded within the endoplasmic reticulum (ER) with peptides derived from intracellular proteins. Alternatively, MHC class I molecules may be loaded with peptides derived from extracellular proteins in a process called MHC class I cross presentation. This pathway is less well defined but can overlap those pathways operating in classical MHC class I presentation and has recently been reviewed elsewhere (1). This review will address the current concepts regarding the intracellular assembly of MHC class I molecules with their peptide cargo within the ER and their subsequent progress to the cell surface.     

DNA content and MHC class II antigen expression in malignant melanoma: clinical course.

To assess the clinical value of two comparatively new properties (DNA content and MHC class II antigen expression (HLA-DR, DP, DQ) of melanoma cells) which have been independently reported to reflect the outlook for patients with malignant melanoma, we investigated retrospectively 50 stage I nodular melanomas in two comparably homogeneous groups of 23 and 27 patients, the course of whose disease differed at five years. Flow cytometry and immunohistology were used on paraffin wax embedded archival material for the analysis of DNA ploidy and detection of class II antigens, respectively. A close association was found between class II antigen expression, detected by monoclonal antibody CR3/43 (antimonomorphic DR, DP, DQ) present in 23 of 50 (46%) melanomas and unfavourable clinical course (p less than 0.005, by log rank test), but no such association was found for DNA ploidy. It is suggested that immunohistology for MHC class II antigen expression may help to predict the behaviour of nodular melanomas whereas the prognostic value of DNA ploidy is more limited. The finding that class II positive cells are found predominantly in melanomas with a substantially increased risk of metastases has implications both for concepts of tumour heterogeneity and host immunity.     

MHC class II antigen expression in normal human epidermis

Monoclonal antibodies consistently demonstrated the presence of MHC class II antigens (HLA-DR,-DP and -DQ) on keratinocytes in normal human epidermis. Reactivity was normally greatest on the keratinocytes of the intraepidermal portion of sweat ducts or the external root sheath of hair follicles, but staining was noted on the surface of some interappendageal keratinocytes in most subjects. The patterns were varied but distinctive and depended on the antibody used. The functional importance of the MHC class II antigens expressed on normal keratinocytes remains to be investigated.     

Axotomy induces MHC class I antigen expression on rat nerve cells

Immunomorphological staining demonstrates that class I major histocompatibility complex (MHC)-coded antigen expression can be selectively induced on otherwise class I-negative rat nerve cells by peripheral axotomy. Induction of class I as well as class II antigen expression was simultaneously seen on non-neural cells in the immediate vicinity of the injured nerve cells. As nerve regeneration after axotomy includes growth of new nerve cell processes and formation of new nerve cell contacts, the present findings raise the question of a role for MHC-coded molecules in cell-cell interactions during nerve cell growth.     

Fc receptors for IgG and antigen presentation on MHC class I and class II molecules

Antigens internalized through specific membrane receptors are presented to helper CD4(+) T cells at antigen concentrations 10(3) to 10(4) fold lower than antigens internalized by fluid phase. B lymphocyte antigen receptors, mannose receptors and receptors for the Fc region of immunoglobulins, promote both internalization and efficient presentation at low antigen concentrations. Thus, binding to specific membrane receptors concentrate antigens on antigen presenting cells and mediates efficient uptake. Is this 'quantitative' concentration of antigens on antigen presenting cells the end of the story? Or may 'quality', i.e. selective intracellular antigen targeting, somehow influence the efficiency or specificity of MHC class I and class II-restricted antigen presentation?     

Routes to manipulate MHC class II antigen presentation

MHC class II molecules (MHC-II) present antigenic fragments acquired in the endocytic route to the immune system for recognition and activation of CD4+ T cells. This ignites a series of immune responses. MHC-II strongly correlates to most autoimmune diseases. Understanding the biology of MHC-II is therefore expected to translate into novel means of autoimmunity control or immune response improvement. Although the basic cell biology of MHC-II antigen presentation is well understood, many novel aspects have been uncovered in recent years including means of antigen delivery, preparation for MHC-II loading, transport processes and vaccination strategies. We will discuss past, present and future of these insights into the biology of MHC-II.     

Enhancement of MHC class II antigen expression by exposure to Candida albicans.

In oral infections with the yeast Candida albicans, the expression of MHC class II antigens on keratinocytes has been reported to be enhanced. In the present experiments, exposure to C. albicans or its products in vitro was found to increase the expression of class II MHC antigens on thioglycollate-induced mouse macrophages, and on LK cells (an antigen-presenting cell line). The implications of this finding for the understanding of immunoregulation and susceptibility to C. albicans infection are discussed.     

Immunohistological relation between DR antigen expression on melanoma cells and infiltration by CD8+ T cells

Previous studies have suggested that class II major histocompatibility (MHC) antigen expression on melanoma cells may influence immune responses against melanoma and the nature of lymphocytic infiltrates against the tumor. This question was examined further by immunoperoxidase studies on sections from 29 primary and 30 metastatic melanoma with monoclonal antibodies (MAbs) against different lymphocyte subsets. The results indicated that expression of MHC class II DR* antigens on melanoma cells was associated with increased overall lymphocytic infiltration and that this applied particularly to the CD8+ subset of T cells. The CR3 receptor (CD 11b) was expressed predominantly on T cells and not macrophages but infiltration by CD11b+ cells did not correlate strongly with DR expression on melanoma cells. Dual staining with MAbs to CD8 and CD11b revealed that, whereas most of the CD8+ cells in DR- primary melanoma and DR+ metastatic melanoma were CD11b+, only approximately 50% of the CD8+ cells in DR+ primary melanoma were also CD11b+. Expression of CD11b on T cells was reported to define a suppressor subset of T cells. If the latter is correct the present results suggest that DR expression on primary melanoma is associated with infiltration by the cytotoxic T cell subset, whereas in the absence of DR antigens and in metastases this subset is absent and the predominant subset appears to be CD8+ CD11b+ T cells with suppressor activity. The biologic and prognostic significance of these findings remains to be determined.     

MHC class I and class II antigen expression in normal human corneas and in corneas from cases of herpetic keratitis

The expression of HLA class I and class II antigens in corneas from normal donors and patients with quiescent herpetic keratitis was investigated using specific monoclonal antibodies. Keratocytes from diseased corneas showed aberrant expression of HLA class I and class II (DR, DP and DQ) antigens. The expression of HLA antigens in these corneas was not associated with immune cell infiltrates or viral antigens.     

MHC class II antigen presentation and immunological abnormalities due to deficiency of MHC class II and its associated genes

Antigen presentation by Major Histocompatibility Complex (MHC) class II molecules plays an important role in controlling immunity and autoimmunity. Multiple co-factors including the invariant chain (Ii), HLA-DM and HLA-DO are involved in this process. While the role for Ii and DM has been well defined, the biological function of DO remains obscure. Our data indicate that DO inhibits presentation of endogenous self-antigens and that developmentally-regulated DO expression enables antigen presenting cells to preferentially present different sources of peptide antigens at different stages of development. Disruption of this regulatory mechanism can result in not only immunodeficiency but also autoimmunity. Despite the fact that deletion of each of the three genes in experimental animals is associated with profound immunological abnormalities, no corresponding human diseases have been reported. This discrepancy suggests the possibility that primary immunodeficiencies due to a genetic defect of Ii, DM and DO in humans are under diagnosed or diagnosed as “common variable immunodeficiency”, a category of immunodeficiency of heterogeneous or undefined etiology. Clinical tests for any of these potential genetic defects are not yet available. We propose the use of multi-color flow cytometry in conjunction with intracellular staining to detect expression of Ii, DM, DO in peripheral blood B cells as a convenient reliable screening test to identify individuals with defects in antigen presentation.     

Transcriptional control of MHC class II antigen expression on mouse T cell lines

We have analysed the factors which regulate MHC class II expression in mouse T cell lines. Two such lines, BW 5147 and PLT-24.2, were used in this study. Using 5-azacytidine (5 AzaC) we have shown that hypomethylation of DNA can induce class II antigen synthesis in BW 5147. The expression of class II in PLT-24.2 cells seems to be under a different control mechanism. Southern blot analysis of I-A beta gene in PLT-24.2 suggests that the expression of class II in this cell line is probably the outcome of a gene rearrangement. We hypothesise that insertion of viral long terminal repeats (LTR) next to the class II genes in transformed T cell lines can act as a promoter for the expression of class II antigens.     

IgG- and IgE-mediated antigen presentation on MHC class II

IgG- and IgE-antibodies have the ability to enhance the production of antibodies directed against the antigen they are specific for. It has been suggested that the mechanism behind IgG- and IgE-mediated feedback enhancement is the ability of these isotypes to induce a more potent antigen-specific T helper cell response, increasing the chances that antigen-specific B cells receive the T cell help they require to become antibody-producing cells. With emphasis on the murine system, we will here focus on the ability of IgG and IgE to capture antigen and facilitate presentation of antigenic peptides to T helper cells. Whether this mechanism underlies feedback enhancement of antibody responses to these antigens will be discussed.     

Structure of MHC class I and class II cDNAs and possible immunodeficiency linked to class II expression in the Mexican axolotl

Summary: Despite the fact that the axolotl (Ambystoma spp. a urodele amphibian) displays a large T-cell repertoire and a reasonable B-cell repertoire, its humoral immune response is slow (60 days), non-anamnestic, with a unique IgM class. The cytotoxic immune response is slow as well (21 days) with poor mixed lymphocyte reaction stimulation. Therefore, this amphibian can be considered as immunodeficient. The reason for this subdued immune response could be an altered antigenic presentation by major histocompatibility complex (MHC) molecules. This article summarizes our work on axolotl MHC genes. Class I genes have been characterized and the cDNA sequences show a good conservation of non-polymorphic peptide binding positions of the a chain as well as a high diversity of the variable amino acids positions, suggesting that axolotl class I molecules can present numerous antigenic epitopes. Moreover, class I genes are ubiquitously transcribed at the lime of hatching. These class 1 genes also present an important polylocism and belong to the same linkage group as the class II B gene, they can be reasonably considered as classical class 1a genes. However, only one class II B gene has been characterized so far by Southern blot analysis. As in higher vertebrates, this gene is transcribed in lymphoid organs when they start to be functional. The sequence analysis shows that the peptide binding region of this class II β chain is relatively well conserved, but most of all does not present any variability in the β domain in inbred as well as in wild axolotls presuming a limited antigenic presentation of few antigenic epitopes. The immunodeficiency of the axolotl could then be explained by an altered class 11 presentation of antigenic peptides, putting into question the existence of cellular co-operation in this lowervertebrate. It will be interesting to analyze the situation in other urodele species and to determine whether our observations in axolotl represent a normal feature in urodele amphibians. But already two different models in amphibians, Xenopus and axolotl, must be considered in our search for understanding immune system and MHC evolution.     

Regulation of IFN-gamma-induced host cell MHC antigen expression by Kirsten MSV and MLV. I. Effects on class I antigen expression

We have reported previously that the Kirsten murine sarcoma virus (Ki-MSV) that carries the v-Ki-ras oncogene prevents C3H10T 1/2 fibroblasts from being able to respond to interferon-gamma (IFN-gamma) with the expression of the class II major histocompatibility complex (MHC) antigen, H-2A. In this report we investigate further as to whether MSV or its parent virus Kirsten murine leukaemia virus (Ki-MLV) is able to reduce host class I MHC antigen expression. The results demonstrate that class I expression is diminished in MSV-infected cells over a time-course of 7 days after exposure to IFN-gamma and over a range of IFN-gamma concentrations. The optimal concentration of IFN-gamma for maximal class I expression remained unchanged. Cells infected with Ki-MLV, which failed to abolish the induction by IFN-gamma of class II antigens, also expressed lower levels of class I antigens, similar to those for cells infected with Ki-MSV, after exposure to IFN-gamma. It is likely therefore that the inhibition of class I induction is due to genetic material shared between the viruses, principally in the long terminal repeats (LTR), and hence that the mechanism of action is distinct from that responsible for the abolition of class II induction by Ki-MSV alone. Since class I antigens are required for CD8+ T cells (mainly cytotoxic T cells) to recognize (foreign) antigen this reduction in class I expression might lead to reduced visibility of infected cells to T cells and thus might contribute to the tumorigenicity of Ki-MSV-infected cells.     

Loss of polymorphic restriction fragments of class I and class II MHC genes in a malignant melanoma

DNAs from human malignant melanoma cells and autologous peripheral blood lymphocytes were evaluated by Southern blot analysis with probes for class I and II HLA genes. DNA of melanoma cells digested with PvuII, EcoRI and BglI and hybridized with a DR beta probe showed a loss of several fragments when compared with DNA from lymphocytes. The same DNAs were not distinguishable when hybridized with a DQ beta probe. Analysis of melanoma and autologous lymphocyte DNAs from the same patient with a class I cDNA, after digestion with several endonucleases, revealed a further loss of fragments in melanoma cells. Comparison of restriction fragment patterns of melanoma and lymphocytes with those of homozygous, serologically-typed cell lines indicated that melanoma cells have lost fragments diagnostic of DR2 and A1 antigens. A densitometric analysis of signals of several oncogenes in comparison with that of DR indicated that a duplication of the remaining DR allele had occurred in melanoma cells.     

Structural principles of MHC class II antigen presentation

Normal immune surveillance depends on the ability of MHC class II molecules to bind peptide antigens and carry them to the cell surface for display to T cells. To do this efficiently, class II molecules must be able to bind peptides from a broad array of antigen sequences and retain them at the cell surface long enough for T-cell recognition to occur. Class II molecules accomplish this task through a combination of clever structural biochemistry and the help of at least two different molecular chaperones: the class II-associated invariant chain (Ii); and a non-peptide binding class II molecule termed H2-DM in mouse and HLA-DM in man (DM). Here, we compare the existing 3-dimensional structures of class II-peptide complexes in order to review the general principles of peptide binding and presentation. We extend this analysis to include the structures of proteins known to interact with MHC class II, focusing primarily on the Ii chain and DM.