Association of H2Ab with resistance to collagen-induced arthritis in H2-recombinant mouse strains: an allele associated with reduction of several apparently unrelated responses

A survey of H2 gene sequences, including new wild-derived genes

A comprehensive collection of mouse major histocompatibility complex (MHC) promoter and exon 2 sequences is here presented and analysed. It covers the three best known class II genes and one class I gene, and includes new wild mouse sequences from the 'w' back-cross strains and from the Jackson collection. All sequences are in GenBank, and the new exon sequences largely confirm previous typing by serology and immune function. As in human leucocyte antigen (HLA), the overall nucleotide diversity is higher in the class II genes, in keeping with their more diverse function. Diversity along the promoters is highest in the region of known transcription factor binding, most notably in and around the CRE and rCAAT sequences. This distribution parallels that of maximum single nucleotide polymorphism impact previously obtained with reporter constructs. Taking into account the low nucleotide diversity of the CIITA promoter, we conclude that MHC promoters are likely to have diversified through co-evolution with their exons, while themselves also directly subject to natural selection. The H2Eb(p) alleles form a distinct group, associated with their lack of the recombination hot spot located between exon 2 and exon 3. The collection is expected to prove useful in guiding functional and evolutionary studies.

Multiple functions for CD28 and cytotoxic T lymphocyte antigen-4 during different phases of T cell responses: implications for arthritis and autoimmune diseases

Chronic T cell responses, as they occur in rheumatoid arthritis, are complex and are likely to involve many mechanisms. There is a growing body of evidence that, in concert with the T cell antigen receptor signal, CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4; CD152) are the primary regulators of T cell responses. Whereas CD28 primarily activates T cell processes, CTLA-4 inhibits them. The mechanism for this dichotomy is not fully understood, especially as CD28 and CTLA-4 recruit similar signalling molecules. In addition, recent studies demonstrate that CD28 and CTLA-4 have multiple functions during T cell responses. In particular, CTLA-4 exerts independent distinct effects during different phases of T cell responses that could be exploited for the treatment of rheumatoid arthritis.

Natural and induced regulation of Th1/Th2 balance

Because Th1/Th2 balance is perturbed during immunological disease, the design of strategies aiming at its rectification has become a priority. The alteration of the balance in pregnancy so as to promote survival of the fetal allograft lends credibility to this aim. Attenuation of the activation signal delivered through the T cell receptor (TCR) represents a promising approach. It is supported by the high level of polymorphism in the MHC class II promoter, which regulates the natural TCR signal and thus modulates Th1/Th2 differentiation. Further support comes from the Th2 shift that occurs in JNK knockout mice, and with kinase inhibitors and anti-CD4 monoclonal antibodies applied in vitro. The approach has implications for nasal tolerance and inhibition of IL-12 production. The further range of options for Th1/Th2 modulation, which are presented throughout this issue of the journal, are here summarised and evaluated.

Recessive expression of the H2A-controlled immune response phenotype depends critically on antigen dose

Major histocompatibility complex (MHC) alleles acting as immune response genes are coexpressed in heterozygous individuals and therefore control of immune responses is usually codominant. As an exception to this rule, however, several examples of recessive immune responses have been ascribed to regulatory, e.g. suppressive, interactions. We report here that the recessive phenotype of both antibody and T-cell responses to the mycobacterial 16 000-MW antigen depends critically on a low antigen dose for immunization. On the basis of similar responses in hemi- and heterozygous mice, we suggest that the mechanism of recessive MHC control does not involve regulation by the low-responder allele. We also demonstrated mixed haplotype restriction of peptide recognition for a significant fraction of high-antigen-dose primed T cells. Their paucity under limiting antigen dose conditions may lead to the recessive expression of MHC control. In conclusion, our results suggest that recessive MHC control can be explained as a simple gene dosage effect under conditions where antigen is limiting, without a need for regulatory mechanisms.

Natural variation in immune responsiveness, with special reference to immunodeficiency and promoter polymorphism in class II MHC genes

This review deals with natural selection operating on heterozygotes as a key factor controlling (a) the frequency of immunodeficiencies, and (b) promoter polymorphism in MHC class II genes. The known difference in frequency distribution of X-linked and autosomal deficiencies lend support to this possibility, and suggest that the frequency of neonatal defect may rise as old-established equlibria between entry and exit of deleterious mutations change. MHC class II gene promoters differ in their capacity to favor Th1 (or reciprocally Th2) responses, thus suggesting that promoter polymorphism is sustained by the greater flexibility in response that this confers on heterozygotes.

Differential surface expression of HLA-DRB1 and HLA-DRB4 among peripheral blood cells of DR4 positive individuals

Increasing interest in the functional consequences of differential expression of MHC class II molecules prompted us to examine the surface expression of HLA class II molecules on fresh peripheral blood mononuclear cells. Differential regulation of DR4 and DRB4 was shown for peripheral blood monocytes. In addition, DR4 expression is upregulated on B cells of patients suffering from chronic inflammation and is reduced under prednisolone-treatment. The expression levels of total DR molecules on a given cell type are almost identical comparing different haplotypes among non-RA controls, suggesting that the alpha-chain determines the level of surface expression. The present findings fit the hypothesis that the differential expression of HLA class II molecules is involved in regulation of the immune response and may thus contribute to determining susceptibility to immunological diseases.

Bystander suppression of murine collagen-induced arthritis by long-term nasal administration of a self type II collagen peptide

Oral and more recently nasal tolerance have attracted attention as potential treatments of autoimmune disease. Arthritis induced by bovine type II collagen (CII) is a widely used animal model of rheumatoid arthritis, which is here used to investigate the efficacy of nasal treatment by a short peptide. The peptide spans residues 707-721 (designated p707), an epitope of mouse CII that is most strongly recognized after immunization of mice with this self-protein. The treatment was partially effective, but almost only when the peptide was administered in large doses over a prolonged period. Mice immunized with bovine CII respond mainly to other peptides, located in the CB11 fragment around amino acid residues 256-270. The tolerance effect therefore results from intramolecular suppression, between epitopes located in different parts of this large protein.

Differential expression of major histocompatibility complex class II genes on murine macrophages associated with T cell cytokine profile and protective/suppressive effects

Protective/suppressive major histocompatibility complex (MHC) class II alleles have been identified in humans and mice where they exert a disease-protective and immunosuppressive effect. Various modes of action have been proposed, among them differential expression of MHC class II genes in different types of antigen-presenting cells impacting on the T helper type 1 (Th1)-Th2 balance. To test this possibility, the expression of H-2 molecules from the four haplotypes H-2(b), H-2(d), H-2(k), and H-2(q) was determined on bone marrow-derived macrophages (BMDMs) and splenic B cells. The I-Ab and I-Ek molecules, both well characterized as protective/suppressive, are expressed at a high level on almost all CD11b+ BMDMs for 5-8 days, after which expression slowly declines. In contrast, I-Ad, I-Ak, and I-Aq expression is lower, peaks over a shorter period, and declines more rapidly. No differential expression could be detected on B cells. In addition, the differential MHC class II expression found on macrophages skews the cytokine response of T cells as shown by an in vitro restimulation assay with BMDMs as antigen-presenting cells. The results indicate that macrophages of the protective/suppressive haplotypes express MHC class II molecules at a high level and exert Th1 bias, whereas low-level expression favors a Th2 response. We suggest that the extent of expression of the class II gene gates the back signal from T cells and in this way controls the activity of macrophages. This effect mediated by polymorphic nonexon segments of MHC class II genes may play a role in determining disease susceptibility in humans and mice.

Collagen-induced arthritis, an animal model of autoimmunity

Collagen induced arthritis (CIA) is an autoimmune model that in many ways resembles rheumatoid arthritis (RA). Immunization of genetically susceptible strains of rodents and primates with type II collagen (CII) leads to the development of a severe polyarticular arthritis that is mediated by an autoimmune response. Like RA, synovitis and erosions of cartilage and bone are hallmarks of CIA, and susceptibility to both RA and CIA is linked to the expression of specific MHC class II molecules. Although not identical to RA, CIA clearly establishes the biological plausibility that an autoimmune reaction to a cartilage component can lead to a chronic, destructive, polyarthritis. Although it is induced in susceptible animals by immunization with heterologous CII, it is the autoreactive component of the immune response that leads to disease. A wealth of evidence indicates that synovitis is initiated by the production of pathogenic autoreactive antibodies capable of fixing and activating complement. The elucidation of the specific amino acid sequences of collagen that are recognized by the MHC molecules has enabled at least two approaches to specific immunotherapy to be considered. Firstly, small synthetic peptides representing dominant epitopes have been used as effectively as the original antigen as a tolerogen. The rather fastidious physicochemical properties of collagen that make it difficult for its routine use in therapy are thereby circumvented by the use of oligopeptides. Secondly, analysis of the specific amino acid side chains that are involved in MHC contact and TCR recognition enables analog peptides to be devised which can specifically and exquisitely inhibit the response to CII, preventing the onset of arthritis. Further investigations involving this model may contribute to the development of specific immunotherapies in the human disorder.

The importance of the back-signal from T cells into antigen-presenting cells in determining susceptibility to parasites

It has long been known that certain MHC class II genes can dominantly suppress immune responses and so increase susceptibility to parasite infections, but the mechanism has been unclear. Recent work has revealed one way in which this form of suppression may operate, through gating by MHC class II molecules of the back-signal from activated T cells into macrophages. The two known suppressive genes of the mouse are expressed in macrophages more extensively than are other class II genes. This is associated with suppression of IL-4 production resulting, we infer, from overproduction in the macrophages of IL-12, the counter-cytokine to IL-4. The lack of IL-4 may itself be immunosuppressive, even for Th2 responses, and excess IL-12 can overinduce the antiproliferative cytokine IFN-gamma. Although this mechanism requires further substantiation, we believe that it offers a reasonable answer to an old conundrum.

Polymorphic MHC class II promoters exhibit distinct expression pattern in various antigen-presenting cell lines

The promoter regions of MHC class II genes are characterized by the presence of conserved sequence motifs called S,X and Y boxes, which are crucial for regulation of transcription of these genes. In humans, promoter polymorphism is known to result in differential transcriptional activity at both inter-locus and inter-allelic levels, but it is not yet known how this relates to tissue-specific expression of MHC class II molecules. We sequenced the 5' regulatory regions of alpha and beta genes of I-A and I-E molecules from four mouse haplotypes and found allelic polymorphisms which were mainly confined to the X box. The promoter sequences of I-Ea genes were non-polymorphic. Transfection of four antigen-presenting cell types with promoter-reporter gene constructs revealed that the promoter sequence polymorphisms result in distinct allele- and tissue-specific activity patterns. Mutagenesis experiments in which the X2 box was reshuffled between I-A beta alleles demonstrated that this box contributes to regulation of differential MHC class II expression in the four cell types. The possibility is discussed that tissue-specific MHC class II expression may control differentiation of T-cell subsets.

Functional significance of polymorphism among MHC class II gene promoters

The functional significance of polymorphism among MHC class II promoters in man and mouse is here reviewed, mainly in terms of the hypothesis of differential expression. The hypothesis proposes that differences between antigen-presenting cells in MHC class II expression exert a co-dominant effect on the Th1-Th2 cytokine balance, such that class II molecules of one type come to control to a greater extent the production of one group of cytokines, and those of another type the production of the alternative group. The survey deals with the influence of signal strength and antigen-presenting cell type on T-cell subset differentiation; functional differences between MHC class II molecules not obviously related to determinant selection; disease protection mediated by HLA alleles; mechanisms possibly responsible for allotypic and isotypic bias; overdominance (heterozygous advantage) in selection for expression of class II alleles; MHC class II promoter structure and function; inter-locus and inter-allele variability within human MHC class II gene upstream regulatory regions; a comparison of these polymorphisms in mouse and man; read-out of class II promoter function; and a comparison with expression of MHC class I. We conclude that the evidence that this variation is functionally active (i.e. controls expression) is increasing, but is not yet compelling. The crucial test still to come, we suggest, is whether or not the biological effects attributable to this polymorphism will line up with molecular studies on expression.

Protective major histocompatibility complex genes and the role of interleukin-4 in collagen-induced arthritis

To investigate the role of interleukin (IL)-4 during the triggering of collagen-induced arthritis, we examined the effects of the I-A(b) and I-E protective/suppressive genes and passively administered anti-IL-4 monoclonal antibody. In contrast to the action of I-E expression on its own, which has mainly a suppressive effect post-triggering, the combination of I-A(b) and I-E had a marked protective effect. Assuming, on the basis of previous experience with the I-A(b) allele, that it might act through suppressing early IL-4 production, we treated mice with the 11B11 IL-4-neutralizing antibody around the time of initial immunization with collagen. Treatment over a period extending to 6 days post-immunization exacerbated the arthritis, but when curtailed to 2 days post-immunization (and tested in pristane-primed animals), the disease was reduced. We conclude that IL-4 plays an essential role in triggering the disease.

Altered Th1/Th2 balance associated with the immunosuppressive/protective effect of the H-2Ab allele on the response to allo-4-hydroxyphenylpyruvate dioxygenase

The H-2Ab allele exerts a dominant down-regulatory effect on the anti-allo-HPPD (4-hydroxyphenylpyruvate dioxygenase) antibody response, through a hitherto unknown mechanism. In the present study, the allo-variable peptide bound to responder H-2Ak molecules with higher affinity than to H-2Ab ones, arguing against the operation of an affinity hierarchy. Quantitative polymerase chain reaction revealed differences in cytokine mRNA expression between suppressed and high-responder mice. Lymph node cells of responder but not suppressed mice contained high levels of interleukin (IL)-4 mRNA as early as 11 h post-immunization and continued to do so for at least 8 days; this early burst was paralleled by a small burst in transforming growth factor (TGF)-beta mRNA level. Differences in IL-12 mRNA were not detected, although an early IL-12 effect could not be excluded. Interferon (IFN)-gamma appeared to contribute to the suppression at later time points. Early treatment of responder mice with anti-IL-4 monoclonal antibody (11B11) down-regulated the antibody response. The proliferative T cell response from hyperimmunized mice was reduced but still detectable in the presence of an H-2Ab allele. Thus, in the presence of this allele, the Th1 response is enhanced and that of Th2 cells suppressed, apparently as a result of the bias of H-2Ab-restricted T cells in favor of the Th1 subset.