Biosynthesis and Function of Class II Molecules
The intracellular assembly of class II molecules occurs within a specialized compartmentalized pathway that differs dramatically from the class I pathway described above. As illustrated in Fig. 2-3B, the class II molecule assembles in the ER in association with a chaperone molecule, known as the invariant chain. The invariant chain performs at least two roles. First, it binds to the class II molecule and blocks the peptide-binding groove, thus preventing antigenic peptides from binding. This role of the invariant chain appears to account for one of the important differences between class I and class II MHC pathways, since it can explain why class I molecules present endogenous peptides from proteins newly synthesized in the ER but class II molecules generally do not. Second, the invariant chain contains molecular localization signals that direct the class II molecule to traffic into post-Golgi compartments known as endosomes, which develop into specialized acidic compartments where proteases cleave the invariant chain, and antigenic peptides can now occupy the class II groove. The specificity and tissue distribution of these proteases appear to be an important way in which the immune system regulates access to the peptide-binding groove and T cells become exposed to specific self-antigens. Differences in protease expression in the thymus and in the periphery may in part determine which specific peptide sequences comprise the peripheral repertoire for T cell recognition. It is at this stage in the intracellular pathway, after cleavage of the invariant chain, that the MHC-encoded DM molecule catalytically facilitates the exchange of peptides within the class II groove to help optimize the specificity and stability of the MHC-peptide complex.
Once this MHC-peptide complex is deposited in the outer cell membrane it becomes the target for T cell recognition via a specific TCR expressed on lymphocytes. Because the endosome environment contains internalized proteins retrieved from the extracellular environment, the class II-peptide complex often contains bound antigens that were originally derived from extracellular proteins. In this way, the class II peptide-loading pathway provides a mechanism for immune surveillance of the extracellular space. This appears to be an important feature that permits the class II molecule to bind foreign peptides, distinct from the endogenous pathway of class I-mediated presentation.
Role of HLA in Transplantation
The development of modern clinical transplantation in the decades since the 1950s provided a major impetus for elucidation of the HLA system, as allograft survival is highest when donor and recipient are HLA-identical. Although many molecular events participate in transplantation rejection, allogeneic differences at class I and class II loci play a major role. Class I molecules can promote T cell responses in several different ways. In the cases of allografts in which the host and donor are mismatched at one or more class I loci, host T cells can be activated by classic direct alloreactivity, in which the antigen receptors on the host T cells react with the foreign class I molecule expressed on the allograft. In this situation, the response of any given TCR may be dominated by the allogeneic MHC molecule, the peptide bound to it, or some combination of the two. Another type of host antigraft T cell response involves the uptake and processing of donor MHC antigens by host antigen-presenting cells and the subsequent presentation of the resulting peptides by host MHC molecules. This mechanism is termed indirect alloreactivity.
In the case of class I molecules on allografts that are shared by the host and the donor, a host T cell response may still be triggered because of peptides that are presented by the class I molecules of the graft but not of the host. The most common basis for the existence of these endogenous antigen peptides, called minor histocompatibility antigens, is a genetic difference between donor and host at a non-MHC locus encoding the structural gene for the protein from which the peptide is derived. These loci are termed minor histocompatibility loci, and nonidentical individuals typically differ at many such loci. CD4 T cells react to analogous class II variation, both direct and indirect, and class II differences alone are sufficient to drive allograft rejection.
Association of HLA Alleles With Susceptibility to Disease
It has long been postulated that infectious agents provide the driving force for the allelic diversification seen in the HLA system. An important corollary of this hypothesis is that resistance to specific pathogens may differ between individuals, based on HLA genotype. Observations of specific HLA genes associated with resistance to malaria or dengue fever, persistence of hepatitis B, and to disease progression in HIV infection are consistent with this model. For example, failure to clear persistent hepatitis B or C viral infection may reflect the inability of particular HLA molecules to present viral antigens effectively to T cells. Similarly, both protective and susceptible HLA allelic associations have been described for human papilloma virus-associated cervical neoplasia, implicating the MHC as an influence in mediating viral clearance in this form of cancer.
Pathogen diversity is probably also the major selective pressure favoring HLA heterozygosity. The extraordinary scope of HLA allelic diversity increases the likelihood that most new pathogens will be recognized by some HLA molecules, helping to assure immune fitness to the host. However, another consequence of diversification is that some alleles may become preferentially selective for recognition of self-antigens as well. Indeed, particular HLA alleles are strongly associated with certain disease states, particularly for some common autoimmune diseases (Chap. 3). By comparing allele frequencies in patients with any particular disease and in control populations, >100 such associations have been identified, some of which are listed in (Table 2-1). The strength of genetic association is reflected in the term relative risk, which is a statistical odds ratio representing the risk of disease for an individual carrying a particular genetic marker compared with the risk for individuals in that population without that marker.
TABLE 2-1
|
SIGNIFICANT HLA CLASS I AND CLASS II ASSOCIATIONS WITH DISEASE |
|||
|
|
MARKER |
GENE |
STRENGTH OF ASSOCIATION |
|
Spondyloarthropathies |
|||
|
Ankylosing spondylitis |
B27 |
B*2702, -04, -05 |
++++ |
|
Reiter’s syndrome |
B27 |
++++ |
|
|
Acute anterior uveitis |
B27 |
+++ |
|
|
Reactive arthritis (Yersinia, Salmonella, Shigella, Chlamydia) |
B27 |
+++ |
|
|
Psoriatic spondylitis |
B27 |
+++ |
|
|
Collagen-Vascular Diseases |
|||
|
Juvenile arthritis, pauciarticular |
DR8 |
++ |
|
|
DR5 |
++ |
||
|
Rheumatoid arthritis |
DR4 |
DRB1*0401, -04, -05 |
+++ |
|
Sjögren’s syndrome |
DR3 |
++ |
|
|
Systemic lupus erythematosus |
|||
|
Caucasian |
DR3 |
+ |
|
|
Japanese |
DR2 |
++ |
|
|
Autoimmune Gut and Skin |
|||
|
Gluten-sensitive enteropathy (celiac disease) |
DQ2 |
DQA1*0501 |
+++ |
|
DQB1*0201 |
|||
|
Chronic active hepatitis |
DR3 |
++ |
|
|
Dermatitis herpetiformis |
DR3 |
+++ |
|
|
Psoriasis vulgaris |
Cw6 |
++ |
|
|
Pemphigus vulgaris |
DR4 |
DRB1*0402 |
+++ |
|
DQ1 |
DQB1*0503 |
||
|
Bullous pemphigoid variant |
DQ7 |
DQB1*0301 |
+ |
|
Autoimmune Endocrine |
|||
|
Type 1 diabetes mellitus |
DR4 |
DQB1*0302 |
+++ |
|
DQ8 |
DRB1*0401, -04 |
||
|
DR3 |
++ |
||
|
DR2 |
DQB1*0602 |
_a |
|
|
Hyperthyroidism (Graves’) |
B8 |
+ |
|
|
DR3 |
+ |
||
|
Hyperthyroidism (Japanese) |
B35 |
+ |
|
|
Adrenal insufficiency |
DR3 |
++ |
|
|
Autoimmune Neurologic |
|||
|
Myasthenia gravis |
B8 |
+ |
|
|
DR3 |
+ |
||
|
Multiple sclerosis |
DR2 |
DRB1*1501 |
++ |
|
DRB5*0101 |
|||
|
Other |
|||
|
Behget’s disease |
B51 |
++ |
|
|
Congenital adrenal hyperplasia |
B47 |
21-OH (Cyp21B) |
+++ |
|
Narcolepsy |
DR2 |
DQB1*0602 |
++++ |
|
Goodpasture’s syndrome (anti-GBM) |
DR2 |
++ |
|
aStrong negative association; i.e., genetic association with protection from diabetes.
The nomenclature shown in Table 2-1 reflects both the HLA serotype (e.g., DR3, DR4) and the HLA genotype (e.g., DRB1*0301, DRB1*0401). It is very likely the class I and class II alleles themselves are the true susceptibility alleles for most of these associations. However, because of the extremely strong linkage disequilibrium between the DR and DQ loci, in some cases it has been difficult to determine the specific locus or combination of class II loci involved. In some cases, the susceptibility gene may be one of the HLA-linked genes located near the class I or class II region, but not the HLA gene itself, and in other cases the susceptibility gene may be a non-HLA gene, such as TNF-α, which is nearby. Indeed, since linkage disequilibrium of some haplotypes extends across large segments of the MHC region, it is quite possible that combinations of genes may account for the particular associations of HLA haplotypes with disease. For example, on some haplotypes associated with rheumatoid arthritis, both HLA-DRB1 alleles and a particular polymorphism associated with the TNF locus may be contributory to disease risk. Other candidates for similar epistatic effects include the IKBL gene and the MICA locus, potentially in combination with classic HLA class II risk alleles.
As might be predicted from the known function of the class I and class II gene products, almost all of the diseases associated with specific HLA alleles have an immunologic component to their pathogenesis. It should be stressed that even the strong HLA associations with disease (those associations with relative risk of ^10) implicate normal, rather than defective, alleles. Most individuals who carry these susceptibility genes do not express the associated disease; in this way the particular HLA gene is permissive for disease but requires other environmental (e.g., the presence of specific antigens) or genetic factors for full penetrance. In each case studied, even in diseases with very strong HLA associations, the concordance of disease in monozygotic twins is higher than in HLA-identical dizygotic twins or other sibling pairs, indicating that non-HLA genes contribute to susceptibility and can significantly modify the risk attributable to HLA.
Another group of diseases is genetically linked to HLA, not because of the immunologic function of HLA alleles, but rather because they are caused by autosomal dominant or recessive abnormal alleles at loci that happen to reside in or near the HLA region. Examples of these are 21-hydroxylase deficiency, hemochromatosis, and spinocerebellar ataxia.
Class I Associations With Disease
Although the associations of human disease with particular HLA alleles or haplotypes predominantly involve the class II region, there are also several prominent disease associations with class I alleles. These include the association of Behget’s disease (Chap. 11) with HLA-B51, psoriasis vulgaris with HLA-Cw6, and, most notably, the spondyloarthropathies (Chap. 9) with HLA-B27. Twenty-five HLA-B locus alleles, designated HLA-B*2701-B*2725, encode the family of B27 class I molecules. All of the subtypes share a common B pocket in the peptide-binding groove, a deep, negatively charged pocket that shows a strong preference for binding the arginine side chain. In addition, B27 is among the most negatively charged of HLA class I heavy chains, and the overall preference is for positively charged peptides. HLA-B*2705 is the predominant subtype in Caucasians and most other non-Oriental populations, and this subtype is very highly associated with ankylosing spondylitis (AS) (Chap. 9), both in its idiopathic form and in association with chronic inflammatory bowel disease or psoriasis vulgaris. It is also associated with reactive arthritis (ReA) (Chap. 9), with other idiopathic forms of peripheral arthritis (undifferentiated spondyloarthropathy), and with recurrent acute anterior uveitis. B27 is found in 50-90% of individuals with these conditions, compared with a prevalence of ~7% in North American Caucasians.The prevalence of B27 in patients with idiopathic AS is 90%, and in AS complicated by iritis or aortic insufficiency it is close to 100%. The absolute risk of spondyloarthropathy in unselected B27+ individuals has been variously estimated at 2-13% and >20% if a B27+ first-degree relative is affected. The concordance rate of AS in identical twins is very high, at least 65%. It can be concluded that the B27 molecule itself is involved in disease pathogenesis, based on strong evidence from clinical epidemiology and on the occurrence of a spondyloarthropathy-like disease in HLA-B27 transgenic rats. Both AS and ReA are associated with the B27 subtypes B*2702, -04, and -05, and anecdotal association has been reported for subtypes B*2701, -03, -07, -08, -10, and -11.
The association of B27 with these diseases may derive from the specificity of a particular peptide or family of peptides bound to B27 or through another mechanism that is independent of the peptide specificity of B27. The first alternative can be further subdivided into mechanisms that involve T cell recognition of B27-peptide complexes and those that do not. A variety of other roles for B27 in disease pathogenesis have been postulated, including molecular or antigenic mimicry between B27 and certain bacteria and reduced killing of intracellular bacteria in cells expressing B27. HLA-B27 has been shown to form heavy chain homodimers, utilizing the cysteine residue at position 67 of the B57 α chain. These homodimers are expressed on the surface of lymphocytes and monocytes from patients with AS, and receptors including KIR3DL1, KIR3DL2, and ILT4 are capable of binding to them. Whether these interactions contribute to disease susceptibility or pathogenesis is currently unknown.
Class II Disease Associations
As can be seen in Table 2-1, the majority of associations of HLA and disease are with class II alleles. Several diseases have complex HLA genetic associations.
Celiac Disease
In the case of celiac disease, it is probable that the HLA-DQ genes are the primary basis for the disease association. HLA-DQ genes present on both the celiac-associated DR3 and DR7 haplotypes include the DQB1*0201 gene, and further detailed studies have documented a specific class Itoß dimer encoded by the DQA1*0501 and DQB1*0201 genes, which appears to account for the HLA genetic contribution to celiac disease susceptibility. This specific HLA association with celiac disease may have a straightforward explanation: peptides derived from the wheat gluten component gliaden are bound to the molecule encoded by DQA1*0501 and DQB1*0201 and presented to T cells. A gliaden-derived peptide that has been implicated in this immune activation binds the DQ class II dimer best when the peptide contains a glutamine to glutamic acid substitution. It has been proposed that tissue transglutaminase, an enzyme present at increased levels in the intestinal cells of celiac patients, converts glutamine to glutamic acid in gliadin, creating peptides that are capable of being bound by the DQ2 molecule and presented to T cells.
Pemphigus Vulgaris
In the case of pemphigus vulgaris, there are two HLA genes associated with disease: DRB1*0402 and DQB1*0503. Peptides derived from desmoglien 3, an epidermal autoantigen, bind to the DRB1*0402- and DQB1*0503-encoded HLA molecules, and this combination of specific peptide binding and disease-associated class II molecule is sufficient to stimulate desmoglien-specific T cells. A bullous pemphigoid clinical variant, not involving desmoglien recognition, has been found to be associated with HLA-DQB1*0301.
Juvenile Arthritis
Pauciarticular juvenile arthritis is an autoimmune disease associated with genes at the DRB1 locus and also with genes at the DPB1 locus. Patients with both DPB1*0201 and a DRB1 susceptibility allele (usually DRB1*08 or -*05) have a higher relative risk than expected from the additive effect of those genes alone. In juvenile patients with rheumatoid factor-positive polyarticular disease, heterozygotes carrying both DRB1*0401 and -*0404 have a relative risk >100, reflecting an apparent synergy in individuals inheriting 5 both of these susceptibility genes.
Type 1 Diabetes Mellitus
Type 1 (autoimmune) diabetes mellitus is associated with MHC genes on more than one haplotype. The presence of both the DR3 and DR4 haplotypes in one individual confers a 20-fold increased risk for Type 1 diabetes; the strongest single association is with DQB1*0302, and all haplotypes that carry a DQB1*0302 gene are associated with Type 1 diabetes, whereas related haplotypes that carry a different DQB1 gene are not. However, the relative risk associated with inheritance of this gene can be modified, depending on other HLA genes present either on the same or a second haplotype. For example, the presence of a DR2-positive haplotype containing a DQB1*0602 gene is associated with decreased risk.This gene, DQB1*0602, is considered “protective” for Type 1 diabetes. Even some DRB1 genes that can occur on the same haplotype as DQB1*0302 may modulate risk, so that individuals with the DR4 haplotype that contains DRB1*0403 are less susceptible to Type 1 diabetes than individuals with other DR4-DQB1*0302 haplotypes.
Although the presence of a DR3 haplotype in combination with the DR4-DQB1*0302 haplotype is a very high risk combination for diabetes susceptibility, the specific gene on the DR3 haplotype that is responsible for this synergy is not yet identified. There are some characteristic structural features of the diabetes-associated DQ molecule encoded by DQB1*0302, particularly the capability for binding peptides that have negatively charged amino acids near their C-termini. This may indicate a role for specific antigenic peptides or T cell interactions in the immune response to islet-associated proteins.
HLA and Rheumatoid Arthritis
The HLA genes associated with rheumatoid arthritis (RA) encode a distinctive sequence of amino acids from codons 67-74 of the DRß molecule: RA-associated class II molecules carry the sequence LeuLeuGluGlnArg-ArgAlaAla or LeuLeuGluGlnLysArgAlaAla in this region, while non-RA-associated genes carry one or more differences in this region. These residues form a portion of the molecule that lies in the middle of the α-helical portion of the DRB1-encoded class II molecule, termed the shared epitope.
The highest risk for susceptibility to RA comes in individuals who carry both a DRB1*0401 and DRB1*0404 gene. These DR4-positive RA-associated alleles are most frequent among patients with more severe, erosive disease. Several mechanisms have been proposed that link the shared epitope to immune reactivity in RA. This portion of the class II molecule may allow preferential binding of an arthritogenic peptide; it may favor the expansion of a type of self-reactive T lymphocyte; or it may itself form part of the pMHC ligand recognized byTCR that initiates synovial tissue recognition.
Molecular Mechanisms For HLA-Disease Associations
As noted above, HLA molecules play a key role in the selection and establishment of the antigen-specific T cell repertoire and a major role in the subsequent activation of those T cells during the initiation of an immune response. Precise genetic polymorphisms characteristic of individual alleles dictate the specificity of these interactions and thereby instruct and guide antigen-specific immune events. These same genetically determined pathways are therefore implicated in disease pathogenesis when specific HLA genes are responsible for autoimmune disease susceptibility.
The fate of developing T cells within the thymus is determined by the affinity of interaction between T cell receptor and HLA molecules bearing self-peptides, and thus the particular HLA types of each individual control the precise specificity of the T cell repertoire (Chap. 1). The primary basis for HLA-associated disease susceptibility may well lie within this thymic maturation pathway. The positive selection of potentially autoreactive T cells, based on the presence of specific HLA susceptibility genes, may establish the threshold for disease risk in a particular individual.
At the time of onset of a subsequent immune response, the primary role of the HLA molecule is to bind peptide and present it to antigen-specific T cells. The HLA complex can therefore be viewed as encoding genetic determinants of precise immunologic activation events. Antigenic peptides that bind particular HLA molecules are capable of stimulating T cell immune responses; peptides that do not bind are not presented to T cells and are not immunogenic.This genetic control of the immune response is mediated by the polymorphic sites within the HLA antigen-binding groove that interact with the bound peptides. In autoimmune and immune-mediated diseases, it is likely that specific tissue antigens that are targets for pathogenic lymphocytes are complexed with the HLA molecules encoded by specific susceptibility alleles. In autoimmune diseases with an infectious etiology, it is likely that immune responses to peptides derived from the initiating pathogen are bound and presented by particular HLA molecules to activate T lymphocytes that play a triggering or contributory role in disease pathogenesis. The concept that early events in disease initiation are triggered by specific HLA-peptide complexes offers some prospects for therapeutic intervention, since it may be possible to design compounds that interfere with the formation or function of specific HLA-peptide-T cell receptor interactions.
When considering mechanisms of HLA associations with immune response and disease, it is good to remember that just as HLA genetics are complex, so are the mechanisms likely to be heterogeneous. Immune-mediated disease is a multistep process in which one of the HLA-associated functions is to establish a repertoire ofpotentially reactive T cells, while another HLA-associated function is to provide the essential peptide-binding specificity for T cell recognition. For diseases with multiple HLA genetic associations, it is possible that both of these interactions occur and synergize to advance an accelerated pathway of disease.
