Rheumatology Principles and Practice Ashit Syngle, SD Deodhar
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Immune System and Immune Basis of Rheumatic Diseases1

Skand Shukla,
Amita Aggarwal
 
THE IMMUNE SYSTEM
Although the concept of immunity has been known to humans since antiquity, the birth of immunology as a science started in the late eighteenth century when Edward Jenner used material from cowpox pustule to develop protection against smallpox. Since the 1960s, there have been rapid advances in the field of immunology in parallel with advances in technology.
Broadly, the immune response is classified into two: innate immune response and an adaptive immune response.1 The innate response is more primitive and comes into play as soon as the body is exposed to any pathogen. It is directed against molecular motifs called pathogen associated molecular patterns (PAMPs) which are common among a large number of organisms; thus it lacks specificity. The receptors, which recognize PAMPs, are called pattern recognition receptors (PRRs). The innate immune response also lacks any memory of previous encounter with the pathogen.
On the other hand, adaptive immune response is more evolved and has specificity and immunological memory. As the receptors of adaptive immune system develop due to gene recombination, they have enormous diversity. The two major players in adaptive immune response are T and B lymphocytes, which generate different but interlinked effector responses. The B lymphocytes provide immunity by making molecules called antibodies, which bind to the extracellular pathogens and cause their elimination. This antibody-mediated immune response is also called humoral immune response. The T lymphocytes play a role in cell-mediated immunity, which mainly offers protection against intracellular microbes.
 
Cells of the Immune System2
Lymphocytes: These cells play an important role in adaptive immune response by generation of antibodies against extracellular pathogens and by killing intracellular pathogens (Fig. 1.1). Naïve lymphocytes are cells which have not encountered an antigen; they are 8-10 µm in size, have a large nucleus and a thin rim of cytoplasm around it. In response to stimulation, the lymphocytes increase in size with increase in cytoplasm and organelles around the nucleus.
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Figure 1.1: The different cells of the immune system. T lymphocytes can be of helper, cytotoxic or regulatory phenotype. NK cells are large and granular. Plasma cells have an eccentric nucleus
The lymphocytes are broadly classified into three categories:
  1. B lymphocytes (bursa or bone marrow derived lymphocytes): These cells are mediators of humoral immunity and produce antibodies, which bind to antigens. They are called B lymphocytes because in birds, they mature in an organ called bursa of Fabricius. However, in mammals no such structure exists and these lymphocytes mature in the bone marrow.
  2. T lymphocytes (thymus-derived lymphocytes): These cells play a role in cellular immunity. T lymphocytes initially develop in the bone marrow but later leave the marrow and migrate to the thymus for further development. The T lymphocytes have been further classified as helper T cells, cytotoxic T cells and regulatory T cells (Fig. 1.1); they have T-cell receptors (TCRs). These TCRs are a complex of two chains, a and β, in most T cells but a rare subgroup of T cells express γ and δ chains. These chains are non-covalently linked to invariant chains, CD3 and ξ which play a role in signal transduction.
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  3. NK cells (natural killer cells): They are a subset of lymphocytes that kill cells infected with intracellular pathogens and tumor cells.
 
Antigen Presenting Cells (APCs)
These cells capture the antigens present in the extracellular environment and present them to the lymphocytes and lead to the proliferation and differentiation of the latter. The function of antigen presentation is done by dendritic cells, macrophages and B cells. Major histocompatibility complexes (MHC) are receptors expressed over various cells for displaying antigens to the T cells. They are of two types, class I and class II.MHC Ipresent peptides to CD8+ T cells and are expressed by all nucleated cells, while MHC II present antigen to CD4+ T cells and are expressed exclusively on antigen presenting cells.
  1. Dendritic cells: They are present in the epithelia and other organs and after capturing the antigens migrate to the lymphoid organs and present the antigens to the naïve T cells. The dendritic cells can be derived from both lymphoid and myeloid lineages. The plasmacytoid dendritic cells are of lymphoid lineage and produce large quantities of anti-viral cytokine IFN-γ. The myeloid-derived dendritic cells include follicular dendritic cells and Langerhans cells.
  2. M acrophages: Macrophages are cells derived from monocytes, which are produced in the bone marrow, and after circulating in the blood these cells enter the tissues and develop into macrophages. Macrophages display the antigenic peptides to the effector T cells, which in turn activate the macrophages for intracellular killing of the microorganisms.
  3. B cells: During the development of humoral immune response, the antigen presentation to T cells is also done by B cells.
 
Lymphoid Tissues
These are divided into two broad groups—primary and secondary. Primary lymphoid organs are the fetal liver, bone marrow (where the development of lymphoid progenitors starts) and the thymus in adults (where further development of T cells occurs). Secondary lymphoid organs comprise lymph nodes, spleen, skin and mucosa-associated lymphoid tissues. After antigen delivery by dendritic cells in the lymph node from draining tissues, the naïve B cells develop into effector and memory B cells with formation of germinal centers in the lymphoid follicles. In contrast, blood-borne antigens are captured in the spleen.
 
Antigens
Immunogens are molecules that elicit an immune response against themselves. Immunogens can be proteins, carbohydrates, lipids or nucleic acids. An antigen may be a large molecule with many antibody-binding regions. These regions are termed epitopes or determinants. Epitopes can be linear, that is, formed by several adjacent amino acid residues, or can be conformational, that is, formed by several amino acid residues which are not adjacent but are in close proximity due to special orientation in a folded protein. The presence of identical multiple antigenic determinants in an antigen is called the polyvalency of that antigen.
 
Antibodies
Antibodies are also known as immunoglobulins because they are the ‘immunity providing globulins’ of the body. Antibodies are distributed in all the body fluids and are present over the epithelial surfaces. A healthy 70-kg adult human produces about 3 grams of antibodies per day, out of which two-thirds is IgA antibody. All antibodies have the same basic structure but display remarkable variability in the antigen-binding regions. An antibody is composed of two identical heavy chains and two identical light chains (Fig. 1.2). There are two types of light chains, κ and λ. A single antibody molecule has either two kappa or lambda chains but cannot have both at the same time. Both these heavy and light chains contain a series of repeating, homologous units, each about 110 amino acids residues in length that fold, forming a globular Ig domain. Both heavy and light chains consist of an amino terminal variable (V) region and a carboxy terminal (C) constant region (Fig. 1.2). The variable regions of the light and heavy chains bind to the antigenic epitope, while the constant region performs effector functions.
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Figure 1.2: Structure of immunoglobulin. Immunoglobulins have two heavy and two light chains. Light chains have two immunoglobulin-like domains whereas a heavy chain has 4–5 immunoglobulin-like domains. The antigen-binding site is made of the variable region of light and heavy chains.
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The variable regions of the heavy and light chains form a surface that is complementary to the three-dimensional structure of the bound antigen, these regions are also called complementarity determining regions.
 
Isotypes of Antibodies
Antibodies can be divided into five classes, also known as isotypes. These are IgG, IgA, IgD, IgM and IgE. In humans, IgA and IgG can be further divided into IgA1 and IgA2 and IgG1, IgG2, IgG3, IgG4 respectively. Each of the five antibody isotypes have different heavy chains α, δ, γ, ε, µ.
Change in the antibody isotype takes place by the process of class (isotype) switching, which occurs after antigen stimulation. This isotype switching is controlled by interaction with T cells and cytokines.
 
Effector Functions
Effector functions of an antibody depend upon the Fc portion of the antibody. Antibodies with different Fc receptors perform different functions.3 The effector functions are only performed by antibodies which are bound to antigens, and not by free antibodies. The Fc portions of the immunoglobulin binds to the Fc receptors (FcRs) on the cells like macrophages, neutrophils and lymphocytes, leading to phagocytosis of the antigen–antibody complexes. Another effector function is activation of the complement which requires binding of the C1q complement protein to the Fc portions of the antigen complexed IgM or IgG.
 
Innate Immune Response
The initial contact with the pathogen triggers the innate immune response. In humans, this response is a function of physical and chemical barriers (epithelia and antimicrobial substances produced at the epithelia), phagocytic cells (neutrophils, macrophages and natural killer cells), complement system and other inflammatory mediators including proteins called cytokines.4
 
Components of Innate Immunity
  1. Epithelial barriers: Intact epithelia of the skin, the gastrointestinal tract and the respiratory tract protect the host by preventing the entry of pathogens into the deeper tissues. Epithelia also produce certain antibiotic substances like defensins, which are toxic for these organisms. Defensins are also present in the neutrophil granules and perform a broad-spectrum antibiotic function. Similarly, the intestinal epithelium secretes peptides known as cryptocidins which perform a similar function in the crypts of the intestine.
  2. Intraepithelial lymphocytes: Apart from naturally occurring antibiotics, a population of cells resides in the epithelial layers and the serous cavities of the body called intraepithelial lymphocytes. The majority of intraepithelial lymphocytes in humans are T lymphocytes with limited TCR diversity and hence are included as part of the innate immune system. Some of the intraepithelial lymphocytes are γδ subgroup of T lymphocytes and some are NK-T cells (that is, they express the receptors of NK cells as well); NK-T cells recognize lipid antigens.
  3. B-1 lymphocytes: The peritoneal cavity contains a population of B lymphocytes known as B-1 cells that express immunoglobulins over their surface but have limited diversity. These B-1 cells constitute 5-10% of the total population of the total B lymphocytes. Similar cells are also present in the marginal zone of the lymphoid follicles of the spleen. These B-1 cells produce IgM antibodies but there is no class switching as seen in other B lymphocytes.
  4. (Natural killer) NK cells: These cells are a subset of lymphocytes that kill cells, which have lost expression of MHC-I, like cells infected by intracellular pathogens or tumor cells. Cytotoxic CD8+ T cells cannot kill them as there are few or no MHC-I–peptide complexes which can bind to TCRs present over the surface of CD8+ T cells. NK cells kill the target by perforin-mediated cell lysis or antibody-dependent cell cytotoxicity
 
The Complement System
This is in an important effector mechanism for both innate immune response and humoral arm of the adaptive immune response.4, 5 The complement system consists of several plasma proteins that are initially activated by immune complexes. Complement activation products cause killing of microbes by lysis and amplification of an inflammatory response. These proteins interact with each other and other molecules of the immune system in a highly regulated manner. Complement response to microorganisms can be in three ways (Fig. 1.3):
The classical pathway: This has been so named because it was the first among the three pathways to be discovered. It is stimulated by the antigen– antibody complex. The complement protein C1 has three components: C1q, C1r and C1s. The C1q is a hexamer of three pairs of chains, while C1r and C1s exist as dimers. After the antigen–antibody interaction has taken place, the C1q binds to the antibody molecules. This leads to the enzymatic activation of the associated C1r and C1s leading to the cleavage of C4 protein subsequently to C4a and C4b.
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Figure 1.3: Complement activation pathway. Immune complexes activate classical pathway by binding to C1q which later binds to C1s and C1r which further activate C4 followed by C2 leading to generation of C3 convertase. In the alternative pathway, low-grade hydrolysis of C3 occurs and activated C3 is stabilized by properdin and Factor D to generate C3 convertase (C3bBb). C3 convertase converts C3 to the activated form and leads to formation of C5 convertase which later leads to generation of C7–9 membrane attack complex.
The C4a diffuses out and stimulates inflammation. C4b stays attached to the antigen–antibody complex and binds to C2 leading to its cleavage to C2a and C2b. The C4b2b acts as a classical pathway C3 convertase and cleaves C3 to C3a and C3b. The C3b component binds to the antigenic surface and the C4b2b complex leads to the formation of C5 convertase. C5 convertase leads to the cleavage of C5 to C5a and C5b.
The alternative pathway: Normally, the C3 in the plasma is continuously hydrolysed at a low rate to generate C3b (known as C3 tickover). A small amount of C3b generated binds to the surface of the microbe. This bound C3b binds factor B, which is subsequently cleaved by factor D to generate a Bb fragment that remains, attached to C3b and is known as alternative pathway C3 convertase. The C3bBb complex is stabilized by a molecule called properdin. The C3 convertase cleaves more and more C3 molecules leading to amplification of the loop. This generates C3BbC3 complexes, also known as C5 convertase.
The lectin pathway: This pathway is activated by the binding of microbial polysaccharides like mannose to the circulating lectins. This leads to activation of C1rC1s enzyme complex and the whole cascade of events leading to generation of C3 convertase. The lectin pathway activation thus occurs in the absence of antibody.
Finally, the activated C5 activates C6 and then downstream activation leads to the formation of a membrane attack complex (C7-C9) that plugs holes into the microbe leading to osmotic lysis.
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Adaptive Immune Response
The major step in generation of adaptive immune response is antigen presentation to T helper cell by APC via MHC in a trimolecular complex of MHC-peptide-TCR.6
MHC-I comprises of three genes A,B and C while MHC-II comprises DP, DQ and DR genes. MHC genes are located over the short arm of chromosome 6 and are expressed codominantly in an individual. MHC-I consists of an α chain bound non-covalently to β2 microglobulin. The α chain has three subunits, α1, α2 and α3. The α1 and α2 take part in the formation of the antigen-binding groove, which binds peptides of 8–11 amino acids in length. Hence, the protein has to be degraded to an appropriate size by the APC before it can be expressed by the APC in conjunction with the MHC I. The α3 subunit binds with the CD8 molecule during the APC–CD8 lymphocyte interaction.
MHC-II comprises an α chain and a β chain bound together non-covalently. The α chain consists of an α1 and an α2 subunit, while the β chain consists of a β1 and a β2 subunit. The α1 and β1 subunit participate in the formation of the antigen-binding groove, which accommodates the antigenic peptide 10–30 residues in length. The β2 subunit has a binding site for the CD4 molecule, while the CD4-lymphocyte–APC interaction takes place.
Though the MHCs are expressed constitutively over the surface of cells, their expression is increased in response to various cytokines secreted during the innate or adaptive response. MHC-I molecules present over the surface of all the nucleated cells of the body bind and present cytosolic protein antigens. These antigens are usually viral proteins or tumor antigens that are first degraded by a multiprotein complex called proteosome. The proteins that are targeted for degradation by the proteosome are first attached to a small polypeptide called ubiquitin; ubiquitinated proteins are then unfolded and are degraded by the proteosome into small peptides. These peptides are then transported to the endoplasmic reticulum via a specialised transporter called TAP (transport associated protein) which mediates active ATP mediated transport of the antigenic peptides to the endoplasmic reticulum. The TAP protein is also bound to the newly formed MHC-I molecules and translocated peptides now bind to these MHC molecules. These MHC-I bound peptides are transported to the cell surface where they interact with the CD8+ T cells.
MHC-II molecules bind to the exogenous peptides that are taken up by the APCs by endocytosis. These endosomes have an acidic environment, which is suitable for the degradation of exogenous proteins into smaller peptides. In addition, some of the endosomes fuse with lysosomes during the process of antigenic degradation. The MHC-II molecules develop in the endoplasmic reticulum and after synthesis are bound to a peptide known as invariant chain, which fits in the antigen-binding groove. After synthesis, the MHC-II molecules exit out of the endoplasmic reticulum in specialised vesicles, which fuse with the endosomes containing antigenic peptides. After fusion, the invariant chain is removed and the peptide binds to the MHC-II, which then moves to the cell surface for display to the CD4+ T cells.
The interaction of APC and the T cell is not only through the MHC–TCR but also takes place through a large number of co-stimulatory molecules. The binding of these accessory molecules to their ligands increases the strength of adhesion between T cells and APCs and is essential for the positive or negative downstream signalling. MHC–TCR interaction in the absence of co-stimulation can lead to T cell energy. One of the important co-stimulatory interactions is B7–CD28 interaction with B7 family members; B7-1 (CD80) and B7-2 (CD86) present on APC interact with the CD28 present over the surface of T cells.
After the MHC–TCR interaction, the naïve CD4+ lymphocytes mature into effector cells. The effector cell provides ‘help’ to the other cells to directly kill the pathogens or aids in killing by producing cytokines or helping B cells produce efficient antibodies. Recognition of antigen by CD8+ lymphocytes results in killing of the infected cell by the granzyme–perforin pathway. The perforins make pores into the cell membrane of the infected cells and granzymes enter these cells through these pores leading to apoptosis.
 
AUTOIMMUNITY
The major function of the immune system is to recognize the foreign pathogens in the body and mount an effective and appropriate immune response so that the pathogen is killed. The immune system can also recognize the self-proteins as antigens. Thus, the immune system has to be educated to react only against foreign pathogens and not against its own self-antigens/tissues. The mechanisms that prevent the immune system from reacting to its own proteins/antigens is called self-tolerance.
Most rheumatic diseases are characterized by the presence of auto-antibodies and immune dysregulation. In these diseases, different mechanisms lead to breakage of this self-tolerance thus leading to autoimmunity. Finally, the immune effector pathways involving cytokines, chemokines and complement system cause tissue damage and disease pathology.
 
 
Generation of Self-tolerance7, 8
The developing T-cell progenitors (thymocytes) move to the thymus.
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Figure 1.4: Negative and positive selection in thymus: When double positive (CD4+CD8+ thymocytes interact with thymic epithelial cells having MHC and self-antigen. If the interaction between T-cell receptor with MHC-antigen is of low level affinity (a) they are selected, whereas those with high affinity (c) or no affinity (b) get deleted by apoptosis
The thymocytes interact with the thymic epithelial cells, macrophages and dendritic cells, which provide the necessary antigenic stimuli for the development of T cells. The thymocytes migrate to the medulla after passing through the thymic cortex and encounter self-MHC–self-peptide complex on APCs. The T cells with high avidity to these complexes undergo cell death by apoptosis to prevent autoimmunity. On the other hand, developing thymocytes, which, bind with self-peptide–self-MHC complex with low avidity, are allowed to develop further (Fig. 1.4).
Initially, when these T cell precursors enter the thymus, they are CD4–CD8− (double negative) and subsequently they pass through the CD4+CD8+ (double positive) stage before finally becoming CD4+ or CD8+ (single positive)
However, this mechanism is not perfect and therefore some self-reactive T cells reach the periphery, where these T cells are kept under check by multiple mechanisms like:
  1. Barrier between the antigen and these cells. This is possible in the brain (blood–brain barrier), eye, etc.
  2. Suppression of these cells by regulatory T cells.
  3. Suppression of these cells by anti-inflammatory cytokines like TGF-β
  4. Prevention of activation of these cells by lack of co-stimulatory signals.
Similarly in B cells, during development in bone marrow, the self-reactive B cells are either deleted by apoptosis or they change their immunoglobulin receptor so that they no longer recognize self-antigens or the self-reactive cells fail to come out of the bone marrow. This seems to be nature's solution to keeping autoimmunity in check by trying to delete all self-reactive cells. However, autoimmunity does occur and so do autoimmune diseases.
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Table 1.1   Immune mechanisms in autoimmune diseases
Autoimmune disease
Target antigen
Pathogenetic mechanism
Graves disease
TSH receptor
Stimulation of TSH receptors by the antibodies
Pemphigus vulgaris
Junctional proteins between the epithelial cells
Activation of enzymes and junctional disruption
Goodpasture's syndrome
Basement membrane protein in kidneys and lung
Complement and Fc receptor mediated damage
Myasthenia gravis
Acetylcholine receptor
Inhibition of binding of acetylcholine to receptors
Pernicious anemia
Intrinsic factor
Neutralization of intrinsic factor
Autoimmune hemolytic anemia
RBC membrane proteins
Fc receptor-mediated phagocytosis
Autoimmune thromocytopenia
Platelet membrane proteins
Fc receptor-mediated phagocytosis
Multiple sclerosis
Autoreactive T cells to myelin basic proteins and proteo-lipid
Insulin-dependent diabetes mellitus
Antibodies to insulin, GAD 65, ICA
T-cell mediated damage of beta cells
Polymyositis
Antibodies to tRNA synthase
CD8+ T-cell mediated muscle damage
 
Autoimmune Diseases
Broadly, autoimmune diseases can be classified as organ-specific or generalized. In organ-specific autoimmune diseases, only one organ is the target for disease (as in Graves’ disease, pernicious anemia, pemphigus, myasthenia gravis), while in systemic autoimmune diseases, multiple organ systems are involved (as in systemic lupus erythematosus, rheumatoid arthritis, systemic necrotising vasculitis).
Organ-specific autoimmune diseases can be mediated by antibodies or by T cells. Table 1.1 gives the major pathogenetic mechanism involved in important organ-specific autoimmune diseases.
 
Systemic Autoimmune Diseases
In these diseases, the antibodies are usually directed against common cellular components like DNA, ribonucleoproteins, immunoglobulins, etc. Similarly, as these antigens are present ubiquitously, these diseases can involve multiple organs. The damage in these diseases is also mediated both by antibodies (like dsDNA-induced glomerular damage in SLE) or T cells (synovitis in rheumatoid arthritis).
Autoimmune diseases occur because of breakage of self-tolerance. The most important factor implicated in this is the environmental triggers. Infection by a microbe activates lymphocytes that recognize the specific microbe. However, if the antigens of the microbe have similarity to the host antigen then the self-reactive cells too may get activated, as in acute rheumatic fever (molecular mimicry between streptococcal M protein and heart tissue). Microbes also contain certain substances like lipo-polysaccharide, peptidoglycans that are recognized by Toll-like receptors (TLRs) present on the macrophages and lymphocytes thus activating them. In this way, autoreactive T cells can get activated. Similarly, antibodies formed against microbes can cross-react with self-antigens like in Guillain–Barre disease(C. jejuni).
Environmental triggers like silica can activate macrophages and other immune cells; smoking can cause conversion of self-antigen into their citrullinated form, thus leading to the generation of an immune response against citrullinated proteins, a hallmark of rheumatoid arthritis.
Some genetic factors also play an important role, for example, mutations in the AIRE gene lead to less expression of self-antigens on thymic epithelia leading to lack of deletion of self-reactive T cells, or deletion of TNF receptor, which prevents death of self-reactive T cells and causes autoimmunity. Similarly, polymorphisms in other genes associated with immune system function can contribute to autoimmunity and immune dysregulation.
 
Immune Mechanisms in Rheumatic Diseases
Rheumatoid arthritis (RA) and spondyloarthropathies are the two most common chronic inflammatory rheumatic diseases that each affect nearly 1% of the population. Other rheumatic diseases include inflammatory polymyositis, SLE, osteoarthritis. The presence of lymphocytic infiltrate at local site of damage, presence of autoantibodies, presence of oligoclonally expanded T cells, significant MHC association, cytokines and chemokines at local mileu, all suggest that the immune system plays a major role in their pathogenesis. Let us first examine the role of MHC in the pathogenesis of rheumatic diseases.
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MHC and Disease Association
Since all antigens are presented to T cells in association with MHC and a strong association is seen in certain autoimmune rheumatic diseases with MHC, the latter may have a pathogenic role to play (Table 1.2).
As HLA B27 has the strongest association with a rheumatic disease, that is, ankylosing spondylitis, it is a good model to study the postulated mechanism for this association. The various mechanisms postulated are: 911
  1. Role in antigen presentation: HLA B27 does not affect the uptake of bacteria but delays the elimination of bacteria in in vitro experiments
  2. Generation of immune response: In experimental studies, it has been observed that empty class I molecules on cell surface can bind exogenous antigen and activate T cells so B27 can act as an antigen presenting molecule.
  3. Molecular mimicry with microbes:11 HLA B27 does share 4–6 amino acid homology with the different proteins of the enteric bacteria. It can either blunt the host response as the body sees it as self and thus prevents an effective immune response to be generated or it can work by breaking the tolerance against the self-antigen thus leading to autoimmunity.
  4. Alteration of HLA B27 by microbes: Can the microbe alter the B27 molecule? Historic experiments by Geczy revealed that filtrate of Klebsiella cultures can modify the B27 molecule and it can then be recognized as non-self. The B27 molecule is unique in having a cystein residue at position 67 of its B pocket, which can react with homocystein produced by these bacteria, which can alter its structure and make it immunogenic. Indeed, B27 mutant transgenic rats, that is, where cystein was replaced by serine, do not develop arthritis even though they have diarrhea.
  5. B27 may itself act as a peptide in perpetuation of disease. Weak linkage is seen with certain DR molecules in ankylosing spondylitis suggesting that those alleles may be presenting the B27 peptides. Initial experiments like development of disease in beta-2 microglobulin deficient B 27 transgenic mice suggested that B27 peptides might be presented by class II molecules leading to disease, but later demonstration of its occurrence in class II deficient rats rules out this possibility.9
  6. B27 heavy chain homodimers: 10 Recently, HLA B27 homodimers have been found on the cell surface of cells; they may present antigen similar to HLA class II antigen to CD4 T cells. Misfolded B27 heavy chains may induce stress response and cause NFkB activation leading to a pro-inflammatory response.
Immunopathogenesis of synovitis Chronic synovitis is a hallmark of most rheumatic diseases including rheumatoid arthritis. The possible sequence of events in the pathogenesis of RA are provided in Table 1.3 and Figure 1.5.
 
INTERACTION OF GENETIC AND ENVIRONMENTAL FACTORS
Genetic factors: Peter Stasny in 1976 first recognized an association between HLA class II antigen DR4 and rheumatoid arthritis. Later, different groups studying different ethnic groups reported an association with HLA DR4, DR1 and DR10. After the availability of DNA-based techniques, it was found that all these alleles share a common sequence in their antigen binding area. This shared sequence was termed the ‘shared epitope’.12
Table 1.2   MHC and disease association
Disease
HLA
Relative risk
Ankylosing spondylitis
HLA B27
90
Reactive arthritis
HLA B27
41
Psoriatic spondylitis
HLA B27
10
Inflammatory disease-associated spondylitis
HLA B27
10
Rheumatoid arthritis
HLA DR0404, 0401,1001,
6
SLE
HLA DR3, HLA DR2
3
Sjögren's syndrome
HLA DR3
6
Oligoarticular JIA
HLA DP
4–5
Polyarticular JIA
HLA DR4
7
Juvenile dermatomyositis
HLA DR3
4
9
Table 1.3   Pathogenesis of rheumatoid arthritis
Phase
Pathological changes
Clinical features
I
Interaction of genetic and environmental agents
None
II
Antigen presentation
None
III
Inflammatory cascade
Polyarthritis, juxta-articular osteopenia
IV
Cartilage and bone destruction
Severe arthritis, erosions, deformities
V
Vasculitis etc.
Extra-articular features
zoom view
Figure 1.5: Steps in pathogenesis of autoimmune disease: Host genetic and environmental interaction leads to break in tolerance and generation of autoreactive cells. These autoreactive cells secrete cytokines (Cy), chemokines (Ch), activate complement (C) through autoantibody production thus initiating inflammation and subsequent tissue damage
Shared epitope has been associated with severity of RA, that is, individuals who are homozygous for shared epitope are more likely to have deformities and joint loss requiring joint replacement and other extra-articular features. Recently, non-HLA genes including cytokine genes, estrogen receptor, corticotropin-releasing hormone have been linked to increased susceptibility to RA. The concordance rates in monozygotic twins is only 15% and, is a little higher if, both have shared epitope. This suggests that heritability factor is only 15-20% and other factors like microbes or sex hormones might be involved in causing the disease.
Environmental factors: Since the clinical featuresof RA mimic viral arthritis, viruses like human T-celllymphotropic virus, rubella, cytomegalovirus, parvovirus, herpes virus have been implicated as the causative agent. Epstein–Barr virus has been linked to RA for long in patients with a high prevalence of antibodies against EBV and increased carriage of EBV in their throat washings. EBV glycoprotein gp110 has also been shown to have homology with the shared epitope of HLA associated with RA. However, the evidence of a role in pathogenesis has never been strong. Smoking has been linked to RA as it induces citrullination of self-proteins and leads to generation of citrullinated antigens.
Autoimmune factors: Both B-cell and T-cell autoimmunity plays a role in the progression of disease. The presence of rheumatoid factor (RF), an autoantibody against the Fc portion of immunoglobulin, is seen in 85% of patients. Antibodies to cyclic citrullinated peptides has been found in two-thirds of patients with RA and has higher specificity than RF. Presence of RF and anti-CCP antibodies is associated with increased risk of joint erosions, morbidity and extra-articular disease.
Even though collagen-induced arthritis is considered a good animal model for RA, auto-antibodies to denatured collagen II are found in only a third of patients with RA and usually occur after the onset of disease. Immune complexes containing these antibodies may have a role in perpetuation of disease.
Expansion of a selective TCR bearing T cells in the synovial biopsies and fluid suggests that these are antigen-driven T cells and the antigen is probably located at the local site.
Inflammatory cascade: Recognition of an unknown antigen present in the HLA groove of antigen-presenting cells by the 10T cells leads to production of pro-inflammatory cytokines like TNF-α, IL-6, GM-CSF from monocytes and IL-2, interferon gamma, IL-4 from T cells. These cytokines lead on to cell proliferation, homing of more inflammatory cells, matrix degradation and bone resorption resulting in synovitis. The cytokine cascade is probably controlled by TNF-α, and biological therapies directed at TNF have shown substantial clinical benefit. Further, inhibitory cytokines like IL-10, IL-1ra and IL-4 are also found in abundance in the synovial tissue. Thus, it is the critical balance between the pro-inflammatory cytokines and the anti-inflammatory cytokines, which determines the degree of tissue damage.
Angiogenesis (formation of new blood vessels) occurs under the influence of vascular endothelial growth factor and other angiogenic stimuli.13 This is essential to support the new cells being formed and this finally results in formation of invasive pannus. Matrix metalloproteinases, collagenase and the recently described IL-17 and RANK/RANKL lead on to bone and matrix resorption resulting in erosions.
 
CONCLUSION
The concerted action of multiple cells of the immune system results in autoimmunity, inflammation and subsequent tissue damage. Understanding basic immune mechanisms helps in better diagnosis, correct prognosis and in planning targeted therapies for better disease outcome. Autoantibodies provide useful tools for diagnosis of various rheumatic diseases, even though their role in pathogenesis is still questionable.
Major histocompatibility complex and other gene association studies help in prediction of outcome, thus identifying patients with poor prognosis. Some of the examples of targeted therapies that have proven benefit include TNF-blockers, IL6 receptor antibodies in RA, juvenile arthritis and spondyloarthropathies; anti-CD20 antibodies to kill B cells in RA, SLE and vasculitis, IL-1 receptor antagonist in systemic onset juvenile arthritis and autoinflammatory syndromes to block excess of IL-1. Further advances in immunology will bring even better molecules so that we can hope to have permanent cures for these patients.
REFERENCES
  1. Medzhitov R, Janeway C Jr. Innate immunity. N Engl J Med 2000;343:338–44.
  1. Delves PJ, Roitt IM. The immune system, Part II. N Engl J Med 2000;343:108–17.
  1. Ravetch JV, Bolland S. IgG Fc receptors. Ann Rev Immunol 2001;19:275–90.
  1. Walport MJ. Complement, Part I. N Engl J Med 2001;344:1058–66.
  1. Walport MJ. Complement, Part II. N Engl J Med 2001;344: 1140–44.
  1. Klein J, Sato A. The HLA system, Part I. N Engl J Med 2000;343:702–9.
  1. Anderton SM, Wraith DC. Selection and fine-tuning of the autoimmune T-cell repertoire. Nature Rev Immunol 2002;2:487–98.
  1. Walker LS, Abbas AK. The enemy within: Keeping self-reactive T cells at bay in the periphery. Nature Rev Immunol 2002;2:11–19.
  1. Khare SD, Luthra HS, David CS. Spontaneous inflammatory arthritis in HLA-B27 transgenic mice lacking beta 2-microglobulin: A model of human spondyloarthropathies. J Exp Med 1995;182:1153–58.
  1. Allen RL, O'Callaghan CA, McMichael AJ, Bowness P. Cutting edge: HLA-B27 can form a novel beta 2-microglobulin-free heavy chain homodimer structure. J Immunol 1999;162:5045–48.
  1. Scofield RH, Kurien B, Gross T, Warren WL, Harley JB. HLA-B27 binding of peptide from its own sequence and similar peptides from bacteria: Implications for spondyloarthropathies. Lancet 1995;345:1542–44.
  1. Gregersen PK, Silver J, Winchester RJ. The shared epitope hypothesis. An approach to understanding the molecular genetics of susceptibility to rheumatoid arthritis. Arthritis Rheum 1987;30:1205–13.
  1. Folkman J. Angiogenesis in cancer, vascular, rheumatoid and other disease. Nature Med 1995;1:27–31.
MULTIPLE CHOICE QUESTIONS
  1. Regarding innate immune response:
    1. It does require prior exposure for its action
    2. It lacks memory
    3. It has specificity as it recognizes microbes by receptors
    4. It is mainly composed of lymphocytes
  1. Regarding T lymphocytes:
    1. They develop in bone marrow
    2. CD4 cells are the cytotoxic T cells
    3. They recognize antigen on APC in conjunction with MHC
    4. T cell receptor has four chains
  1. Regarding immunoglobulins all the following are true except:
    1. They are produced by plasma cells
    2. IgG is the most abundant immunoglobulin in blood
    3. Each Ig molecule has two heavy and two light chains
    4. Variable region is responsible for effector functions
  1. Regarding complement pathway:
    1. Complement activation occurs only by immune complexes
    2. C5 and C3 complement components are part of the classical pathway
    3. Properdin and factor B are important for alternative pathway
    4. Membrane attack complex kills the cells by apoptosis
  1. Regarding MHC all the following are true except:
    1. The genes for MHC are located on chromosome 6
    2. MHC class I comprises A, B and C
    3. MHC class I molecule present extracellular antigens
    4. MHC class I alpha chain is bound to β∓2 microglobulin11
  1. Regarding MHC-disease association, which is incorrect:
    1. Rheumatoid arthritis: HLA DR4
    2. Ankylosing spondylitis: HLA B27
    3. Sjögren's syndrome: HLA DR3
    4. SLE: HLA DR4
  1. Regarding tolerance one of the following is FALSE:
    1. Central tolerance mainly occurs due to deletion of autoreactive T cells
    2. Central T cell tolerance occurs in bone marrow
    3. Peripheral tolerance can be mediated by regulatory T cells
    4. Receptor editing is an important way of generating tolerance in B cells
  1. Regarding rheumatoid arthritis all the following are true except:
    1. Rheumatoid factor is present in 75-85%
    2. Anti-CCP antibodies have lower specificity than rheumatoid factor
    3. TNF-α is the most important cytokine in pathogensis
    4. IL-17 plays an important role in bone resorption
  1. Regarding HLA B27 all the following are true except:
    1. It can form homodimers
    2. It is expressed only by antigen presenting cells
    3. It has sequence similarity with certain bacteria
    4. Rats expressing human HLA B27 develop disease like ankylosing spondylitis
  1. Breakage of self-tolerance can occur due to all the following except:
    1. Microbial infection
    2. Toxins
    3. Genetic susceptibility
    4. Mental stress
Answers:
1d,
2b,
3b,
4d,
5b,
6c,
7c,
8a,
9b,
10d