http://www.nature.com/cgi-taf/DynaPage.taf?file=/ni/journal/v2/n9/special_full/ni0901-781_r.html
Nature Immunology 2, 781 - 784 (2001)
© Nature America, Inc.
Veena Taneja & Chella S. David
Department of Immunology, Mayo Clinic,
Rochester, MN 55905, USA. (http://www.nature.com/email_response/email.taf?address=david.chella%40mayo.edu)
A variety of different animal models are used to study autoimmune disease. What have we learned?
Scientific literature contains a plethora of mouse models touted to be valuable in studies of autoimmune states with unknown etiology and pathogenesis. Although no single animal model perfectly mimics a human disease, experimental animal models afford opportunities to study the role of individual genes in order to investigate the genetic, environmental and pathogenic aspects of an autoimmune disease. Different animal models of the same autoimmune disease have unique values: some models highlight genetic factors, whereas others emphasize effector mechanisms. What is important, however, is the integration of the diverse data the models generate into a coherent framework for understanding autoimmunity. The parameters of autoimmunity are the same: genetic predisposition, the "trigger" that causes disease onset and the affected target tissue, which determines the specificity of the disease (Fig. 1). Here we will discuss some of the major spontaneous and induced animal models for human autoimmune diseases, their contributions and inadequacies, what future animal models we need and how animal models can be used in the management of human diseases.
Figure 1. The basic parameters of autoimmunity are the same.
In the thymus, a self-peptide with high binding affinity for HLA molecules leads to negative selection of T cells. In contrast, a self-peptide with low binding affinity can lead to positive selection of T cells, some of which could be potentially autoreactive. Some of the HLA molecules are able to positively select more autoreactive T cells and may predispose an individual to autoimmunity. Viral or bacterial antigens that mimic self-antigens are presented in the context of HLA molecules and can activate these autoreactive T cells in the periphery, thus triggering an autoimmune response. The response may be directed to an organ that overexpresses costimulatory molecules or cytokines and expresses an antigen that matches the specificity of autoreactive T cells. This in turn leads to inflammation, expansion of B cells, pathogenesis and destruction of the organ.
Multigene models
Among the multigenic spontaneous
models for human autoimmunity, two models—NOD (nonobese diabetic, for diabetes)
and NZB/W (lupus)—are particularly significant. A thoroughly studied model
of spontaneous autoimmunity is autoimmune diabetes in the NOD mouse, which
closely resembles human type 1 diabetes mellitus (also called insulin-dependent
diabetes mellitus, or IDDM) (1). They share a similar pathology and immunological
basis, and islet-reactive CD4+ T cells and CD8+ T cells are implicated
in both diseases. Although the autoantigen is not known, both NOD mice
and humans show responses to glutamic acid decarboxylase (GAD), insulin
and insulinoma-associated protein 2 (Ia2). The T cell response is predominantly
restricted by the NOD major histocompatibility complex (MHC) class II molecule,
I-Ag7, which shares homology with the human IDDM-predisposing molecule
HLA-DQ8. Both I-Ag7 and DQ8 carry non–aspartic acid residues at position
57 on the b chain.
In germ-free colonies of NOD mice, the onset of disease occurs spontaneously
and within a defined time frame. However, in conventional facilities, the
onset of the disease is variable and the contributions of other non-MHC
genes critical. Thus, this model is strain-specific and requires the involvement
of multiple genes. Heterozygosity in general leads to abrogation of disease,
which suggests that most of the genes critical for onset may be recessive.
In human type 1 diabetes, class II genes play a dominant role and, in some
cases, there is actually a higher relative risk of disease for heterozygous
individuals, which involves two predisposing haplotypes: HLA-DQ8–HLA-DR4
and HLA-DQ2–HLA-DR3, for example. Thus, although several of the non-MHC
genes implicated in human type 1 diabetes may be important within a population,
in an individual, the presence of the predisposing class II gene combined
with one or two environmental or genetic influences can cause the onset
of disease. Although the NOD mouse has provided considerable information
about the multiplicity of genes that contribute to type 1 diabetes, an
animal model in which individual genes and environmental influences can
be identified is warranted.
Lupus is a complex disease that is
characterized by autoantibody production and involves many organs and many
antigens. Several murine models of spontaneous lupus-like disease have
been described, with NZBWF1 being the best characterized model. (2) NZB/WF1
hybrids develop lupus-like autoimmunity characterized by immunologlobulin
G autoantibody production and progressive severe glomerulonephritis that
closely resembles human systemic lupus erythematosus (SLE). As in human
lupus, associations with MHC alleles correlate with nephritis susceptibility
in certain mouse strains. Genetic analysis in the NZB/WF1 mice and their
derivatives have provided a wealth of information on how various genes
contribute to specific pathological events in the disease process. Similar
to human lupus, data from these studies indicate that the disease expression
requires interactions between several non-MHC and MHC genes. Studies with
the "speed congenics" have identified several genomic intervals that contribute
to different degrees of SLE pathogenesis (3). The identification of murine
susceptibility genes should provide insights into the genetic mechanisms
involved in the development of this complex disease in predisposed individuals
and an opportunity to identify syntenic genes in humans.
MHC-transgenic models
Among the MHC transgenic (Tg) mice
that develop spontaneous diseases, two animal models—the HLA-B27–expressing
rat and mouse models (for human spondyloarthropathies) and the HLA-DQ8
rat insulin promoter (RIP).B7-1 C57BL/6 (for type 1 diabetes)–Tg mice—are
noteworthy. The gene encoding HLA-DQ8 is the predominant predisposing gene
in human type 1 diabetes. Mice expressing the HLA-DQ8 transgene are not
susceptible to spontaneous diabetes. However, they do lose their tolerance
to self-GAD and potential autoreactive T cells in the periphery cause insulitis
in the pancreas, although further pathogenesis is absent. The same phenomena
could occur in humans with predisposing type 1 diabetes MHC genes. The
onset of diabetes may require a second insult in the pancreas, perhaps
in the form of a viral or bacterial infection, overproduction of a cytokine
or overexpression of an accessory molecule. To simulate such conditions,
HLA-DQ8 (type 1 diabetes–predisposing)- and the HLA-DQ6 (diabetes protective)-Tg
mice were crossed with RIP.B7-1–Tg mice, which—in the context of RIP—express
the costimulatory molecule B7-1 in the b
cells of islets (3). HLA-DQ8 RIP.B7-1–Tg mice develop spontaneous diabetes,
whereas HLA-DQ6 RIP.B7-1–Tg mice do not (4). Thus, the HLA-DQ8 RIP.B7-1–Tg
mice represent an excellent model for human type 1 diabetes. The disease
is dominant in these mice so that the contribution of various genetic and
environmental elements to the onset, modulation and protection of the disease
can be investigated. Interestingly, one class II molecule (HLA-DR4) can
down-regulate diabetes in the DQ8 RIP.B7-1–Tg mice, which suggests interactions
between various class II molecules in the disease process (5).
The first spontaneous disease models
that expressed a human HLA were HLA-B27–Tg rats and mice (6, 7). When animals
are moved from pathogen-free environments to conventional colonies, animals
from both species develop spondylitis-like features, which indicates that
the onset of the disease requires an environmental trigger. The disease
primarily affects male rodents, as with the human disease. These models
confirmed that the gene encoding HLA-B27 itself, and not other closely
linked genes, is responsible for spondylitis. But the model also generated
controversies and additional questions. Why did the B27-Tg rats require
more than 100 copies of the genes encoding HLA-B27 and human b2-microglobulin
(b2M) for the onset
of disease and why did HLA-B27 mice develop disease only in the absence
of the mouse b2M?
Studies in mice have suggested that the B27 molecule may reach the cell
surface as an empty or free heavy chain in the absence of b2M
and present an exogenous antigen to activate autoreactive T cells. Data
from studies on human cells have shown that b2M-free
B27 homodimers are transported to the cell surface and have the capacity
to load exogenous peptides (8). These homodimers resemble a class II molecule
that can present peptides in a classical class II antigen-processing pathway.
Thus, the studies in rodent models and humans are converging in delineating
spondylitis.
TCR-transgenic models
There are two well studied T cell
receptor (TCR)-Tg mice that develop spontaneous autoimmune disease. BDC2.5-Tg
mice express rearranged genes encoding a
and b TCR chains
that are derived from the islet-reactive diabetogenic CD4+ T cell clone
BDC2.5. Although these mice provide valuable information about what processes
make T cells diabetogenic, disease cannot be transferred, and only 20–25%
of the TCR-Tg NOD mice develop disease when housed in specific pathogen-free
conditions (9). Other studies have shown differences in the T cell clones
generated from T cells of NOD mice and TCR-Tg mice (10), limiting the use
of TCR-Tg mice in disease pathogenesis. This Tg mouse is an excellent model
with which to study the role of the TCR, the specificity of T cells and
the peptides involved in disease. But caution needs to be exercised in
extrapolating this information to human diseases because diabetic patients
do not have T cells expressing a predominant TCR.
KRN mice express rearranged TCR genes
from a T cell hybridoma that recognizes amino acids 41–61 of bovine pancreas
ribonuclease in the context of H-2Ak. Crossing KRN mice with a NOD strain
(H-2Ag7) leads to the development of spontaneous systemic arthritis, which
shares many features with rheumatoid arthritis (RA), in the transgene-positive
offspring (11). The disease is triggered by recognition of self-MHC–peptide
complexes by Tg T cells, which leads to a breakdown in tolerance and self-reactivity.
The molecular target of both T and B cells is an enzyme of the glycolytic
pathway, glycosyl-phosphatidyl inositol (GPI), which is essentially expressed
in all the tissues including joint cartilage. However, it differs from
the human disease by the absence of rheumatic factor (RF), the presence
of inflammation of the spine and absence of germinal centers in the lesions.
Also, the disease is more aggressive in mice, with an excess of myeloid
cells infiltrating the synovial membrane. The disease is restricted by
H-2Ag7, a molecule very similar to human HLA-DQ8 which is linked to RA.
GPI is expressed on the surface of synovial lining and anti–GPI IgG can
form immune complexes with GPI with subsequent immune response leading
to development of joint pathology (12). This model may be important in
dissecting human disease even though the requirement of oligoclonal T cells
may not be true for humans.
Knockout models
Mice deficient in interleukin 2 (IL-2),
IL-2 receptor-a,
IL-10 and with mutated genes encoding TCRa
develop spontaneous inflammatory bowel disease (IBD), which has similarities
to human Crohn's disease. IL-10–deficient mice develop chronic enterocolitis,
due to aberrant immune responses to normal enteric antigens, that progresses
with age (13). Chronic enterocolitis shares immunological, pathological
and physiological similarities with human IBD in its pattern of inflammation,
increased colonic aerobic bacteria with accompanying decreased lactobacilli,
increased gut and intestinal permeability and endotoxemia. Studies in some
models have led to clinical trials that use various cytokines to treat
the human disease. As human IBD seems to result from a genetically determined
defect, it would be useful to have a mouse model of this disease to identify
triggering antigens, characterize negative and positive TCR thymic selection
and dissect the complex network of cytokines and their regulation.
Induced models
Experimental studies in induced models
have the advantage over those in spontaneous models in that the onset and
progression of disease can be controlled. Viral and other microbial infections
are implicated in triggering immune responses to host autoantigens that
are cross-reactive. Histories of viral infection preceding the onset of
disease support this theory. In the context of autoimmune diseases, attention
has focused on the search for the autoantigen that initiates the immune
response. Most of the induced animal models use viral antigens or the self-antigen
of the disease-specific tissue target. The induced models also have the
advantage that they can focus on single gene effects with the use of transgene
or gene-deletion models.
Although it has been proposed that
some autoimmune diseases may have a viral etiology, virus-induced autoimmunity
remains a controversial topic. It is possible that many viruses can set
the stage for generating autoimmune processes. Evolution has led to selection
of efficient MHC molecules, which can present multiple epitopes of infectious
agents to activate T cell populations and clear infection. Although these
molecules can effectively present viral antigens, they may predispose an
individual to autoimmunity. Infectious agents may have a role in the onset
of some human autoimmune diseases. Several virus-induced models for autoimmune
diseases have provided important information.
Epidemiological studies of multiple
sclerosis (MS) provide the strongest evidence for the involvement of viral
etiology in the onset of disease. Theiler virus–induced demyelination,
a model for human MS, has several features that are similar to the human
disease: an immune-mediated demyelination, involvement of CD4+ T helper
cells, delayed type hypersensitivity response to antigens and pathology.
The Theiler virus was isolated from mice with demyelinating disease. In
the presence of some class I molecule alleles, the virus is immediately
cleared and the mice remain healthy. In the context of other class I molecules,
the virus is not cleared and replicates in the brain. This leads to three
possible scenarios. In the first, the immune system is incapable of clearing
the infection and the massive viral load kills the animal. In the second,
the immune system eventually resolves the viral load and the animal lives
a normal life. In the third, a stand-off leads to a chronic infection in
the brain and activation of self-myelin–reactive T cells and autoantibodies,
which causes demyelination and disease. This mouse model may provide a
scenario that closely resembles the human disease. In HLA-Tg mice, class
II molecules altered the severity of demyelination without influencing
viral load (14).
Several animal models have been developed
in which a potential or known autoantigen is used to induce disease. In
humans, such autoantigens are seldom involved in the onset of disease,
but are potential targets. Loss of tolerance to autoantigens would result
in the positive selection of self-reactive T cells. In most cases, induction
of disease requires the use of complete Freund's adjuvant, thus indirectly
involving a microbial agent. Although induced models may be artificial,
the resultant clinical signs and pathology resemble human diseases of unknown
etiology. Over the past decade, numerous studies and approaches have led
to identification of candidate autoantigen for many diseases: myelin basic
protein (MBP), proteolipid protein (PLP) and myelin oligodendrocyte glycoprotein
(MOG) in experimental autoimmune encephalomyelitis (EAE); thyroglobulin
in thyroiditis; acetylcholine receptor in myasthenia gravis; interphotoreceptor
retinoid-binding protein in uveitis and type II collagen in RA.
RA is characterized by synovial inflammation
and the destruction of joints. Immunization of some mouse strains with
heterologous type II collagen leads to development of collagen-induced
arthritis (CIA) with joint histopathology similar to that seen in RA. In
both mice and humans, pathology is mediated by T and B cells. The disease
susceptibility in mice is restricted to H-2A loci (q,r) (the human analog
of DQ), whereas protection is mediated by a polymorphism in the I-Eb loci
(the human analog of DR). RA has been explained on the basis of the "shared
epitope": the third hypervariable region (HV3) (amino acids 67–74 of DRB1*0401),
and other alleles sharing this region, present a putative arthritogenic
epitope that results in initiation of autoimmunity. On the basis of extensive
studies in CIA, we propose an alternative hypothesis: the DRB1 "shared
epitope" shapes the T cell repertoire in the thymus by serving as a self-peptide
presented by DQ molecules. Thus, a low-affinity binding DRB1 HV3 peptide
would positively select potential autoreactive T cells, whereas high-affinity
binding peptide would lead to negative selection (15). Our model implies
a role for both the DR and DQ haplotype in RA predisposition. In humans,
it is difficult to study the function of individual class II genes, as
they are expressed in haplotypes and strong linkage disequilibrium occurs
between DR and DQ alleles. Thus, HLA class II DR and HLA class II DQ-transgenic
mice that lack endogenous class II molecules were generated to study the
role of MHC in arthritis. These human-mouse models have helped to define
the role of HLA in arthritis (16). An important role of HLA-DQ in arthritis
was evident when Ao.DQ8-Tg mice developed severe CIA. Studies with Tg mice
that express both DR and DQ alleles, show that polymorphism in DRB1 loci
could modulate the DQ-restricted disease by being protective, permissive
or neutral, thus supporting our hypothesis (17). The limitations of this
model are that it bypasses the role of class I molecules and type II collagen
may not be the only target in the human disease.
The rodent model of EAE is the most
commonly studied model for MS. Despite intensive study and knowledge of
the target cells, genetic factors that influence the severity of MS and
the etiology of the disease are not understood. The autoantigens used to
induce EAE are components of myelin, either crude myelin extract or purified
basic MBP, PLP or MOG. Although these models imitate some histopathological
aspects of MS, the extent of demyelination and distribution of lesions
are different. Like MS, susceptibility to EAE is restricted by class II
genes. Studies with HLA-Tg and human TCR–Tg mice show that HLA class II
molecules can mediate disease by presenting an MBP self-peptide to T cells
(18). However, the variations of the MHC alleles associated with MS in
different ethnic populations make it difficult to assess the role of these
alleles in the severity of or predisposition to disease. EAE induced with
MOG is characterized by chronic (relapsing and progressive) disease with
central nervous system demyelination. This model suggests an important
role for autoantibodies in the disease. As in human disease, genetic background
and gender of the animals also have an impact. The initial event that leads
to T cell activation in MS remains elusive, although cross-recognition
between infectious microbes and myelin antigens have been implicated.
Future models
We need to improve the existing models
to make them more closely resemble human disease. Models with human MHC,
without endogenous mouse alleles should be developed so that immune-mediated
processes are restricted by human molecules. Mice expressing two or more
HLA genes that simulate human haplotypes might reflect variations in human
disease. New spontaneous disease models that combine HLA-Tg mice with cytokines,
costimulatory molecules, viral Tg and knockout mice would further refine
human disease models. Use of these models and the availability of the mouse
and human genome maps should help decipher the role of non-MHC genes in
human autoimmune disease.
References
©
2001 Nature Publishing Group