A methodological review of induced animal models of autoimmune diseases (1)
Animal models of autoimmune diseases refer to two groups, sponta- neous and induced models. In the former group, animals with or with- out genetic modifications develop disease spontaneously, while in the latter group disease is artificially induced.
Immunization with autoantigens
Immunization with tissue extracts
The rapid and acute disseminated encephalomyelitis (now termed as experimental autoim- mune encephalomyelitis, EAE) was induced in rhesus monkeys by immunization with heterologous and homologous brain tissue extract in presence of CFA. Since then, immunization with tissue extracts has been applied to induce various autoimmune disorders, including multi- ple sclerosis (MS), autoimmune hepatitis (AIH), autoimmune thyroiditis (AIT), glomerulonephritis, myasthenia gravis (MG), and autoimmune myocarditis (AIM).
Immunization with cells
Autoimmune thrombocytopenia (AITP) is a disorder of low blood platelet counts in which platelets are destroyed by autoantibodies. Mice immunized with rat platelets by repeated intraperitoneal injection developed a transient thrombocytopenia and autoantibodies capable of binding to and destroying platelets. Rats immunized with human umbilical vein endothelial cells (HUVEC) produce autoantibodies against EC and develop autoimmune emphysema.
Immunization with specific protein
Heterologous antigens have been extensively used for immunization to induced animal models of many autoimmune diseases, including MG, rheumatoid arthritis, autoimmune uveitis, glomerulonephritis, and pemphigus vulgaris (PV). Recombinant homologous antigens supplemented with a GST-tag have been used for induction of experimental epidermolysis bullosa acquisita (EBA) and experimental bullous pemphigoid (BP). In addition, immunization with homologous antigen conjugated to a peptide containing a T cells epitope has also been reported to be efficient for induce autoimmunity.
Peptide-based immunizations
Tt should be noted that applicability of this strategy has been limited to experimental autoimmune diseases which are predominantly mediated by T cell immune responses, such as EAE, mouse cardiac myosin peptide induced AIM, human IRBP peptide induced AIU, and human G6PI peptide induced arthritis. Immunizing mice with a peptide containing a predominant T cells epitope of Ro60 antigen can induce the production of pathogenic autoantibodies via intermolecular epitope spreading and the consequent primary Sjögren’s syndrome-like disease.
Immunization of knock out animals with the outknocked protein
A powerful approach for breaking immune tolerance is immunization of knock-out animals with the outknocked protein. Today, several diseases such as experimental PV, systemic vasculitis (SV) and primary Sjögren’s syndrome (pSS) have been modeled by this technique.
Genetic immunization
The fact the antibodies raised in this model were not always able to recognize the relevant conformational epitopes which were not present in the recombinant protein used for immunization. Inoculating a cDNA encoding human TSH-R gene into the mice intramuscularly led to the expression of human TSH-R in mice as a heterologous antigen. The mice generated stimulating autoantibodies against the TSH-R and developed hyperthyroidism.
Virus immunization
Mice were infected with adenovirus Ad5 expressing human cytochrome P450 2D6 (CYP2D6), the autoantigen of AIH. The infected mice developed severe and chronic autoimmune liver damage resembling human AIH, while the control mice infected with adenovirus Ad5 only did not.
Transfer of autoimmunity
Transfer of immune cells
Transfer of dendritic cells (DC) pulsed with autoantigen can be used to induce autoimmunity and consequent autoimmune disease, providing a novel modeling strategy. It has been successfully applied for induction of several other experimental autoimmune diseases, including autoimmune diabetes, EAE, experimental AIT, experimental AIM, and experimental AIU. The transfer of human PBMC from patients has been used to establish animal models for several autoimmune diseases including SLE, MG, and PV. The success of this strategy relies on the cross reactivity of human autoimmunity to animal antigens and, thus, it works only in case of a sufficient similarity in the pathogenic autoantigen between human and recipient animals.
Transfer of serum/autoantibodies
Injection of AIT sera collected from the donor mice immunized with homologous thyroid extract intravenously showed a defined pathology of experimental thyroiditis in approximately 50% of the recipient mice. IgG isolated from rabbits immunized with recombinant murine COL7 IgG can be transferred into mice to induce experimental EBA. Specific treatments like intracerebral injection or creation of inflammatory conditions increasing the permeability of the BBB are required to bring the lymphocytes or autoantibodies into the central nervous system (CNS) for the induction of autoimmune diseases targeting the CNS.
Induction by environmental factors
Infection
Mice infected with Theiler’s murine encephalomyelitis virus (TMEV) developed spontaneous encephalomyelitis. The first virus-induced animal model for MS was established by inoculating Swiss mice with TMEV. Infection with murine CMV and gamma herpesvirus were also used to establish the animal model for pSS and AITP. It has been demon- strated that TMEV infection can trigger autoimmunity via molecular mimicry and epitope spreading in mice, suggesting an autoimmune basis of this model truly representative for an autoimmune disorder.
Drugs/chemicals
DNA methylation inhibitors such as procainamide and hydralazine can cause a lupus-like disease. A mouse model of systemic sclerosis (SSc) was also established by repetitive injection of bleomycin.
Adjuvant
Adjuvants, which prolong the lifetime of antigens, enhance the antigen delivery to antigen presenting cells and stimulate the innate immune system, are essential components for the induction of immune responses. Injection of the strong adjuvant pristane can provoke autoimmune arthritis in susceptible rats, mice and hamsters. Besides arthritis, a SLE-like disease phenotype was reported to be induced in mice via injection of pristane.
Strategies for establishing new animal models
Selection of animals
To select a proper inbred mouse strains for modeling autoimmune diseases, some factors need to be taken in consideration. First of all, since most autoimmune diseases are associated with specific HLA loci and mouse models of autoim- mune diseases induced by immunization are often MHC-dependent. Outbred mice have genetic heterogeneity which may interfere with disease susceptibility. A critical example for this is a loss-of-function 2-bp deletion in complement component 5 gene in multiple inbred strains, including FVB/N, DBA/2, A/He, AKR, DBA/2, NZB/B1N, and SWR.
Selection of strategies
Principally, serum/autoantibodies transfer should be applied for autoimmune diseases predominantly mediated by autoantibodies, while transfer of T cells is applicable for disorders predominantly mediated by autoreactive T cells. Since for many autoimmune diseases the contribution of both, autoreactive T cells and autoantibodies, are relevant, the transfer of splenocytes, PBMC or DCs are good options be- cause they mediate cellular as well as humoral autoimmunity. Immunization with whole tissue or cells as an antigen can be applied when the relevant autoantigen of a disease has not been identified while immunization with specific protein should be a priority for modeling a disease with a known autoantigen. An immunization with a peptide is recommended only when the peptide contains the disease relevant T cell epitope. Selection of a homologous or heterologous autoantigen is also a key issue for induction of autoimmune diseases in animals. Usually, immunization with heterologous or modified homologous autoantigens are powerful for breaking self-tolerance, while homologous autoantigens are applied for diseases whose autoantigens are not tolerant.