Urs Christen

CD4 + T cells are required for secondary expansion and memory in CD8 + T lymphocytes

A long-standing paradox in cellular immunology concerns the conditional requirement for CD4+ T-helper (TH) cells in the priming of cytotoxic CD8+ T lymphocyte (CTL) responses in vivo. Whereas CTL responses against certain viruses can be primed in the absence of CD4+ T cells, others, such as those mediated through ‘cross-priming’ by host antigen-presenting cells, are dependent on TH cells. A clearer understanding of the contribution of TH cells to CTL development has been hampered by the fact that most TH-independent responses have been demonstrated ex vivo as primary cytotoxic effectors, whereas TH-dependent responses generally require secondary in vitro re-stimulation for their detection. Here, we have monitored the primary and secondary responses of TH-dependent and TH-independent CTLs and find in both cases that CD4+ T cells are dispensable for primary expansion of CD8+ T cells and their differentiation into cytotoxic effectors. However, secondary CTL expansion (that is, a secondary response upon re-encounter with antigen) is wholly dependent on the presence of TH cells during, but not after, priming. Our results demonstrate that T-cell help is ‘programmed’ into CD8+ T cells during priming, conferring on these cells a hallmark of immune response memory: the capacity for functional expansion on re-encounter with antigen.

Molecular Mimicry, Bystander Activation, or Viral Persistence: Infections and Autoimmune Disease

SUMMARY Virus infections and autoimmune disease have long been linked. These infections often precede the occurrence of inflammation in the target organ. Several mechanisms often used to explain the association of autoimmunity and virus infection are molecular mimicry, bystander activation (with or without epitope spreading), and viral persistance. These mechanisms have been used separately or in various combinations to account for the immunopathology observed at the site of infection and/or sites of autoimmune disease, such as the brain, heart, and pancreas. These mechanisms are discussed in the context of multiple sclerosis, myocarditis, and diabetes, three immune-medicated diseases often linked with virus infections.

Among CXCR3 Chemokines, IFN-γ-Inducible Protein of 10 kDa (CXC Chemokine Ligand (CXCL) 10) but Not Monokine Induced by IFN-γ (CXCL9) Imprints a Pattern for the Subsequent Development of Autoimmune Disease

Infection of the pancreas with lymphocytic choriomeningitis virus results in rapid and differential expression among CXCR3 chemokines. IFN-γ-inducible protein of 10 kDa (IP-10), in contrast with monokine induced by IFN-γ and IFN-inducible T cell-α chemoattractant, is strongly expressed within 24 h postinfection. Blocking of IP-10, but not monokine induced by IFN-γ, aborts severity of Ag-specific injury of pancreatic β cells and abrogates type 1 diabetes. Mechanistically, IP-10 blockade impedes the expansion of peripheral Ag-specific T cells and hinders their migration into the pancreas. IP-10 expression was restricted to viruses infecting the pancreas and that are capable of causing diabetes. Hence, virus-induced organ-specific autoimmune diseases may be dependent on virus tropism and its ability to alter the local milieu by selectively inducing chemokines that prepare the infected tissue for the subsequent destruction by the adaptive immune response.

A Dual Role for TNF-α in Type 1 Diabetes: Islet-Specific Expression Abrogates the Ongoing Autoimmune Process When Induced Late but Not Early During Pathogenesis

We report here that islet-specific expression of TNF-α can play a dual role in autoimmune diabetes, depending on its precise timing in relation to the ongoing autoimmune process. In a transgenic model (rat insulin promoter-lymphocytic choriomeningitis virus) of virally induced diabetes, TNF-α enhanced disease incidence when induced through an islet-specific tetracycline-dependent promoter system early during pathogenesis. Blockade of TNF-α during this phase prevented diabetes completely, suggesting its pathogenetic importance early in disease development. In contrast, TNF-α expression abrogated the autoimmune process when induced late, which was associated with a reduction of autoreactive CD8 lymphocytes in islets and their lytic activities. Thus, the fine-tuned kinetics of an autoreactive process undergo distinct stages that respond in a differential way to the presence of TNF-α. This observation has importance for understanding the complex role of inflammatory cytokines in autoimmunity.

Breaking tolerance to the natural human liver autoantigen cytochrome P450 2D6 by virus infection.

Autoimmune liver diseases, such as autoimmune hepatitis (AIH) and primary biliary cirrhosis, often have severe consequences for the patient. Because of a lack of appropriate animal models, not much is known about their potential viral etiology. Infection by liver-tropic viruses is one possibility for the breakdown of self-tolerance. Therefore, we infected mice with adenovirus Ad5 expressing human cytochrome P450 2D6 (Ad-2D6). Ad-2D6–infected mice developed persistent autoimmune liver disease, apparent by cellular infiltration, hepatic fibrosis, “fused” liver lobules, and necrosis. Similar to type 2 AIH patients, Ad-2D6–infected mice generated type 1 liver kidney microsomal–like antibodies recognizing the immunodominant epitope WDPAQPPRD of cytochrome P450 2D6 (CYP2D6). Interestingly, Ad-2D6–infected wild-type FVB/N mice displayed exacerbated liver damage when compared with transgenic mice expressing the identical human CYP2D6 protein in the liver, indicating the presence of a stronger immunological tolerance in CYP2D6 mice. We demonstrate for the first time that infection with a virus expressing a natural human autoantigen breaks tolerance, resulting in a chronic form of severe, autoimmune liver damage. Our novel model system should be instrumental for studying mechanisms involved in the initiation, propagation, and precipitation of virus-induced autoimmune liver diseases.

Molecular anatomy of antigen-specific CD8(+) T cell engagement and synapse formation in vivo.

Antigen-specific CD8+ T cells are required for the clearance of most viral infections and several cancers. However, it is not clear in vivo whether CD8+ T cells can engage multiple targets simultaneously, engagement results in the formation of an immunologic synapse or molecules involved in CD8 function are redistributed to the synapse. We used here high-resolution microscopy to visualize interactions between virus-specific effectors and target cells in vivo. Using either in situ tetramer staining or green fluorescent protein–labeled virus-specific T cells, we have shown that a single CD8+ T cell can engage two or three targets, a synapse occurs at the site of engagement and molecules involved in attachment (lymphocyte function–associated antigen 1), signaling (Lck) and lytic activity (perforin) are differentially positioned on the T cell. In addition, we have established an in vivo approach for assessing the intricacies of antigen-specific T cell activation, migration, engagement, memory and other defining elements of adaptive immunity.

Cure of prediabetic mice by viral infections involves lymphocyte recruitment along an IP-10 gradient

Viruses can cause but can also prevent autoimmune disease. This dualism has certainly hampered attempts to establish a causal relationship between viral infections and type 1 diabetes (T1D). To develop a better mechanistic understanding of how viruses can influence the development of autoimmune disease, we exposed prediabetic mice to various viral infections. We used the well-established NOD and transgenic RIP-LCMV models of autoimmune diabetes. In both cases, infection with the lymphocytic choriomeningitis virus (LCMV) completely abrogated the diabetic process. Interestingly, such therapeutic viral infections resulted in a rapid recruitment of T lymphocytes from the islet infiltrate to the pancreatic draining lymph node, where increased apoptosis was occurring. In both models this was associated with a selective and extensive expression of the chemokine IP-10 (CXCL10), which predominantly attracts activated T lymphocytes, in the pancreatic draining lymph node, and in RIP-LCMV mice it depended on the viral antigenic load. In RIP-LCMV mice, blockade of TNF-alpha or IFN-gamma in vivo abolished the prevention of T1D. Thus, virally induced proinflammatory cytokines and chemokines can influence the ongoing autoaggressive process beneficially at the preclinical stage, if produced at the correct location, time, and levels.

Infections and Autoimmunity—Good or Bad?

The relationship between infections and autoimmunity is complex. Current evidence indicates that microbes can initiate, enhance, or, conversely, abrogate autoimmunity. In this paper, we will review experimental examples illustrating mechanisms involved in these three scenarios. Microbial infections can act as environmental triggers inducing or promoting autoimmunity resulting in clinical manifestations of autoimmune disease in genetically predisposed individuals. However, increasing evidence suggests the opposite outcome, which is the prevention or amelioration of autoimmune processes following microbial encounters. These latter observations support conceptually the “hygiene hypothesis,” suggesting that cleaner living conditions will lead to enhanced incidence of autoimmune disorders, asthma, and allergies. Because proof of concept in humans is difficult to obtain, we will discuss relevant animal model data in context with likely or proven human associations. Knowledge of mechanisms that underlie either positive or negative effects of infections on autoimmunity will facilitate exploration of molecular details for prospective clinical studies in the future.

A viral epitope that mimics a self antigen can accelerate but not initiate autoimmune diabetes

We document here that infection of prediabetic mice with a virus expressing an H-2Kb-restricted mimic ligand to a self epitope present on beta cells accelerates the development of autoimmune diabetes. Immunization with the mimic ligand expanded autoreactive T cell populations, which was followed by their trafficking to the islets, as visualized in situ by tetramer staining. In contrast, the mimic ligand did not generate sufficient autoreactive T cells in naive mice to initiate disease. Diabetes acceleration did not occur in H-2Kb-deficient mice or in mice tolerized to the mimic ligand. Thus, arenavirus-expressed mimics of self antigens accelerate a previously established autoimmune process. Sequential heterologous viral infections might therefore act in concert to precipitate clinical autoimmune disease, even if single exposure to a viral mimic does not always cause sufficient tissue destruction.

Islet-Specific Expression of CXCL10 Causes Spontaneous Islet Infiltration and Accelerates Diabetes Development

During inflammation, chemokines are conductors of lymphocyte trafficking. The chemokine CXCL10 is expressed early after virus infection. In a virus-induced mouse model for type 1 diabetes, CXCL10 blockade abrogated disease by interfering with trafficking of autoaggressive lymphocytes to the pancreas. We have generated transgenic rat insulin promotor (RIP)-CXCL10 mice expressing CXCL10 in the β cells of the islets of Langerhans to evaluate how bystander inflammation influences autoimmunity. RIP-CXCL10 mice have islet infiltrations by mononuclear cells and limited impairment of β cell function, but not spontaneous diabetes. RIP-CXCL10 mice crossed to RIP-nucleoprotein (NP) mice expressing the NP of the lymphocytic choriomeningitis virus in the β cells had massively accelerated type 1 diabetes after lymphocytic choriomeningitis virus infection. Mechanistically, we found a drastic increase in NP-specific, autoaggressive CD8 T cells in the pancreas after infection. In situ staining with H-2Db(NP396) tetramers revealed islet infiltration by NP-specific CD8 T cells in RIP-NP-CXCL10 mice early after infection. Our results indicate that CXCL10 expression accelerates the autoimmune process by enhancing the migration of Ag-specific lymphocytes to their target site.