*2.2.3. Ljungan virus and human parechoviruses*

The Ljungan virus (LV) is a RNA virus discovered in Sweden in the mid-1990s in rodents (Myodes glareolus; formerly Clethrionomys glareolus called "bank vole"). This virus be‐ longs to the Parechovirus genus within the Picornaviridae family. Niklasson et al. described the occurrence of T1D in 67 wild bank voles after 1 month of captivity in laboratory: diabetic animals showed clinic signs of diabetes (persistent hyperglycemia with weight loss, ketosis, and hyperlipidemia) and specific β-cell destruction associated with signs of autoimmunity: increased levels of autoantibodies to GAD65, IA-2, and insulin. The disease was correlated with LV antibodies. Moreover, LV antigen was detected by immunocytochemistry in the is‐ lets of diabetic bank voles. In parallel, two groups of new onset diabetic children were stud‐ ied: the first group represented a total of 53 children which were diagnosed with T1D between 1992 and 1995, and the second group was composed of 289 children with newly di‐ agnosed T1D between 1995 and 2000. The study showed increased levels of LV antibodies in newly diagnosed T1D children indicating a possible zoonotic relationship between LV infec‐ tion and human T1D [109].

In addtion to type 1 diabetes, viruses could be involved in the development of type 2 diabe‐ tes. Indeed, Niklasson et al. demonstrated that a type 2 diabetes-like disease could be in‐ duced by LV in a CD-1 mouse model. Pregnant CD-1 mice were infected with LV and kept under not stressful conditions. After weaning, puberty male mice were kept under stress (all males in the same cage) or not (animals in individual cage). All male mice received glucose (100 g/l) in the drinking water. Only animals infected in utero and kept under stress devel‐ oped diabetes. Thus, in these animals, viral infection in utero, in combination with stress in adult life could induce diabetes in males [110]. In 2007, Blixt et al. investigated the functional characteristics of pancreatic islets, isolated from female and male bank voles considered as infected by LV. About 20% of all specimens were classified as glucose intolerance/diabetes (GINT/D) following a glucose tolerance test. Of these animals the majority became diabetic by 20 weeks of age, and GINT/D animals had increased serum insulin levels. Functional dif‐ ferences, concerning insulin content, capacity to synthesize (pro) insulin, secrete insulin and metabolize glucose, were observed between normal and GINT/D animals as well as between genders. The increased serum insulin level and the increased basal islet insulin secretion in GINT/D animals suggests that the animals had developed a type 2 diabetes probably due to LV infection associated with stress in laboratory [12].

Human parechoviruses, like LV, belong to the Parechovirus genus; they have also been im‐ plicated in the development of T1D in humans. In a recent nested case-control study, the "Environmental Triggers of Type 1 Diabetes: The MIDIA study", stool samples from 27 chil‐ dren who developed islet autoimmunity (repeatedly positive for two or three autoantibod‐ ies) and 53 children matched for age and community of residence (control group) were analyzed for human parechovirus using a semi-quantitative real-time polymerase chain re‐ action every month from the 3rd to the 35th month. Sera of children were tested for autoan‐ tibodies against GAD, IA-2, and insulin every 3 months until the age of 1 year and every 12 months thereafter. There was no significant difference in the number of infection episodes between the two groups. There was also no significant difference in the prevalence of hu‐ man parechovirus in stool samples throughout the study period, except in samples collected 3 months prior to seroconversion, in which 16/77 samples (20.8%) from cases had an infec‐ tion as opposed to 16/182 (8.8%) samples from controls (OR = 3.17, p = 0.022) [148].

and decreased blood levels of IL-12p40. These data indicates that the TLR9 -signaling path‐ way is implicated in the KRV-induced innate immune activation and participates to the de‐

EMC and KRV are natural viral pathogens of rodent that brought a lot of information as far as the virus–induced pathogenesis of T1D. The role of these viruses in the human T1D has not been reported, however, the Ljungan virus is another rodent virus that has been suspect‐

The Ljungan virus (LV) is a RNA virus discovered in Sweden in the mid-1990s in rodents (Myodes glareolus; formerly Clethrionomys glareolus called "bank vole"). This virus be‐ longs to the Parechovirus genus within the Picornaviridae family. Niklasson et al. described the occurrence of T1D in 67 wild bank voles after 1 month of captivity in laboratory: diabetic animals showed clinic signs of diabetes (persistent hyperglycemia with weight loss, ketosis, and hyperlipidemia) and specific β-cell destruction associated with signs of autoimmunity: increased levels of autoantibodies to GAD65, IA-2, and insulin. The disease was correlated with LV antibodies. Moreover, LV antigen was detected by immunocytochemistry in the is‐ lets of diabetic bank voles. In parallel, two groups of new onset diabetic children were stud‐ ied: the first group represented a total of 53 children which were diagnosed with T1D between 1992 and 1995, and the second group was composed of 289 children with newly di‐ agnosed T1D between 1995 and 2000. The study showed increased levels of LV antibodies in newly diagnosed T1D children indicating a possible zoonotic relationship between LV infec‐

In addtion to type 1 diabetes, viruses could be involved in the development of type 2 diabe‐ tes. Indeed, Niklasson et al. demonstrated that a type 2 diabetes-like disease could be in‐ duced by LV in a CD-1 mouse model. Pregnant CD-1 mice were infected with LV and kept under not stressful conditions. After weaning, puberty male mice were kept under stress (all males in the same cage) or not (animals in individual cage). All male mice received glucose (100 g/l) in the drinking water. Only animals infected in utero and kept under stress devel‐ oped diabetes. Thus, in these animals, viral infection in utero, in combination with stress in adult life could induce diabetes in males [110]. In 2007, Blixt et al. investigated the functional characteristics of pancreatic islets, isolated from female and male bank voles considered as infected by LV. About 20% of all specimens were classified as glucose intolerance/diabetes (GINT/D) following a glucose tolerance test. Of these animals the majority became diabetic by 20 weeks of age, and GINT/D animals had increased serum insulin levels. Functional dif‐ ferences, concerning insulin content, capacity to synthesize (pro) insulin, secrete insulin and metabolize glucose, were observed between normal and GINT/D animals as well as between genders. The increased serum insulin level and the increased basal islet insulin secretion in GINT/D animals suggests that the animals had developed a type 2 diabetes probably due to

Human parechoviruses, like LV, belong to the Parechovirus genus; they have also been im‐ plicated in the development of T1D in humans. In a recent nested case-control study, the

velopment of autoimmune diabetes in the BBDR rat [179, 13].

ed to be involved in human type 1diabetes.

32 Type 1 Diabetes

*2.2.3. Ljungan virus and human parechoviruses*

tion and human T1D [109].

LV infection associated with stress in laboratory [12].

Various viruses were reported to be associated with human T1D: rubella and mumps virus, rotavirus, retrovirus, human parechovirus, cytomegalovirus and parvovirus B19 (table 1). In addition, viruses were reported to be associated with animal T1D: EMCV, KRV and LV, the role of which in human type 1 diabetes has also been studied (figure 2). Using animal mod‐ els, as BB-rats, NOD mice or specific transgenic mouse strains, studies suggested different mechanisms by which viruses may be involved in the initiation or modulation of autoim‐ mune process. These models suggested that a direct infection of islets, responsible for the release of autoantigens, could explain the activation of T-cells and the development of auto‐ immunity. Another hypothesis supported by some studies was the concept of molecular mimicry between virus and β-cells: a normal immune response against a viral antigen would become pathogenic for β cells due to the existence of structural homologies with the pancreatic antigen. In addition to their possible role in the activation of β-cell-reactive T cells, viruses can reduce the capacity of Treg cells to maintain tolerance. Together, these studies suggest that viruses through diffent mechanisms may trigger T1D and/or may par‐ ticipate in the amplification of the autoimmune process. In addition to the viruses presented in this section, the major candidates are enteroviruses. Therefore the rest of this review will be focused on these viruses.


**Table 1.** Viruses involved in human and animal type 1 diabetes grouped according to their genome and their family (in red).
