Non-obese diabetic (NOD) mice, which spontaneously develop autoimmune T1D starting around week 12 of life, have been used to model a variety of aspects of T1D47
. Although the model is criticized as incomplete and/or of limited translational value,70
the NOD mouse is an excellent system for studying the impact which HEV infections have upon T1D development. A female NOD mouse population will develop T1D in 70–100% of the animals by week 15–25 of life if kept in a clean environment but if the mice are not often transferred to fresh sterile cages, water and food, incidences can drop to as low as 20–30%. After inoculating 4 week old NOD mice with group B coxsackievirus (CVB), mice were well protected from developing host-driven autoimmune T1D with of just 0–20%.71
But when older NOD mice were used, which in this system of genetically closely identical mice corresponds to the extent of autoimmune insulitis, CVB rapidly induced T1D onset.72
The age of the mouse (extent of islet inflammation) was key: virus was only demonstrable in islets of older mice, not in young mice. It was shown that as NOD mice aged and insulitis increased, the mice became susceptible—not to protection from T1D onset—but to rapid induction of T1D. This volte face—protection from, to induction of, disease—occurred as a function of the extent of the host's own autoimmune islet inflammation. The strain of the virus used to inoculate the mice, defined in this case by how rapidly it replicates, was also important73
: a more rapidly replicating CVB strain required logs fewer infectious particles to induce T1D than a more slowly replicating strain. Thus, virus strain and infectious dose were linked, as is the case for many infectious disease agents. From these observations with a highly defined system, a link between exposure to HEV in general and how it might affect human T1D was proposed.10
A concern regarding the studies mentioned above is that NOD (or any) mice lack a naturally-occurring infection that triggers T1D onset, in contrast to accumulating evidence that supports a role for HEV as a trigger of human T1D. In this context, it must be recalled that mouse models of various and diverse human diseases have been extremely useful to aid in the understanding of pathogenesis. Two excellent models exist, for example, for human HEV diseases. Though PV is not an infection of mice, mouse-adapted strains74,75
exist and have been used to understand the mechanisms underlying PV-induced paralysis. The construction of mice expressing the human poliovirus receptor (CD155) facilitated these studies further.76,77
Similarly, the CVB replicate well in all mice and have been used in numerous studies to elucidate mechanisms underlying HEV myocarditis, for example.38,78,79
In each of these cases, despite the lack of a natural enterovirus that induces these diseases in mice, the models have proven exceptionally useful and capable of recapitulating observations from humans.
But viruses alone do not comprise the entire cast of T1D-linked infectious agents. Acknowledging the above arguments, one should consider that a failure to develop a sufficiently robust protective immune response against fecal microbiota—bacterial and viral—beginning early in life, may be a primary influence on whether or not a genetically predisposed individual develops T1D. This lack would fail to suppress, either significantly or totally, the destructive naturally-occurring autoimmune response in a genetically prone individual, thereby leading to T1D onset. Moreover, recent insights attribute a major role to intestinal microbiota in the post-thymic functional education of various T cell subsets.80,81
Specifically, it appears that reduced intestinal bacterial diversity is associated with a propensity to develop inappropriate T cell responses against self antigens and development of autoimmune disease. The emerging thought is that intestinal microbiota, as well as viruses, are instrumental in shaping the functionality of T cells in the periphery. Evidence is mounting to suggest that a diverse gut microbiome associates with protection from T1D onset82,83
in genetically at-risk individuals. It is not surprising, then, that a better name for the hygiene hypothesis has been suggested to be the “microbial deprivation hypothesis”84
. Consequently, an HEV infection which meets key criteria49
to trigger T1D in an at-risk individual, might better be understood as an example of an infectious disease, much like a PV infection might induce polio. Because for polio, fewer than 1% of PV infections led to paralytic disease,60
it is reasonable to assume that similar infection/disease ratios hold for other HEV. In the case of a modern human population genetically at risk for developing T1D, only the rare HEV infection might trigger T1D onset. The genetic susceptibility factor supplied by the potential T1D patient may well be modulated by the impact of having a stimulatory diverse gut microbiome from birth onward.
We have argued for the necessity of active insulitis as a key part of the mechanism of HEV-induced destruction of islet β cells and subsequent T1D onset.49
We suggest broadening this to considering that any case of T1D may involve a significant lack of prior infectious agent-induced immune stimulation. The genetic landscape of the host is pivotal in deciding whether viruses and/or bacteria will be effectively cleared or ignored (in the gut), or whether deleterious responses are initiated that lead to T1D. Lacking autoimmune infiltration of islets, humans (based on work in the NOD mouse system71
) might be more resistant to a productive HEV infection of the islets. Were this not the case, we would expect the incidence of T1D to be higher simply based on the large number of HEV infections occurring annually in any population.66,85
Of course, insulitis patterns in NOD mice differ significantly from those seen in humans86
and as little is known about insulitis in individuals without overt disease,87
we must infer more than is desirable. It is, however, worth noting that a recent study suggests that detection of HEV in human islets may be dependent upon insulitis,88
in line with results from the NOD mouse system.72,89
The “fertile field hypothesis”90
originally proposed that environmental infections can prepare the way for subsequent autoimmune process that initiate the disease. However, it has been argued49
that at least in the specific case of T1D, HEV may turn this approach on its head, by taking advantage of a pre-existing fertile field (islets assaulted by the host's own autoimmune disorder) in which to replicate and cause disease. This was based on two experimental results. Experimental infection of young NOD mice without insulitis by CVB show neither islet infections nor can be induced into developing T1D.71
In contrast and in older NOD mice with active autoimmune islet inflammation, CVB readily replicate within islet cells and can rapidly cause T1D onset.72
Seen in the light of a necessity for a pre-existing robust gut microbiome which helps to prevent T1D onset, this model can be theoretically modified to depend on the history of the individual's microbiological exposure as well. Importantly, the host participates in its own protection or demise in either case.
It is interesting to speculate that purely genetically-driven T1D, as well as virus-induced T1D, might now be more common than it was in our past because of a lowered chance of exposure to diverse fecal associated infectious agents early in life. Routine exposure, which surely was a constant part of life in our human past, has now been, at least in part, interdicted by higher levels of hygiene. By this argument, it may be no coincidence that T1D incidences are increasing. While HEV infections in early life may indeed help to lower the risk for T1D development later on, we must assume that either this has (1) enormous importance for the developing immune system in at-risk people, out of proportion to HEV infection frequency, or (2) that it is contributory toward health but not the sole arbiter of protection from T1D. It would therefore seem that another key arbiter early in life in at-risk individuals, must be the nature of one’s gut bacterial population.