Temporal Divergence of Viral Insult and Clinical Onset
Despite decades of investigation, no evidence exists for the involvement of a particular viral strain with T1D. The search for a correlation with certain viral agents can be expected to be complex for a variety of reasons. In both mouse models and T1D patients, development of clinical hyperglycemia is thought to represent the final stage of the autoimmune process. Therefore, it can be assumed that the event that initiates the loss of tolerance against islet antigens likely precedes the onset of diabetes by several months or years. This temporal discrepancy poses considerable restraints in studying the role of viral infection in T1D development, as the vast majority of patients are traditionally sampled after diagnosis. Taking into account that inciting viral agents may use a “hit-and-run” strategy, or act by repeated, sequential infection, analyses around clinical onset may at least in some occasions miss out on the culprits.
Alternatively, viral infection may only serve as an accelerating factor that, superimposed onto advanced insulitis, leads to rapid culmination into hyperglycemia. The latter scenario would suggest that detection of viral particles around onset is an achievable goal in determining a causal relationship.
Multiple Viruses May Provoke Disease in Similar Fashion
The fact that no absolute association has been identified with certain viral strains or even viral genuses or families indicates that, if T1D is indeed caused by viruses, multiple infectious strains may result in the same disease phenotype. Historically, samples from T1D patients have been probed by a “one test-one pathogen” approach that unavoidably introduces experimental bias. Whether the assay is based on detection of virus-specific antibodies or nucleic acid sequences, such strategies are costly, inefficient, and time-consuming and generally make poor usage of the limited sample volumes that are available from T1D patients. To cast the net more widely in the evaluation of a viral etiology, emerging nucleic acid technologies to detect pathogens on a broad-spectrum basis should be applied on blood and pancreas samples from T1D patients at various stages pre- and postdiagnosis. Indeed high-density microarrays, such as the Virochip pan-viral microarray and deep sequencing, can test for thousands of potential pathogens simultaneously (Wang et al. 2002
Alignment with the Hygiene Hypothesis
Based on our present knowledge, enteroviruses would appear associated with at least a fraction of T1D cases. But if enteroviruses are indeed a major contributor to T1D pathogenesis, how can we explain the increase in T1D incidence in countries where exposure to enteroviruses has been dropping (e.g., Finland) (Viskari et al. 2005
)? In other words, is the theory that T1D can be caused by a viral infection compatible with the hygiene hypothesis? Based on the findings in the NOD mouse, one could argue that the lack of exposure to enteroviruses in developed countries results in a reduced frequency of individuals with protective immunity through early childhood infections. When genetically driven islet inflammation occurs in these unprotected individuals, they would be more susceptible to an enteroviral infection that has the potential to initiate overt autoreactivity and β-cell damage.
Immunization Strategies: Why Not Now?
So why don’t we initiate population-wide vaccination programs to more thoroughly and directly evaluate the role of enterovirus in T1D? Theoretically, virologists deem the development of enterovirus vaccines relatively straightforward and achievable (S Tracy, pers. comm.). The main limitation at present is that the enterovirus genus of the Picornavirida family consists of five virus species. These virus species in turn contain many different strains and serologically distinct viruses (Fauquet 2005
). Any one or a combination of these viruses could be the virus detected by, for example, the anti-VP1 antibody that is commonly used in immunohistochemical analysis. Finnish groups are currently attempting to delineate which enteroviral strains are most prevalent in T1D patients to clarify serotypes that should be immunized against (Roivainen 2006
A disturbing observation related to the idea of prophylactic vaccination is the finding that CVB infection protects against diabetes development in the young NOD mouse. This protection is orchestrated via at least two distinct suppressive immune mechanisms, the up-regulation of the inhibitory PD-1/PD-L1 pathway and increasing numbers of circulating T cells with regulatory capacities (Filippi et al. 2009
). Such data illustrate the dual role of viral infections in autoimmunity, and portray T1D development as a balancing act between immune “education” by viruses (see “hygiene” hypothesis) and the induction of aberrant immunity in response to these agents. Moreover, they suggest that the protective effect of viral infections is a proactive mechanism that involves the emergence of regulatory mechanisms and thus, exceeds the achievement of sterile immunity which would be the ultimate goal in vaccination programs.
As a final note, the introduction of childhood immunizations programs and the growing prevalence of T1D in developed countries have also provided rationale for assessing a possible correlation between the two entities. Multiple large-scale studies found no support for any causal relation between childhood vaccination and T1D (Blom et al. 1991
; EURODIAB Substudy 2 Study Group 2000; DeStefano et al. 2001
; Hviid et al. 2004
). As there appears to be no significant association between vaccination and T1D, the risk-benefit ratio as of today balances strongly in favor of continued protection efforts by means of immunization.