Predicting the outcome of immunization with islet antigens is complex. The resulting immune response depends not only on the dose, frequency, and route of administration but also on the precise context, in which the use of suitable adjuvants and inflammation can profoundly influence the resulting immune response or lack thereof. In addition, one should expect interindividual variations in the autoreactive repertoire of T-cells: some islet-reactive T-cells might already be activated at the time of immunization and their avidities can be expected to vary depending on central (thymic) and peripheral tuning events, which in turn will influence the character (magnitude and cytokine production) of the resulting antigen-specific response. The underlying mechanisms are more apparent in murine studies, but evidence is also beginning to emerge from human studies in vivo, both in autoimmunity and comparable inflammatory settings. The two main outcomes involve the induction or augmentation of beneficial (regulatory) immune responses (1
) and the elimination (deletion) of deleterious islet–specific effector responses (4
), both of which are context-dependent and will be discussed below (). It is self-evident that both outcomes could be beneficial in type 1 diabetes () where there may be both a regulatory T-cell (Treg) defect and effector cells that are relatively resistant to regulation (7
Mechanisms of action of antigen-specific immunotherapy and predicted nature of response
FIG. 1. Beneficial effects of antigen-specific immunotherapy (ASI) on the pathological immune responses that result in type 1 diabetes. β-cell damage is a result of the combined actions of proinflammatory helper T cells (TH1) and cytotoxic T lymphocytes (more ...)
Studies in mice have clearly documented the enhancement of Tregs and the skewing of cytokine responses (immune deviation) after mucosal (oral, nasal) or peripheral (peptide with and without adjuvant, DNA vaccination) insulin, proinsulin, or other autoantigenic peptide administration (8
). In these experiments, repeated administration of β-cell autoantigens, most notably insulin or its peptides, led to increased interleukin (IL)-4, IL-10, and transforming growth factor (TGF)-β production by insulin-specific polyclonal T-cell populations isolated from the spleen, pancreatic lymph nodes, and, in some cases, the islets. Unfortunately the numbers of autoreactive T-cells in the blood are low and, therefore, little information is available about their frequency and function in peripheral blood, which would be very helpful to guide human biomarker efforts. The antigen-induced or enhanced cell populations can actively suppress effector immune responses as evidenced by adoptive transfer studies. Unless they are enabled by certain chemokines or chemokine receptors or integrins to home to solid organs, the antigen-induced modulating cells are usually found in lymph nodes and spleens following transfer into pre-diabetic recipient mice. Thus, antigenic immunization can endow islet-reactive T-cells with the ability to regulate and suppress deleterious effector responses, which entitles them to be called adaptive regulatory T-cells (aTregs). In some cases, the transcription factor forkhead box P3 (FoxP3), a good marker for naturally occurring Tregs (nTregs), shows increased expression. FoxP3 is of value as a bona fide Treg marker in the mouse, but its utility in humans is much more limited because it is also expressed on recently activated effector T-cells. Thus, successful ASIs that prevent type 1 diabetes in animal models are associated with the induction of cytokines, which can be considered as protective from type 1 diabetes (immune deviation) and are produced by CD4+ T-cells that can function as aTregs. In this context, the difference between immune deviation and Treg induction is mainly a semantic argument. The control of effector responses of various specificities (bystander suppression) likely occurs through modulation of antigen-presenting cells (APCs) resulting in a lack of anti-islet effector T-cell expansion, but not their deletion. New alternative models have also been described recently in which APCs are perhaps incapacitated from their role of antigen presentation to autoreactive effector T-cells through a direct APC-lytic process mediated by Tregs both in vitro in man and in vivo in murine studies (10
Similarly, immune deviation indicative of Treg generation has been documented in humans (for example, the induction of a proinsulin peptide–specific IL-10 response after low dose intradermal administration of the peptide in type 1 diabetic patients) (12
), reminiscent of observations in studies of peptide immunotherapy in clinical allergy (13
). Other examples are the significant increases in GAD-specific γ-interferon, IL-5,-13,-10,-17,-6, tumor necrosis factor-α, FoxP3, and TGF-β mRNA responses as well as autoantibody induction (15
) following subcutaneous administration of 20 μg of recombinant human GAD65 adsorbed onto alum (in a classic prime-boost regimen, n
= 35/group). In other clinical trials where the limited effects of ASI have been documented, for example after daily dosing of oral insulin (16
) or after a proinsulin-expressing DNA vaccine (17
), no such clear effects as of yet have been documented and will have to await future biomarker studies.
The strong clinical advantage of modulating the β-cell–specific immune response and redirecting its aggressive nature to a more regulatory function is that the resulting aTregs can suppress heterologous islet–specific effector responses, and they can do so in a site-specific manner because they are predicted to become active only at sites where islet antigens are being presented. Thus, in a sense, ASI offers a site-specific immune modulatory drug. The clinical disadvantage is that antigen-specific therapies may have less potency than directly immunosuppressive strategies, as evidenced by the fact that they tend only to work prior to onset of diabetes in preclinical models. Translation to man may therefore require deployment at the early stages of pre-diabetes or enhancement with other complementary strategies (see below). Thus, optimization of dosing and delivery regimens for ASI in parallel with the development of suitable adjunct therapies needs to be considered as a major priority area (see the detailed discussion below).