Type 1 diabetes (T1D) is a chronic auto­immune disease in which insulin­producing β­cells located in the pancreatic islets of Langerhans are gradually destroyed by autoreactive T cells. Prospective studies with pre­diabetic relatives of patients with T1D have shown that the process of β­cell degeneration can take more than 3 years before the disease manifests itself clinically, and its progression can occur in waves or cycles (FIG. 1a)1. Because of the limited access to samples of the human pre­diabetic pan­creas, very little is known about the precise nature and kinetics of events that take place in humans during these early phases of T1D, and, therefore, our understanding of the natural progression of the disease is largely based on studies in murine models. Because of the complex nature of this disease, predic­tive factors, environmental contributions and immunological therapeutic targets have been difficult to identify.

The mechanism by which the progressive loss of β­cell mass occurs is multifaceted and as a result has become a matter of significant debate. Scientists agree that there is a strong immunological component to the progres­sion of the disease. However, the temporal and compositional details of the immune regulation that is associated with the onset of T1D remain controversial. The central question is whether the chronic aspect of the disease is a feature of the autoreactive

T cells that are involved in β­cell destruction or of the antigenic targets that are present. In other words, is the chronic progression of T1D a result of the recognition of a small number of autoantigens by a limited number of autoreactive effector T cells that, because of their small number, require a long period of time to eliminate the β­cells and/or a result of changes in the number and/or function of regulatory T cells, which control autoimmunity? Or, alternatively, is the chronic state of the disease a result of the enhanced exposure over time of autoreactive T cells to an increasing number of antigenic determinants (as a result of epitope spreading) which leads to an increase in the immuno­logical severity and targeting of β­cells on each new antigenic exposure?

Moreover, secondary events such as viral infections, environmental factors or even metabolic stress might contribute to the expression of β­cell­associated antigens, thereby enhancing the vulner­ability of β­cells to immune destruction. The expression of neo­antigens might even be enhanced by the inflammatory process that occurs in the pancreatic islets per se, as well as by the proliferation of β­cells or hypertrophy of β­cell mass that has been observed to occur in response to inflamma­tion in animal models of the disease. The result of this increase in neo­antigens or in β­cell mass would be a cyclical progression

of pancreatic islet destruction contingent on new or repeated (auto)antigenic expo­sure2,3. Therefore, our hypothesis is that the immunological response that leads to T1D is cyclic in nature and therefore potentially involves similar mechanisms to those observed during other relapsing–remitting autoimmune disorders, such as multiple sclerosis, or during chronic viral infections. We propose that the factors that are respon­sible for the cyclic nature of T1D include the appearance and spreading of antigenic determinants, β­cell proliferation and the actions of regulatory T cells.

The speed of T1D progression would depend on the degree of epitope spreading, the rate of proliferation of β­cells in response to immune attack, and the regulation of auto­reactive effector T­cell responses — which are intrinsic to the immunological process that leads to T1D — by regulatory T cells. If the regulatory response is insufficient, the recurring immune events would eventually lead to the destruction of the majority of the islet β­cell mass and thereby reduce the body’s capacity to produce insulin (FIG. 1).

Understanding and mapping the precise kinetics of the immune response to the disease has implications for the design of immunomodulatory therapeutics, and the regulatory immune component of this disease could be therapeutically exploited to ultimately suppress the progression of T1D. Here, we discuss the still unresolved contro­versies and implications of this hypothesis (if proven correct) for immune­based interven­tions in T1D. To test this hypothesis, and if correct the clinical applications thereof, assays that reliably indicate the status of effector and regulatory mechanisms are vital for complete endpoint analysis.

Cyclical eventsCyclical processes (also known as relapsing­remitting processes) in general, including biological systems, require feedback circuits. For this reason, complex organisms have developed a multitude of feedback systems with synergic and antagonistic features that facilitate the adaptation of the organism to a variety of environmental and intrinsic disturbances, such as infections. Therefore, the more feedback circuits, and redundancy,

O P I N I O N

Type 1 diabetes as a relapsing–remitting disease?Matthias von Herrath, Srinath Sanda and Kevan Herold

Abstract | Chronic immunological processes that underlie persistent viral infections and autoimmune disorders such as multiple sclerosis can be relapsing–remitting in nature. The progressive loss of β‑cell mass during the development of autoimmune type 1 diabetes (T1D) can also be non‑linear, but the exact nature and kinetics of the immunological processes that govern T1D are not known. Here, we propose that the immunological process that is at the root of T1D is relapsing–remitting in nature and discuss the unresolved controversies and therapeutic implications of this hypothesis.

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PersPeCTives

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Nature Reviews | Immunology

Islet antibody 1

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that control a certain process, the greater the stability of the system. The beneficial conse­quence of these feedback circuits is usually a rapid and complete return to baseline parameters.

The immune system is a prime example of this type of system and is comprised of numerous feedback mechanisms that are cyclical in nature. The positive and negative feedback circuits form a mechanism to mount immune responses and eliminate foreign antigens in a highly regulated

manner, such that minimal damage to the host occurs. The duration of these cycles is determined by the specific role of each of the molecules involved, by cellular traf­ficking and entry processes, and by the potential need for clonal expansion and cell death. Because immunological proc­esses, such as the killing of a target cell or the presentation of a microbial antigen, will frequently take a specific reproducible amount of time, so will the cycling of the system as a whole.

Constituents of cyclical immune processesEpitope spreading. Prime examples of the cyclical nature of the immune system have been observed during chronic viral infec­tions, such as with HIv, in which the devel­opment of viral escape variants (that is, new epitopes and viral determinants) typically results in the expansion and the subsequent decrease of the viral load, thereby driving the rise and subsequent fall of the T­cell responses.

Similar events take place during autoimmune diseases. For example, in experimental autoimmune encephalo­myelitis (eAe), the mouse model of multiple sclerosis, a relapsing–remitting disease course has been documented4,5. Interestingly, during each relapse, a different and new autoantigenic epitope is recognized. relapsing–remitting eAe does not occur in mouse strains that can only present one anti­genic determinant to the immune system, which indicates that, in mouse models, the genetic background also has a fundamental role in the response mounted by the immune system4. Therefore, in principle, it is conceiv­able that an in vivo immunological process undergoes cyclical behaviour based on the recognition of new antigenic specificities by autoreactive T cells, and the consequential action of regulatory T cells, thereby leading to the ebb and flow of autoimmune attacks that are typically seen in autoimmune disease. environmental factors could also have a central role in this dynamic process by damaging target cells (for example by stimulating the expression of co­stimulatory molecules that cause immune activation), by enhancing antigen presentation and by stimulating the production of inflammatory cytokines.

The targeting of neo­autoantigens and epitope spreading during the immune response appears to be a prerequisite for the development of cyclical immunological proc­esses and the relapsing–remitting nature of a disease. To date, most isolated autoantibodies linked to T1D are targeted to proteins secreted by β­cells. In addition, modifica­tion of β­cell proteins, or neo­autoantigen formation, can occur during inflammation, infection and metabolic stress. Inflammatory cytokines such as interferon­γ (IFNγ) increase the overall efficiency of antigen presentation by upregulating the expression of MHC class I and class II molecules and by unmasking β­cell antigens for immune recognition6. Whether additional processing and cleavage of intracellular proteins also occurs during T1D is not known, but it is entirely possible based on observations in

Figure 1 | Type 1 diabetes as a relapsing–remitting disease? a | Graph showing the stepwise, non‑linear decline of β‑cell mass over time, as well as the development of autoantibodies that are associated with hyperglycaemia; that is, the onset of type 1 diabetes (T1D)51. b | The immunological response to T1D is cyclic. An increase in the numbers of autoreactive effector T cells is controlled by an increase in the number of regulatory T cells. However, over time, a gradual disequilibrium of the cyclical behaviour could occur, leading to the number of autoreactive effector T cells surpassing the number of regulatory T cells, which would no longer be capable of containing autoreactive effec‑tor T‑cell responses and thereby lead to a decline in pancreatic islet function. c | β‑cell proliferation increases in a cyclical fashion over time. This figure indirectly depicts the biological trends of the development of T1D, which may be attributed to the cyclical nature of the immunological events that lead to the attack or protection of β‑cells. such a phenomenon is usually the result of feedback‑loop mechanisms, which, in the case of T1D, could be due to misdirected effector T cells that are not easily controlled by regulatory T cells. The inflammatory process of the pancreatic islets themselves may enhance β‑cell proliferation and antigenic presentation, ultimately leading to the generation of more effector and regulatory T cells. in addition, as β‑cell mass declines, the pressure on each β‑cell to produce insulin increases, which may be sufficient to alter the recognition of β‑cells by the immune system and to alter their ability to regenerate and increase insulin production.

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Autoantigen repertoire

Time

other experimental systems. In the case of systemic lupus erythematosus, spontaneous non­enzymatic modifications of aspartic­ acid residues alter protein structure, possibly exposing neo­epitopes7.

Antigen processing, which begins as a simple biochemical cleavage of proteins by intracellular proteases, may be severely altered by the presence of post­translational protein modifications. Cytokines (such as IFNγ and interleukin­1β (IL­1β)) and metabolic stress have been shown to lead to β­cell death and to possible alterations in β­cell protein expression by endoplasmic­reticulum stress8–10. Metabolic stresses and immune activation may also lead to the unmasking of previously cryptic  epitopes, to which the immune system has not been tolerized by thymic and peripheral selec­tion mechanisms11. These insults to β­cells may not be unrelated.

The host factors expressed during inflammation might increase susceptibil­ity to viral infection by, for example, the upregulation of receptors for viruses such as the coxsackie virus12. recently, Dotta et al. described the isolation of coxsackie B4 virus from patients who had died at the time of the onset of clinical symptoms of T1D13. The histological findings in the pancreatic islets included evidence of natural killer (NK)­cell infiltration, which could be a sign of local infection. viral infection is frequently associ­ated with inflammation and functional immune impairment. During the process of autophagy — a cellular process that

involves the catabolic degradation of cel­lular components by the lysosomal pathways — the presentation of self antigens may occur by misdirecting some proteins from the lysosomal pathways into the antigen­presentation pathways. Autophagy is increased in β­cells under glycaemic stress and therefore it might have a prominent role in revealing neo­autoantigens as T1D progresses14. Finally, cellular processes unique to β­cells may also be involved in neo­autoantigen for­mation during the onset of T1D.

So, in individuals with T1D an auto­immune response could develop against neo­autoantigens formed either by meta­bolic stress or by inflammatory events that are due to, for example, viral infection; both of which may be exacerbated by ineffective tolerization mechanisms15.

The acquisition of autoantibody specifici­ties in human T1D supports this model. As illustrated in FIG. 2, the number of autoantigens recognized by autoantibodies, and therefore probably also by autoreactive T cells, during the development of T1D increases over time in humans who develop the disease16 and this has also been docu­mented in the non­obese diabetic (NOD) mouse model. In the NOD mouse model, intramolecular and intermolecular spreading of reactivity to antigens — including insulin, glutamic­acid decarboxylase 65 (GAD65) and islet­specific glucose­6­phosphatase catalytic­subunit­related protein (IGrP) — have been identified in diabetogenic T cells5. Among these isolated antigens, a primary role for insulin in initiating the autoimmune process has emerged. This concept is based on the absence of insulitis and diabetes in NOD mice deficient in normal murine insulin and also on the early appearance of insulin­specific antibodies in subjects with T1D, and on the high frequency of insulin­specific T cells in the pancreatic and draining lymph nodes of individuals with autoimmune diabetes16–18. However, recent studies have suggested a sequential hierarchy in the reactivity to these antigens. eliminating immune responses to insulin completely prevents the development of the disease, whereas diabetes still occurs at a normal rate in NOD mice that do not express IGrP19,20. The link between antigenic spreading and disease pathogenesis is strong, because the risk of developing T1D increases proportionately with the number of autoantibodies present (for example those specific for insulin, islet­cell antigen 2 (IA2), GAD65 or IGrP) (BOX 1)20,21. It is therefore reasonable to assume that the process of antigenic and epitope spreading will be applicable to autoreactive T­cell responses, which can directly kill β­cells, leading to

Figure 2 | Repertoire evolution in type 1 diabetes as a driver for relapses. in the early stages of autoimmune disease, very few autoantigens are being presented and recognized by the immune system (left panel). Many studies have shown that chronic or recurrent inflammatory events lead to alterations in antigenic processing and presentation, which leads to a more extensive array of neo‑autoantigens and epitopes that are displayed (as depicted by the panels furthest to the right). This epitope spreading has been well documented in the mouse model of experimental autoimmune encephalomyelitis, in which the relapsing–remitting nature of the disease is due to the involvement of neo‑autoantigens11,52. in type 1 diabetes (T1D), insulin appears to be a crucial primary antigenic target18. During β‑cell destruc‑tion, the debris and antigens of β‑cells are transported to the pancreatic lymph node and presented there by antigen‑presenting cells. A more diverse autoreactive repertoire develops over time, which ultimately results in a more robust and less reversible autoimmune response. it follows that the longer the disease progresses, the higher the number of autoreactive effector T cells. A delay in treatment will make epitope‑specific antigenic tolerance significantly less probable21.

Box 1 | Antigenic targets in type 1 diabetes

Beginning with the identification of autoantibodies, a number of clinical studies have shown that progression to type 1 diabetes (T1D) can be identified in relatives of patients with the disease. Insulin, glutamic-acid decarboxylase 65 (GAD65), islet-cell antigen 2 (IA2) and islet-specific glucose-6-phosphatase catalytic-subunit-related protein (IGRP) are widely viewed as key antigens in the autoimmune response in T1D and autoantibodies specific for each of these antigens have been detected in humans. These antigens are typically associated directly with pancreatic islet β-cells, but some studies have also shown the presence of non-β-cell antigens, such as glial fibrillary acidic protein (GFAP), in the development of T1D. The sequential responses to these antigens imply an evolution of the autoimmune response over time. Although the use of autoantibody production for the early detection of disease has not been widely studied, the presence of multiple autoantibodies has been used to identify individuals at risk of disease. Autoantibody production has been shown to be as high as 80% over 5 years before any hyperglycaemic events, indicating that autoimmune processes involving multiple β-cell antigens precede the occurrence of clinically manifested hyperglycaemia in human T1D by many years.

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the release of new antigens that can then, in turn, be presented to the immune system. This then would lead to new T­cell responses specific for these new antigens and to further spreading, particularly in the context of a more general inflammatory response.

Regulatory T cells. A disrupted balance between the actions of regulatory and effec­tor T cells is a probable cause of the relaps­ing–remitting progression of T1D (FIG. 1b). Autoreactive T cells have a crucial role at the onset of autoantigen­specific β­cell destruc­tion, whereas regulatory T cells are able the restore homeostasis, thereby counteracting the autoreactive immune response following inflammation. There have been reports of defective thymic­derived regulatory T cells (Treg cells) in patients with T1D22, but the significance of these findings in disease progression remains unclear. In addition to these abnormalities in the number and func­tion of Treg cells, adaptive regulatory T cells or other regulatory phenomena might also be abnormal. T­cell activation by pancreatic­islet­associated antigens is characterized by the production of IL­10 in healthy control subjects, but by the production of both IL­10 and IFNγ in individuals with T1D23.

Animal models clearly support the notion that much more rapid T1D occurs when regulatory T cells are absent24. However, paradoxically, regulatory T cells accumulate in the pancreatic islets of NOD mice over time, and the highest frequency of these cells in pancreatic islets is seen at the time of diagnosis, which suggests that inflamma­tion stimulates the formation of regulatory T cells that would be expected to depress the autoimmune response25. However, the actions of these regulatory T cells may be insufficient, especially late during T1D pathogenesis. In addition, regulatory T cells have been shown to function within the pancreatic islets, where they regulate effector T­cell function directly, thereby raising fur­ther uncertainty about the significance of the finding of increased numbers of these cells in pancreatic islets at the time of the clinical onset of T1D26,27.

experimental evidence would suggest that these specific regulatory T cells are functionally ineffective because even a brief exposure to transforming growth factor­β (TGFβ), purported to be a stimulus for regulatory T­cell growth and function, can block progression of the disease. In addition, induction of CD4+CD25+ TGFβ­dependent regulatory T cells with a CD3­specific mono­clonal antibody at the time of T1D diagnosis can allow for the functional recovery of

β­cells28–30. Therefore, regulatory T­cell func­tion can clearly modulate the progression of the disease in animal models.

The function and number of these cells are likely to contribute to the relapsing­ remitting nature of T1D progression but further understanding of their precise tim­ing of emergence and the lack of functional effects of these cells later during disease is needed. Nonetheless, induction of regulatory T cells is likely to be part of a key regulatory mechanism that could be readily harnessed for therapeutic intervention.

β-cell regeneration. In NOD mice, β­cells respond to autoimmune attack by proliferat­ing31,32 (FIG. 1c). This response appears to be directly related to the inflammatory response because it can be transferred to NOD–scid (severe combined immunodeficiency) mice with diabetogenic splenocytes and inhibited by immune regulatory treatments such as a CD3­specific monoclonal antibody or transfer of regulatory T cells32. However, the cellular mechanisms that are responsible are still under investigation.

enhanced vascular supply, the direct effects of cytokines or other factors includ­ing those produced by the inflammatory response, or even an immune response to a declining β­cell mass are possible inducers of β­cell proliferation. Although the result of β­cell proliferation is the generation of new β­cells, β­cell mass still declines because β­cells are destroyed owing to the progression of the autoimmune process.

As discussed earlier, this high rate of cell turnover creates an environment in which proteins may be modified and new peptides may be presented as antigens. Moreover, the enhanced cellular machinery may also increase cell susceptibility to viral infec­tions33. Therefore, β­cell regeneration may have two effects that contribute to the relaps­ing–remitting process of the disease progres­sion: the replenishment of β­cells that are lost during autoimmune attack and the restimulation of the autoimmune response by creating new antigenic epitopes.

Therefore, as illustrated in BOX 2, anti­gen availability and epitope spreading is probably the most crucial ‘ingredient’ for cyclical behaviour within the immune sys­tem. Modification of β­cell proteins during inflammation and β­cell regeneration may lead to the presentation of new antigens to autoreactive T cells and in this way reignite the autoimmune response, even after it has been successfully downregulated, for example through the action of Treg cells.

The honeymoon phaseGiven our limited ability to study the target tissue (the pancreas), does clinical evidence exist to support the notion that T1D is a relapsing–remitting disease process? In other autoimmune diseases, such as systemic lupus erythematosus and multiple sclerosis, clinical data indicate that there are periods of disease flare followed by quiescence. The ‘honeymoon’ phase of T1D, also referred to as the partial remission phase of T1D, might in fact represent a period of quiescence following an acute phase of pancreatic islet autoimmunity.

The honeymoon phase usually begins within weeks of T1D diagnosis, initiation of subcutaneous insulin therapy, and correc­tion of hyperglycaemia. It is characterized by a temporary reduction in insulin require­ments (patients need less than 0.5 units of insulin per kg per day) and improved glycaemic control34. Although the presence of the honeymoon phase does not provide direct evidence of prior remissions in the pre­diabetic stage, the temporary improve­ment followed by worsening of the disease resembles the remission and relapse cycle of other autoimmune diseases.

Studies indicate that between 43–56% of children with T1D will experience a significant honeymoon phase, which can last for up to 24 months after diagnosis35–37. Invariably, metabolic function deteriorates over time. In fact, out of all the subjects who were under the age of 18 at the time of enrolment in the Diabetes Control and Complications Trial (DCCT) only 3% had

Box 2 | Factors that may contribute to relapsing–remitting cycles

• Antigen availability and epitope spreading (BOX 1; FIG. 2).

• Modification of β-cell proteins as a mechanism for generating neo-autoantigens to which tolerance has not been acquired.

• The role of β-cell regeneration in exposure or presentation of new antigens.

• Cellular processes that have been implicated in antigen generation including those that are unique to β-cells. Among the processes unique to β-cells is the effect of metabolic control as a modulator of autoimmunity (an explanation of the effects of strict glycaemic control in C-peptide loss).

• Regulatory T cells — their location, number, specificity and function.

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clinically significant insulin production (stimulated values of >0.2 pmol per ml) after 5 years of disease diagnosis38. Several mechanistic aetiologies for this phenomenon (including a modulating effect of insulin and a decrease in insulin resistance) have been proposed, although none have been entirely proved in large cohorts of patients and it is known that re­establishment of normal glu­cose levels and β­cell rest through exogenous insulin administration is certainly not the only contributing factor here39.

The involvement of the immune system in the honeymoon phase has only begun to be studied. Investigations of NOD mice treated with a CD3­specific monoclonal antibody support the theory that most of the honeymoon phase that follows treatment is due to the recovery of existing β­cells in the pancreatic islets32. Moreover, immunologi­cal remission studies in NOD mice treated with CD3­specific monoclonal antibody indicate that the majority of the functional β­cells following insulin treatment are β­cells that were present at the time of diagnosis of diabetes but metabolically decompensated. These observations raise the possibility that during the honeymoon phase, metabolic control per se, and/or insulin administration, may affect recovery of insulin secretion. This was suggested by findings from the DCCT in which improved connecting peptide (C­peptide) responses were maintained in subjects randomized to strict metabolic control compared to conventional treatment1.

The administration of insulin itself could therefore be a factor, but the results of the Diabetes Prevention Trial­1 (DPT­1) in which insulin itself failed to prevent progres­sion to disease in individuals at high risk, would suggest that the major effect of insulin is more likely to be on glucose metabolism rather than as an immune modulator during the late stages of disease40.

Part of the difficulty in translating these studies is that a solid link between metabolic function and immunology has yet to be fully established in humans. Some studies suggest that there may be a connection between the two, once a hyperglycaemic state has been experienced, as insulin treatment of pre­diabetic patients before onset of hyper­glycaemia did not prevent the development of T1D40. Also, the presence of insulitis or hyper­expression of MHC class I molecules on pancreatic biopsies 3 months after diagnosis predicted worse clinical outcomes for up to 2 years after diagnosis compared to patients without insulitis or hyper­expression of MHC class I molecules41.

More intriguing are data from CD3­specific monoclonal antibody and other clinical trials, which have shown that C­peptide responses to mixed meal­tolerance tests increase with treatment, suggesting that immune modulation can in fact affect metabolic parameters such as the functional capacity of β­cells42–44. Such an effect on C­peptide preservation has been seen with several other immune intervention agents, such as vaccination with heat­shock protein­60 peptide in T1D and with GAD peptide43,45 in latent autoimmune diabetes in adults (LADA). It is interesting to note that early experience with the GAD­peptide vaccine did show alterations in the ratio of CD4+CD25+ to CD4+CD25– T cells in the treated group, suggesting that alterations in immune function may be associated with metabolic changes. However, in the absence of a reliable biomarker of the autoimmune process, it is difficult to differentiate the role of metabolic control, immune remission, or even insulin administration in the develop­ment of the honeymoon phase. It is likely that each of these factors contribute to the clinical picture1.

Our current understanding of these mechanisms that are involved in disease remission during the honeymoon phase suggests that there may at least be a transient arrest of the ongoing autoimmune response and may be consistent with the relapsing–remitting process of this disease. In most cases, the naturally occurring honeymoon phase suggests that it is the quiescent phase in a ‘tug of war’ between inflammation and immune regulation, which is reminiscent of other relapsing–remitting diseases.

An important aspect to study is the overlap between the immunological and metabolic events that occur during

the honeymoon phase and those in the pre­diabetic state. If there are parallels, interventions that are effective during the honeymoon phase should also bear hope for patients at risk of developing T1D. However, unlike in pre­diabetic individuals, who are autoantibody positive but often do not progress to disease, the disease in individu­als that are in the honeymoon phase of T1D invariably progresses.

Future therapeutic implications To determine whether our hypothesis that T1D is a relapsing–remitting disease is correct, a precise immunological study of the honeymoon phase needs to be imple­mented. This phase is considered the last quiescent period before full­blown disease onset and possibly resembles earlier cycles found in the pre­diabetic phase in mouse models, before most of the β­cell mass is lost. To confirm the cyclical nature of the onset of T1D, precise temporal measure­ments of autoantibodies, β­cell mass and T­cell responses (including effector and regulatory T cells) to multiple pancreatic islet antigens at optimal intervals must be obtained (BOX 3). This will also require the refinement of immunology laboratory techniques that can reliably correlate effec­tor and regulatory immune mechanisms with clinical status, although recently published studies in this area have yielded mixed results23,46. However, should such studies yield a clear immunological shift during the honeymoon phase, a stronger platform by which to temporally introduce immunomodulatory therapeutics will be achieved.

Most important for therapeutic intervention will be the disease status (pre­diabetic versus early onset versus late

glossary

Connecting peptide(C‑peptide). Insulin is synthesized by β‑cells as a hormone precursor known as pro‑insulin. When released from the pancreas into the blood, pro‑insulin is cleaved into insulin and a small peptide known as C‑peptide. C‑peptide can be used as a measure of endogenous insulin secretion (one C‑peptide is released for each insulin molecule secreted).

Cryptic epitope A cryptic epitope is an antigenic peptide generated or ‘unmasked’ under altered conditions, such as inflammation or autophagy. When cryptic epitopes become visible to the immune system they become good candidates for eliciting an immune response responsible for autoimmune disease.

Epitope spreadingThe de novo activation of (autoreactive) T cells by antigens that have been released after damage of target cells (in this context, β‑cells) has occurred.

Glycaemic controlThis is a medical term that refers to the typical levels of blood sugar (glucose) in a person with diabetes mellitus. Good glycaemic control, in the sense of a ‘target’ for treatment, has become an important goal of diabetes care.

Honeymoon phaseThis is a partial remission phase in type 1 diabetes that usually begins within weeks of diagnosis, initiation of subcutaneous insulin therapy and correction of hyperglycaemia. It is characterized by a temporary reduction in insulin requirements (patients need less  than 0.5 units per kg per day of insulin) and improved glycaemic control.

Intramolecular and intermolecular spreadingA term that refers to the recognition of new determinants or epitopes by T or B cells during the development of an (auto)immune response.

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onset), which will probably determine the therapeutic protocol used. For example, the induction of antigen­specific regulatory T cells might be most useful during the period when the numbers of these T cells specific for certain pancreatic islet antigens are declining. By contrast, the most effi­cient reduction of effector T cells would occur at times when islet­specific effector responses are at their peak, and might be required to allow for control of the disease by a more limited repertoire of regula­tory T cells47. Similarly, supporting β­cell replication and increase of β­cell mass, for example with glucagon­like peptide 1 (GLP1) agonists, might be best applied at times when β­cells are not optimally rep­licating, for example following treatment with an immune modulator48,49. Overall, phases of relapse might justify the use of a stronger immune suppression regimen to curb aggressive T­cell responses, whereas phases of remission might most optimally profit from enhanced regulatory T­cell induction and increasing the immune suppression regimen may not be necessary during these times.

Last, a central and unresolved question is whether drugs that show promise in patients with early­onset T1D (during the honeymoon phase) will also be effective when given to patients who have not yet developed clinical symptoms of T1D but are at risk from the disease. In fact, recent studies demonstrate clinical effectiveness of CD3­specific monoclonal antibody treat­ment during the early stages of T1D48,50. The therapeutic potential for antigen­ specific immune modulation is highest, provided that the specificities and/or actions of the regulatory T cells are suf­ficiently broad to silence the disease even in its later stages.

Matthias von Herrath* is at La Jolla Institute for Allergy and Immunology, 9420 Athena Circle, La Jolla,

California 92037, USA.

Srinath Sanda is at the Benaroya Research Institute at Virginia Mason, Seattle, Washington 98101, USA.

Kevan Herold* is at the Yale School of Medicine, New Haven, Connecticut 06509, USA.

*These authors contributed equally to this work.

Correspondence to M.H. and K.H. e-mails: matthias@liai.org; Kevan.herold@yale.edu

 doi:10.1038/nri2192Published online 2 November 2007

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Box 3 | Therapeutic implications

If the nature of the effector and regulatory immune response changes (cycles) over time, antigen-specific immune-based therapies for type 1 diabetes (T1D) might need to be tailored to each individual. For example, induction of regulatory T cells might best target an existing regulatory-T-cell response and not target existing memory effector T cells.

Assays need to be developed that precisely assess and stage individual patients in respect to existing immunoreactivities to pancreatic islet antigens.

It might be best to foster and sustain increased β-cell regeneration during those phases of disease progression when β-cells are not already replicating at their peak.

Great therapeutic advantage might lie within the ‘honeymoon’ phase that constitutes a last-ditch effort to stabilize the system and increase β-cell mass.

If events during the honeymoon phase in humans reflect the recurrence of similar cycles that may take place during pre-diabetes, those drugs that show promise in preserving C-peptide levels in patients with recent-onset T1D should be tested aggressively in pre-diabetic patients. By contrast, if this proves not to be the case (as it seems to be in non-obese diabetic mice43), one might have to proceed with more caution.

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DATABASESOMiM: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMiMType 1 diabetes

FURTHER INFORMATIONMatthias von Herrath’s homepage: http://www.liai.orgDcct: http://diabetes.niddk.nih.gov/dm/pubs/controlDPt-1: http://www.diabetesmonitor.com/dpt‑1.htm

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