Pathogenesis of Type 1 Diabetes

Question 1

Outline the epidemiology of type 1 diabetes

Answer 1

Highest incidence in people of european (northern) or scandinavian decent

In europe, about 3-4% increase in incidence per annum

Peak incidence at 5-7 years of age and again at puberty.

Question 2

Outline the risk factors for T1DM

Answer 2

Polygenic condition with >40 genes identified from GWAS

HLA genes (chromosome 6) contribute to >50% of risk

Class II genes (DQ and DR) are the most importnat deteminants of T1DM)

Expressed on antigen presenting cells and present antigen to CD4 T-Lymphocytes

Linkage disequlibrium between DR and DQ genes

()% of individuals with T1DM have DR4/DQ8 or DR3/DQ2 haplotypes

Overall, 69% of patients have HLA-DQ phenotypes

Theere are laso non-HLA genes involved eg. insulin, VTNR, PTN22

Question 3

What are HLA genes and what do they do?

Answer 3

The human leukocyte antigen (HLA) system or complex is a gene complex encoding the major histocompatibility complex (MHC) proteins in humans. These cell-surface proteins are responsible for the regulation of the immune system in humans.

Question 4

Majority of individuals with high risk HLA haplotypes do not go on to develop T1DM, there are searchesa for the environmental triggers of this autoimmune process. Outline some.

Answer 4

Viral infection - enteroviruses (coxsackie B), CMV, rotavirus all implicated. The development of autoimmunity follows a seasonal enterovirus infection pattern. ENterovirus positively associated with development of autoantibodies in T1DM cohort.

North South hypothesis - Vitamin D supplementation during pregnancy reduces T1DM in offspring.

Multiple additional hypotheses

Question 5

Briefly outline the pillars of the natural hostory of T1DM

Answer 5

Longstanding model of a chronic autoimmune process

Break in immunological tolerance with loss of beta cell mass

Initial loss of glucose stimulated insulin release

Later development of insulin deficiency

Question 6

• Islet autoantibodies are currenlty the only available biomarkers of iset autoimmunity and T1DM pathogenesis. What are they?

Answer 6

Autoantibodies against β-cell protein:

• Insulin
• GAD65
• IA-2
• ZnT8 transporter

Individuals may harbour one or more of these autoantibodoes, risk of T1DM increasing wioth each additional one.

These autoantibodies are ddetectible many years before the onset of clinical T1DM

Question 7

What percentage of people with T1DM haev at least 1 islet autoantibody present? What is the advantage to detecting these autoantibodies in asymptomatic individuals?

Answer 7

93%.

Detection identifies at risk groups who can be monitored to study natural history of the disease or for inclusion in prevention trials.

Question 8

The rate if progression to diabetes is determoned by the number of autoantibodies - what is the risk of diabetes development in individuals with 4 AABs at 5 years?

Answer 8

Around 60%

Question 9

Outline the immune mechanisms involved in T1DM

Answer 9

Immun destruction of beta cells in pancreatic islets involving bnoth adaptive and immune repsonse.

• Pancreatic ilste sdemonstrate infiltration by CD8+ T cells
• These cells demonstrate exlcusive sepcificity to islet autoantigens (Prototpyic tissue specific autoimmune disease)
• There is makred hetrogeneity in the pathological findings and phenoypte in T1DM
• Beta cells function does not necessarily equate with beta cell mass in T1DM
• There is possibilty of immune moduklation as a theepeutic strategy

Question 10

Autoreactive t-cells that mediate ther destruction of beta cells in t1DM are detected in high numberer in people who do not have T1DM or other autoimmune disease. Why do people with T1DM therefore aquire the disease?

Answer 10

T1DM occurs when the tolerance to autoreactive T-cells becomes imbalanced:

After birth ikkmune balance is established in which autoreactive T-cells are kept in check by immune regulation, prevemnting them from becoming active.

Follwing an unknown trigger, regulatory T-cells can no longer control autoreactive t cells.

This imbalance results in activation of B-cells (producing islet autoantibodies) and effector T-cells

Over time immune regulation is outweighed by islet autoreactivity leading to destruction of beta cells and emergence of clinical features.

Question 11

FUndamentally T1DM occurs due to the imbalance in autoreactive T-effector cells and T-regulatory cells. What are the 2 possibilities that can lead to this occuring?

Answer 11

1. Defect in T regulatory cells - failure to suppress autoreactive T-cells
2. Effector T-cells resistant to immune regulation

Question 12

Describe the therapeutic goals of T1DM therapy?

Answer 12

• Prevention of autoimmune destruction
• Preservation of beta cells mass
• Replacement or regeneration of beta cells
• Automated insuilin delivery

Question 13

Describe the difference between primary prevention, secondary prevention and intervention and give examples of each approach.

Answer 13

1° prevention - Targeting high risk individuals before islet autoimmunity develops.

Autoantigen dependent approaches - promoting immune tolerance against islet protein cpuld prevent development of autoantibodies.

Oral insulin trials to induce immune tolerance.

2° prevention - after islet autoimmunity develops. Very large number of potential strategies.

Non-autoantigen approaches - Multiple studies ongoing, cyclosporin, BCG vaccination, gluten free diet etc all show no effect on progression to T1DM

Autoantigen-specific approaches - Many based on insulin administration with the hypothesis that it reduces beta cell load and induces immunological tolerance.

Intervention - Targeting newly diagnosed individual with T1DM.

Immune suppression agents currently being studied - anti-CD3 and anti-CD20 monoclonal antibodies

Alternative approaches using autoantigen specific approaches also studied.

Question 14

Which Anti-CD3 antibody is showing clinical promise in the prevention of development of T1DM? (2019)

Answer 14

Teplizumab - number of high risk indiviuals who proceeded to develop T1DM after a 2 week course od the drug was reduced compared to placebo.

Question 15

Why is beta cell replication important?

Answer 15

Since beta cell mass is reduced in both type 1 and 2 duabetes, beta cell regenration offers atreactive potential therapeutic strategies.

Effective approach for T2DM, but in T1DM number of cadaveric islets available is small, promting considerable interest in alternatuive sources (hES, iPSC)

Question 16

Outline the main points in human beta cell proliferation.

Answer 16

Beta cells in the embryonic pancreas develop by neogenesis - differentiation forom precursor cells. There is negligable beta cell replication in the human embryo.

Marked increase in replication after birth which peaks at 1 year before falling off into early childhood.

Studies from adult cadeveric pancreata indicate a proliferation rate close to 0.

Question 17

Rodent studies have provided insight into the development, funciton, differentiation and survival of b-cells in a mammallian setting., however there are fundamentl adifferences between beta cell physiology in rodents and humans reflecting key nutritional and metabolic needs. Outline the differences between rodent and human beta cell populations and functions

Answer 17

Rodent islets consist of 80% beta cells formed in a central core, human islets consist of only 50% beta cells and are diffusely scattered.

Rodent peak beta cell proliferation os much higher than humans, with around 30% observed in the neonatal period.

Very large rnumber of egents able to replicate rodent b cells, but this is not replicable in human populations.

Question 18

Despite low to absent proliferation of human beta cell proloferation, all of the key cell cycle regulators are present. Why is the proliferation rate therefore so low?

Answer 18

Beta cells contain large number of upstream inhibitory molecules which prevent entry of positive cell cycle regulators to enter the cell cycle - Balance in favour of inhibitory molecules

Such inhibitory molecules need to be overridden to facilitate proliferation.

In contract to the majority of cell tpyes, many of the key cell cycle regulators are present in the cytoplasm of beta cells rather than the nucleus - perhaps failure to access the nuclear compartment is the reaon for near absent proliferartion

Question 19

Studying disorders in which abnormal bets cell replication is demonstrated may provide insight into beta cell regulation. One such condition is MEN 1. Summarise this condition.

Answer 19

Multiple endocrine neoplasia type 1 (MEN1) is characterised by the development of pancreatic neuroendocrine tumours (amongst others e.g. pituitart, parathyroid) arising from the pancreatic islets (insulinoma, glucagonoma), and results from a mutation on the MEN1 gene, which acts as a tumour suppressor.

Recent studies have indicated Menin as a key determinant of proliferation pathways in pancreatic endocrine cells.

Question 20

What does Menin do?

Answer 20

Menin is involved in poroliferation pathways in pancreatic endocrine cells - Blocks proliferation. It determines KRAS signalling output in these cells, which is considered an oncogene due to activation of MAPK pathways.

i.e. removal of menin removes block on MAPK pathways in KRAS singalling, thus resulting in net proliferation - Seen in MEN1 endocrine cell.

Question 21

KRAS/RAF/MEK/ERK pathways inhibitors can be used as treatment against which condition?

Answer 21

MEN1

Question 22

What happens to beta cell mass durinf pregnancy and how does this change come about?

Answer 22

Beta cell mass increases - thought to do so to meet increase in insulin demand during pregnancy.

Lactogenic and serotinergic signalling.

Question 23

Mitogenic signalling pathwyas (JAK-STAT, AKT/PI3K, mTOR) have been shown to be involved in pancreatic islet proliferation in rodents.

What are the observations of miitogenic signalling in human beta cell populations?

What are potenital impediments to human beta cell proliferation?

Answer 23

However the relevance of such pathways in human beta cells reamins uncertain, reflecting lack of physiologically relevant beta cell populations to study. It appears that the activation of mitogenic pathways in human beta cells appears challenging and only produce modest effects in aboslute terms of proliferation.

• Wrong receptors or transporters
• Wrong growth factors or nutrients
• Inadequate vascularisation
• IMpaiored nucelar trafficking of cell cycle molecules
• Epigenetic silencing or activation of key factors
• Different cell cycle machinery compared to that of rodents
• Excessive levels of cell cycle inhibitors that result in senescence (loss in cell division or growth/deterioration with age)
• Inneffective or different signalling pathways

Question 24

What are limitation of current islet cell transplantation?

Answer 24

• Severe shortage of donor pancreases
• Inconsistency of islet isolation
• Alloimmunity and autoimmunity - need for immunosuppresion

Question 25

What is the rationale for using islet cell transplantation?

Answer 25

To transplant endocrine only tissue into thr pancreas, restoring physiological glucose homeostasis in T1DM.

Question 26

What are possible sources of human beta cells

Answer 26

hESC and iPSCs - appear to hold the most promise

Cellular reprogramming and transdifferentiation

In vivo beta cell regeneration