Immune Mechanisms of Diabetes Lecture (Dr. Shnyra) Flashcards
Diabetes and Insulin
- Diabetes was considered a TERMINAL CONDITION before Insulin was available for therapy
- History created on January 11, 1922 when a 14 year old boy with T1D become the FIRST RECIPIENT OF INSULIN
- Fredrick Grant Banting received the Nobel Prize for Insulin at the age of 32
Classification of Diabetes
Diabetes Mellitus Type I A: 1) Immune Mediated 2) Immune Etiology Distinction 3) Results in Insulin Dependence, with a loss of Beta Cells
Type I B:
1) Idiopathic
2) Unknown Etiologic Distinction
3) Results in Insulin dependent, with loss of Beta Cells
Type II:
1) Insulin Resistance and relative Insulin Deficiency is the Etiologic Distinction
2) Oral Hypoglycemic agents are Effective EARLY in Life
Genetics, Behavioral Factors, and Environment Triggers have impact on the development of T2D
- Genetic susceptibility, sedentary lifestyle, high-fat diet, and psychological stress have Complementary effects on the Development of T2D
Changes in Immune Cell Populations in Adipose Tissue
- LEAN Adipose Tissue contains GREATER portion of M2/M1 Macrophages
- It also contains a Large Number of REGULATORY T CELLS (Treg Cells)
- OBESITY Leads to Adipocyte NECROSIS and an INCREASE in M1 Macrophages
- There is also a REDUCTION in TREG CELLS and an INCREASE in B CELLS, CD4+ Th1 Cells, and CD8+ T Cells
Obesity results in INFLAMMATION of ADIPOSE TISSUE
- Lean Adipose tissue has ELEVATION Fractions of ANTI-INFLAMMATORY M2-like Macrophages and Treg Cells
- The Local Environment is DOMINATED by ANTI-INFLAMMATORY CYTOKINES IL-10, IL-4, IL-13
- Long term NUTRIENT EXCESS leads to APOPTOTIC and NECROTIC DEATH of Adipocytes
- Upon Obesity, the ADIPOSE Tissue has a Mixed M1/ M2 Phenotype of ATMs, more Cd8+ T Cells than CD4+ Th1 Cells, and fewer Treg Cells
- The Immune Cells promote CHRONIC INFLAMMATION through the production of Pro-Inflammatory Cytokine and Chemokines such as IL-1, TNF-Alpha, IL-6, CCL2, CCL3, and CXCL8 (IL-8)
What is Type I Diabetes
- T1D is characterized by IMMUNE-MEDIATED Destruction of PANCREATIC Beta Cells resulting in INSULIN DEFICIENCY
- Patentis with T1D are prone to KETOACIDOSIS, a Dangerously HIGH levels of Ketones in the Blood
- Most cases are Characterized by AUTOANTIBODY markers of Beta Cells DESTRUCTION and Strong HLA ASSOCIATIONS
- Type I Diabetes is a T CELL MEDIATED AUTOIMMUNE DISORDER
- The onset of T1D is associated with INFILTRATION of the ISET of LANGERHANS by MONONUCLEAR CELLS and CD8+ T Cells. This infiltrate is termed INSULITIS!!!!!!!!
Which Factors Contribute to T1D?
- T1D is the SECOND MOST FREQUENT Autoimmune Disease in Childhood
- The LONG-TERM Micro and Macro-Vascular complication son Diabetes are associated with he leading causes of Disability and even Morality in Young Adults
- T1D Development involved Genetic and Environment Factors, such as BIRTH Delivery Mode, use of Antibiotics, and Diet
- Gut MICROBIOTA could be the Link between Environmental Factors, the Development of AUTOIMMUNITY, and T1D
Environment Factors of T1D
- The CONCORDANCE RATE for T1D is MONOZYGOTIC TWINS is not 100% but about 30 to 50%, SUGGESTING ADDITIONAL NON-GENETIC INFLUENCES!!!
- T1D INCIDENCE is INCREASING about 3% per year, the Rate too HIGH to be attributable to Changes in Susceptibility Genes
- Evidence linking the Environment and T1D in humans in INDIRECT (EPIDEMIOLOGICAL and ANIMAL STUDIES)
- There is also a 350 fold VARIATION in the INCIDENCE of T1D in different countries Worldwide
Breast Feeding vs Cow Milk
- A variety of studies have shown AN INVERSE CORRELATION BETWEEN A DECREASE IN BREAST- FEEDING and the INCREASE IN TYPE1 DIABETES RISK
- EARLY EXPOSE to Cow Milk in life (Ex: because of lack of Breast Feedgin) may CONTRIBUTE to T1D:
a) IMMUNE TOLERANCE TO INSULIN might also be compromised by early exposer to Cow Milk which CONTAINS MUCH LESS INSULIN than does Human Milk - However, there is INCONSISTENCY in results of those studies which may attribute to:
1) VARIATIONS in COMPOSITION of Milk
2) GEENTIC VARIATION in Cow Proteins
3) Vitiations in MILK-SENSITIVE DIABETES-PRONE INDIVIDUALS in Studies
Diet Factors
- WHEAT GLUTEN is a POTENT DIABETOGEN
- The risk of T1D is HIGHER in patients with GLUTEN SENSITIVE ENTEROPATHY
- Other Environmental factors linked with T1D include VITAMIN D, an Immune Modulator and Suppressant
a) The NORTH-SOUTH Gradient of T1D incidence in Europe, with Lower Mean SUNSHINE HOURS in the North - PSYCHOLOGICAL STRESS has also been suggested as a Trigger for Diabetes, but data are sparse and Inconsistent
Hygiene Hypothesis and T1D
- Exposure to a variety of Infectious Agents during early Childhood might be PROTECTIVE in T1D as well
- A Constant INCREASE in the Incidence of T1D contrasted by a GRADUAL DECREASE in the Incidence of INFECTIOUS DISEASES such as TB, Mumps, Measles, Hepatitis A, and Enterovirus Infections
- A Variety of studies have shown an INVERSE CORRELATION between a Decrease in Breast-Feeding and the INCREASE in T1D Risk.
Role Infections in T1D
- STREPTOZOCIN and BIFILOMYCIN A1 form Streptomycin are CYTOTOXIC FOR BETA CELLS
- BACTERIA may also act as ADJUVANTS for the Immune Response to food Ags
- VIRUSES may act against Beta Cells by a Mechanisms that include:
a) Direct CYTOTOXICITY
b) Triggering of an Autoimmunity by MOLECULAR MIMICRY - Viruses which have been Implicated with T1D:
a) MUMPS
b) RUBELLA
c) Cytomegalovirus
d) Enteroviruses
e) Retroviruses
Genetics of T1D
- Familial CLUSTERING in T1D
- The RISK of developing T1D before age 20 years:
a) 6% int he Sibling of an Affected Patient
b) 0.4% in General Population
- The CONCORDANCE:
a) 30 to 50% in Monozygotic Twins
b) 10% in Dizygotic Twins
What genes determine T1D Susceptibility
- Type 1 Diabetes is a MULTIFACTORIAL AUTOIMMUNE Disease
- T1D patient with ant their relatives are at INCREASED RISK for OTHER AUTOIMMUNE DISEASE (Thyroid Autoimmunity and Addison’s Disease
- There may be NO SPECIFIC DIABETES GENE, but….
- Only specific “WRONG” COMBINATIONS of Normal POLYMORPHISMS!!!!
What genes are Associated with T1D?
- About 18 GENES of varying potency are associated with Susceptibility to T1D
MOST SIGNIFICANT
1) The HLA Region (the MHC Gene on CHROMOSOME 6): Presentation of INSULIN Ags for CD8+ T Cells
2) The INSULIN GENE (CHROMOSOME 11): Ag for Autoimmune Response
3) REGULATORS of Insulin Gene expression in the Thymus (AIRE)
4) The CTLA-4 GENE (CHROMOSOME 2): Regulation of Autoimmune REsponse
Role of HLA in T1D
- HLA Alleles DQ2/ DQ8 and DR3/ DR4 are the HIGH-RISK Alleles
- Alleles DQ2/ DQ8 (Haplotypes with DR3DQ2 or DR4DQ8) found in more than 90% OF INDIVIDUALS with T1D!!!!!!!!!
- Heterozygous Genotypes DR3/ Dr4 are most COMMON IN CHILDREN diagnosed with T1D prior to the age of 5 (50%)
- HLA Class II molecules that LACK ASP57 OF THE BETA CHAIN are often found among individuals with T1D
- HAL Class II Haplotypes such as DR2/ DQ6 confer dominant PROTECTION
The Insulin Gene (IDDM2)*****
Check Lecture
- Mapped to a Region contains the VARIABLE NUMBER OF TANDEM REPEATS (VNTR) in the PROMOTER REGION of the Insulin Gene
- the VTNR POLYMORPHISM is categorized into Class I, II, and II
- The SUSCEPTIBLE CLASS I Alleles of the Insulin VNTR are associate with LOWE INSULIN mRNA SYNTEHSIS resulting in:
a) Low Ag (Insulin) Synthesis
b) Low Ag Presentation in the Thymus
c) Failure of Deleting Self-Reactive CD8 T Cells
- The CENTRAL TOLERANCE is BROKEN with Class I Alleles
Transcription AutoImmune Regulator (AIRE)
- Transcriptional EXPRESSION of INSULIN in the Thymus is controlled by AIRE
- Malfunctioning of AIRE results in LOWER LEVELS OF INSULIN mRNA in the Thymus
- The Absence of Insulin results in FAILURE OF DELETING Insulin-Reactive T Cells and the CENTRAL TOLERANCE is Broken!!!!!!
- AIRE is a CRITICAL Factor in the Induction of Central Tolerance against Insulin
CTLA -4 Gene (IDDM12)
- CTLA4 (Cytotoxic T Lymphocyte Antigen-4) is the SUSCEPTIBILITY LOCUS on Chromosome 2 to be associated with T1D
- CTLA-4 (CD152) encodes a Glycoprotein that is a CD28 HOMOLOGUE and bind B7 PROTEIN (CD80/ 86)
- CTLA-4 may COUNTER-REGULATE the CD28- Dependent TCR ACTIVATION of T CELLS
- The Function of CTLA-4 is SUPPRESSION of T CELL ACTIVATION and activation of its APOPTOSIS
Mechanisms of Action of CTLA-4
- Engagement of CTLA-4 on a T Cell may deliver INHIBITORY SIGNALS that Terminate further activation of that cell (Cell-Intrinsic Function of CTLA-4)
- CTLA-4 on Regulatory or responding T Cells binds to B7 Molecules on APCs or removes these molecules form the surface of APCs, making the B7 costimulators unavailable to CD28 and Blocking T Cell Activation
CTLA-4 Controls the PERIPHERAL TOLERANCE
- CTLA-4 competes with CD28 for binding CD80 leading to Cell-Cycle arrest that prevents Expansion of activated T Cells
- Failure of T Cells to express the CTLA-4 gene due to Mutation may Contribute to ABERRANT IMMUNE RESPONSES seen in T1D
- Soluble Recombinant CTLA4 (sCTLA4) has been used in Clinical Trials or TREATMENT of AUTOIMMUNE DISEASES
CTLA-4 on the Board Exams
IN ACTIVATED T CELLS CD152 (CTLA4):
a) Biomes Sequestered within GOLGI
b) Binds to Appropriate MHC
c) Induces PROGRESSION through the Cell Cycle
d) Stimulate Transcription of IL-2 mRNA
e) BEGINS TO MOVE TO THE MEMBRANE AND BINDS CD80/CD86!!!!! (THIS IS THE ANSWER TO THE QUESTION!!)
Autoantibodies in T1D
- ISLE CELL AUTOANTIBODIES (ICA) are detected in Individuals with T1D are present with INCREASED FREQUENCY among individuals recently diagnosed with T1D
- Autoantibody production APPEARS IN ADVANCE (Months to years) of the Metabolic Changes T1D can be used to predict Disease
- Their present CONFIRMS a DIAGNOSIS of Type IA Diabetes
- THE SPECIFICITIES of Several Identified ICA include:
1) Glutamic Acid Decarboxylase (GAD65)
2) Insulinoma Antigen-2 (IA-2, TYROSINE PHOSPHATASE)
3) Insulin AutoAntibodies (IAA)
Pathogenic Role of Auto-Abs
- Despite the utility of Autoantiboes for Diagnostic Purposes, their PATHOGEN CONTRIBUTION IS CONTROVERSIAL:
a) T1D cannot be transferred using Serum from Diabetic Humans —-> PLASMAPHERESIS PRIVETS LITTLE THERAPEUTIC BENEFIT
b) T1D was reported in a 14 year old male with X LINKED AGAMMAGLOBULINEMIA
WHAT IS THE PATHOGEN ROLE?
- Autoantibodies MAY AFFECT THE TIME COURSE of Disease Development
Autoantibodies
- The presence of 2 OR MORE distinct Antibody specificities is HIGHLY PREDICTIVE of future T1D (Five year risk = 28- 66%)
- When COMBINED WITH HLA typing, Autoantibody Screening is also USEFUL FOR PREDICTION disease in General Populations
T1D
- T1D is Th1-MEDIATED DISEASE!!!!
Th1/ Th2 Paradigm
- The T1D AUTOREACTIVE T CELLS belong to the Th2 SUBSET
Pathogenic Role of T Cells
- T Cells are ACTIVATED IN THE LYMPH NODES that drain the Pancreas
- Once activated, ISLET Specific T Cells TRAFFIC TO THE PANCREAS where they PROLIFERATE and accumulate resulting in Organ Specific INFLAMMATION
- LOCAL APCs capable of presenting Ag in the Context of CLASS II MHC molecules and Secreting IL-12 play an important role in the Pathogenesis if T1D
- These PACs activate Ag-specific CD4 T CELLS and further stimulated IFN-GAMMA
- IFN-Gamam INHIBITS Th2 CYTOKINE PRODUCTION (IL-4, IL-5, IL-10) and enhance IL-1Beta, TNF-Alpha, and free radical production of Macrophages which are all toxic to Islet Beta Cells
Cell-Mediated Immunity and T1D
- A counter regulatory role of Th1/ Th2 subsets would suggest that T1D WOULD NOT OCCUR WITH ASTHMA
- However, ASTHMA IS MORE PREVALENT in Children with T1D than Non-Diabetes Children
- What is the COMMON DENOMINATOR?: The Failure of Regulatory Mechanisms controlled by TREG CELLS
- SUSCEPTIBILITY to T1D may be greatly enhanced when TREG CELLS FAIL TO PREVENT ACTIVATION/ Expansion of Auto-Reactive T Cells
- Islet Specific Pathogenic Clones of T Cells are found in Healthy Individuals, but T1D i prevented and the PERIPHERAL SELF TOLERANCE is maintained by Treg Cells
T Regulatory Cells
- Treg cells can ACT locally IN TISSUES and draining LYMPH NODES
- Treg cells become ACTIVATED BY LOCAL APC presenting Auto-Ag
- Treg cells SUPPRESS APCs DIRECTLY through Cell- Cell interactions or INDIRECTLY (Cytokines or Chemokines)
- Alternatively, Treg Cells might act directly on T CELL EFFECTORS
Mechanisms of Action of Regulatory T Cells
- Regulatory T Cells appear to suppress Immune Responses at MULTIPLE STEPS. They may also directly suppress B CELL ACTIVATION and Inhibit the Proliferation and Differentiation of NK Cells.
- Although SEVERAL MECHANISMS of Suppression have been proposed, the following are the best supported by available data:
1) PRODUCTION of the IMMUNOSUPPRESSIVE Cytokine IL-10 and TGF-Beta
2) REDUCED ABILITY of APCs to STIMULATE T CELLS. One proposed Mechanisms of this action is Dependent ob Binding of CTLA-4 on Regulatory Cells to B7 Molecules on APCs
3) CONSUMPTION of IL-2 because of the HIGH Levels of Expression of the IL-2 Receptor, these cells may absorb IL-2 and deprive other T Cell populations of tis Growth Factor, resulting in Reduced Proliferation and Differentiation of other IL-2 Dependent Cells
Cd4+/ Cd25+ Treg Cells in T1D
- ISLET Specific Ags are released and presented in PANCREATIC LYMPH NODES
- The Ags can activate damaging CD4+ and CD8+ T CELLS
- The Ags can also activate CD4+/ CD25+ T reg Cells
- Treg Cells PREVENT ACTIVATION of both CD4+ and CD8+ T Cells
- Treg Cells are shown to PREVENT DIABETES in NOD Mice (Non-Obese Diabetic)
Key Concepts
- REGULATORY T CELLS ARE IMPORTANT
- The Loss of Treg in IPEC Syndrome leads to NEONATAL DIABETES:
1) IMMUNODYSREGUALTION, POLYENDOCRINOPATHY, ENTEROPATHY, X-Linked SYNDROME, a rare disease linked to the Dysfunction of the TRANSCRIPTIONAL ACTIVATOR FOXP3
2) FoxP3 (Forehead box P3) is a SPECIFIC MARKERY of Natural T REGULATORY CELLS - FOXP3 appears to Function as the MASTER REGULATOR in the DEVELOPMENT and FUNCTION of Regulatory T Cells
Key Remaining Questions in T1D
BASIC SCIENCE:
- What genes determine Type 1A Diabetes SUSCEPTIBILITY?
- What triggers or Suppresses the activation of Autoimmunity?
- What are the MAJOR TARGET Molecules and is there a primary Auto-Ag?
- What are the EFFECTOR MECHANISMS for Islet Destruction?
CLINICAL
- Is Type 1A Diabetes PREDICTABLE?
- Is the rate of PROGRESSION to over Diabetes Predictable?
- Is Type 1A Diabetes PREVENTABLE?
Three Major Islet Auto-Ags in T1D
1) INSULIN/ PROINSULIN:
a) Tissue Distribution:
- Beta Cells, Islet of Langerhans
b) Function:
- Regulator of Blood Glucose
c) PREVALENCE of AUTOANTIBODIES at DIAGNOSIS
- 50 to 70%
d) Comments:
- Most PREVALENT in Children with Type 1 DM
2) GLUTAMIC ACID DECARBOXYLASE -65 (GAD65)
a) Tissue Distribution:
- Endocrine Cells in Islets of Langerhan
- Central and Peripheral Nervous System
b) Function:
- GABA Synthesis
c) PREVALENCE of AUTOANTIBODIES at DIAGNOSIS
- 60 to 80%
d) Comments:
- Most prevalent in ADULT-Onset Type 1 DM
3) ISLET PROTIEN TYROSINE PHOSPHATASE (PTPs) IA-2 and IA-2Beta (Phogrin)
a) Tissue Distribution:
- Endocrine Cells in Islets of Langerhans
- Central and Peripheral Nervous System
b) Function:
- Unknown
c) PREVALENCE of AUTOANTIBODIES at DIAGNOSIS
- 40 to 60%
d) Comments:
- None
