Mechanisms of b Cell Destruction

Slide 1 · Mechanisms of β-Cell Destruction

Autoimmunity and the natural history of type 1 diabetes

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Trace how failure of T-cell tolerance leads to islet injury.

Distinguish the three stages of autoimmune type 1 diabetes.

Explain the roles of CD8⁺ T cells, Th1 cells and islet autoantibodies.

Connect loss of β-cell reserve with insulitis and clinical manifestations.

Tolerance failure

Islet-directed immunity

β-cell depletion

Clinical diabetes

Slide 3 · The central pathological mechanism

Immune dysregulation

Self-reactive T cells escape control

Recognition

Islet antigens become targets

Effector response

T cells attack β cells

Outcome

Insulin secretory capacity declines

Type 1 diabetes is an immune-mediated β-cell disease.

Slide 4 · Why pancreatic β cells matter

β cells are insulin-producing endocrine cells in the islets of Langerhans.

Insulin promotes tissue glucose utilization and lowers circulating glucose.

Loss of the β-cell population progressively restricts insulin secretion.

The consequences emerge when functional reserve can no longer maintain glucose balance.

Islet β cell

Insulin release

Glucose homeostasis

Slide 5 · The pancreatic islet is the immune target

The endocrine pancreas contains multiple hormone-producing cell types.

Immune injury in type 1 diabetes is centered on insulin-producing β cells.

An islet contains β cells alongside other endocrine cells and local vasculature.

Schematic islet; Spatial distribution is not a histologic measurement

Slide 6 · Insulin reserve links pathology to phenotype

Intact β-cell population

Adequate insulin secretory reserve

Progressive islet injury

Declining β-cell functional capacity

Metabolic impairment

Glucose tolerance becomes abnormal

Symptomatic deficiency

Classic type 1 diabetes appears

Clinical presentation is late relative to the initiation of autoimmunity.

Slide 7 · Type 1 and type 2 diabetes: Different mechanisms

Type 1 diabetes

Primary immune-mediated β-cell loss

Breakdown of islet self-tolerance

Insulitis and β-cell depletion

Type 2 diabetes

Insulin resistance with inadequate compensation

No established autoimmune basis

Islet amyloid may be present

This lecture addresses autoimmune β-cell destruction, not every cause of β-cell dysfunction.

Slide 8 · Clinical onset may conceal years of pathology

The appearance of type 1 diabetes can be clinically abrupt.

The islet-directed autoimmune process begins well before symptoms.

Insulin reserve is progressively depleted during a clinically silent interval.

Symptoms mark a later phase of an ongoing biological process.

Autoimmune initiation

Presymptomatic injury

Metabolic dysfunction

Symptomatic disease

Slide 9 · The three-stage model of type 1 diabetes

Stage 1

Autoimmunity

Normoglycemia

No symptoms

Stage 2

Autoimmunity

Dysglycemia

No symptoms

Stage 3

Autoimmunity

Dysglycemia

Symptoms

Autoimmune activity may precede both dysglycemia and symptoms.

Slide 10 · Stage classification uses three distinct dimensions

Islet autoimmunity: Disease-associated circulating autoantibodies.

Glycemic status: Normoglycemia versus dysglycemia.

Clinical status: Absence or presence of symptoms.

A stage is defined by a pattern, not by a single isolated finding.

Autoantibody status

Glycemic state

Clinical symptoms

Slide 11 · Stage 1: Presymptomatic autoimmunity

Immune feature

At least two islet autoantibodies are present

Evidence of established islet autoimmunity

Metabolic feature

Glucose values remain in the normal range

Insulin reserve still maintains glycemia

Clinical feature

No classic diabetes symptoms

Disease is biologically present but clinically silent

Slide 12 · How stage 1 remains normoglycemic

Islet-specific immunity

Recognizable autoantibody response

Residual β cells

Sufficient functional reserve persists

Glucose regulation

Normoglycemia maintained

Autoimmunity and hyperglycemia are not equivalent states.

Slide 13 · Why multiple autoantibodies are informative

The stage 1 definition requires two or more islet autoantibodies.

Their coexistence supports a disease-associated autoimmune process.

Autoantibodies may be detectable before any symptoms or dysglycemia.

Their presence does not establish that antibodies are the direct mediators of cell death.

≥2 islet autoantibodies

Normoglycemia

Presymptomatic

Slide 14 · Stage 2: Dysglycemia without symptoms

Immune status

Islet autoimmunity persists

The process remains presymptomatic

Physiologic change

Glucose tolerance becomes increasingly impaired

Remaining β-cell reserve no longer fully compensates

Clinical interpretation

Dysglycemia precedes classic symptoms

Biological progression is underway

Slide 15 · Stage 2 reflects loss of compensatory reserve

Continuing β-cell loss

Insulin-producing capacity falls

Restricted response

Insulin output insufficient for glycemic demands

Dysglycemia

Glucose handling becomes abnormal

A symptom-free person may have measurable metabolic disease.

Slide 16 · The presymptomatic period is not a static state

Stage 1 and stage 2 both represent autoimmune diabetes before symptoms.

Stage 2 demonstrates worsening glucose regulation beyond stage 1.

Progression is linked to a declining functional β-cell population.

Classifying presymptomatic disease therefore requires both immunologic and metabolic information.

Stage 1: Normoglycemia

Stage 2: Dysglycemia

Stage 3: Symptomatic

Slide 17 · Stage 3: Clinical type 1 diabetes

Immunologic process

Islet-directed autoimmunity persists

Major β-cell loss is established

Metabolic state

Dysglycemia is clinically manifest

Insulin deficiency is severe

Manifestations

Polyuria, polydipsia, polyphagia

Ketoacidosis may occur

Slide 18 · β-cell depletion underlies stage 3 disease

Symptomatic disease typically after destruction of more than 90% of β cells.

This is an approximate textbook statement, not an individual staging test.

Large loss of cell number makes normal insulin production unsustainable.

Schematic β-cell depletion; Cell counts are illustrative, not measured

Slide 19 · Polyuria follows marked hyperglycemia

Insufficient insulin leads to persistent elevation of plasma glucose.

Hyperglycemia can cause glucose-associated osmotic diuresis.

Increased urine volume is therefore a metabolic consequence of β-cell failure.

Polyuria signals clinical decompensation rather than the start of autoimmunity.

Severe insulin deficit

Hyperglycemia

Osmotic diuresis

Slide 20 · Polydipsia is secondary to fluid loss

Hyperglycemia

Osmotic diuresis develops

Fluid deficit

Water is lost in urine

Thirst response

Polydipsia becomes evident

A clinical symptom can be traced to a specific failure of endocrine function.

Slide 21 · Polyphagia accompanies an insulin-deficient state

Glucose is abundant in blood but cannot be used normally by key insulin-responsive tissues.

Anabolic actions of insulin are reduced.

Disrupted nutrient utilization helps explain hunger despite hyperglycemia.

This is a downstream metabolic effect—not a mechanism of autoimmune recognition.

Insulin deficiency

Reduced nutrient utilization

Polyphagia

Slide 22 · Why ketoacidosis can develop

Severe insulin lack

Normal metabolic restraint is lost

Catabolic shift

Lipolysis and ketone production rise

Ketoacidosis

Potential life-threatening complication

Autoimmune cell loss creates the endocrine deficit; Ketoacidosis is its consequence.

Slide 23 · Five-year progression estimates in the source

Stage 1

Two or more islet autoantibodies

Normoglycemia

Less than 50% progress to symptomatic T1D in five years

Stage 2

Islet autoimmunity

Dysglycemia without symptoms

Approximately 75% progress in five years

Estimates as reported (2026); Not an individualized prediction.

Slide 24 · Progression risk rises with dysglycemia

The stage 2 estimate exceeds the stage 1 estimate in the Robbins account.

Abnormal glucose handling indicates reduced reserve closer to clinical failure.

The risk figures describe groups defined by disease stage.

Do not infer an individual patient’s exact time to presentation from stage alone.

Stage 1: <50% / 5y

Stage 2: 75% / 5y

Slide 25 · T-cell self-tolerance is the decisive defect

Normal safeguard

Self-reactive T cells deleted or controlled

Tolerance failure

Islet-reactive T cells persist

T-cell activation

Endogenous β-cell antigens are recognized

Autoimmune injury

Effector lymphocytes injure β cells

Failure of self-tolerance is the central immune abnormality.

Slide 26 · Two complementary layers of T-cell tolerance

Central tolerance

Deletion of autoreactive clones during thymic development

Insufficient deletion permits escape of self-reactive T cells

Peripheral tolerance

Regulatory mechanisms restrain escaped T cells

Defective regulation or suppression resistance enables immune attack

Slide 27 · Defective thymic clonal deletion

Immature T-cell repertoire

Potential self-reactivity is generated

Negative selection

Normally removes dangerous clones

Selection failure

Some islet-reactive clones survive

The source presents defective deletion as one possible contributor, not as a proven universal defect.

Slide 28 · The AIRE association supports the tolerance model

AIRE participates in central immune tolerance.

Inherited defects of AIRE cause autoimmune polyendocrine syndrome type 1 (APS-1).

Type 1 diabetes can occur in syndromes with profound immune-regulatory defects.

This association supports—but does not reduce all type 1 diabetes to—one gene defect.

AIRE abnormality

Central tolerance failure

Autoimmunity

Slide 29 · INS promoter variation is a susceptibility clue

Variable number tandem repeats occur in the insulin gene promoter.

The textbook proposes altered insulin expression in thymic antigen-presenting cells as a possibility.

Such alteration might influence deletion of insulin-reactive T cells.

The underlying mechanism of the genetic association is not established.

INS promoter VNTR

Possible thymic expression effect

Possible selection effect

Slide 30 · Regulatory T cells suppress effector responses

Escaped autoreactive clone

Potential to attack self antigen

Regulatory T-cell control

Normally restrains activation or function

Regulatory defect

Insufficient suppression permits injury

Peripheral regulation complements central deletion.

Slide 31 · Effector cells can resist regulatory suppression

Regulation may fail because regulatory T-cell function is defective.

Alternatively, activated effector T cells may become abnormally resistant to suppression.

Either failure can permit persistence of a destructive islet-directed response.

The source presents these mechanisms as nonexclusive possibilities.

Weak regulation

Suppression resistance

Persistent effector T cells

Slide 32 · T-cell tolerance: Three mechanistic vulnerabilities

Thymic selection

Defective deletion of islet-reactive clones

Central escape

Treg effectiveness

Reduced suppressive function

Peripheral control failure

Effector responsiveness

Abnormal resistance to regulation

Continued autoreactivity

Slide 33 · Genetic regulation intersects autoimmune susceptibility

T1D has a strong genetic susceptibility component.

CTLA4 and PTPN22 polymorphisms are linked to disease risk in the source.

Both loci connect susceptibility with the regulation of T-cell responses.

Genetic association by itself does not demonstrate a single causal pathway in an individual.

CTLA4

PTPN22

T-cell regulation

Slide 34 · HLA variation implicates antigen presentation

HLA class II variation

Susceptibility alleles differ in peptide-binding regions

Peptide display

Potential presentation of selected antigens

Immune recognition

Possible effect on islet-specific T-cell responses

The exact basis of susceptibility conferred by particular HLA alleles remains incompletely defined.

Slide 35 · HLA is the leading inherited susceptibility locus

The HLA gene cluster as the strongest T1D susceptibility locus.

The textbook estimates a contribution as high as approximately half of genetic susceptibility.

DR3/DR4 and certain DQ8-linked haplotypes are enriched in affected populations.

These alleles alter risk; They are not sufficient for a diagnosis.

HLA association

Peptide binding

Altered immune risk

Slide 36 · Genes create susceptibility, not certainty

Inherited factors

HLA class II loci

INS, CTLA4, PTPN22 and immune regulators

Different pathways can contribute to loss of tolerance

Non-genetic influences

Twin concordance is incomplete

Potential environmental interactions

Triggers remain uncertain

This is a multifactorial model, not single-gene determinism.

Slide 37 · Environmental triggers remain unsettled

The source notes incomplete concordance even among monozygotic twins.

Viruses have been considered possible initiators of islet autoimmunity.

Neither a specific causal virus nor a universal mechanism is established.

Some infectious exposures have also been proposed to confer protection.

Genetic susceptibility

Unknown environmental influence

Immune activation?

Slide 38 · Molecular mimicry is a hypothesis

Viral antigen

May share epitopes with an islet antigen

Antiviral response

Cross-reactive T cells could be generated

Islet injury

Autoimmune attack may follow

Conceptual possibility—not a confirmed obligatory trigger.

Slide 39 · A proposed site of T-cell priming

Damaged islet

Antigens may be released

Peripancreatic lymph node

Islet antigens are encountered

Autoreactive T cells

Activated clones acquire effector function

The site and initiating sequence are presented as proposed mechanisms, not direct observations.

Slide 40 · Antigen release may amplify the immune process

β-cell injury can expose endogenous islet components to immune recognition.

The textbook proposes antigens from damaged islets as possible activating stimuli.

T cells that escaped tolerance may respond when antigen is encountered.

A reinforcing injury–antigen-exposure loop is a conceptual inference, not proven here.

Islet injury

Antigen exposure

T-cell response

Slide 41 · Activated effector T cells reach the pancreas

Local lymph node

Autoreactive T cells become activated

Systemic movement

Activated populations migrate

Islet tissue

Effector cells mediate β-cell injury

The pancreatic islet is the end-organ target of the activated immune response.

Slide 42 · Insulitis: Inflammatory targeting of islets

Insulitis is an inflammatory cell infiltrate involving pancreatic islets.

T lymphocytes and macrophages among the cells found in T1D.

Inflammation and selective β-cell depletion are characteristic pathologic findings.

Original schematic of islet-directed infiltration; Not a micrograph

Slide 43 · Two T-cell effector populations injure β cells

CD8⁺ cytotoxic T cells

Directly kill β cells

Cellular cytotoxicity is a principal destructive mechanism

Th1 helper T cells

Produce inflammatory cytokines

IFN-γ and TNF may contribute to β-cell injury

Slide 44 · CD8⁺ cytotoxicity is a direct injury pathway

CD8⁺ cytotoxic T cell

Recognizes relevant islet target

Effector contact

Cytotoxic attack on β cell

β-cell death

Functional cell mass declines

Robbins explicitly identifies CD8⁺ CTLs as direct β-cell killers.

Slide 45 · Th1-derived cytokines can injure β cells

Islet-directed Th1 response

Helper T-cell activation

Cytokine milieu

IFN-γ and TNF are released

β-cell injury

Inflammatory damage contributes to loss

Cytokine-mediated injury complements direct cytotoxicity.

Slide 46 · Interferon-γ belongs to the inflammatory pathway

Interferon-γ (IFN-γ) among Th1-derived mediators.

Its role in this account is immune-cell-associated β-cell injury.

A named cytokine is not equivalent to a stand-alone diagnostic biomarker.

The source does not provide a quantitative contribution of IFN-γ to cell loss.

Th1 cell

IFN-γ

β-cell injury

Slide 47 · Tumor necrosis factor is another injurious signal

Tumor necrosis factor (TNF) is included among the relevant Th1 cytokines.

Inflammatory mediators can coexist with direct CD8⁺ cytotoxic killing.

Different pathways may operate concurrently within an affected islet.

The textbook does not establish a fixed hierarchy between TNF and IFN-γ.

Th1 cell

TNF

β-cell injury

Slide 48 · Direct killing versus cytokine-associated injury

Contact-dependent route

CD8⁺ cytotoxic T cells

Direct killing of β cells

Immune-effector cellular pathway

Inflammatory route

Th1 T cells

IFN-γ and TNF release

Contributes to β-cell injury

Both pathways are derived from the Robbins mechanism section.

Slide 49 · Insulin is itself an islet autoantigen

Insulin is one antigen considered a target of autoimmune recognition.

Its production by β cells creates antigenic specificity for the endocrine target.

Insulin-related genetic susceptibility may intersect with thymic tolerance.

Autoantigen recognition and direct tissue injury are separate mechanistic steps.

Insulin antigen

Autoreactive T cell

β-cell attack

Slide 50 · Glutamic acid decarboxylase is an autoantigen

The β-cell enzyme glutamic acid decarboxylase (GAD) is another antigenic target.

Anti-GAD65 autoantibodies are listed among circulating T1D-associated autoantibodies.

Antigen specificity explains why an islet-focused autoimmune phenotype is recognizable.

Antibody detection does not by itself establish antibody-mediated killing.

GAD antigen

Anti-GAD65

Immune biomarker

Slide 51 · Common islet autoantibodies in the source

Marker

Antigen or label

Interpretation

Anti-GAD65

Glutamic acid decarboxylase

Marker of islet autoimmunity

Anti-IA2

IA2-associated antigen

Marker of islet autoimmunity

Anti-ZnT8

Zinc transporter 8

Marker of islet autoimmunity

Names are drawn from the Robbins comparison table; Test performance is not specified.

Slide 52 · Why the role of antibodies remains unresolved

What is demonstrated

Most patients may have circulating islet autoantibodies

Antibodies can precede clinical onset

Useful evidence of immune activation

What is not established

Direct antibody-driven β-cell destruction

That every positive antibody causes cell injury

That antibodies alone determine disease stage

The source explicitly distinguishes association from proven pathogenicity.

Slide 53 · Checkpoint blockade illustrates the role of restraint

Immune checkpoints

Normally limit excessive immune responses

Checkpoint inhibitor therapy

Disrupts inhibitory immune regulation

Adverse autoimmunity

Type 1 diabetes can emerge in some treated patients

A clinical perturbation of tolerance supports the mechanistic model.

Slide 54 · β-cell depletion alters islet composition

The type 1 diabetes comparison table describes β-cell depletion and islet atrophy.

Other endocrine cell types may remain within the depleted endocrine structures.

A schematic can explain selective loss but cannot establish microscopic diagnosis.

Original illustrative islet composition, not authentic tissue

Slide 55 · Morphology helps distinguish diabetic mechanisms

Type 1 diabetes

T cells and macrophages may infiltrate islets

Insulitis is characteristic

β-cell depletion and islet atrophy

Type 2 diabetes

No autoimmune islet-destruction mechanism established

Islet amyloid is described

β-cell depletion usually milder

Patterns summarize Robbins; Neither column is a stand-alone diagnostic algorithm.

Slide 56 · Histology is not a routine staging surrogate

The three stages are defined using autoimmunity, glycemic status and symptoms.

They are not defined by direct pancreatic biopsies or β-cell counts.

Functional reserve is inferred from clinical and metabolic behavior.

Islet lesions explain biology but should not be confused with stage-defining tests.

Autoantibodies

Glucose state

Symptoms

Slide 57 · Slow progression is compatible with autoimmune disease

Typical course

Long presymptomatic phase

Symptoms may appear abruptly after major loss of reserve

LADA

Adult presentation with islet autoantibodies

Slower progression

Insulin may not be needed immediately

LADA is discussed in the parent Type 1 Diabetes section.

Slide 58 · Type 1 diabetes is not restricted to childhood

The source emphasizes that type 1 diabetes can develop at any age.

The term juvenile-onset diabetes is therefore not preferred.

Insulin dependence describes treatment need, not an exclusive pathologic category.

Immune-pathogenesis evidence is more informative than age alone.

Childhood onset

Adult-onset autoimmunity

Shared immune mechanism

Slide 59 · Case study: Autoimmunity before hyperglycemia

A person being monitored because of familial susceptibility has two distinct islet autoantibodies on repeat testing. Glucose regulation remains normal, and no symptoms are present.

This is a fictional educational scenario; No patient data are represented.

What is the correct stage?

Stage 1: Multiple autoantibodies, normoglycemia, no symptoms.

Slide 60 · Case study: Silent metabolic progression

An individual with established islet autoimmunity develops abnormal glucose tolerance but reports no polyuria, thirst or other classic symptoms.

The aim is to classify biological stage independently of symptom burden.

Which finding defines progression?

Stage 2: Dysglycemia despite absence of symptoms.

Slide 61 · Case study: Symptomatic β-cell insufficiency

A patient presents with polyuria, polydipsia and marked hyperglycemia. Prior tests documented multiple islet autoantibodies. An insulin-deficient phenotype has now become clinically apparent.

This scenario intentionally omits invented numerical laboratory values.

What is the stage?

Stage 3: Dysglycemia with clinical symptoms.

Slide 62 · When interpreting autoantibodies, avoid a causal leap

Observed

Disease-associated antibody detected

Supported interpretation

Islet autoimmunity is present

Unsupported inference

Antibody itself must be killing β cells

Biomarkers and effectors occupy different positions in a causal model.

Slide 63 · T1D versus T2D: Mechanistic discrimination

Question

Autoimmune T1D

Typical T2D

Primary mechanism

Immune effector destruction

Insulin resistance plus inadequate compensation

Islet antibodies

Commonly present

Typically absent

Islet lesion

Insulitis, β-cell depletion

Amyloid, relatively mild β-cell loss

Role of HLA

Strong susceptibility association

No analogous HLA linkage reported

Slide 64 · Evidence, proposals, and unknowns

Established in the source

T-cell self-tolerance failure

CD8⁺ cytotoxicity and Th1 injury

Progressive β-cell depletion

Proposed / unresolved

Exact initiating antigenic event

Specific environmental trigger

Whether autoantibodies directly injure β cells

The categories describe certainty conveyed by the textbook, not an exhaustive literature review.

Slide 65 · Integrating the pathogenesis

Susceptibility

Immune regulation and HLA associations

Tolerance failure

Autoreactive islet-specific T cells

Effector injury

Cytotoxicity plus inflammatory signals

β-cell loss

Insulin reserve declines

Clinical diabetes

Dysglycemia then symptoms

One causal narrative connects inherited risk, tissue injury and phenotype.

Slide 66 · Three conclusions for pathology practice

Mechanistic

Type 1 diabetes is predominantly a T-cell-mediated autoimmune disorder

Antibody pathogenicity remains unresolved

Temporal

Autoimmunity precedes overt hyperglycemia

Stage 1 → stage 2 → stage 3 describes clinical evolution

Morphologic

Insulitis involves inflammatory islet infiltration

β-cell depletion underlies insulin deficiency

Slide 67 · Principal textbook and source context

Robbins, Cotran & Kumar: Pathologic Basis of Disease

11th edition (2026), Chapter 24 — The Endocrine System

Primary topic: Mechanisms of β-Cell Destruction (printed pp. 1002–1003)

Context: Pathogenesis of Type 1 Diabetes and adjacent endocrine pancreas sections (pp. 997–1003)

Staging concept in textbook Fig. 24.31 is credited to Insel RA et al., Diabetes Care (2015); diagrams here are newly authored schematics.