Role of Infections and Other Environmental Factors

Slide 1 · Role of Infections and Other Environmental Factors

Infectious stimuli

APC costimulation

Molecular mimicry

Environmental exposures

Microbiome and tissue damage

Ultraviolet radiation; smoking

Immune balance

Potential protection by infection

Sex-related susceptibility

Autoimmunity emerges from interactions between immune tolerance, susceptibility, and environmental context.

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Explain infection-induced APC costimulation and loss of T-cell anergy.

Distinguish molecular mimicry from nonspecific microbial B-cell activation.

Trace tissue injury, ultraviolet radiation, and smoking to proposed autoimmune mechanisms.

Evaluate microbiome, protective infection, and sex effects without overstating causality.

Integrate environmental triggers with genetic susceptibility and immune tolerance.

Slide 3 · Autoimmunity requires failure of self tolerance

Self-reactive lymphocytes may exist

Peripheral tolerance restrains activation

Susceptibility and environmental signals disturb control

Immune responses damage self tissues

Environmental stimuli are enabling factors, not sufficient causes in every individual.

The same self antigen can be innocuous or immunogenic depending on presentation context.

Slide 4 · Why infection is an attractive trigger

Immune activation

Microbes activate antigen-presenting cells

Costimulatory capacity can increase

Antigenic overlap

Microbial structures may resemble self

Cross-reactivity becomes possible

Inflammatory damage

Tissue injury exposes self constituents

Cytokines recruit additional cells

Slide 5 · Genetic susceptibility does not determine destiny

Inherited predisposition

HLA alleles influence peptide display.

Non-HLA variants alter immune regulation.

Most carriers do not necessarily develop disease.

Environmental contribution

Infections can activate APCs.

Tissue damage can change antigen access.

Exposure interacts with an already susceptible host.

Genetic association and an environmental exposure are not individually equivalent to causation.

Slide 6 · Central tolerance and peripheral tolerance

Central tolerance

Immature autoreactive lymphocytes are removed.

B cells may undergo receptor editing.

Some regulatory T cells develop centrally.

Peripheral tolerance

Anergy silences antigen recognition without activation.

Regulatory T cells suppress responses.

Inhibitory pathways and apoptosis restrain self-reactive cells.

Slide 7 · The two-signal requirement for T-cell activation

Signal 1: Self peptide–MHC presented to T cell

Signal 2: B7 on APC engages CD28

Adequate signals permit T-cell activation

Antigen recognition without sufficient costimulation normally favors functional unresponsiveness.

Infection can change the second-signal context of an otherwise familiar self antigen.

Slide 8 · Anergy: Recognition without effective response

Resting APC context

Self antigen may be presented.

B7 expression may be inadequate.

Autoreactive T cells remain functionally anergic.

Infection-activated context

APCs increase costimulatory signals.

Self-antigen presentation may continue.

A previously unresponsive T cell can be activated.

The critical change is immune context, not necessarily the identity of the self peptide.

Slide 9 · T-cell costimulation is a checkpoint

APC

Displays peptide on MHC

Provides B7 costimulation

T lymphocyte

Recognizes peptide–MHC

Receives B7 via CD28

Regulation

Limits response to self

Failure increases autoimmune potential

Slide 10 · Two principal infection-linked mechanisms

Costimulation mechanism

Microbe activates an APC.

APC displays self antigen and increases B7.

Anergy may break; Autoreactive T cell responds.

Molecular mimicry mechanism

Microbial antigen resembles self antigen.

Pathogen-directed lymphocytes may cross-react.

Self tissue becomes a target.

Slide 11 · Microbes can increase APC costimulation

Microbial infection

Activation of antigen-presenting cells

Increase in costimulatory molecules

Activation of self-reactive T cells

The source proposes costimulator induction as an explanation for infection-associated autoimmunity.

Activation is favored when self antigens are simultaneously presented.

Slide 12 · What changes on the antigen-presenting cell?

Before infection

May display tissue-derived self peptides.

Inadequate costimulation limits response.

T-cell anergy can be preserved.

During infection

APC activation increases costimulatory molecule expression.

Self peptide can still be displayed.

The same T-cell specificity may now respond.

Slide 13 · B7–CD28 links innate signals to adaptive activation

Infection activates APC

B7 costimulators are expressed

B7 binds CD28 on T cell

Self-reactive T cell becomes activated

B7 on APCs and CD28 on T cells provide an illustrative second-signal interaction.

This is a proposed pathway to loss of anergy, especially with impaired regulation.

Slide 14 · Self-antigen presentation is the crucial coincidence

Self peptide available

APC presents tissue antigen

Autoreactive T cell can recognize it

APC activated

Microbial stimulus increases costimulation

Antigen encounter becomes activating

Regulation inadequate

Protective brakes may be defective

Autoimmune reaction becomes more likely

Slide 15 · Regulatory defects amplify infection-triggered activation

Self-reactive T cell persists

Regulatory mechanisms are defective

Infection supplies stronger costimulation

Activation becomes possible

The textbook explicitly emphasizes that deficient regulation makes this mechanism more plausible.

An infectious trigger need not directly damage the target tissue to alter APC function.

Slide 16 · An antigen-specific response can be context dependent

Nonactivating encounter

Self peptide recognized.

Insufficient second signal.

Anergy or other tolerance persists.

Activating encounter

Same self peptide may be recognized.

Infection changes APC costimulation.

Autoreactive T-cell activation is possible.

Distinguish altered antigen presentation context from a newly created self antigen.

Slide 17 · Costimulation model: Clinical interpretation

A temporal association between infection and autoimmune symptoms is biologically plausible.

The sequence is not proof that the infection caused disease in an individual patient.

Host susceptibility and regulatory pathways modify the response.

The pathogen need not share an antigen sequence with the affected organ.

Slide 18 · How to identify the costimulation model

A microbial infection activates antigen-presenting cells; A self-reactive T cell recognizes a tissue peptide during this response.

Which change provides the second T-cell signal? B7 expression on the APC.

Which checkpoint may fail? Peripheral T-cell anergy.

What remains uncertain? Whether this sequence causes a particular human autoimmune disorder.

Slide 19 · Molecular mimicry: Antigen resemblance

Microbial antigen resembles a self antigen

Host mounts immune response to microbe

Cross-reactive lymphocyte or antibody recognizes self

Autoimmune tissue injury may follow

Shared amino acid sequences are one proposed basis of antigenic resemblance.

The source describes molecular mimicry as an important proposed mechanism.

Slide 20 · Mimicry differs from nonspecific activation

Molecular mimicry

Antigenic similarity is central.

Antimicrobial specificity overlaps a self target.

Cross-reaction can involve cells or antibodies.

Bystander costimulation

APC activation is central.

Self antigen is displayed during infection.

No shared microbial/self sequence is required.

Slide 21 · The microbial immune response is the starting point

Microbial antigen encounter

Immune recognition and activation

Antimicrobial effector response

Potential cross-reaction with self

The original stimulus is foreign, even when part of the resulting response recognizes self.

Recognition of self is an unintended consequence of overlapping specificity.

Slide 22 · Humoral mimicry: Cross-reactive antibodies

Microbial protein stimulates B-cell response

Antibody recognizes microbial determinant

Similar self determinant is also recognized

Antibody-mediated tissue reaction

The source uses rheumatic heart disease to demonstrate antibody cross-reactivity.

An antibody to a pathogen does not automatically imply clinically important autoimmunity.

Slide 23 · Cellular mimicry: Cross-reactive T-cell recognition

APC displays microbial peptide

T cell recognizes foreign peptide

Similar self-peptide recognition may occur

Self-reactive T-cell response

Robbins Fig. 6.22 depicts a T cell that recognizes a microbial peptide resembling self.

The illustration is mechanistic; Individual disease causality still requires evidence.

Slide 24 · Rheumatic heart disease: Classic example

Trigger

Streptococcal proteins initiate antibody responses

Antibodies target microbial epitopes

Cross-reaction

Some antibodies recognize myocardial proteins

Shared antigenic features underlie recognition

Consequence

Cardiac inflammatory injury can follow

The textbook links this process with myocarditis

Slide 25 · Rheumatic cross-reactivity: Causal chain

Streptococcal infection

Antistreptococcal antibodies

Recognition of myocardial proteins

Myocarditis

This example supports the plausibility of pathogen–self antigenic cross-reactivity.

The full pathology of rheumatic heart disease is outside this source unit.

Slide 26 · Do not generalize the rheumatic example uncritically

Rheumatic heart disease supplies a concrete textbook example of molecular mimicry.

Robbins suggests subtler forms may operate in classic autoimmune disorders.

The text does not establish specific microbial mimics for every autoimmune disease.

Shared sequences alone do not prove a pathogenic autoimmune mechanism.

Slide 27 · What would support a mimicry hypothesis?

Proposed link

Mechanistic observation

Interpretive limitation

Resemblance

Microbial and self antigens share relevant features

Similarity alone is insufficient

Immune response

Host develops pathogen-reactive lymphocytes or antibody

Response may remain pathogen-specific

Cross-reactivity

Same immune specificity recognizes self

Pathogenicity still needs demonstration

Tissue injury

Self-tissue reaction follows immune recognition

Other inflammatory mechanisms may coexist

Slide 28 · Molecular mimicry: Applied classification

A patient develops cardiac inflammation after streptococcal infection; Antistreptococcal antibodies bind myocardial proteins.

Identify the mechanism: Antigenic cross-reactivity (molecular mimicry).

Identify the effector highlighted in the source: Antibody.

Exclude the unsupported leap: Not all postinfectious myocarditis is caused by mimicry.

Slide 29 · Viruses may activate B cells polyclonally

Viral infection

Polyclonal activation of B lymphocytes

Broad antibody secretion

Autoantibodies may be produced

Epstein–Barr virus and HIV as examples of viral B-cell activation.

This pathway does not require a single microbial antigen to resemble a self antigen.

Slide 30 · Epstein–Barr virus and HIV: Source examples

Epstein–Barr virus

Listed as capable of polyclonal B-cell activation.

The broad response can include autoantibody production.

No specific autoimmune diagnosis is inferred here.

Human immunodeficiency virus

Also listed as causing polyclonal B-cell activation.

Autoantibodies may result from altered B-cell activation.

Autoantibody detection does not establish a disease mechanism.

Slide 31 · Polyclonal activation versus molecular mimicry

Dimension

Polyclonal B-cell activation

Molecular mimicry

Initiating process

Broad B-cell activation

Response to resembling microbial antigen

Antigenic similarity

Not required by model

Central to model

Potential product

Autoantibodies among other antibodies

Cross-reactive antibody or lymphocyte

Interpretation

Autoantibodies may be nonpathogenic

Cross-reactivity may damage tissue

Slide 32 · Autoantibodies do not equal autoimmune disease

Microbial activation may produce antibodies that recognize self antigens.

A detectable autoantibody is not, by itself, proof of organ injury.

Assess whether an immune response can plausibly account for tissue pathology.

Avoid attributing every postinfection autoantibody to molecular mimicry.

Slide 33 · Infection-associated tissue injury releases self antigens

Infection damages tissue

Previously contained self components are released

Immune system encounters exposed self antigens

Autoreactive responses may be activated

Infections may expose self antigens independently of microbial–self similarity.

Antigen availability changes when cells or tissues are injured.

Slide 34 · Damage can change the antigen itself

Infectious tissue injury

Structural modification of self constituents

Formation or exposure of neoantigens

Activation of responsive T cells

The textbook links tissue injury with both antigen release and structural modification.

Neoantigen formation is mechanistically distinct from unmodified antigen exposure.

Slide 35 · Antigen release and neoantigen formation

Release of self antigens

Damage liberates previously sequestered components.

Antigen becomes available for immune recognition.

Sequence does not require chemical modification.

Formation of neoantigens

Tissue injury alters the structure of self components.

Modified structures can create novel immune determinants.

T cells may recognize these determinants.

Slide 36 · Cytokines create a permissive inflammatory site

Microbial infection

Production of inflammatory cytokines

Lymphocyte recruitment into affected tissue

Self-reactive lymphocytes encounter self antigens

Inflammation can bring potentially autoreactive lymphocytes into antigen-rich tissue.

Recruitment is a proposed enabling event, not definitive evidence of autoimmunity.

Slide 37 · Recruitment links systemic immunity to local injury

Systemic compartment

Self-reactive lymphocytes may circulate

Tolerance normally restrains activity

Inflamed tissue

Cytokines recruit lymphocytes

Self antigens may be released

Outcome

Cells and antigens encounter each other

Tissue-directed reactions may result

Slide 38 · Multiple microbial mechanisms can coexist

APC activation

Costimulation rises

Previously anergic T cells may respond

Antigenic overlap

Pathogen-specific recognition overlaps self

Cross-reactivity is possible

Damage and inflammation

Self antigens exposed or modified

Cytokines recruit lymphocytes

These are complementary proposed pathways—not mutually exclusive diagnostic categories.

Slide 39 · Environmental insults can change self-antigen display

Physical or chemical insult

Tissue-cell stress or death

Self-antigen exposure or alteration

Autoimmune responses may occur

Robbins explicitly extends this mechanism beyond infection.

The tissue microenvironment and antigen display are central to the hypothesis.

Slide 40 · Ultraviolet radiation and systemic lupus erythematosus

Sunlight / ultraviolet radiation

Death of exposed cells

Exposure of nuclear antigens

Potential pathologic lupus immune response

This as a proposed explanation for photosensitive lupus flares.

It does not claim every flare follows UV exposure or that UV is sufficient by itself.

Slide 41 · Why nuclear antigens matter after UV injury

Cell injury

Ultraviolet exposure may cause cell death

Nuclear material becomes accessible

Antigen availability

Exposed nuclear constituents can be immunogenic

Existing autoimmune recognition becomes relevant

Clinical association

Lupus flares may follow sunlight

Proposed mechanistic link: Antigen exposure

Slide 42 · Sunlight-associated lupus flare: Interpretation

An individual with systemic lupus erythematosus develops a flare after substantial sunlight exposure.

Proposed initiating event: Ultraviolet-induced cellular injury.

Potential antigenic consequence: Exposure of nuclear antigens.

Causal caution: The vignette illustrates the textbook hypothesis, not a unique diagnostic test.

Slide 43 · Smoking is a risk factor for rheumatoid arthritis

Smoking as an environmental risk factor for rheumatoid arthritis.

A proposed mechanism is chemical modification of self antigens.

Modified self determinants could become targets of an immune response.

Risk-factor status is not equivalent to inevitability or to proof of an individual mechanism.

Slide 44 · Smoking: Proposed antigen-modification pathway

Exposure to tobacco smoke

Chemical modification of self antigens

Altered immune recognition may occur

Increased rheumatoid arthritis susceptibility

The precise chemical modification is not specified in the supplied topic excerpt.

Avoid presenting molecular intermediates absent from the assigned source.

Slide 45 · UV radiation and smoking: Distinct routes

Feature

Ultraviolet radiation

Smoking

Disease association

Lupus flares following sunlight

Rheumatoid arthritis risk

Emphasized perturbation

Cell death and nuclear-antigen exposure

Possible chemical modification of self antigens

Proposed mechanism

More antigen available to immune system

Altered self-antigen structure

Inference limit

Not a sufficient cause of lupus

Not a sufficient cause of RA

Slide 46 · Local tissue injury is a generalizable trigger

Infection, UV, or another insult

Local tissue injury

Release or modification of self antigens

Possible autoimmune activation

Tissue damage from various causes can release self antigens.

It is the loss of normal antigenic context that unifies multiple exposures.

Slide 47 · Environmental association is not mechanistic proof

Observation

What it supports

What it does not establish

Exposure precedes flare

Temporal compatibility

Unique causation

Exposure associated with disease

Population-level relationship

Sufficiency in an individual

Plausible immune pathway

Biological mechanism

Clinical pathogenicity in all cases

Multiple host factors

Multifactorial susceptibility

Single-trigger explanation

Slide 48 · The microbiome is an environmental interface

Gut microbiome

Commensals coexist with mucosal immune cells

May influence immune differentiation

Skin microbiome

Commensals interact with barrier immunity

May modulate immune responses

Autoimmune relevance

Balance of effector and regulatory T cells may shift

Human disease-specific effects remain uncertain

Slide 49 · Microbes can potentially shift effector–regulatory balance

Differences in commensal communities

Altered immune-system signals

Relative effector / regulatory T-cell balance changes

Susceptibility may shift toward or away from autoimmunity

Robbins frames this as a possibility rather than a proven causal map for human diseases.

The direction of effect depends on host–microbe interactions.

Slide 50 · Effector and regulatory T cells have opposing roles

Effector T-cell response

Can drive tissue-directed inflammation.

May become pathogenic when directed at self.

Microbial influences could alter its prominence.

Regulatory T-cell response

Suppresses inappropriate immune activation.

Contributes to peripheral self tolerance.

Changes in its maintenance may alter disease risk.

Slide 51 · Which microbial communities cause human autoimmunity?

The source does not identify definitive causal microbes for specific human autoimmune diseases.

Experimental or population associations need cautious interpretation.

Host predisposition can confound apparent microbiome–disease links.

Robbins leaves disease-specific causal attribution unresolved.

Slide 52 · Microbiome manipulation: Therapeutic uncertainty

Biological hypothesis

Commensals help shape immune response.

Changing communities could alter effector/Treg balance.

This makes prevention or treatment attractive conceptually.

What Robbins does not establish

Specific causal organisms in people.

A validated microbiome manipulation to prevent disease.

A proven intervention for treating these autoimmune disorders.

Slide 53 · Microbiome and pathogen effects are not synonymous

Feature

Acute microbial infection

Commensal microbiome

Typical exposure model

Infection with immune activation

Resident microbial communities

Mechanisms emphasized

APC activation; Mimicry; Injury

Longer-term shaping of immune balance

Potential direction

Can promote autoimmune activation

May shift toward or away

Causality

Several mechanisms postulated

Specific human links unresolved

Slide 54 · Infections may also protect against autoimmunity

Potential pathogenic effect

APC activation increases costimulation.

Molecular mimicry may create cross-reactivity.

Tissue injury promotes antigen exposure.

Potential protective effect

Some infections reduce disease in animal models.

Protective immune regulation may increase.

Mechanism is incompletely established.

Slide 55 · A population paradox: Infection control and autoimmunity

Autoimmune disease incidence has increased in some high-resource countries.

This trend has occurred alongside improved infection control.

This as epidemiologic context for a protective infection hypothesis.

Parallel trends alone do not show that reduced infection exposure causes autoimmunity.

Slide 56 · Animal models show infection-associated protection

Experimental observation

In some type 1 diabetes animal models, infections reduce disease incidence.

Possible explanation

Infections may support regulatory pathways instead of triggering autoimmunity.

Translation limit

Results in animal models do not prove protection in humans.

Slide 57 · IL-2 may help maintain regulatory T cells

Certain infections

Low-level interleukin-2 production may occur

Regulatory T-cell maintenance is supported

Autoimmune reactions may be restrained

This pathway is explicitly described as one possibility; The mechanism remains unclear.

IL-2 is important for regulatory T-cell maintenance in the tolerance context.

Slide 58 · Triggering and protection can both be plausible

Pathway

Immune consequence

Autoimmune implication

Costimulator induction

Activates self-reactive T cells

Potential triggering

Mimicry

Cross-reactive immune recognition

Potential triggering

Inflammatory injury

Self-antigen exposure and recruitment

Potential triggering

Regulatory maintenance

Supports Treg function

Potential protection

Slide 59 · Sex bias is a major feature of autoimmunity

Many autoimmune diseases occur more frequently in females than in males.

This with a sex-distribution figure across several disorders.

The underlying biological explanations remain incompletely understood.

A population sex bias does not determine risk for an individual patient.

Slide 60 · Sex hormones and X-linked genes: Hypotheses

Hormonal contribution

Sex hormones may modify immune responses.

Potential relevance to tolerance and disease bias.

The source does not establish a singular hormonal mechanism.

X-chromosome contribution

Genes carried on the X chromosome may contribute.

Potential relevance to sex-associated susceptibility.

Specific causal gene pathways are not resolved in the excerpt.

Slide 61 · Multiple exposures converge on self tolerance

Inherited susceptibility

Tolerance and immune regulation vary

Infection or environmental tissue injury

Autoreactive lymphocytes become active

Genetic background may alter how an infection or insult is interpreted by immunity.

Autoimmune disease as a multifactorial result, not one exposure with one outcome.

Slide 62 · Chronic autoimmune injury can outlast a trigger

Environmental event helps initiate response

Autoreactivity develops

Self antigens remain present

Recurrent or progressive tissue damage may follow

The general-features subsection describes chronicity and relapsing/remitting courses.

Persistence of self antigens helps explain why disease may continue after an initiating event.

Slide 63 · One trigger can intersect several checkpoints

Recognition

Self-antigen availability or mimicry

Determines what can be recognized

Activation

APC costimulation and cytokines

Determines whether response proceeds

Regulation

Anergy, Tregs, inhibitory pathways

Determines whether it is restrained

Slide 64 · Interpreting environmental links: Three questions

Question

Example

Why it matters

What changed?

APC costimulation / antigen exposure

Identifies the proposed initiating step

What responds?

Cross-reactive antibody / self-reactive T cell

Identifies the proposed immune effector

What is demonstrated?

Association / model / tissue damage

Prevents overstating human causation

Slide 65 · Case: Postinfectious cardiac inflammation

Antibodies raised against a streptococcal protein also recognize myocardial proteins, followed by myocarditis.

Most direct concept: Molecular mimicry.

Effector pathway highlighted: Cross-reactive antibody recognition.

A distinct mechanism from APC costimulation without shared epitopes.

Slide 66 · Case: Ultraviolet-associated lupus flare

A person with lupus experiences a flare temporally associated with intense sunlight exposure.

The implicated insult is ultraviolet radiation, not microbial mimicry.

The proposed sequence is cell death followed by nuclear-antigen exposure.

The association is plausible but not sufficient to prove causation in that episode.

Slide 67 · Case: Infection without antigen resemblance

An activated APC displays a self peptide and expresses strong B7 costimulators during a microbial infection.

Which molecular interaction changes? B7–CD28 costimulation.

Which tolerance process is challenged? T-cell anergy.

Must microbial peptide resemble self? No.

Slide 68 · Case: Unexpected autoantibody during viral infection

Autoantibodies are detected after an infection known to cause broad B-cell stimulation, without evidence of organ injury.

Consider polyclonal B-cell activation among possible explanations.

Do not equate autoantibody detection with established autoimmune disease.

Do not infer molecular mimicry without demonstrated cross-reactivity.

Slide 69 · Integrated comparison of environmental mechanisms

Mechanism

Immediate immune change

Textbook example or context

APC costimulation

Self-reactive T-cell activation

Infection-activated APCs

Molecular mimicry

Cross-reactive recognition

Streptococcal proteins / myocardium

Polyclonal B activation

Broad autoantibody production

EBV; HIV

Tissue injury

Self-antigen release / change

Infection; UV; Smoking (proposed)

Regulatory shifts

Effector/Treg balance

Microbiome; Possible IL-2 effects

Slide 70 · What is established, and what remains proposed?

Direct textbook statements

Infections may precipitate autoimmune reactions.

Smoking is a rheumatoid arthritis risk factor.

Female predominance characterizes many autoimmune diseases.

Explicit mechanistic uncertainty

Microbial contributions to specific human diseases are incompletely defined.

Microbiome interventions are not established here.

Protective IL-2 and sex-linked explanations remain hypotheses.

Slide 71 · Principal teaching conclusions

Microbial costimulation can convert self-antigen recognition into T-cell activation.

Molecular mimicry is cross-recognition of microbial and self antigens.

Viral B-cell activation, tissue injury, and cytokine recruitment offer additional routes.

Ultraviolet radiation, smoking, and commensal microbiota may change immune context.

Potential protective effects and sex bias emphasize multifactorial, incompletely resolved causation.