T Cell Maturation in the Thymus

Overview

Stage 0: The Arriving Cell — Double Negative (DN)

Correction on your note: You mentioned the cell arrives as double positive — technically it arrives as double negative and becomes double positive within the cortex after TCR rearrangement. The DP stage is where positive selection then acts on the cell.

Stage 1: Positive Selection (Cortex)

• If TCR recognizes MHC Class I → downregulates CD4 → becomes CD8+ single positive
• If TCR recognizes MHC Class II → downregulates CD8 → becomes CD4+ single positive

Stage 2: Negative Selection (Medulla)

1. Medullary dendritic cells
2. Medullary thymic epithelial cells (mTECs)

The AIRE Gene — The Key to Negative Selection in the Medulla

What is AIRE?

Correction on your note: You referred to it as "IR" — the correct name is AIRE (Autoimmune Regulator gene), located on chromosome 21.

Function of AIRE

AIRE Mutation → Autoimmune Polyglandular Syndrome Type 1 (APS-1)

Mechanism

AIRE mutation (loss of function)
        ↓
mTECs fail to express tissue-specific self-antigens
        ↓
Autoreactive T cells are NOT deleted during negative selection
        ↓
Autoreactive T cells escape to periphery
        ↓
Attack endocrine (and other) glands → APS-1

Genetics

Classic APS-1 Clinical Triad (Whitaker's Triad)

Mnemonic: "The HAC" — Hypoparathyroidism, Adrenal insufficiency, Candidiasis

Full T Cell Maturation Flowchart

Bone Marrow
   → Blood
      → Thymus CORTEX
           ↓
      T cell precursor
           ↓ (TCR rearrangement)
      DOUBLE NEGATIVE (CD4⁻ CD8⁻)
           ↓ (upregulate both co-receptors)
      DOUBLE POSITIVE (CD4⁺ CD8⁺)
           ↓
      ┌─────────────────────────────────────┐
      │      POSITIVE SELECTION (cortex)    │
      │  cTECs test: can TCR bind self-MHC? │
      │  Fail → die by neglect              │
      └─────────────────────────────────────┘
           ↓
      SINGLE POSITIVE (CD4⁺ OR CD8⁺)
           ↓ migrates to medulla
      ┌──────────────────────────────────────────┐
      │       NEGATIVE SELECTION (medulla)        │
      │  mTECs + DCs test: too autoreactive?      │
      │  AIRE → ectopic self-antigen expression   │
      │  High avidity → apoptosis (clonal delete) │
      │  Low avidity → survives                   │
      └──────────────────────────────────────────┘
           ↓
      Naive, self-tolerant T cell → Periphery

High-Yield USMLE Summary

T Cell Maturation in the Thymus

Origin

• T cell precursors arise from hematopoietic stem cells in the bone marrow
• They migrate via blood to the thymus for maturation

Stage 1 — Double Negative (DN) Phase

• Arriving precursors are CD4⁻ CD8⁻ (double negative)
• Inside the thymic cortex, they undergo TCR gene rearrangement (β-chain first, then α-chain)
• After successful rearrangement, they upregulate both CD4 and CD8 → become Double Positive (DP)

Stage 2 — Positive Selection (Cortex)

• Occurs in the thymic cortex
• Mediated by cortical thymic epithelial cells (cTECs)
• Question asked: "Can your TCR bind self-MHC with at least moderate affinity?"
  • Yes → cell survives and proceeds
  • No → cell dies by neglect (apoptosis, no survival signal)
• DP cell now commits to a lineage:
  • TCR recognizes MHC Class I → loses CD4 → becomes CD8⁺ single positive
  • TCR recognizes MHC Class II → loses CD8 → becomes CD4⁺ single positive
• Single positive (SP) cell then migrates to the medulla

Stage 3 — Negative Selection (Medulla)

• Occurs in the thymic medulla
• Mediated by two cell types:
  • Medullary dendritic cells
  • Medullary thymic epithelial cells (mTECs)
• Question asked: "Are you binding self-antigens too strongly?"
  • High avidity binding → clonal deletion (apoptosis) — eliminates autoreactive T cells
  • Low/moderate avidity → cell survives → exits thymus as a mature, naive, self-tolerant T cell

AIRE Gene — The Engine of Negative Selection

• AIRE = Autoimmune Regulator gene, located on chromosome 21
• Expressed in mTECs
• AIRE is a transcriptional regulator that forces mTECs to ectopically express tissue-specific peripheral self-antigens (e.g., insulin, thyroglobulin) inside the thymus
• This exposes developing T cells to peripheral antigens they would otherwise never see
• If a T cell binds these antigens too strongly → deleted
• This process is called central tolerance

AIRE Mutation → APS-1

• Inheritance: Autosomal recessive (classic); some dominant mutations in the PHD1 domain
• Chromosome: 21
• Higher frequency in: Finns, Sardinians, Iranian Jews, Norwegians, Irish

APS-1 Clinical Triad

• Chronic mucocutaneous candidiasis — appears first, in childhood
• Hypoparathyroidism — most common endocrine feature
• Primary adrenal insufficiency (Addison's disease) — anti-21-hydroxylase antibodies

Full Maturation Flow

Peripheral Tolerance

Why Peripheral Tolerance Exists

• Central tolerance (thymus) is not perfect — some self-reactive T cells escape into the periphery
• Peripheral tolerance acts as the second safety net to silence or eliminate these cells
• Two main mechanisms: Anergy and Apoptosis (AICD)

Mechanism 1 — Anergy

• A T cell in the periphery encounters an antigen presented on MHC by a dendritic cell
• For full T cell activation, two signals are required:
  • Signal 1: TCR binds antigen-MHC complex
  • Signal 2: CD28 on the T cell binds B7 (CD80/CD86) on the dendritic cell
• B7 is only upregulated on dendritic cells during active inflammation / infection (danger signal)
• If a T cell recognizes a self-antigen in a non-inflammatory environment:
  • Signal 1 fires (TCR binds)
  • Signal 2 does NOT fire (no B7 upregulation, no danger signal)
• Result: T cell enters anergy — it is alive but permanently unresponsive, functionally silenced
• Anergic T cells cannot proliferate or produce cytokines even if stimulated again later

Mechanism 2 — Activation-Induced Cell Death (AICD) via Fas/FasL Pathway

• Also called the extrinsic apoptosis pathway
• When a T cell receives Signal 1 repeatedly without Signal 2, it upregulates two molecules on its own surface:
  • Fas (CD95) — a death receptor
  • FasL (CD95L) — the ligand for Fas
• The same cell (or a neighboring T cell) expresses both → Fas on one cell binds FasL on itself or adjacent cell
• This interaction activates caspase cascade → apoptosis
• This is called fratricidal killing or suicide depending on whether it is the same or neighboring cell

Correction on Your Note

• You called it "fast ligand" and "fast receptor" — the correct names are FasL (Fas Ligand) and Fas (CD95), also written as CD95 / CD95L
• Fas = the receptor (death receptor) → CD95
• FasL = the ligand → CD178 / CD95L

Additional Peripheral Tolerance Mechanisms (USMLE High-Yield)

• CTLA-4 — expressed on activated T cells; also binds B7 but with higher affinity than CD28, and delivers an inhibitory signal → brakes T cell activation
• Regulatory T cells (Tregs) — CD4⁺ CD25⁺ FoxP3⁺ cells that actively suppress autoreactive T cells via IL-10 and TGF-β
• PD-1 / PD-L1 pathway — PD-1 on T cells binds PD-L1 on peripheral tissues → suppresses T cell activity (exploited by tumors to evade immunity)

When Peripheral Tolerance Fails → Autoimmune Lymphoproliferative Syndrome (ALPS)

Mechanism

• Normally, autoreactive T cells that escape to periphery are eliminated by the Fas-FasL apoptosis pathway
• In ALPS, there is a mutation in Fas (CD95) or FasL
• Fas-FasL interaction fails → apoptosis does not occur → autoreactive lymphocytes accumulate and proliferate unchecked

Clinical Features of ALPS

• Massive non-malignant lymphadenopathy (lymph node enlargement)
• Splenomegaly (often massive)
• Hepatomegaly
• Autoimmune cytopenias — autoimmune hemolytic anemia, thrombocytopenia, neutropenia
• Elevated double-negative T cells (DNTs) — CD4⁻ CD8⁻ αβ T cells in peripheral blood — this is the diagnostic hallmark
• Increased risk of lymphoma (Hodgkin and non-Hodgkin)

Genetics

• Most commonly: FAS gene mutation (autosomal dominant, variable penetrance)
• Also: FasL, Caspase-10, Caspase-8 mutations
• Presents in childhood

Full Peripheral Tolerance Summary Flow

Antiphospholipid Antibody Syndrome (APS)

What Are Antiphospholipid Antibodies?

• Antibodies directed not against phospholipids directly, but against proteins that are bound to phospholipids
• The phospholipid acts as a scaffold — the real antigenic target is the protein sitting on top of it
• Main protein target: β2-glycoprotein I (β2-GPI) bound to phospholipid membranes
• These antibodies exert a procoagulant effect → paradoxically cause clotting despite appearing as anticoagulants in lab tests

The Three Major Antiphospholipid Antibodies

1. Anti-β2-Glycoprotein I (Anti-β2-GPI)

• The most pathologically significant antibody
• β2-GPI is a plasma protein that naturally binds to phospholipid membranes (especially on platelets and endothelial cells)
• Normally β2-GPI inhibits coagulation — it is anticoagulant by nature
• Anti-β2-GPI antibody binds to the β2-GPI/phospholipid complex → neutralizes its anticoagulant function → tips the balance toward thrombosis
• Also activates platelets and endothelial cells directly → further procoagulant state
• Detected by ELISA

2. Lupus Anticoagulant (LA)

• Misleading name — it causes clotting in vivo, not anticoagulation
• It is called "anticoagulant" only because of what it does in the test tube (in vitro)
• Mechanism in vitro: LA antibody binds to phospholipids in the coagulation assay reagent → blocks phospholipid-dependent clotting steps (Factor X, prothrombin activation) → falsely prolongs PTT
• In vivo: binds phospholipids on platelets and endothelium → activates them → thrombosis
• Detected by: prolonged PTT that does NOT correct with mixing study (1:1 mix with normal plasma)
  • If PTT corrects → factor deficiency (e.g., hemophilia)
  • If PTT does not correct → inhibitor present → lupus anticoagulant
• Confirmed by: dilute Russell's viper venom time (dRVVT)

3. Anticardiolipin Antibody (aCL)

• Cardiolipin is a phospholipid found in mitochondrial membranes and also used as an antigen in syphilis screening tests
• Anti-cardiolipin antibodies (IgG, IgM, IgA) bind to the cardiolipin-β2-GPI complex
• Detected by ELISA
• IgG subtype most strongly associated with thrombosis and pregnancy loss

The Syphilis Connection — False Positive VDRL/RPR

• VDRL and RPR are non-treponemal screening tests for syphilis
• They work by detecting antibodies against a cardiolipin-lecithin-cholesterol antigen complex
• The reagent antigen is cardiolipin
• In APS, the patient already has anticardiolipin antibodies circulating in their blood
• These antibodies react with the cardiolipin in the VDRL/RPR reagent → false positive result
• The patient does NOT have syphilis

• How to confirm true syphilis vs false positive: use treponemal-specific tests — FTA-ABS or TPPA
  • True syphilis → treponemal test positive
  • APS false positive → treponemal test negative

Clinical Features of APS

• Arterial thrombosis — stroke, TIA, MI in young patients
• Venous thrombosis — DVT, pulmonary embolism (most common)
• Obstetric complications:
  • Recurrent pregnancy loss (especially second/third trimester — due to placental thrombosis)
  • Preeclampsia, IUGR
• Thrombocytopenia — mild, due to platelet consumption
• Livedo reticularis — mottled, net-like skin discoloration
• Libman-Sacks endocarditis — sterile, non-bacterial endocarditis (especially in SLE-associated APS)

Primary vs Secondary APS

• Primary APS — occurs alone, no underlying disease
• Secondary APS — occurs with an underlying autoimmune disease, most commonly SLE (up to 30–40% of SLE patients have antiphospholipid antibodies)

Diagnosis — Sapporo/Sydney Criteria

• Clinical: vascular thrombosis (arterial/venous) OR pregnancy morbidity
• Laboratory (any one of):
  • Lupus anticoagulant positive
  • Anticardiolipin IgG or IgM (medium/high titer)
  • Anti-β2-GPI IgG or IgM (medium/high titer)
  • Must be positive on two occasions at least 12 weeks apart (to exclude transient positivity)

Lab Summary Flow

High-Yield One-Liners

• APS antibodies target proteins bound to phospholipids, not phospholipids directly
• Lupus anticoagulant prolongs PTT in vitro, causes clots in vivo
• Anticardiolipin antibody causes false positive VDRL and RPR
• Confirm syphilis with FTA-ABS or TPPA (treponemal tests)
• Confirm lupus anticoagulant with mixing study (no correction) + dRVVT
• Recurrent second/third trimester pregnancy loss in a young woman → think APS
• APS + SLE = secondary APS; treat with heparin + aspirin in pregnancy, long-term warfarin for thrombosis

PTT Mixing Study — Detailed Explanation

Why We Do a Mixing Study

• A prolonged PTT has two possible causes:
  • Factor deficiency — not enough clotting factor to complete the intrinsic pathway
  • Inhibitor present — something in the blood is actively blocking the coagulation pathway
• A mixing study distinguishes between these two by adding normal plasma (which contains all factors at 100%) to the patient's plasma

How the Mixing Study Works

• Take patient plasma (prolonged PTT) + normal plasma in a 1:1 ratio
• Normal plasma contains all clotting factors in full quantity
• Re-run the PTT on the mixed sample

Result Interpretation

If PTT Corrects After Mixing → Factor Deficiency

• Normal plasma donated its factors to the patient's plasma
• The missing factor is now present in sufficient quantity → coagulation proceeds normally → PTT normalizes
• The patient's plasma had no factors to give, but had nothing blocking the donated factors either
• Diagnoses: Hemophilia A (Factor VIII deficiency), Hemophilia B (Factor IX deficiency), Factor XI deficiency

If PTT Does NOT Correct After Mixing → Inhibitor Present

• Normal plasma donated its factors — but something in the patient's plasma destroyed or blocked those donated factors too
• The inhibitor is so potent it overpowers even the normal plasma's contribution
• This confirms an inhibitor is circulating in the patient's blood

Why Lupus Anticoagulant Prolongs PTT — The Mechanism

• The PTT test works by adding phospholipid reagent + activator + calcium to plasma and measuring how long clotting takes
• The phospholipid in the reagent acts as a surface on which Factor X and prothrombin activation complexes assemble
• Lupus anticoagulant binds and occupies these phospholipids in the reagent
• Without a free phospholipid surface, the coagulation complexes cannot assemble → reaction slows → PTT artificially prolonged
• This is purely a test tube artifact — the phospholipid in the reagent is not the same as what happens inside blood vessels

Why LA Causes Clotting In Vivo Despite Prolonging PTT In Vitro

• Inside the body, LA binds phospholipids on platelet surfaces and endothelial cell membranes
• β2-GPI sits on these phospholipid membranes acting as a natural anticoagulant
• LA neutralizes β2-GPI → removes the brake on coagulation → procoagulant state
• Also directly activates platelets and endothelial cells → further clot formation
• Result: arterial and venous thrombosis in vivo

Inhibitor Titre — Bethesda Assay

• If mixing study does not correct, the next step is to identify and quantify the inhibitor
• For Factor VIII inhibitor (e.g., acquired hemophilia): Bethesda assay — measures inhibitor units
• For lupus anticoagulant: confirmed by dilute Russell's viper venom time (dRVVT)
  • dRVVT uses very low phospholipid concentration → LA has maximum effect → prolonged clotting time
  • Then repeat with excess phospholipid → LA gets neutralized → time corrects
  • Correction with excess phospholipid = confirms LA

Factor VIII Inhibitor vs Lupus Anticoagulant — Key Distinction

• Both prolong PTT and do not correct on mixing study
• Factor VIII inhibitor:
  • Causes bleeding (destroys Factor VIII)
  • Seen in hemophilia patients who developed antibodies after factor replacement, or in acquired hemophilia (middle-aged, autoimmune)
  • Bethesda assay positive
• Lupus anticoagulant:
  • Causes thrombosis (not bleeding)
  • Seen in SLE, APS, infections, drugs
  • dRVVT confirms

Full Mixing Study Algorithm Flow

Antiphospholipid Antibodies — Quick Revision

Anticardiolipin Antibody

• Binds cardiolipin-β2GPI complex
• Cardiolipin is the antigen used in VDRL and RPR syphilis screening
• Anticardiolipin reacts with reagent → false positive VDRL/RPR
• Confirm true syphilis with FTA-ABS or TPPA (treponemal) → negative = false positive

Anti-β2-Glycoprotein I

• β2-GPI normally sits on phospholipid membranes and inhibits coagulation
• Anti-β2-GPI neutralizes it → removes anticoagulant brake → thrombosis
• Most pathologically significant antibody in APS
• Causes arterial/venous thrombosis + recurrent pregnancy loss

Lupus Anticoagulant

• Binds phospholipid in the PTT reagent → blocks coagulation complex assembly → PTT falsely prolonged
• In vitro = anticoagulant | In vivo = pro-thrombotic
• Confirmed by mixing study + dRVVT

PTT Mixing Study

• Mix patient plasma + normal plasma (1:1)
• Normal plasma has all factors at full levels

• Patient bleeding + no correction → Factor VIII inhibitor → Bethesda assay
• Patient clotting + no correction → Lupus anticoagulant → confirm with dRVVT