Unit 1_4 Notes
Leukocyte Circulation & Migration
Purpose of movement
Movement is critical for protection against threats: tissue-resident immune cells detect threats and alert; dendritic cells (DCs) move to alert/activate lymphocytes; responding cells (activated lymphocytes) move to sites of antigen detection.
Movement must be controlled so that the right cells go to the right locations.
Barriers and sentinels (epithelial barrier)
Barrier breach triggers detection by receptors on tissue-resident sentinels (DCs, macrophages) and by epithelial/endothelial cells.
Tissue-resident cells express cytokines and chemokines in response to antigen: pro-inflammatory signals recruit leukocytes.
Key early mediators include IL-1 and TNF-α, which stimulate cytokine production.
Neutrophils are the first responders; polarization begins with CXCL8 (IL-8) creating a chemokine gradient that attracts and recruits circulating leukocytes (neutrophils first, then monocytes).
E. coli is often a model organism illustrating barrier breach and response.
Chemokines and chemokine receptors
Chemokines are chemotactic cytokines; four families based on conserved cysteine patterns:
CC: adjacent cysteines
CXC: separated by one amino acid (X)
C: single cysteine
CX3C: separated by three amino acids
Naming convention for ligands: CCLxx, CXCLxx (C = chemokine), ligand family plus a number; examples include CCL19/CCL21 and CXCL13.
Chemokine receptors are GPCRs and are named in parallel to their ligand families:
CCRs (e.g., CCR7)
CXCRs (e.g., CXCR5)
Important note: the numbers in receptor names do not correspond to the chemokine numbering of ligands.
Leukocyte movement: stopping signals and brakes
Blood flow is rapid; leukocytes require molecular brakes to slow and stop at sites of infection.
This slowing is achieved through increased expression of adhesion molecules on endothelial cells and through adhesion interactions.
Major classes of adhesion molecules involved:
Selectins (L-, P-, E-selectin)
Immunoglobulin superfamily: ICAMs (e.g., ICAM-1, ICAM-2)
Integrins (e.g., LFA-1, Mac-1, VLA-4)
Selectins bind carbohydrate ligands; they mediate initial, low-affinity tethering/rolling to slow cells.
Selectins and selectin ligands
Selectins are transmembrane proteins that bind carbohydrates (lectins) and mediate weak, low-affinity adhesion:
L-selectin (CD62L) is expressed on leukocytes.
P-selectin and E-selectin are expressed on endothelial cells.
Selectin ligands are carbohydrate-containing ligands on cell surfaces.
Epithelial barrier and early extravasation steps
Epithelial barrier breach and antigen detection trigger a cascade of events:
Pro-inflammatory cytokines (e.g., IL-1, TNF-α) stimulate endothelial cells to express E- and P-selectins.
Leukocyte margination: blood vessels dilate and blood flow slows; leukocytes come closer to the endothelium.
CXCL8 (IL-8) and other chemokines recruit leukocytes via a chemokine gradient.
Rolling and weak adhesion (two-step process)
Epithelial barrier context: neutrophils express E- and P-selectin ligands and L-selectin; these bind to ligands on endothelium with low affinity, producing rolling along the vessel wall.
Shear forces from blood flow disrupt initial bindings, but allow new selectin–ligand interactions to form, creating a rolling behavior.
Rolling helps leukocytes survey the endothelium for activation signals before firm adhesion.
Firm adhesion and activation of integrins
After rolling, leukocytes require stronger adhesion to stop firmly at the endothelium:
Chemokines induce changes in integrins on leukocytes from a low-affinity to a high-affinity state and promote clustering of integrins, increasing avidity.
Integrins undergo activation by chemokine signaling (inside-out signaling) that changes their conformation and increases binding strength.
Key integrin–ICAM pairs involved:
LFA-1 (LFA-1 = αLβ2 = CD11a/CD18) on leukocytes binds ICAM-1 and ICAM-2 on endothelial cells.
Mac-1 (αMβ2 = CD11b/CD18) on monocytes also binds ICAMs.
VLA-4 (α4β1) on leukocytes binds VCAM-1 on endothelium.
Other endothelial ligands include MadCAM-1 for α4β7 (on certain T cells and DCs) and MAdCAM-1.
Important notes:
ICAM-1 is a primary endothelial ligand for LFA-1; VCAM-1 ligates VLA-4.
Ligand expression on endothelium is upregulated in response to inflammatory signals.
Chemokine-induced integrin activation and firm adhesion (tight binding)
Chemokines expressed on endothelium induce high-affinity integrin conformations and clustering on leukocytes, enabling firm adhesion to endothelium.
Example: CXCL8 (IL-8) signaling promotes activation of LFA-1 and VLA-4, enabling tight binding to ICAM-1 and VCAM-1, respectively.
This firm adhesion occurs after rolling and prior to transmigration.
Transmigration (diapedesis) and tissue entry
After firm adhesion, leukocytes transmigrate across endothelial cells (diapedesis) into tissues:
Chemokines stimulate leukocytes to squeeze between endothelial cells (transmigration) to reach the tissue.
Additional mediators released by leukocytes (e.g., prostaglandins, leukotrienes) can increase vascular permeability to facilitate passage.
Outcome: leukocytes reach the site of infection or injury to execute effector functions.
Neutrophil extravasation and early innate response
Neutrophil extravasation occurs within minutes of infection and is a hallmark of acute inflammation.
Neutrophils are recruited early by chemokines such as CXCL8 (IL-8) and by cytokines like IL-1 and TNF-α.
Granulocyte Colony Stimulating Factor (G-CSF) promotes neutrophil production and release from bone marrow to maintain innate response.
The movement of neutrophils to the site of injury begins with margination, rolling, adhesion, and transmigration, followed by migration toward the chemokine gradient at the infection site.
LAD (Leukocyte Adhesion Deficiency) Type I
LAD Type I is an immune disorder caused by defects in the β subunit of LFA-1 and Mac-1, impairing leukocyte adhesion and recruitment into tissues.
Consequence: recurrent bacterial and fungal infections due to impaired extravasation.
Neutrophil behavior after tissue entry
Once in tissue, neutrophils move along chemokine gradients toward the actual site of infection/injury.
This gradient-guided movement constitutes the core of the early innate response.
Lymphocytes and the adaptive immune response: overview
Lymphocytes (B and T cells) support the adaptive immune response.
The adaptive response requires:
1) Antigen transport from the injury site to peripheral lymphoid organs (pl. lymph nodes, LNs).
2) Naïve lymphocytes must circulate through LNs to encounter antigen.
3) Activated (effector) lymphocytes must migrate to the site of infection to exert effector functions.
Lymphocyte movement and recirculation (naïve lymphocytes)
Mature, naïve lymphocytes are produced/educated in primary lymphoid organs (bone marrow for B cells, thymus for T cells).
They enter the blood and circulate through peripheral lymphoid tissues (lymph nodes, LNs).
Within lymph nodes, lymphocytes migrate to specific regions and stay there until they encounter cognate antigen or receive an exit signal.
Naïve lymphocytes recirculate through LNs and return to blood, then move to other LNs in series via lymphatics; they continue this cycle until they encounter their antigen or die.
Naïve T cells typically pass through all LNs at least once per day (recirculation pattern).
Lymph node (LN) organization and antigen trafficking
Lymph enters LN through afferent lymphatics; mature antigen-carrying cells (e.g., DCs) present antigen to T cells in specific LN zones.
In the LN cortex: DCs capture antigen and move toward areas near high endothelial venules (HEVs) to interact with circulating T and B cells.
Follicles in the LN are rich in B cells; T cells localize primarily to the paracortex (T cell zone).
B cells are attracted to follicles via CXCL13 signaling (CXCR5 on B cells).
T cells are attracted to the T cell zones via CCR7 signaling (CCR7 on T cells responds to CCL19 and CCL21 produced by LN stromal and resident cells).
Mature, naïve T cells upregulate CCR7 and enter the LN via HEVs, migrating first to the cortex and then toward the T cell zone; mature, naïve B cells express CXCR5 and home to follicles via CXCL13.
Summary of receptor–ligand pairs:
Naïve T cells: CCR7 + CCL19/CCL21; movement into LN via HEVs; migration to T cell zone.
Naïve B cells: CXCR5 + CXCL13; movement into follicles via HEVs; interactions with follicular DCs and T cells.
Lymphocyte recirculation and entry into LNs via HEVs
Mature T cells leave the thymus; mature B cells leave the bone marrow.
Both naïve T and B cells are recruited to LNs by chemokines that match their receptors:
Naïve T cells: Chemokine receptor CCR7; chemokines CCL19/CCL21 produced by LN cortex; migration through HEV walls into the LN cortex; leads to T cell localization.
Naïve B cells: Chemokine receptor CXCR5; chemokine CXCL13 produced in follicles by follicular DCs; migration through HEV toward follicles.
Retention in LNs and exit from LNs (S1P axis)
Retention mechanism:
Sphingosine-1-phosphate (S1P) acts as a lipid chemoattractant; its receptor on lymphocytes is S1PR1.
In the LN, surface S1PR1 is kept low to retain cells; in blood/lymph, S1P levels are high, promoting exit when S1PR1 is expressed.
Retention logic:
Naïve T cells entering LN have low S1PR1 because high S1P in blood causes S1PR1 internalization.
In LN, low S1P allows S1PR1 to gradually re-express on the surface over hours, enabling eventual egress when the cell has not encountered its antigen.
Exiting LN:
Upon full surface re-expression of S1PR1, T cells exit along the S1P gradient via efferent lymphatics and return to the bloodstream.
For both naïve and activated T cells, exiting LN involves following the S1P gradient; however, activated T cells downregulate homing receptors that drive LN entry (e.g., L-selectin, CCR7) and upregulate adhesion molecules and chemokine receptors that direct them to peripheral sites of infection.
Movement of activated lymphocytes (effector phase)
After activation by DCs, effector lymphocytes no longer preferentially return to LNs.
They must traffic to the site of threat to exert their functions (cytokine production, cytotoxic activity, help for B cells, etc.).
Activation-related changes include:
Downregulation of L-selectin and CCR7 reduction of LN homing signals.
Upregulation of integrins that bind ligands on peripheral endothelium (e.g., LFA-1, VLA-4) to permit exit from LN and trafficking to infected tissues.
T cell migration to LNs: mechanistic details
Naïve T cell entry
L-selectin on T cells binds PNAd (peripheral node addressin) on HEVs; initial rolling is slow but stable enough to support entry.
Coreceptor engagement and chemokine signaling (CCR7 with CCL19/CCL21) activate integrins (LFA-1) to promote firm adhesion and entry into the LN paracortical (T cell) zone.
HEV migration: Tight binding via activated LFA-1 to ICAM-1 on HEV, enabling entry into the LN.
DCs vs T cells: Both express CCR7 and are drawn to LN regions; DCs are generally stationary but extend dendrites to contact multiple T cells and provide antigen presentation.
If a T cell’s TCR encounters a cognate antigen presented by a DC (in the tissue or LN), T cells stop migrating and form stable interactions with the DC for activation via multiple receptor–ligand interactions.
Retention and exit of T cells from LNs during activation
Naïve T cells retain in LN to encounter antigen; if no recognition occurs, they recirculate and exit via S1P-mediated egress.
S1P gradient and S1PR1 control exit:
Naïve T cells entering LN have low S1PR1 due to internalization triggered by high [S1P] in blood; S1PR1 gradually re-expressed in LN, permitting exit when ready.
Activated T cells: S1PR1 is downregulated by CD69 (which is induced by antigen and IFN signaling), preventing exit while activation completes.
After activation, CD69 levels drop, S1PR1 re-expression increases, and activated T cells exit toward the site of infection following the S1P gradient; these cells are no longer destined for LNs and instead traffic peripherally to the infection site.
B lymphocytes: movement, retention, and activation
Naïve B cells entering LNs
Express CXCR5; respond to CXCL13 produced by follicular stromal cells and follicular dendritic cells in follicles; migrate into B cell follicles via HEVs.
Some CCR7 expression allows limited movement relative to T cell zones for interactions with T cells.
Retention and exit of naïve B cells
Retained in LN by S1P/S1PR1 axis; exit along S1P gradient when ready.
Activated B cells
Retention mechanisms exist to sustain interactions with T helper cells and to support affinity maturation and germinal center formation; eventual exit via S1P gradient toward peripheral tissues.
Distinct adhesion/migration patterns
Naïve and activated B cells display distinct combinations of adhesion molecules and chemokine receptors that direct them to specific LN regions and peripheral sites.
Reinforcement: additional notes on movement and signaling
Lymphocyte recirculation is a reiterative process transporting naive lymphocytes through multiple LNs via blood and lymphatic routes until antigen encounter or death occurs.
DCs vs T cells within LN: both express CCR7; DCs can present antigen to multiple T cells while T cells actively scan for cognate antigen; upon antigen recognition, T cells stop moving and focus on activation.
Lymph node architecture and antigen processing are central to efficient adaptive responses: antigen capture, presentation, and clonal expansion occur in the LN, followed by targeted effector cell migration to infected sites.
Quick reference for key cell types and molecules
Cells: DCs, macrophages, neutrophils, monocytes, naïve and activated T cells, naïve and activated B cells.
Endothelial adhesion molecules: E-selectin, P-selectin, L-selectin (on leukocytes); ICAM-1, ICAM-2, VCAM-1, MadCAM-1 (on endothelium).
Integrins: LFA-1 (CD11a/CD18, αLβ2), Mac-1 (CD11b/CD18, αMβ2), VLA-4 (α4β1).
Ligands: ICAM-1/ICAM-2, VCAM-1, MadCAM-1, PNAd (on HEVs).
Chemokine receptors and ligands (examples): CCR7 with CCL19/CCL21; CXCR5 with CXCL13.
S1P and S1PR1: governs retention in LN and egress; CD69 negatively regulates S1PR1 by internalization during activation.
Mathematical and conceptual notes (LaTeX formatting)
Chemokokine family patterns:
CC: adjacent cysteines
CXC: cysteines separated by 1 amino acid:
C: single cysteine
CX3C: cysteines separated by 3 amino acids
S1P gradient concept: a directional cue for egress
Gradient representation: [S1P]{ ext{blood}} > [S1P]{ ext{LN}}
ightarrow ext{S1PR1 activation and lymphocyte egress}Receptor–ligand binding strength transitions (inside-out signaling): chemokine signaling converts integrins from a low-affinity to a high-affinity state, increasing adhesion avidity and enabling firm arrest on endothelium.
Antigen encounter dynamics: once a cognate antigen is encountered, T cells may stop migrating and form stable interactions with DCs for activation; this can be viewed as a switch from motile surveillance to a localized activation state.
Practical and practical-implication connections
Therapeutic contexts: understanding S1P–S1PR1 axis has implications for immunomodulation (e.g., drugs that influence S1P signaling can affect lymphocyte trafficking).
Immunodeficiencies: LAD Type I highlights the crucial role of integrins and adhesion molecules in effective immune surveillance and infection control.
Vaccine design considerations: efficient antigen transport to LNs and prompt engagement of naïve lymphocytes are essential for robust adaptive responses; exploiting LN homing signals can improve vaccine efficacy.
Real-world relevance and broader concepts
The coordinated movement of immune cells ensures rapid innate responses, followed by a tailored adaptive response that targets the specific threat with specificity and memory potential.
The balance between retention (to allow antigen encounter) and egress (to enable surveillance and systemic immune readiness) is a fundamental principle of immune surveillance.
Video resources and wrap-up notes
A Khan Academy video overview illustrates leukocyte and lymphocyte movement and clarifies that dendritic cells, not macrophages, present antigen to T cells in LN contexts (highlighted as a note from the video).
Wrap-up emphasizes understanding signaling pathways, signaling molecules, brakes to slow cells in circulation, and retention mechanisms in LNs; next lecture promises deeper coverage of the innate immune system and the initiation/resolution of immune responses.
Quick glossary (for exam recall)
Margination: leukocytes moving toward and lining the endothelium in inflamed vessels.
Rolling: leukocytes roll along the endothelium via low-affinity selectin interactions.
Firm adhesion: high-affinity integrin–ICAM interactions arrest leukocytes securely on endothelium.
Transmigration/Diapedesis: passage of leukocytes through endothelium into tissue.
S1P/S1PR1: lipid chemoattractant signaling axis governing retention in LNs and egress to blood/lymph.
HEV: high endothelial venule; specialized LN venules where naïve lymphocytes enter from blood.
PNAd: peripheral node addressin; ligands for L-selectin on HEV endothelium.
CXCL13: chemokine that recruits CXCR5-expressing B cells to follicles.
CCL19/CCL21: chemokines that recruit CCR7-expressing T cells to LN T cell zones.
CD69: activation-induced marker that inhibits S1PR1 and retention in LN during activation.
Title
Leukocyte Circulation & Migration – Comprehensive Study Notes (Markdown Summary)