Paper - Review

DOI: 10.1016/j.cell.2020.08.048

Graphical Abstract

Highlights

• Epithelium in the distal colon
  → requires macrophages (MΦs) → for survival
• Distal colon MΦs insert "ballon-like" protrusion (BLPs) ← in the epithelium
• BLPs sense fungal toxins
  ← within the fluids absorbed → through epithelial cells
• BLP+ MΦs prevent the absorption of fungal toxins → protecting the barrier integrity

Summary

The colon
→ is primarily responsible
→ for absorbing fluids

Colon
→ contains → a large number of micro-organisms
← which are enriched in (its distal segment)

∴ The colonic mucosa
→ must tightly regulate → fluid influx
→ to control absorption of (fungal metabolites)
← can 1⃣ be toxic → to epithelial cells 2⃣ lead to (barrier dysfunction)

❗: The innate immune system
→ allows (rapid quality check) ← of absorbed fluids
→ to avoid intoxication of colonocytes

This mechanism
→ relies ← on a population of (distal colon macrophages)
← that are equipped ← with "balloon-like" protrusions (BLPs)
← which sample (absorbed fluids)

Epithelial cells
→ keep absorbing fluids
→ leading to 1⃣ their death 2⃣ subsequent loss of (epithelial barrier integrity)

❗: Essential role ← of macrophages
← in the maintenance of (colon-microbiota interactions)

Introduction

The intestinal tract
→ provides → a unique environment
∵ it is continuously exposed → to 1⃣ food antigens 2⃣ the commensal microbiota

The intestinal tract
→ comprises → different regions
← with 1⃣ distinct anatomies 2⃣ physiological roles

Maintenance of (the intestinal barrier)
→ is critical
∵ It controls
→ the absorption of 1⃣ nutrients 2⃣ electrolytes 3⃣ water
← from 1⃣ the gut lumen 2⃣ blood circulation
∵ It prevents → the absorption of (toxic luminal substances)

Disruption of (the intestinal barrier)
→ leads to → multiple pathological situations ranging
← from 1⃣ nutrient deprivation 2⃣ inflammatory bowel disease 3⃣ sepsis 4⃣ multi-organ failure

The distal colon
→ is important → for (fluid absorption)

The distal colon epitheliums
→ is exposed → to the elevated osmotic pressure
← which needed to induce (water absorption)
→ to 1⃣ dehydrate 2⃣ solidify feces

The distal colon
→ faces → a tremendous amount of micro-organisms
← e.g. 1⃣ bacteria 2⃣ fungi 3⃣ archaea 4⃣ viruses

Fungi
→ are more abundant ← in the distal colon
→ can produce → metabolites
← that trigger apoptosis of (intestinal epithelial cells)

∴ The colonic mucosa
→ tightly regulate (fluid absorption)
→ to avoid (potential entry) of (toxic fungal metabolites)
→ into 1⃣ epithelial cells 2⃣ the blood circulation

Fluid absorption (← in the distal colon)
→ is achieved
← by a single-layered epithelium
← which displaying a selective permeability

Permeability is controlled
← by the presence
← 1⃣ an abundant mucus ← which (consist of net-like polymers) & (acts ← as a physical mesh → separating the colon luminal content ← from the epithelium)
← 2⃣ junctional complexes ← which seal the space (← between epithelium cells)

Fluid absorption (← through epithelial cells)
→ is tightly coordinated
← by 1⃣ sodium pumps 2⃣ ionic channels 3⃣ transporters 4⃣ Aquaporins

These components
→ control → the selective absorption
← of 1⃣ nutrients 2⃣ electrolytes 3⃣ water

This process
→ continuously adapts
→ 1⃣ to fluctuations ← in the availability of (nutrients & electrolytes) 2⃣ to the body (salt & water) balance

Intestinal permeability
→ is regulated
← by 1⃣ the microbiota 2⃣ immune cells

❓: The dialog
← between 1⃣ intestinal epithelial cells 2⃣ microbes 3⃣ immune cells
→ is orchestrated
→ to maintain (intestinal permeability) ← in homeostatis

The most abundant types ← of (intestinal immune cells)
→ are macrophages (MΦs)
← mainly differentiate ← from monocytes
← in response → to local cues

Intestinal MΦs
→ reside → within 1⃣ lamina propria 2⃣ the muscle layer
← where they participate ← in a variety of (biological processes)
← e.g. 1⃣ the degradation of micro-organisms 2⃣ silent clearance of (apoptotic bodies) 3⃣ tissue repair 4⃣ gastro-intestinal motility

Intestinal MΦs
→ limit inflammation
→ facilitate → the survival of (local FOXP3+ T-regulatory cells)

The function of MΦs
→ is regulated ← by the microbiota

They are more abundant
← in the colone

MΦs
→ can be found
← in close association ← with epithelial cells

Disruption
← of MΦs' sub-epithelial localization
→ participates ← in the loss of (intestinal barrier integrity)

∴ MΦs
→ are ideally positioned
→ to orchestrate epithelial cell-microbiota interactions
→ for maintenance of (colon homeostasis)

Investigate → the role of MΦs
← in the integrity & function ← of the colon epithelium

Results

MΦ control epithelial cell survival and barrier integrity in the distal colon

The gut immune system
→ is highly compartmentalized
← with different cell types distributed ← in gradient along the intestine

∴ This particularly
→ concerns MΦs
← which are present ← in greater amount
← in the colon > in the small intestine

Colonic MΦs
→ display → an anergic phenotype

Their loss
→ correlates
← with 1⃣ the development ← of ulcerative colitis 2⃣ Crohn's disease
∴ A contribution → to the maintenance (← of epithelium integrity)

❗: 1⃣ depleted MΦs ← using the mouse model 2⃣ evaluated → the state of the epithelium
→ to address the question

Analyzed → both 1⃣ proximal 2⃣ distal colons
← which are known to exhibit differences
← both in 1⃣ physiology 2⃣ microbiota composition

The efficient depletion ←of (colonic MΦs)
→ was verified
← by 1⃣ flow cytometry 2⃣ immunostaining
← in both colon segments

MΦ depletion
→ led → to (massive apoptosis ← of epithelial cells)
← in ⭕ the distal colon
← not ❌ in the proximal colon

Death of (epithelial cells) ← in the distal colon
→ was observed
← when MΦs were depleted
← by injecting anti-CSF1 receptor antibodies

∴ MΦs → facilitate → the survival of (epithelial cells)
← in the distal colon

1⃣ Infused → (CD64-WT & CD64-DTR) mice
← with a hypotonic solution ← containing the small fluorescent molecule hydrazide
2⃣ Measured → its appearance ← in the blood
→ to evaluate → the impact of (epithelial cell death)
← on epithelium permeability

∴ Hydrazide
→ was more abundant
← in the blood of MΦ-depleted mice
∴ An increase ← in (barrier permeability)

∴ MΦs are needed
→ for 1⃣ epithelial cell survival 2⃣ intestinal barrier integrity

Distal colon MΦ insert "balloon-like" protrusion in between epithelial cells

To investigate
→ distribution of (MΦs & their physical interaction)
← with epithelial cells
← in 1⃣ the proximal 2⃣ distal colon

⁉: Performed → whole-mount immuno-staining
← of 1⃣ proximal 2⃣ distal colon sections

All cells
← that express CD11c → throughout (their differentiation)
← which include 1⃣ intestinal MΦs 2⃣ dendritic cells 3⃣ DCs
→ switch → from membrane-tomato → to membrane-GFP expression
∴ Allowing better visualization of (these phagocytes)

Observed → a phagocyte population ← in the distal colon
← that physically interacted ← with epithelial cells
← through (membrane protrusion) ← which shaped as balloons

The (cell bodies) ← of these cells
→ were localized → around the opening of the crypts

Each cell displayed (three BLPs)
← which crossed → the basement membrane
← which contacted → the base of epithelial cells

❗: BLPs
→ do NOT → correspond to the "trans-epithelial dendrites"
∵ they do NOT reach → the lumen of the colon

BLP+ cells
→ resembled bona fide intestinal MΦs
→ were lost ← in toxin-injected CD64-DTR mice
∵ immunostaining

Reduced number of BLPs
→ were observed
← in the proximal colon
Proximal crypts
→ contained a similar number of MΦs
← as their distal counter-part

Proximal colon MΦs
→ displayed protrusions
← which were rater thin & without ballons
∵ Observation of (proximal colon MΦs)

∴ The distal colon is enriched ← with MΦs
← which equipped with peculiar balloon-like membrane protrusions
← which inserted at the base of epithelial cells

BLPs are filled with epithelial cell membranes and endo-lysosomal compartments

Analyzed → the inner structure of BLPs
← using high-resolution confocal microscopy

BLPs
→ were approx. 1-5 μm in diameter
→ some of them forming (thin extensions)
← than extended ← between epithelial cells

BLPs
→ were filled
← with membranes & vesicles devoid of actin

Most BLP internal vesicles
← in CD11c-Cre/R26-mTmG mice
→ were Tomato+
∴ They originated ← from cells
← that do NOT express CD11c

⁉: these membranes
→ might come ← from epithelial cells
∴ Generated → Villin: Cre-ER-T2/R26-mTmG mice
← in which (intestinal epithelial cells) → express (membrane GFP (mGFP))

BLPs
→ were filled ← with GFP-positive membranes
∵ Analysis of (distal colon sections)
∴ They contain membranes & vesicles of (epithelial origin)

BLP inner vesicles
→ varied ← [10, 200] nm in diameter
→ were partly filled ← with electron-dense material
∵ correlative light-transmission electron microscopy analysis
∴ They might contain lysososmes

Most BLPs
→ were positive
→ for 1⃣ LAMP1 2⃣ LAMP2 3⃣ Rab7A late-endosomal/lysosomal markers

EEA1
← the marker of early endosomes
→ was barely detected ← in BLPs

All BLPs
→ were enriched ← in MHC class II
→ were associated ← invariant chain ← e.g. 1⃣ CD74 2⃣ Li

The endo-lysosomal markers
← which found within BLPs
→ were most likely expressed
← in the MΦs themselves
∵ Not observing → these two molecules ← in the epithelium

∴ BLPs → are filled ← with endo-lysosomal compartments
← that contain membranes internalized ← from epithelial cells

BLP+ MΦ are CD11c-high

❓: whether BLP+ MΦs
→ constitute → a unique cells population
← which distinct from its BLP- counterpart

FACS-sorted terminally differentiated MΦs
← from both 1⃣ proximal 2⃣ distal colons ← of a C57BL/6J mouse
Compared → their gene expression profiles
← using single-cell RNA-seq

Adopted → a semi-supervised approach
→ to group the cells ← based on their gene expression profile
→ to reliably identify (transcriptionally different MΦ sub-populations)

Explored → different solutions
← which obtained by varying the input ← with a graph-based clustering algorithm
→ to identify the cluster ← whose relative abundance was greater ← in the distal colon

Selected (feasible solutions)
← as the ones ← where the fraction of (putative BLP+ MΦs)
→ was [50%, 70%] ← in the distal colon
→ was below 40% ← in the proximal colon

Scored → the solutions
← using the silhouette width
← a measure of the similarity of (each cell) → to the cells of the same cluster
← compared to the cells ← belonging to the neighboring cluster

The optimal solution
→ identified ← with this approach
→ contained five clusters

Only two of them
→ expressed (high levels of all MΦ markers)
∴ the other three
→ likely contained contaminants
→ were therefore removed

∴ In total
→ collected 2106 MΦs

The distal colon sample
→ included 58% of cells ← from cluster 1
The proximal colon
→ only included 18%

∴ 1⃣ cluster 1 → is enriched → for BLP+ MΦs
2⃣ cluster 0 → is enriched → from BLP- MΦs

Stained (tissue sections)
← with anti-CD11c antibodies
← as the CD11c gene → was found → to be more expressed
← in cluster 1

These antibodies
← which specifically labeled sub-epithelial BLPs+ MΦs
← in the distal colon

The percentage ← of (BLP+ MΦs)
← found by tissue staining
→ was similar → to the percentage of CD11c-high MΦs
← defined ← by 1⃣ flow cytometry 2⃣ imaging ← in the distal colon

∴ Tissue disaggregation
→ leads →to the limited death of (BLP+ MΦs)

∴ Cluster 1 → is enriched ← for BLP+ MΦs

BLP+ MΦs exhibit a distinct gene expression profile

Extracted → the genes
← which are differentially expressed ← in clusters
→ to identify (the gene specific) → for BLP+ MΦs

88 & 122 genes
→ were respectively up- & down-regulated
← in BLP+ MΦs ← as compared to BLP- MΦs

Several genes
→ could be directly & indirectly linked
← with 1⃣ microbiota recognition 2⃣ inflammatory disorders ← in the colon
← e.g. 1⃣ lgal3 2⃣ plaur 3⃣ ll1b 4⃣ llr2 5⃣ cd9

Compared → our results
→ to a recently published study
← in which single-cell RNA-seq was used → to identify the presence of (distinct MΦ sub-population) ← in the murine colon

∴ Cluster 4
← which corresponded → to terminally differentiated MΦs
→ shared 60 genes ← with the signature of our BLP+ MΦs

Cluster 6
→ strongly resembled → BLP- MΦs
← with 59 overlapping genes

These two sub-populations
→ were found → to be spatially segregated
← within the colonic stroma

Both cluster 4 & 6
→ were decreased ← in germ-free mice

BLP+ colonic MΦs
→ correspond → to specific MΦ sub-populations
← which is enriched ← in the distal colon
→ forms (tight interactions) ← with epithelial cells
→ might be influenced ← by the microbiota

Intestinal fungi augment the formation of BLPs

Evaluate → the impact of the microbiota
← on BLP+ MΦs

The colon
→ harbors → the largest amount of micro-organisms ← in the body
← where commensal bacterial & fungal population → share the intestinal niche

Treated → mice
← with 1⃣ a broad-spectrum antibiotic cocktail 2⃣ anti-fungal agents
→ to assess (the potential effect) of (1⃣ bacteria 2⃣ fungi)

There was NO effect of antibiotics
Two anti-fungal agents
← 1⃣ fluconazole 2⃣ amphotericin B
→ significantly decreased the number of BLPs
← without altering the total number of MΦs

These drugs
→ efficiently reduce → the amount of (intestinal fungi) ← in mice
← using distinct mechanisms of action

The number of sub-epithelial MΦs
→ was unchanged ← in mice treated with anti-fungal drugs
→ displayed → long thin protrusion ← instead of BLPs
∵ tissue staining

The combination
← of 1⃣ fluconazole 2⃣ amphotericin B
← with antibiotics
→ showed → no additive effects
∴ These two anti-fungal agents
→ do NOT act → on BLPs indirectly
← i.e. by leading → to the expansion of (colonic bacteria)

∴ Distal colon-resident fungi
→ might promote → the formation of BLPs
← in sub-epithelial MΦs

Colonized → germ-free mice
← with altered Schaedler flora (ASF) ← a well-defined community of eight bacteria species
← with the gut fungal pathobiont ← Candida albicans
→ to directly assess (the role of fungi) ← in the induction of BLPs

Fungal colonization
→ induced strong BLP formation
← by MΦs ← in the distal colon of germ-free mice
→ that were otherwise devoid the these structure
Bacterial colonization
→ did NOT influence → the formation of BLPs

Bacterial colonization
→ did NOT influence → the formation of BLPs

No change
← in the number of sub-epithelial MΦs
→ was observed

∴ These results
→ support a role → for the mycobiota
← in the formation of BLPs ← in the distal colon

Intestinal fungi are responsible for epithelial cell death in the distal colon of MΦ-depleted mice

There is a population of CD11c-high MΦs
→ that form BLPs ← in response to local fungi

MΦ depletion
→ was associated ← with massive epithelial cell apoptosis
← in the distal colon
← NOT in the proximal one

⁉: MΦ might protect epithelial cells
← from fungi-induced death
← by forming BLPs

Fungi
→ produce → many toxic molecules
← including 1⃣ toxins 2⃣ metabolites
← which can be deleterious for the host

Pre-treated → CD64-DTR mice
← with anti-fungal agents
← before MΦ depletion
Assessed → the impact of such treatment
← on epithelial cell survival

Anti-fungal treatment
→ rescued → the epithelial cells
← from death in MΦ-depleted animals

Epithelial cell death
→ was NOT rescued
← when MΦ-depleted mice → were treated with antibiotics

∴ Fungi
→ are most likely responsible
→ for the death of epithelial cells
← observed in the distal colon of MΦ-depleted mice

∴ MΦs
→ might protect epithelial cells
← from fungi-induced cell death

BLP+ MΦs sample the fluids absorbed through epithelial cells

Colonic MΦs
→ use C-type lectin receptors
← such as 1⃣ Dectin-1 2⃣ Dectin-2 3⃣ Mincle
→ 1⃣ to recognize fungi cell-wall constitutents 2⃣ to orchestrate anti-fungal immunity

NOT observe → any difference
← in the level of expression of 1⃣ Dectin-1 2⃣ Dectin-2
← in MΦs sorted ← from the distal and proximal colons

Dectin-1-KO MΦs
→ formed silmilar number of BLPs
← in the distal colon
← as compared to wild-type animals

BLP+ MΦs
→ might not respond → to intestinal fungi
→ through the recognition of β-glucan ← by Dectins

BLPs
→ not reaching→ 1⃣ the colon lumen 2⃣ the micro-organisms it contains

BLP characterization
→ were filled ← with membranes from (epithelial cells)
→ enriched ← in endo-lysosomal compartments

⁉: BLPs
→ might sample → fungal metabolites & toxins indirectly
← through the fluids absorbed ← by the epithelial cells
← that they are in contact with

Epithelial cells
← in the distal colon
→ possess (specific mechanisms)
← which 1⃣ optimize the absorption of water 2⃣ facilitate stool dehydration

BLP+ MΦs
→ sample the environment
← in the absence of (direct contact) ← with the local microbiota
∵ such mechanisms

❓: whether BLP+ MΦs
→ respond to fluid absorption ← by epithelial cells

Infused
→ mice ← that intra-rectally with a hypotonic solution

Such treatment
→ increased → the number of BLPs ← as fast as 10 min after infusion
→ returned → to steady state ← in 30 min

This increase
→ was observed
← when fluid absorption was stimulated ← by aldosterone injection
← a corticosteroid hormone ← increases 1⃣ sodium 2⃣ water absorption ← in the distal colon

Observed → a significant decrease ← in the number of BLPs
← when inhibiting (water absorption)
← by treating animals ← with the laxative Bisacodyl

This decrease
→ was abrogated
← when mice were pre-treated with indomethacin
← which inhibits the action of Bisacodyl

❗: an important accumulation of the dye
← inside the BLP+ MΦ networks
∵ tracing (the fluid absorbed) ← through epithelial cells

∴ The fluid absorbed
← through the epithelium of the distal colon
→ stimulates the formation of BLPs
← in associated MΦs ← wherein this fluid acculmulates

❗: The formation of BLPs
← upon fluid absorption
→ was increased ← when fungi were present

MΦs protect epithelial cells from being poisoned by fungal toxins

Searched → for a molecule
← which to be used as 1⃣ a generic fungal metabolite 2⃣ toxic
→ for epithelial cells
∴ Gliotoxin

This fungi metabolite
→ induce apoptosis ← in epithelial cells
→ is produced ← by both 1⃣ pathobionts 2⃣ food spoilage fungi

Infused fungi-depleted C57BL/6J mice
← with a hypotonic solution
← which 1⃣ containing 2⃣ not containing gliotoxin
→ to investigate → whether MΦs can sense (fungal metabolites)
← present in the fluids absorbed ← by epithelial cells

Gliotoxin-containing solution
→ stimulated BLP-formation
← 5 min after the infusion

The epithelium
→ kept absorbing → the gliotoxin-free hypotonic solution
→ stopped → the absorption of (the one containing gliotoxin)
← 20 min upon infusion

∴ Epithelial cells
→ sense & stop (absorbing fluids)
← that are poisoned ← with fungal toxins

Performed → a similar experiment ← in 1⃣ CD64-WT 2⃣ CD64-DTR littermates
← 1⃣ injected with diphtheria toxin (DT) 2⃣ infused with the gliotoxin-containing hypotonic solution
→ ❓: 1⃣ this is an intrinsic capacity of (epithelial cells) 2⃣ requires associated BLP+ MΦs

∴ 1⃣ DT-injected CD64-WT mice
→ stopped → the absorption of (the gliotoxin-containing solution)
2⃣ MΦ-depleted CD64-DTR mice
→ kept absorbing

Infused 1⃣ CD64-WT 2⃣ CD64-DTR littermates
← with the gliotoxin-containing hypotonic solution
→ ❓: whether (this uncontrolled absorption of gliotoxin) → had long-term effects
← on epithelium homeostasis

Distal colon epithelial cells
→ underwent (massive apoptosis)
← when MΦs were depleted

∴ These phagocytes → protect epithelial cells
← from being poisoned ← by the fungal toxin

∴ MΦs → endow (epithelial cells)
← with the ability 1⃣ to recognize (toxic fluids) 2⃣ to stop absorption
∴ Maintaining 1⃣ epithelial integrity 2⃣ local homeostasis

BLPs are needed for MΦs to prevent epithelium poisoning by fungi toxins

The essential role of MΦs
← in stopping the absorption of toxin-containing fluids
← through epithelial cells

BLPs
→ are enriched
→ for CD74+ endo-lyosomal compartments
CD74 expression
→ was slightly increased
← in cluster 1

CD74
→ controls (MHC class II trafficking) → to endocytic compartments
→ leads → to the expansion of (endolysosomal compartments)
→ enhances → the internalization of (extra-cellular fluid) ← by macropinocytosis

⁉: CD74 might help → BLP+ MΦs internalizing
← within endo-lysosomes 1⃣ the fluid 2⃣ small vesicles
← that released by epithelial cells

Infused
→ fluconazole-treated 1⃣ CD74-WT 2⃣ CD74-KO mice
← with (a hypotonic solution) ← that containing gliotoxin

The number of BLPs
→ was equivalent
← in the two groups of mice
← at steady states

∴ 1⃣ the number of BLPs → increased ← in wild-type mice
2⃣ it did NOT ← in CD74-KO animals

∴ A decrease
← in uptake of extracellular fluids

CD74-KO sub-epithelial MΦs
→ exhibited → (reduced level) ← of hydrazide internaliztion

CD74
→ is required → for BLP formation
← in response → to (fluid absorption) → through (epithelial cells)

∴ CD74-KO mice
→ constitute (a valuable model)
→ to evaluate the implication of BLPs

1⃣ Absorption of (gliotoxin-containing solution)
→ stopped 20 min after infusion
← in wild-type mouse
2⃣ This decrease ← in absorption
→ was not observed
← in the epithelium of CD74-KO mice

∴ BLPs
→ orchestrate (fluid sampling)
← by distal colon MΦs
∴ Critical → to protect (epithelial cells)
← from absorbing fluids ← enriched in 1⃣ fungi toxins 2⃣ fungi metabolites

Discussion

The intestinal barrier
→ separates → the gut lumen
← from the internal milieu

The intestinal barrier
→ acts ← as a selectively permeable filter
← that allows the absorption of 1⃣ nutrients 2⃣ electrolytes 3⃣ water
← which can then reach (the blood circulation) → for organ irrigation

The intestinal lumen
→ contains (many toxic substances)
← which produced by the microbiota
∴ Absorption
→ must be tightly regulated
→ to prevent 1⃣ intoxication 2⃣ disease

Dysregulation of (intestinal barrier permeability)
→ is a leading cause of (sepsis-related mortality)
← in 1⃣ critically ill patients 2⃣ IBD

A mechanism
← which driven ← by a particular population of CD11c-high sub-epithelial MΦs
→ ensures (rapid quality check) ← of (absorbed fluids)
→ to maintain (barrier integrity)

MΦs
→ instruct (epithelial cells)
→ to stop absorption
∴ Preventing → epithelial cell 1⃣ poisoning 2⃣ death

Epithelial cells
→ absorb fluids
← independently of their fungal 1⃣ toxin 2⃣ metabolite load
← the absence of 1⃣ MΦs 2⃣ BLPs
∴ Compromising (barrier integrity)

The permeability of (the intestinal barrier)
→ is differentially regulated
← depending on (the local physiological function)
← of (the specific intestine segment) & on its microbial content

∴ MΦs
→ emerge ← as key players
← in the orchestration of (such regulation)

MΦs
→ form transient (trans-epithelial dendrites)
← which visible by (live imaging)
← in the small intestine

These protrusions
→ extend ← between (epithelial cells)
→ reach (the gut lumen)
← where they capture 1⃣ bacteria 2⃣ dietary antigens

These protrusions
→ express (tight-junctional proteins) → to form adhesions ← with epithelial cells
← during crossing the barrier
→ ∴ ensures → the maintenance of (the epithelium integrity)

BLPs
→ penetrate → the basement membrane
→ occupy → the inter-cellular space ← of the epithelium
→ do NOT directly contact → the colon lumen

Paracellular permeability
→ is elevated ← in the small intestine epithelium
∴ its physiological roles
← in 1⃣ nutrient absorption 2⃣ establishment of food tolerance

The colonic epithelium
→ exhibits (limited paracellular permeability)
→ helps → (the colonic epithelium)
→ to resist (local mechanical stress) ← imposed by 1⃣ elevated osmotic pressure 2⃣ stool solidification
∴ Preventing loss ← in barrier integrity

∴ MΦs
→ might have developed → an alternative sampling strategy
← in the context of (such tightly sealed colonic epithelium)

MΦs
→ form BLPs
← which sample (fluids absorption) → through & between (epithelial cells)

The paracellular route
→ allows sampling of the fluid absorbed ← through the epithelium
The trans-cellular route
→ might detect transient events ← of (barrier leakage)
← that result from 1⃣ stretch 2⃣ shear mechanical insult

∴ Sampling mechanisms
← by the peripheral immune system
→ are adapted → to both 1⃣ the local cues 2⃣ physiological functions

Distal colon BLP+ MΦs
→ respond → to the presence of (fungi products)

Fungi
→ being particularly enriched
← in the distal colon

Dectins
→ might NOT be involved
← in this process ← in (homeostatic conditions)

The distal colon
→ contains → a thick mucus layer
← which physically separates (the microbiota) ← from the epithelial cells
∴ Inner parts of this mucus
→ are sterile

∴ NO contact
← between 1⃣ sub-epithelial MΦs 2⃣ the colon lumen

MΦs
→ use BLPs
→ to sample the fluids ← which absorbed through (epithelial cells)
→ carry (the whole spectrum) ← of fungal metabolites
∴ Providing (a comprehensive picture) ← of (local mycobiota composition)

MΦs
→ physically contact fungi
→ use Dectin-1 → to mount (effective anti-fungi immune responses)

BLPs
→ respond
→ 1⃣ to fungi compounds ← produced by pathogenic species 2⃣ to a broad variety of (fungi metabolites)

∴ The BLP response
→ may be critical
→ 1⃣ to sense potentially (dangerous fungi species) 2⃣ to detect (the over-growth of commensal fungi)

Commensal fungi
→ are indeed responsible
→ for epithelial cell apoptosis ← in MΦ-depleted mice

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