This work was supported financially by NIH (1 R01 AI095338-01A1).

The authors declare that they have no competing financial interests.

Neutrophil migration across mucosal epithelial surfaces is a common feature in disease pathology following infection with bacterial pathogens3. The methodology described herein offers a rapid, straightforward approach to experimentally isolate this discrete event using a human cell derived in vitro coculture assay system that models a feature of the inflammatory process triggered by bacterial infections. This system was originally developed using polarized intestinal epithelial cells infected with ent...

Mucosal surfaces serve as physical and immunological barriers providing protection against external threats pervasive in the environment1,2.&#160;This protective epithelial barrier can be compromised when pathogenic organisms invade2.&#160;In the case of a bacterial pathogen, this encounter often instigates an inflammatory process by activating the innate immune system and triggering a rapid mobilization of first responder granulocytes known as neutrophils2-4. Chemotactic agents facilitating neutrophil recruitment are produced in part by the mucosal epithelial cells seeking to rid the host of the offending pathogen2-4.&#160;Excessive or unresolved neutrophil infiltration of the mucosal epithelial surface can cause significant pathology1,5.&#160;This is a consequence of nonspecific tissue damage caused by the anti-bacterial neutrophil arsenal5-7.&#160;In such cases, bacterial clearance capacity of neutrophils is overshadowed by destruction of host tissue during an infectious insult.&#160; Disruption of the protective epithelial barrier function can lead to enhanced exposure of underlying tissue to microorganisms and/or toxins, further exacerbating disease pathology8,9.&#160;These consequences can be observed in multiple organ systems including the lung and digestive tract1,5.&#160;Furthermore, noninfectious inflammatory conditions such as severe bouts of asthma, chronic obstructive pulmonary disease (COPD), acute respiratory distress syndrome (ARDS), and inflammatory bowel disease (IBD) are marked by the pathological breach of the mucosal epithelial barrier by an excessive neutrophilic response4,5,10-12.
The complex process of neutrophil recruitment following mucosal infection involves several compartmentalized steps1,5,13,14.&#160;First, neutrophils must depart from circulation via a series of cell-to-cell interactions that facilitate trans-endothelial migration1,13.&#160;Neutrophils next navigate existing interstitial space containing extracellular matrix1,14.&#160;To reach the lumen of the infected mucosa, neutrophils must then migrate across the epithelial barrier1,4,5.&#160;This intricate multistep phenomenon is often investigated in aggregate using in vivo animal models of infection15.&#160;Such models are useful for establishing the necessity of specific factors, such as chemokines, adhesion molecules, or signaling pathways that participate in the overall process but are largely inadequate for resolving molecular contributions critical for each distinct compartmentalized step16.&#160;Cocultured in vitro systems modeling trans-endothelial, trans-matrix, or trans-epithelial migration of neutrophils have been particularly useful in this regard1,14,16,17.&#160;
A robust coculture assay system has been developed for the purpose of deciphering mechanisms responsible for neutrophil trans-epithelial migration in response to pathogenic infection18-22.&#160;This model involves infecting the apical surface of polarized human epithelial cell layers with a bacterial pathogen followed by application of freshly isolated human neutrophils to the basolateral surface18-22.&#160;Neutrophils migrate across the epithelial barrier in response to epithelial-derived chemotactic products secreted following pathogenic infection18,21-23.&#160;This model system has been employed using intestinal and lung epithelial cultures exposed to appropriate tissue specific bacterial pathogens and has unveiled novel molecular mechanisms likely important to the neutrophil recruitment process during mucosal infection3,8,19,24-28.&#160;The strength of this in vitro coculture model is that a reductionist approach enables the investigator to experimentally manipulate the pathogen, epithelial barrier, and/or neutrophil in a well-controlled, highly reproducible, fairly inexpensive system.&#160;Insight gathered from this approach can be effectively leveraged to conduct focused analysis of compartmentalized events during neutrophil recruitment using in vivo infection models22,29,30.
This article demonstrates the multiple steps necessary for the successful establishment of this reproducible model to explore pathogen induced neutrophil trans-epithelial migration.&#160;Lung epithelial barriers infected with the pathogen Pseudomonas aeruginosa are featured in this article; however, other tissue epithelia and pathogens can be substituted with minor modifications.&#160;Seeding and culturing of polarized lung epithelial cell layers on inverted collagen coated permeable transwell filters is detailed herein, as is the growth of pathogenic P. aeruginosa and the isolation of neutrophils from whole blood.&#160;How these components are combined to observe pathogen induced neutrophil trans-epithelial migration is presented along with appropriate positive and negative controls to establish a reproducible assay.&#160;The versatility of this approach to examine various aspects of pathogen induced neutrophil trans-epithelial migration is discussed with reference to specific studies in the literature.

<table border="0" cellpadding="0" cellspacing="0" width="1470">
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	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">NCl-H292 cells</td>
			<td style="width:147px;">ATCC</td>
			<td style="width:137px;">CRL-1848</td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">RPMI-1640 medium</td>
			<td style="width:147px;">ATCC</td>
			<td style="width:137px;">30-2001</td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Pseudomonas aeruginosa PAO1</td>
			<td style="width:147px;">ATCC</td>
			<td style="width:137px;">#47085</td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Escherichia coli MC1000</td>
			<td style="width:147px;">ATCC</td>
			<td style="width:137px;">#39531</td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">D-PBS (1x) liquid</td>
			<td style="width:147px;">Invitrogen</td>
			<td style="width:137px;">14190-144</td>
			<td style="width:667px;">without calcium and magnesium</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Heat Inactivated Fetal bovine serum</td>
			<td style="width:147px;">Invitrogen</td>
			<td style="width:137px;">10082-147</td>
			<td style="width:667px;">10% added to culture medium</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Penicillin-Streptomycin</td>
			<td style="width:147px;">Invitrogen</td>
			<td style="width:137px;">15140-122</td>
			<td style="width:667px;">100x: 10,000 units of penicillin and 10,000 &#181;g of streptomycin per ml.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Trypsin-EDTA (0.05%)</td>
			<td style="width:147px;">Invitrogen</td>
			<td style="width:137px;">25300-062</td>
			<td style="width:667px;">50 ml aliquots are stored frozen at -20 &#186;C.&#160; Aliquot in use can be stored at 4 &#186;C short-term.&#160;&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Hank&#39;s Balanced Salt Solution - HBSS(-)</td>
			<td style="width:147px;">Invitrogen</td>
			<td style="width:137px;">14175-079</td>
			<td style="width:667px;">Sterile, without calcium and magnesium</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Trypan Blue Solution</td>
			<td style="width:147px;">Invitrogen</td>
			<td style="width:137px;">15250-061.</td>
			<td style="width:667px;">Stock = 0.4%</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Collagen, Rat Tail</td>
			<td style="width:147px;">Invitrogen</td>
			<td style="width:137px;">A10483-01</td>
			<td style="width:667px;">Can also be isolated in the laboratory directly from the tails of rats using standard protocols</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Citric acid</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">C1909-500G</td>
			<td style="width:667px;">Component of 1 M citrate buffer and acid citrate dextrose (ACD) solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Sodium Citrate</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">S4641-500G</td>
			<td style="width:667px;">Component of 1 M citrate buffer</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Dextrose anhydrous</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">D8066-250G</td>
			<td style="width:667px;">Component of acid citrate dextrose (ACD) solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Ammonium Chloride</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">213330-500G</td>
			<td style="width:667px;">Component of red blood cell (RBC) lysis buffer</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Sodium bicarbonate</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">S6014-500G</td>
			<td style="width:667px;">Component of red blood cell (RBC) lysis buffer</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">EDTA</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">ED-100G</td>
			<td style="width:667px;">Component of red blood cell (RBC) lysis buffer</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">HBSS(+) powder</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">H1387-10L</td>
			<td style="width:667px;">Key component of HBSS+</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">HEPES</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">H3375-500G</td>
			<td style="width:667px;">Component of HBSS+</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Sigmacote</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">SL2-25ML</td>
			<td style="width:667px;">Follow vendor instructions to coat glass pipette tips</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Triton X-100</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">T-9284</td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">2,2&#39;-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS)</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">A9941-50TAB</td>
			<td style="width:667px;">Key component of ABTS substrate solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">30% Hydrogen Peroxide Solution</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">H1009-100ML</td>
			<td style="width:667px;">Component of ABTS substrate solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">N-Formyl-Met-Leu-Phe (fMLP or fMLF)</td>
			<td style="width:147px;">Sigma-Aldrich&#160;</td>
			<td style="width:137px;">F-3506</td>
			<td style="width:667px;">A Stock solution of 10 mM in DMSO should be prepared and aliquots stored at -20 &#186;C.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Gelatin Type B</td>
			<td style="width:147px;">Fisher Scientific</td>
			<td style="width:137px;">M-12026</td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Pseudomonas isolation agar&#160;</td>
			<td style="width:147px;">Fisher Scientific</td>
			<td style="width:137px;">DF0927-17-1</td>
			<td style="width:667px;">Follow manufacturer&#8217;s instructions to make PIA plates</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Ficoll-Paque PLUS&#160;</td>
			<td style="width:147px;">Fisher Scientific</td>
			<td style="width:137px;">45-001-749</td>
			<td style="width:667px;">Optional, can improve neutrophil purity</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;"></td>
			<td style="width:147px;"></td>
			<td style="width:137px;"></td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;"></td>
			<td style="width:147px;"></td>
			<td style="width:137px;"></td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;"></td>
			<td style="width:147px;"></td>
			<td style="width:137px;"></td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;"></td>
			<td style="width:147px;"></td>
			<td style="width:137px;"></td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;"></td>
			<td style="width:147px;"></td>
			<td style="width:137px;"></td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Name of Material / Equipment</td>
			<td style="width:147px;">Company</td>
			<td style="width:137px;">Catalog Number</td>
			<td style="width:667px;">Comments</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">24-well migration plate</td>
			<td style="width:147px;">Corning Incorporated</td>
			<td style="width:137px;">#3524</td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">24-well wash plate</td>
			<td style="width:147px;">Falcon</td>
			<td style="width:137px;">35-1147</td>
			<td style="width:667px;">Can be reused if soaked in 70% ethanol and washed thoroughly prior to reuse</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">96-well plate</td>
			<td style="width:147px;">Fisher Scientific</td>
			<td style="width:137px;">#12565501</td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:520px;">Transwell Permeable Supports&#160;</td>
			<td style="width:147px;">Corning Incorporated</td>
			<td style="width:137px;">#3415</td>
			<td style="width:667px;">Polycarbonate; Diameter: 6.5 mm; Growth area: 0.33 cm2; Dish style: 24-well plate; Pore size: 3.0 &#181;m</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Petri dish</td>
			<td style="width:147px;">Falcon</td>
			<td style="width:137px;">35-1013</td>
			<td style="width:667px;">Each Petri dish holds 24 inverted 0.33 cm2 Transwells.&#160;&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">500 ml 0.2 &#956;m filter / flask</td>
			<td style="width:147px;">Fisher Scientific</td>
			<td style="width:137px;">09-740-25A</td>
			<td style="width:667px;">To sterilize acid citrate dextrose (ACD) solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">5-3/4 in glass Pasteur pipette</td>
			<td style="width:147px;">Fisher Scientific</td>
			<td style="width:137px;">13-678-20A</td>
			<td style="width:667px;">Coat tips with Sigmacote prior to use</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Hemostat</td>
			<td style="width:147px;">Fisher Scientific</td>
			<td style="width:137px;">13-812-14</td>
			<td style="width:667px;">Curved, Serrated</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Invertoskop Inverted Microscope</td>
			<td style="width:147px;">Zeiss</td>
			<td style="width:137px;">#342222</td>
			<td style="width:667px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:520px;">Versa-Max Microplate Reader</td>
			<td style="width:147px;">Molecular Devices</td>
			<td style="width:137px;">#432789</td>
			<td style="width:667px;"></td>
		</tr>
	</tbody>
</table>

in vitro coculture assay to assess pathogen induced neutrophil trans-epithelial migration

Mucosal surfaces serve as protective barriers against pathogenic organisms.&#160;Innate immune responses are activated upon sensing pathogen leading to the infiltration of tissues with migrating inflammatory cells, primarily neutrophils.&#160;This process has the potential to be destructive to tissues if excessive or held in an unresolved state.&#160; Cocultured in vitro models can be utilized to study the unique molecular mechanisms involved in pathogen induced neutrophil trans-epithelial migration.&#160;This type of model provides versatility in experimental design with opportunity for controlled manipulation of the pathogen, epithelial barrier, or neutrophil.&#160;Pathogenic infection of the apical surface of polarized epithelial monolayers grown on permeable transwell filters instigates physiologically relevant basolateral to apical trans-epithelial migration of neutrophils applied to the basolateral surface.&#160;The in vitro model described herein demonstrates the multiple steps necessary for demonstrating neutrophil migration across a polarized lung epithelial monolayer that has been infected with pathogenic P. aeruginosa (PAO1).&#160;Seeding and culturing of permeable transwells with human derived lung epithelial cells is described, along with isolation of neutrophils from whole human blood and culturing of PAO1 and nonpathogenic K12 E. coli (MC1000).&#160; The emigrational process and quantitative analysis of successfully migrated neutrophils that have been mobilized in response to pathogenic infection is shown with representative data, including positive and negative controls.&#160;This in vitro model system can be manipulated and applied to other mucosal surfaces.&#160;Inflammatory responses that involve excessive neutrophil infiltration can be destructive to host tissues and can occur in the absence of pathogenic infections.&#160;A better understanding of the molecular mechanisms that promote neutrophil trans-epithelial migration through experimental manipulation of the in vitro coculture assay system described herein has significant potential to identify novel therapeutic targets for a range of mucosal infectious as well as inflammatory diseases.

Several studies have demonstrated that pathogen-infected epithelial layers facilitate neutrophil trans-epithelial migration3,8,19,24-28,31,32. This occurs via a pathogen-specific induction of an epithelial cell-derived neutrophil chemotactic gradient3,23. For example, pathogenic P. aeruginosa interacting with the apical surface of lung epithelial cells causes a substantial number of neutrophils to migrate across the epithelial layer18,22,25,26,33,34. This clinically relevant assa...

Watch this Scientific Journal Video about In vitro Coculture Assay to Assess Pathogen Induced Neutrophil Trans-epithelial Migration at JoVE.com

In vitro Coculture Assay to Assess Pathogen Induced Neutrophil Trans-epithelial Migration

Neutrophil trans-epithelial migration in response to mucosal bacterial infection contributes to epithelial injury and clinical disease.&#160;An in vitro model has been developed that combines pathogen, human neutrophils, and polarized human epithelial cell layers grown on transwell filters to facilitate investigations towards unraveling the molecular mechanisms orchestrating this phenomenon.

In vitro Coculture Assay to Assess Pathogen Induced Neutrophil Trans-epithelial Migration

Mucosal surfaces serve as protective barriers against pathogenic organisms.&#160;Innate immune responses are activated upon sensing pathogen leading to ...

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Immunology and Infection

17.3K Views.  Harvard Medical School. Neutrophil trans-epithelial migration in response to mucosal bacterial infection contributes to epithelial injury and clinical disease. An in vitro model has been developed that combines pathogen, human neutrophils, and polarized human epithelial cell layers grown on transwell filters to facilitate investigations towards unraveling the molecular mechanisms orchestrating this phenomenon.