We would like to thank Dr. David Olivieri for critical review of this manuscript, Dr. Nuno Moreno for the logistic help to build our animal holder and heating pads and Dr. Vijay K. Kuchroo for the kind donation of Foxp3-KIgfp/gfp mice. This work is supported by "Funda&ccedil;&atilde;o para Ci&ecirc;ncia e Tecnologia" (FCT, Portugal), grant # PTDC/EBB-BIO/115514/2009.

1. Animal preparation
Important notes: BM cell suspensions and thymic transplantations were performed in aseptic conditions. The BM suspensions were prepared inside the hoods of our cell culture room, while the thymic transplantation was performed in the surgical room located within our animal facility. In order to keep and guarantee these aseptic conditions, we were not allowed to record our video in these places. However, all the surgical materials were autoclaved and the surgical bench was previously cleaned with Virkon (Pharmacal Research Laboratories Inc.) and 70% ethanol. All procedures were approved by the Institutional Animal Care and Use Committee (IACUC) and they were in agreement with the Federation of European Laboratory Animal Science Associations (FELASA) directives. The approval ID number is AO10/2010. All the image experiments were terminal procedures and the animals were euthanized immediately following the end of image acquisition. 
The detailed procedure is as follows: 
<ol>
 <li>Irradiate (with a &gamma;-irradiator) 8 week old Nude mice with 900 rads to destroy the endogenous hematopoietic precursors inside the bone marrows (BM). After irradiation, return the animals to their cages until you prepare the donor BM cells for intravenous injection and further BM reconstitution. This BM transfer was performed 1 h after irradiation. </li>
 <li>Separate the BM donor animals. Tibias and femurs of hind limbs of one animal are generally enough to reconstitute up to 3 recipient animals. After euthanasia with CO2, remove the hind limbs and then remove the skin and muscle from the bones. </li>
 <li>Cut the closed end of each bone and flush it with PBS (or RPMI) or crush them all in a mortar, using a pestle, with 2 ml of PBS (or RPMI) to remove the BM. </li>
 <li>Disrupt the BM structure into a single-cell suspension by repetitions of vigorous aspiration using a P1000 pipette. Alternatively, you can also pass the BM throughout a 21G syringe needle several times. Centrifuge (400g, 5 min, R.T.), re-suspend cells in PBS, and inject intravenously into your nude-irradiated recipients. In our experiments we used BM from Foxp3-KIgfp/gfp mice where all regulatory T cells (Tregs) will express GFP1. </li>
 <li>Acceptance of the donor BM occurs in the first days after transfer, since non-BM-reconstituted animals cannot survive more than 14 days. However, one should wait for 4 to 6 weeks to allow fully recovery of all BM compartments. Bactrim (Roche) was added to the water (2 mg/ml) during the first week after irradiation to decrease the risk of bacterial infections. </li>
 <li>Embryonic thymus transplantation was already described2. Briefly, start breeding pairs of isogenic mice and check for plugging (evidence of coitus) every day. Separate plugged females and CO2-euthanize them on day 15 of pregnancy (observation of plug is day 1 of pregnancy). Remove the embryos and the thymuses. Place the embryonic thymuses in cold PBS until transfer. </li>
 <li>To perform the thymuses transplantation, anesthetize BM-recipient Nude mice with ketamin (120 &mu;g/g of mouse weight) / xylazine (16 &mu;g/g of mouse weight) and keep them on top of a heating pad at 37&deg;C until they get unconscious. </li>
 <li>Surgically expose the kidney to perform the transplantation of the thymuses. First scrub the skin with ChloraPrep (Carefusion Inc.) and 70% ethanol. Then, make a 1 cm cut on the skin of the back-lateral body part of the animal, 2 mm bellow the last thoracic rib. </li>
 <li>Cut the peritoneal cavity and pull out the kidney of this cavity. Make a small superficial cut in the kidney capsule with a scalpel blade. Using forceps with a very thin tip make a pouch underneath the kidney capsule of the recipient mouse and add up to 4 thymic lobes into this pouch. </li>
 <li>Put the kidney back to its place, suture the peritoneal cavity with one or two stitches, and then close the mouse skin using the same suture method. Alternatively, sutures can be done using surgical glue. </li>
 <li>Inject analgesic (Butorphanol at 5 mg/kg) subcutaneously right after finishing the surgery to avoid animal pain and keep the animals in a heating pad until they start to recover from anesthesia. Mice are caged individually for the first 3 days after transplantation to prevent cage mates from removing the stitches. Remove stitches after 1 week. </li>
 <li>Nude mice do not have T cells. Therefore, you can evaluate the thymus graft acceptance by monitoring the percentage of CD4+ or CD8+ T cells in the blood. Once per week, 2 weeks after thymus transplantation, bleed the animals and stain the blood with anti-CD4 and anti-CD8 monoclonal antibodies (MAbs). When the CD4:CD8 ratio is approximately 2:1, the transplanted thymus is fully functional (Figure 1). </li>
</ol>
2. Intravital image acquisition
Prepare all the materials you will need in advance and put them aside.
<ol>
 <li>Anesthetize animals with ketamin/xylazine (same doses described at item 1.5) and place them on top of a heating pad at 37&deg;C. Inject 100 &mu;l of Rhodamine B isothiocyanate-Dextran (20 mg/ml in PBS) intravenously to allow future visualization of blood flow. </li>
 <li>Expose the kidney containing the transplanted thymus according to the protocol previously described in item 1.6. </li>
 <li>To prevent the kidney from returning to its original position, close the lateral sides of the skin incision with stitches. </li>
 <li> Place a piece of PBS-soaked cotton on the top of the organ to keep it moist. </li>
 <li>Put the heating pad on top of the animal holder stage (Figure 2A). </li>
 <li>Put the animal on top of the heating pad in the animal holder, organ up (Figure 2B). </li>
 <li>Pinch the organ at both sides with a clamp made of thin aluminum foil (Figure 2C). </li>
 <li>Close the animal holder and place the top heater probe as close as possible to your tissue (Figure 2D). This heater probe constantly measures the organ temperature and sends this information to the heater to adjust the electrical current, keeping it at 37&deg;C. </li>
 <li>Remove the PBS-soaked cotton and add low-melting agarose (2% in PBS) at 30&deg;C on top of your preparation. </li>
 <li>Cover the whole preparation with the top heater (Figure 2E). In the aperture on this heater there is a coverglass isolating the agarose from the objective (Figure 2F). Monitor the mouse anesthesia and inject a half-dose boost every 40 min. After the image acquisition, euthanize the animal with CO2.</li>
 </ol>
 Note: pictures of the aluminum foil clip and the top heater are presented in Figure 3.
 
3. Two-photon imaging acquisition
 We used an upright "Prairie Ultima X-Y" two-photon microscope. Our system is equipped with a Ti:Sapphire laser, four top PMTs for simultaneous up to 4 channel acquisitions and a 20x water immersion objective.
 <ol>
 <li>Turn on the Ti:Sapphire laser and the whole microscope system. </li>
 <li>Add dichroic mirrors and filter sets according to the wavelengths desired. In our case we used an Olympus-BX2 Chroma holder containing a 565 nm dichroic mirror, one 525/50 nm filter (for Foxp3-GFP signal), and one 595/50 nm filter (for Dextran-Rhodamine-B signal inside blood vessels). The laser wavelength range used was between 880 to 900 nm. </li>
 <li>Add the animal holder to the microscope, connect and turn on all the heaters, add water on the top heater aperture, carefully lower the objective into the water, and focus on a vessel or some GFP-Tregs. </li>
 <li>With the microscope x,y,z external control, quickly survey the tissue to locate a region of interest. </li>
 <li>Adjust the gain on the PMTs to optimize color separation and minimize the amount of laser required to achieve sufficient signal over background. Try to use the minimal amount of laser to avoid photo-damage of your tissue. </li>
 <li>Once the region of interests is located, begin time-lapse imaging. In our case, a typical 5D (x, y, z, t, and color) acquisition protocol consists of acquiring sequential images of a 50 &mu;m-depth tissue volume, divided into 4.0 &mu;m z-steps, x and y length of 140 &mu;m each. Each volume takes around 30 seconds to be acquired. Therefore, 60 acquisitions will perform around 30 min of image acquisition. </li>
 <li>Transfer the data to the institutional server and use ImageJ, Imaris, or Volocity software to perform multi-dimensional rendering and cell tracking. </li>
 </ol>
4. Representative Results
<img alt="Figure 1" src="/files/ftp_upload/3504/3504fig1.jpg" /> 
Figure 1. Embryonic thymus acceptance and function. After BM recover, embryonic thymuses were transplanted to BM reconstituted animals; two weeks after thymus transplantation, these mice were bled once per week to monitor the percentages of T cell subtypes by staining with anti-CD4 or anti-CD8 MAbs. We considered the thymus was successfully accepted in animals with a CD4:CD8 ratio around 1.5-2.0:1 (A). To confirm its functionality, we sacrificed some thymus-transplanted animals and compared the percentage of different thymocyte subpopulations with WT mice (B). The percentages of double-negative (DN; CD4-CD8- thymocytes) subpopulations (by staining with anti-CD25 and anti-CD44 MAbs), double-positive (DP; CD4+CD8+ thymocytes) and single-positive (SP) CD4+ or CD8+ thymocytes were similar between these animals.
<img alt="Figure 2" src="/files/ftp_upload/3504/3504fig2.jpg" /> 
Figure 2. Animal holder assembly. After deeply anesthetized, the animal is put on top of a heating pad, previously mounted on top of the animal holder stage (A). The kidney containing the transplanted thymus is facing up (B). An aluminum foil clamp delicately pinches the whole kidney (C) to keep it in place. Fig. 1C insert shows in detail the clamp. The top part of the animal holder is put in place (D). The PBS-soaked cotton is removed from the top of the organ, replaced by warm low-melting agarose, and the top heater is added (E). Finally, the whole assembly is transferred to the microscope, here represented by the objective (F).
<img alt="Figure 3" src="/files/ftp_upload/3504/3504fig3.jpg" /> 
Figure 3. Details of holder parts. These pictures show the aluminum foil clamp (A) holding the kidney (B). Note also the region where the transplanted thymus is located. This approximately indicates the region to cut the thymus capsule and make the pocket to put the embryonic thymus. The top heater coverglass (C) was fixed with silicone glue to its bottom part (D).
Movie 1. Intravital imaging of Foxp3-GFP+ thymocytes inside the thymus where the physiological levels of oxygenation and temperature (37&deg;C) were kept during the whole process. Note the blood flow and the movement of Tregs. These speeds can be measured and compared with published data. <a href="https://www.jove.com/files/ftp_upload/3504/3504movie1.avi">Click here to view the movie.</a>
Movie 2. Intravital imaging of Tregs inside a thymus where the images were acquired at 30&deg;C. Note the absence of movement and the round shape of the cells despite the maintenance of blood flow. <a href="https://www.jove.com/files/ftp_upload/3504/3504movie2.avi">Click here to view the movie.</a>

<ol>
	<li>Bettelli, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reciprocal+developmental+pathways+for+the+generation+of+pathogenic+effector+TH17+and+regulatory+T+cells.">Reciprocal developmental pathways for the generation of pathogenic effector TH17 and regulatory T cells.</a> Nature. 441, 235-238 (2006).</li><li>Ramsdell, F., Z&#250;&#241;iga-Pfl&#252;cker, J. C., Takahama, Y., Coligan, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vitro+systems+for+the+study+of+T+cell+development:+fetal+thymus+organ+culture+and+OP9-DL1+cell+coculture.">In vitro systems for the study of T cell development: fetal thymus organ culture and OP9-DL1 cell coculture.</a> Current protocols in immunology. , (2006).</li><li>Tadokoro, C. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Regulatory+T+cells+inhibit+stable+contacts+between+CD4++T+cells+and+dendritic+cells+in+vivo.">Regulatory T cells inhibit stable contacts between CD4+ T cells and dendritic cells in vivo.</a> J. Exp. Med. 203, 505-511 (2006).</li><li>Mempel, T. R., Henrickson, S. E., Von Andrian, U. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=T-cell+priming+by+dendritic+cells+in+lymph+nodes+occurs+in+three+distinct+phases.">T-cell priming by dendritic cells in lymph nodes occurs in three distinct phases.</a> Nature. 427, 154-159 (2004).</li><li>Miller, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+the+single+cell+dynamics+of+CD4++T+cell+activation+by+dendritic+cells+in+lymph+nodes.">Imaging the single cell dynamics of CD4+ T cell activation by dendritic cells in lymph nodes.</a> J. Exp. Med. 200, 847-856 (2004).</li><li>Shakhar, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stable+T+cell-dendritic+cell+interactions+precede+the+development+of+both+tolerance+and+immunity+in+vivo.">Stable T cell-dendritic cell interactions precede the development of both tolerance and immunity in vivo.</a> Nat. Immunol. 6, 707-714 (2005).</li><li>Hugues, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Distinct+T+cell+dynamics+in+lymph+nodes+during+the+induction+of+tolerance+and+immunity.">Distinct T cell dynamics in lymph nodes during the induction of tolerance and immunity.</a> 5, 1235-1242 (2004).</li><li>Tang, Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Visualizing+regulatory+T+cell+control+of+autoimmune+responses+in+nonobese+diabetic+mice.">Visualizing regulatory T cell control of autoimmune responses in nonobese diabetic mice.</a> Nat. Immunol. 7, 83-92 (2006).</li><li>Le Borgne, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+impact+of+negative+selection+on+thymocyte+migration+in+the+medulla.">The impact of negative selection on thymocyte migration in the medulla.</a> Nature immunology. 10, 823-830 (2009).</li><li>Baez, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+open+cremaster+muscle+preparation+for+the+study+of+blood+vessels+by+in+vivo+microscopy.">An open cremaster muscle preparation for the study of blood vessels by in vivo microscopy.</a> Microvascular research. 5, 384-394 (1973).</li><li>Wang, B., Zinselmeyer, B. H., McDole, J. R., Gieselman, P. A., Miller, M. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Non-invasive+Imaging+of+Leukocyte+Homing+and+Migration+in+vivo.">Non-invasive Imaging of Leukocyte Homing and Migration in vivo.</a> J. Vis. Exp. (46), e2062-e2062 (2010).</li><li>Barretto, R. P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Time-lapse+imaging+of+disease+progression+in+deep+brain+areas+using+fluorescence+microendoscopy.">Time-lapse imaging of disease progression in deep brain areas using fluorescence microendoscopy.</a> Nature medicine. 17, 223-228 (2011).</li></ol>

We have nothing to disclose.

In this paper we demonstrated the procedures for two-photon imaging of thymocytes inside a living animal. We also described some parameters that one should carefully control, such as the continuation of blood flow and the maintenance of organ temperature during the imaging procedures. Nonetheless, despite careful efforts to keep the organ stable, motion artifacts such as "organ drifting" can occur. Posterior image correction can be performed by the development of algorithms specifically designed for this purpose. Further image analysis could also be the source of new protocols development which seeks to minimize errors.
The thymus is the organ where all T cells are produced and, therefore, it is the organ where immunologists interested in understanding the generation of &#x03B3;&delta;, CD4, or CD8 T cells will focus their attention. Most studies concerning T cells are based upon differences in the numbers and/or stability of these cells after different in vitro/in vivo manipulations. However, only after the in vivo visualization we could observe the interaction between cells of the immune system involved in maintaining homeostasis3-7. Therefore, the in vivo observation of thymocytes is probably one of the most important missing information to better understand T cell biology. Intravital TPM provides a detailed picture of T cell movements and interactions and we demonstrate here how it can be used for detailed thymocyte studies. However, every technique has its limitations. While intravital imaging acquisition is the most accurate system for reflecting cells behavior inside the body, it is also true that explanted image acquisition of organs is less laborious and has been used to collect important information about the immune system8,9. Moreover, one cannot deny intravital imaging methods require surgery to expose tissues and blood vessels in anesthetized animals, which per se could cause an alteration in the whole organ physiology10. Nevertheless, there are non-invasively methods that abolish the artifacts caused by the surgical procedure11 and new methods are being developed that better prepare in advance the animals to be used12. Therefore, new surgical procedures and tolls will minimize or bypass actual limitations of intravital imaging acquisition and become more and more accessible to the scientific community.
We have demonstrated that the method we have described is feasible and it reports all in vivo systemic manipulations, such drug administration, that we have used. Thus, we suggest the use of this method together with ex vivo techniques already available in order to complement and strengthen further studies concerning thymocytes development.

<table border="1">
<thead>
<tr>
<td>Name of the reagent</td>
<td>Company</td>
<td>Catalogue number</td>
<td>Comments</td>
</tr>
</thead>
<tbody>
<tr>
<td>Rhodamine B ishothiocyanate-Dextran</td>
<td>Sigma-Aldrich</td>
<td>R9379</td>
<td>prepare stock at 20 mg/ml</td>
</tr>
<tr>
<td>Two-photon microscope</td>
<td>Prairie Technologies Inc.</td>
<td>Prairie Ultima X-Y</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>Ti:Sapphire laser</td>
<td>Coherent, Inc.</td>
<td>Chameleon Ultra Family</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>20x/1.00 NA immersion objective</td>
<td>Olympus Inc.</td>
<td>XLUMPLFLN 20XW</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>Holder (Filters/Dichroic)</td>
<td>Chroma Technology Corp.</td>
<td>91018 BX2 (U-MF2)</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>525 nm/50 filter</td>
<td>Chroma Technology Corp.</td>
<td>ET525/50m</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>595 nm/50 filter</td>
<td>Chroma Technology Corp.</td>
<td>ET595/50m</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>565 nm dichroic</td>
<td>Chroma Technology Corp.</td>
<td>565dcxr</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>Imaris software</td>
<td>Bitplane AG Inc.</td>
<td>Imaris</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>Volocity</td>
<td>PerkinElmer Inc.</td>
<td>Volocity</td>
<td>&nbsp;</td>
</tr>
<tr>
<td>ImageJ</td>
<td>NIH, USA</td>
<td>ImageJ</td>
<td>&nbsp;</td>
</tr>
</tbody>
</table>

intravital imaging of the mouse thymus using 2-photon microscopy

Two-photon Microscopy (TPM) provides image acquisition in deep areas inside tissues and organs. In combination with the development of new stereotactic tools and surgical procedures, TPM becomes a powerful technique to identify "niches" inside organs and to document cellular "behaviors" in live animals. While intravital imaging provides information that best resembles the real cellular behavior inside the organ, it is both more laborious and technically demanding in terms of required equipment/procedures than alternative ex vivo imaging acquisition. Thus, we describe a surgical procedure and novel "stereotactic" organ holder that allows us to follow the movements of Foxp3+ cells within the thymus.
Foxp3 is the master regulator for the generation of regulatory T cells (Tregs). Moreover, these cells can be classified according to their origin: ie. thymus-differentiated Tregs are called "naturally-occurring Tregs" (nTregs), as opposed to peripherally-converted Tregs (pTregs). Although significant amount of research has been reported in the literature concerning the phenotype and physiology of these T cells, very little is known about their in vivo interactions with other cells. This deficiency may be due to the absence of techniques that would permit such observations. The protocol described in this paper provides a remedy for this situation.
Our protocol consists of using nude mice that lack an endogenous thymus since they have a punctual mutation in the DNA sequence that compromises the differentiation of some epithelial cells, including thymic epithelial cells. Nude mice were gamma-irradiated and reconstituted with bone marrows (BM) from Foxp3-KIgfp/gfp mice. After BM recovery (6 weeks), each animal received embryonic thymus transplantation inside the kidney capsule. After thymus acceptance (6 weeks), the animals were anesthetized; the kidney containing the transplanted thymus was exposed, fixed in our organ holder, and kept under physiological conditions for in vivo imaging by TPM. We have been using this approach to study the influence of drugs in the generation of regulatory T cells.

Watch this Scientific Journal Video about Intravital Imaging of the Mouse Thymus using 2-Photon Microscopy at JoVE.com

Intravital Imaging of the Mouse Thymus using 2-Photon Microscopy

We have developed novel laboratory tools and protocols for intravital imaging acquisition of the thymus. Our technique should help in the identification of &#x201C;niches&#x201D; within the thymus where T cell development occurs.

Two-photon Microscopy (TPM) provides image acquisition in deep areas inside tissues and organs. In combination with the development of new stereotactic ...

intravital-imaging-of-the-mouse-thymus-using-2-photon-microscopy

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JoVE Journal

Immunology and Infection

16.8K Views.  Instituto Gulbenkian de Ciência. We have developed novel laboratory tools and protocols for intravital imaging acquisition of the thymus. Our technique should help in the identification of “niches” within the thymus where T cell development occurs.

Video: Intravital Imaging of the Mouse Thymus using 2-Photon Microscopy

Intravital Microscopy of the Inguinal Lymph Node

Lymph nodes (LN's), located throughout the body, are an integral component of the immune system. They serve as a site for induction of adaptive immune response and therefore, the development of effector cells. As such, LNs are key to fighting invading pathogens and maintaining health. The choice of LN to study is dictated by accessibility and the desired model; the inguinal lymph node is well situated and easily supports studies of biologically relevant models of skin and genital mucosal infection.
The inguinal LN, like all LNs, has an extensive microvascular network supplying it with blood. In general, this microvascular network includes the main feed arteriole of the LN that subsequently branches and feeds high endothelial venules (HEVs). HEVs are specialized for facilitating the trafficking of immune cells into the LN during both homeostasis and infection. How HEVs regulate trafficking into the LN under both of these circumstances is an area of intense exploration. The LN feed arteriole, has direct upstream influence on the HEVs and is the main supply of nutrients and cell rich blood into the LN. Furthermore, changes in the feed arteriole are implicated in facilitating induction of adaptive immune response. The LN microvasculature has obvious importance in maintaining an optimal blood supply to the LN and regulating immune cell influx into the LN, which are crucial elements in proper LN function and subsequently immune response. 
The ability to study the LN microvasculature in vivo is key to elucidating how the immune system and the microvasculature interact and influence one another within the LN. Here, we present a method for in vivo imaging of the inguinal lymph node. We focus on imaging of the microvasculature of the LN, paying particular attention to methods that ensure the study of healthy vessels, the ability to maintain imaging of viable vessels over a number of hours, and quantification of vessel magnitude. Methods for perfusion of the microvasculature with vasoactive drugs as well as the potential to trace and quantify cellular traffic are also presented. 
Intravital microscopy of the inguinal LN allows direct evaluation of microvascular functionality and real-time interface of the direct interface between immune cells, the LN, and the microcirculation. This technique potential to be combined with many immunological techniques and fluorescent cell labelling as well as manipulated to study vasculature of other LNs.

A technique for performing intravital microscopy of the inguinal lymph node (LN) is outlined. Such technique allows for real-time, in vivo study of the lymph node microvasculature and structure both during homeostasis and infection. This technique can be adapted to cell trafficking studies and to other lymph node sites.

Lymph nodes (LN's), located throughout the body, are an integral component of the immune system. They serve as a site for induction of adaptive immune ...

Direct Observation of Phagocytosis and NET-formation by Neutrophils in Infected Lungs using 2-photon Microscopy

After the gastrointestinal tract, the lung is the second largest surface for interaction between the vertebrate body and the 
environment. Here, an effective gas exchange must be maintained, while at the same time avoiding infection by the multiple pathogens that are inhaled 
during normal breathing. To achieve this, a superb set of defense strategies combining humoral and cellular immune mechanisms exists. One of the most 
effective measures for acute defense of the lung is the recruitment of neutrophils, which either phagocytose the inhaled pathogens or kill them by 
releasing cytotoxic chemicals. A recent addition to the arsenal of neutrophils is their explosive release of extracellular DNA-NETs by which bacteria 
or fungi can be caught or inactivated even after the NET releasing cells have died. We present here a method that allows one to directly observe neutrophils, 
migrating within a recently infected lung, phagocytosing fungal pathogens as well as visualize the extensive NETs that they have produced throughout the 
infected tissue. The method describes the preparation of thick viable lung slices 7 hours after intratracheal infection of mice with conidia of the mold 
Aspergillus fumigatus and their examination by multicolor time-lapse 2-photon microscopy. This approach allows one to directly investigate antifungal 
defense in native lung tissue and thus opens a new avenue for the detailed investigation of pulmonary immunity.

We show, how to use 2-photon microscopy for the observation of the dynamics of neutrophil granulocytes in infected lungs while they phagocytose pathogens or produce neutrophil extracellular traps (NETs).

After the gastrointestinal tract, the lung is the second largest surface for interaction between the vertebrate body and the 
environment. Here, an ...

Intravital Microscopy of the Spleen: Quantitative Analysis of Parasite Mobility and Blood Flow

The advent of intravital microscopy in experimental rodent malaria models has allowed major advances to the knowledge of parasite-host interactions 1,2. Thus, in vivo imaging of malaria parasites during pre-erythrocytic stages have revealed the active entrance of parasites into skin lymph nodes 3, the complete development of the parasite in the skin 4, and the formation of a hepatocyte-derived merosome to assure migration and release of merozoites into the blood stream 5. Moreover, the development of individual parasites in erythrocytes has been recently documented using 4D imaging and challenged our current view on protein export in malaria 6. Thus, intravital imaging has radically changed our view on key events in Plasmodium development. Unfortunately, studies of the dynamic passage of malaria parasites through the spleen, a major lymphoid organ exquisitely adapted to clear infected red blood cells are lacking due to technical constraints.
Using the murine model of malaria Plasmodium yoelii in Balb/c mice, we have implemented intravital imaging of the spleen and reported a differential remodeling of it and adherence of parasitized red blood cells (pRBCs) to barrier cells of fibroblastic origin in the red pulp during infection with the non-lethal parasite line P.yoelii 17X as opposed to infections with the P.yoelii 17XL lethal parasite line 7. To reach these conclusions, a specific methodology using ImageJ free software was developed to enable characterization of the fast three-dimensional movement of single-pRBCs. Results obtained with this protocol allow determining velocity, directionality and residence time of parasites in the spleen, all parameters addressing adherence in vivo. In addition, we report the methodology for blood flow quantification using intravital microscopy and the use of different colouring agents to gain insight into the complex microcirculatory structure of the spleen. 
Ethics statement
All the animal studies were performed at the animal facilities of University of Barcelona in accordance with guidelines and protocols approved by the Ethics Committee for Animal Experimentation of the University of Barcelona CEEA-UB (Protocol No DMAH: 5429). Female Balb/c mice of 6-8 weeks of age were obtained from Charles River Laboratories.

We show the method for performing intravital microscopy of the spleen using GFP transgenic malaria parasites and the quantification of parasite mobility and blood flow within this organ.

The advent of intravital microscopy in experimental rodent malaria models has allowed major advances to the knowledge of parasite-host interactions 1,2. ...

Intravital Imaging of the Mouse Popliteal Lymph Node

Lymph nodes (LNs) are secondary lymphoid organs, which are strategically located throughout the body to allow for trapping and presentation of foreign antigens from peripheral tissues to prime the adaptive immune response. Juxtaposed between innate and adaptive immune responses, the LN is an ideal site to study immune cell interactions1,2. Lymphocytes (T cells, B cells and NK cells), dendritic cells (DCs), and macrophages comprise the bulk of bone marrow-derived cellular elements of the LN. These cells are strategically positioned in the LN to allow efficient surveillance of self antigens and potential foreign antigens3-5. The process by which lymphocytes successfully encounter cognate antigens is a subject of intense investigation in recent years, and involves an integration of molecular contacts including antigen receptors, adhesion molecules, chemokines, and stromal structures such as the fibro-reticular network2,6-12. 
Prior to the development of high-resolution real-time fluorescent in vivo imaging, investigators relied on static imaging, which only offers answers regarding morphology, position, and architecture. While these questions are fundamental in our understanding of immune cell behavior, the limitations intrinsic with this technique does not permit analysis to decipher lymphocyte trafficking and environmental clues that affect dynamic cell behavior. Recently, the development of intravital two-photon laser scanning microscopy (2P-LSM) has allowed investigators to view the dynamic movements and interactions of individual cells within live LNs in situ12-16. In particular, we and others have applied this technique to image cellular behavior and interactions within the popliteal LN, where its compact, dense nature offers the advantage of multiplex data acquisition over a large tissue area with diverse tissue sub-structures11,17-18. It is important to note that this technique offers added benefits over explanted tissue imaging techniques, which require disruption of blood, lymph flow, and ultimately the cellular dynamics of the system. Additionally, explanted tissues have a very limited window of time in which the tissue remains viable for imaging after explant. With proper hydration and monitoring of the animal's environmental conditions, the imaging time can be significantly extended with this intravital technique. Here, we present a detailed method of preparing mouse popliteal LN for the purpose of performing intravital imaging.

Recent advances in 2-photon microscopy have enabled real-time in situ imaging of live tissues in animal models, thereby enhancing our ability to investigate cellular behavior in both physiologic and pathologic conditions. Here, we outline the preparations required to perform intravital imaging of the mouse popliteal lymph node.

Lymph nodes (LNs) are secondary lymphoid organs, which are strategically located throughout the body to allow for trapping and presentation of foreign ...

Highly Resolved Intravital Striped-illumination Microscopy of Germinal Centers

Monitoring cellular communication by intravital deep-tissue multi-photon microscopy is the key for understanding the fate of immune cells within thick tissue samples and organs in health and disease. By controlling the scanning pattern in multi-photon microscopy and applying appropriate numerical algorithms, we developed a striped-illumination approach, which enabled us to achieve 3-fold better axial resolution and improved signal-to-noise ratio, i.e. contrast, in more than 100 &#181;m tissue depth within highly scattering tissue of lymphoid organs as compared to standard multi-photon microscopy. The acquisition speed as well as photobleaching and photodamage effects were similar to standard photo-multiplier-based technique, whereas the imaging depth was slightly lower due to the use of field detectors. By using the striped-illumination approach, we are able to observe the dynamics of immune complex deposits on secondary follicular dendritic cells &#8211; on the level of a few protein molecules in germinal centers.

High-resolution intravital imaging with enhanced contrast up to 120 &#181;m depth in lymph nodes of adult mice is achieved by spatially modulating the excitation pattern of a multi-focal two-photon microscope. In 100 &#181;m depth we measured resolutions of 487 nm (lateral) and 551 nm (axial), thus circumventing scattering and diffraction limits.

Monitoring cellular communication by intravital deep-tissue multi-photon microscopy is the key for understanding the fate of immune cells within thick ...

Intravital Microscopy Imaging of the Liver following Leishmania Infection: An Assessment of Hepatic Hemodynamics

Intravital microscopy (IVM) is a powerful optical imaging technique that has made possible the visualization, monitoring and quantification of various biological events in real time and in live animals. This technology has greatly advanced our understanding of physiological processes and pathogen-mediated phenomena in specific organs.
In this study, IVM is applied to the mouse liver and protocols are designed to image in vivo the circulatory system of the liver and measure red blood cell (RBC) velocity in individual hepatic vessels. To visualize the different vessel subtypes that characterize the hepatic organ and perform blood flow speed measurements, C57Bl/6 mice are intravenously injected with a fluorescent plasma reagent that labels the liver-associated vasculature. IVM enables in vivo, real time, measurement of RBC velocity in a specific vessel of interest. Establishing this methodology will make it possible to investigate liver hemodynamics under physiological and pathological conditions. Ultimately, this imaging-based methodology will be important for studying the influence of L. donovani infection&#160;on hepatic hemodynamics.
This method can be applied to other infectious models and mouse organs and might be further extended to pre-clinical testing of a drug&#8217;s effect on inflammation by quantifying its effect on blood flow.

Intravital Microscopy Imaging of the Liver following Leishmania Infection: An Assessment of Hepatic Hemodynamics

This article reports on a detailed method for the dynamic measurement and quantification of blood flow velocity within individual blood vessels of the mouse liver vasculature using intravital microscopy imaging in combination with a specific methodology for image acquisition and analysis.

Intravital microscopy (IVM) is a powerful optical imaging technique that has made possible the visualization, monitoring and quantification of various ...

Long Term Intravital Multiphoton Microscopy Imaging of Immune Cells in Healthy and Diseased Liver Using CXCR6.Gfp Reporter Mice

Liver inflammation as a response to injury is a highly dynamic process involving the infiltration of distinct subtypes of leukocytes including monocytes, neutrophils, T cell subsets, B cells, natural killer (NK) and NKT cells. Intravital microscopy of the liver for monitoring immune cell migration is particularly challenging due to the high requirements regarding sample preparation and fixation, optical resolution and long-term animal survival. Yet, the dynamics of inflammatory processes as well as cellular interaction studies could provide critical information to better understand the initiation, progression and regression of inflammatory liver disease. Therefore, a highly sensitive and reliable method was established to study migration and cell-cell-interactions of different immune cells in mouse liver over long periods (about 6 hr) by intravital two-photon laser scanning microscopy (TPLSM) in combination with intensive care monitoring.
The method provided includes a gentle preparation and stable fixation of the liver with minimal perturbation of the organ; long term intravital imaging using multicolor multiphoton microscopy with virtually no photobleaching or phototoxic effects over a time period of up to 6 hr, allowing tracking of specific leukocyte subsets; and stable imaging conditions due to extensive monitoring of mouse vital parameters and stabilization of circulation, temperature and gas exchange.
To investigate lymphocyte migration upon liver inflammation CXCR6.gfp knock-in mice were subjected to intravital liver imaging under baseline conditions and after acute and chronic liver damage induced by intraperitoneal injection(s) of carbon tetrachloride (CCl4).
CXCR6 is a chemokine receptor expressed on lymphocytes, mainly on Natural Killer T (NKT)-, Natural Killer (NK)- and subsets of T lymphocytes such as CD4 T cells but also mucosal associated invariant (MAIT) T cells1. Following the migratory pattern and positioning of CXCR6.gfp+ immune cells allowed a detailed insight into their altered behavior upon liver injury and therefore their potential involvement in disease progression.

Stable intravital high-resolution imaging of immune cells in the liver is challenging. Here we provide a highly sensitive and reliable method to study migration and cell-cell-interactions of immune cells in mouse liver over long periods (about 6 hours) by intravital multiphoton laser scanning microscopy in combination with intensive care monitoring.

Liver inflammation as a response to injury is a highly dynamic process involving the infiltration of distinct subtypes of leukocytes including monocytes, ...

Analysis of Yersinia enterocolitica Effector Translocation into Host Cells Using Beta-lactamase Effector Fusions

Many gram-negative bacteria including pathogenic Yersinia spp. employ type III secretion systems to translocate effector proteins into eukaryotic target cells. Inside the host cell the effector proteins manipulate cellular functions to the benefit of the bacteria. To better understand the control of type III secretion during host cell interaction, sensitive and accurate assays to measure translocation are required. We here describe the application of an assay based on the fusion of a Yersinia enterocolitica effector protein fragment (Yersinia outer protein; YopE) with TEM-1 beta-lactamase for quantitative analysis of translocation. The assay relies on cleavage of a cell permeant FRET dye (CCF4/AM) by translocated beta-lactamase fusion. After cleavage of the cephalosporin core of CCF4 by the beta-lactamase, FRET from coumarin to fluorescein is disrupted and excitation of the coumarin moiety leads to blue fluorescence emission. Different applications of this method have been described in the literature highlighting its versatility. The method allows for analysis of translocation in vitro and also in in vivo, e.g., in a mouse model. Detection of the fluorescence signals can be performed using plate readers, FACS analysis or fluorescence microscopy. In the setup described here, in vitro translocation of effector fusions into HeLa cells by different Yersinia mutants is monitored by laser scanning microscopy. Recording intracellular conversion of the FRET reporter by the beta-lactamase effector fusion in real-time provides robust quantitative results. We here show exemplary data, demonstrating increased translocation by a Y. enterocolitica YopE mutant compared to the wild type strain.

Analysis of Yersinia enterocolitica Effector Translocation into Host Cells Using Beta-lactamase Effector Fusions

Effector translocation into host cells via a type III secretion system is a common virulence strategy among gram-negative bacteria. A beta-lactamase effector fusion based assay for quantitative analysis of translocation was applied. In Yersinia infected cells, conversion of a FRET reporter by the beta-lactamase is monitored using laser scanning microscopy.

Many gram-negative bacteria including pathogenic Yersinia spp. employ type III secretion systems to translocate effector proteins into eukaryotic target ...

Imaging Neutrophils and Monocytes in Mesenteric Veins by Intravital Microscopy on Anaesthetized Mice in Real Time

Efficient immune response is dependent on rapid mobilization of blood leukocytes to the site of infection or injury. Investigating leukocyte migration in vivo is crucial for understanding the molecular basis of leukocyte transendothelial migration and interaction with vascular endothelium. One powerful approach involves intravital microscopy on transgenic mice expressing fluorescent proteins in cells of interest.
Here we present a protocol for imaging monocytes and neutrophils in the CX3CR1gfp/wt mouse i.v. injected with orange dye-labeled neutrophils with an inverted confocal microscope. Time-lapse movies gathered from 30 min&#160;to several hours of imaging allow the analysis of leukocyte behavior in mesenteric veins under both steady state and inflammatory conditions. We also describe the steps to locally induce blood vessel inflammation with TLR2/TLR1 agonist Pam3SK4 and monitor the subsequent recruitment of neutrophils and monocytes.
The presented technique can also be used to monitor other populations of leukocytes and investigate molecules implicated in leukocyte recruitment or trafficking using other stimuli or transgenic mice.

We detail a protocol to monitor the behavior of neutrophils and monocytes in mesenteric veins under steady state and inflammatory conditions using intravital confocal microscopy on anaesthetized mice.

Efficient immune response is dependent on rapid mobilization of blood leukocytes to the site of infection or injury. Investigating leukocyte migration in ...

Intravital Imaging of Neutrophil Priming Using IL-1&#946; Promoter-driven DsRed Reporter Mice

Neutrophils are the most abundant leukocytes in human blood circulation and are quickly recruited to inflammatory sites. Priming is a critical event that enhances the phagocytic functionality of neutrophils. Although extensive studies have unveiled the existence and importance of neutrophil priming during infection and injury, means of visualizing this process in vivo have been unavailable. The protocol provided enables monitoring of the dynamic process of neutrophil priming in living animals by combining three methodologies: 1) DsRed reporter signal &#8212; used as a measure of priming 2) in vivo neutrophil labeling &#8212; achieved by injection of fluorescence-conjugated anti-lymphocyte antigen 6G (Ly6G) monoclonal antibody (mAb) and 3) intravital confocal imaging. Several critical steps are involved in this protocol: oxazolone-induced mouse ear skin inflammation, appropriate sedation of animals, repeated injections of anti-Ly6G mAb, and prevention of focus drift during imaging. Although a few limitations have been observed, such as the limit of continuous imaging time (~ 8 hr) in one mouse and the leakage of fluorescein isothiocyanate-dextran from blood vessels in the inflammatory state, this protocol provides a fundamental framework for intravital imaging of primed neutrophil behavior and function, which can easily be expanded to examination of other immune cells in mouse inflammation models.

Intravital Imaging of Neutrophil Priming Using IL-1&amp;#946; Promoter-driven DsRed Reporter Mice

This current protocol employs fluorescent reporters, in vivo labeling, and intravital imaging techniques to enable monitoring of the dynamic process of neutrophil priming in living animals.

Neutrophils are the most abundant leukocytes in human blood circulation and are quickly recruited to inflammatory sites. Priming is a critical event that ...