小鼠肺脏大约有4x10⁶个肺泡，人类约有3x10⁸个肺泡，而肺泡中巨噬细胞的空间分布比约为0.3，即平均每三个肺泡中存在一个巨噬细胞。肺泡巨噬细胞（AM）是组织驻留巨噬细胞，在出生前后定殖于肺部，并且在成年生物体中能够长期自我维持而无需单核细胞的参与。AM 位于肺泡中，可以通过支气管肺泡灌洗（BAL）的方法冲洗肺部来获取。

AM是肺的组织驻留巨噬细胞，对免疫调节和表面活性物质稳态至关重要。由于它们位于肺泡的气腔中，AM 直接暴露于吸入的空气、病原体或其他雾化颗粒。因此，AM 在炎症反应的启动或抑制中起着关键作用，并且是众多探索肺部疾病机制的研究的对象。有趣的是，小鼠的 AM 起源于胎儿单核细胞，并且在稳态条件下能够在体内自我维持其数量而无需骨髓来源的单核细胞的贡献。AM 的特别之处在于它们位于身体表面之外，直接暴露于外部环境。因此，可以使用支气管肺泡灌洗（BAL）以最小的组织干扰就可以分离富集它们。

Materials and Reagents

1. 15-ml conical tubes (Corning, catalog number: 352196)

2. Bottle-top vacuum filter with 0.22 μm membrane (Corning, catalog number: 431161)

3. Plastic storage bottle (Corning, catalog number: 430281)

4. 70-μm sterile cell strainer (BD, catalog number: 340633)

5. 1-ml syringe (Braun, catalog number: 9161406V)

6. 18-G cannula (Braun, catalog number: 4667123)

7. Petri dish 94/16 mm (Greiner Bio-one, catalog number: 633181)

8. Non-treated 6-well plate (NuncTM, catalog number: 150239)

9. C57BL/6 mice (aged 6-10 weeks)

10. PBS, pH 7.2 (Thermo Fisher Scientific, GibcoTM, catalog number: 20012019)

11. EDTA stock solution (e.g., 0.5 M, pH 8.0)

12. Hemolysis buffer (self-made or commercial, e.g., Morphisto, catalog number: 12146)

13. Trypan Blue solution 0.4% (Sigma-Aldrich, catalog number: T8154)

14. RPMI 1640 Medium, no glutamine (Thermo Fisher Scientific, GibcoTM, catalog number: 31870025)

15. Fetal bovine serum (Testing of different batches is recommended)

16. Gentamicin sulphate 50 mg/ml in aqueous solution (Lonza, catalog number: BE02-012E)

17. Penicillin-Streptomycin (10,000 U/ml) (Thermo Fisher Scientific, GibcoTM, catalog number: 15140122)

18. Sodium Pyruvate (100 mM) (Thermo Fisher Scientific, GibcoTM, catalog number: 11360070)

19. GlutaMAXTM Supplement (Thermo Fisher Scientific, GibcoTM, catalog number: 35050038)

20. Conditioned medium from J558L cell line transfected with murine GM-CSF cDNA as a source for GM-CSF (Zal et al., 1994; Stockinger et al., 1996; Rayasam, 2015)

21. ESGRO Complete Accutase (Merck, catalog number: SF006)

22. EGTA stock solution (e.g., 0.5 M, pH 8.0)

23. UltraComp eBeadsTM Compensation Beads (Thermo Fisher Scientific, Invitrogen, catalog number: 01-2222-41)

24. Zombie Violet Fixable Viability Kit (BioLegend, catalog number: 423113)

25. FACS antibodies (as indicated in Table 2)

26. BAL buffer (see Recipes)

27. Complete medium (see Recipes)

28. AM culture medium (see Recipes)

29. Detachment medium (see Recipes)

Equipment

1. Pipettes

2. Mouse dissection tools (scissors, forceps)

3.Water bath set to 37 °C

4. Refrigerated benchtop centrifuge for spinning conical tubes

5. Hemocytometer (Roth, catalog number: T729.1)

6. Incubator (37 °C, 5% CO2)

7. Inverse microscope

Procedure

A. Harvest alveolar macrophages by bronchoalveolar lavage (BAL)

1.For each mouse, prepare a 15-ml conical tube filled with 3 ml complete medium (see Recipes).

2.Warm-up BAL buffer (see Recipes) to 37 °C in a water bath. Keep warm during the whole procedure.

3.Euthanize the mouse by cervical dislocation without rupturing the jugular vein or the trachea to avoid exposing AM to CO2 or isoflurane, which could affect functional properties of AM.

4.Using dissection tools, remove the skin, ribcage and muscles to expose both lungs and trachea. Avoid cutting or rupturing blood vessels.
Note: Since methods for the surgical exposure of lungs and the trachea have been published previously in this journal, the reader is referred to those protocols for instructions (Han and Ziegler, 2013; Tibbitt and Coquet, 2016; Jhingran et al., 2016; Sun et al., 2017).

5.Use a fine scissor to make a small incision in the upper part of the trachea just below the larynx. The part of the trachea facing downwards (away from the experimenter) should remain intact, do not cut through the whole trachea.

6.Use the incision to insert a slightly blunted 18-G cannula and direct the cannula 5 mm deeper into the trachea towards the lungs. Take care not to damage lung tissue.

7.Attach a 1 ml syringe filled with 1 ml warm BAL buffer onto the inserted cannula.

8.Inject 1 ml buffer while fixating the cannula position with the other hand.

9.Pull the plunger to collect BAL fluid in the syringe. About 800-900 μl can be recovered. Observe that the pressure should not be too high, otherwise the alveoli will burst and BAL fluid will be lost. Upon injection and collection, the lungs should visibly inflate and deflate.

10.Filter collected BAL fluid through a 70 μm cell strainer into the 15-ml tube with 3 ml complete medium from Step A1.

11.Repeat Steps A6-A10 for 9 more times each time with fresh warm BAL buffer. Pool cells into the same 15-ml tube.

12.Collect cells by centrifugation at 300 x g, 5 min at 4 °C. Remove supernatant. The cell pellet should be white. A red/pink color indicates that blood was accidentally collected during the BAL.

13.Add 1 ml hemolysis buffer for 2 min incubation at room temperature to lyse residual erythrocytes. Fill up tube with complete medium to stop lysis and collect cells by centrifugation as before. Remove supernatant. The color of the cell pellet should be white now.

14.Resuspend cell pellet in 500 μl BAL buffer and take a sample for counting using a hemocytometer chamber after staining with Trypan Blue to exclude dead cells. Count only live (Trypan-Blue negative) cells.

15.Calculate the total cell number per BAL. Typically, 5 x 105-7 x 105 live cells per adult wild-type mouse aged 6-8 weeks can be recovered when using pre-warmed BAL buffer, containing PBS with 2 mM EDTA and 0.5% serum.

16.Proceed to cell staining and flow cytometry analysis or in vitro culture.

B. Flow cytometric analysis of alveolar macrophages

1. Block unspecific binding sites on cells with TruStain fcX and concomitantly stain with Zombie Violet in 200 μl cold PBS (without FBS) at 4 °C in the dark for 15 min (see Table 2).

2. Wash cells with cold BAL buffer by centrifugation at 300 x g for 5 min at 4 °C.

3. Stain cells in a volume of 100 μl per 1 million cells according to Table 2 for 30 min at 4 °C in the dark using BAL buffer.

Table 2. FACS reagents used for staining BAL AM

4. Prepare compensation beads for each antibody conjugate.

5. Wash cells with BAL buffer, resuspend in 200 μl BAL buffer for recording.

6. Record cells by flow cytometry after acquiring the compensation beads. AM are double-positive for SiglecF and CD11c (Figures 2A-2C), and > 98% viable (Figure 2D).


Figure 2. FACS analysis of BAL AM. A. Simple gating strategy for exclusion of doublets, dead cells. AM are SiglecF- and CD11c-positive. B-C. BAL cells harvested with pre-warmed BAL buffer containing EDTA are phenotypically not different from BAL cells harvested using pre-cooled PBS. Each symbol denotes 1 mouse. Typically, > 95% of BAL cells are AM. D. Viability analysis of BAL singlet cells assessed by staining with Zombie Violet Fixable Dye.

C. Culture of alveolar macrophagesNote: Cells will adhere fully within a few hours after the first plating and we do not keep cells in suspension at the first medium exchange. However, for subsequent medium exchanges, the cells in the supernatant are collected as well since a typical AM culture consists of both adherent and suspended cells (see also Notes section).

7. Change medium every 2 days until the cell culture reaches confluency.

8. To change medium, transfer the medium and suspension cells into a 15 ml-tube. Add 2 ml warm AM culture medium to the well with adherent cells to prevent drying-out. Collect the suspension cells using centrifugation at 300 x g 5 min. Resuspend the pelleted cells in 1 ml warm AM culture medium and combine with adherent cells.
Note: Freshly harvested primary AM will double every 7-10 days (Soucie et al., 2016). If the majority of AM appear stretched (spindle-like) and activated, increasing the amount of conditioned medium or adding recombinant GM-CSF might help; however, proliferative capacity will be limited and it might advisable to start a new culture (see Figure 3 for exemplary images of early AM culture).


Figure 3. Representative images of AM culture within the first days after plating the cells. A. AM culture with predominantly round-shaped cells that are partly floating and partly adherent on Day 1 after plating. B. Same culture as (A) on Day 2. C. Same culture as (A) on Day 4. D. Example for an AM culture with a large fraction of elongated, dark cells on Day 4 after plating. Arrowheads indicate dividing cells, 100x magnification.

9. To detach cells from a confluent well, collect suspension cells into a 15-ml tube.

10. Add 750 μl detachment medium (see Recipes) to 1 well of a 6-well plate (or 3 ml to a 94 mm Petri dish) and incubate for 10-30 min at 37 °C.
Note: AM are very adherent and prone to rupture when using too harsh detachment procedures. Thus, the use of non-treated plastic ware and proper detachment medium is important (see Recipes). Ruptured cells in the culture medium might affect both activation status and proliferative capacity of AM.

11. Detachment of cells can be supported by pipetting on the plastic bottom gently to avoid cellular damage (see Notes).

12. Pool detached cells with cells in suspension and centrifuge cells at 300 x g for 5 min.

13. Resuspend cell pellet in 1 ml warm AM cultured medium and take a sample for counting using a hemocytometer chamber after staining with Trypan Blue to exclude dead cells. Count only live (Trypan-Blue negative) cells.

14. If the cell number has doubled, add 5 ml warm AM culture medium and split into 2 wells of a 6-well plate (or correspondingly to 2x 94 mm Petri dishes). In general, the cell number plated is maintained around the values indicated above in the note to Step C2.
Note: Earlier, we could show that AM culture remains proliferative for at least 10 passages (Soucie et al., 2016). Since then, we have experience with AM cultures that remain proliferative even beyond 20 passages with no indication of a decline in proliferative capacity.

1. Collect cells by centrifugation as before. Remove supernatant.

2. Plate 3 x 105-4 x 105 cells per well of a non-treated 6-well plate in 3 ml pre-warmed AM culture medium (see Recipes).
Note: Typically, 3 x 105-4 x 105 cells are plated in 1 well of a 6-well plate. If BAL cells of several mice are pooled, 1.1-1.2 million cells can be plated in a non-treated 94 mm Petri dish in 10 ml pre-warmed AM culture medium.

3. Add gentamicin to the AM culture (1:1,000).
Note: Gentamicin is omitted after the first medium change.

4. Incubate at 37 °C, 5% CO2.

5. Replace culture supernatant after 6-18 h with fresh AM culture medium and discard the supernatant.

Data analysis

Harvested cells were counted manually using a hemocytometer and considering only Trypan-Blue-negative cells. Stained cells were recorded on a BD LSRFortessa with 5 lasers using BD FACSDiva software and analyzed using FlowJo v10. Microscope images were acquired on an inverse microscope (Leica DMi1) equipped with a digital camera (MC120). Gating was performed as indicated in Figure 2A. To test for statistically significant differences between the means of three groups (Figure 1), one-way ANOVA with Tukey’s multiple comparisons test was performed using GraphPad Prism 7. No data points were excluded.

Notes

1. Proliferative AM are round-shaped and semi-adherent. Re-plating of suspension AM will result in part of the cells attaching to the new well, while the other part remains in suspension. Take care to not lose the suspended cells when changing medium as this will reduce the number of proliferative cells and slow the expansion of the culture.

2. When detaching cells, do not pipet the cell suspension up and down extensively, this might affect the viability of the culture; if cells do not detach readily, collect detachment medium containing already detached cells and perform another round of incubation with fresh detachment medium and/or increase the incubation time. Late-passage cells require shorter incubation times (~ 5 min) than early-passage cultures (up to 30 min).

3. The percentage of conditioned medium should be titrated after preparation of each batch by testing the growth of AM in the presence of various amounts of conditioned medium (e.g., 1, 2, 5, 10% in complete medium). In our batches, we use typically 1%-3% conditioned medium diluted into complete medium (e.g., 100 μl in 10 ml), which corresponds roughly to 2-5 ng/ml purified recombinant mouse GM-CSF.

4. We successfully replaced conditioned medium with 20 ng/ml recombinant GM-CSF (Peprotech) for long-term culture. Lower GM-CSF concentrations might be sufficient but have not been tested.

Recipes

1. BAL buffer
PBS
2 mM EDTA (dilute 1:250 from 0.5 M EDTA stock solution)
0.5% Fetal bovine serum (FBS)
Sterile-filter using vacuum filtration and keep at 4 °C until use

2. Complete medium
RPMI 1640
1x GlutaMAX
1x Pyruvate
1x Penicillin/Streptomycin
10% FBS
Sterile-filter using vacuum filtration and keep at 4 °C until use

3. AM culture medium
Supplement complete medium with 1-5% conditioned medium containing mouse GM-CSF (needs to be titrated)
Pre-warm an aliquot in a water bath to 37 °C before use

4.Detachment medium
ESGRO Complete Accutase
1 mM EGTA (dilute 1:500 from 0.5 M EGTA stock solution)
Aliquot and freeze at -20 °C. Pre-warm an aliquot in the water bath to 37 °C before use

原始参考文章：

Busch, C. J., Favret, J., Geirsdóttir, L., Molawi, K. and Sieweke, M. H. (2019). Isolation and Long-term-culture of Mouse Alveolar Macrophages. Bio-protocol 9(14): e3302. DOI: 10.21769/BioProtoc.3302.