library_construction_protocol: Fetal lung xenografts were processed as previously described (Kenney et al (2022) Cell Reports). Briefly, lung tissues were placed in a 60 mm dish, minced using a disposable scalpel, then transferred into a 15 mL conical tube containing 3 mL of digestion buffer (HBSS minus Ca2+, Mg2+, and phenol red, 0.5 mg/mL Liberase TM, 1 mg/mL DNase I) and incubated at 37°C for 30 min with agitation every 10 min. After digestion, tissue was passed through a 70 μm strainer on a 50 mL tube, mashed using the plunger of a 3 mL syringe and washed twice with 1 mL FACS buffer. Cell suspension was centrifuged at 300 x g for 5 minutes at 4°C, suspended in 1 mL of ACK lysing buffer, and incubated for 2 min at room temperature.  After incubation, lysis was quenched with 12 mL of FACS buffer, cell suspension was centrifuged at 300 x g for 5 min at 4°C, and the cell pellet was suspended in 1 mL of FACS buffer prior to downstream processing.Cells were frozen down in a 90% FBS (Bio-Techne, R&D systems) 10% DMSO solution (ThermoFisher Scientific) and kept at -80°C. Four to five days following freezing, cells were thawed, and viability was assessed using Trypan blue (Fisher Scientific). Samples with ≥90% viability were then processed using the Chromium Next GEM Single Cell 3' GEM, Library & Gel Bead Kit v3.1, as per manufacturer instructions, and single cell barcoding was performed using a Chromium instrument (10x genomics) located in the NEIDL BSL-3. Reverse transcription of RNAs was performed in the BSL-3 using a thermocycler (Applied Biosciences), and cDNA was then removed from containment. Full-length, barcoded cDNAs were then amplified by PCR to generate sufficient mass for library construction. Enzyme fragmentation, A tailing, adaptor ligation and PCR were then performed at the Boston University single-cell sequencing core to obtain final libraries containing P5 and P7 primers used in Illumina bridge amplification. Size distribution and molarity of resulting cDNA libraries were assessed via Bioanalyzer High Sensitivity DNA Assay (Agilent Technologies, USA). All cDNA libraries were sequenced on an Illumina NextSeq 500 instrument at the Boston University microarray and sequencing core according to Illumina and 10x Genomics guidelines with 1.4-1.8pM input and 1% PhiX control library spike-in (Illumina, USA).