Characteristics of Human Donor Lungs Used for Research

To the Editor:

Tissues from organ donors that are not used for transplantation provide a valuable resource for investigating human disease and are used as complementary models to animal studies. Many fundamental discoveries have been conducted using primary cells derived from such tissue (1, 2). These primary cells behave differently to injury when there are underlying comorbidities, such as diabetes mellitus (DM), hypertension, aging, and exposures and chronic alcohol use (3–7).

Some of the results of these studies have been previously reported in the form of a preprint (https://doi.org/10.1101/2024.08.10.607431).

To investigate the various comorbidities and exposures in lung donors, we procured data from 1,686 donors with various outcomes at Mid-America Transplant from January 2017 until July 2023. Deidentified data of deceased donors was obtained after review and approval of an application from the Research Advisory Committee of Mid-America Transplant. The lungs were characterized based on their use into three groups: lungs used for research, lungs used for transplant, and lungs that were not recovered from donors. The data procured included demographics, medical history, drug exposure, serologies, chest radiograph readings, and microbiological culture data. Expanded criteria donors (ECDs) were defined as brain-dead donors >60 years old or donors 50–59 years old who had two or more of the following: history of hypertension, creatinine ⩾1.5 mg/dl, and/or death from a stroke or cardiovascular accident. We identified donors as “diabetes history positive” who had a documented history of DM before the current hospitalization; the identification of donors with a history of chronic obstructive pulmonary disease, asthma, and hypertension was made similarly. A subset of those who had diabetes history positive had a documented history of insulin use for diabetes control and were classified as “diabetes with insulin use.” Donors whose computed tomography scans indexed to the organ donation showed evidence of pulmonary embolism were included under “positive history of pulmonary embolism.” High-risk behavior was present if the deceased donor had factors associated with an increased risk for disease transmission, including blood-borne pathogens, and if the deceased donor met the criteria for increased risk for human immunodeficiency virus, hepatitis B, and hepatitis C transmission set forth in the current U.S. Public Health Services Guideline. Donors included under “intravenous drug use” were donors who had ever abused or had a dependency on intravenous drugs that were not prescribed and were used for nonmedical reasons. Donors were included under “history of nonintravenous drug use” if the donor had ever abused or had a dependency on nonintravenous street drugs, such as crack, marijuana, or prescription narcotics, sedatives, hypnotics, or stimulants. A comparative analysis of chest radiographs was done by adapting a lung donor scoring system used to predict outcomes in transplant recipients (8). Microbiological culture data of lung donors was obtained at the time closest to harvest, which included cultures of sputum and BAL and/or washings. Data from the three donor categories were analyzed using IBM SPSS Statistics Version 29.0.2.0.

The mean age of donors used for research was 41 ± 18 years, of whom nearly 10% were age ⩾65 years (Table 1). A higher prevalence of ECD was noted in those used for research, with 25% of the cohort being ECD, whereas only 10% of the donors in the transplant cohort were ECD (Table 1). The mean maximum Pa_O2/Fi_O2 ratio among donors whose lungs were used for research was 404, compared with 504 in donors whose lungs were used for transplant (Table 1). A total of 14% of the donors whose lungs were used for research had a history of DM, of whom 4% had insulin-dependent DM, comparable to those with rejected lungs (Table 2). In comparison, only 8% of the donors whose lungs were transplanted had a history of DM. In terms of exposure history, a quarter of the research donor population had a positive history of cigarette use exceeding 20 pack-years, whereas it was noted in only 10% of the donors whose lungs were transplanted (Table 2). Nine percent of donors in the research cohort had a positive vaping history, compared with 7% among donors whose lungs were used for transplantation (Table 2). Only 5% of the donors whose lungs were used for research had a history of intravenous drug use, compared with 17% of the donors used for transplant. Of note, at least 40% of the donors were found to have a positive history of nonintravenous drug use, of whom at least 30% had a history of continued nonintravenous drug use (Table 2). Ten percent of the donors whose lungs were used for research had a history of chronic obstructive pulmonary disease, compared with only 1% of those whose lungs were used for transplantation; the prevalence of asthma and pulmonary embolism were similar in research and transplant cohorts (Table 2). Three percent of the donors belonging to the research cohort and 1% of the donors of the transplant cohort showed positive hepatitis B core antigen antibody, indicating a previous hepatitis B infection.

Table 1.

Donor Demographics

Characteristics	Research Lungs (n = 128)	Transplanted Lungs (n = 391)	Rejected Lungs (n = 1,167)
Age
Mean ± SD	41.04 ± 18	34.86 ± 14	42.29 ± 17
Median (IQR)	41.5 (29)	33 (21)	44 (24)
Age ⩾ 65 yr, %	9.38	1.53	7.97
PaO2/FiO2 ratio, minimum
Mean ± SD	209 ± 108	268 ± 107	185 ± 109
Median (IQR)	192 (149)	267 (155)	167 (158)
PaO2/FiO2 ratio, maximum
Mean ± SD	404 ± 161	504 ± 139	353 ± 273
Median (IQR)	395 (158.5)	486 (90)	350 (208)
BMI
Mean ± SD	28.52 ± 9	26.92 ± 6	29.04 ± 9
Median (IQR)	26.76 (8.9)	25.86 (6.79)	27.38 (9.88)
Race, %
White	71.88	74.17	77.72
Black/African American	24.22	21.99	19.36
Hispanic	2.34	2.55	1.29
Others	1.56	1.27	1.60
Sex, %
Male	56.25	59.59	62.98
Female	43.75	40.40	37.01
Blood group, %
O	50	48.59	45.15
A	38.26	41.42	38.03
B	10.15	8.69	12.68
AB	1.56	1.28	4.11
Cause of death, %
Anoxia	51.56	39.39	52.70
Head trauma	22.66	38.87	22.62
Cerebrovascular/stroke	20.31	20.72	20.65
Other	5.46	1.02	4.02
Death type, %
Brain death	70.31	95.14	60.33
Asystole	29.69	4.86	39.67
Mechanism of death, %
Intracranial hemorrhage/stroke	18.75	20.72	20.91
Cardiovascular	17.19	5.63	19.11
Blunt injury	17.19	22.51	17.99
Asphyxiation	11.72	5.12	7.71
Drug/intoxication	10.16	23.27	18.68
Gunshot wound	7.03	17.39	6.08
Others	17.97	5.36	9.48
Circumstances of death, %
Death from natural causes	53.91	30.18	50.04
Accident, non-MVA	18.75	31.19	25.36
MVA	10.94	15.09	12.43
Alleged suicide	10.16	16.11	6.86
Alleged homicide	4.68	6.91	3.68
Others	1.56	0.51	1.63
Organ donor type, %
SCD	46.88	85.17	41.05
DCD	25.13	4.85	39.33
ECD	25	9.72	19.45
Null	0	0.26	0.72

Definition of abbreviations: BMI = body mass index; DCD = donor after circulatory death; ECD = expanded criteria donor; Fi_O2 = fraction of inspired oxygen; IQR = interquartile range; MVA = motor vehicle accident; Pa_O2 = partial pressure of oxygen; SD = standard deviation; SCD = standard criteria donor.

Table 2.

History of Noncommunicable Diseases, Exposures, and Microbiological Data

Characteristics	Research Lungs (n = 128)	Transplanted Lungs (n = 391)	Rejected Lungs (n = 1,167)
History of diabetes
Diabetes	14.06	7.93	13.88
No diabetes	85.94	91.30	85.43
Diabetes with insulin use	3.91	3.32	5.23
History of pulmonary embolism
Positive history	5.50	5.90	6.90
Negative history	78.10	90.80	72.40
Unknown	16.40	3.30	20.70
History of COPD
Positive history	10.20	1	12.40
Negative history	89.80	99	87.60
History of asthma
Positive history	13.30	13.50	18.60
Negative history	86.70	86.50	81.40
History of hypertension
Positive history	41.41	22.76	44.04
Negative history	57.81	76.47	55.44
Unknown	0.78	1.02	0.51
History of high-risk behavior
Positive history	14.84	25.58	22.37
Negative history	85.16	74.17	77.63
Unknown	0	0.26	0
History of cigarette use of at least 20 pack-years
Positive history	27.34	10.49	33.16
Negative history	70.31	87.47	63.41
Unknown	2.34	2.05	3.43
History of cigarette use in the last 6 mo
Positive history	21.87	9.46	28.62
Negative history	67.18	78	62.46
Unknown	10.95	12.54	8.92
History of vaping
Positive history	8.60	6.60	3.80
Negative history	91.40	93.40	96.20
History of cocaine use
Positive history	11.72	13.55	17.40
Negative history	87.50	84.91	81.49
Unknown	0.78	1.53	1.11
History of continued cocaine use
Positive history	8.59	6.65	7.37
Negative history	91.41	90.28	89.80
Unknown	0	3.07	2.83
History of intravenous drug use
Positive history	5.47	16.88	16.20
Negative history	94.53	81.59	82.52
Unknown	0	1.53	1.29
History of nonintravenous drug use
Positive history	46.09	41.94	45.84
Negative history	53.91	57.54	53.73
Unknown	0	0.51	0.43
History of continued nonintravenous drug use
Positive history	32.54	36.06	36.76
Negative history	63.49	60.61	58.18
Unknown	3.97	2.81	5.06
Gram-positive bacteria
MRSA	6.25	6.14	7.20
MSSA	22.66	23.27	19.40
Other Staphylococcus sp.	0.78	0	0.42
Other Streptococcus sp.	4.69	8.18	3.94
Streptococcus pneumoniae	3.13	4.35	3.85
Other sp.	0	0	0.77
Gram-negative bacteria
Acinetobacter sp.	0	1.02	1.71
Escherichia coli	1.56	1.02	2.14
Enterobacter sp.	3.91	6.14	3.08
Hemophilus influenzae	6.25	6.91	6.16
Klebsiella sp.	3.91	2.81	3.68
Pseudomonas sp.	2.34	1.02	3.59
Other sp.	7.03	4.60	4.37
Viruses
SARS CoV-2	0	0.26	1.02
Rhinovirus	0.78	0.77	1.28
Influenza	1.56	0	0.34
Parainfluenza	0	0	0
Adenovirus	0	0.26	0.25
Enterovirus	0.78	0.77	1.03
Other virus	1.56	0	0.09
Fungi
Aspergillus	0.78	0.26	0.09
Candida	0.78	0.51	0.69
Yeast	0	1.28	1.37
Other fungi	0	0	0.43
Chest radiograph score
0 (clear)	16.41	39.64	13.28
1 (minor changes)	23.44	27.37	27.76
2 (opacity in ⩽1 lobe)	22.66	22.76	19.79
3 (opacity in >1 lobe)	35.94	9.21	38.39

Definition of abbreviations: COPD = chronic obstructive pulmonary disease; MRSA = methicillin-resistant Staphylococcus aureus; MSSA = methicillin-sensitive Staphylococcus aureus; SARS CoV-2 = severe acute respiratory syndrome coronavirus 2; sp. = species.

Data are given as percentages.

The most prevalent microorganism detected on respiratory cultures among all three cohorts was methicillin-susceptible Staphylococcus aureus, which was found in ∼23% of lungs used for research and transplantation (Table 2). Among gram-negative organisms isolated from lungs used for research, Hemophilus influenzae was seen in 6%, and Enterobacter and Klebsiella were each observed in 4% of donor lungs (Table 2). Chest radiographs having opacities in more than one lobe (scored 3) were highest in donors whose lungs were rejected at 38%, closely followed by 36% in donors whose lungs were used for research and only 9% of the donors whose lungs were transplanted (Table 2). Only a total of eight donors (two transplant donors and six donors whose lungs were rejected) were subject to normothermic regional perfusion.

No strict selection criteria or protocols for human donor lungs are used for ex vivo research. However, there are various pathophysiological mechanisms by which donor age, DM, and a history of substance abuse or smoking affect human lung tissue, which can influence the outcomes of ex vivo lung tissue experiments. For example, a decline in adaptive and innate immune responses is observed in lungs obtained from older donors because of mitochondrial dysfunction, reduced T-cell subpopulations, and impaired B-cell generation (3). In addition, DM is known to induce inflammatory and fibrotic changes in the lung (4). DM also leads to decreased innate immune responses in the lung, as evidenced by a marked reduction in TLR2 and TLR4, together with thickening of alveolar epithelial and capillary basal laminae (5, 6). Exposure to cigarette smoke accelerates senescence in lung epithelial cells, impairs IL-8 production, and leads to an altered macrophage phenotype associated with aberrant tissue remodeling, and an inability to kill pathogens and remove dead cells (7, 9, 10). Exposure to vaping can result in subacute lung injury with histological evidence of diffuse alveolar damage and increased infiltration of inflammatory cells in lung tissue (11). However, other factors, such as length of time on a ventilator and cold ischemia time, can affect the quality of lung tissue because of ventilator-induced lung injury and ischemic lung injury, respectively (12, 13). In addition, various occupational exposures can affect the lung structure and function (14). Nonavailability of data for the above variables, including not having biomarker or cellular response data that can be used for correlation analysis with comorbidities (e.g., DM history), are limitations of this study.

However, based on our existing observations, we conclude that comorbidities should be carefully considered when evaluating data derived from donor lungs used for research. Regarding lungs from older donors, those with DM or with a history of smoking have significant variations in cellular pathology, composition, grades of inflammation, and varied immunological responses; we propose that accounting for these comorbidities in donors of lung tissue used for research is important when designing ex vivo studies and interpreting their results. In this context, reporting comorbidities based on objective data, for example obesity (using body mass index), DM [using Hba1c (glycated hemoglobin) or insulin use], a history of drug use (using urine drug screens), and microbiological data where available, would be useful. Other history, including smoking, vaping, or occupational exposure, is based on either data obtained from the charts or from the family and should be reported when available; however, the absence of this history does not preclude it and, thus, should be acknowledged as a limitation. Given that normothermic regional perfusion is being increasingly used, particularly in donors after circulatory death, and it may add extended warm ischemic times to the lung harvest, we recommend reporting the presence or absence of its use when lungs are used for research (15). Ideally, it would be helpful to select and/or match lungs for research based on donor factors or clearly report these characteristics to the extent of the available information. In addition, an alternative source of relatively “normal lung” can be excess adjacent lung tissue after lobectomy for nonsmoking cases of isolated lung cancer, and studies comparing these sources of lung tissue for ex vivo studies could be useful. Finally, we hope that our report will facilitate appropriate power calculations required for research studies that use donor lungs, such as those investigating pneumonia or aging.