Overcoming the cystic fibrosis sputum barrier to leading adeno-associated virus gene therapy vectors

Abstract

Gene therapy has not yet improved cystic fibrosis (CF) patient lung function in human trials, despite promising preclinical studies. In the human CF lung, inhaled gene vectors must penetrate the viscoelastic secretions coating the airways to reach target cells in the underlying epithelium. We investigated whether CF sputum acts as a barrier to leading adeno-associated virus (AAV) gene vectors, including AAV2, the only serotype tested in CF clinical trials, and AAV1, a leading candidate for future trials. Using multiple particle tracking, we found that sputum strongly impeded diffusion of AAV, regardless of serotype, by adhesive interactions and steric obstruction. Approximately 50% of AAV vectors diffused >1,000-fold more slowly in sputum than in water, with large patient-to-patient variation. We thus tested two strategies to improve AAV diffusion in sputum. We showed that an AAV2 mutant engineered to have reduced heparin binding diffused twice as fast as AAV2 on average, presumably because of reduced adhesion to sputum. We also discovered that the mucolytic N-acetylcysteine could markedly enhance AAV diffusion by altering the sputum microstructure. These studies underscore that sputum is a major barrier to CF gene delivery, and offer strategies for increasing AAV penetration through sputum to improve clinical outcomes.

Figure 1

Effect of AlexaFluor 647 (AF647) dye labeling on adeno-associated virus (AAV) transduction in BEAS-2B cells and on nanoparticle transport in cystic fibrosis (CF) sputum. (a–d) BEAS-2B cells transduced (a) with AAV2 or (b) with AF647-labeled AAV2 (AAV2-AF647) are compared with (c) untreated control cells. The AAV packaged a green fluorescent protein (GFP) reporter gene; green indicates GFP expression. Scale bars represent 100 µm. (d) Flow cytometry comparing GFP expression in BEAS-2B cells transduced with AAV2 versus AAV2-AF647. Results shown are mean cell fluorescence, in arbitrary units. Error bars denote standard error of the mean (n = 3). The difference in transduction between AAV2 and AAV2-AF647 is not statistically significant. (e–f) Multiple particle tracking in CF sputum samples of (e) 100-nm PS-PEG particles versus (f) 100-nm PS-PEG particles labeled with AF647 (PS-PEG-AF647). Graphs show distribution of individual particles' mean squared displacement (MSD) at a time scale of 1 second. Data represent six sputum samples, with an average of >1500 particles of each type tracked per sample.

Figure 2

Transport in cystic fibrosis (CF) sputum samples of adeno-associated virus (AAV)1, AAV2, and AAV5, compared with 100-nm PS-PEG control particles. Distribution of individual particle mean squared displacement (MSD) values at a time scale of 1 second for (a) 100-nm PS-PEG particles, (b) AAV1, (c) AAV2, and (d) AAV5. Data represent the average of 10 sputum samples, with each sample equally weighted, and with an average of >500 particles of each type tracked per sample. Percentage of particles that moved rapidly, defined as log₁₀MSD ≥ 0 at a time scale of 1 second, is shown for each particle type (dashed boxes).

Figure 3

Patient-to-patient variation in adeno-associated virus (AAV) transport. (a) Box-and-whisker plots of mean squared displacement (MSD) values (at a time scale of 1 second) of AAV1, AAV2, AAV5, and 100-nm PS-PEG control particles in sputum samples from 10 cystic fibrosis (CF) patients. Maximum whisker length is 1.5 times the interquartile range; outliers are shown as dots. Patients are numbered in ascending order according to the median MSD of 100-nm PS-PEG particles in their sputum sample. The dashed line at log₁₀MSD = 0 is a visual aid to emphasize fast-moving particles, which we define as log₁₀MSD ≥ 0 at a time scale of 1 second. (b) Representative trajectories of AAV1, AAV2, AAV5, and 100-nm PS-PEG control nanoparticles in sputum samples from 3 of the aforementioned 10 CF patients. Trajectories show 1 second of motion. The MSDs of the trajectories presented are within the middle 50 percentile for the given patient sample and particle or virus type.

Figure 4

Effect of mutation in adeno-associated virus (AAV)2 heparin-binding domain. (a) Effect of soluble heparin on transduction of BEAS-2B cells by AAV2 versus AAV2 mutant, which was engineered to reduce heparin binding. Green fluorescent protein (GFP) expression by transduced cells was measured by flow cytometry. Values shown are relative to GFP expression in the absence of soluble heparin for the respective AAV serotype. Error bars indicate standard error of the mean (n ≥ 3). Difference is statistically significant (P < 0.01) at 10, 25, and 100 µg/ml. (b) Transport in 17 cystic fibrosis (CF) sputum samples of AAV2 versus AAV2 mutant. Each marker represents the median mean squared displacement at a timescale of 1 second in one patient sample. Lines connect pairs of data from the same patient's sample. Difference is statistically significant (P < 0.01). See methods section for details on statistics.

Figure 5

Effect of mucolytic agent N-acetylcysteine (NAC) on adeno-associated virus (AAV)1 transport in cystic fibrosis (CF) sputum and on AAV1 transduction in BEAS-2B cells. (a–b) Multiple particle tracking of AAV1 in CF sputum samples either (a) untreated or (b) pretreated with 5 mmol/l NAC. Graphs show distribution of individual particles' mean squared displacement (MSDs) at a time scale of 1 second. Data represent five sputum samples, with an average of >900 AAV particles tracked per sample. Percentage of particles that moved rapidly, defined as log₁₀MSD ≥ 0 at a time scale of 1 second, is shown for both conditions (dashed boxes). The sputum samples used for the NAC study were different from the sputum samples used for Figures 2 and 3, and thus the percentage of fast AAV1 particles differs between Figures 2b and 5a. (c) Effect of 5 mmol/l NAC in the cell culture media on AAV1 transduction of BEAS-2B cells. Results show mean cell GFP fluorescence, measured by flow cytometry, in arbitrary units. Error bars represent standard error of the mean (P = 0.027, n = 8).