A dose escalation study to evaluate the safety of an aerosol BCG infection in previously BCG-vaccinated healthy human UK adults

Abstract

Introduction: Tuberculosis (TB) is the leading cause of death worldwide from a single infectious agent. Bacillus Calmette-Guérin (BCG), the only licensed vaccine, provides limited protection. Controlled human infection models (CHIMs) are useful in accelerating vaccine development for pathogens with no correlates of protection; however, the need for prolonged treatment makes Mycobacterium tuberculosis an unethical challenge agent. Aerosolised BCG provides a potential safe surrogate of infection. A CHIM in BCG-vaccinated as well as BCG-naïve individuals would allow identification of novel BCG-booster vaccine candidates and facilitate CHIM studies in populations with high TB endemicity. The purpose of this study was to evaluate the safety and utility of an aerosol BCG CHIM in historically BCG-vaccinated volunteers.

Methods: There were 12 healthy, historically BCG-vaccinated UK adults sequentially enrolled into dose-escalating groups. The first three received 1 × 10⁴ CFU aerosol BCG Danish 1331 via a nebuliser. After safety review, subsequent groups received doses of 1 × 10⁵ CFU, 1 × 10⁶ CFU, or 1 × 10⁷ CFU. Safety was monitored through self-reported adverse events (AEs), laboratory tests, and lung function testing. Immunology blood samples were taken pre-infection and at multiple timepoints post-infection. A bronchoalveolar lavage (BAL) taken 14 days post-infection was analysed for presence of live BCG.

Results: No serious AEs occurred during the study. Solicited systemic and respiratory AEs were frequent in all groups, but generally short-lived and mild in severity. There was a trend for more reported AEs in the highest-dose group. No live BCG was detected in BAL from any volunteers. Aerosol BCG induced potent systemic cellular immune responses in the highest-dose group 7 days post-infection.

Discussion: Aerosol BCG infection up to a dose of 1 × 10⁷ CFU was well-tolerated in historically BCG-vaccinated healthy, UK adults. No live BCG was detected in the BAL fluid 14 days post-infection despite potent systemic responses, suggesting early clearance. Further work is needed to expand the number of volunteers receiving BCG via the aerosol route to refine and establish utility of this aerosol BCG CHIM.

Clinical trial registration: https://clinicaltrials.gov/, identifier NCT04777721.

Keywords: BCG; CHIM; aerosol; tuberculosis; vaccine.

Figure 1

Consort diagram for TB044.

Figure 2

Number and maximum grade reported of respiratory (A) and systemic (B) adverse (AE) occurrence on daily volunteer diary in the 2 weeks following BCG aerosol challenge. AE’s reported split by group, dose of aerosol BCG delivered; Group 1 = 1 × 10⁴ CFU; Group 2 = 1 × 10⁵ CFU; Group 3 = 1 × 10⁶ CFU; Group 4 = 1 × 10⁷ CFU. Data obtained once daily in all cases, and volunteers in each group n=3. SOB, Shortness Breath.

Figure 3

Percentage of predicted TLCO (TLCO_ADJ) at screening and also 7 days (D7) and 84 days (D84) after aerosol BCG for Group 1 (A), Group 2 (B), Group 3 (C) and Group 4 (D) for each volunteer enrolled. TLCO, transfer capacity of the lung, for the carbon monoxide. All percentage predicted accounting for age, sex, ethnicity and height and adjusted for most recent laboratory haemoglobin. N.B. volunteer in blue TLCO repeated at D168 rather than D84.

Figure 4

Percentage change in FEV1 (A) and FVC (B) compared to pre-challenge (D0) testing, separated by group. Plotted points are the median percentage change for each group. Bars show range. FEV1 - Forced Expiratory Volume in 1^st second, FVC, Forced Vital Capacity. Dotted line is time of BCG administration. D0 spirometry was completed on the day of enrolment immediately prior to challenge. 1hr- repeated spirometry taken one hour after challenge with BCG was completed. N.B Spirometry for one individual in group 1 completed at D168 rather than D84 visit.

Figure 5

Enumeration of BCG from clinical vaccine vials (A) and dose loaded into the nebulizer (B). Dots represent individual vaccine vials/dose per volunteer. Red dot shows a different batch of BCG used. Lines show median numbers per group, solid lines show the manufacture’s stated range for amount of BCG contained in a vaccine vial and the dotted line shows value used for dose calculations.

Figure 6

Heatmap of qPCR results using IS1081 probe, from PVA masks. Volunteers separated by dosing group (Group 1 = 1 x 10⁴ CFU, Group 2 = 1 x 10⁵ CFU, Group 3 = 1 x 10⁶ CFU and group 4 = 1 x 10⁷ CFU). Single assay positive probes highlighted in green boxes, where secondary IS6110 probe was not also positive.

Figure 7

IFN-γ ELISpot responses to PPD (A), BCG (B) and a pool of M.tb peptides - MTB300 (C) and ELISA PPD-specific IgA (D) and IgG (E) in serum at baseline and multiple time points post-aerosol BCG and in concentrated BAL fluid 14 days post-aerosol BCG (F). Dots represent individual values, lines represent a volunteers response over time. Colours represent the different dose groups - black = 1 x 10⁴ CFU, red = 1 x 10⁵ CFU, green = 1 x 10⁶ CFU and blue = 1 x 10⁷ CFU. Stars denote significance *p = <0.05, **p = <0.01.

Figure 8

Ratio of BCG growth after a 96 hour incubation with PBMC, collected and frozen on day of aerosol BCG and 7, 28 and 56 days post-aerosol BCG, in the mycobacterial growth inhibition assay (MGIA). A group of BCG naïve volunteers (open circles) were run as a control for the assay. Colours represent the different dose groups - black = 1 x 10⁴ CFU, red = 1 x 10⁵ CFU, green = 1 x 10⁶ CFU and blue = 1 x 10⁷ CFU. Stars denote significance *p = <0.05.