BCG vaccination provides protection against IAV but not SARS-CoV-2

Abstract

Since the vast majority of species solely rely on innate immunity for host defense, it stands to reason that a critical evolutionary trait like immunological memory evolved in this primitive branch of our immune system. There is ample evidence that vaccines such as bacillus Calmette-Guérin (BCG) induce protective innate immune memory responses (trained immunity) against heterologous pathogens. Here we show that while BCG vaccination significantly reduces morbidity and mortality against influenza A virus (IAV), it fails to provide protection against severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). In contrast to IAV, SARS-CoV-2 infection leads to unique pulmonary vasculature damage facilitating viral dissemination to other organs, including the bone marrow (BM), a central site for BCG-mediated trained immunity. Finally, monocytes from BCG-vaccinated individuals mount an efficient cytokine response to IAV infection, while this response is minimal following SARS-CoV-2. Collectively, our data suggest that the protective capacity of BCG vaccination is contingent on viral pathogenesis and tissue tropism.

Keywords: BCG vaccination; SARS-CoV-2; animal models; hematopoietic stem cells; influenza virus; lung pathology; monocytes; trained immunity.

Graphical abstract

Figure 1

In mice, BCG vaccination induces unspecific protection against IAV infection, but not against SARS-CoV-2 (A) Experimental model of BCG vaccination and intranasal (i.n.) IAV infection in C57BL/6J mice. (B) Mortality (top) and morbidity (bottom) of 1-month BCG-i.v.-vaccinated or nonvaccinated control C57BL/6J mice after i.n. infection with a lethal dose of 90 plaque-forming units (PFU) influenza A/Puerto Rico/8/34 (IAV-PR8), n = 7–10/group. (C) Lung viral load in BCG-i.v.-vaccinated and control nonvaccinated C57BL/6J mice at day 3 post sublethal IAV-PR8 infection (50 PFU), n = 13/group. (D) Experimental model of BCG vaccination and i.n. or intratracheal (i.t.) SARS-CoV-2/SB2 infection in B6.Cg-Tg(K18-ACE2)2Prlmn/J mice. (E) Mortality (top) and morbidity (bottom) (n = 8/group) of BCG-vaccinated and control K18-hACE2/J mice after i.n. infection with 1 × 10⁴ TCID50/mL SARS-CoV-2. (F) Lung viral load at day 3 and day 5 post sublethal i.t. SARS-CoV-2/SB2 infection (4,000 TCID50/mL), n = 3–5/group. Stars in morbidity curves (B and E) indicate significant weight loss compared with day 0. Data are displayed as mean ± SEM. ^∗p < 0.05, ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001, ^∗∗∗∗p ≤ 0.0001 (survival analyses and two-way ANOVA). See also Figure S1 and Table S1.

Figure 2

BCG vaccination protects Syrian golden hamsters against IAV, but not SARS-CoV-2 infection (A) Experimental model of BCG vaccination and i.n. infection with IAV-H3N2 in Syrian golden hamsters. (B) Lung viral load in BCG-i.v.-vaccinated and nonvaccinated control Syrian golden hamsters at day 3 post i.n. infection with 1 × 10⁵ PFU IAV-H3N2, n = 8/group. (C) Experimental model of BCG vaccination and i.n. infection with SARS-CoV-2/SB2 or/RIM-1 in Syrian golden hamsters. (D–F) Morbidity (n = 7–10/group) (D), lung viral load by TCID50/mL measurement (n = 3–5/group) (E), and representative images and quantification of viral N protein staining in the lungs of golden hamsters infected with 1 × 10⁵ PFU SARS-CoV-2/SB2 (n = 3–5/group) (F). (G and H) Morbidity (n = 7–10/group) (G) and lung viral load determined by TCID50/mL assay (n = 3–5/group) (H) in BCG-vaccinated and control Syrian golden hamsters at 3 and 5 days post i.n. infection with 1 × 10⁵ PFU SARS-CoV-2/RIM-1. Stars in morbidity curves (D and G) indicate significant weight loss compared with day 0. Data are displayed as mean ± SEM. ^∗p < 0.05, ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001, ^∗∗∗∗p ≤ 0.0001 (t test and two-way ANOVA). See also Figure S2, Table S2.

Figure 3

In Roborovski hamsters, BCG vaccination provides no protection against sublethal and lethal SARS-CoV-2 infection (A) Experimental model of BCG vaccination and i.n. infection with SARS-CoV-2/RIM-1 in Roborovski hamsters. (B and C) Mortality and morbidity of BCG-vaccinated and control Roborovski hamsters after i.n. infection with a sublethal (B) or lethal dose (C) of SARS-CoV-2/RIM-1, n = 7/group. (D) Lung viral load in BCG-vaccinated and nonvaccinated Roborovski hamsters at day 3 post infection with a sublethal (low dose) or lethal dose (high dose) of SARS-CoV-2/RIM-1 as determined by TCID50/mL assay, n = 4–5/group. Stars in morbidity curves (B and C) indicate significant weight loss compared with day 0. Data are displayed as mean ± SEM. ^∗p < 0.05, ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001, ^∗∗∗∗p ≤ 0.0001 (two-way ANOVA). See also Figure S3 and Table S1.

Figure 4

SARS-CoV-2 induces significantly worse lung histological damage than IAV (A) One-month BCG-vaccinated and nonvaccinated (PBS) C57BL/6J mice were i.n. infected with a sublethal or lethal dose of IAV-PR8. Representative images of lung histopathology on days 0, 3, and 6 post IAV infection (left) and scoring (right), n = 4–5/group, refer to Table S1. (B) One-month BCG-vaccinated and nonvaccinated (PBS) K18-hACE2 mice were i.t. infected with 4,000 TCID SARS-CoV-2/SB2. Representative images of lung histopathology on days 1, 3, and 5 post SARS-CoV-2 infection (left) and scoring (right), n = 3–4/group, refer to Table S1. (C) BCG-vaccinated and control Syrian golden hamsters were i.n. infected with 1 × 10⁵ PFU SARS-CoV-2 and exhibited a clinically mild disease phenotype. Representative images of lung histopathologies on days 0, 3, and 5 post SARS-CoV-2 infection (left) and scoring (right), n = 3–6/group, refer to Table S1. (D) RNAscope analysis of SARS-CoV-2 viral particles in human lung cells. Heatmap data are displayed as mean. See also Figure S4 and Table S1.

Figure 5

BCG-induced trained immunity provides protection against IAV, but not SARS-CoV-2 (A) IAV viral load in the BM of C57BL/6J mice (day 3 and 6 post infection with 90 PFU IAV-PR8, n = 3/group). (B and C) SARS-CoV-2 viral load in the BM of K18-hACE2/J mice (day 3 and 5 post i.t. infection with 4,000 TCID SARS-CoV-2/SB2, n = 3–5/group). (D) SARS-CoV-2 viral load in the BM of Roborovski hamsters (day 3 post infection with 1 × 10⁵ PFU SARS-CoV-2/RIM-1, n = 3–4/group). (E) Experimental model of BCG vaccination in humans and infection of blood monocyte-derived macrophages (MDMs) with IAV and SARS-CoV-2/RIM-1 for assessment of cytokine responses. (F) Expression of IL-1β, TNF-α, CCL2, type I IFNs, and IL-8 in MDMs from human donors before and after BCG vaccination, at 24 h post in vitro IAV-H3N2 or SARS-CoV-2/RIM-1 infection, n = 5/group. Data are displayed as mean ± SEM. ^∗p < 0.05, ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001, ^∗∗∗∗p ≤ 0.0001 (t test and one-way ANOVA). See also Figure S5.