Airway Memory CD4(+) T Cells Mediate Protective Immunity against Emerging Respiratory Coronaviruses

Abstract

Two zoonotic coronaviruses (CoVs)-SARS-CoV and MERS-CoV-have crossed species to cause severe human respiratory disease. Here, we showed that induction of airway memory CD4(+) T cells specific for a conserved epitope shared by SARS-CoV and MERS-CoV is a potential strategy for developing pan-coronavirus vaccines. Airway memory CD4(+) T cells differed phenotypically and functionally from lung-derived cells and were crucial for protection against both CoVs in mice. Protection was dependent on interferon-γ and required early induction of robust innate and virus-specific CD8(+) T cell responses. The conserved epitope was also recognized in SARS-CoV- and MERS-CoV-infected human leukocyte antigen DR2 and DR3 transgenic mice, indicating potential relevance in human populations. Additionally, this epitope was cross-protective between human and bat CoVs, the progenitors for many human CoVs. Vaccine strategies that induce airway memory CD4(+) T cells targeting conserved epitopes might have broad applicability in the context of new CoVs and other respiratory virus outbreaks.

Figure 1

Intranasal Vaccination with VRP-SARS-N Results in CD4⁺ T-Cell-Dependent Protection against Lethal SARS-CoV Infection 6-week-old BALB/c mice were vaccinated with VRP-GFP or VRP-SARS-N subcutaneously (s.c.) in the footpad or intranasally (i.n.) and boosted 6–7 weeks later. (A) Vaccinated mice were infected with SARS-CoV at the indicated times. Cells from airway, lung, and spleen were stimulated with SARS-N353 peptide. Numbers of IFN-γ⁺CD4⁺ T cells are shown. (B) Survival after infection with 500 PFU SARS-CoV 4–6 weeks after boosting. n = 12, SARS-N s.c.; n = 77, SARS-N i.n.; n = 45, GFP i.n. (C) Intranasally vaccinated mice were infected with various doses of SARS-CoV. n = 4, GFP 100 PFU; n = 4, SARS-N 100 PFU; n = 10, SARS-N 500 PFU; n = 8, SARS-N 10⁴ PFU. (D) Lungs were removed at the indicated times p.i., and sections were stained with hematoxylin and eosin. (E) To obtain virus titers, lungs were homogenized at the indicated time points and titered on Vero E6 cells. Titers are expressed as PFU/g tissue. n = 3 mice/group/time point. ^∗p < 0.05. Data are representative of two independent experiments. (F) VRP-SARS-N-vaccinated mice were treated intraperitoneally (i.p.) with 1 mg anti-CD4 antibody (clone GK1.5) or rat IgG (rIgG) at day −2 and day 0 p.i. (day 2); some mice were rested for 98 days before SARS-CoV infection (day 98). n = 5, rIgG i.p. day −2; n = 9, αCD4 i.p. day −2; n = 9, rIgG i.p. day −98; n = 5, αCD4 i.p. day −98. (G) Lungs were removed from VRP-SARS-N-vaccinated and antibody i.p.-treated mice at day 4 p.i. Sections were stained with hematoxylin and eosin. (H) Numbers of IFN-γ⁺CD4⁺ T cells at several times after boosting are shown. (I) Survival after infection with 100 PFU SARS-CoV at 11 and 41 weeks after boosting. n = 9, GFP; n = 5, SARS-N 11 weeks; n = 6, SARS-N 41 weeks. (J) 300 μL of immune serum were transferred into VRP-GFP-vaccinated mice 1 day before infection. n = 4, GFP serum i.p.; n = 5, SARS-N serum i.p.; n = 5, SARS-S serum i.p. (K) Mice were vaccinated with rVV-SARS-N or irrelevant rVV intravenously (i.v.) or i.n. and boosted 6–7 weeks later. Mice were infected with SARS-CoV 4–6 weeks after boosting. n = 5/group. (L) 15-month-old VRP-SARS-N-vaccinated mice were infected with 100 PFU SARS-CoV. n = 5, GFP i.n.; n = 10, SARS-N i.n. Error bars in (A) and (E) represent SEM. See also Figures S1, S2, and S6.

Figure 2

Airway-Derived N-Specific CD4⁺ T Cells Are Superior Effector Cells (A) To localize memory CD4⁺ T cells in the respiratory tract at 5–7 weeks after boosting, 0.25 μg of fluorochrome-conjugated CD45 and 0.5 μg of fluorochrome-conjugated CD90.2 antibody were injected i.n. and i.v., respectively, as described in the Experimental Procedures. (B) CD4⁺ T cells from airway, parenchyma, and vasculature were stimulated with SARS-CoV N353 peptide. Data are representative of five independent experiments. (C) Cells from vaccinated mice were stained with SARS-N353 tetramer and phenotypic markers at 5–7 weeks after boosting after simultaneous i.n./i.v. labeling. Data are representative of two independent experiments. (D–G) Vaccinated mice were infected with SARS-CoV at the indicated times. Cells were stimulated with SARS-N353 peptide. Frequency, numbers, and mean fluorescence intensity (MFI) of IFN-γ expression (D) of IFN-γ⁺CD4⁺ T cells or cells expressing IFN-γ and TNF, IL-10, or IL-2 (E and F) are shown. ^∗p < 0.05. Data are representative of three independent experiments. (G) Functional avidity of N353-specific CD4⁺ T cells (left) and the amount of peptide required for half-maximum response (EC₅₀) are shown (right). ^∗p < 0.05. Data are representative of four independent experiments. Error bars in (B), (D), (F), and (G) represent SEM. See also Figures S3 and S5.

Figure 3

N-Specific Airway Memory CD4⁺ T Cells Mediate Protection by Local Expression of IFN-γ (A and B) For local depletion of CD4⁺ T cells from the airway, mice were treated with 10 μg anti-CD4 antibody (clone GK1.5) i.n. in 75 μL PBS at day −1. (A) Mice were simultaneously i.n. and i.v. labeled. BALF and lungs were harvested 2 days after depletion. Control mice received equivalent doses of rIgG in each experiment. Depletion efficiency was calculated. (B) N353 CD4⁺ T cell numbers were monitored after depletion. n = 3 mice/group/time point. Data are representative of 2–4 independent experiments. (C) Airway CD4⁺ T cells were depleted from VRP-SARS-N-vaccinated mice with anti-CD4 antibody or rIgG i.n. 1 day prior to SARS-CoV challenge. n = 5, rIgG i.n.; n = 24, αCD4 i.n. (D) IFN mRNA levels in the lungs were measured at the indicated time points. n = 3–4 mice/group/time point. ^∗p < 0.05. Data are representative of two independent experiments. (E) CD4⁺ T cells were depleted from VRP-SARS-N-vaccinated mice prior to SARS-CoV infection by i.n. or i.p. administration of anti-CD4 antibody. IFN-γ RNA levels in the lungs were measured at the indicated time points. n = 3–6 mice/group/time point. ^∗p < 0.05. Data are representative of two independent experiments. (F) For in vivo ICS, VRP-SARS-N-vaccinated mice were treated i.n. with BFA or vehicle at day 4 p.i. After 6 hr, airway CD4⁺ T cells were analyzed for IFN-γ expression. Frequency of IFN-γ⁺CD4⁺ T cells is shown. ^∗p < 0.05. Data are representative of two independent experiments. (G) VRP-SARS-N-vaccinated mice were treated with 10 μg IFN-γ neutralizing antibody (clone XMG1.2) or rIgG i.n. at day −2 and every 2 days thereafter. Lungs were harvested at indicated time points and interferon-related gene RNA levels in the lungs were measured. n = 3–6 mice/group/time point. ^∗p < 0.05. Data are representative of two independent experiments. (H) VRP-SARS-N-vaccinated mice were treated with anti-IFN-γ antibody i.n. Control mice received equivalent doses of rIgG in each experiment. n = 5, rIgG; n = 9, αIFN-γ i.p.; n = 7, αIFN-γ i.n. (I) Lungs were removed from VRP-SARS-N-vaccinated and antibody i.n.-treated mice at day 4 p.i. Sections were stained with hematoxylin and eosin. (J) To obtain virus titers, mice were treated with antibody i.n. and sacrificed at the indicated time points. Titers are expressed as PFU/g tissue. n = 3 mice/group/time point. ^∗p < 0.05. Data are representative of two independent experiments. (K) Naive mice were treated with 200 ng rTNF or rIFN-γ 12 hr before SARS-CoV infection. n = 4, PBS i.n.; n = 5, rIFN-γ i.n.; n = 4, rTNF i.n. Error bars in (A), (B), (D)–(G), and (J) represent SEM. See also Figure S4.

Figure 4

Airway CD4⁺ T-Cell-Derived IFN-γ Is Required for Optimal rDC Migration to the MLNs (A and B) To determine rDC migration from the lung to MLN, vaccinated mice were treated with anti-CD4 antibody, IFN-γ neutralizing antibody, or rIgG i.n. CFSE was then administered i.n. 6 hr before SARS-CoV infection. The percentage of CFSE⁺ cells within the CD11c⁺MHCII⁺ DC population (A, left), total CFSE⁺ DC numbers per LN (A, right), and total LN cell numbers (B) at 18 hr p.i. are shown. ^∗p < 0.05. Data are representative of four independent experiments. (C) CD40, CD80, CD86, and CCR7 expression on CFSE⁺ rDCs in the MLN at 18 hr p.i. Isotype control is shaded. (D) Naive mice were treated with 200 ng rIFN-γ i.n. for 12 hr. The frequency of CCR7⁺ rDCs in lungs is shown. Data are representative of two independent experiments. Error bars in (A), (B), and (D) represent SEM.

Figure 5

Airway CD4⁺ T-Cell-Derived IFN-γ Promotes CXCR3-Dependent Mobilization of Virus-Specific CD8⁺ T Cells to the Infected Lung (A and B) Vaccinated mice were infected with SARS-CoV. At day 6 and 8 p.i., cells from lungs were stimulated with SARS-S366 peptide for IFN-γ (A) and granzyme B or CD107a/b expression (B). (C) In vivo cytotoxicity assays in the lungs were performed on day 6 p.i. using SARS-CoV CD8⁺ T cell peptide S366 as described in the Experimental Procedures. ^∗p < 0.05. Data are representative of three independent experiments. (D and E) For systemic depletion of CD8⁺ T cells, VRP-SARS-N-vaccinated mice were injected i.p. with anti-CD8 or control antibody at day −2 and day 0 p.i. Mice were then infected with SARS-CoV and monitored for survival (D) (n = 5, rIgG i.p.; n = 7, αCD8 i.p.) or virus titers (E) (n = 3 mice/group/time point. ^∗p < 0.05). Data are representative of two independent experiments. (F) SARS-CoV S366-specific IFN-γ⁺CD8⁺ T cell frequency and numbers in the lungs of VRP-SARS-N-vaccinated and antibody i.n.-treated mice at day 6 p.i. (G) Chemokine receptor expression on SARS-CoV S366-specific CD8⁺ T cells at day 6 p.i. (H) VRP-SARS-N-vaccinated mice were treated with anti-CD4 antibody or IFN-γ neutralizing antibody i.n. and infected with SARS-CoV. Lungs were assayed for chemokine RNA levels. n = 4–6 mice/group/time point. ^∗p < 0.05. Data are representative of two independent experiments. (I) SARS-CoV-challenged, VRP-SARS-N-vaccinated mice were treated with CXCR3 blocking antibody (clone CXCR3-173) i.p. at day 3 and 5 p.i. Lung cells were stimulated with SARS-S366 peptide at day 6 p.i. Error bars in (A), (C), (E), (F), (H), and (I) represent SEM. See also Figures S2 and S4.

Figure 6

The N-Specific CD4⁺ T Cell Epitope Is Conserved in MERS-CoV and Is Presented by Human HLA-DR2 and DR3 Molecules (A) Sequences of SARS-CoV N353 epitope and homologous epitopes in other CoVs are shown. Underlined letters are anchor residues. (B) Airway-derived cells were prepared from MERS-CoV-infected (1 × 10⁵ PFU) mice at day 6 p.i. and stimulated with peptide. Frequencies of MERS-CoV-specific T cells (determined by IFN-γ intracellular staining) are shown. (C) Control and VRP-MERS-N-vaccinated mice were infected with MERS-CoV and sacrificed at the indicated time points. Airway cells were stimulated with MERS-N350 peptide for intracellular IFN-γ. (D and E) Virus titers were obtained from the lungs of VRP-MERS-N-vaccinated mice (D) or vaccinated and antibody i.n.-treated mice (E) at the indicated time points p.i. Titers are expressed as PFU/g tissue. n = 3 mice/group/time point. ^∗p < 0.05. Data are representative of two independent experiments. (F) Human HLA class II transgenic mice were immunized s.c. with N-specific peptides (100 μg) from SARS-CoV (left) or MERS-CoV (right). At 10 days after immunization, lymphocytes prepared from draining lymph nodes were stimulated with N or control proteolipid protein (PLP) peptides in vitro. The results are presented as stimulation indices (cpm of test sample/cpm of the control). ^∗p < 0.05. Data are representative of two independent experiments. (G) HLA-DR2 mice were vaccinated with VRP-SARS-N (left) or VRP-MERS-N (right) i.n. and infected with SARS-CoV or MERS-CoV. Lung virus titers are expressed as PFU/g tissue. n = 3 mice/group/time point. ^∗p < 0.05. Data are representative of two independent experiments. Error bars in (C)–(G) represent SEM. See also Figure S7.

Figure 7

Cross-reactive Memory CD4⁺ T Cells Are Protective against SARS-CoV and MERS-CoV (A) Airway-derived cells from VRP-MERS-N-vaccinated mice were stimulated with various CoV-specific peptides and assayed for IFN-γ production 5 days after booster. The percentage of cross reactivity between MERS-N peptide and other coronavirus N peptides is shown. (B) VRP-MERS-N- or VRP-GFP-vaccinated mice were infected with SARS-CoV. Airway-derived cells were stimulated with SARS-N or MERS-N peptides and assayed for IFN-γ production at day 6 p.i. Numbers of CD4⁺ T cells responding to MERS-N350 or SARS-N353 peptides are shown (right). ^∗p < 0.05. Data are representative of six independent experiments. (C) VRP-MERS-N- or VRP-GFP-vaccinated mice were infected with 100 PFU or 500 PFU of SARS-CoV. n = 4, GFP-immunized + 100 PFU SARS-CoV; n = 4, GFP + 500 PFU; n = 12, MERS-N + 100 PFU; n = 8, MERS-N + 500 PFU. (D) Mice were vaccinated with VRP-HKU4-N. Airway-derived cells were stimulated with CoV-specific peptides and assayed for IFN-γ production. The percentage of cross reactivity between HKU4-N peptide and other coronavirus N peptides is shown. (E) VRP-HKU4-N- or VRP-GFP-vaccinated mice were infected with MERS-CoV. Airway-derived cells were stimulated with HKU4-N and MERS-N peptides. Numbers of IFN-γ⁺ SARS-N353- and MERS-N350-specific CD4⁺ T cells are shown (right). ^∗p < 0.05. Data are representative of three independent experiments. (F) Virus titers in the lungs are expressed as PFU/g tissue. n = 3 mice/group/time point. ^∗p < 0.05. Data are representative of two independent experiments. Error bars in (A), (B), and (D)–(F) represent SEM.