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. 2016 Mar 15;44(3):647-658.
doi: 10.1016/j.immuni.2016.02.006. Epub 2016 Mar 2.

Gut Microbiota-Induced Immunoglobulin G Controls Systemic Infection by Symbiotic Bacteria and Pathogens

Melody Y Zeng  1 , Daniel Cisalpino  2 , Saranyaraajan Varadarajan  1 , Judith Hellman  3 , H Shaw Warren  4 , Marilia Cascalho  5 , Naohiro Inohara  1 , Gabriel Núñez  6
Affiliations

Gut Microbiota-Induced Immunoglobulin G Controls Systemic Infection by Symbiotic Bacteria and Pathogens

Melody Y Zeng et al. Immunity. .

Abstract

The gut microbiota is compartmentalized in the intestinal lumen and induces local immune responses, but it remains unknown whether the gut microbiota can induce systemic response and contribute to systemic immunity. We report that selective gut symbiotic gram-negative bacteria were able to disseminate systemically to induce immunoglobulin G (IgG) response, which primarily targeted gram-negative bacterial antigens and conferred protection against systemic infections by E. coli and Salmonella by directly coating bacteria to promote killing by phagocytes. T cells and Toll-like receptor 4 on B cells were important in the generation of microbiota-specific IgG. We identified murein lipoprotein (MLP), a highly conserved gram-negative outer membrane protein, as a major antigen that induced systemic IgG homeostatically in both mice and humans. Administration of anti-MLP IgG conferred crucial protection against systemic Salmonella infection. Thus, our findings reveal an important function for the gut microbiota in combating systemic infection through the induction of protective IgG.

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Figures

Figure 1
Figure 1. Gut Microbiota Induces Antigen-Specific IgG in the Steady State
(A) ELISA of serum IgG, IgA, and IgM against fecal bacteria (FB) in naive SPF JH−/− and WT mice and GF WT mice. 6–10 mice were used for each genotype. (B) ELISA of serum IgG1, IgG2c, IgG2b, and IgG3 against fecal bacteria in 6- to 8-week-old naive SPF WT mice. Six WT mice were used. (C) ELISA of serum IgG, IgA, and IgM against fecal bacteria in 6- to 8-week-old WT and Tcrb−/− naive mice. 6–10 mice were used for each genotype. (D) ELISA of serum IgG against fecal bacteria of 4-, 6-, 10-, and 40-week-old mice. (E) Peritoneal B1 and B2 cells and splenic marginal zone (MZ) and follicular (FO) B cells were stimulated ex vivo with LPS, heat-killed fecal bacteria, or E. coli for 3 days, and cells producing IgG that recognized fecal bacteria were detected by ELISpot. Data represent two to three independent experiments. Error bars indicate SD. *p < 0.05, **p < 0.01, ***p < 0.001. See also Figure S1.
Figure 2
Figure 2. Spontaneous Systemic Dissemination of Gram-Negative Symbiotic Bacteria
(A) qPCR of bacterial 16S rRNA gene in the spleens of SPF and GF WT mice. Each dot represents one mouse. (B) Compositions of bacterial communities in mesenteric lymph nodes (MLNs), spleens (SPL), and fecal bacteria (FB) of same WT SPF mice. (C) Abundance of gram-negative Enter-obacteriaceae, Porphyromondaceae, and Pre-votellaceae in MLNs, spleens, and feces from WT SPF mice. Each dot represents one mouse. (D) Immunoblotting of bacterial antigens from fecal bacteria (FB), Escherichia coli (EC), Klebsiella pneumoniae (KP), Enterococcus faecalis (EF), and Clostridium bifermentans (CB) that were recognized by IgG in the serum of a 4-, 6-, or 10-week-old SPF WT mouse. A SDS-PAGE gel was separately stained with Coomassie blue to indicate total amounts of cell lysates loaded. Data are representative of two to three independent experiments. Error bars indicate SD. *p < 0.05, **p < 0.01, ***p < 0.001.
Figure 3
Figure 3. Gut Microbiota-Induced IgG Confers Protection against DSS-Induced Bacteremia
(A) ELISA of fecal IgG, IgA, and IgM against fecal bacteria in feces of naive (mock) or day 7 DSS-treated mice (mock = 5 mice; day 7 DSS = 7 mice). (B) Flow cytometry for IgG on fecal bacteria from naive (mock) or day 7 DSS-treated mice. (C) WT and JH−/− mice were treated with DSS for 7 days and CFU of aerobes and anaerobes in the blood were determined. Each dot represents one mouse. (D) Survival of WT and JH−/− mice, treated with or without antibiotics, after administration of 2.5% DSS in drinking water for 7 days. (E) Percentages of bacterial isolates from spleens, livers, and blood of day 7 DSS-treated JH−/− mice. (F) WT and JH−/− mice were i.p. injected with 5 × 107 CFU of ECM6L4 (an isolate from a DSS-treated JH−/− mouse), and E. coli was recovered from the peritoneum 6 hr after infection and analyzed for IgG or IgA coating by FACS. (G) Survival of WT, JH−/−, and Fcer1g−/− mice after i.p. injection with 107 CFU of M6L4 (WT n = 9; JH−/− n = 8; Fcer1g−/− n = 8). (H) Survival of untreated JH−/− mice or JH−/− mice that were administered 400 mg of purified serum IgG from WT or QM mice 24 hr prior to i.p. infection with 107 CFU of ECM6L4 (JH−/− n= 6; JH−/− +WT IgG n = 7; JH−/− +QM IgG n = 6). Data represent two to three independent experiments. Error bars indicate SD. *p < 0.05, **p < 0.01, ***p < 0.001. See also Figure S2.
Figure 4
Figure 4. TLR4 Signaling Is Required for Induction of Microbiota-Specific IgG
(A) Serum concentrations of IgG against fecal bacteria in age-matched WT, Tlr2−/−, Tlr2−/−Tlr4−/−, Nod1−/−Nod2−/−, and Nlrp3−/− mice (n = 4–8 per genotype). (B) Serum concentrations of IgG, IgA, and IgM against fecal bacteria in age-matched and cohoused WT and Tlr2−/−Tlr4−/− mice (n = 8–10 per genotype). (C) Immunoblotting for IgG in sera from age-matched WT and Tlr2−/−Tlr4−/− mice that bound to gram-negative C. rodentium (CR), Salmonella (ST), E. coli (EC), and K. pneumoniae (KP). Coomassie-stained SDS-PAGE gels loaded with lysates are shown. (D) Serum concentrations of IgG, IgA, and IgM against fecal bacteria in age-matched and co-housed WT, Myd88−/−, Trif−/−, and Myd88−/−Trif−/− mice. (E) Survival of WT and Tlr2−/−Tlr4−/− mice and of Tlr2−/−Tlr4−/− mice after treatment of 2.5% DSS for 7 days (WT n = 9; Tlr2−/−Tlr4−/− n = 7). (F) Survival of WT and Tlr2−/−Tlr4−/− mice and of Tlr2−/−Tlr4−/− mice that were administered 400 mg of purified serum IgG from WT or QM mice 24 hr prior to i.p. infection with ECM6L4 (WT n = 5; Tlr2−/−Tlr4−/− n= 6; Tlr2−/−Tlr4−/− +WT IgG n = 6; Tlr2−/−Tlr4−/− +QM IgG n = 5). (G) Serum concentrations of IgG, IgA, and IgM against fecal bacteria in co-housed bone marrow chimeras that were transplanted with JH−/−, WT+ JH−/−, or Tlr2−/− Tlr4−/− + JH−/− bone marrow cells. (H) Peritoneal B1 and B2 cells, and splenic marginal zone (MZ) and follicular (FO) B cells from WT or Tlr2−/−Tlr4−/− mice were stimulated ex vivo with heat-killed fecal bacteria for 72 hr before analysis by ELISpot to determine the number of cells secreting IgG that recognized fecal bacteria. Data represent two to three independent experiments. Error bars indicate SD. *p < 0.05, **p < 0.01, ***p < 0.001. See also Figure S3.
Figure 5
Figure 5. Gram-Negative Murein Lipoprotein Is a Major Microbiota-Derived Antigen to Induce Steady-State IgG Response
(A) Immunoblotting for serum IgG from a 10-week-old WT naive mice that bound to E. coli total cell lysate, cytosolic membrane proteins (CM), and outer membrane proteins (OM). (B) Reactivity of serum IgG in WT SPF or GF mice to WT or MLP-deficient E. coli (ΔMLP). Eight WT SPF mice were used; three GF mice were used. (C) Reactivity of serum IgG in a WT SPF mouse to particles in culture supernatants from WT or ΔMLP E. coli. Eight WT SPF mice were used. (D) Immunoblotting for IgG in the serum from a 6-week-old WT SPF mouse that bound to bacterial antigens from gram-negative C. rodentium (CR), Salmonella (ST), E. coli (EC), and K. pneumoniae (KP) and gram-positive E. faecalis (EF) and C. bifermentans (CB). Immunoblotting was performed separately for MLP using an anti-MLP monoclonal antibody. (E) ELISA of human serum IgG against fecal bacteria (n = 5). (F) Binding of human serum IgG to WT or MLP-deficient E. coli. Each dot represents one person. (G) Immunoprecipitation of MLP from WT E. coli lysate by anti-MLP or isotype and immunoblotting with human serum or anti-MLP. Data represent two to three independent experiments. Error bars indicate SD. *p < 0.05, **p < 0.01, ***p < 0.001. See also Figure S4.
Figure 6
Figure 6. Anti-MLP IgG Promotes Killing of E. coli In Vitro and In Vivo
(A) Neutrophil killing of WT or ΔMLP E. coli that were opsonized with PBS, WT, or JH−/− serum (fold of killing over killing of non-opsonized E. coli). (B) 5 × 107 CFU WT or ΔMLP E. coli were i.p. injected into 8-week-old WT mice and harvested 6 hr later to analyze surface IgG or IgA by flow cytometry. (C and D) WT mice of 6–8 weeks were i.p. injected with 5 × 107 CFU WT and or ΔMLP E. coli. The absolute numbers of monocytes/macrophages (CD11b+LY6C+LY6Glo), neutrophils (CD11b+LY6Ghi), T cells (CD3+), and B cells (B220+) were determined by flow cytometry at 24 hr after infection (C). Bacterial numbers of E. coli in spleens and livers were determined at 24 hr (D). Each dot represents one mouse. (E) WT and ΔMLP E. coli were incubated with anti-MLP or isotype-matched control IgG for 30 min. The bacterial numbers of IgG-coated bacteria were determined by flow cytometry. (F) Survival of JH−/− mice that were administered 1 mg of monoclonal anti-MLP IgG or isotype 24 hr prior to i.p. infection with 107 of ECM6L4 (isotype n = 7; anti-MLP n = 10). Data represent two to three independent experiments. Error bars indicate SD. *p < 0.05, **p < 0.01, ***p < 0.001. See also Figure S5.
Figure 7
Figure 7. IgG against Murein Lipoprotein Confers Protection against Salmonella Infection
(A–C) GF mice were i.p. administered PBS or 107 WT or ΔMLP E. coli. 2 weeks later, the concentrations of serum IgG, IgA, and IgM against fecal bacteria were determined by ELISA (A), and the presence of anti-MLP IgG in the sera of these mice were also confirmed by immunoprecipitation of MLP from WT E. coli lysates and immunoblotted using sera from these mice or monoclonal anti-MLP (B). 2–3 weeks after administration of PBS or 107 CFU of WT and or ΔMLP E. coli, GF mice were i.p. infected with 104 CFU of Salmonella M525P and the levels in spleens and livers were determined at 72 hr (C). (D and E) Salmonella were incubated with anti-MLP IgG or isotype for 30 min, and then assessed for surface IgG coating (D) and killing by neutrophils in vitro (E). 6- to 8-week-old WT mice were i.p. administered 1 mg of anti-MLP IgG or isotype for 24 hr before i.p. infection with 4 × 104 ST M525P. CFU of ST M525P in spleens and livers were determined at 72 hr. Data represent two to three independent experiments. Error bars indicate SD. *p < 0.05, **p < 0.01, ***p < 0.001. See also Figure S6.
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