Natural killer cell-mediated host defense against uropathogenic E. coli is counteracted by bacterial hemolysinA-dependent killing of NK cells

Abstract

Uropathogenic Escherichia coli (UPEC) are a common cause of urinary tract infections (UTIs) in humans. While the importance of natural killer (NK) cells in innate immune protection against tumors and viral infections is well documented, their role in defense against bacterial infections is still emerging, and their involvement in UPEC-mediated UTI is practically unknown. Using a systematic mutagenesis approach, we found that UPEC adheres to NK cells primarily via its type I fimbriae and employs its hemolysinA toxin to kill NK cells. In the absence of hemolysinA, NK cells directly respond to the bacteria and secrete the cytokine TNF-α, which results in decreased bacterial numbers in vitro and reduction of bacterial burden in the infected bladders. Thus, NK cells control UPEC via TNF-α production, which UPEC counteracts by hemolysinA-mediated killing of NK cells, representing a previously unrecognized host defense and microbial counterattack mechanism in the context of UTI.

Figure 1. UPEC Kill Human NK Cells

(A) Flow cytometry analysis of various GFP-expressing strains of E. coli (indicated above the quadrants) incubated with primary human NK cells and stained with PI (y axis). The bacterium to NK ratio was 30:1, and incubations were performed for 3 hr either at 4°C (upper quadrants) or at 37°C (lower quadrants). The percentages of NK cells in each quadrant are indicated. (B) Forward and side scatter analysis of NK cells incubated with or without UPEC CFT073, at 4°C or at 37°C (indicated inside the quadrants). Gate 1 shows live human NK cells, and gate 2 shows the nonviable ones. (C) Ratios ×100 of viable NK cells present in gate 1 following 3 hr of incubation with the indicated bacteria at 37°C were divided by the number of NK cells incubated with the same bacteria for 3 hr at 4°C. (D) Percentages of PI-positive NK cells present in gate 2 following 3 hr incubation at 37°C with the various GFP expression E. coli strains, indicated in the x axis. (E) Confocal microscopy of PI-stained human NK cells incubated with different GFP expressing E. coli strains for 3 hr at 37°C. The different bacteria are indicated by green staining, DAPI is in blue, and PI is in red. Magnification ×1,800. (F) Confocal microscopy, SEM, and TEM of human NK cells incubated with UPEC CFT073 for 3 hr at 37°C. The bacterium to NK cells ratio was 30:1. The bacteria seen in the confocal microscopy are indicated by green staining, PI is in blue, and the anti-CD56 staining is in red. The TEM and SEM pictures are marked. (G) Shown are percentages of PI-positive NK cells following incubation of NK cells at 37°C with the indicated bacteria at various time points (indicated in the x axis). The bacterium to NK cells ratio was 30:1. (H) Percentages of PI-positive NK cells following 3 hr incubation at 37°C with the indicated E. coli strains at several bacteria to NK cells ratios (indicated at the x axis). Figures show one representative experiment out of at least three independent experiments performed and an average ±SD of duplicate in (C), (D), (G), and (H). For (C) and (D), *p < 0.00001 for UPEC CFT073 versus all other bacteria. For (G) and (H) *p < 0.01 for UTI536 versus all other bacteria. 25922 represent ATCC 25922. See also Table S1 and Figure S1.

Figure 2. UPEC CFT073 Preferentially Kills NK Cells

(A) Adhesion of UPEC CFT073 to human NK and T cells was determined by percentages of GFP-positive cells following 3 hr incubation at 4°C. (B) Killing of human NK and T cells following 3 hr incubation at 37°C with UPEC CFT073 was determined by the percentages of cells stained with PI. Figures show a representative of three independent experiments, and an average ±SD of duplicate is presented. *p < 0.0005.

Figure 3. Transposon-Based Mutagenesis Identifies Type I Fimbriae and HemolysinA

(A) More than 1,000 GFP-expressing UPEC CFT073 mutants were screened for adhesion to primary human NK cells at 4°C (A and C, determined by percentages of GFP-positive NK cells) and for killing of human NK cells at 37°C (B and D determined by PI-positive NK cells). B5, an irrelevant mutant of UPEC CFT073 was used as a control. Data are representative of at least three independent analyses of the relevant mutants. Data are represented as mean ± SEM. *p < 0.001. (E) Table summarizing the transposon location in the disrupted genes of relevant mutants. The adhesion mutants are in white, the hemolysinA mutants are in dark gray, and the irrelevant mutant B5 (bottom) is in light gray. See also Figure S2.

Figure 4. Increased Type I Fimbriae-Mediated Attachment Leads to an Increase in Killing of Human NK Cells

(A) Percentages of binding of human NK cells following 3 hr of incubation with GFP-expressing bacteria indicated in the x axis. Binding was performed at 4°C and was calculated by the percentages of GFP-positive NK cells. *p < 0.0001 for Fim on versus all other bacteria, **p < 0.01 for B5 and PAP– versus A15, D33, and Fim off. (B and C) Killing of human NK cells at 37°C following incubation with the GFP-labeled bacteria, indicated in the legend, at several time points (B) and at various bacteria to NK cells (E:T) ratios (C). The percentages of NK cell killing by the various bacteria were determined by PI staining. Data are a representative of three independent experiments and an average ± SD of duplicate is shown. For (B) and (C), *p < 0.001 for Fim on bacteria versus all other bacteria.

Figure 5. Hemolytic Bacteria Kill NK Cells

(A) Hemolysis of sheep RBCs was mediated by CFT073, ATCC 25922, UTI89, UTI536, and SR71 UPEC strains streaked on blood agar plates. (B) Quantification of sheep RBCs hemolysis (represented in the y axis by O.D) induced by UPEC CFT073, the Fim on and Fim off mutants, and the UPEC UTI536 strains. RBCs treated with 1% triton were used as a positive control, and treatment with RPMI (phenol red minus) was used as a negative control. *p < 0.01 for UTI536 and 1% triton versus all other bacteria. (C) Hemolysis was mediated by the hemolysinA mutants (C93, D57, and E49, upper panels) and by the complemented hemolysinA bacteria (C93C, D57C, and E49C, lower panels). (D and E) Killing of NK cells by wild-type UPEC CFT073 bacterium, by the hemolysinA mutants (C93, D57, and E49) and by the complemented bacteria (C93C, D57C, and E49C) at 37°C, determined either by flow cytometry (D) or by using confocal microscopy (E). One of three (A, C, and E) or four (B and D) independent experiments is shown. An average ±SD of duplicate is shown in (B) and (D). In (D), *p < 0.0001 for UPEC CFT073 and E49C, D57C, and C93C versus E49, D57, and C93. See also Figure S3.

Figure 6. NK Cells Accumulate in the Bladder following UPEC CFT073 Infection

(A) Percentages of PI-positive murine NK cells incubated at 37°C for 3 hr with wild-type hemolytic (CFT073, ATCC 25922, UTI89, UTI536) and nonhemolytic (XL1, SR71) UPEC bacteria, with the irrelevant mutant B5 or with the hemolysinA-deficient mutants (C93, D57, and E49). *p < 0.005 for CFT073, B5, ATCC 25922, UTI89, and UTI536 versus all other bacteria. (B) Number of GFP-positive NK cells in the bladder on day 1 and on day 2 following infection with UPEC CFT073 or with the hemolysinA-deficient C93 mutant. Injection of PBS was used as control (five to eight mice were used in each group). *p < 0.005 for NK cells present in the bladders injected with C93 on day 2 versus day 1 and versus day 2-injected UPEC CFT073 bladders. (C) Representative confocal microscopy images of uroepithelial cells obtained from murine bladders injected with PBS (top), with wild-type UPEC CFT073 (middle), or with the hemolysinA-deficient mutant C93 (bottom). NK cells are visualized by GFP. Left panels depict merge staining of DAPI-stained bladder cells with the GFP-labeled NK cells; right panels show the NK cells only. For (A) and (B), data are a representative of three independent experiments, and an average ±SD of triplicate is shown. Slides for confocal microscopy were prepared from four to eight bladders of mice in each group and were taken from two independent experiments on day 2 of UTI induction. 25922 stands for ATCC 25922. See also Figure S4.

Figure 7. NK Cells Secrete TNF-α to Protect against UTI

(A) ELISA assays for TNF-α secretion (concentration represented as picogram/ml, pg/ml) from NK cells following 5 hr of incubation with the hemolytic (CFT073, ATCC 25922, UTI89, UTI536) and nonhemolytic (XL1, SR71) UPEC bacteria at 37°C. *p < 0.005 for XL1 versus all other hemolytic bacteria. Data are represented as mean ± SEM. **p < 0.01 for SR71 versus all other hemolytic bacteria. (B) FACS staining of UPEC CFT073, the hemolysinA-deficient mutant C93, the irrelevant mutant B5, PD1.6 and YAC-1 cells with NCR1-Ig, and with the NKG2D-Ig fusion proteins (staining is indicated by the black empty histograms). The gray-filled histogram represents staining with control Ig-fusion protein. (C) ELISA assays for TNF-α secretion from murine NK cells following 5 hr of incubation with the irrelevant mutant B5 and the hemolysinA-deficient mutant C93. NK cells incubated with the C93 bacterium were coincubated with or without blocking anti-TLR-4 mAb. Data are represented as mean ± SEM. *p < 0.01 for C93 in the absence of anti-TLR-4 mAb versus C93 in the presence of anti-TLR-4 mAb. (D) Numbers of GFP-positive NK cells in the blood, kidneys, and bladders of mice treated with sterile PBS only or injected with the hemolysinA-deficient mutant C93 with or without NK cell depletion using anti-NK1.1 mAb. *p < 0.01 for mice injected with the bacteria versus no bacteria. Data are represented as mean ± SEM. **p < 0.0001 for NK1.1-treated mice versus all other mice. ***p < 0.001 for bacteria in the bladders following UTI versus no bacteria or versus NK cell depletion. (E) Schematic representation of the in vivo experiment. The catheter in the figure indicates the day of UTI induction. (F and G) Bacterial loads in the bladders (F) and kidneys (G) as determined by colony-forming units (CFU) on day 2 following bacterial injection. Mice were treated with anti-NK1.1 (NK1.1), with Etanercept (TNF-α), or with PBS as described in (E) and infected with UPEC CFT073, the mutant B5, or the hemolysinA-deficient C93 mutant. Data are represented as mean ± SEM. *p < 0.05 for mice inoculated with C93 only versus all other injections. (H) ELISA assays for the presence of TNF-α in the infected bladders derived from (F). Three to five bladders were obtained from each group. Data are represented as mean ± SEM. **p < 0.01 for C93-injected treated mice versus all other mice. (I) TEM was performed 2 days following C93 injections into the bladder. Data in (A)–(D) and (I) is a representative of two independent experiments. The data presented in (F)–(H) is a summary of two independent experiments. See also Figure S5.