| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Previous Article | Next Article ![]()
Clinical and Vaccine Immunology, March 2008, p. 412-417, Vol. 15, No. 3
1071-412X/08/$08.00+0 doi:10.1128/CVI.00373-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Division of Infectious Diseases, Department of Medicine, Helsinki University Central Hospital, Helsinki, Finland,1 Department of Bacteriology and Immunology, Haartman Institute, University of Helsinki, Helsinki, Finland,2 National Public Health Institute, Turku, Finland,3 Department of Pediatrics, Turku University Central Hospital, Turku, Finland,4 Department of Medical Microbiology and Immunology, University of Turku, Turku, Finland5
Received 10 September 2007/ Returned for modification 19 November 2007/ Accepted 17 December 2007
| ABSTRACT |
|---|
|
|
|---|
| INTRODUCTION |
|---|
|
|
|---|
UTIs are mostly of the ascending type, and therefore, the first defense line the pathogen encounters is the local immune system on the mucosa of the urinary tract, especially urinary antibodies. It has been shown that local antibodies can bind to the infecting bacteria and prevent their adherence to mucosal surfaces (29, 31, 33, 34) and the following infection. Accordingly, in animal experiments, local immunization eliciting local antibodies has been shown to protect against disease (36, 37). Recently, in adults, a vaginally administered vaccine has proven to be protective against reinfections in phase II trials (9, 35). If a urinary pathogen is able to cross the mucosal barrier, it must be cleared by the systemic immune mechanisms. It appears that an optimal defense state would involve both local and systemic immunity. The fact that 75% of previously infected women do not become reinfected (7) indicates that the immune mechanisms in patients experiencing their first episode of UTI may prove worth mimicking in developing vaccines against UTI. The view that local immunity is most effectively induced by local immunization (21) appears to apply to the genitourinary tract as well, as suggested by the protective efficacy of the vaginally administered vaccine against UTI (9, 35).
Knowledge on the human immune defense against UTI in children is sparse. Specific antibody responses to the infecting pathogen have been observed both in sera and in urines from both adults and children with UTIs (6, 17, 18, 20, 22, 25, 26, 28), with the levels being considerably lower in children than in adults. The local immune response to UTI has been investigated in adults by examining urinary tract-originating, newly recruited, pathogen-specific B cells in the peripheral blood. These cells are expected to be on their way back to the urinary mucosa to secrete antibodies locally (17, 18). These cells have proven to have unique homing potentials, as interpreted by their expression of homing receptors (Kantele et al., unpublished results). The present study was undertaken to look for circulating antibody-secreting cells (ASC) specific to urinary pathogens in children with UTIs. The results are expected to provide information on the immune response that will be useful in the development of vaccines against UTIs in children.
| MATERIALS AND METHODS |
|---|
|
|
|---|
Patients. Thirty-seven children (24 girls and 13 boys; aged 3 weeks to 15.4 years) suffering from PN caused by Escherichia coli participated in the study. None of the children had any known underlying diseases. One patient had a previous history of UTI. PN was diagnosed by the following criteria: typical clinical symptoms, rectum temperature over 38°C, C-reactive protein level of >40 mg/ml or blood leukocytosis of >10 x 109 cells/ml, leukocyturia (>10/µl), and a pathogenic bacterium isolated from urine (at least 105 CFU/ml) taken at the time of admission to the hospital. The urine samples for microbiological analysis were collected with help of a urine collection bag in four cases and by suprapubic aspiration in 33 cases. The E. coli strains of 23 patients were found to be P fimbriated (P+), as judged by a commercial P fimbria latex test carried out on the isolated E. coli (Orion Diagnostica, Helsinki, Finland).
Healthy laboratory personnel (five women, aged 18 to 32 years) volunteered as controls.
Renal ultrasonography (RUS) was performed on all 37 patients within 1 week of diagnosis. Voiding cystourethrography was performed on 30 patients approximately 1 month after UTI, and 99mTc-dimercaptosuccinic acid (DMSA) scanning was performed on 28 patients 6 to 18 months after the infection.
The study was approved by the ethics committees of the participating hospitals and institutions. Informed consent was obtained from parents of all patients. Human experimentation guidelines of the U.S. Department of Health and Human Services and those of the authors' institutions were followed in the conduct of clinical research.
Collection of blood samples. Samples of blood were collected in the acute and in the convalescent phase of the disease. The difficulty of getting acute-phase samples at comparable stages of the disease (because the onset of symptoms varies and patients seek medical care after various times) was solved by taking two "acute-phase" samples. In order to catch the peak (or near the peak) of the ASC response, which according to our studies on oral vaccines (13, 14) is expected 7 days after antigen exposure, the sampling was carried out as follows. The acute-phase I specimens were collected 1 to 3 days after admission to the hospital, acute-phase II specimens were collected 1 week later, and convalescent-phase samples were collected 3 to 7 weeks after the onset of the disease. For patients not seeking medical care until 1 week after the onset of symptoms, only one acute-phase sample was taken. The definition "acute-phase sample" is later used for that acute-phase sample that yielded the larger number of ASC. Four healthy volunteers (all females, aged 23 to 38 years) belonging to the laboratory personnel served as controls. All blood samples were assayed by enzyme-linked immunospot (ELISPOT) assay immediately after they were drawn.
Preparation of antigen. The following two types of antigen preparations were used: a preparation of the E. coli strain isolated from each patient's own urine and a preparation of isolated P fimbria.
Each patient's own E. coli strain was identified with conventional methods, grown on nutrient agar plates, suspended in phosphate-buffered saline (PBS; pH 7.4), and formalin killed (formaldehyde was added to a final concentration of 1%). The concentration of bacteria in the stock solution was estimated by the McFarland standard, and the bacteria were diluted to a final concentration of 108 bacteria/ml PBS.
The P fimbria isolate used in the ASC assay was obtained from Auli Pere (Department of General Microbiology, University of Helsinki). It had been isolated from strain IH 11086. This strain is of serotype O4:K12:H1 and was originally isolated from a patient with PN (24). The method of isolation of the P fimbria (19) and a detailed description of the IH 11086 strain (24) have been published elsewhere.
Isolation of mononuclear cells. Mononuclear cells containing mainly lymphocytes were obtained by Ficoll-Paque (Pharmacia, Uppsala, Sweden) centrifugation of heparinized venous blood. The isolated cells were washed three times and suspended in culture medium to a concentration of 2 x 106 cells/ml as described earlier (12).
Assay of specific ASC. Specific ASC were enumerated by ELISPOT assay (5, 12, 17, 18, 27). In this assay, isolated lymphocytes are allowed to secrete antibodies into microtiter plate wells previously coated with the antigen of interest. The secreted antibodies react with the antigen in the immediate vicinity of the secreting cell. These antibodies are visualized with enzyme-conjugated antisera followed by a substrate overlay in agarose; the latter immobilizes the decaying substrate and turns the area of antibody into a colored spot.
The plates (Maxisorp; Nunc, Copenhagen, Denmark) were coated by incubating a solution of whole bacteria prepared from each patient's own E. coli strain (see above) or a preparation of P fimbria at a concentration of 5 µg/ml PBS in the wells for 2 h at 37°C or overnight at 20°C. The next steps were described in detail earlier (12, 17, 18). Briefly, the nonspecific binding sites were blocked with 1% bovine serum albumin-PBS (30 min, 37°C), and then 105 cells in culture medium were incubated in each well (2 to 3 h, 37°C). The antibodies secreted during this time were detected with alkaline phosphatase-conjugated antisera (diluted 1:100 in 1% bovine serum albumin-PBS; Orion Diagnostica, Helsinki, Finland) followed by the substrate in 50°C agarose. For each patient, a total of 96 wells were studied, which means that 1.6 million cells were studied for each antigen-immunoglobulin (Ig) isotype combination.
Statistical methods. The mean numbers of ASC are given as geometric means ± standard errors of the means (SEM). In order to be able to calculate the geometric means, zero values were replaced by the value 1. The significance of differences in the ASC responses was analyzed with Student's t test, using logarithmically transformed data. A person was regarded as a responder when having more than two specific ASC/106 cells. Nonresponders were included in all of the analyses as if they had one specific ASC/106 peripheral blood mononuclear cells (PBMC) in each Ig class or a total of three ASC/106 PBMC (IgA plus IgG plus IgM).
| RESULTS |
|---|
|
|
|---|
Imaging studies and recidivism. RUS was abnormal for 6 (16%) of the 37 patients. All abnormalities were mild to moderate hydronephrosis (posteroanterior diameter of >10 mm). Two of these patients had vesicoureteral reflux (VUR). None had obstructive uropathy. DMSA scans were abnormal for 9/28 (32%) patients, including 5 patients with VUR. Voiding cystourethrography was not performed on two patients with abnormal DMSA scans. Seven (19%) of the 37 patients had recidivist infections. Three of them had abnormal DMSA scans, one had VUR, and one had abnormal RUS.
ASC specific to each patient's own E. coli strain. In the acute phase (less than 2 weeks after the estimated onset of the disease), pathogen-specific ASC were found in the blood of 33/37 children with PN. All four nonresponders were under 1 year old. The number of ASC was found to increase with increasing age (Fig. 1). The geometric mean (±SEM) of the ASC response was 63 ± 2,135 ASC/106 PBMC. In 18/33 (54.5%) cases, the response was dominated by IgM-secreting ASC, while IgA dominated in 15/33 (45.5%) cases and none of the patients had an IgG-dominated response. When the patients were divided into two groups according to age, the IgM dominance was found to be more frequent among children under 2 years (62.5%) than among those over 2 years (30%). For IgM-ASC, the geometric mean (±SEM) response of all patients was 19 ± 246 ASC/106 PBMC, that for IgA-ASC was 16 ± 1,348 ASC/106 PBMC, and that for IgG-ASC was 6 ± 815 ASC/106 PBMC (Fig. 2). Figure 2 shows separately the Ig isotype distributions for patients under and over 2 years.
|
|
|
Since the present study was planned to explore the immune response to UTI in children of various ages, it revealed the increase in numbers of ASC with increasing age, but the number of patients of each age remained too low to allow a further statistical comparison between the numbers of ASC and the numbers of recidivist infections, signs of renal scarring, or presence of VUR. Of those with a response exceeding 10 ASC/106 PBMC, 5/24 had recidivist infection and 6/17 had signs of renal scarring. If the group with recidivism was compared to that with no recidivism, the geometric means of ASC responses did not differ from each other (58 versus 89 ASC/106 PBMC; P = 0.98), nor did those of the groups with signs of or no signs of renal scarring in DMSA scans (45 versus 208 ASC/106 PBMC; P = 0.19).
ASC specific to P fimbria. ASC specific to P fimbria were found in the blood of 18/23 patients with a P-fimbriated E. coli strain (P+) and in 12/14 patients with a non-P-fimbriated E. coli strain (P–) (Fig. 4). The geometric means (±SEM) of the P fimbria-specific ASC responses were 16 ± 52 and 9 ± 4 ASC/106 PBMC for the P+ and P– groups, respectively. For 13/18 P+ patients, a response was seen by more than one isotype: in most cases, the response was dominated by IgM; in 4/18 cases, it was dominated by IgA; and in 1 case, it was dominated by IgG. Among the P– patients, 11/12 patients had only an IgM response and 1/12 patients had only an IgA response.
|
| DISCUSSION |
|---|
|
|
|---|
A response was found in 33/37 children with upper UTIs caused by E. coli. In our recent study, we found a response in 20/20 adult patients. The magnitude of the response in children (63 ± 2,135 ASC/106 PBMC) was found to be significantly lower than that observed in adults (1,643 ± 3,272 ASC/106 PBMC) (P < 0.001). The findings of nonresponding cases and lower magnitudes of responses than those of adults are suggested to be due to the less developed state of the immune system in children. This is supported by our notion that the magnitude of the response in children increased with increasing age. In fact, all of the nonresponding children were infants. A slow maturation of the urinary IgA system with age was also suggested by previous studies (30) showing lower amounts of total IgA in the urines of children than in those of adults. While these studies suggest a slower development of mucosal immune responses in the urinary tracts of children, previous studies by others have shown low systemic responsiveness in children, as measured by urinary IgG, serum antibodies (30), and interleukin-6 (2). The slow development of the immune system may account for the increased number of UTIs in infants compared to older children. Moreover, it certainly is a fact that needs to be considered in determining the timing of administration of possible future UTI vaccines for children.
The general pattern of the ASC response in children was found to be similar to that observed for adults: pathogen-specific ASC were observed in the acute phase of the disease, and a decline in their number was seen in the convalescent phase. However, the Ig isotype distribution of ASC in children was found to differ significantly from that for adults, even if both the previous study with adults and the present study with children examined the ASC response after the first episode of UTI. In adults, the dominating Ig class is IgA in most cases, and both the IgA- and IgG-ASC responses are more vigorous in magnitude than the IgM-ASC response (P < 0.001 for both) (18). In children, the responses were dominated by IgM in most cases. This difference in isotype distribution combined with the significantly lower responses observed in children accounts for the fact that the absolute numbers of IgM-ASC did not differ statistically between adults and children, even if a significant difference was found in the total response (IgA- plus IgG- plus IgM-ASC) (P < 0.003) as well as in IgA- and IgG-ASC responses (P < 0.003 and P < 0.002, respectively). In adults, the low IgM-ASC response appears to be a typical feature of the urinary tract (18), since the IgM-ASC response has been shown to be almost as prominent as the IgA-ASC response in adults when the antigen is administered via another mucosal surface, the gastrointestinal tract (13-15), even after booster immunization (16). Earlier studies have suggested that IgM might have only a minor role in the local immune system of the urinary tract (10), as IgM is rarely, if ever, present in the urine of healthy humans (8). Large macromolecules such as IgM do not usually pass the glomerular filter under normal conditions (32), and increased urine IgM is usually regarded as a predictor of impaired renal function (1). An increase in urinary IgM can be seen occasionally in UTI (28). The high IgM-ASC response could reflect a more systemic nature of the infection in children than in adults, and IgM-ASC could belong to the systemic part of the total ASC response, which is composed of local and systemic responses combined, as suggested previously (18).
We have shown recently for adults that an ASC response to P fimbria is elicited in UTI patients with a P+ E. coli strain, while those with a P– E. coli strain have no response (17). In the present study, 18/23 patients with a P+ E. coli strain had a P fimbria-specific ASC response. However, 12/14 patients with a P– E. coli strain also exhibited a response. In 11/12 cases, the latter was only an IgM response. Those with the highest responses tended to belong to the P+ group, yet most of the responses in both groups were low, with no statistical difference between the groups. The observation of a few significant P fimbria-specific ASC responses in P– patients can be explained partly by a possible failure to detect P fimbria with the latex test, as it is known that E. coli cells are subject to fimbrial phase variation in vivo in the urinary tract (23, 24): some strains have been shown to have a predominantly P fimbria-positive phase (e.g., 95%) in urine samples and a poor expression of P fimbriae (e.g., 1%) after growth on agar plates (23, 24). Unfortunately, the bacterial strains of the present study could not be tested for pap (3), the operon for P fimbria, as the strains were no longer available. Regarding the patients discussed above, the possibility exists that another, undetected P+ E. coli strain was simultaneously present in the urinary tract. A third explanation is a nonspecific stimulation of the immune system, as suggested by the increased numbers of Ig-secreting cells: in the course of a serious infection, the numbers of Ig-secreting cells far exceed the numbers of pathogen-specific ASC, as shown in our previous studies of adults with diarrhea (15) or PN (18). Polyclonal stimulation has been suggested as an explanation for the phenomenon (17, 18), and in fact, small numbers of ASC specific to an irrelevant antigen, trinitrophenyl, have also been found (18). In the present study, cells originally encountering a P-fimbriated E. coli strain in the normal microbial flora of the child's intestine could have been activated polyclonally.
Renal scarring may be associated with recurrences of PN more frequently in children than in adults. The present study revealed two major differences in the ASC response in children compared to that in adults, i.e., the lower magnitude of the response and the IgM dominance. Even if the data in the present study are not sufficient to allow such an analysis, in the future it would be interesting to determine if there exists a connection between a low ASC response or IgM dominance and renal scarring.
We conclude that the ASC assay can be used to measure the immune response to UTI in children. The response in children resembles that of adults, but it is lower in magnitude and the Ig isotype distribution of the response is different. IgM is the dominating Ig class and seems to have a more significant role than IgA in the defense against UTI in childhood.
| ACKNOWLEDGMENTS |
|---|
This work was supported by a special Finnish governmental subsidy for health sciences research and by the Finnish Medical Association.
We do not have a commercial or other association that might pose a conflict of interest.
| FOOTNOTES |
|---|
Published ahead of print on 9 January 2008. ![]()
| REFERENCES |
|---|
|
|
|---|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Antimicrob. Agents Chemother. | Clin. Microbiol. Rev. | Infect. Immun. |
|---|---|---|
| J. Clin. Microbiol. | J. Virol. | ALL ASM JOURNALS |