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Clinical and Diagnostic Laboratory Immunology, September 2004, p. 913-918, Vol. 11, No. 5
1071-412X/04/$08.00+0 DOI: 10.1128/CDLI.11.5.913-918.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Microbiology and Immunology,1 S. Daniel Abraham International Center for Health and Nutrition,2 Department of Epidemiology,5 Division of Health in the Community, Ben-Gurion University,3 Department of Pediatrics, Soroka Medical Center, Beer-Sheva, Israel,2 W. Harry Feinstone Department of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland6
Received 28 March 2004/ Returned for modification 5 May 2004/ Accepted 13 May 2004
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The Negev region of Israel provides a unique opportunity to study the immune responses to measles vaccination in relatively homogeneous populations with different ethnic backgrounds and living conditions. Bedouin Arabs have a high birth rate, with an average of nine children in each family, live in relatively crowded conditions, and suffer from a substantial burden of infectious diseases. The birth rate is lower in Jewish families (2.7 children/family), living conditions are generally less crowded, and the burden of infectious diseases is less. Both populations have similar access to modern health care services. The current study evaluated immune responses to primary MMR vaccination at 12 months as well as the generalized effects of primary MMR vaccination on immune function in Bedouin and Jewish Israeli children.
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Ninety-one infants with adequate samples included 77 Bedouin Arab infants (59% males) and 14 Jewish infants (71% males). All infants had blood drawn immediately before receiving MMR and were asked to return for repeat venipuncture 30 days later. The dates of the follow-up samples ranged from 28 to 111 days after the administration of MMR vaccine. There were no cases of measles reported in the Negev region from the time of birth of the study infants until the time of the second venipuncture.
Collection of blood samples. Blood (5 to 6 ml), collected by venipuncture, was placed into a 15-ml tube containing 50 IU of heparin and immediately transferred to the research laboratory, where total blood cell counts and flow cytometry assays were performed and mononuclear cells were separated. Assays requiring live cells were run within 4 h. Plasma samples were stored in aliquots at 20°C for up to 1 year until tested. Because of small and variable sample volumes, not all assays could be run on all samples. Priority was given to antibody assays and to leukocyte (WBC) counts.
Total WBC counts. One milliliter of blood was used for WBC counts, which were performed with a Coulter counter (CELL DYNE 1600 system; Abbott Laboratories, Diagnostic Division, Abbott Park, Ill.) following the manufacturer's protocol.
Cell cultures. Monolayer cultures of Vero cells, grown in Dulbecco's modified Eagle's medium supplemented with 10% fetal calf serum (FCS) (Maagar Biological Industries, Beit Haemek, Israel), glutamine, and antibiotics, were used for the growth of measles virus stocks and for the plaque reduction neutralization (PRN) assay. K562 erythroleukemia cells, grown in RPMI 1640 medium (Maagar) supplemented with 10% FCS, glutamine, and antibiotics, were used as target cells for the measurement of natural killer (NK) cell cytotoxic activity.
MV neutralizing antibody. Paired sera from before and after vaccination were run in parallel and tested for MV neutralizing antibody with a modified 50% PRN assay (25). Briefly, serial twofold dilutions (1:4 to 1:4,096) of heat-inactivated sera (56°C for 30 min) were prepared in 96-well flat-bottom microtiter plates. Equal volumes of the Edmonston strain of MV containing 50 to 100 PFU/ml were added to duplicate wells of each dilution. The mixtures were incubated for 1 h at room temperature, and then 5 x 104 Vero cells were added to the serum-virus mixtures. After 5 days of incubation in a humidified atmosphere at 37°C in 5% CO2, plaques were counted. A reference serum calibrated against an international reference serum (5) was included in each run. PRN titers were expressed as the reciprocal of the highest dilution that gave 50% plaque reduction and adjusted to the international standard. A fourfold rise in titer was considered seroconversion.
Virus-specific ELISA-reactive IgG antibody. Mumps and rubella virus-specific immunoglobulin G (IgG) antibodies were assayed with commercial enzyme-linked immunosorbent assay (ELISA) kits (Human mbH, Aachen, Germany) according to the manufacturer's instructions. Seroconversion was defined as a change from negative to positive as defined by the manufacturer.
Ig levels. Total IgM and IgG levels in sera were determined in the clinical immunology laboratory of Soroka Medical Center, using the Beckman immunochemistry system according to the manufacturer's instructions (Beckman Instruments, Inc., Galway, Ireland). Total IgE levels were determined by the IMX system (Abbott).
Lymphoproliferation assays.
Peripheral blood mononuclear cells (PMBCs) were separated from heparinized blood by centrifugation on Ficoll-Hypaque (Pharmacia Fine Chemicals, Piscataway, N.J.) density gradients. PMBCs were washed in phosphate-buffered saline (PBS) and resuspended at 106/ml in RPMI 1640 supplemented with 5% heat-inactivated FCS, 10 mM HEPES (Gibco BRL), and antibiotics. Spontaneous proliferation and proliferative responses to phytohemagglutinin (PHA; Wellcome HA15) and preservative-free tetanus toxoid (TT), a gift from S. Udem (Lederle Labs) were tested on fresh cells. Spontaneous proliferative activity was measured by culturing 2 x 105 (200 µl) unstimulated PBMCs in triplicate in flat-bottom 96-well microtiter plates (Corning, Inc.) at 37°C in 5% CO2 for 1, 3, and 6 days. Data are expressed as cpm incorporated into the cells. Proliferative activity of lymphocytes to PHA or TT was determined after culturing the cells with 2.5-µg/ml PHA for 3 days or with 1:10,000 TT for 6 days. The samples were pulsed with 1 µCi of [methyl-3H]thymidine (5,000 mCi/mmol; NRCN, Beer-Sheva, Israel) 18 h prior to harvesting. Data are expressed as cpm incorporated into PHA- or TT-stimulated cells cpm of unstimulated cells (
cpm).
Flow cytometry. Whole-blood samples (150 µl) were reacted with 20 µl of anti-CD4/CD8 or anti-CD56 antibodies (Becton Dickinson, Mountain View, Calif.) according to the manufacturer's instructions and incubated for 30 min on ice in the dark. To remove erythrocytes, 1.4 µl of fluorescence-activated cell sorter (FACS) lysing solution was added to each tube and mixed gently. After incubation (10 min, room temperature, in the dark), samples were pelleted (300 x g, 7 min, room temperature) and supernatant fluids were aspirated, leaving approximately 50 µl of fluid. Cell pellets were gently resuspended and washed twice in f-PBS (Becton Dickinson). After final resuspension, 800 µl of FACS storing solution was added, and samples were analyzed immediately or stored up to 14 days at 4°C in the dark. Before the analysis, samples were gently mixed and filtered through silk filters. FACS analysis was done with a FACStar Plus flow cytometer (Becton Dickinson). Data are reported as percentage of lymphocytes positive.
NK cell lysis. NK cytotoxic activity was tested in a 51Cr-release assay by incubating lymphocyte effector cells at ratios of 1:100, 1:50, and 1:25 in 96-well U-bottom microtiter plates (Corning) at 37°C with 2 x 104 51Cr-labeled K562 erythroleukemia target cells. After 4 h of incubation, the plates were centrifuged for 10 min at 300 x g, and 0.1 ml of supernatant fluid was harvested from each well for determination of 51Cr release. For maximal 51Cr release, 0.1 of 2 N HCl was added to target cells. The percentage of specific lysis was calculated according to the formula % specific lysis = [(test cpm spontaneous cpm)/(maximum lysis cpm spontaneous cpm)] x 100.
Spontaneous release did not exceed 20%. Results are expressed as mean percent specific 51Cr release.
Statistical analysis. Statistical analysis was performed with SPSS or Statview software. Wilcoxon's signed-rank test or Student's paired t test was used to estimate the significance of the differences between post- and preimmunization values. Comparison between groups was performed with the Mann-Whitney U, Student's t, or Fisher's exact test, as indicated. Quantitative data are reported ± standard error.
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TABLE 1. Seroconversion rates for Jewish and Bedouin infants to measles, mumps, and rubella viruses after MMR vaccination at 12 months of age
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To determine whether the antiviral titers achieved after vaccination were similar in the Jewish and Bedouin infants, titers were compared for measles and mumps, the two viruses for which the tests used provide quantitation of the amount of antibody (Fig. 1). Measles neutralizing antibody titers after vaccination were lower in Jewish children (122 ± 60 mIU/ml) than in Bedouin children (333 ± 39 mIU/ml) (P = 0.0016; Student's t test on log-transformed data), and this was primarily due to differences in the children tested more than 6 weeks after immunization. If only the titers of seroconverters are compared the difference remains, but is no longer statistically significant (240 ± 55, Jewish; 347 ± 38, Bedouin; P = 0.19). Mumps antibody titers were not different between the groups.
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FIG. 1. Levels of antibody to measles and mumps viruses after vaccination. Antibody was measured in plasma by plaque reduction neutralization (PRNT) for measles virus and by quantitative enzyme immunoassay for mumps. Measles antibody data were log transformed for analysis. Data are shown for all Bedouin (n = 77) and Jewish (n = 14) children and stratified by time after vaccination for those for whom this information was available. Jewish children had lower levels of antibody to measles than did Bedouin children. HU, units as defined by the manufacturer. *, P < 0.05; **, P < 0.01 (Student's t test).
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FIG. 2. Levels of Ig in Bedouin and Jewish children before and after vaccination. Total levels of IgM, IgG, and IgE were measured on plasma from Bedouin (n = 46 to 48) and Jewish (n = 13) children before and after vaccination. Levels of IgM and IgE were similar between groups and did not change after immunization. Levels of IgG were higher in Bedouin children than Jewish children at both time points and increased after vaccination. **, P < 0.01 comparing Bedouin and Jewish children (Student's t test); ***, P < 0.001 comparing levels before and after vaccination (Wilcoxon's signed-rank test).
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FIG. 3. WBC counts in Bedouin and Jewish children before and after vaccination. Numbers of WBCs and lymphocytes and percentages of CD4+ and CD8+ T lymphocytes were measured before and after vaccination in Bedouin (n = 25 to 75) and Jewish (n = 9 to 13) children. There were no changes in these parameters. WBCs were higher in Bedouin children than Jewish children before and after vaccination. Lymphocyte counts were higher and the percentages of CD4+ and CD8+ T lymphocytes were lower in Bedouin children than Jewish children. CD4/CD8 ratios were not different. *, P < 0.05; **, P < 0.01; ***, P < 0.001 (Mann-Whitney U test for comparison between groups and Wilcoxon's signed-rank test for comparison of values before and after vaccination).
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TABLE 2. In vitro proliferation of PBMCs before and after vaccinationa
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FIG. 4. NK cell numbers and lytic activity in Bedouin and Jewish children before and after vaccination. Numbers of NK cells were determined by quantitating the percentage of lymphocytes that were CD56+ in Bedouin (n = 31 to 41) and Jewish (n = 11 to 13) children before and after vaccination. No differences were detected. Activity of NK cells was measured by lysis of K562 cells. There were no differences between Bedouin and Jewish children, but NK activity increased after vaccination for Bedouin children. **, P < 0.01; Wilcoxon's signed-rank test.
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MV is a highly immunosuppressive virus, and most of the morbidity and mortality is caused by increased susceptibility to other infectious diseases (7). Documented immunologic abnormalities include leukopenia, loss of delayed-type hypersensitivity skin test responses to recall antigens, suppressed in vitro lymphoproliferative responses to mitogens and recall antigens, and generalized immune activation with skewing of the immune response toward type 2 cytokine production (7, 9, 10, 16). In addition, measles leads to an increased production of IgE and increased susceptibility to the autoimmune disease acute disseminated encephalomyelitis (8, 15). The standard-dose live attenuated measles vaccine virus is safe and generally free of significant side effects, but experience with adverse effects associated with the use of the high-titer vaccine in infants (12, 18) and transient abnormalities in in vitro proliferative responses after primary vaccination of infants (13) have encouraged further study of the effects of vaccination on immune function of infants.
Lymphopenia is common during measles (26) and was observed after primary vaccination with the earliest versions of the measles vaccine (Edmonston B), which frequently induced a fever and rash (2). Also, a decrease in WBCs was observed in Canadian and Israeli children 1 month after revaccination with MMR (24, 32). However, WBCs were unchanged 2 and 12 weeks after primary vaccination of 6- to 9-month-old South African infants with the Edmonston-Zagreb or Schwarz strains of measles vaccine (13) or after primary vaccination of 1- to 8-year-old children with MMR (19). We found no effect of primary vaccination on WBCs, lymphocyte percentages, or CD4/CD8 ratios after primary vaccination of 1-year-old infants with MMR, further suggesting that primary vaccination with the current vaccine has little effect on these parameters.
Despite the fact that leukocyte counts do not change after primary vaccination, lymphocyte function has been noted to change. In our study, increased spontaneous proliferation of cultured PBMCs indicated immune activation and correlates with previous observations of higher expression of activation markers on lymphocytes (27) and increases in ß-2 microglobulin, neopterin, and soluble CD8 in plasma after measles vaccination (13). A number of studies have also reported altered T-cell function after primary measles vaccination. For instance, in Bangladeshi infants, Candida delayed-type hypersensitivity was reduced after measles vaccination (27) and many, but not all, studies have reported decreased in vitro lymphoproliferative responses to PHA, concanavalin A, TT, or purified protein derivative (13) after measles or MMR primary immunization (19, 21, 29). In the present study, proliferative responses to PHA and TT were unchanged. The differences between these various studies may reflect differences in the age of vaccination, time of analysis after vaccination, or populations studied.
The effect of vaccination on NK cells and their function has received limited attention. During wild-type MV infection, NK cell lytic activity is suppressed and type 1 interferon, an important activator of NK cell activity, is not detectable (11, 28). In this study, we have shown that NK cell numbers and function are preserved after vaccination. In Bedouin children, as in a previous study of Canadian children (21), there was an increase in NK cells or activity.
The differences between Bedouin and Jewish infants were unexpected. The low rate of seroconversion to measles is consistent with a previous study of Jewish school children in the same region in which 24% of 6-year-old children with a documented history of receiving MMR were seronegative (24). The differences in seroconversion, as well as in IgG levels and WBC counts, could reflect genetic or environmental influences. A few previous studies have shown differences in serologic responses to measles vaccine in children of different ethnic backgrounds. Native Canadians have a higher rate of seropositivity (83%) than Caucasian Canadians (76%) and higher mean antibody levels (22). A lower proportion of African children seroconverted after receiving an aerosol-administered vaccine in South Africa than Indian children (4). Furthermore, twin studies of measles antibody levels suggest a role for genetic influences in the variation of responses to measles immunization (31). In American children, HLA class I and class II alleles have been linked to measles vaccine responsiveness. HLA class I alleles B8, B13, and B44 (14) and class II alleles DRB1*03 and DQA1*0201 (23) and HLA homozygosity (30) are all associated with measles seronegativity. Further study in larger numbers of individuals will be needed to determine the factors leading to differences in immune parameters and seroconversion to measles in Bedouin and Jewish children.
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