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Journal of Clinical Microbiology, November 2005, p. 5784-5786, Vol. 43, No. 11
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.11.5784-5786.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Centre National de Référence des Arbovirus, Institut Pasteur de la Guyane, Cayenne, French Guiana,1 Epidemiology Unit, Institut Pasteur de la Guyane, Cayenne, French Guiana2
Received 18 March 2005/ Returned for modification 2 June 2005/ Accepted 24 August 2005
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IgG avidity tests have been shown to be useful for distinguishing primary from chronic or recurrent infections for a few infectious diseases (6, 11, 12, 17), including dengue virus infection (5). We previously demonstrated that the dengue IgG avidity index, determined from a single acute-phase serum sample, discriminates between primary and secondary dengue virus infections (15). We developed this test with the DEN-2 antigen, prepared by the sucrose acetone method (4). Here we extended this test for use in any laboratory using one of the four types of dengue antigen by carrying out the previously described dengue IgG avidity test (15) with all four dengue antigens (DEN-1 to DEN-4) simultaneously.
Sera from the collection of the Centre National de Reference des Arbovirus from the Institut Pasteur de la Guyane were used for this study. We tested 105 pairs of serum specimens from patients infected with the DEN-3 serotype. All sera were collected from patients living in French Guiana with basic clinical symptoms of dengue (fever, headache, myalgia, arthralgia) associated or not with rash and minor hemorrhagic manifestations. A first serum sample was collected during the acute phase (day 1 to day 5), with the first day of fever considered the first day of the disease. Dengue virus infection was diagnosed virologically, based on the acute-phase serum samples, by means of culture from AP 61 mosquito cells (Aedes pseudocustellaris) or reverse transcription-PCR by routine methods (14, 16). A second sample was collected during the convalescent phase, which was 7 days or more after the onset of fever. The HI test was also performed with all of the serum samples analyzed by using our IgG avidity test. According to the HI test criteria (18), 59 pairs of serum specimens were classified as having primary dengue virus infection and 46 were classified as having secondary dengue virus infection. We then compared the IgG avidity index for each type of dengue antigen (DEN-1 to DEN- 4) by testing 105 pairs of serum samples. Using the DEN-1 antigen, we assessed the levels of avidity of 27 pairs of serum samples (15 primary infections and 12 secondary infections). In the same way, using the DEN-2 antigen, we assessed 28 pairs of serum samples (16 with primary infections and 12 with secondary infections). Using the DEN-3 and DEN-4 antigens, we tested 14 pairs of serum samples from primary infections and 11 pairs of serum samples from secondary infections, respectively. Six measures of avidity were performed with each serum sample for each antigen used.
The IgG avidity test was performed with 105 pairs of serum samples, as described previously (15). The sera were first diluted (1:100) in phosphate-buffered saline (PBS) supplemented with 0.1% Tween 20 and 5% nonfat milk powder (the reagents were from Sigma Laboratories l'Isle d'Abeau Chesnes, France) and then added to two separate microplates previously coated with one of the four dengue antigens (DEN-1, DEN-2, DEN-3, or DEN-4). The plates were incubated for 2 h at 37°C. One of the plates was washed with 0.1% Tween 20 (PBS-T) in PBS, and the other was washed with 8 M urea in PBS-T (Sigma Laboratories). Bound IgG was detected at 650 nm (A650), and an avidity index corresponding to (A650 assay with urea/A650 assay without urea) x 100 was calculated for each sample. Normal mouse brain antigen was used as a control in all tests. Sera from patients being vaccinated against yellow fever (six serum samples from patients vaccinated against yellow fever with no history of dengue virus infection) were included as a negative control to rule out the possibility of cross-reaction with other flaviviruses, such as the virus that causes yellow fever (1).
In a first step, we compared for each dengue antigen the levels of avidity of the antibodies according to the patient's immune status using the Student's t test. Thus, in order to prove that the level of avidity is related for each patient to the day that blood was drawn and to the immune status, we performed a covariance analysis (15). The avidity was significantly higher (P < 0.001) for secondary dengue virus infections than for primary dengue virus infections for the four antigens tested (Table 1). Analysis of covariance further showed that the avidity increased significantly (P < 0.001) in relation to the time that blood was drawn and was significantly higher (P < 0.001) for secondary dengue virus infections than for primary dengue virus infections (Fig. 1). Thus, discriminatory analysis is possible regardless of the dengue antigen used for the determination of avidity index.
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TABLE 1. Comparison of the mean IgG avidity between primary and secondary dengue virus infections with the four types of dengue antigen
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FIG. 1. Temporal changes in IgG antibody avidity in sera from patients with primary and secondary dengue virus infections, determined on the basis of the criteria of the HI test. Each serum sample was tested with the four dengue antigens: sera from patients with primary infections ( , DEN-1; , DEN-2; , DEN-3; and , DEN-4) and from patients with secondary infections ( , DEN-1; , DEN-2; , DEN-3; and , DEN-4). Avidity trends for a primary or a secondary infection with each dengue antigen: shaded dashed lines, DEN-1; shaded solid lines, DEN-2; black dashed lines, DEN-3; and black solid line, DEN-4.
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