Secretion of the NS1 protein in plasma is a hallmark of flavivirus infections

We investigated NS1 secretion in clinical flavivirus infections other than DEN, such as YF, and more recently WN. With the use of YF NSl-specific antibodies (generous gift from JJ Schlesinger), it was possible to capture the protein by ELISA in 7 out of l8 serum samples recovered from patients infected with YFV in Senegal (kindly provided by C. Mathiot) (Table 3). The NSl-positive sera clustered in the

Days post-onset of fever

FIG. 3. Status of the NS1-specific IgG response during the course of DENV-1 infections. The NS1-specific response was analysed by ELISA for paired acute and convalescent sera using purified DEN-1 NS1 antigen in coating. Relative values are reported on the graph (with a mean value of the negative control of 0.1). The dotted line represents the average intensity of the NS1-specific IgG response over time.

Days post-onset of fever

FIG. 3. Status of the NS1-specific IgG response during the course of DENV-1 infections. The NS1-specific response was analysed by ELISA for paired acute and convalescent sera using purified DEN-1 NS1 antigen in coating. Relative values are reported on the graph (with a mean value of the negative control of 0.1). The dotted line represents the average intensity of the NS1-specific IgG response over time.

FIG. 4. Inverse correlation between the levels of NS1 antigen and NS1-specific IgG in DENV-1 infected patients. The NS1 antigen and the NS1-specific IgG were quantified by ELISA among the group of acute specimens recovered between day 0 and day 6. Relative values are reported on the graph (with a mean value of the negative controls of 0.05 for 'NS1 Ag' and of 0.1 for 'NS1-specific IgG').

lowest—albeit positive—concentrations of IgM. For the detection of the NS1 protein of WNV in natural infections, we made use of monoclonal antibodies raised against DENV NS1 that cross-reacted with WNV NS1 or against NS1 of a closely related flavivirus, Murray Valley encephalitis virus (collaboration with R.

TABLE 3 Detection of NS1 antigen in sera from patients infected with yellow fever virusa

Patient reference #

IgM MAC-ELISA (OD)

NS1 Ag-Capture (OD)

Virus isolation

1

0.31

1.13

+

2

0.53

1.20

+

3

0.36

0.60

4

0.58

0.65

5

0.74

0.75

6

0.97

7

1.01

8

1.10

9

1.16

10

1.28

0.61

11

1.34

12

1.45

13

1.47

14

1.50

15

1.52

1.19

16

1.59

17

1.94

18

2.11

a For each patient, serum was tested by the IgM MAC-ELISA or the NS1 antigen-capture ELISA assays. Positive values of the corresponding optical densities (OD) are reported, as well as positive virus isolation (+). NS1-positive sera are highlighted in bold.

a For each patient, serum was tested by the IgM MAC-ELISA or the NS1 antigen-capture ELISA assays. Positive values of the corresponding optical densities (OD) are reported, as well as positive virus isolation (+). NS1-positive sera are highlighted in bold.

Hall), to generate optimized conditions of NS1 capture. We processed biological samples (serum and cerebrospinal fluid, CSF) from 10 patients who developed a meningo-encephalitic syndrome during a recent WN outbreak in Romania (1996, collaboration with C. Ceianu and A. Ungureanu-Alexse) (Table 4). All samples, apart from one that remains unknown, have been recovered 2—5 days after the first clinical symptoms appeared and WNV infections were diagnosed by a positive signal for specific IgM. Interestingly, WNV NS1 could be detected in half of the serum samples but in none of the corresponding CSF samples.

In conclusion, secreted NS1 can be readily detected in acute flavivirus-associated hepatic or encephalitic infections in humans.

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