Production of the NS1 antigen in blood is virus and host dependent

In order to quantify the levels of the NS1 viral antigen in DENV-1, -2 and -3 infections, we made use of purified NS1 protein standards for each of the corresponding viral serotypes (Fig. 1A). Even in a comparable epidemiological setting, such as for DENV-1 and DENV-2 that co-circulated in French Guiana during the same period, NS1 concentrations varied depending on the individual, the viral serotype and the course of infection (Fig. 2). Concentrations of the NS1 antigen in plasma frequently reached values above 10-100 ng/ml, although for DENV-3-infected patients, the amounts of protein could be substantially lower. Accordingly, the mean value of NS1 concentration calculated over a 6 day period from the onset of fever was three orders of magnitude higher for type 1 compared to type 3, and 7.5-fold higher for type 2 than for type 3 (mean values of 285, 700 and 93 ng/ml

z rri a

10,000 9,000 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0

012345678 Days after onset of fever

10 15 20

days after onset of fever

5,000 4,5004,0003,5003,0002,500 2,000 1,500 1,000 500 0

10 15 20

days after onset of fever

10 15 20

Days after onset of feve

600 400 200 ,000 800 600 400 200 0

10 15 20

Days after onset of feve

10 15 20

Days after onset of fever

FIG. 2. NS1 concentrations in plasma from patients infected with (A) DENV-1, (B) DENV-2 and (C) DENV-3. Specimens were tested by the NS1 antigen-capture ELISA at different dilutions (1 : 2, 1 : 10 and 1 : 100) and the levels of NS1 estimated using values falling within the linear range of the assay.

2,000

for type 1, 2 and 3, respectively). It is not clear why the range of NS1 concentrations in DENV-3 infections is substantially lower than that of the other serotypes while the breadth of NS1 secretion is at least as wide for type 3 as for the two other viral serotypes (positive type 3 specimens found over a period of 8 days, Fig. 2). One possibility is that DENV-3 does not replicate as efficiently, either due to the genetic background of the population living in Guadeloupe compared to that of French Guiana, or to the particular strain involved in the epidemics. Another possibility is that DEN-3 NS1 might have a shorter half-life in vivo, or bind more efficiently to target cells or soluble host factors, thereby lowering the detectable pool of NS1 proteins in plasma. Quite notably, several individuals from the different serogroups peaked in the range of 1—10 |ig NS1/ml, significantly above the average values. However, high NS1 concentrations did not seem to correlate with specific clinical symptoms or haematological markers, suggesting that NS1 level may be of poor prognostic value.

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