Avidity determination of IgG in diagnosis of tick-born encephalitis

Hana ZelenáJiří JanuškaJan Raszka

Virology department, National Reference Laboratory for Arboviruses,Institute of Public Health, Ostrava, Czech Republic

Diagnosis of tick-born encephalitis

• 1st phase (fever): direct detection- PCR- virus isolation from blood

• 2nd phase (neurological): indirect detection- IgG, IgM ELISA, IFA- Complement fixation test (CFT)- Virus neutralizing antibodies (VNA)

Dynamics of diagnostic markers in tick-born encephalitis

Dynamics of diagnostic markers in tick-born encephalitis

Avidity of IgG antibodies• Avidity is a measure of the strenght of the

antibody-antigen interactions, it increases with their binding affinity and with their valence

• Avidity reflects the maturity of the antibodies. Low avidity antibodies are synthetized during the primary infection, in time avidity gradually increases.

• High avidity antibodies are produced by memory B-cells during the secondary infection or reactivation or infection in people that were vaccinated.

Low avidity occures also in immunosupression.

Usefullness of IgG avidity measurement

Dynamics of diagnostic markers in tick-born encephalitis

Principle of IgG avidity measurement

• Test of the strenght of antigen-antibody interaction by incubation with denaturing agent (urea)

• Low avidity antibodies dissociate from complexes and are washed away.

• High avidity antibodies withstand urea treatment and remain bound to the antigen.

Protocol for the determination of anti-TBEV IgG avidity

• Anti-TBEV IgG test kit is based on indirect ELISA.• Patient serum is tested in two parallel wells.• During the 1st incubation step antibodies found in

the serum sample bind to the antigen. One of the two wells is incubated with urea solution (8 mol/L) for 5 minutes, while the other well remains empty.

• Absorbance ratio between the two parallel wells is calculated at the end of the test (the well with urea/the well without urea)

IgG avidity measurementresults and interpretation

• Avidity (%) = absorbance of the well with urea/absorbance of the well without urea

Avidity Result Interpretation

<40% low avidity Recent infection (< 3 weeks)

40-60% „gray zone“

>60% high avidity

Infection in the past, infection in people that were vaccinated

IgG avidity in time(patients)

Dynamics of anti-TBEV antibody levels (patient with typical TBE)

Anti-TBEV positive samples (april to september 2010)29 samples IgG+ IgM+ 59 samples IgG+ IgM-

Pitfalls in TBE serology – relevance of avidity testing

• Infection in vaccinated people- min. 4-fold increase in CFT and VNA – must be paired samples- IgM positive or negative- high IgG avidity

• Elevated IgM pesistence after primary infection (up to 1 year)- IgG and IgM positive- high IgG avidity

• Atypical or subclinical course of the disease- IgM and IgG positive- low IgG avidity

• Early disappearance of IgM in primary infection– low IgG avidity

Conclusion

• Avidity determination of IgG is a complementary tool that makes serological diagnosis of tick-born encephalitis more accurate

• It increases the testing reliability in acute infections,posses high importance in cases of:- atypical course of the disease- atypical antibody response- infection in vaccinated people

• Interpretation of serological result that is complemented with IgG avidity measurement has clearly higher validity.