Incidence immunoassay for distinguishing recent from established HIV-1 infection in therapy-naive populations.

Objective: To identify a specific marker of recent HIV-1 infection.

Design: The humoral immune response in individuals recently infected with HIV-1 was followed by analysing the antibody isotype-specific response generated to HIV-1 antigens in sequential samples collected during and following seroconversion.

Methods: Antibody isotype-specific HIV-1 Western blots were analysed to identify interactions indicative of recent HIV-1 infection. These responses were further quantified using an antibody isotype-specific enzyme-linked immunoabsorbent assay based on recombinant HIV-1 antigens.

Results: During maturation of the immune response to HIV-1 infection, a rapid and enduring IgG1 isotype response was seen to all the major proteins transcribed by env, gag and pol. An early transient peak of IgG3 reactivity to p24 was observed over an interval of approximately 1-4 months following HIV-1 infection. The presence of IgG3 reactivity to p24 permitted established infection to be distinguished from recently infected individuals during this time period.

Conclusion: An assay for anti-p24 IgG3 reactivity would provide an estimate of the incidence of HIV infection that may be applicable for epidemiological surveys as well as for monitoring new infections during vaccine trials and for managing treatment programmes.