Background:  In the early stages of HIV infection, p24 antigen is present before the production of anti-HIV antibodies and is the first serological marker detected. Antibody tests performed during the first 2 to 3 weeks of infection may be negative. The objective of this study was to identify and characterize HIV-1 p24 antigen-positive, antibody-negative, plasma specimens collected in a region where HIV-1 non-B subtypes are endemic.

Methods:  We analyzed for HIV-1 p24 antigen by ELISA, 226 plasma specimens collected in Cameroon between 2000 and 2004. These specimens were negative for anti-HIV antibodies using the Abbott HIVAB HIV-1/2 (rDNA) EIA (3A77) and a rapid strip assay that identifies HIV-1 group M, N, and O and HIV-2 infections. Based on the p24 ELISA signal, specimens were selected for real-time polymerase chain reaction (RT-PCR) amplification using pol primers that amplify viruses in the HIV-1/SIV_cpz and HIV-2/SIV_smm lineages and env gp41 primers for HIV-1 group M.

Results:  We identified 2 p24 antigen ELISA reactive specimens that were positive by RT-PCR. Both the pol and env primer sets amplified viral sequences from specimen 496-5; sequence analysis identified the virus as HIV-1 group M CRF02_AG, which was confirmed by amplification and analysis of a gag p24 sequence. From specimen ABB/NW/245/01, only the pol primers amplified a PCR product; sequence analysis of this fragment showed the infecting virus was HIV-1 group O. Subsequently, sequence analysis of gag p24, pol IN, and env gp41, amplified using group O specific primers, confirmed the presence of a group O virus in ABB/NW/245/01.

Conclusions:  The antigen-positive, antibody-negative specimens, confirmed by PCR, represent recently acquired HIV infections. The identification of an early infection by CRF02 is not unexpected since CRF02 accounts for at least 60% of HIV-1 infections in Cameroon. However, group O infections represent <5% of HIV-1 infections in Cameroon. Identification of an early group O infection shows that while rare, group O is being transmitted within the population. Due to the lack of commercially available seroconversion panels for HIV-1 non-subtype B infections, these specimens are valuable for evaluating the ability of fourth-generation antigen/antibody combination assays to detect early non-subtype B infections. These results also demonstrate that there are early HIV p24 antigen-positive infections that are not identified by assays that detect only anti-HIV antibodies.