Early detection of HIV infection is the best way to prevent spread of the disease and to improve the efficiency of the antiretroviral therapy. Spain’s top research institution said it has patented an HIV test that can detect the AIDS-causing virus within a week of infection, the fastest yet. Researchers develop a sandwich immunoassay based on nanomechanical and optomechanical transduction for detecting p24 antigen in human serum samples that only involves two immunoreaction steps . The technique achieves a detection limit of 10−5 pg/mL. The technique could dramatically shorten the window period of HIV infection, in which the disease is undetectable, to just a single week.
Without treatment, HIV advances in stages, overwhelming immune system and getting worse over time. The three stages of HIV infection are: acute human immunodeficiency virus infection(AHI), clinical latency, and AIDS (acquired immunodeficiency syndrome). Acute human immunodeficiency virus infection (AHI) can be defined as the time from HIV acquisition until seroconversion, i.e., the appearance of detectable antibodies to HIV in the blood. The duration of this stage is of about four weeks. Detection of AHI is crucial for improvement of the individual’s health and is also critical for prevention of HIV transmission. Subjects with acute infection are maximally contagious, because HIV replicates without being checked by the immune system and the amount of virus in blood and genital secretions rises rapidly. In addition, the infectivity potential of the virus during the early infection stage is much higher than in later stages.
Acute HIV Testing Methods
There are two methods to detect the the acute human immunodeficiency virus, one way is by detecting in blood either the viral RNA by nucleic acid amplification tests (NAATs), this method exhibits a detection limit of 20±35 RNA copies/mL, i.e. 10±18 virions/mL, a concentration that typically occurs ~2 weeks after HIV acquisition. NAATs are very sensitive, but these technologies are technically complex and expensive.
Another way is to detect HIV capsid antigen p24 by the fourth-generation immunoassays. Fourth-generation immunoasssays achieve a p24 detection limit of 10pg/mL, a concentration approximately reached between three and four weeks after infection. Immunoassays for p24 detection are simpler and cheaper and have the potential for implementation in low-resource settings where the prevalence of HIV infection is very high. Moreover, HIV-1 p24 antigen should be a more sensitive indicator of the virus presence as a virion approximately contains 2,000±3,000 p24 antigens vs only 2 RNA copies.
Detection of p24 in human serum
Immunoassays for p24 detection are based on ELISA. Briefly, the targeted antigen is bound to a surface by specific capture antibodies, and subsequently the captured antigen is specifically linked to a primary antibody. Enzyme-linked secondary antibodies are then used to bind a region of the primary antibody. In the final step, an enzyme substrate is added to produce a change of color of the solution that is used as the detection signal. Numerous efforts have been made to lower the limit of detection of p24, mostly focused on improving the signal generation mechanism. In 2012 the best detection limit was obtained by Rica & Stevens using enzymes that control the growth of gold nanoparticles that generate coloured solutions achieving a striking detection limit of 10−6 pg/mL. However, these assays are complex, involving several steps in the signal amplification. Recently, the generation of an amplified fluorescent signal by nuclease-linked fluorescence oligonucleotide assay has provided a detection limit 1±2 pg/mL and the combination of ELISA with thio-NAD cycling that change the light absorbance at 405 nm has demonstrated a detection limit of 0.1 pg/mL. The new technique achieves a detection limit of 10−5 pg/mL and could dramatically shorten the window period of HIV infection, in which the disease is undetectable, to just a single week.
The improved limit of detection of the optoplasmonic transduction method is attributed to its higher specificity to the gold nanoparticles. In the nanomechanical method, although the relative change of the resonance frequency is dominated by the mass of the nanoparticle labels, the mass arising from the functionalization steps, nonspecific adsorption and contamination provides a noise floor that ultimately limits the detection limit.
The potential for HIV infectivity in the first stage of infection (acute human immunodeficiency virus infection) is much higher than in the later stages. Therefore, initiating antiretroviral therapy prior to seroconversion improves immune control and has been associated with benefits in CD4 cell count, a reduction in systemic inflammation, the preservation of cognitive function, and a reduction of the latent reservoir. Logically, its detection is critical to the prevention of HIV transmission.
Priscila M. Kosaka, Valerio Pini, Montserrat Calleja and Javier Tamayo. Ultrasensitive detection of HIV-1 p24 antigen by a hybrid nanomechanical-optoplasmonic platform with potential for detecting HIV-1 at first week after infection. PLOS ONE, 2017.