Human immunodeficiency viruses are two species of Lentivirus that have silently reshaped the modern world, causing acquired immunodeficiency syndrome and leaving a legacy of scientific discovery and social upheaval. These microscopic entities, roughly 120 nanometers in diameter, are about 100,000 times smaller in volume than a single red blood cell, yet they possess the power to dismantle the human immune system over time. Without treatment, the average survival time after infection is estimated to be 9 to 11 years, depending on the specific subtype of the virus. The virus operates by infecting vital cells in the human immune system, such as helper T cells, macrophages, and dendritic cells, leading to a progressive failure that allows life-threatening opportunistic infections and cancers to thrive. In most cases, HIV is a sexually transmitted infection, occurring through contact with or transfer of blood, pre-ejaculate, semen, and vaginal fluids, but it can also be passed from an infected mother to her infant during pregnancy, childbirth, or through breast milk. The virus exists within these bodily fluids as both free virus particles and virus within infected immune cells, creating a complex and persistent threat to human health.
The Molecular Deception
The molecular structure of HIV is a masterpiece of biological deception, designed to evade the immune system while ensuring its own survival. The virus is composed of two copies of positive-sense single-stranded RNA that codes for nine genes, enclosed by an ovoidal capsid made of 2,000 copies of the viral protein p24. This RNA is tightly bound to nucleocapsid proteins and enzymes needed for development, including reverse transcriptase, proteases, and integrase. Surrounding the capsid is a matrix of the viral protein p17, which ensures the integrity of the particle, and this is in turn surrounded by a viral envelope taken from the membrane of a human host cell. The envelope contains relatively few copies of the HIV envelope protein, which consists of a cap made of three molecules known as glycoprotein 120 and a stem consisting of three gp41 molecules. This envelope protein is a major target for HIV vaccine efforts, yet over half of its mass is N-linked glycans that form a dense shield, preventing the normal maturation process of glycans and hiding the underlying viral protein from neutralization by antibodies. This high density of glycans means that almost all broadly neutralizing antibodies identified so far must bind to or cope with these envelope glycans, making the development of a vaccine an exceptionally difficult challenge.The Replication Cycle
The replication cycle of HIV is a multi-step process that begins with the entry of the virus into a host cell and ends with the release of new virus particles. The HIV virion enters macrophages and CD4+ T cells by the adsorption of glycoproteins on its surface to receptors on the target cell, followed by the fusion of the viral envelope with the target cell membrane. The first step in fusion involves the high-affinity attachment of the CD4 binding domains of gp120 to CD4, which triggers a structural change in the envelope complex, exposing the chemokine receptor binding domains and allowing them to interact with the target chemokine receptor. This allows for a more stable two-pronged attachment, which enables the N-terminal fusion peptide gp41 to penetrate the cell membrane. Once inside, an enzyme called reverse transcriptase liberates the positive-sense single-stranded RNA genome and copies it into a complementary DNA molecule. The process of reverse transcription is extremely error-prone, and the resulting mutations may cause drug resistance or allow the virus to evade the body's immune system. The integrated viral DNA may then lie dormant, in the latent stage of HIV infection, before being transcribed into RNA and packaged into new viral particles.