HIV-1 is the virulent lentivirus behind most AIDS
Two lentiviruses, HIV-1 and HIV-2, infect humans and can end in AIDS as helper T cells collapse. HIV-1, once also called LAV or HTLV-III, is more infectious and virulent than HIV-2 and drives the global majority of cases.
HIV-2 spreads so poorly that it stays largely confined to West Africa, while HIV-1 causes most infections worldwide. In 2020 the international virus taxonomy committee gave them formal species names, Lentivirus humimdef1 and humimdef2. Like other lentiviruses, which infect many animal species, they cause slow illnesses with long incubation; untreated, survival after infection averages an estimated nine to eleven years depending on subtype.
Each particle is a rough sphere about 120 nanometres across; a red blood cell holds roughly 100,000 times its volume. Inside a cone of some 2,000 copies of the protein p24 sit two strands of RNA carrying nine genes, which encode 19 proteins, together with enzymes such as reverse transcriptase and integrase. On entering a cell, the virus copies its RNA into DNA and stitches it into the host genome, where it can lie dormant and invisible to the immune system for up to ten years.
Its targets are CD4 helper T cells, macrophages and dendritic cells. CD4 numbers fall through several routes: abortively infected cells burn out by pyroptosis, uninfected neighbours die by apoptosis, the virus kills its hosts directly and cytotoxic CD8 cells destroy infected ones. Below a critical level, cell-mediated immunity fails and opportunistic infections and cancers take hold.
Vaccine makers focus on the envelope spike, built from gp120 caps on gp41 stems, because it is the only viral protein on the surface. More than half its mass is sugar, a dense glycan shield that deflects antibodies, and nearly every broadly neutralising antibody found so far has adapted to it. Stabilised SOSIP trimers now mimic the native spike for vaccine trials. Meanwhile, studies of both same-sex and opposite-sex couples show no sexual transmission when the positive partner keeps an undetectable viral load.
Source: HIV
