HIV replicates by hijacking a specific type of immune cell, the CD4+ T cell, and turning it into a factory for producing new copies of the virus. The process involves a series of tightly coordinated steps: the virus docks onto the cell surface, fuses with its membrane, converts its RNA genome into DNA using an enzyme called reverse transcriptase, stitches that DNA permanently into the host cell’s chromosomes, and then commandeers the cell’s own machinery to churn out fresh viral particles. Each step is a potential point of failure for the virus and a potential drug target for medicine, which is why understanding the life cycle has been so central to treating HIV.
Locking On to the Cell Surface
HIV does not infect cells at random. It targets cells that display a protein called CD4 on their surface, primarily a class of immune cells known as helper T cells. The virus carries its own surface proteins, gp120 and gp41, which sit on its outer envelope like spikes. The first step of infection begins when gp120 binds tightly to CD4 on the host cell. That initial contact triggers a shape change in gp120, exposing a hidden region that can then latch onto a second receptor, called a coreceptor.
Two coreceptors matter most: CCR5 and CXCR4. During early infection, the virus tends to use CCR5, while strains that emerge later in the course of untreated disease often switch to using CXCR4.1PubMed. The HIV coreceptors CXCR4 and CCR5 are differentially expressed and regulated on human T lymphocytes The binding of CD4 reshapes gp120, revealing the coreceptor binding sites and priming the virus’s fusion machinery for the next step.2PubMed. HIV-1 Entry Mechanisms: Protein-Host Receptor Interactions and Membrane Fusion Dynamics This two-step handshake is why people who carry a natural mutation that disables CCR5 (a variant called CCR5-delta32) are highly resistant to most strains of HIV. The virus simply cannot complete the docking sequence.
Fusing with the Cell Membrane
Once gp120 has engaged both CD4 and a coreceptor, the second envelope protein, gp41, takes center stage. Gp41 undergoes a dramatic structural rearrangement, driving a short stretch of amino acids called the fusion peptide into the host cell membrane like a harpoon. The protein then folds back on itself, pulling the viral membrane and the cell membrane together until they merge.3Journal of Biological Chemistry. Conserved Polar Segment and Membrane-proximal Region Contribute to the Function of Human Immunodeficiency Virus Type 1 gp41 in Distinct Stages of the Fusion Cascade
Structural studies have shown that this folded form of gp41, sometimes called a six-helix bundle, is remarkably stable, and that stability is what provides the energy to force two separate membranes into one. Regions flanking the bundle’s core, including the fusion peptide proximal region and the membrane proximal external region, form helical extensions that splay apart and help generate the membrane curvature needed for the viral and cell membranes to meet.4PLoS Pathogens. Crystal Structure of HIV-1 gp41 Including Both Fusion Peptide and Membrane Proximal External Regions Evidence supports that the bundle forms before a fusion pore opens, meaning the membranes are brought together first, and only then does a channel open to let the viral contents spill inside.5eLife. Structure of HIV-1 gp41 with its membrane anchors targeted by neutralizing antibodies This is the moment the virus’s inner contents, collectively called the core, enter the host cell.
Reverse Transcription and the Journey to the Nucleus
Inside the cell, the viral core faces an unusual problem. HIV’s genome is made of RNA, but to become a permanent part of the host cell, it needs to be converted to DNA. The enzyme responsible is reverse transcriptase, which is what gives retroviruses their name: the information flows “backward” from RNA to DNA, the reverse of the usual direction in biology.6PubMed Central. HIV-1 reverse transcription Reverse transcriptase carries out two jobs: it acts as a DNA-building enzyme and also degrades the original RNA template as it goes, ultimately producing a double-stranded DNA copy of the viral genome.7PubMed Central. Structure of HIV-1 reverse transcriptase cleaving RNA in an RNA/DNA hybrid
For years, scientists assumed the viral core had to shed its protective protein shell, the capsid, soon after entering the cell, with reverse transcription happening in the cytoplasm. That picture has been overturned. Recent research shows that the intact, cone-shaped capsid actually travels through the cell and passes through the nuclear pore, the gateway into the nucleus, while still largely assembled.8PubMed Central. Cone-shaped HIV-1 capsids are transported through intact nuclear pores Reverse transcription appears to be completed inside the nucleus, sheltered by the capsid, and uncoating, the disassembly of that protective shell, happens at or near the site where the new DNA will be inserted into the host chromosomes.9PubMed Central. Nuclear Import of HIV-1 When the capsid does break apart, it does so by cracking open rather than gently disassembling into individual subunits.8PubMed Central. Cone-shaped HIV-1 capsids are transported through intact nuclear pores Experiments blocking the nuclear pore confirmed that HIV remains sensitive to capsid-destabilizing compounds after nuclear import, further supporting the idea that uncoating is completed inside the nucleus rather than in the cytoplasm.10PubMed Central. Nuclear pore blockade reveals that HIV-1 completes reverse transcription and uncoating in the nucleus
Stitching Into the Host’s DNA
With a fresh double-stranded DNA copy of the viral genome now inside the nucleus, the viral enzyme integrase takes over. Integrase cuts both ends of the viral DNA and splices it directly into a chromosome of the host cell. This integrated form, called the provirus, becomes a permanent part of the cell’s genetic material. Every time that cell divides, it copies the proviral DNA along with everything else.
Integration is not entirely random. HIV strongly favors inserting itself into regions of the chromosome that are actively being used by the cell, which makes sense from the virus’s perspective because active regions are more likely to be read and turned into new viral RNA.11PubMed Central. HIV DNA integration The virus achieves this targeting in part through a cellular protein called LEDGF/p75, which acts as a tether, binding to integrase on one end and to active gene regions on the other. When researchers introduced a single point mutation into integrase, the pattern changed drastically: more than a tenfold increase in integrations landing in centromeric repeat sequences compared to the normal enzyme, which almost never inserts there.12PubMed Central. A point mutation in HIV-1 integrase redirects proviral integration into centromeric repeats That kind of experiment reveals how precisely the integration machinery is tuned.
Making New Virus Particles
Once the provirus is embedded in the host genome, the cell’s own enzymes begin reading it, producing messenger RNA that encodes all the proteins the virus needs. Some of this RNA will also serve as the genome for the next generation of viral particles. The main structural protein, called Gag, orchestrates the assembly process. Gag collects the viral RNA, recognizing specific unpaired guanosine bases within a highly structured region near the start of the RNA molecule.13PubMed Central. HIV-1 RNA genome packaging: it’s G-rated
Packaging happens at the inner surface of the cell’s outer membrane. Gag molecules gather on the RNA and then cluster together, forming a growing shell that pushes outward against the membrane like a bubble. Both the ability of Gag to anchor to the membrane and its ability to stack up in organized arrays on the RNA are essential: mutations that disrupt either property abolish genome packaging even when viral particles still form.14PubMed Central. Plasma Membrane Anchoring and Gag:Gag Multimerization on Viral RNA Are Critical Properties of HIV-1 Gag Required To Mediate Efficient Genome Packaging The virus borrows the cell’s own membrane-sculpting machinery, known as the ESCRT pathway, to pinch off the budding particle and release it from the cell surface.15PubMed Central. ESCRT-II functions by linking to ESCRT-I in human immunodeficiency virus-1 budding
Freshly budded particles are not yet infectious. They need a final step called maturation, during which the viral protease enzyme cleaves the large Gag precursor into its individual components. This cleavage reorganizes the internal structure of the particle, converting it from an immature, roughly spherical arrangement into the characteristic cone-shaped capsid found in mature, infectious HIV. Protease inhibitors, a long-established class of HIV drugs, work by blocking this exact step.
The Latent Reservoir
If every infected cell were actively churning out virus, treatment would be far simpler. The complication is latency. A small fraction of cells that carry integrated proviral DNA go quiet: they do not produce new virus, and because they look like ordinary resting immune cells, neither the immune system nor antiviral drugs can identify them. This pool of silent, long-lived memory CD4+ T cells harboring intact, replication-competent HIV genomes is the major barrier to curing the infection.16PubMed Central. HIV persistence in subsets of CD4+ T cells: 50 shades of reservoirs
Latency is established astonishingly early. Research in people treated within days of developing symptoms of acute HIV infection showed that even aggressive antiretroviral therapy started that soon could not prevent the formation of latently infected cells carrying integrated viral DNA.17PubMed. Early establishment of a pool of latently infected, resting CD4(+) T cells during primary HIV-1 infection Lab experiments have confirmed that both resting and activated T cells can become latently infected, meaning the virus does not need a fully activated cell to set up a silent foothold.18PLoS Pathogens. HIV Latency Is Established Directly and Early in Both Resting and Activated Primary CD4 T Cells If the cell later wakes up in response to some immune signal, the provirus can reactivate and start producing virus again, which is why people who stop antiretroviral therapy nearly always see viral rebound.
Why HIV Mutates So Rapidly
Reverse transcriptase is fast but sloppy. Unlike the cell’s own DNA-copying enzymes, it has no built-in proofreading ability, so it introduces mistakes at a rate estimated between roughly one error per hundred thousand bases and one per thousand bases copied per replication cycle.19PubMed Central. The Determination of HIV-1 RT Mutation Rate, Its Possible Allosteric Effects, and Its Implications on Drug Resistance Given that HIV’s genome is about 10,000 bases long and billions of new particles can be produced every day in an untreated person, the virus generates an enormous variety of slightly different versions of itself constantly.
This high mutation rate is a double-edged sword for the virus. Most mutations are harmful or neutral, but the sheer number of rolls of the dice means that drug-resistant variants can emerge quickly whenever selective pressure is applied. That genetic diversity is the primary reason why combination antiretroviral therapy, using drugs that target different steps of the life cycle simultaneously, is so much more effective than any single drug alone.20PubMed Central. Avoiding Drug Resistance in HIV Reverse Transcriptase It is also a central obstacle to developing an effective vaccine: the virus changes fast enough to outrun many immune responses.
How the Body Tries to Fight Back
Human cells are not defenseless against retroviruses. Over millions of years of evolution alongside viruses, we have developed a set of built-in antiviral proteins, sometimes called restriction factors, that can interfere with various steps of the HIV life cycle. APOBEC3G, for example, can lethally mutate the viral genome during reverse transcription by editing cytosine bases to uracil in the newly synthesized DNA. BST-2 (also known as tetherin) can physically trap new virus particles on the cell surface, preventing their release. SAMHD1 can starve the virus of the raw building blocks it needs for reverse transcription.
HIV has evolved countermeasures to each of these defenses. Its accessory proteins, Vif, Vpu, and others, act as molecular adaptors that redirect the cell’s own protein-disposal systems to destroy these restriction factors or shunt them to the wrong part of the cell where they cannot function.21PubMed Central. HIV accessory proteins versus host restriction factors Vif, for instance, tags APOBEC3G for degradation before it can get packaged into new virus particles. Vpu sends BST-2 to the cell’s recycling machinery. None of these accessory proteins have enzymatic activity of their own; they work entirely by manipulating the cell’s existing systems. This molecular arms race is ongoing and has shaped both the virus and the human genome over evolutionary time.
Cell-to-Cell Spread and Metabolic Hijacking
The classical image of HIV replication involves free-floating virus particles drifting through the bloodstream and bumping into a new cell. That happens, but it is not the only, or even the most efficient, route. HIV can also pass directly from an infected cell to an uninfected one through a structure called a virological synapse, a tight adhesive junction that forms between the two cells.22PubMed Central. HIV-1 Virological Synapse is not Simply a Copycat of the Immunological Synapse This cell-to-cell transfer concentrates the virus at the point of contact, making infection more efficient and potentially helping the virus dodge antibodies that would neutralize free-floating particles.
HIV also reshapes the metabolism of the cells it infects. CD4+ T cells with active HIV infection show boosted glycolysis, a shift toward rapidly burning glucose for energy that resembles the metabolic rewiring seen in some cancer cells.23PubMed Central. Metabolic Reprogramming in HIV+ CD4+ T-Cells: Implications for Immune Dysfunction and Therapeutic Targets in M. tuberculosis Co-Infection This metabolic reprogramming likely serves the virus by increasing the supply of building blocks and energy needed for rapid viral production, but it may also contribute to the immune dysfunction seen in HIV disease, since T cells running a warped metabolic program do not function normally.
HIV-1 Versus HIV-2
Most global attention focuses on HIV-1, but HIV-2, found mainly in parts of West Africa, follows the same basic life cycle. The critical difference is output. HIV-2 establishes a stable integrated proviral infection just as HIV-1 does, with similar amounts of total viral DNA in infected cells, but it produces significantly less viral RNA. That lower rate of viral gene expression translates directly into lower levels of virus in the blood and is thought to explain why HIV-2 progresses more slowly and transmits less easily between people.24PubMed Central. Direct evidence of lower viral replication rates in vivo in human immunodeficiency virus type 2 (HIV-2) infection than in HIV-1 infection
Competition experiments in the lab have shown that HIV-1 group M isolates, the strains responsible for the global pandemic, are roughly a hundred times fitter than either HIV-2 strains or the rarer HIV-1 group O strains when grown alongside each other in human blood cells.25PubMed Central. The replicative fitness of primary human immunodeficiency virus type 1 (HIV-1) group M, HIV-1 group O, and HIV-2 isolates That dramatic fitness advantage helps explain why HIV-1 group M dominates the global epidemic while HIV-2 and group O remain geographically contained.
How the Virus Crossed Into Humans
HIV-1 originated from a simian immunodeficiency virus (SIV) that infected chimpanzees, and the chimpanzee virus itself was a hybrid that arose from SIV strains infecting other primate species. Jumping from one host species to another is not trivial for a virus because each new host has slightly different versions of the restriction factors described earlier. Research into one of the ancestral SIV lineages found that its Vif protein could partially counteract chimpanzee APOBEC3G but managed only about 16% of the job compared to the SIV strains already adapted to chimpanzees. The same ancestral virus also could not counteract chimpanzee A3D, a restriction factor that had gained heightened antiviral activity through natural selection in the chimpanzee lineage.26Cell Host & Microbe. Gene Loss and Adaptation to Hominids Underlie the Ancient Origin of HIV-1 The virus had to evolve solutions to these specific barriers, including possibly losing an entire gene (vpx), before it could thrive in chimpanzees and eventually make the further jump into humans. Each species boundary is essentially a new set of locks that the virus must pick, and the molecular traces of that lock-picking are still visible in the viral genome today.
Drugs Targeting Multiple Steps at Once
Traditional antiretroviral drugs each target a single enzyme or step: reverse transcriptase inhibitors block the RNA-to-DNA conversion, integrase inhibitors prevent chromosomal insertion, and protease inhibitors stop maturation. In 2022, a fundamentally different kind of drug reached the market. Lenacapavir is a capsid inhibitor: it binds to the structural protein that forms the viral capsid shell and interferes with multiple stages of the life cycle rather than just one.27PubMed Central. Pharmacological outlook of Lenacapavir: a novel first-in-class Long-Acting HIV-1 Capsid Inhibitor
Lenacapavir binds at a conserved interface between adjacent capsid protein subunits, stabilizing the capsid lattice and disrupting the nuclear import of the capsid, its disassembly at the right moment, and the proper formation of capsids in newly released particles.28PubMed Central. Discovery of Lenacapavir: First-in-Class Twice-Yearly Capsid Inhibitor for HIV‑1 Treatment and Pre-exposure Prophylaxis Because it hits the capsid at several different points in the life cycle, the drug has potent antiviral activity and a high barrier to resistance. It also has a remarkably long duration of action, requiring only a subcutaneous injection twice a year, which has obvious advantages for adherence compared to daily pills.29PubMed. Lenacapavir: A Novel Long-Acting Capsid Inhibitor for HIV The existence of a drug that disrupts early infection, nuclear transport, and late assembly simultaneously underscores how central the capsid is to the entire replication cycle.