Why Some People Are Naturally Protected From HIV

A small fraction of people carry genetic traits that make them highly resistant to HIV infection or, if infected, allow them to control the virus without medication for years or even decades. The best-known example is a mutation in the CCR5 gene, but protection turns out to involve a surprisingly broad set of mechanisms, from the shape of immune-system molecules to antibodies lurking in mucosal tissue to tiny proteins inside cells that shred viral DNA before it can do any harm. Understanding how these defenses work has already led to real cures for a handful of patients and is reshaping the search for vaccines and gene therapies.

The CCR5-Delta32 Mutation

HIV needs two things to enter a human immune cell: the CD4 receptor and a co-receptor, usually a protein called CCR5. A deletion mutation known as CCR5-Delta32 removes 32 base pairs from the CCR5 gene, which prevents the receptor from appearing on the cell surface at all. Without that doorway, the most common strains of HIV simply cannot get inside.

People who inherit two copies of this deletion (one from each parent) lack functional CCR5 entirely and are strongly resistant to infection. A meta-analysis pooling data across multiple studies found that people with two copies had roughly a 75 percent reduction in risk of acquiring HIV compared with those who had normal CCR5 genes.¹ The mutation essentially locks the front door on the virus.²

One Copy Versus Two

Carrying a single copy of Delta32 does not block infection outright, but it does slow the disease down. Among people already living with HIV, those with one copy of the mutation are over-represented in the group called long-term nonprogressors, people who maintain relatively stable immune function for many years without treatment. One study found that about 36 percent of long-term nonprogressors carried a single copy, compared with roughly 13 percent of people whose disease advanced quickly.³ The likely reason is straightforward: with fewer CCR5 receptors on the surface of their immune cells, these individuals give the virus fewer entry points, which slows its spread through the body.

Two copies of Delta32 are rare in most populations. The mutation is concentrated in people of Northern European ancestry, where roughly one in a hundred people is homozygous (carrying two copies) and about one in ten carries a single copy. In most African, East Asian, and Indigenous American populations, the mutation is extremely uncommon or absent. That geographic skew matters: it means this particular form of genetic protection is far from universal.

How HLA Molecules Shape Immune Control

CCR5-Delta32 is the most famous form of natural protection, but plenty of people control HIV without it. A major reason involves HLA molecules, the proteins on cell surfaces that display fragments of invaders to the immune system. Certain versions of these molecules are especially good at flagging HIV for destruction.

HLA-B57 stands out. People carrying this gene variant can present a wide range of conserved fragments from the virus’s core protein (Gag) to both killer T cells and natural killer (NK) cells.⁴ “Conserved” is the key word here: these are parts of the virus that it cannot easily change without crippling itself. When HLA-B57 forces HIV to mutate in those regions to escape immune attack, the resulting mutant virus is often weaker and replicates more slowly. Research has described this as a dual mechanism: the virus loses fitness through escape mutations, and the immune system mounts strong responses even to the mutated versions.⁵

The result is that HLA-B57 carriers are heavily over-represented among so-called elite controllers, the rare group of HIV-positive people who maintain undetectable viral loads for years without antiretroviral therapy. Their immune systems essentially wage a war of attrition the virus cannot win.

Natural Killer Cells and the KIR Connection

The immune system’s first-responder cells, natural killer (NK) cells, also contribute. NK cells are regulated by a family of receptors called KIRs, and the combination of specific KIR genes with specific HLA molecules can either enhance or dampen NK cell activity against infected cells.

One combination that matters is KIR3DS1 paired with HLA-B molecules carrying a motif called Bw4-80I. NK cells with this combination were shown to strongly inhibit HIV replication in infected cells, in a dose-dependent and contact-dependent manner. The inhibition was specific: it only worked when the target cells displayed the right HLA molecule.⁶ The broader interaction between KIR and HLA-B genes has been recognized as a significant modifier of HIV disease progression.⁷

A recent study of post-treatment controllers, people who started antiretroviral therapy early and then maintained viral suppression even after stopping it, found that a specific combination of HLA-B*35 with a genotype associated with NK cell education was common among them. The findings suggest NK cells play a direct role in sustaining HIV remission, not just in slowing initial disease.⁸

Cellular Restriction Factors

Even before the immune system mobilizes T cells or NK cells, individual human cells carry built-in antiviral proteins that can block HIV at different stages of its life cycle. These restriction factors are part of the innate immune system, and variation in their activity among individuals likely contributes to differences in susceptibility.

APOBEC3G and APOBEC3F are enzymes that sabotage HIV’s genetic material during replication by introducing so many errors into the viral DNA that the resulting copies are nonfunctional. TRIM5-alpha intercepts incoming virus particles and routes them to the cell’s recycling machinery before the virus can even begin copying itself.⁹ A third factor, SAMHD1, starves the virus of the raw materials it needs to build DNA copies in certain immune cells.¹⁰

HIV has evolved countermeasures against all of these, which is part of why it is so difficult to defeat. But the balance between restriction factors and viral counter-proteins varies from person to person. Someone whose APOBEC3G is expressed at higher levels, or whose TRIM5-alpha variant is slightly more efficient, may have a meaningful edge.

Mucosal Antibodies at the Point of Entry

Most HIV transmission occurs across mucosal surfaces, so immune defenses at those sites are the virus’s first obstacle. Some people who remain uninfected despite repeated sexual exposure to HIV produce a specific type of antibody, IgA, in their genital or oral mucosa that can neutralize the virus before it crosses the epithelial barrier.

Research on HIV-exposed but uninfected individuals found that IgA purified from cervicovaginal fluid and saliva could block HIV from passing through epithelial cells in the lab. This inhibitory effect showed up in about half of saliva samples and half of cervicovaginal samples from exposed individuals, but in none of the samples from unexposed controls.¹¹ A separate study found that women taking pre-exposure prophylaxis (PrEP) who were also sexually exposed to HIV had IgA with significantly higher neutralizing ability than women on placebo, suggesting that both drug exposure and repeated viral contact can prime these mucosal defenses.¹²

Elite controllers also show stronger immune responses in mucosal tissue. Their gut-lining CD8 T cells mount more complex, multi-functional responses than those seen in people whose virus is controlled by antiretroviral drugs.¹³ This hints that for some people, the battlefront where the virus meets the strongest resistance is right at the body’s surfaces.

When the Virus Itself Is Defective

Sometimes protection comes not from the host but from the pathogen. A small number of long-term nonprogressors turned out to be infected with viruses carrying broken versions of the nef gene, which HIV normally needs to replicate efficiently and evade immune detection. A well-known group of patients in Australia were infected with nef-deleted viruses for over 11 years and maintained extremely low viral loads, in some cases fewer than 20 copies per milliliter of blood.¹⁴

This is not a permanent guarantee, though. HIV is remarkably good at repairing itself. In one case, a virus with a 36-base-pair deletion in nef was observed to partially restore function through a nearby duplication that recovered some, but not all, of the protein’s activity.¹⁵ So while a crippled virus can give a person years of apparent protection, the virus may slowly evolve its way out of the handicap.

Another wrinkle involves a less common form of HIV called X4-tropic virus, which uses a different co-receptor (CXCR4) instead of CCR5. In a documented case, a person with normal CCR5 genes was infected by a purely X4-tropic strain through the mucosal route and experienced unusually rapid immune cell depletion.¹⁶ This underscores that CCR5-based protection has a hard limit: it guards against the dominant R5-tropic strains but cannot stop a virus that has evolved to use a different door.

Epigenetic Silencing and Post-Treatment Control

Beyond inherited genetic traits, some people appear to silence the virus at a molecular level after infection. In long-term nonprogressors and elite controllers, the HIV DNA that integrates into their cells tends to be more heavily methylated, a chemical modification that effectively mutes the virus’s ability to produce new copies. The degree of methylation also correlated with how long a person had been infected, suggesting the silencing deepens over time.¹⁷

A related phenomenon involves post-treatment controllers. The French VISCONTI study identified individuals who started antiretroviral therapy very early after infection and then maintained viral remission for years after stopping treatment. These patients had extremely small viral reservoirs, partly because their long-lived CD4 T cells were infected at very low rates.¹⁸ Their profile differed from classic elite controllers: they did not typically carry the protective HLA alleles associated with strong T cell responses. Instead, their early treatment seemed to have prevented the virus from establishing a deep foothold, and their innate immune systems, particularly NK cells, helped maintain control afterward.

Evolutionary Origins of CCR5-Delta32

HIV has only been circulating in humans since the early twentieth century, so the Delta32 mutation did not arise because of HIV. Something else drove it to high frequency in European populations long before the modern pandemic. Researchers have long debated what that selective pressure was, with bubonic plague and smallpox among the leading candidates.¹⁹

A 2025 study using ancient DNA has pushed the timeline much further back. By screening hundreds of ancient genomes, researchers traced the deletion to at least 6,700 years ago in the Western Eurasian Steppe region and found strong evidence that positive selection acted on the mutation between roughly 8,000 and 2,000 years ago, during the late Neolithic and Bronze Age. The mutation’s presence in Latin American populations can be explained by post-Columbian genetic mixing rather than independent origin.²⁰ Whatever pathogen originally favored this mutation, it was active thousands of years before plague or smallpox pandemics hit Europe, which complicates both of those popular hypotheses.

The West Nile Tradeoff

Lacking CCR5 is not entirely free of consequences. The same receptor that HIV exploits also plays a role in the immune response to other infections. The clearest example involves West Nile virus. Two independent cohorts found that people homozygous for Delta32 were significantly more likely to develop symptomatic West Nile infection, with one cohort showing more than four-fold higher odds and the other more than nine-fold higher odds compared with people who had at least one working CCR5 gene.²¹ In one of those cohorts, the Delta32 homozygous state was also associated with more than thirteen-fold higher odds of dying from the infection.

Before this discovery, people who lacked CCR5 were thought to be essentially healthy, with no obvious downside.²² The West Nile data changed that view and added an important cautionary note for gene-editing approaches that aim to knock out CCR5 as a therapeutic strategy. Deleting one receptor to block one virus might open a vulnerability to others.

Clues From Primates and Children

Humans are not the only primates that deal with immunodeficiency viruses. Sooty mangabeys and African green monkeys carry SIV (simian immunodeficiency virus) at high levels but rarely get sick. They avoid the runaway immune activation that destroys the immune systems of HIV-infected humans and HIV-infected non-natural-host primates alike. One of their key adaptations is reduced CCR5 expression on long-lived CD4 T cells, which limits viral access to the most important immune cells.²³ They also dampen the interferon response that, in humans, fuels chronic inflammation. Sooty mangabey immune cells produce far less interferon-alpha in response to viral stimulation than human cells do.²⁴

Remarkably, a subset of HIV-infected children appears to have independently arrived at a similar strategy. Among 170 untreated children who maintained normal immune cell counts despite years of infection, immune activation levels were low even though their viral loads were substantial (a median of around 26,000 copies per milliliter). Like the mangabeys, these children also showed low CCR5 expression on their central memory CD4 T cells.²⁵ The parallel suggests that the path to living with HIV without getting sick may involve tolerating the virus rather than trying to eliminate it, a fundamentally different strategy from the aggressive immune responses seen in adult elite controllers.

From Biology to Treatment

The CCR5-Delta32 mutation has already produced actual cures, though in extraordinary circumstances. The first was a patient with both HIV and acute myeloid leukemia who received a bone marrow transplant from a donor homozygous for Delta32. After the transplant, the patient stopped antiretroviral therapy and showed no viral rebound more than 20 months later.²⁶ Several additional patients have since been cured through the same approach, all of whom needed stem cell transplants for blood cancers and received cells from Delta32 homozygous donors.²⁷

These cases are proof of concept, not a scalable treatment. Bone marrow transplants are dangerous, expensive, and only justified when someone also has a life-threatening blood cancer. The real interest lies in replicating the effect without the transplant. CRISPR gene-editing technology has been used to knock out CCR5 in laboratory cell lines and animal models, achieving significant resistance to the common R5-tropic strains of HIV.²⁸ Early clinical trials are underway.

Meanwhile, researchers studying broadly neutralizing antibodies (bNAbs) have taken a different lesson from natural protection. A small percentage of people living with HIV naturally produce antibodies capable of neutralizing many different strains by targeting conserved spots on the viral envelope. Clinical trials have found that combinations of these antibodies, administered as drugs, can keep the virus suppressed even after stopping standard antiretroviral therapy, though pre-existing resistance in the patient’s viral population remains a hurdle.²⁹ The goal is to make what a few people’s immune systems achieve naturally available to everyone through engineered biology.

Why No Single Gene Tells the Whole Story

It is tempting to look for a single “HIV resistance gene,” but the evidence points to a layered system. CCR5-Delta32 blocks viral entry. HLA-B57 and related alleles shape how powerfully T cells and NK cells attack the virus. KIR gene variants determine how effectively NK cells recognize and kill infected cells. Restriction factors inside cells degrade viral DNA. Mucosal IgA antibodies intercept the virus before it even reaches a vulnerable cell. Epigenetic silencing can mute integrated viral DNA. And in some cases, the virus itself arrives in a crippled form.

Most elite controllers and long-term nonprogressors carry some combination of these advantages, not all of them. A person with protective HLA alleles but normal CCR5 can still become an elite controller. A person with Delta32 heterozygosity but unremarkable HLA genes can still be a long-term nonprogressor. The small fraction of people who appear fully resistant to infection, sometimes despite years of high-risk exposure, likely have multiple layers of defense operating simultaneously. Ongoing research into the microRNA profiles of nonprogressors has identified sets of small regulatory molecules that differ between people who control HIV and those who do not, adding yet another layer of biological variation to the picture.³⁰ Each new layer identified brings the field closer to understanding exactly which knobs to turn to give everyone the protection that a fortunate few inherited.