The subject had an unusual combination of factors that inhibited HIV replication.
By: Gus Cairns –
At: http://www.aidsmap.com/page/3511496/
Researchers in Sydney, Australia, have identified a patient who appears to have spontaneously cured his own HIV infection without any medication, many years after the initial infection.
However, "Subject C135" appears to have a unique combination of fortunate circumstances that only serve to illustrate how difficult it is to eliminate HIV infection once it has taken hold. Researchers caution that we do not yet know whether these same characteristics could be recreated in other people with HIV through artificial means, such as genetic engineering and vaccines.
The Sydney Blood Bank Cohort
Subject C135 was infected in 1981, at age 34, when he needed a blood transfusion after a car accident. Now 72, he belongs to a group of eight people who were infected by the same donor; these, plus the donor, form a group called the Sydney Blood Bank Cohort (SBBC).
The SBBC patients initially attracted attention because they all appeared to be elite controllers: they were asymptomatic and had stable CD4 counts despite not taking medication. Most of them were identified in 1991, although C135 was not identified until 1996, when all transfusion recipients from the donor, D36, were traced.
Another member was identified posthumously: this person died at age 22 in 1987 from the autoimmune disease lupus, which required immunosuppressive therapy; because of this, HIV-related conditions may have contributed to their death. Two members of the cohort died between 1994 and 1999, from causes unrelated to HIV, at ages 77 and 83.
The other six and the donor are still alive, but only three remain elite controllers, maintaining undetectable viral loads on therapy. The donor began antiretroviral therapy in 1999 after a decline in CD4 count and the onset of neurocognitive symptoms, and the other three since then.
In 2011, a study of the three remaining elite controllers found that the factor that most distinguished them from other members of the cohort was a strong CD4 cell response to the HIV capsid protein p24. That article noted that “only one patient, C135, has identifiable genetic polymorphisms that likely contributed to non-progression.” He was described as “unique” even then.
Case C135
The researchers now feel they have sufficient evidence, so they say that C135 represents "a probable case of elimination of HIV infection," following repeated polymerase chain (PCR) testing of lymphocytes (T cells) taken from blood and intestinal and lymph node tissue.
In fact, the last time detectable HIV DNA could be recovered from this patient was in March 1997, 22 years ago. Since then, all attempts to detect HIV have failed, so he may have actually been HIV-free in 1997. The caution in stating this reflects the difficulty of finding HIV in cells and the fact that in some cases of people with repeatedly undetectable HIV tests, the virus has reappeared over time.
C135 was definitely infected: The 1996 Western blot test, which detects specific HIV proteins, showed positive results for p24, the p18 shell protein, and the gp160 envelope protein. However, these were much weaker than normal, indicating that HIV was replicating unusually slowly. At this point, her CD4 count averaged around 500; it has slowly increased over 20 years to approximately 750. Her CD4:CD8 ratio has remained at around 1.1 throughout. Although within normal ranges, these are on the lower end, reflecting the fact that her immune system may have sustained some damage in the initial infection. Her CD38 percentage is within the normal range, varying in recent years between 0.6% and 2%. The CD38 percentage is the proportion of CD8 (suppressor T) cells that are activated and fighting infections at any given time.
Five pieces of good luck.
So these are all normal results: what was atypical about C135? It turns out that, aside from the bad luck of receiving a transfusion containing HIV in the first place, he had the good fortune to benefit from five separate factors that suppress HIV replication and encourage a strong, specific immune response against it.
Subject C135 appears to have a unique combination of fortunate circumstances that only serve to show how difficult it is to eliminate HIV infection once it is established.
The first is the defining characteristic of patients with SBBC: the virus shared between the donor and recipients has a missing piece of DNA that codes for a viral gene called nef (negative regulatory factor). Nef amplifies T cell activation, thus providing new cells for the virus to infect, degrades a cellular antiviral defense called CTLA-4, and sends a false activation signal to cells that keeps the immune system on high alert. The virus also lacked a section called the LTR (long terminal repeat), which acts like the end cap on a shoelace, preventing the viral DNA from unraveling.
Although the nef-deficient virus replicates slowly and people who have it tend to have lower viral loads, it is not disabled and will eventually cause immune damage to most people.
However, the patient had several other characteristics that further slowed viral replication. The second characteristic was that he was what is called heterozygous for the CCR5 gene. This means that, like 8–10% of people of Northern European descent, he had only one copy of the gene that coats the surfaces of CD4 cells with the co-receptor molecule CCR5. People without CCR5 genes, approximately 1% of Northern Europeans, are virtually immune to HIV. C135 had 48% of the normal complement of CCR5. It was still possible to infect cultured cells from him in the lab dish with novel HIV strains, but his HIV-suppressing CD8 cells had to be eliminated before infection could occur.
The third characteristic is that although their overall immune system wasn't overly sensitive to HIV, their CD4 cells had a very potent and specific response to a 15-amino-acid length of the HIV gag (shell) protein. These peptides, or short sections of protein, are what virus-infected cells display on their surfaces to "call for help" and signal that they are infected. When these trigger an immune response, they are called epitopes. In other words, C135's CD4 cells were unusually alert to a particular and very specific signal of viral infection. In response, they quickly released the cell-signaling chemical Interleukin-2 (IL-2) to direct CD8 cells to destroy them.
The fourth characteristic of C135 is that it had two cellular immune genes that ensured its response to HIV was particularly efficient: HLA-B57 and HLA-DR13. HLAs (human leukocyte antigens) are the cell surface molecules that "present" epitopes to the immune system, and some can do so more efficiently than others.
HLA-B57, in particular, is known for two things. First, people with the B5701 variant of this gene are allergic to the antiretroviral drug abacavir and cannot take it. Second, however, HLA-B57 and HLA-DR13 are associated with lower HIV viral loads and slower progression. Fifty percent of long-term non-progressors who remain off ART for many years have HLA-B57, even though only 1.5% to 5% of most populations have it.
Subject C135's acute CD4 response to HIV was not caused by having HLA-B57. Rather, the HLA type acted as an intermediary. Once his CD4 cell fired IL-2 as an instruction to destroy HIV-infected cells, the efficiency with which his HLA-B57 displayed HIV on the cell surface meant that the effect of IL-2 was amplified and accelerated, ensuring his fifth and final characteristic—and the one that likely eliminated all HIV-infected cells from his body: a strong and widespread CD8 (T-cell suppressor) cell response to HIV-infected cells. This response was weak unless the CD8 cells were cultured alongside the CD4 cells, demonstrating that the CD4 response to HIV had to occur first, before the CD8 cells "figured out" that they had to kill the HIV-infected cells.
So, C135's response to HIV was so potent, probably in part because his virus replicated slowly, and therefore, during the initial infection, his immune system had a chance to develop an effective and specific response against HIV before HIV had a chance to mutate away from that vulnerability—what has been called "immune escape." HIV usually wins the "arms race" between the body's immune response and its ability to evade it, but it can occasionally lose the race in situations like this, or during very early treatment, or if the body is already primed to recognize HIV with a vaccine.
Furthermore, however, she had genetic factors that ensured this early immune response was particularly rapid and efficient, which may have ensured that her slow-acting HIV never had time to develop resistance to the immune response. Instead, that response did what it does with most non-HIV viruses: it eliminated her.
Could scientists make it happen in others? Probably not yet.
Therefore, a very specific chain of characteristics had to come together and act sequentially to ensure that the immune response to what was already a weakened virus in a person's system was strong, specific, and fast enough to do what has never been seen before: allow someone to spontaneously eliminate all HIV from their body, without the need for medicine.
There may be other C135 cases out there. He was sought out and tested because he had received blood from an HIV-positive donor, but he was otherwise not in one of the high-risk HIV populations and was asymptomatic. Therefore, there could be other elite controllers who eliminated their infection whom we don't know about because we never knew they had HIV. On the other hand, testing programs haven't identified similar patients.
Could vaccines or gene therapies be designed to induce changes that mimic what happened naturally in this person? The answer is no, not at present. Although scientists can, and have, designed vaccines that recognize the p24 segments to which their CD4 cell was sensitive, they have no idea how to alter human HLA molecules to act with the same efficiency as they did in this case, or even if it's possible. There have also been attempts to block nef function before, but nothing that has worked.
However, the C135 case offers us something that the Timothy Ray Brown case also did in 2008: a proof of concept. It shows that circumstances can arise, albeit rarely, that allow for a spontaneous cure or deep remission of HIV, and the proof that it can happen is what prompts researchers to make it happen more often.