The vast majority of humans carry the gene CCR5.
In many ways, it is incredibly unhelpful.
It affects our odds of surviving and recovering from a stroke, according to recent research.
And it is the main access point for HIV to overtake our immune systems.
But some people carry a mutations that prevents CCR5 from expressing itself, effectively blocking or eliminating the gene.
Those few people in the world are called ‘elite controllers’ by HIV experts. They are naturally resistant to HIV.
If the virus ever entered their body, they would naturally ‘control’ the virus as if they were taking the virus-suppressing drugs that HIV patients require.
Both the Berlin patient and the London patient received stem cells donated from people with that crucial mutation.
WHY HAS IT NEVER WORKED BEFORE?
‘There are many reasons this hasn’t worked,’ Dr Janet Siliciano, a leading HIV researcher at the Johns Hopkins University School of Medicine, told DailyMail.com.
1. FINDING DONORS
‘It’s incredibly difficult to find HLA-matched bone marrow [i.e. someone with the same proteins in their blood as you],’ Dr Siliciano said.
‘It’s even more difficult to find the CCR5 mutation.’
2. INEFFECTIVE TRANSPLANT LEADS TO CANCER RELAPSE
Second, there is always a risk that the bone marrow won’t ‘take’.
‘Sometimes you don’t become fully “chimeric”, meaning you still have a lot of your own cells.’
That is one of the two most common reasons for previous attempts failing: their immune system is not fully replaced, then the cancer comes back and they can’t survive it.
3. GRAFT-VERSUS-HOST DISEASE: THE OLD IMMUNE SYSTEM ATTACKS THE NEW ONE
The other most common reason this approach has failed is graft-versus-host disease.
That is when the patient’s immune system tries to attack the incoming, replacement immune system, causing a fatal reaction in most.
4. UNKNOWN QUANTITIES
Interestingly, both the Berlin patient and the London patient experienced complications that are normally lethal in most other cases.
And experts believe that those complications helped their cases.
Timothy Ray Brown, the Berlin patient, had both – his cancer returned and he developed graft-versus-host disease, putting him in a coma and requiring a second bone marrow transplant.
The London patient had one: he suffered graft-versus-host disease.
Against the odds, they both survived, HIV-free.
Some believe that, ironically, graft-versus-host disease might have helped both of them to further obliterate their HIV.
But there is no way to control or replicate that safely.