The signature effect of HIV infection, and the cause of AIDS, is disruption of the T-lymphocyte branch of the immune system and in particular the destruction of CD4+ T-helper cells.
A team of researchers at the US National Institute of Allergies and Infectious Diseases (NIAID) has now found that HIV also causes a very specific form of damage to the other half of the adaptive immune system, the B-cells, and in particular the memory B-cells, which recognise previously-experienced infections and generate antibodies against them.
By using probes to delete specific genes within B-cells, they discovered that HIV infection creates an unusual population of exhausted, non-responsive cells called tissue-like B-memory cells. In previous experiments with cells taken from HIV-negative people, they found that that these cells are characterised the activation of genes which cause the cell to produce proteins that inhibit the cell’s function and that two of these inhibitory proteins had an especially strong effect on B-cell function.
Now, in cells taken from people with HIV, they have found that, by deleting the genes that manufactured these inhibitory proteins, they could restore the anti-HIV activity of these B-cells, at least in the test tube, that this rejuvenated activity was long-lasting, and that the cells exhibited a number of other markers of increased immune activity.
Although the gene-therapy techniques used in these experiments were sophisticated and can cause unpredictable immune reactions in themselves, the inhibitory proteins thus identified could become new therapeutic targets.
One of the puzzles of HIV infection has always been that, while the immune system does mount an antibody response to HIV – indeed it is these antibodies that are detected in the standard HIV test – this response only partially controls viral replication, and eventually fails to entirely.
B-lymphocytes are the bone-marrow cells and their job is to secrete antibodies. Antibodies are soluble protein molecules that either directly destroy foreign invaders, render them harmless, or tag them for destruction by other parts of the immune system.
A strong antibody response to a pathogen can either prevent an infection happening altogether or can clear it from the system. Once an infection is experienced, the body creates a population of ‘memory’ B-cells that swiftly mount an antibody response if the invading pathogen is encountered again.
Vaccines generally work by imitating an infection and thus setting up a memory B-cell response in advance of an actual infection. T-cells, the thymus cells, work in a similar way but destroy infected cells rather than manufacture antibodies.
In HIV infection, the body mounts a very strong antibody response in the first few weeks that partially works, bringing the viral load down from millions to, on average, about 50,000 copies/ml. However it does not contain viral replication any further or eliminate HIV infection, and eventually weakens so that the viral load increases again.
Dr Lela Kardava and her team from NIAID discovered that people with HIV had an unusual subset of B-memory cells called tissue-like cells that were characterised by the presence, on their surface, of a variety of inhibitory receptor molecules. The cells behaved much the same as exhausted T-cells do in HIV infection: they were sluggish and failed to react to foreign substances and to HIV itself.
In a series of experiments, Kardava’s team knocked out specific genes coding for these inhibitory proteins and found that by doing so they were able to restore some of the B-cells’ antibody responses. They did so by incubating cells with pieces of small interfering RNA (siRNA), molecules that target and interfere with specific genes. Previous experiments in cells taken from HIV-negative people had shown that ‘downregulating’ the inhibitory proteins with siRNA led to an 80-90% increase in the ability of the B-cells to proliferate.
They deleted nine inhibitory molecules in turn in cells taken from a group of people with chronic HIV infection. These individuals were either not taking ARV therapy or had only recently started and had an average viral load of 2096 copies/ml, with an average CD4 count of 427 cells/mm3.
They found that the deletion of two inhibitory receptors called FCRL4 and SIGLEC6 had particularly strong rejuvenating effects. The siRNA targeting the genes coding for these proteins led to a 30-66% reduction in the expression of these proteins in the cell. This in turn led to a doubling of the number of cells which, in response to standard immune stimuli, secreted anti-HIV antibodies.
These responses were long-lasting; the anti-HIV antibody responses of the cells that had had FCRL4 and SIGLEC6 ‘downregulated’ (reduced) demonstrated a similar increase in responsiveness to HIV several weeks later. Cells with the downregulated inhibitory proteins also secreted five times as much of the powerful pro-inflammatory chemical (cytokine) interleukin-6 and 50% more of the chemokine MIP-1α, indicating that modulating B-cell exhaustion may have a number of other immune-modulating effects.
The NIAID team are working on the hypothesis that the exhaustion seen in the B-cells of people with HIV is very similar to that seen in T-cells: the cells essentially stop working as a defensive manoeuvre against a virus whose constant stimulation would otherwise cause more damage by exciting the body into a constantly inflammatory state.
If, however, therapies could be devised than enabled B-cells to mount better antibody responses to HIV without undesirable side-effects, they could in theory form part of a ‘functional cure’ that rendered HIV infection less harmful – or might even be part of a way to eliminate HIV from the body.
The NIAID team say: “Our findings suggest that the development of strategies aimed at reversing the deleterious effects of these inhibitory receptors may improve immune responses against...persisting viruses.”
Kardava L et al. Attenuation of HIV-associated human B cell exhaustion by siRNA downregulation of inhibitory receptors. Journal of Clinical Investigation, early online edition doi: 10.1172/JCI45685. 2011.