Researchers have found that the HIV virus increases the potency of the tuberculosis bacterium (Mtb) by affecting a central function of the immune system.
In most people who are exposed to tuberculosis infection, the immune defence deals with the bacteria by enclosing them in a special scar tissue. In this condition the tuberculosis is said to be "latent". Around 10 per cent of those with latent tuberculosis go on to develop active disease.
"The risk of infection progressing to active tuberculosis is around 30 times higher for people who are HIV-positive.
But the reason for this has not been known at the cellular level," said lead researcher Robert Blomgran from Linkoping University in Sweden.
The new study, published in the American Journal of Pathology, describes how the researchers looked in more detail at what happens in one particular type of immune cell, known as dendritic cells.
These play an important role in the immune defence. Dendritic cells break down tuberculosis bacteria and other foreign microorganisms, and display the bacteria fragments at the cell surface.
Other cells of the immune system, in particular T-cells, recognise the fragments and bind to them.
The dendritic cell then activates the T-cell such that it can kill the tuberculosis bacteria efficiently.
In this way, dendritic cells act as a communication link between the innate immune defence and the specific immune defence, of which the T-cells are part.
The researchers infected human dendritic cells with both Mtb and the HIV virus. They showed that co-infection reduced the ability of the dendritic cells to present foreign molecules to the immune defence.
They also had a lower capacity to activate tuberculosis-specific T-cells than was the case when the dendritic cells were infected with Mtb alone.
"We have now shown that HIV has a clear effect also on the innate immune defence, in particular the dendritic cells, which link the innate and the adaptive immune defences," Blomgran said.