HEALTH: How Measles Damages The Immune System

By IPS Correspondents Reprint | | Print |

Judith Perera

LONDON, Aug 20 1996 (IPS) - One of the reasons why so many still die from measles — more than a million children a year in the developing countries — is because the virus weakens the immune system and lay the patient open to other, more lethal diseases.

“Over a million children die of measles and complications arising from it every year in developing countries,” says Dr Jean- Marc Olive, who is responsible for measles prevention at the World Health Organisation in Geneva.

Measles frequently leads to respiratory disease, diarrhoea, otitis media and even blindness. “We know there is a synergistic effect with vitamin A deficiency and WHO recommends supplements to prevent blindness following measles,” he says.

“Measles can also push children over the borderline into malnutrition, especially if they suffer from post-measles diarrhoea,” he adds.

Although it has long been known that the measles virus can weaken the immune system, the mechanism has remained a mystery.

Now, however, researchers at Johns Hopkins Medical School in Maryland, United States, think they may have the answer.

Immunologist Christopher Karp believes the virus has the effect of suppressing production of a particular protein known as interleukin-12 (Il-12).

Normally Il-12 stimulates white blood cells — ‘killer T-cells’ — to attack invading bacteria or viruses. The Il-12 is produced by other cells known as macrophages, a kind of scavenger cell which mop up dead cells and microorganisms.

They are often the first immune cells to come into contact with a bacteria or virus, and it is their job to signal to the other immune cells so that a proper defence can be mounted. Il-12 is the messenger protein which passes on that signal.

Karp and his colleagues found that a macrophage which had taken up a measles virus produced 10,000 times less IL-12 than normal. Even dead viruses have the effect of shutting off production of this vital protein.

The researchers discovered, on further investigation, that the key to this shut-down effect was a protein on the surface of the macrophage known as CD46. This is the protein that the measles virus latches on to in order to infect the cell. They found that any blockage of CD46 had the same effect.

This discovery has opened the way to more effective vaccines for measles.

“At present, measles immunisation is not given until an infant is nine months old,” explains Dr Olive. It is the last of the routine immunisations to be administered to babies and may sometimes even be forgotten.

It is not given earlier because of some evidence that the vaccines used at present may themselves impair immune function. In any case attempts to give standard measles vaccinations to infants under six months are usually not successful because protection from breast-feeding mothers prevents the vaccine virus from taking hold.

If stronger doses are given the vaccination is more successful — but trials had shown that the result was an increase in death from other disease, especially among baby girls. This is also thought to be a result of reduced immunity after contact with the measles virus, although it is not clear why girls should be more affected than boys.

However, now that the mechanism is better understood it may be possible to alter the measles virus to produce a strain which does not bind to CD46 and to use this strain to make a vaccine. In this way the weakening effect on the immune system would be avoided and it would be safe to give to younger infants.

For the past four years worldwide immunisation against measles has remained at an average of 78 percent ranging from 54 percent in Africa to 88 percent in the western Pacific. Over 26 million cases occur every year, many of which cause serious complications.

The immunisation programme has been most successful in the Americas where targets have been achieved and where eradication is now set for 2000.

New vaccines that could safely be given to infants under a year, however, would be of great benefit in poorer countries, especially in areas where there is a high risk of secondary infection from malaria and other tropical diseases — all more difficult to control and treat.