Antibody cocktail to fight bacteria.

Antibody cocktail to fight bacteria

A mixture of seven human antibodies, produced by a new laboratory procedure, is proposed as an effective protectant against a deadly bacterium, The bacterium, called Pseudomonas aeruginosa, is the most lethal of the microorganisms that patients commonly acquire while hospitalized. Mark E. Lostrom and his colleagues at Genetic Systems Corp. in Seattle focused on this bacterium in devising a new strategy to devellop prophylactic "cocktails' made of human antibodies.

The bacterium P. aeruginosa comes in at least 17 varieties, each having a characteristic surface molecule. The Genetic Systems scientists decided to work only with the seven varieties that are responsible for 90 percent of hospital infections, Lostrom said last week in Washington, D.C., at the American Society for Microbiology Conference on Biotechnology.

Invading bacteria expose many surface components to a host's immune system. Some of these components are shared by all bacteria of a species, whereas others provide the means of distinguishing the varieties, called serotypes. Using standard methods for producing large amounts of specific mouse antibodies (monoclonal antibodies), Lostrom and his colleagues employed a laboratory model of an immune system attack directed at each of the different surface components of P. aeruginosa. The serotype-specific antibodies, only one of which attacks a given bacterium, were the most efficient at triggering destruction of the bacteria. The antibodies against components found on all the serotypes were not effective.

The serotype-specific antibodies were also effective at protecting live mice against a bacterial attack. The mice given antibody prophylactically, before a large dose of the bacterium of the same serotype, all survived and showed few symptoms. Mice not given the antibody were killed in 1 to 3 days by the same dose of bacteria.

Antibodies against one shared bacterial product did confer some protection. P. aeruginosa makes a potent toxin; a monoclonal antibody that binds this toxin protected mice against a toxin dose that kills unprotected mice within a day. Lostrom says the toxin's role in human disease is unclear.

Lostrom proposes that a preparation of seven or eight monoclonal antibodies --one for each of the seven important serotypes and perhaps one for the toxin --should protect patients against most hospital infections.

Because administration of mouse antibodies to patients may create an undesirable immune response, the next step was to make "equivalent human monoclonal antibodies with those same marching orders,' Lostrom says. The technique they used, called cell-driven viral transformation, is the "newest advance' in antibody production, according to Lostrom.

This surprising technique avoids the cell fusion step of most other methods to create antibody-producing cells that survive indefinitely in the laboratory. Instead of merging a cell making the appropriate antibody and a cancer cell to create a cell called a hybridoma, the new technique mixes normal blood cells, called B lymphocytes, from patients likely to have hospital-acquired bacterial infections, with other cells infected with the cancer-causing Epstein-Barr virus. The virus moves into the blood cells, and with some biochemical sleight-of-hand the scientists destroy the cells originally infected with Epstein-Barr virus. The scientists then choose from among the transformed cells the ones producing an antibody of interest.

The number of "immortal' cells that result from this procedure is far greater than those from other methods. Lostrom estimates that 1 in 50 of the B lymphocytes present is transformed, whereas with the cell-merging technique only 1 human cell in 10 million is transformed.

"Cell-driven viral transformation gives us the broadest view of the antibody repertoire,' Lostrom says.

Animal trials examining the protective effect of the human monoclonal antibodies gave results so clear-cut that Lostrom admits being self-conscious about showing the graphs. All 10 mice given the human monoclonal antibody survived a high dose of bacterium. But the unprotected mice all died the first day.

The research at Genetic Systems was performed under contract to Cutter Laboratories of Emeryville, Calif. Cutter is now moving toward the product development stage, Lostrom says.

Lostrom suggests that the strategy used to develop the antibodies against P. aeruginosa can be used for fighting other bacteria. But other cases may be more complicated. For example, Escherichia coli, another major cause of hospital-acquired infections, has more than 150 serotypes, among which 14 are responsible for about 80 percent of human E. coli disease.

Hospital-acquired infections are an increasing problem, Lostrom says. He estimates that they strike about 5 percent of U.S. hospital patients, adding more than $1 billion annually to medical costs. As bacterial resistance to antibiotics increases, the fatality rates for the infections also are increasing.

"The best hope [against hospital-acquired infections] is prophylaxis,' Lostrom says. "It is a very viable alternative to antibiotic therapy.'