Enlisting The Immune System To Attack Bacteria

A team of scientists has demonstrated that they can pin molecular bull’s-eyes on pathogenic bacteria. They fed microbes modified D-amino acids that the bacteria incorporated into their cell walls, making them targets for immune cells (ACS Chem. Biol. 2014, DOI: 10.1021/cb5002685). The molecules are a step toward a new antibacterial strategy, the researchers say, one that doesn’t kill bacteria directly but recruits the immune system to do the dirty work.

In recent years, scientists have developed possible cancer treatments that convince the immune system to target tumors. Marcos M. Pires of Lehigh University wondered if a similar approach would work on bacteria.

The anticancer strategies work by getting cancer cells to display molecules on their surfaces that catch the attention of immune cells. So Pires’s team focused on the outer cell walls of gram-positive bacteria. These walls contain peptidoglycan, a cross-linked polymer made from sugars and peptides. Some microbial enzymes grab D-amino acids from the environment and incorporate them into surface peptidoglycans. Since mammalian cells don’t typically use D-amino acids in their proteins, Pires thought this process may be “an interesting way to target bacteria.”

So Pires and his team designed D-amino acids that would recruit antibodies to bind to bacterial cell walls. They synthesized a series of D-amino acids containing the antigen 2,4-dinitrophenyl (DNP). Most people naturally have antibodies that recognize DNP, Pires says.

To test their strategy, the researchers incubated Bacillus subtilis with the D-amino acids for four hours, then added fluorescently labeled antibodies that bind DNP. The bacteria incubated with one amino acid, DNP attached to D-lysine, glowed the brightest, indicating that the bacteria were displaying D-Lys(DNP) on their cell walls. Experiments with a pathogenic strain of bacteria, Staphylococcus aureus, showed similar results. Meanwhile, human cells fed D-Lys(DNP) showed no decrease in viability, demonstrating the amino acid probably isn’t toxic to host cells.

Next, the researchers tested whether the DNP-displaying bacteria would provoke macrophages, immune cells that gobble up antibody-tagged invaders. The researchers added macrophages to a culture containing D-Lys(DNP), a DNP antibody, and fluorescently labeled B. subtilis. These macrophages glowed twice as bright as those in cultures without the D-amino acid, indicating the macrophages were more likely to engulf bacteria that incorporated D-Lys(DNP) into their peptidoglycans.

Daniel B. Kearns of Indiana University loves the idea of using D-amino acids to recruit the immune system. But it has some drawbacks, he says. For example, friendly bacteria in the body might fall prey to the method. Also, he wonders whether the effect would be potent enough to conquer a bacterial infection. Pires is working to boost the immune response by increasing the number of antigens on the bacterial surface and identifying more effective antigens.