HAMILTON, MONT—How group A Streptococcus (GAS) can escape destruction by human polymorphonuclear leukocytes (PMNs) to cause infection has not been well elucidated. To reveal the pathogen’s tricks, a recent study screened GAS genes up-regulated during contact with PMNs in vitro.[1] Several up-regulated genes encoded proteins that helped the microbe evade PMNs; at least one was highly expressed in human pharyngitis.

The ability of PMNs to phagocytose pathogens is a crucial component of human innate immunity. PMNs destroy ingested microbes by producing reactive oxygen species (ROS) and other antimicrobial agents. Yet pathogens such as GAS somehow escape destruction. Extracellular proteins conferring phagocytosis resistance had already been identified, “but the screen clearly identified many new factors that were not known to play a role in evading host defense,” said Frank R. DeLeo, PhD, principal investigator of the study. The new findings provide “some potential for therapeutics down the road once we characterize the proteins,” suggested Dr. DeLeo, an investigator at the National Institute of Allergy and Infectious Diseases Laboratory of Human Bacterial Pathogenesis in Hamilton, Montana.

EXPRESSION, PHAGOCYTOSIS ALTERED

Dr. DeLeo and colleagues found that after a 30-minute incubation, 33% of human PMNs contained ingested GAS. After an hour, however, phagocytosis rates leveled off at about 49%. Killing rates also dropped sharply with time. After one hour, almost one quarter of the streptococci had been destroyed; two hours later, only an additional 12% of the GAS had been killed.

To help explain the organisms’ survival, the researchers used a DNA microarray to detect changes in GAS gene regulation after contact with PMNs. After 30 and 60 minutes of contact, expressions of 28 and 69 genes, respectively, were altered. But the greatest changes in gene expression occurred after three hours, when phagocytosis and killing had dropped off markedly: 276 genes (approximately 16% of the genome) showed changes in transcription.

SURVIVAL GENES

“One of the key findings was that a two-component gene regulatory system called ihk-irr facilitates evasion of killing by the human innate immune response,” Dr. DeLeo noted. Indeed, streptococci with a mutant irr gene suffered significantly more deaths after three hours than did wild-type GAS (62.7% vs 13.6%). Further, irr was highly expressed in a number of human pharyngitis GAS isolates; the genes may control expression of other survival genes.

At least 11 of the genes induced by PMNs were already linked with GAS virulence; their products mediate a variety of survival strategies. For instance, up-regulated proteins encoded by sic (streptococcal inhibitor of complement) and mac (membrane attack complex of complement) block phagocytosis. In addition, said Dr. DeLeo, “there are genes like hydroperoxidases … that deal with detoxifying hydrogen peroxide, superoxides, and ROS.” He added, “Genes involved in DNA repair were also induced: One of them specifically handles DNA damage that is the result of oxidative stress.” Enzymes for repairing damaged cell walls were also up-regulated. Other induced genes encode secreted proteins that help GAS elude phagocytosis; their immunogenicity suggests potential as components of an effective vaccine.

—Mimi Zucker, PhD

References
1. Voyich JM, Sturdevant DE, Braughton KR, et al. Bacterial pathogen genome-wide protective response to human innate immunity: molecular strategies used by group A Streptococcus to evade destruction by human polymorphonuclear leukocytes. Proc Nat Acad Sci. 2003;100:1996-2001.

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