Staph Infection Immune Response Pinpointed

A new study mapping the gene profiles of children with severe Staphylococcus aureus infections1, shows immune systems respond to this strain of bacteria by activating genes involved in immediate defense mechanisms, while deactivating genes involved in long term immune defense memory. The study seems to raises a few questions of its own. For example, does the immune response uncovered here apply to all bacterial infections, or just Staph infections? Would the pattern reverse toward the end of the infection? And what implications does this have for improving diagnosis and treatment of virulent strains like MRSA?

The study was undertaken because most recent research on this pathogen2,3 has focused on determining exactly what the bacterium S. aureus does to disrupt the immune system. Very little is known, however, about how the immune system responds to staph infection, or why certain people tend to get more severe staphylococcal infections than others.

“The beauty of our study is that we were able to use existing technology to understand in a real clinical setting what’s going on in actual humans – not models, not cells, not mice, but humans,” says Dr. Monica Ardura, instructor of pediatrics at UT Southwestern and lead author of the study. “We have provided the first description of a pattern of response within an individual’s immune system that is very consistent, very reproducible and very intense.”

Gene Expression Profiling

Using gene expression profiling4, a process that reviews how specific genes are activated or suppressed in response to the infection, the researchers at UT Southwestern established how an individual’s immune system responds to a S. aureus infection at the genetic level.

For this study, ribonucleic acid was extracted from blood and placed on special gene chip called microarrays, which analyses the entire human genome to see which genes are switched on or off.

The study team found that in children with invasive staphylococcal infections, the genes involved in the body’s innate immune response are overactivated while those associated with the adaptive immune system, such as T Memory cells, are suppressed. (There are two components in the immune system: the innate system, providing immediate defense against infection; and the adaptive system, whose memory cells are called into action to fight off subsequent infections.)

“It’s a very sophisticated and complex dysregulation of the immune system, but our findings prove that there’s consistency in the immune response to the staphylococcus bacterium,” Dr. Ardura said. “Now that we know how the immune system responds, the question is whether we can use this to predict patient outcomes or differentiate the sickest patients from the less sick ones. How can we use this knowledge to develop better therapies?”

Methodology

Blood used was from samples collected between 2001 and 2005 from 77 children; 53 hospitalized at Children’s Medical Center Dallas with invasive S. aureus infections and 24 controls. Those with underlying chronic diseases, multiple infections, immunodeficiency, and who received steroids or other immunomodulatory therapies were excluded from the study.

Ages ranged from a few months to 15 years and there were 43 boys and 34 girls. Children with S. aureus infections, both methicillin-resistant (MRSA) and methicillin-susceptible (MSSA), were matched with healthy controls for age, sex and race. Researchers also characterized both the extent and type of infection in each patient to make sure that the strain of bacteria didn’t influence the results.

Microarray analysis was done by using the Affymetrix cDNA Synthesis and In Vitro Transcription kits to prepare the samples for Affymetrix HG-U133A and B GeneChips. The HG-U133 set contains 44,760 probe sets representing over 39,000 transcripts derived from ~33,000 human genes.

The resulting data was imported into GeneSpring software from Agilent to perform the gene expression analyses, statistical testing, hierarchical clustering, and classification of samples with transcriptional module analysis methods5.

Next Step- Dynamic Data

Dr. Ardura noted that further research is needed; these results represent a static snapshot of the cell during an invasive staphylococcal infection.

“The median time to get the blood sample was day four because we wanted to make sure the hospitalized children had a S. aureus infection, and its takes four days to have final identification of the bacterial pathogen,” she said. The next step, she added, is to study the cell dynamics in patients before, during and after infection. They also hope to understand better how various staph-infection therapies affect treatment.

“This is a very important proof-of-concept that the information is there for us to grab,” Dr. Ardura said. “Now we have to begin to understand what that data tells us.”

References

1. Ardura MI, Banchereau R, Mejias A, Di Pucchio T, Glaser C, et al. (2009) Enhanced Monocyte Response and Decreased Central Memory T Cells in Children with Invasive Staphylococcus aureus Infections. PLoS ONE 4(5): e5446. doi:10.1371/journal.pone.0005446
2. Francis JS, Doherty MC, Lopatin U, Johnston CP, Sinha G, et al. (2005) Severe community-onset pneumonia in healthy adults caused by methicillin-resistant Staphylococcus aureus carrying the Panton-Valentine leukocidin genes. Clin Infect Dis 40: 100–107.
3. Klevens RM, Morrison MA, Nadle J, Petit S, Gershman K, et al. (2007) Invasive methicillin-resistant Staphylococcus aureus infections in the United States. Jama 298: 1763–1771.
4. Whitney AR, Diehn M, Popper SJ, Alizadeh AA, Boldrick JC, et al. (2003) Individuality and variation in gene expression patterns in human blood. Proc Natl Acad Sci U S A 100: 1896–1901.
5. Chaussabel D, Quinn C, Shen J, Patel P, Glaser C, et al. (2008) A modular analysis framework for blood genomics studies: application to systemic lupus erythematosus. Immunity 29: 150–164.

Photo: U. S. Department of Agriculture. Bacterial cells of S. aureus.