Antimicrobial peptide gene expression in human tonsils

Abstract

The human palatine tonsils play important roles in host immunity and provide a barrier against invading pathogens. However the Gram-positive bacterium, Streptococcus pyogenes often penetrates their defences. This results in tonsillitis, in which sufferers present with swollen and painful palatine tonsils. Moreover tonsillitis is often recurrent. Despite tonsillitis being relatively common, the immunological factors that allow this infection to perpetuate remain obscure. Antimicrobial peptides (AMPs), expressed at epithelial and mucosal surfaces, provide a first-line of defence against potential pathogens. Moreover these molecules have also been reported to be up-regulated in response to infection, but their roles in the defence of the human palatine tonsils are not well known. Studies were therefore initiated to test the hypothesis that a defect in the innate immune response involving host AMPs, is a cause or contributory factor to recurrent acute tonsillitis (RAT). The first study investigated and compared AMP gene expression patterns in palatine tonsils excised from patients undergoing tonsillectomy for RAT to those of control subjects whose tonsils were excised for conditions such as snoring. To date it was the largest investigation of its kind using over ninety tonsils. The levels of LL-37, HBDl, HBD2 and LEAP-2 AMP mRNA expression were quantified by real-time PCR assays developed and optimised specifically for the study. The data indicated that all AMP genes examined were expressed, that considerable variability was detected between the AMP expression levels of individual subjects but that the mean AMP expression levels between the RAT and control groups were not statistically different. This study was conducted using tonsils excised from RAT patients at the time of their surgery when they were probably not suffering a streptococcal infection. To answer the question of whether RAT was due to a failure of the AMPs to up-regulate in response to infection, an in vitro model was adopted. HaCaT cells were used as the in vitro model of the tonsil and challenged with Group A streptococci. Quantitative real-time expression data suggested that in response to the S. pyogenes MI serotype, HBDI gene expression was decreased, suggesting that GAS down regulates the expression of this gene, whereas that of LEAP-2 was induced. The results of challenge experiments performed using pili-defective mutants also suggested that such changes in the host response occurred in the absence of streptococcal binding. The HaCaT cells were not however derived from tonsil. Thus to further investigate streptococcal-tonsil AMP responses an ex vivo tonsil model was used. Following challenge of the control (non-RAT) and RAT tonsil sections with S. pyogenes, a statistically significant increase in HBDI gene expression and a decrease in HBD2 gene expression were observed in the control (non-RAT) tonsils. No comparable statistically significant changes were identified in the RAT tonsils. These data therefore highlighted differences between the AMP expression profiles of the control (non-RAT) and RAT tonsils in response to a S. pyogenes M I challenge. Although speculative these data indicate that the RAT tonsils were less able to respond to the S. pyogenes challenge, which in part may help to explain the susceptibility of RAT patients to infection.