Amy L. Aldrich

Biofilms are complex communities of bacteria encased in a matrix composed primarily of polysaccharides, extracellular DNA, and protein. Staphylococcus aureus can form biofilm infections, which are often debilitating due to their chronicity and recalcitrance to antibiotic therapy. Currently, the immune mechanisms elicited during biofilm growth and their impact on bacterial clearance remain to be defined. We used a mouse model of catheter-associated biofilm infection to assess the functional importance of TLR2 and TLR9 in the host immune response during biofilm formation, because ligands for both receptors are present within the biofilm. Interestingly, neither TLR2 nor TLR9 impacted bacterial density or inflammatory mediator secretion during biofilm growth in vivo, suggesting that S. aureus biofilms circumvent these traditional bacterial recognition pathways. Several potential mechanisms were identified to account for biofilm evasion of innate immunity, including significant reductions in IL-1β, TNF-α, CXCL2, and CCL2 expression during biofilm infection compared with the wound healing response elicited by sterile catheters, limited macrophage invasion into biofilms in vivo, and a skewing of the immune response away from a microbicidal phenotype as evidenced by decreases in inducible NO synthase expression concomitant with robust arginase-1 induction. Coculture studies of macrophages with S. aureus biofilms in vitro revealed that macrophages successful at biofilm invasion displayed limited phagocytosis and gene expression patterns reminiscent of alternatively activated M2 macrophages. Collectively, these findings demonstrate that S. aureus biofilms are capable of attenuating traditional host proinflammatory responses, which may explain why biofilm infections persist in an immunocompetent host.

Somatic gene mutations are key determinants of outcome in patients with myelodysplastic syndromes (MDS) and secondary AML (sAML). In particular, patients with TP53 mutations represent a distinct molecular cohort with uniformly poor prognosis. The precise pathogenetic mechanisms underlying these inferior outcomes have not been delineated. In this study, we characterized the immunological features of the malignant clone and alterations in the immune microenvironment in patients with TP53-mutant and wild-type MDS or sAML. Notably, PDL1 expression is significantly increased in hematopoietic stem cells of patients with TP53 mutations, which is associated with MYC upregulation and marked downregulation of MYC's negative regulator miR-34a, a p53 transcription target. Notably, patients with TP53 mutations display significantly reduced numbers of bone marrow-infiltrating OX40+ cytotoxic T cells and helper T cells, as well as decreased ICOS+ and 4-1BB+ natural killer cells. Further, highly immunosuppressive regulatory T cells (Tregs) (ie, ICOShigh/PD-1-) and myeloid-derived suppressor cells (PD-1low) are expanded in cases with TP53 mutations. Finally, a higher proportion of bone marrow-infiltrating ICOShigh/PD-1- Treg cells is a highly significant independent predictor of overall survival. We conclude that the microenvironment of TP53 mutant MDS and sAML has an immune-privileged, evasive phenotype that may be a primary driver of poor outcomes and submit that immunomodulatory therapeutic strategies may offer a benefit for this molecularly defined subpopulation.

The management and treatment of tissue infection, especially chronic infection, represents a significant challenge. Application of autologous platelet-rich plasma (PRP) has emerged as a promising adjunct therapy for facilitating the healing of surgical wounds and tissue injuries. PRP is extracted from whole blood using a sequential centrifugation technique and when activated, can release a vast array of antimicrobial proteins, cytokines, and growth factors. These bioactive molecules are responsible for the ability of PRP to kill pathogens, resolve necrotic tissue, and promote wound healing. PRP is emerging as a useful supplement to prevent postoperative infection and treat chronic wound or bone infections. PRP displays a synergistic effect with antibiotics, which provides unique advantages when treating antibiotic-resistant bacteria. This review will describe the method for PRP preparation and its antibacterial properties, as well as discuss both preclinical in vivo results and evidence from clinical practice of PRP use for the treatment of wound and bone infections. Impact Statement The clinical application of platelet-rich plasma (PRP) has been widely studied for its effects on trauma or injury repair/regeneration, however the antibacterial property of PRP has been overlooked. Increasing evidence suggests PRP as a good antibacterial agent and that it could help prevent/treat tissue infection. This review emphasizes the importance of PRP's antibacterial property and summarizes the preclinical and clinical findings regarding the application of PRP in the prevention and treatment of wound and bone infection. The use of biocompatible PRP may be advantageous for tissue infection treatment due to its inherent antibacterial and healing promoting properties.

Abstract Interleukin‐1β ( IL ‐1β) is essential for eliciting protective immunity during the acute phase of Staphylococcus aureus ( S. aureus ) infection in the central nervous system ( CNS ). We previously demonstrated that microglial IL ‐1β production in response to live S. aureus is mediated through the Nod‐like receptor protein 3 ( NLRP 3) inflammasome, including the adapter protein ASC (apoptosis‐associated speck‐like protein containing a caspase‐1 recruitment domain), and pro‐caspase 1. Here, we utilized NLRP 3, ASC , and caspase 1/11 knockout ( KO ) mice to demonstrate the functional significance of inflammasome activity during CNS S. aureus infection. ASC and caspase 1/11 KO animals were exquisitely sensitive, with approximately 50% of mice succumbing to infection within 24 h. Unexpectedly, the survival of NLRP 3 KO mice was similar to wild‐type animals, suggesting the involvement of an alternative upstream sensor, which was later identified as absent in melanoma 2 ( AIM 2) based on the similar disease patterns between AIM 2 and ASC KO mice. Besides IL ‐1β, other key inflammatory mediators, including IL ‐6, CXCL 1, CXCL 10, and CCL 2 were significantly reduced in the CNS of AIM 2 and ASC KO mice, implicating autocrine/paracrine actions of IL ‐1β, as these mediators do not require inflammasome processing for secretion. These studies demonstrate a novel role for the AIM 2 inflammasome as a critical molecular platform for regulating IL ‐1β release and survival during acute CNS S. aureus infection. image The AIM2 inflammasome is protective during acute CNS bacterial infection. A disconnect in phenotypes between the inflammasome sensor Nod‐like receptor protein 3 (NLRP3) and its adaptor ASC (apoptosis‐associated speck‐like protein containing a caspase‐1 recruitment domain) during acute CNS Staphylococcus aureus ( S. aureus ) infection led to the discovery of absent in melanoma 2 (AIM2) as a critical inflammasome sensor. The AIM2 inflammasome is potentially triggered by dsDNA in cells harboring intracellular S. aureus , leading to ASC and caspase 1 recruitment, resulting in pro‐IL‐1β processing and cytokine secretion. This cascade, in turn, is protective to the host during acute infection. The NLRP3 inflammasome is also activated in response to S. aureus challenge by α‐hemolysin ( hla ); however, it is not critical for host survival. ASC also regulates the production of other inflammatory mediators, presumably via indirect effects mediated by IL‐1β action.