Date Published: August 13, 2018
Publisher: Public Library of Science
Author(s): Stephanie L. Brandt, Nathan Klopfenstein, Soujuan Wang, Seth Winfree, Brian P. McCarthy, Paul R. Territo, Lloyd Miller, C. Henrique Serezani, Alice Prince.
The early events that shape the innate immune response to restrain pathogens during skin infections remain elusive. Methicillin-resistant Staphylococcus aureus (MRSA) infection engages phagocyte chemotaxis, abscess formation, and microbial clearance. Upon infection, neutrophils and monocytes find a gradient of chemoattractants that influence both phagocyte direction and microbial clearance. The bioactive lipid leukotriene B4 (LTB4) is quickly (seconds to minutes) produced by 5-lipoxygenase (5-LO) and signals through the G protein-coupled receptors LTB4R1 (BLT1) or BLT2 in phagocytes and structural cells. Although it is known that LTB4 enhances antimicrobial effector functions in vitro, whether prompt LTB4 production is required for bacterial clearance and development of an inflammatory milieu necessary for abscess formation to restrain pathogen dissemination is unknown. We found that LTB4 is produced in areas near the abscess and BLT1 deficient mice are unable to form an abscess, elicit neutrophil chemotaxis, generation of neutrophil and monocyte chemokines, as well as reactive oxygen species-dependent bacterial clearance. We also found that an ointment containing LTB4 synergizes with antibiotics to eliminate MRSA potently. Here, we uncovered a heretofore unknown role of macrophage-derived LTB4 in orchestrating the chemoattractant gradient required for abscess formation, while amplifying antimicrobial effector functions.
Staphylococcus aureus is a significant cause of severe skin and soft tissue infections that can progress to life-threatening infections, such as osteomyelitis or sepsis, if left untreated [1–3]. The inflammatory response to S. aureus infection is orchestrated by the interaction among structural cells of the skin and both resident and recruited phagocytes . Upon skin infection, keratinocytes and resident immune cells produce antimicrobials to clear the pathogens and generate cytokines, chemokines, and lipid mediators to further activate dermal macrophages and promote neutrophil recruitment to the site of infection . Most studies have focused on the role of cytokines and chemokines as central regulators of skin host defense. Although IL-1β and CXCL2 have been implicated as regulators of neutrophil chemotaxis  and IFN-γ and TNFα as potent inducers of phagocyte antimicrobial effector functions [6, 7], the early events that lead to the production of these molecules upon bacterial recognition remain elusive. We and others have shown that the lipid mediator leukotriene B4 (LTB4) directly enhances neutrophil migration to inflammatory sites and also amplifies in vitro pathogen recognition and antimicrobial effector functions . LTB4 is produced within seconds to minutes upon phagocyte activation from the multistep metabolism of arachidonic acid (AA) by 5-lipoxygenase (5-LO) to form leukotriene A4 (LTA4), which is then hydrolyzed to LTB4 by LTA4 hydrolase . The effects of LTB4 are mainly mediated by its high-affinity receptor BLT1 , but the low-affinity receptor BLT2 also exerts essential functions in skin homeostasis, cancer, and recruitment of T cells [8, 9]. In vitro studies show that LTB4/BLT1 also allows TLR activation by increasing MyD88 expression and activating downstream effectors, such as IKK-α and -β, p38 MAPK, IRAK4, and transcription factors such as NFκB, AP-1, and PU.1 [10–13]. We have also shown that aerosolized LTB4 increases clearance of lung Streptococcus pneumoniae, demonstrating its safety and potential therapeutic application . Furthermore, Yamamoto et al.  have shown that treatment of mice with LTB4 increased the clearance of MRSA peritoneal infection, but the cellular players and molecular mechanisms involved in in vivo host defense are unknown.
MRSA, previously a nosocomial pathogen, has reached epidemic proportions and now is commonly found in both community and hospital settings [22, 23]. Infections with antibiotic-resistant pathogens significantly limit treatment options and could lead to irreversible tissue damage and co-morbidities associated with chronic infections . The development of host-derived immunotherapeutics that boost innate immune response and limit antibiotic resistance to avoid alterations in microbiome populations is much needed. Therefore, combination therapies using endogenous molecules along with the use of antibiotics represent a new frontier in the control of antibiotic-resistant pathogens .