Evolving concepts in bone infection: redefining "biofilm", "acute vs. chronic osteomyelitis", "the immune proteome" and "local antibiotic therapy".

Osteomyelitis is a devastating disease caused by microbial infection of bone. While the frequency of infection following elective orthopedic surgery is low, rates of reinfection are disturbingly high. Staphylococcus aureus is responsible for the majority of chronic osteomyelitis cases and is often considered to be incurable due to bacterial persistence deep within bone. Unfortunately, there is no consensus on clinical classifications of osteomyelitis and the ensuing treatment algorithm. Given the high patient morbidity, mortality, and economic burden caused by osteomyelitis, it is important to elucidate mechanisms of bone infection to inform novel strategies for prevention and curative treatment. Recent discoveries in this field have identified three distinct reservoirs of bacterial biofilm including: Staphylococcal abscess communities in the local soft tissue and bone marrow, glycocalyx formation on implant hardware and necrotic tissue, and colonization of the osteocyte-lacuno canalicular network (OLCN) of cortical bone. In contrast, S. aureus intracellular persistence in bone cells has not been substantiated in vivo, which challenges this mode of chronic osteomyelitis. There have also been major advances in our understanding of the immune proteome against S. aureus, from clinical studies of serum antibodies and media enriched for newly synthesized antibodies (MENSA), which may provide new opportunities for osteomyelitis diagnosis, prognosis, and vaccine development. Finally, novel therapies such as antimicrobial implant coatings and antibiotic impregnated 3D-printed scaffolds represent promising strategies for preventing and managing this devastating disease. Here, we review these recent advances and highlight translational opportunities towards a cure.

MyD88 signaling regulates both host defense and immunopathogenesis during pneumocystis infection.

The immune response protects against Pneumocystis infection but is also a key component of Pneumocystis pneumonia (PcP)-related immunopathogenesis. Signaling through myeloid differentiation factor 88 (MyD88) is critical for activation of immune pathways downstream of TLRs and IL-1R. To determine whether MyD88 regulates normal host defense against Pneumocystis, nonimmunosuppressed wild-type (WT) and MyD88-deficient mice were infected. MyD88(-/-) mice had higher early Pneumocystis burdens than did WT mice but mounted an effective adaptive immune response and cleared Pneumocystis similarly to WT. However, MyD88(-/-) mice displayed a more intense and prolonged pulmonary immune response than did WT mice. To determine the role of MyD88 in the development of PcP-related immunopathogenesis, WT and MyD88(-/-) mice were rendered susceptible to PcP by depletion of CD4(+) T cells. At 4 wk postinfection, CD4-depleted WT and MyD88(-/-) mice harbored similar organism burdens, but MyD88(-/-) mice were protected from the PcP-related respiratory impairment observed in WT mice. Improved pulmonary physiology in MyD88(-/-) mice correlated with lower lung CCL2 levels and reduced cell recruitment. However, by 5 wk postinfection, the overall health of MyD88(-/-) mice began to deteriorate rapidly relative to WT, with accelerated weight loss, impaired lung function, and exacerbated alveolar inflammation. This physiological decline of MyD88(-/-) mice was associated with increased TNF-α and IFN-γ in the lung, and by the inability to control Pneumocystis burden. Thus, MyD88 is not required for resistance to Pneumocystis infection, but limits the adaptive immune response in immunocompetent mice. In the setting of active PcP, MyD88 signaling contributes to both immunopathogenesis and control of fungal burden.

The alveolar epithelial cell chemokine response to pneumocystis requires adaptor molecule MyD88 and interleukin-1 receptor but not toll-like receptor 2 or 4.

Pneumocystis is an opportunistic fungal pathogen that causes pneumonia in a variety of clinical settings. An early step in Pneumocystis infection involves the attachment of organisms to alveolar epithelial cells (AECs). AECs produce chemokines in response to Pneumocystis stimulation, but the upstream host-pathogen interactions that activate AEC signaling cascades are not well-defined. MyD88 is an adaptor molecule required for activation of proinflammatory signaling cascades following Toll-like receptor (TLR)-dependent recognition of conserved molecular patterns on pathogens. To determine whether the TLR/MyD88 pathway is required for the AEC chemokine response to Pneumocystis, wild-type (WT) and MyD88-deficient AECs were incubated with Pneumocystis. As expected, WT AECs produced CCL2 and CXCL2 following Pneumocystis stimulation. In contrast, MyD88-deficient AECs were severely impaired in their ability to respond to Pneumocystis. MyD88-deficient AECs did not display Pneumocystis-induced Jun N-terminal protein kinase activation and produced much less chemokine than Pneumocystis-stimulated WT AECs. Using a panel of TLR agonists, primary murine AECs were found to respond vigorously to TLR2 and TLR4 agonists. However, the AEC chemokine response to Pneumocystis did not require TLR2 or TLR4. Surprisingly, the interleukin-1 receptor (IL-1R) was required for an AEC chemokine response to Pneumocystis. The role of MyD88 in early responses during Pneumocystis infection was supported by in vivo studies demonstrating that MyD88-deficient mice showed impaired Pneumocystis-stimulated chemokine production and impaired inflammatory cell recruitment. These data indicate an important role for MyD88 in the AEC inflammatory response to Pneumocystis.