Osteomyelitis is a complex infection of bone and marrow tissues caused by microbial agents that presents an escalating clinical challenge and substantial healthcare and economic burdens [1], [2]. Bone infection is caused by three clinical mechanisms: (1) osteomyelitis caused by the propagation of a contiguous source (i.e., trauma or surgical contamination), (2) osteomyelitis secondary to vascular insufficiency or neuropathy such as diabetic foot ulceration, and (3) acute hemorrhagic osteomyelitis [3]. Staphylococcus aureus is the principal pathogen that causes most cases of osteomyelitis [4]. This bacterium adheres to host tissues, evades immune surveillance by disrupting immune cell function, and persists within mammalian cells [5]. Recent reports have revealed that smoking is one of the highest risk factors for osteomyelitis, potentially exacerbating infections and complicating treatment due to recurrence [6], [7], [8].While a clinical association between smoking and the risk of osteomyelitis has been established, the exact molecular mechanisms remain undefined.

Cigarette smoke comprises over 4500 chemical entities, including particulate and vapor phases. Chronic exposure to these components is known to detrimentally affect immune function. Notably, the lack of immunosuppressive effects after chronic exposure to the vapor phase suggests the presence of an immunosuppressive agent within the particulate phase [9]. Nicotine is a component of the particulate phase that is thought to be responsible for the immunomodulatory effects induced by cigarette smoke [10]. Previous studies have demonstrated the connections of nicotine to several cancers’ poor prognosis, such as breast cancer, esophageal cancer and lung cancer [11], [12], [13]. Previous studies have demonstrated an association between nicotine and poor prognosis in several cancers, including breast, esophageal, and lung cancers. Moreover, nicotine can promote and aggravate certain bacterial infectious diseases, including periodontitis and gingivitis [14]. However, research on the relationship between nicotine and S. aureus-induced osteomyelitis and its underlying mechanisms is limited.

Macrophages are integral to the innate immune system and constitute the front line of defense against pathogenic invasion. Previous studies have reported that macrophages play an important role in the development of S. aureus-induced osteomyelitis, with macrophage polarization being pivotal for controlling host defenses against pathogens and injurious agents [15], [16], [17]. Under different stimuli, macrophages polarize to either M1 or M2 phenotypes, where M1 macrophages are stimulated by bacterial components like lipopolysaccharide (LPS) to elicit pro-inflammatory responses by releasing cytokines including interleukin (IL)-6, IL-12, and tumor necrosis factor alpha (TNF-α) that contribute to pathogen removal [18], [19]. The M2 polarization is associated with tissue repair, immunoregulation, and tumor-promoting processes [20], [21]. Recent studies indicate that nicotine promotes M2 macrophage polarization and reduces inflammation by inhibiting the production of pro-inflammatory cytokines such as IL-6 and TNF-α [14], [22]. Inflammation is a crucial component of the innate immunity that aids tissue repair and pathogen eradication [23]. However, the premature suppression of inflammation may reduce macrophage pathogen clearance, which increases the survival of invading pathogens. Previous research has demonstrated that nicotine increases Mycobacterium tuberculosis (Mtb) burden in macrophages via nicotinic acetylcholine receptor (nAChR) interactions [24]. Therefore, we hypothesized that the anti-inflammatory properties of nicotine aggravate osteomyelitis.

In this study, we explored the interplay between nicotine and S. aureus-induced osteomyelitis. We discovered that nicotine impaired the bactericidal capabilities of mouse bone marrow-derived macrophages (BMDMs) infected with S. aureus and reduced antimicrobial efficacy in mice with S. aureus-induced osteomyelitis. Using high-throughput sequencing, two crucial genes, NF-E2-related factor 2 (Nfe2l2/Nrf2) and solute carrier family 7 member 11 (Slc7a11), were identified. Nrf2 is a key gene that governs inflammation and cellular redox homeostasis, while Slc7a11 is a member of the solute carrier family that encodes a cystine/glutamate transporter and plays an essential role in the iron overload-ferroptosis pathway [25], [26], [27]. Nrf2 enhances the expression of Slc7a11, functioning as a transcriptional activator [28], [29]. We demonstrated that the Nrf2/Slc7a11 signaling axis was a potential mechanism by which nicotine promoted S. aureus-induced osteomyelitis, where the subsequent inhibition of Slc7a11 expression using Erastin significantly enhanced the phagocytosis/killing abilities and antimicrobial activity of BMDMs in mice. Furthermore, Erastin attenuated the detrimental effects of nicotine by modulating glutamate/glutathione metabolism. This study revealed the profound molecular impact of nicotine on S. aureus-induced osteomyelitis.