Bacterial keratitis represents a serious ocular concern, carrying the potential for sight-threatening complications, including corneal scarring, perforation, endophthalmitis, and subsequent blindness. In young patients, contact lens (Cheng et al., 1999) and ocular trauma emerged as more prevalent risk factors, while prior ocular surface disease and previous ocular surgery were comparatively more frequent risk factors among older patients (Green et al., 2008).

Pseudomonas aeruginosa (P. aeruginosa) stands out as the prevailing gram-negative microorganism consistently identified in most cases of ocular infections (Astley et al., 2023; Kate et al., 2023). P. aeruginosa holds substantial clinical significance in the global healthcare domain and is classified within the ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogen group, frequently implicated in alarming upsurges of antimicrobial resistance (Pendleton et al., 2013). In the southern part of India, P. aeruginosa is responsible for approximately 8%–21% of keratitis cases (Gopinathan et al., 2009), whereas, in the United States, it accounts for up to 39% of such cases (Varaprasathan et al., 2004).

Bacterial keratitis may lead to irreversible vision impairment if timely and adequate treatment is not initiated (Garg et al., 1999). Levofloxacin, Ciprofloxacin, and Amikacin exhibited the highest efficacy among therapeutic agents for ocular infections (Subedi et al., 2018). Previous investigations have indicated an increasing trend in antibiotic resistance rates among ocular isolates ((Smitha et al., 2005; Thomas et al., 2019; Bispo et al., 2022,; Marasini et al., 2023) especially, for fourth-generation fluoroquinolones, Moxifloxacin and Gatifloxacin) as well as cell lines (Ryu et al., 2017). Since the threat of antibiotic resistance among bacteria causing corneal infections is on the rise, alternative therapeutic interventions and new antibiotic approaches for treatment require attention (Thomas et al., 2019; Bispo et al., 2022; Marasini et al., 2023).

Mesenchymal stem cells (MSCs), on the other hand, have been reported to produce antimicrobial molecules, including LL-37 (Krasnodembskaya et al., 2010), lipocalin-2 (LCN-2) (Cortés-Araya et al., 2018), Interleukin-6 (IL-6) (Bouffi et al., 2010), etc. which display inhibitory effects on bacterial growth. This secretion of antimicrobial molecules by MSCs appears to underlie their inherent antibacterial properties. Additionally, MSCs demonstrate an indirect mechanism of augmenting the antibacterial activity of innate immune effector cells, such as neutrophils and macrophages (Chow et al., 2020). Through the release of specific factors, MSCs activate these immune cells, thereby bolstering their capacity to effectively eliminate bacteria (Chow et al., 2020). While the antibacterial effect of MSCs has been investigated in diseases like leukaemia (Meisel et al., 2011), acute lung injury (Krasnodembskaya et al., 2010), cystic fibrosis (Sutton et al., 2016), etc., the application of MSCs in treating bacterial infections in the eye is not widely studied. MSC-secreted hepatocyte growth factor is reported to potentially act as a suppressor of lipopolysaccharide (LPS)-induced corneal opacity in mice (Elbasiony et al., 2022), whereas MSCs derived from limbus (Basu et al., 2014), bone marrow (Mittal et al., 2016, 2019; Shukla et al., 2019), and umbilical cord (Kacham et al., 2021) have been effective in non-infectious corneal wound healing. Subconjunctival injection of equine muscle-derived and bone marrow-derived MSCs improved the ocular lesions while a series of repeated injections were effective in treating the refractory form of equine immune-mediated keratitis (Narinx et al., 2023; Davis et al., 2019). While further clinical trials are needed to determine the optimal dosage and delivery routes of MSCs for treating different cornel epitheliopathies (Soleimani et al., 2023), topically administered umbilical cord-derived MSCs reduced corneal scarring and opacity when combined with anti-fungal (Natamycin) eye drops in a murine model of fungal keratitis (Zhou et al., 2019).

This study aims to investigate the anti-bacterial and immunomodulatory properties of bone marrow MSC-derived conditioned medium (MSC-CM) in an in vitro model of P. aeruginosa-induced corneal epithelial damage and to explore the proof-of-concept for using MSCs as an adjunctive therapy for P. aeruginosa-induced keratitis.