Activation of Natural Killer Cells in Patients with Chronic Bone and Joint Infection due to Staphylococci Expressing or Not the Small Colony Variant Phenotype

Chronic bone and joint infections (BJI) are devastating diseases. Relapses are frequently observed, as some pathogens, especially staphylococci, can persist intracellularly by expressing a particular phenotype called small colony variant (SCV). As natural killer (NK) cells are lymphocytes specialized in the killing of host cells infected by intracellular pathogens, we studied NK cells of patients with chronic BJI due to staphylococci expressing or not SCVs (10 patients in both groups). Controls were patients infected with other bacteria without detectable expression of SCVs, and healthy volunteers. NK cell phenotype was evaluated from PBMCs by flow cytometry. Degranulation capacity was evaluated after stimulation with K562 cells in vitro. We found that NK cells were activated in terms of CD69 expression, loss of CD16 and perforin, in all infected patients in comparison with healthy volunteers, independently of the SCV phenotype. Peripheral NK cells in patients with chronic BJI display signs of recent activation and degranulation in vivo in response to CD16-mediated signals, regardless of the type of bacteria involved. This could involve a universal capacity of isolates responsible for chronic BJI to produce undetectable SCVs in vivo, which might be a target of future intervention. 1. Introduction Chronic bone and joint infections (BJI) are devastating diseases and staphylococci (S. aureus and coagulase-negative staphylococci) are the most frequent bacteria involved in such diseases [1]. Various mechanisms of persistence have been described in vivo and in vitro, such as biofilm formation (especially in implant-associated infections) and expression of small colony variants (SCVs) [2, 3]. SCVs are a naturally occurring subpopulation of staphylococci, which (i) correspond to a particular fastidious phenotype in vitro, expressing slow-growing capacities; (ii) have been described in few bacterial chronic diseases, such as BJI or cystic fibrosis; (iii) are associated with intracellular persistence ex vivo; and (iv) are associated with clinical recurrence of the infection. Few data are available about SCVs in vivo [2, 4, 5]. In particular, host or bacterial factors that lead to in vivo expression of SCVs are unknown. Natural killer (NK) cells are innate lymphocytes that are specialized in the recognition and killing of host cells infected by intracellular pathogens [6, 7]. Cytotoxicity is mediated via the release of prestored granules containing proteins such as perforin and granzymes. NK cell degranulation can be induced through the engagement of various