Pathogenesis and toxinsImmune discrepancies during in vitro granuloma formation in response to Cutibacterium (formerly Propionibacterium) acnes infection
Introduction
Cutibacterium acnes is an important part of the normal flora of human skin, living in and around sweat glands and sebaceous follicles. The pathogenicity of C. acnes has long been restricted to skin conditions [1]. Although described as a commensal bacterium with a low pathogenicity, its involvement has been reported in many clinical entities [2]. In the context of chronic/low-grade pathologies, the two main features of C. acnes infection are sarcoidosis and prosthetic joint infection (PJI). The link between these two pathologies is the presence of granulomatous structures in response to the infection.
Sarcoidosis is a systemic disease with an unknown etiology, which is characterized by non-caseating granulomas found primarily in the lung [3]. While the accurate cause of sarcoidosis remains unknown, it seems that the pathogenesis results from the combination of genetic susceptibility and exposure to specific antigens, either environmental or infectious [3]. Nevertheless, C. acnes has been suspected of being involved in the development of sarcoidosis and has frequently been cultured out of lymph nodes from sarcoidosis patients [4], [5] and localized within sarcoidosis granulomas [6]. Additionally, C. acnes has been shown to drive differential cytokine responses in the peripheral blood mononuclear cells of sarcoidosis patients [7]. In 2–14% of cases, C. acnes has also been identified as the cause of various implant-associated infections, including PJI, particularly in shoulder prostheses, spine implant surgery, and hip and knee prostheses [8], [9]. We previously reported that C. acnes isolates belonging to clonal complex (CC) 36 were more frequently observed in PJI [10]. These PJI are also characterized by granulomatous structures in the tissues next to the material [11], [12], [13].
Some granuloma-producing animal models of sarcoidosis have been proposed using different bacteria and bacterial products [14], [15], [16]. On the other hand, host-bacterial interaction in PJI is usually assessed using a foreign-body infection model with guinea pigs or rabbits [17], [18]. To date, no study has focused on the granuloma formation next to the material implanted. Moreover, animal models remain technically and ethically unsuitable to evaluate a large collection of isolates.
Thus, the cellular organization and host-bacteria interactions within complex granulomas have not been well-described to date. To our knowledge, there is no relevant model that illustrates in vitro granulomas leading to clinical extrapolation. To develop such a model, we used freshly collected human peripheral blood mononuclear cells (PBMCs) from healthy individuals. This enabled us to evaluate the relevance of an in vitro persistence model of C. acnes in human blood cell phagocytes in a cellular and physiological environment that mimics the in vivo situation. In this study, we induced a physiological granulomatous reaction in response to different C. acnes isolates in order to investigate the cellular process during granuloma formation.
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Bacterial strains
The C. acnes ATCC6919 isolate (CC18, phylotype IA1) came from the American Type Culture Collection (ATCC). Its clinical origin is acne lesion and its genome has been sequenced (Genebank accession number 1174878). The C. acnes BL clinical isolate (CC36, phylotype IB) was isolated at Nantes University Hospital from a PJI. Its genome was recently sequenced [19]. Lastly, the C. acnes S8 strain (CC28, phylotype IA2) was kindly provided by Professor Y. Eishi of the Department of Human Pathology,
PBMCs infected by the sarcoidosis C. acnes isolate generated a higher number of granulomatous structures
The first step in the development of an in vitro human model of C. acnes granulomas was to induce recruitment of PBMCs around live C. acnes. We first determined the optimal multiplicity of infection (MOI) of C. acnes for use in subsequent experiments. The monocyte:bacteria MOI ratios studied were 10,000:1, 1000:1, 100:1 and 10:1. The MOIs 100:1 and 10:1 caused premature destruction of granulomatous structures due to an uncontrolled increase in the bacterial load. By contrast, at an MOI of
Discussion
To the best of our knowledge, we have developed for the first time an in vitro model of granuloma formation in response to C. acnes infection. In our experimental conditions, we were able to generate granulomatous structures easily with all the clinical strains tested. Granulomatous structures are immune responses to control intracellular bacterial development [24], even though C. acnes is not strictly considered an intracellular pathogen compared to Mycobacterium tuberculosis or Listeria
Acknowledgment
We thank the Cytometry Facility Cytocell for expert technical assistance.
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contributed equally to this work.