Bartonella henselae invades human mesenchymal stromal cells and modulates secretion of pro-angiogenic cytokines

Background: Bartonella henselae causes different human diseases, the most prominent being cat scratch disease, a persistent, necrotizing lymphadenitis characterized by a typical granuloma that manifests in immunocompetent patients. Instead, in immunocompromized patients B. henselae can cause bacillary peliosis hepatis and bacillary angiomatosis. Endothelial cell infection represents an important step in the pathogenesis of cat scratch disease and bacillary angiomatosis. Experimental studies in a rat model showed that a brief bacteremia is followed by a 3–5-day bacteremia-free window before resurgence of persistent intraerythrocytic bacteremia that lasts for 8–11 weeks. This model highlights the possible existence of a sanctuary site where the initial infection gets established before persistent intraerythrocytic infection manifests. Mesenchymal stromal cells (MSC) are multipotent progenitor cells that can differentiate into various cell lineages such as osteocytes, condrocytes, and adipocytes, and have a proven ability to augment the neovascularization processes. Additionally, MSC also have antimicrobial proprieties and recently their role in the persistence of viable non replicating M. tuberculosis has been established. The aim of this study was to determine the possibility that MSC represent a reservoir for B. henselae. The effect that the bacteria have on the release of pro-angiogenic and pro-inflammatory cytokines, and on TLR expression of MSC were also evaluated. Material/methods: Human adipocyte-derived mesenchymal stromal cells (Ad-MSC) isolated from adipose tissue obtained by lipoaspiration from healthy donors were stimulated with MOI 100 of B. henselae for 48-96 hr and 7 days. VEGF-A, IL-8 and IL-6 secretion upon B. henselae infection was measured by ELISA kits. The internalization of bacteria in Ad-MSC was demonstrated by immunohistochemical and immunofluorescence analysis. Cells were also evaluated by RT-PCR and FACS analysis for the expression of TLR-2/TLR-4 after B. henselae stimulation. Results: Infection of Ad-MSC with B. henselae resulted in an increase of VEGF, IL-8 and IL-6 secretion in a time-dependent manner (Figure 1). Bacteria can enter the intracellular space of MSC and we noted solitary bacteria in the perinuclear area and clusters inside vacuolic compartments after 96 hr of infection and bacteria persist at 7 days post infection, as demonstrated by immunofluorescence. Moreover B. henselae can augment TLR-2 expression on cell surface in a time dependent manner as demonstrated by RT-PCR and FACS analysis. Conclusions: Our results indicate that B. henselae induces the production of pro-angiogenic factors and can invade MSC modulating TLR expression. It remains to be evaluated the capacity of survive/proliferation of bartonella in MSC and the bacterial effects on Ad-MSC proliferation and differentiation. These cells could represent a potential habitat of B. henselae trough their differentiation into cells with phenotypic and functional features of endothelial cells.