Ly6C(+) monocytes are important components of the innate immune defense against infections. These cells have been shown to proliferate in the bone marrow of mice with systemic infections. However, the proliferative capacity of Ly6C(+) monocytes in infected peripheral tissues as well as the associated regulatory mechanisms remain unclear. In this study, we analyzed the proliferative capacity of Ly6C(+) monocytes in the urinary bladder after infection with uropathogenic E. coli, one of the most prevalent pathogen worldwide, and in LPS-induced peritonitis. We show that Ly6C(+) monocytes proliferated in the bladder after infection with uropathogenic E. coli and in the peritoneum after intraperitoneal injection of LPS. We identified IL-6, a molecule that is highly expressed in infections, as a crucial regulator of Ly6C(+) monocyte proliferation. Inhibition of IL-6 via administration of antibodies against IL-6 or gp130 impeded Ly6C(+) monocyte proliferation. Furthermore, repression of IL-6 trans-signaling via administration of soluble gp130 markedly reduced the proliferation of Ly6C(+) monocytes. Overall, this study describes the proliferation of Ly6C(+) monocytes using models of urinary tract infection and LPS-induced peritonitis. IL-6 trans-signaling was identified as the regulator of Ly6C(+) monocyte proliferation.
