Macrophage responses to bacterial toxins: a balance between activation and suppression

Toxins secreted by bacteria can impact the host in a number of different ways. In some infections, toxins play a crucial and central role in pathogenesis (i.e., anthrax), while in other bacterial infections, the role of toxins is less understood. The cholesterol-dependent cytolysins (CDCs), of which streptolysin O is a prototype, are a class of pore-forming toxins produced by many gram-positive bacteria and have only been studied in a few experimental infection models. Our laboratory has demonstrated that CDCs have effects on macrophages that are both pro- and anti-inflammatory. Here, we review evidence that CDCs promote inflammation by driving secretion of IL-1β and HMGB-1 from macrophages in a NLRP3-dependent manner, while also causing shedding of membrane microvesicles from cells that can interact with macrophages and inhibit TNF-α release. CDCs thus impact macrophage function in ways that may be both beneficial and detrimental to the host.