Early immune responses to intradermal lipopolysaccharide in healthy volunteers: prednisolone's impact on TLR4-mediated inflammation.

Background: Intradermal lipopolysaccharide (LPS) administration is a well-established model to investigate Toll-like receptor 4 (TLR4)-mediated inflammation in human skin. Previous studies have shown strong immune activation following LPS injection, including neutrophil influx and cytokine release. However, these studies lacked early time-point assessments, limiting understanding of initial inflammatory kinetics. Additionally, corticosteroids, while known to suppress some inflammatory effects, did not inhibit early neutrophil recruitment. Objectives: This study aimed to characterize the early innate immune response to intradermal LPS in healthy volunteers, with specific focus on IL-1β, neutrophil infiltration, neutrophil extracellular trap (NET) formation, and inflammasome activation. We also evaluated the impact of oral prednisolone on these early responses. Methods: In a randomized, placebo-controlled, double-blind trial, 36 healthy male volunteers received oral prednisolone or placebo (1:1) for 2.5 days prior to LPS challenge. Participants were injected with intradermal LPS (5 ng/injection) on the volar forearm and monitored at multiple time points (1, 3, 6, 24 h). Vascular responses were assessed by multispectral imaging and laser speckle contrast imaging. Immune responses were measured through cytokine analysis of suction blister fluid, flow cytometry, and immunohistochemistry of skin punch biopsies. Results: Intradermal LPS triggered a rapid increase in skin perfusion and erythema, detectable from 1 hour post-injection. IL-8 peaked at 1 hour, while IL-6 and IL-1β peaked at 3-6 hours. A marked neutrophil influx and NET formation were observed as early as 1 hour, with peak NET formation at 24 hours. Classical monocyte infiltration followed neutrophil recruitment. NLRP3 inflammasome activation, as evidenced by ASC and NLRP3 expression, peaked at 3 hours. Prednisolone reduced vascular responses and infiltration of intermediate monocytes but had limited effect on early cytokine levels, neutrophil influx, NET formation, or inflammasome activation. Conclusions: The intradermal LPS model reveals a rapid sequence of inflammatory events, beginning within one-hour post-challenge. While prednisolone attenuates some late-phase responses, early neutrophil-driven and inflammasome-mediated events remain largely unaffected. These findings enhance the utility of this model for studying early innate immune responses and evaluating new anti-inflammatory therapies targeting IL-1β and NET formation specifically.