Immuno-modulation and anti-inflammatory benefits of antibiotics: the example of tilmicosin

Abstract

Exaggerated immune responses, such as those implicated in severe inflammatory reactions, are costly to the metabolism. Inflammation and pro-inflammatory mediators negatively affect production in the food animal industry by reducing growth, feed intake, reproduction, milk production, and metabolic health. An ever-increasing number of findings have established that antibiotics, macrolides in particular, may generate anti-inflammatory effects, including the modulation of pro-inflammatory cytokines and the alteration of neutrophil function. The effects are time- and dose-dependent, and the mechanisms responsible for these phenomena remain incompletely understood. Recent studies, mostly using the veterinary macrolide tilmicosin, may have shed new light on the mode of action of some macrolides and their anti-inflammatory properties. Indeed, research findings demonstrate that this compound, amongst others, induces neutrophil apoptosis, which in turn provides anti-inflammatory benefits. Studies using tilmicosin model systems in vitro and in vivo demonstrate that this antibiotic has potent immunomodulatory effects that may explain why at least parts of its clinical benefits are independent of anti-microbial effects. More research is needed, using this antibiotic and others that may have similar properties, to clarify the biological mechanisms responsible for antibiotic-induced neutrophil apoptosis, and how this, in turn, may provide enhanced clinical benefits. Such studies may help establish a rational basis for the development of novel, efficacious, anti-microbial compounds that generate anti-inflammatory properties in addition to their antibacterial effects.

Figure 1

Light micrographs from cytospin slides of bronchoalveolar lavages (A, B) or from histological preparations (eosin and hematoxylin; C,D) from calves 24 h after intratracheal inoculation with sterile endotoxin-free saline (controls, A, C), or with 2 × 10⁸ live M. haemolytica in saline (infected, B, D). Infection induces a rapid migration of neutrophils to the bronchoalveolar space (B). In homeostatic conditions, neutrophils serve to protect the lung against the invading bacteria; however, when unchecked, this accumulation of neutrophils is responsible for severe inflammatory injury. In lesional areas (D), inflammation during this infection produces classical pathological signs of bovine respiratory disease (BRD), including alveolar obstruction by inflammatory infiltrates and fibrin deposition. Bars = 100 μm.

Figure 2

Transmission electron micrographs (A and B) of bovine neutrophils (N) exposed for 2 h to tilmicosin (0.5 μg/mL), and co-incubated for 2 h with bovine monocyte-derived esterase-positive macrophages (M). Neutrophils exposed to the antibiotic exhibit characteristic signs of apoptosis, including nuclear membrane delamination, chromatin condensation, and cytoplasmic vaculation, while keeping their plasma membrane and cytoplasmic organelles intact. Exposure to tilmicosin significantly enhanced phagocytic uptake of apoptotic neutrophils by macrophages, as illustrated by tight membrane contacts (arrowhead) and full phagocytic inclusion of neutrophils in advanced stages of apoptosis withtin the macrophages (arrows). Bar = 1 μm. (Modified from reference 83).

Figure 3

Schematic illustration demonstrating how tilmicosin, by promoting neutrophil apoptosis, also called programmed cell death (PCD), may generate clinical benefits. By inducing neutrophil apoptosis, antibiotics such as tilmicosin prevent severe tissue damage secondary to leukocyte necrosis. Induction of neutrophil apoptosis confers anti-inflammatory properties to the antibiotic, without interfering with cell diapedesis, migration, and pulmonary infiltration.