Flagellin or Lipopolysaccharide Treatment Modified Macrophage Populations after Colorectal Radiation of Rats

Abstract

Radiation-induced acute intestinal toxicity remains a major limitation to the delivery of tumoricidal doses of colorectal irradiation. Recent reports indicate that Toll-like receptor (TLR) agonists TLR4 and TLR5 protect against toxicity due to intestinal irradiation. The phenotype (M1 or M2) of macrophages expressing TLRs may play a role in tissue repair. The aim was to investigate whether administration of TLR4 agonist lipopolysaccharide (LPS) or TLR5 agonist flagellin after irradiation modified the recruitment and phenotype of colonic macrophages and improved tissue damage. Rats were exposed to single 20-or 27-Gy doses of colorectal irradiation. TLR4 agonist LPS or TLR5 agonist flagellin (at 50 or 200 mg/rat) was administered i.p. 3 days after irradiation. Flow cytometric analysis, immunostaining, and real-time polymerase chain reaction analysis were used to assess the M1/M2 phenotype and crypt cell proliferation 7 days after irradiation. Irradiation (20 and 27 Gy) increased TLR4 1 and TLR51 macrophage frequency in the mucosa. LPS or flagellin administration maintained this elevated frequency after the 27-Gy irradiation. LPS and flagellin drove macrophages toward the anti-inflammatory M2 phenotype by increasing Arg1 and CD163 expression and microenvironmental effector molecules (C-C motif chemokine 22, transforming growth factor-b1, and interleukin-10). Proliferating cell nuclear antigen immunostaining, Ki67 expression, and antimicrobial factor Reg3g showed that the M2 shift correlated with epithelial regeneration. In conclusion, administration of either LPS or flagellin after colorectal irradiation may provide effective protection against epithelial remodeling. This tissue repair was associated with an M2 macrophage shift. Using TLR agonists to moderately activate innate immunity should be considered as a strategy for protecting healthy tissue from irradiation.

RETRACTED: Macrophage phenotypic plasticity in atherosclerosis: The associated features and the peculiarities of the expression of inflammatory genes

“…The classic M1 phenotype of activated macrophages is induced by inflammatory cytokines including tumor necrosis factor (TNF)-α and interferon (IFN)-γ and by stimulation with bacterial constituents such as lipopolysaccharides (LPS) and flagellin or in response to an intracellular parasite infection [27,28]. M1 macrophages are characterized by the expression of a broad spectrum of proinflammory cytokines (TNF-α, interleukin (IL)-1β, IL-12, and IL-23) and chemokines (C-X-C motif chemokine (CXCL9, CXCL10, and CXCL11)) [19] (Table 1).…”

Stimulating Innate Immunity to Enhance Radiation Therapy–Induced Tumor Control

“…While LPS can induce potent pro-inflammatory responses from macrophages in particular contexts, as discussed above LPS treatment of tumor macrophages following radiation therapy results in secretion of cytokines that are anti-inflammatory and support tumor growth and tissue repair 27–29,65 . These data suggest that while LPS is a potent immune adjuvant, and while endogenous TLR4 adjuvants contribute to tumor control following radiation therapy, TLR4 ligation can have anti-inflammatory as well as proinflammatory effects, depending on the differentiation of the responding cells.…”

Addressing the Symptoms or Fixing the Problem? Developing Countermeasures against Normal Tissue Radiation Injury

“…Interestingly, because free radical generation has been shown to be elevated under conditions of low oxygen concentration (154), the presence of hypoxia may provide additional support for the role of oxidative stress in radiation injury. Furthermore, it has been suggested that the presence of both ROS and hypoxia regulates macrophage activation and polarization (155, 156), conditions shown to be associated with late radiation injury (148, 157). …”