Premetastatic milieu explained by TLR4 agonist-mediated homeostatic inflammation

Decreased resistance to bacterial cold-water disease and excessive inflammatory response in ayu (Plecoglossus altivelis) reared at high water temperature

Introduction

Temporal elevation of water temperature positively affects immune activity and disease resistance in poikilothermic teleost fish. The ayu, Plecoglossus altivelis, an important fish species for Japanese freshwater fisheries, is usually produced under higher water temperatures than the natural conditions to facilitate rapid growth. However, it has been reported that rearing fish at higher water temperatures inhibits the development of the thymus, suggesting that resistance to infectious diseases is reduced in ayu reared at higher water temperatures. Here, we show that decreased resistance to bacterial cold-water disease and excessive inflammatory responses occurred in ayu reared at 22°C compared with those reared at lower temperatures.

Methods

Ayu larvae were reared at 12°C, 15°C and 22°C for 77 days and fed 3% of their body weight. Thymus index and condition factor was calculated after the fish rearing. Then, ayu reared at the different temperatures were challenged with Flavobacterium psychrophilum and the fish were sampled for histopathology and gene expression analyses. Further, the fish were vaccinated with formalin-killed F. psychrophilum and continuously reared at the three different water temperatures. Serum antibody titer was determined by ELISA and cumulative mortality in each group was recorded after the bacterial challenge.

Results

Ayu reared at 22°C showed a significantly lower thymus index and higher condition factor than those reared at lower temperatures. Infiltrated leukocytes and many melanin pigments were frequently observed in the adipose tissues and spleens of ayu reared at 22°C, respectively, but not in those reared at 12°C. The gene expression levels of inflammatory cytokines such as IL-1β, IL-8 and TNFα in the spleen were significantly higher in the 22°C group than in the 12°C group. The cumulative survival rate after challenge with Flavobacterium psychrophilum was 51.7%, 40.0% and 13.3% in the 12°C, 15°C and 22°C groups, respectively. The relative percent survival values of vaccinated fish reared at 15°C and 22°C groups were lower than those reared at 12°C. Moreover, the specific antibody titer of the vaccinated fish was the lowest in the 22°C group and the highest in the 12°C group.

Discussion

These results suggest that rearing the fish under high water temperature causes excessive inflammatory responses similar to metabolic inflammation in human obesity, resulting in a decrease of disease resistance. In addition, thymic involution induced by higher water temperature probably leads the poor response to vaccination. The present study provides insights into the physiological and immunological changes of fish under global warming.

Premetastasis

Sterile inflammation within primary tumor tissues can spread to distant organs that are devoid of tumor cells. This happens in a manner dependent on tumor-led secretome, before the actual metastasis occurs. The premetastatic microenvironment is established in this way and is at least partly regulated by hijacking the host innate immune system. The biological manifestation of premetastasis include increased vascular permeability, cell mobilization via the blood stream, degradation of the extracellular matrix, immunosuppression, and host antineoplastic activities.

Stimuli-induced organ-specific injury enhancement of organotropic metastasis in a spatiotemporal regulation

The relationship between inflammation and tumorigenesis has been established. Recently, inflammation is also reported to be a drive force for cancer metastasis. Further evidences show that various stimuli directly induced-injury in a specific organ can also promote metastasis in this organ, which include epidemiological reports, clinical series and experimental studies. Each type of cancer has preferential sites for metastasis, which is also due to inflammatory factors that are released by primary cancer to act on these sites and indirectly induce injuries on them. Host factors such as stress,fever can also influence distant metastasis in a specific site through stimulation of immune and inflammatory effects. The five aspects support an idea that specific-organ injury directly induced by various stimuli or indirectly induced by primary tumor or host factors activation of proinflammatory modulators can promote metastasis in this organ through a spatiotemporal regulation, which has important implications for personalized prediction, prevention and management of cancer metastasis.

Serum amyloid A3 binds MD-2 to activate p38 and NF-κB pathways in a MyD88-dependent manner

Serum amyloid A (SAA) 3 is a major component of the acute phase of inflammation. We previously reported that SAA3 served as an endogenous peptide ligand for TLR4 to facilitate lung metastasis. Because these experiments were performed with SAA3 recombinant proteins purified from Escherichia coli or mammalian cells, we could not rule out the possibility of LPS contamination. In this study, we used SAA3 synthetic peptides to eliminate the presence of LPS in SAA3. We found that the SAA3 synthetic peptide (aa 20-86) (20-86) stimulated cell migration and activated p38 in a manner dependent on TLR4, MD-2, and MyD88. SAA3 (20-86) also activated NF-κB and Rho small GTPase. Using surface plasmon resonance analysis, the binding constant KD values between SAA3 (20-86) or SAA3 (43-57) and TLR4/MD-2 protein highly purified by the baculovirus system were 2.2 and 30 μM, respectively. FLAG-tagged SAA3 tightly bound to protein A-tagged MD-2, but not to TLR4 in baculovirus coinfection experiments. Although SAA3 (20-86) caused a low, but appreciable level of endocytosis in TLR4, it induced the upregulation of both IL-6 and TNF-α, but not IFN-β1. An i.v. injection of SAA3 (43-57) induced the lung recruitment of CD11b(+)Gr-1(+) cells at an estimated serum concentration around its KD value toward TLR4/MD-2. Taken together, these results suggest that SAA3 directly binds MD-2 and activates the MyD88-dependent TLR4/MD-2 pathway.

Imbalance of Clara cell-mediated homeostatic inflammation is involved in lung metastasis

We have previously shown that tumor necrosis factor (TNF)α produced from primary tumor-induced expression of two endogenous Toll-like receptor 4 (TLR4) ligands, S100A8 and serum amyloid A3 (SAA3), in pre-metastatic lungs. However, mechanistic details of the signaling network and relevance to pulmonary physiology are poorly understood. Here, we identify Clara cells as a control tower of the network. Clara cell ablation by naphthalene suppressed pulmonary recruitment of CD11b+TLR4+ cells and spontaneous lung metastasis. Clara cells turned out to express TLR4 through which SAA3 was auto-amplified. Reciprocal bone marrow transplantation between wild-type and TLR4 knockout mice demonstrated that pulmonary TLR4+ Clara cells could be derived from bone marrow. SAA3-induced TNFα expression in both alveolar type II cells and macrophages. Primary co-cultures of alveolar type II cells and Clara cells revealed that the induction of TNFα in alveolar type II cells was dependent on the Clara cell-mediated amplification of SAA3. SAA3 induction by bacterial endotoxin also required both Clara cells and TLR4. Thus, pulmonary metastatic soil may feature deregulation of homeostatic inflammatory responses to constant assaults of microbes with endotoxin.