Urate is a key factor inducing acute gouty arthritis, and it is also a major factor inducing recurrent gout and joint deformities [2, 3]. It plays a key role in joint swelling, temperature rise, and joint limitation of gouty arthritis. Previous researchers have isolated urate from the joints of the limbs of patients with hyperuricemia , and it affects the development of gouty arthritis. With the exception of humans, primates, and birds, almost all animals synthesize and secrete uric acid oxidase which function is to decompose uric acid into water-soluble products such as allantoin . Currently, artificially recombinant uricase is used in clinical trials, such as rasburicase and pegloticase. However, there are many restrictions on the use of uricase, such as large side effects, and uricase injected into the blood can induce an immune response. In addition, as the number of uses increases, the effect of the enzyme will become lower and eventually useless [26, 27].
To this end, Specific pathogen Free(SPF) chickens were used to make animal models to induce acute attacks of gouty arthritis, which could simplify the acute onset of gouty arthritis. Some reports have documented events that urate crystals were added to phagocytes in vitro, that is, the crystals were rapidly absorbed by leukocytes, followed by degranulation, phagosome membrane lysis, and cell rupture with release of the crystals. It is assumed that a hydrogen donor site on the crystal is attached to an acceptor site on the outer surface of the cell membrane lipid bilayer. The interaction between the crystal and the cell membrane can also alter cell metabolism and cause the secretion of inflammatory mediators . Because only MSU is used to induce arthritis[2, 4], this model can simulate symptoms such as joint swelling, local joint temperature rise, and joint limitation in acute episodes of gouty arthritis, and can be viewed as a simple arthritis. Both IL-1β and IL-1beta are the important indicators [20, 22].TNFα is the earliest and most important inflammatory medium produced by activated macrophages. It can activate neutrophils and lymphocytes, increase the permeability of vascular endothelial cells, regulate the metabolic activity of other tissues and promote the synthesis and release of other cytokines [24, 25]. Interleukin-1β (IL-1β) is a pivotal proinflammatory cytokine that plays important roles in regulating immune responses and in inducing a series of inflammatory reactions in response to infection.
We have investigated the expression of IL-1β, TNFα and the symptoms of acute arthritis induced by monosodium urate, and have investigated the relation between joint circumference, joint temperature, synovial fluid TNFα and IL-1β and arthritis symptoms, before both before and after onset of clinical symptoms of arthritis. With the development of arthritis, the local temperature of the joints gets higher and higher, and the circumference turns longer and longer, and at the same time, the level of TNFα and IL-1β increase, reaching the highest level after 8 hours approximately. We found that the degree of TNFα and IL-1β produced in the joint depends on the severity of the disease, and the joint circumference becomes longer and the temperature rises with the increase of TNFα and IL-1β, so the level of TNFα and IL-1β is closely related to inflammation. Therefore, in the use of MSU-induced arthritis, the production of detectable T TNFα and IL-1β is closely related to the onset of clinical symptoms. Although TNFα is not a diagnostic and therapeutic indicator of gout, it is a crucial indicator that cannot be ignored in human arthritis .
Studies have shown that the use of recombinant uricase can decompose uric acid in serum, reduce the level of serum uric acid to reduce the probability of gouty arthritis [26, 27]. The occurrence and development of gouty arthritis probably result from the excessively high level of serum uric acid which becomes monosodium urate after encountering monosodium ion. Monosodium urate is hardly dissolved in the blood and will gradually deposit on joints and other parts [2–4]. If it deposites on joints, arthritis can be induced. Intravenous injection of uricase is an important way to reduce uric acid levels in current clinical trials, but there are many limitations. From the analysis of the molecular structure of monosodium urate and uric acid, the core genes and functional bonds of the two have not changed, which provides a theoretical basis for the reaction of uricase and monosodium urate.
The principle of the reaction between uric acid and monosodium urate is shown in Fig. 7, represented by yellow and green arrows. There is a dynamic balance between monosodium urate and synovial fluid, and the separated monosodium urate ion immediately produces a chemical reaction when it encounters uricase . Although monosodium urate is difficult to dissolve, it still has dissolution equilibrium with the aqueous solution. For example, monosodium urate is broken down into monosodium urate anion and monosodium positive ion (Monosodium urate ⇋ Monosodium urate ion + Na+). After encountering uricase, under the catalysis of uricase, monosodium urate ion reacts with oxygen to form 5-hydroxyisourate (C5H4N4O4) (Monosodium urate ion + H2O + O2→ 5-hydroxyisourate + H2O). The resulting 5-hydroxyisourate is automatically decomposed into allantoin (C4H8N4O4) and carbon dioxide (5-hydroxyisourate + H2O → Allantoin + CO2) due to unstable existence. Allantoin is easily soluble in water and excreted through the urinary system.
If uricase is injected into the joint, many adverse reactions are reduced at the same time, and the effect may not decrease with the increase of the number of times. Because uricase enters the bloodstream and spreads throughout the body with blood circulation, it is easier to trigger the immune system. To this end, we injected uricase into the joints that induced inflammation with MSU, and observed the correlation between its efficacy and the symptoms of experimental animals and the level of IL-1β and TNFα. It is also encouraging that with the continuous injection of uricase into the joints, the symptoms of the experimental animals improved significantly, including reduced joint swelling, decreased temperature and decreased TNFα and IL-1β level. This treatment effect is similar to the treatment of arthritis by nonsteroidal anti-inflammatory drugs(NSAIDS), so uricase can inhibit or alleviate urate-induced inflammation.
However, in the experiment, according to the observed experimental phenomena and analysis of the experimental data, it was found that the therapeutic effect of 10 mg uricase was not significantly different from that of 5 mg uricase (p > 0.05), as shown in Fig. 4,5,6. The reason for this phenomenon may be the "saturation" of the activity of uricase to break down monosodium urate or the inhibitory effect of degradation products on uricase. But unfortunately this is only our inference, and no research has proved this conjecture.
In this study, chicken knee joints were observed under CT and no significant results were obtained in all groups. As reported by Lisa et al. , CT can be used to diagnose gouty arthritis. Although 50 mg / ml monosodium urate can be observed on a polarized light microscope as shown in Fig. 8, the concentration of monosodium urate suspension injected into the joints of chicken was 50 mg / ml, which was not much different from the concentration of joint fluid. At the same time, the injected monosodium urate suspension was greatly diluted with synovial fluid, leading to its density was more similar to that of synovial fluid, which could not be distinguished under CT. A report by Lisa et al.  suggests that the test object is a patient who has been exchanging gouty joint fluid for some time, and thus the density and size of monosodium urate deposition have reached a level different from that of joint fluid, so CT detection can be used. Furthermore, according to Alexis et al. , CT or ultrasonography is not sensitive to the detection of patients with early gout.
We have shown in this study that uricase can indeed treat MSU-induced inflammation. Uricase breaks down urate, which improves symptoms and suppresses the increase in TNFα and IL-1β. IL-1β is a key factor. Of course, if combined with anti- TNFα, anti- TNFα treatment will be better. Uricase joint injection is a new way to treat arthritis induced by urate deposition. It can bring long-term benefits to patients, if combined with the therapy of lowering serum uric acid.