Low-dose aspirin and the resolution of acute inflammation

Inflammation is the major driving force behind rheumatoid arthritis, psoriatic arthritis, systemic lupus erythematosus and Crohn s disease, for example. However, such diseases are non-resolving or recurrent in nature. By contrast, there are other inflammatory diseases such as streptococcal-induced pneumonia, which typically resolve without causing lasting tissue injury. Therefore, it is fair to say that some inflammatory responses are under the control of endogenous factors that signal their cessation, which may be dysregulated in others. The idea behind this MRC-funded research project comes from the observation that acute inflammation, elicited, for example, by a finger prick or bee-sting injury has a natural progression from start (swelling, pain and redness) to finish (reduction in swelling and pain and restoration of the tissue to its prior form, collectively called resolution). However, it has remained unappreciated for many years that resolution of inflammation is under the control of factors manufactured by the injured tissue. Our philosophy is that we could use these factors or develop drugs that mimic their action to trick chronic inflammatory diseases into resolving or switching off. This approach would allow the inflammatory response to progress as normal and neutralize the injurious agent, which is the basis and function of the inflammatory response, but bring about resolution of the event in a timely manner with minimal tissue injury. In pursuit of this thinking, there are certain hormones called lipoxins that are preferentially expressed during the resolution phase of acute inflammation. Of the lipoxin family of eicosanoids are a sub-group triggered by aspirin called aspirin-triggered epi-lipoxins. In a series of experiments, including those carried out in man, we found that aspirin at low, cardioprotective doses alters the expression of these protective factors resulting in a significant impact on the innate immune response in man. These findings are particularly important given the widespread use of low-dose aspirin, not alone for its cardio-protective but also potential anti-cancer effects. In which event, we are now uncovering novel properties of low-dose aspirin on the innate immune response in man that will not only change the way we view how inflammation occurs but also how it may be treated more effectively.

While pro-inflammatory signals initiate and drive acute inflammation, it is becoming apparent that the resolution phase of acute inflammation is also actively controlled by endogenous factors. However, these factors and the mechanisms by which they control resolution remain poorly investigated. Moreover, no therapies exist to specifically enhance resolution. Of the factors described to date, the lipoxin family of eicosanoids, in particular aspirin-triggered 15-epi-lipoxin A4, is gaining recognition for its pro-resolution properties as, for instance, it accelerates macrophage phagocytosis of apoptotic leukocytes. 15-epi-lipoxin A4 is generated upon the acetylation of the active site of inducible cyclooxygenase (COX-2) by aspirin, resulting not it?s inhibition but in the conversion of arachidonic acid to 15rHETE, which is metabolized, in a transcellular manner, by 5-lipoxygenase to 15-epi-lipoxin A4. On this theme of resolution we found that phagocytosis of apoptotic leukocytes further promotes macrophage clearance to draining lymph nodes. To add further insight into the pro-resolution properties of epi-lipoxins, we recently reported that through 15-epi-lipoxin A4, aspirin elicits nitric oxide (NO) synthesis. Lymphatics generate NO through constitutive nitric oxide synthase (eNOS), which increases lymphatic fluid velocity. Therefore, we hypothesise that at least two mechanisms exist whereby aspirin-triggered 15-epi-lipoxin A4 signals resolution ? increased phagocytosis of apoptotic leukocytes coupled with NO-dependent enhanced lymphatic drainage. Central to this application is the fact that aspirin triggers 15-epi-lipoxin A4 synthesis in acute inflammation at doses far below its anti-inflammatory levels in man and rodents. Based on this we carried out preliminary experiments in various acute inflammatory models finding that while low-dose aspirin had no effect on the onset of inflammation, it hastened resolution through increased macrophage lymphatic clearance. Conventional anti-inflammatory doses of aspirin impeded resolution, a property shared with other nonsteroidal anti-inflammatory drugs. We aim to characterise the effect of low-dose aspirin and that of a stable 15 epi-lipoxin A4 analogue on mechanisms underlying acute inflammatory resolution, utilizing experimental inflammatory models with COX-2 and eNOS knockout mice as well as adoptive transfer experiments and fluorescent cell tracking, all well-characterised in our laboratory. We will finally examine the pro-resolution effects of low-dose aspirin in man in a simple model that allows leukocyte kinetics to be tracked. By understanding the novel anti-inflammatory properties of low-dose aspirin and the epi-lipoxins we will not only gain further insight into mechanisms that control resolution but possibly, for the first time, propose new and more effective regimes for the treatment of recurrent or inappropriate inflammatory responses.