Neutrophilic granulocytes (white blood cells), vital in innate immunity, are fundamental in inflammatory intestinal diseases, particularly Crohn’s disease; they are considered the first line of defense against germs, which they destroy through their oxidative metabolism. However, these inflammatory cells have been less studied than other immune cells such as T cells. This is because autoimmune disease discoveries made in the 1990s and 2000s focused on T cells and the inflammatory cytokine network. With the even more recent discovery of the various lymphocyte subpopulations (Th1, Th2, Th17, Th19, Tregs, ILCs, etc.), the interest in the lymphocyte “world” has exploded even more. In contrast, studies on neutrophilic granulocytes had completely fallen asleep.
They continued on an experimental basis, with investigations involving their cellular survival and the underlying molecular mechanisms. Granulocytes, in fact, are cells that are continuously renewed by the bone marrow, given that their average lifespan is between 12-36 hours depending on situations of stasis or bacterial attack. The reason is to be found in their own biological system: being cells that destroy bacteria through oxidative enzymatic reactions (mediated by oxidative stress originating from membrane enzymes), they themselves suffer many of those biological lesions in their half-life that prevents them from surviving the self-inflicted damage. Their survival mechanisms against the phenomenon of programmed cell death (apoptosis) induced by pharmacological agents such as methyl-xanthines or glucocorticoids, notoriously used as anti-inflammatories and immunosuppressants, were investigated.
It was thus established that their exposure to peroxide (hydrogen peroxide) produced by their oxidative metabolism is capable of triggering their programmed death, but this phenomenon seems reduced in the genetic disease chronic granulomatosis, where neutrophils have difficulty letting go of the inflammatory phenomena . Their death, accelerated by the Fas membrane receptor or by degradation products of their membranes (ceramide), is partially delayed or suppressed by antioxidants such as N-acetyl-cysteine, glutathione or ascorbic acid. This once again indicates that it is the their own biological system to determine how long they will resist in the field, whether longer under surveillance conditions or dying “on the battlefield”. On the contrary, certain cytokines (e.g. IL-2, IL-15 and TNF-alpha) or growth factors delay their elimination.
Growth factors have a cellular “nutrition” effect and promote survival through different mechanisms. They can stimulate DNA synthesis, protein synthesis, stabilize the energy produced by mitochondria and suppress the production of lethal proteins that serve to execute programmed death. Practically, the same principle by which cancer cells exploit them to their advantage. Neutrophil granulocytes use some growth factors (G-CSF and GM-CSF), which activate cellular pathways with an anti-lethal effect (such as the mitogenic kinases ERK1 and ERK2 and the PI3K/c-Akt kinase axis which directly suppresses the phenomenon apoptotic). Even some cellular messengers such as the famous cyclic AMP (cAMP) appear to delay their spontaneous death. This occurs via two cellular pathways: the direct cAMP/PKA axis and the Epac/ERK2 axis.
Manipulation of the cAMP/PKA system can be achieved pharmacologically in various ways. Many drugs in clinical use are capable of altering it. Just think of all the adrenergic drugs used in vascular and cardiology clinics and also natural products such as caffeine from coffee and theophylline from green tea. These are also capable of altering cellular levels of cAMP, but before the discovery of adenosine receptors it was thought that they affected its cellular degradation by blocking the PDE4 phosphodiesterase enzyme. When it was discovered, however, that they were actually natural antagonists of the A2a and A2b receptors, it was understood why in some cases these substances lowered cellular levels of cAMP, instead of raising them, given that not all adenosine receptors possess the same downstream transducers.
Theophylline is a drug still used in both cardiac and pulmonary clinical practice, although not as much as in the past. In the first case it is a cardiac tonic useful in acute pump failure, it is a regulator of vascular tone and can also have a diuretic effect. This made it a cornerstone in the treatment of congestive heart failure. At the pulmonary level, before the advent of more modern bronchodilators, theophylline was used to treat chronic asthma and chronic obstructive pulmonary disease (COPD), because it has a dilating effect on the bronchi. Theophylline also acts on adenosine receptors present on neutrophilic granulocytes and accelerates their spontaneous death. This could contribute to its anti-inflammatory effect of neutrophil recruitment during bronchial asthma, especially in chronic forms. Likewise, the effect of corticosteroids on the viability of these cells was investigated.
Steroids currently in clinical use such as dexamethasone and prednisone accelerate the spontaneous death of neutrophils, interfering with the “positive” action that mediators such as G-CSF, TNF-alpha and prostaglandin E2 (PGE2) have, which instead support their vitality. PGE2 is a well-known mediator of inflammation whose antagonists are NSAIDs (ketoprofen, ibuprofen, nimesulide, etc. up to the more modern coxib derivatives). Through the membrane receptor EP2R1 it activates the previously mentioned cAMP/PKA and Epac/ERK cascades which stimulate the survival of these cells. Therefore, the rationale for using NSAIDs and/or corticosteroids when there is chronic inflammation is not fundamentally wrong. The major practical problem arises due to the appearance of side effects of these drugs, from gastrointestinal symptoms to blood sugar or kidney function disorders.
Fortunately, inflammation is a very complex phenomenon that involves other mediators and therefore other more or less targeted types of drugs. However, when there is an acute lung injury/event that potentially puts organ function or the patient’s life at risk (acute lung injury, violent asthma attack, etc.) the use of corticosteroids is advisable. obligation, also because fortunately these molecules have a good supporting role for lung function. In fact, remember that, at birth, the maturation of the lungs of premature babies is achieved with a small therapy based on glucocorticoids. These stimulate lung cells to secrete “surfactant”, the liquid that allows the lungs to flow into the pleurae. And in a severe pediatric asthma attack, there is no hesitation in using a steroid when other options fail.
By Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.
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