Could a simple biomarker as neutrophil-to-lymphocyte ratio reflect complex processes orchestrated by neutrophils?

The complex pathological mechanisms of autoimmune diseases have now been discovered and described, including interactions between innate and adaptive immunity, the principal cells of which are neutrophils and lymphocytes. Neutrophil-to-lymphocyte ratio (NLR) was proposed as a biomarker for inflammation that reflects the balance between these aspects of the immune system. NLR is widely studied as a prognostic or screening parameter in quantity diseases with important inflammatory components such as malignancies, trauma, sepsis, critical care pathology, etc. Although there are still no consensually accepted normal values for this parameter, there is a proposal to consider an interval of 1-2 as a normal value, an interval of 2-3 as a grey area indicating subclinical inflammation and values above 3 as inflammation. On the other hand, several studies have been published indicating that a particular morphological type of neutrophils, low-density neutrophils (LDNs), play a pathological role in autoimmune diseases. Probably, the LDNs detected in patients with different autoimmune diseases, mostly than normal density neutrophils, are involved in the suppression of lymphocytes through different pathways: inducing of lymphopenia through neutrophil depending overproduction of type I interferon (IFN)-α/β and direct suppression by a hydrogen-peroxide-dependent mechanism. Their functional features involvement in IFN production is of particular interest. IFN is one of the critical cytokines in the pathogenesis of many autoimmune diseases, primarily systemic lupus erythematosus (SLE). An interesting and important feature of IFN involvement in the pathogenesis of SLE is not only to be directly related to lymphopenia but also its role in the inhibition of the production of C-reactive protein (CRP) by hepatocytes. The CRP is the primary acute phase reactant, which in SLE often does not correlate with the extent of inflammation. NLR in such a case can be an important biomarker of inflammation. The study of NLR as a biomarker of inflammation also deserves attention in other diseases with established interferon pathways, as well as in hepatopathies, when CRP does not reflect the proper inflammation activity. Also, it may be interesting to study its role as a predictor of relapses in autoimmune diseases.