Background
Inflammation is an important component of cancers and chronic diseases and many inflammatory mediators have shown to have potential prognostic roles. Healthy individuals produces five classes of immunoglobulins: IgG, IgM, IgA, IgD and IgE. Immunoglobulins, normally, consist of two identical heavy chains and two identical light chains. The heavy chains select which class the immunoglobulin belongs to. Different conditions are characterized by the overproduction of monoclonal immunoglobulins. In some patients, only light chains are produced. More studies have investigated the role of immunoglobulin free light chains (FLCs), that can trigger mast cell activation in an antigen-specific manner. Increased expression of FLCs has been observed within the stroma of many human cancers including breast, colon, lung, pancreas, kidney and skin. Increased serum concentration of polyclonal FLCs in inflammatory diseases has been correlated with the degree of inflammation (1).
The ability of free light chains to activate mast cells and then to become an active part of the pathogenetic mechanisms of chronic inflammatory diseases led to an increase of interest in their clinical use, both as an attractive therapeutic target and as a biochemical marker of disease evolution or remission (2). It has been hypothesized that differences in serum immunoglobulins, FLCs and secretory IgA (sIgA) could exist between subjects with asthma of varying severity and non-asthmatic subjects; moreover, the circulating FLCs levels could correlate with lung function, symptoms and airway inflammation (3).
FLCs mediate antigen-specific hypersensitivity responses in the presence of a not yet identified FLCs receptor on mast cells, independently of complement activation. γ-chain associated receptors, such as FcεRI and FcγRIII, are not involved in FLC-triggered mast-cell activation. Redegeld at al. have isolated a mast-cell membrane associated protein that interacts with FLCs (4).
Mast cells and neutrophils are fundamental cells in the sensitization and effector phases of chronic inflammatory immune responses in the lung, therefore a possible link between mast cells and FLCs suggest their potential role in the pathophysiology of asthma and might be a novel biomarker involved in the humoral immune response to antigens (4-6).
FLCs extend neutrophil lifespan suggesting an effective contribution to chronic neutrophilia associated with chronic obstructive pulmonary disease (COPD). Increased levels of FLCs in serum and lung tissues have been associated with increased blood neutrophil count in COPD patients. FLCs binding to neutrophils induces CXCL8 inflammatory chemotactic mediator, increasing neutrophils recruitment into the airways with enhanced blood neutrophilia in COPD (7,8).
In the mammalian immune system, two isotypes, k and λ, of FLCs are produced. The k/ λ ratio significantly varies among species. Serum FLCs levels are elevated in autoimmune diseases as systemic lupus erythematosus, rheumatoid arthritis and Sjögren syndrome and changes in their levels are associated with disease activity (9).
Both k and λ FLCs share a common binding site on Tamm–Horsfall protein (THP), a monomeric glycoprotein produced by cells in the ascending limb of Henle of the kidney. The responsible of this binding is F99, a 9-mer peptide derived from the amino-acid sequence in THP (10). Kraneveld et al. have shown that the highly selective FLCs antagonist (F-991) could be used to inhibit the development of airway hyperreactivity and inflammation (6) and has demonstrated to have remarkable biological activity in a number of animal models of allergic diseases, representing a potential treatment of allergic diseases in humans (11). Therefore, the presence and the characterization of FLCs in atopic patients could be of interest as they may provide an alternative approach to the treatment.
Recent studies evaluated the presence of FLCs in SARS-CoV-2 infected patients. SARS-CoV-2 affects the upper respiratory tract, preferentially nasal ciliated cells, mucus-producing cells and ciliated cells in the bronchial epithelium.
The damage of the epithelial barrier is the basis of chronic inflammatory diseases, including the most severe forms of asthma. Firstly, Malecka-Gieldowska et al. (12) observed that FLCs levels were markedly elevated in COVID-19 patients in comparison to non COVID-19 intensive care unit (ICU) patients. Importantly, the k/ λ ratio was similar in those groups, but λ concentration was higher and the k/ λ ratio decreased in SARS-CoV-2-infected but non-hospitalized in ICU group, compared to the non-infected patients from ICU. There was also a difference in the k concentration and k /λ ratio between tested groups with the highest values in COVID-19 patients.
Recently, Napodano et al. (13) compared salivary levels of immunoglobulin A subclasses (IgA1 and IgA2) FLCs in a cohort of 29 SARS-CoV-2 patients and 21 healthy subjects, describing the role of λFLC as an ideal indicator of patient conditions, that effectively could monitor patients’ fluctuation in real-time.
It is well known that healthy airway epithelium produces inducible Nitric Oxide Synthase (iNOS) (14), but its expression is significantly upregulated in asthmatic patient’s airways, mainly in epithelial and inflammatory cells such as eosinophils, neutrophils, and macrophages. Nitric oxide (NO) acts as a messenger molecule and its activity depends on factors such as oxidative stress, antioxidants, and the amount and activity of NOS. NO has a role in muco-ciliary function and ciliary movement frequency, in epithelial ion transport, in restoring barrier dysfunction by damage repair processes after barrier injury and in modulating inflammation by regulating epithelial production of inflammatory mediators, contributing to the patient’s innate defense.
Increased NO levels contributes to bronchial hyperreactivity and mucus hypersecretion, increases vascular permeability, reduces ciliary heartbeat, and promotes free radical production, airway inflammation, and tissue damage (15). Fractional exhaled nitric oxide (FeNO) has gained great clinical importance as a biomarker of type 2 inflammation in chronic airway diseases such as asthma and it is also very useful to identify those severe asthma patients that might benefit from personalized therapies with monoclonal antibodies.
Aim of this study was to describe clinical and laboratory characteristics of a population of asthmatics patients and evaluate the presence and isotypes distribution of FLCs in asthmatic and healthy subjects, aiming to investigate their potential role as quantitative and objective serum biomarkers of this condition. Additionally, the study seeks to evaluate any clinical correlations with disease severity.