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.