Discussion
Elevated levels of sFLC have been found in a variety of ADs, including systemic lupus erythematosus, rheumatoid arthritis, Sjögren’s syndrome, ankylosing spondylitis, psoriatic arthritis, vasculitis although many studies included a small number of patients [5, 18, 19].
In these ADs, sFLC correlated strongly with inflammatory mediators, activity and severity index demonstrating their potential for predicting disease activity [19, 20].
Few studies have quantified the FLC in SSc and only one in the serum [10]. Bosello et al found elevated k-sFLC vs HC that correlated with restrictive lung disease, disease activity and inflammatory biomarkers such as ESR, CRP and interleukin-6 (IL-6) [6]. IL-6 is produced by activated B cells and the authors suggest that sFLC may play a role in the pathogenetic mechanisms of SSc. As multifunctional cytokine, IL-6 plays a well-recognized role in inflammatory, immunoregulatory and haemopoietic responses. Thanks to a trans-signaling mechanism mediated by its soluble receptor, IL-6 may orchestrate a wide range of inflammatory-immune properties [21-23]. Among biological properties of IL-6, the stimulation of collagen synthesis by dermal fibroblasts is well recognized [24].
In fact, high levels of IL-6 are found in the skin and serum of SSc patients connecting the role of this cytokine in the onset of inflammation and fibrosis through the activation of B cell and the subsequent production of sFLC [25]. Moreover, Lanteri et al, analyzing 134 SSc patients found a correlation between sFLC and BAFF levels [10] The BAFF family, play a pivotal fundamental role in B cell development, maturation, and survival and it is found higher in the skin and serum and of SSc patients [25].
Serum FLC is a marker of polyclonal activation and here we showed that in our patients both serum free k and λ and k + λ sum correlate with ESR and CRP. In addition, a linear correlation was found between serum free k with DAI and DSS. In addition, when specific cut-off values for normal range of sFLC were considered, SSc patients with abnormal serum free k and k+ λ-sum showed a positive correlation with DAI and DSS.
Thus, supporting the role of B cell activation in the pathophysiology of SSc, an overproduction of FLCs may occur because of chronic immune stimulation. This fit well with the proposed role of FLC as a biomarker of B cell activity as suggested in many studies on Ads [5].
To our knowledge, this is the first study that includes the evaluation also of urinary FLC in patients with SSc.
The main result of our study is that urinary FLC levels were found to be elevated in comparison to the HC. Higher urine free k levels, k+ λ sum, k/λ ratio>1.80 were found in SSc patients, while urine λ levels were comparable to HC.
Since in SSc the clinical features are heterogeneous and disease course variable, the introduction of new biomarkers is crucial to early determine systemic complications.
Biomarkers are useful to predict disease activity and progression, to identify patients at high risk of flares and monitor response to treatment. In a large cohort of individual without plasma cell disorder, the findings of FLC levels above the normal range proved to be predictors of worse overall survival in the general population of individual without plasma cell disorders [26]. The urinary levels of polyclonal FLC strongly depend on the rate of production and serum concentrations, on the range of proteinuria, presence of tubular damage and the affinity of the tubules for all different FLC [27]. The system of reabsorption by the proximal tubular is very efficient but saturable. Measurement of uFLCs is most used in course of plasma cell malignancy as multiple myeloma, with an excess of monoclonal k or λ FLC [28].
On the other hand, the clinical significance of polyclonal FLC is to monitor activity of the disease, especially in those characterized by activation of the B such as ADs and chronic inflammatory disorders [5].
In SSc, subclinical renal involvement is characterized by abnormal resistive index [29], isolated reduction in glomerular filtration rate (GFR) rate and reduced kidney reserve [30]. Livi et al evaluated the renal functional reserve from response of the kidney to an amino acid challenge in normotensive patients with normal renal function and no urinary abnormalities [31]. They found in SSc patients an abnormal activation of renal functional reserve (RFR). RFR is a parameter to measure the ability of the undamaged kidney to increase GFR in response to protein overload and it depends on the activation of preglomerular vasodilating mechanisms. In SSc, a functional derangement of vascular tone control is evident, favouring the prevalence of vasoconstrictor over vasodilating factors.
The lack of correlation between serum and urinary FLC suggests the kidney’s inability to reabsorb the sFLC produced. It is well known that a high prevalence of silent renal changes is observed in SSc kidneys, including a reduced renal blood flow [32].
In clinical practice, the polyclonal elevations of FLCs are observed in patients with chronic kidney disease and high blood pressure [33]. However, in our study the mean value of eGFR in SSc patients was 93 ml/min and no differences were found between patients with and without arterial hypertension with both serum and urinary FLC levels. Thus, we can confirm that the clearance and loss of FLC is not GFR and comorbidity dependent.
The continuous reabsorption and accumulation of sFLC within the proximal tubule lumen led to inflammation, tubular cells apoptosis, activation of redox pathways and production of profibrotic cytokines [34, 35]. In the present study SSc patients with abnormal urine free k have higher DAI, although there is a weak positivity. The analysis of both urine and serum for the assessment of FLC levels offer a simplified route to evaluating the presence of activity and severity disease in ADs. The increased levels of uFLC in these patients could be the result of non-specific polyclonal B cell activation as occurs in other ADs.
The FLC production observed in different systemic rheumatic autoimmune diseases reflect the clone expansion of B cells with specific autoreactivities [4]. Due to their short half-life, FLC may be considered a direct biomarker of B cell activity, recently described also in organo-specific autoimmune disorders [36]. Increased FLC levels maybe a marker of disease severity and can be useful for therapeutic monitoring or to assess recurrences investigating and reporting the association between pathogenic viruses (SARS-Cov2) with the development of inflammatory condition in autoimmune diseases.
In conclusion, we suggest that both serum and urinary levels of FLC could be considered a reliable biomarker to early diagnosis and monitoring of disease activity, leading to an optimization of specific patients’ management.