Our findings indicate that plasma FN concentration was significantly lower in SLE than in RA patients and normal individuals (Table 2). Moreover, the low value of FN concentration was more frequently associated with the unusual FN immunoblotting patterns characterized by the presence of FN fragments and/or the high-molecular weight band series which appeared in immunoblotting of the most SLE, but in none of the RA and normal plasmas (Table 3, Fig. 3).

However, previous reports indicated elevated plasma FN level in SLE patients is [13] significantly higher in patients with active SLE than with nonactive disease [14]. It should be underlined that our results of FN determination by the set of domain-specific monoclonal antibodies do not correspond to total FN level measured by polyclonal antibodies used in previous studies. It may be because the FN domain epitopes can be masked by interfering with antibodies, other molecules, the FN ligands, such as respective cellular integrins, collagen and its fragments, fibrin or glucosaminoglycans [15]. Additionally, the degradation of FN molecule by endogenous metalloproteinases, active during inflammatory diseases, can lead to damage of domain epitopes and/or to exposition of cryptic functional sites. Moreover, a conformational alteration in one domain could have an effect on conformation and stabilization of the nearest neighbor sites [16].

Among analyzed FN domains, the most helpful seems to be the determination of ^{Fibrin–Heparin}FN, ^CBDFN and ^CtFN levels. The low expression of ^{Fibrin–Heparin}FN (Table 2) was found to be a common characteristic parameter for SLE and RA. Moreover, the low ^CBDFN and ^CtFN can help to differentiate SLE with RA. However, we rather recommend the ^CtFN determination. Using it seems to be more universal than ^CBDFN determination, because on the one hand, the low ^CtFN expressions can help to differentiate SLE from RA and normal samples, and on the other hand, the high level RA from the normal individuals. According to the ROC analysis, the ^CtFN concentration lower than 350 mg/l in SLE and higher than 460 mg/l in RA can predict the diseases with quite good sensitivity (58 and 70%, respectively) as well as specificity (58 and 73%, respectively). As the previous report indicates, the low ^CtFN concentration and the concomitant presence of FN fragments have been observed by us in synovial fluid of RA patients [9]. The FN fragments were also reported in the plasma of patients with active SLE [17]. Here, we have also shown that about 60% of analyzed SLE samples showed besides the 230 and 240-kDa FN bands corresponding to native FN, the presence of normally absent four FN fragments (having molecular weight from 158 to 209 kDa) and three to five high-molecular weight FN forms (from 260 to 310 kDa). They formed, respectively, about 16 and 24% of all FN forms reactive with anti-FN monoclonal antibody (immunopatterns B and C in Fig. 3 and Table 3). In contrast, in RA, the plasma FN was untouched (Fig. 3—lane 2), and its significantly higher level resulted probably from the increased total FN production by hepatic cells in a response to systemic inflammation. It could not be excluded that the plasma FN molecules in autoimmune patients can represent apart the FN derived from hepatic cells, the significant part of the FN of nonhepatic origins, that is, epithelial and/or endothelial cell productions. They could be synthesized by highly active immunological cells and/or epithelial and/or endothelial cells as native, intact molecules, and/or additionally they can appear as fragments derived from degraded extracellular matrix of blood vessel cells in SLE. The tissue erosive process happens exclusively in joints and around the joints of RA patients, where many extracellular components, among them synovial, but not plasma FN, undergo degradation [18, 19]. The findings are important because SLE and RA differentiation may be sometimes difficult, particularly at the beginning of SLE when patients have joint pain without any systemic indicators [20].

The FN fragments are thought to have many diverse activities [21–23] and among them an ability to form cross-linking complexes with FN, various extracellular matrix components and some blood plasma components such as fibrin and fibrinogen [24, 25]. A formation of multimeric cross-linking polymers lies on the base of repair processes at which FN takes a main role [26]. In 60% SLE patient plasma samples, we observed the presence of FN forms having molecular weight from 260 to 310 kDa (Fig. 3, Table 3). The high-molecular FN forms may partly result from the expression of alternatively spliced (ED-A, ED-B and IIICS) segments, either are highly glycosylated glycoforms, or may be formed by an FN linkage with some blood plasma, extracellular matrix components, or their fragments [27, 28], or even smaller molecules such as homocysteine [29].

In conclusion, the data presented here pointed out that the analysis of FN level based on its domain epitope expression and of FN molecular status revealed by immunoblotting can be helpful to differentiate SLE in respect to RA and healthy individuals. The observed FN molecular pattern can be related to the general molecular processes by which the plasma FN is used for tissue needs.