2.2. Anti-DNA antibodies

Anti-DNA antibodies are a subgroup of ANAs that can recognize and bind cellular DNA, and their detection is almost exclusive to SLE making them the serological hallmark for the disease [27, 51, 54]. Whether anti-DNA antibodies are also released in the context of other human diseases is a question that remains unanswered [27]. Detection of anti-DNA antibodies in the sera of SLE patients has been included as a separate criterion in an optimized version of SLE classification criteria that were set to enhance the sensitivity of the old criteria, as will be described below, which highlights their importance for SLE diagnosis [50] as they exhibit 95% specificity for SLE and are detected in at least 70% of SLE patients [45]. In addition, their value is not confined to their diagnostic role, but they are also quite valuable in probing the molecular basis of lupus autoreactivity [27] and in theranostic investigations where they can be used as deterministic factors for eligibility in clinical trials and in directing the clinical use of certain therapeutics [55, 56]. In normal immune responses, antibodies that target DNA can be seen in some cases such as that in response to viral or bacterial infections [54, 57]. However, anti-DNA antibodies in normal immunity differ from SLE anti-DNA antibodies in many aspects. In normal immunity, anti-DNA antibodies are of the IgM isotype that can recognize and bind single-stranded DNA (ssDNA) with low affinity and are nonpathogenic in nature. But in SLE pathogenesis, there is an isotype shift of the expressed anti-DNA antibodies from IgM to IgG antibodies which are detected in the majority of SLE patients. These class-switched IgG antibodies exhibit a high affinity toward double-stranded DNA that resulted from specific somatic mutations in the variable regions of these antibodies mostly in the complementarity-determining regions (CDRs) that generated positively charged amino acid residues such as asparagine, arginine, and lysine that promote enhanced binding affinity to the negatively charged DNA [27, 54].

The origin of such IgG anti-DNA antibodies was suggested to derive from antigen-specific Bcell clonal expansion where DNA was used as the selecting antigen [54]. However, the various studies that investigated the release of high-affinity anti-DNA antibodies in SLE settled on the prerequisite association of DNA with proteins to be able to elicit anti-DNA immune responses [54, 58]. This implied a role for T cells that are reactive against histones, the proteins constituting octamer complexes around which stretches of DNA are wrapped inside the nucleus, and nucleosomes which are the basic structural unit of chromatin [27, 54]. A mechanism was proposed to start with presenting DNA in complexation with a foreign protein antigen (e.g., viral or bacterial) to T cells specific to this antigen. However, in a way that is not yet fully characterized, a shift in recognition takes place toward the complexed DNA that triggers the activation of T cells specific to histones and nucleosomes [27, 54].

Various studies have demonstrated differences in selectivity patterns exhibited between anti-DNA antibodies of normal individuals and that of SLE patients. Normal anti-DNA antibodies were found to exhibit high specificity toward species-specific DNA which suggests that they bind at DNA regions that are not shared with human DNA. In contrast, SLE anti-DNA antibodies were found to nonselectively bind to a variety of investigated structurally different DNA antigens. This suggests that SLE anti-DNA antibodies might specifically bind the highly conserved phosphate backbone rather than specific nucleotide sequence [27, 54].

Therefore, as will be described later in the section of diagnostic technologies, assays that are able to detect high-affinity anti-DNA antibodies are described as assays with high specificity to SLE as it is inferred that such high-affinity anti-DNA antibodies are more reflective of SLE immune-deregulated responses [27].

Despite this high specificity of anti-DNA antibodies to SLE, their value in disease prognosis is compromised as they are poor predictors of disease activity [45]. This was demonstrated by the detection of tenacious levels of anti-DNA antibodies in SLE patients in remission [59–61] or the presence of normal levels in patients with active disease [62]. Therefore, new biomarkers are continuously emerging, and some of the promising biomarkers are discussed below. For more detailed discussions, interested readers can be directed to these reviews [43, 46].
