The Role of CXCL4 in Systemic Sclerosis: DAMP, Auto-Antigen and Biomarker

Abstract

Systemic sclerosis (SSc) is an autoimmune disease characterized by specific autoantibodies, vasculopathy and fibrosis of the skin and internal organs. In SSc, chronic activation of the immune system is largely sustained by endogenous inflammatory mediators that act as damage-associated molecular patterns (DAMPs), which activate Toll-like receptors (TLRs). Major autoantigens are nucleic acids or molecules that are able to bind nucleic acids. It is important to identify solid and predictive biomarkers of both disease activity and disease subtype. CXCL4 has been regarded as a new biomarker for early SSc in recent years, and here, we discuss its modulation over the course of a disease and after pharmacological interventions. Moreover, we provide evidence that CXCL4, in addition to being a biomarker of SSc subtypes and a prognostic marker of disease severity, has a dual pathogenic role in SSc: on the one hand, in complex with self-nucleic acids, CXCL4 acts as a DAMP for IFN-I and pro-inflammatory cytokines' release by innate immune cells (such as dendritic cells); on the other hand, CXCL4 is a target of both antibodies and T cells, functioning as an autoantigen. CXCL4 is certainly an interesting molecule in inflammation and autoimmunity, not only in SSc, and it may also be considered as a therapy target.

Keywords: CXCL4; autoimmunity; systemic sclerosis.

Figure 1

Pleiotropic functions of CXCL4. CXCL4 is a member of kinocidins, with direct capacity for bacterial killing. The image illustrates the most important functions of CXCL4, which are also relevant in autoimmunity and in SSc.

Figure 2

CXCL4 organizes dsDNA into immunogenic liquid–crystalline complexes suitable for TLR9-mediated IFN-α production. Inter-DNA spacing close to the steric size of TLR9 (d = 3–4 nm) allows the optimal binding of columnar DNA lattices to TLR9-clustered arrays. CXCL4 organizes DNA into liquid–crystalline columnar lattices at an inter-DNA spacing compatible with TLR9 amplification. DNA fragmentation increases the total number of discrete DNA fragments for the same mass of DNA, with optimal close packing of DNA ligands [38].

Figure 3

CXCL4–DNA and CXCL4–RNA complexes activate immune cells via TLRs. As with other well-known DAMPs like Tenascin-C, acting via TLR4 [49], fibronectin, S100A8 and S100A9 (alarmins), when CXCL4 binds to DNA/RNA, it acts as a DAMP and participates in the inflammation process.

Figure 4

Model for B-cell activation via CXCL4–DNA/RNA complexes. After stimulation through TLR7/8/9 by CXCL4–DNA/RNA complexes, pDCs become activated and start to produce IFN-I, which, together with BAFF, IL-15, IL-2, help plasma cell differentiation (pDCs can also produce BAFF, [58], whereas IL-15 and IL-2 can be produced by T cells but also by stroll cells, monocytes, etc. [59]). Afterward, B-cell transition and differentiation in antibody-secreting plasma cells occurs. In SSc, activated B cells start to produce antibodies, among which there could be anti-CXCL4 antibodies.

Figure 5

CXCL4 as an autoantigen in SSc. When complexed with nucleic acids, CXCL4 stimulates pDCs and mDCs via TLRs, which activates both cell types and induces the production of IFN-I, IL-12, IL-23 and TNF-α. IFN-I released by pDCs implements the antigen-presenting cell capacity of mDCs, whereas pro-inflammatory cytokines can activate CD4 T cells (including Th17 cells and perhaps CD8 T cells [64]). T cells specific to CXCL4 are likely to provide B-cell help for autoantibody production [41] or directly induce an inflammation that favor fibrosis. An algorithm that predicts the binding capacity of CXCL4-derived epitopes to HLA molecules showed that the sequence of CXCL4 possesses “binding motifs” for several HLA-DR alleles represented in Caucasians (DR4, DR1 and DR11), which may explain the recognition by CD4 T cells [41].