The use of leukocytes’ secretome to individually target biological therapy in autoimmune arthritis: a case report

The immunological disease continuum, that includes all autoinflammatory and autoimmune diseases, as proposed by McGonagle & McDermott [10], allows to better understand the complexity of immune factors that can be associated with the numerous immunological disorders. In this regard, the present case report can possibly be related to polygenic autoinflammatory diseases and mixed pattern diseases, both conceptual classification in which psoriatic arthritis can be found. The investigation of the patient’s secretome, as we report here, remains non-exhaustive, with a major focus on the biologics currently available to treat autoimmune arthropathies such as RA and PsoA (Table 2). This focus could easily be extended to other factors depending on new biologics, as well as other stimuli to activate relevant immune signalling as the field progresses further. Nonetheless, the targeted secretome allows deciphering whether a factor could be largely produced over others to be able to adapt more precisely the treatment. This case report exemplifies the personalized treatment option based on the patient’s medical history and the determination of the secretome of the patient’s blood leukocytes in response to immune activators that revealed a unique overproduction of the IL-6 cytokine. In fact, the overproduction of IL-6 in the present case report of a refractory PsoA was in line with reports of mediators from synovial inflammation of PsoA where anti-IL-6 biologics were effective treatment choices [11‐15].

Another consideration is at the level of a unique overproduction of IL-6 in the present case of refractory PsoA, which could indicate a predominant autoinflammatory pattern over the autoimmune part of PsoA. In this regard, genomic DNA samples were isolated from the patient’s peripheral blood, and whole exome sequencing and data analysis were performed on the proband as previously described [16]. Publicly-available databases (ExAC, 1000 genomes, dbSNP, NHLBI GO Exome Sequencing Project and ClinSeq), as well as an in-house exome database, were used to filter for rare candidate variants. We did not identify any pathogenic mutations in known disease-causing genes. We analyzed the sequencing data for novel causes of immune dysregulation accounting for different modes of inheritance. Ideally, whole exome sequencing should be performed in trios to help with filtering a long list of candidate variants that are typically found when doing exome sequencing in singleton cases. Nonetheless, we did not identify any strong candidate variants to explain the phenotype. We also performed Sanger sequencing to analyze specific non-coding polymorphisms in the IL6 promoter and 5′UTR that could potentially account for some of the patient’s phenotype since the clinical spectrum of PsoA, more precisely the active peripheral arthritis, has been linked to IL6 (-174G/C) polymorphism [17]. Coding regions as well as flanking 5′ untranslated regions of the IL6 gene (RefSeq: NM_000600.5) were amplified by AmpliTaq Gold Fast PCR Master Mix (Thermo Fisher Scientific Inc., Waltham, Massachusetts, USA) and sequenced on SeqStudio Genetic Analyzer (Applied Biosystems Inc., Foster City, California, USA). We found that the patient had reference alleles at -597G, -572G, -473A8/T12 and -174G [18]. We finally analyzed the data for common low-penetrance coding variants associated with psoriatic or rheumatoid arthritis. We found that the patient is a heterozygous carrier for the single nucleotide polymorphism (SNP) in TRAF3IP2 rs33980500 (NM_001164281.2:c.28G>A; p.Asp10Asn; NM_147200.2:c.55G>A, NP_671733.2;p.Asp19Asn), which was identified by two genome-wide association studies (GWAS). The susceptibility allele rs33980500 has been shown to cause altered TRAF6 binding and thereby affect multiple immune pathways [19, 20]. Because TRAF3IP2 binding motifs have been associated with multiple TRAF proteins, the inhibition of the TRAF6 pathway could upregulate the TRAF2/5 pathway, leading to an enhanced immune response. Interestingly, studies showed that, in the TRAF6-independent pathway, TRAF2 and TRAF5 transduce the IL-17 signals to stabilize mRNA transcripts of chemokines and cytokines, such as CXCL1/GROα and IL-6 [21, 22]. Although these results did not identify a high penetrance genetic cause, they did identify a risk allele in TRAF3IP2, which in combination with additional environmental and genetic factors may help to explain this patient’s phenotype and suggest a further link between the PsoA, IL-6 production and TRAF3IP2/TRAFs signalling (Fig. 3).

While we examined the secretome of blood cells, one limitation of this study is that many other cell types in autoimmune arthritis are known to secrete cytokines, such as synoviocytes, fibroblasts, chondrocytes, osteoblasts/osteoclasts and epithelial cells (the latter ones in psoriatic arthritis for example), and that these cells, through their cytokine secretion, could contribute to the local pathology [23‐26]. The cytokine pattern could evidently be different from the ones produced by the blood cells. However, the use of these cells often requires invasive techniques to obtain them and their purification, for subsequent activation, is more labour-intensive than blood cells [27]. It is also useful to stress that the presence of a SNP in TRAF3IP2 rs33980500, as shown in this case report, should be present in all cell types using this signalling pathway [28, 29].