Here, we provide evidence that monocytes, cells of innate immune system, may contribute to the development of arthritis in peripheral SpA and therefore might be at play when considering chronic inflammation, one of the most significant features of this disease.
There is a substantial evidence concerning the role of the innate immune system in the pathogenesis of a variety of SpA features, including chronic inflammation, repair and new bone formation [11, 12]. It seems that peripheral blood monocytes in SpA are functionally primed and/or functionally reprogrammed by not yet well identified factor(s) and exhibit molecular or cellular features characteristic for SpA [13–15]. Monocytes are classified into three subpopulations – classical (CD14++CD16−), intermediate (CD14++CD16+) and nonclassical (CD14+CD16++); the latter two subpopulations being referred to as “proinflammatory” . The “proinflammatory” subsets form app. 5–15% of circulating monocytes, express and secrete upon stimulation by various factors large amounts of pro-inflammatory cytokines, e.g. TNFα, interleukins (IL) – IL-12 and IL-1, but insignificant amount of anti-inflammatory IL-10. Contrariwise, classical monocytes produce relatively small amounts of TNFα, but they are a robust source of IL-10 . Under physiological conditions most of classical monocytes (ca. 80–90%) leave the bloodstream after a circulating lifespan of 1 day, whereas remaining fraction further mature into intermediate cells and finally convert (in app. 12 days) to nonclassical monocytes before leaving the circulation . Then extravasated monocytes supplement the population of resting tissue macrophages, but they may differ functionally in respect to their stage of maturation, pre-priming, and the type of target tissue mileu they migrate to. Therefore, these cells may influence inflammatory processes by factors released inside blood vessels or locally, upon their migration.
Currently, there are only a few studies (based on the analysis of single blood samples) that investigated differences in transcriptome/proteome profile of monocyte subpopulations isolated from healthy individuals [e.g. 19–26]. These studies indicate, on molecular basis, the significant differences in genetic profiles between these three monocyte subpopulations, confirming and extending most of the former phenotypic and functional observations. However, there is no data so far comparing gene expression in monocyte subsets in SpA patients exhibiting axial or peripheral signs.
Vascular endothelial growth factor-A (VEGFA) is one of the most important growth factors engaged in vascular development and angiogenesis. Since bone is a highly vascularized organ and angiogenesis plays an important role in osteogenesis, it has been established that VEGFA also influences skeletal development and postnatal bone repair . Process of bone remodeling is based on a balance between bone formation and resorption . Disturbance of this balance may strongly depend on both osteoclastic and osteoblastic activity. Liu et al  suggested, that VEGFA and TNFα might directly participate in the differentiation of fibroblasts into osteoblasts, and anti-VEGFA has been suggested as a possible new therapy preventing osteogenesis in SpA patients .
We have shown that VEGFA-mRNA expression in classical monocytes positively correlated with the number of swollen and painful joints. Our results also confirmed higher VEGFA level in SpA patients’ sera when compared to healthy blood donors (data not shown) supporting the previous findings by Lin et al . This may identify the classical monocytes subpopulation as the main source of VEGFA, which, upon migration to the synovium of peripheral joints, may promote local inflammation in SpA patients. It was already shown that VEGFA serum and synovial fluid levels are elevated in patients with ankylosing spondylitis expressing features of peripheral arthritis [31, 32]. Moreover, VEGFA may be secreted by various cell types, including macrophages, which are present in the synovial membrane and entheses in patients with SpA, but no association with particular monocyte subpopulation (as a source of particular tissue macrophages) has been specified .
MSX2 is a transcription factor with a homeobox domain, presumably involved in bone development and ectopic calcifications, although its role in these processes is still controversial. Furuichi et al. examined a total of 45 single nucleotide polymorphisms (SNPs) in 15 genes by sequential screening and reported promising evidence for the association between MSX2 polymorphisms and SpA in Japanese population . Moreover, in the basic studies involving animal models, the MSX2 knockout mice display remarkable decrease in mineralization of the axial skeleton, reduced proliferation of osteoprogenitors defecting skull ossification and abnormal calvarial development , whereas transgenic mice overexpressing MSX2 show enhanced proliferation of calvarial cells [35, 36]. A loss-of-function mutation of MSX2 in humans, which reduces DNA binding activity, causes defect in skull ossification . These observations contrast with a gain-of-function mutation of MSX2, which results in an autosomal dominant disorder, Boston-type craniosynostosis [38, 39]. These findings demonstrate that MSX2 expression is critical for human skull development and suggest its positive ossific role in bone development. However, MSX2 protein suppresses the expression of bone marker genes, including RUNX2 (a master regulator of osteoblast differentiation) and osteocalcin, and negatively regulates bone development and ectopic calcification [40–42]. The roles of MSX2 may vary depending on cell type and/or cell differentiation stage.
We have demonstrated that MSX2-mRNA overexpression in non-classical monocytes is positively associated with the number of swollen and painful joints, and therefore question the canonical role of this protein expressed preferentially in non-classical subset of monocytes. It was shown that TNFα may induce MSX2 expression and MSX2 mediates the inhibitory action of TNFα in osteoblast differentiation [43, 44]. It may not be excluded that MSX2 expression in non-classical monocytes is secondary to their proinflammatory functions related to TNFα auto- or paracrine action. Moreover, upon migration into soft tissue, non-classical monocytes may contribute to local inhibition of the BMP2-regulated osteoblast differentiation within inflamed peripheral joints  and therefore being possibly involved in new bone formation and joint remodeling. These concepts must be verified applying the appropriate mouse model (e.g. SKG mice) of SpA associated peripheral arthritis, which is currently underway.
Our study has some limitations. Obviously, the number of patients is small, but it is our belief that this pilot observation is interesting although requires further investigation. Also, this is a cross-sectional study and we do not know whether the discovered findings are durable enough to be attributed to chronic pain and synovitis characteristic for peripheral SpA. Finally, we were exploring the peripheral blood monocytes only, having no matching data considering the local environment of synovial tissue.