Biomarkers hold promise to expedite diagnosis and stratify the risk of adverse multiple sclerosis prognosis, which would aid personalisation of therapy. Candidate multiple sclerosis biomarkers have been proposed but are often studied in isolation, and comparing studies that have used different clinical outcomes is challenging.12 Given the complexity of multiple sclerosis biology, a combination of markers may improve predictions versus any marker in isolation. Here we have shown that combinations of up to six biomarkers measured in CSF or serum provide incremental improvements in the prediction of diagnosis and prognosis in multiple sclerosis (Fig. 2). Importantly, we found that predictions using combinations of blood-based biomarkers rivalled those using CSF markers, offering promise for less invasive approaches to monitoring and personalising multiple sclerosis care.
Neurofilament light is a well-established candidate biomarker in multiple sclerosis and in our study was among the optimum single biomarkers to distinguish multiple sclerosis from control cases in both serum and CSF. Meta-analysis and systematic review have both highlighted evidence supporting neurofilament light as a marker to predict future multiple sclerosis disease activity, brain atrophy and to monitor treatment response to disease modifying therapies.17,55 Our model confirms the predictive power of neurofilament light in CSF to distinguish multiple sclerosis from control cases (AUC = 0.85), but the addition of four more CSF markers; C9, chitinase-3-like-1, TNFR1, sCD27, increased the AUC to 0.94. (Fig. 2, Table 3). The limitations of ELISA in detecting low concentrations of neurofilament light in blood have been overcome using highly sensitive SiMoA technology, and some clinics are starting to incorporate neurofilament light into practice.8 In our study, while serum neurofilament light was one of the top biomarkers to distinguish multiple sclerosis from control cases, it did not feature in the final serum model of diagnosis. Some studies have demonstrated utility in combining neurofilament light with GFAP,18 which can also be measured using SiMoA. We used an ELISA assay for GFAP and were unable to adequately detect GFAP in CSF; serum GFAP detected using ELISA did not feature in any of our predictive models.
Chitinase-3-like-1 featured in CSF predictive models of multiple sclerosis versus control status in our cohort, and also as a serum marker in the combined CSF/serum model predicting time to EDSS 6. Chitinase-3-like-1 is a marker of macrophage and astrocyte activation and is potentially neurotoxic.19 It has been found to be elevated in other studies of multiple sclerosis CSF,20–22 and has shown promise in predicting conversion from CIS to multiple sclerosis and future multiple sclerosis disease activity.22–23 Other markers that predicted multiple sclerosis versus control status in our study were chemokines/cytokines known to promote a pro-inflammatory milieu such as osteopontin, MCP1, CCL27, TNFR1 and soluble CD27. Osteopontin is a cytokine produced by a range of cells including macrophages, lymphocytes and dendritic cells, providing cross talk between the innate and adaptive immune system. Osteopontin regulates the differentiation of pro-inflammatory lymphocytes, inhibits apoptosis of inflammatory cells,24 and has a role in microglia-mediated synaptic engulfment.25 CCL27 and MCP1 (CCL2) are both chemokines involved in regulating migration of macrophages/monocytes.26 MCP1 is abundantly expressed by microglia located at the active rim of multiple sclerosis lesions,27 and is thought to play a role in multiple sclerosis pathogenesis,28 although we and others found it to be significantly lower in serum and CSF of people with multiple sclerosis than controls.29 CCL27 induces the homing of memory T-cells to sites of inflammation and has been found by others to be elevated in the serum of people with multiple sclerosis.30 Tumour necrosis factor alpha (TNFα) is recognised as a key function in autoimmune disease, where excessive activation of TNFα mediates cytotoxic and pro-inflammatory responses via TNFR1.31 CD27 is a T-cell activation marker, whose soluble form has been shown by others to be significantly elevated in people with multiple sclerosis,32 and predictive of future multiple sclerosis disease activity,33 including transition from CIS to multiple sclerosis.34
CSF biomarkers that combined to predict time to next relapse also included neurofilament light and MCP1. Vitamin D binding protein, a regulator of the distribution, stability and bioavailability of vitamin D, was one of the markers whose expression differed significantly between multiple sclerosis and control cases in serum and also featured in the CSF and serum models predicting relapse. There is evidence for an immunomodulatory role of vitamin D in multiple sclerosis.35 Other studies have demonstrated some utility of vitamin D binding protein in distinguishing people with multiple sclerosis versus controls,36,37 or risk of developing multiple sclerosis.38 Vitamin D binding protein has been shown to be expressed on spinal cord neurons, pia mater and grey matter within the brains of people with multiple sclerosis, and in an animal model of multiple sclerosis, high vitamin D binding protein appeared to mitigate beneficial effects of vitamin D3 supplementation and inhibit recovery.37 CXCL12, a chemoattractant protein for T-cells as well as monocytes appeared in the CSF and serum models predicting relapse, and the CSF model predicting time to EDSS 6. CXCL12 has been found highly expressed in active multiple sclerosis lesions and appears to play a role in enhancing the inflammatory response in multiple sclerosis.39
Our models also demonstrated evidence of complement activation and consumption. In our cohort, significantly differences in the concentration of several complement proteins between the CSF and serum of multiple sclerosis versus controls suggested dysregulation of this pathway in multiple sclerosis. The complement molecules Factor B (component) and iC3b (activation product) featured in our final model to predict multiple sclerosis status. C1inh/C1s complex also featured prominently in the prediction of relapses and disability in our cohort. This is an indication of activation of the classical pathway of the complement cascade,40 and in line with our earlier work suggesting this is relevant to multiple sclerosis biology.16 C3 and Factor H both featured in our serum model predicting relapses. We have previously shown both to be present at high levels in multiple sclerosis lesions,41 while others have shown these complement proteins to be elevated in the blood and CSF of people with multiple sclerosis versus controls, and the be relevant in predictions of disability outcomes.42,43 Overall, our data supports previous findings that imply ongoing local and systemic complement dysregulation in multiple sclerosis.16,44
Others have explored the potential utility of combining more than one protein biomarker to predict multiple sclerosis diagnosis or prognosis. Lucchini et al. measured a panel of 8 protein candidates in the CSF of people with multiple sclerosis or controls. They found the combination of chitinase-3-like-1, CXCL10, CXCL12, CXCL13 increased the AUC for predicting conversion to clinically definite multiple sclerosis after first attack above any single biomarker in isolation.22 Bielekova et al.45 also found that the combination of CSF IL-12/IL-23p40, CXCL13 and IL-8 in CSF was more predictive of multiple sclerosis versus control status than any of these markers in isolation.
There are some practical considerations around combining biomarkers in clinical practice. Some biomarkers were not statistically significant in univariate analyses but still provided a small differential addition to the final model. Conversely, some of the biomarkers that were most discriminatory in univariate analysis autocorrelated to some extent and therefore did not all appear in final combination models. There may be a balance between selecting the model that was statistically optimal versus a choosing smaller combination of markers that gives similar predictive value but is simpler to measure in multiplex (Supplementary Tables 5 and 6). While optimum models sometimes combined CSF and serum markers, there are practical advantages in using a single sample type, ideally serum, which can be serially sampled more easily than CSF.
The present study is subject to some limitations or caveats. Using research operating procedures, we aimed for all blood and CSF samples to reach the freezer within 2 hours. Variations in sample handling in routine clinical practice may introduce the risk of degradation of small molecules such as complement.46 The demographics of our multiple sclerosis and control group differed, which could affect the results we are observing, even though we adjusted for sex and age in all analyses. Body mass index (BMI) data were unavailable for this cohort, so were not included, despite emerging evidence that correction of some biomarkers for BMI improves correlations.47 Our cohort was too small to explore biomarker signatures of different multiple sclerosis subtypes. We included all people with multiple sclerosis in the analysis of time to relapse, even though people with progressive disease are somewhat less likely to experience relapses. While over fitting of statistical models is a potential limitation, our test and train AUCs were very similar which suggests this was not the case in our study. However, our results require validation in an independent cohort.
In conclusion, we have used well-optimised assays for 24 candidate protein biomarkers in the blood and 20 CSF to demonstrate models that are highly predictive of multiple sclerosis diagnosis and prognosis. We demonstrated for the first time that combination serum models rivalled those of CSF, holding promise for a non-invasive approach. Although further validation of our findings is needed in different cohorts, this study suggests that combining several biomarkers in a single test will aid diagnostic and prognostic accuracy in the future.