The goals of this study were to examine circulating levels of inflammatory and degenerative
mediators associated with IVD disease and to evaluate changes in serum levels based
on clinical factors, including diagnosis, age, gender, duration of symptoms, and two
features associated with poor outcomes of treatment of LBP- BMI and tobacco use. Our
findings indicate that the serum level of IL-6 is significantly higher in participants
with LBP. Participants with LBP due to SS or DDD (Other Dx) had significantly higher
levels of IL-6 than participants with DH and controls. Positive correlations were
found between IL-6 levels and BMI, symptom duration, and age. We also observed that
MMP-1 levels were lower in subjects with LBP, specifically subjects with DH, relative
to controls. MMP-1 levels were found to be positively correlated with age. We did
not observe significant differences in IL-6 or MMP-1 serum levels with respect to
disease severity as classified by Pfirrmann grade. MRI findings have been shown to
have poor correlation with symptomatic degeneration and do not correlate to functional
impairment or pain 56], 57].
To evaluate the significance of cytokine modulation in pathological conditions, it
is necessary to establish the physiological range of these molecules in healthy subjects.
Serum cytokine levels vary by age, gender, sample type, and type of assay used, among
many other variables. The levels of IL-6 measured in the present study are within
the reported range for normal human sera, suggesting that LBP subjects have low-grade
systemic inflammation. Elevation of circulating IL-6 levels in participants with DDD
(part of Other Dx) compared with DH are consistent with observed differences in disc
tissue cytokine levels based on diagnosis 58]. Differential protein levels of IL-6 in tissues from participants with DH or DDD
have been observed in vitro, with higher levels in samples from subjects with confirmed
discogenic pain caused by DDD than in those with DH or sciatica 58]. While we did not segregate patients based on the nature of pain (i.e., discogenic,
neuropathic, or mixed), the findings based on diagnosis suggest that patients with
discogenic back pain due to DDD or SS may have higher circulating levels of IL-6 than
patients with radicular or neuropathic pain associated with herniated discs. Nevertheless,
the direct relationship between IVD tissue and blood levels of IL-6 is unknown and
future studies investigating the relationship between local (i.e. disc) and circulating
cytokine levels are warranted to more conclusively identify the source of elevated
systemic cytokine levels. An elevated circulating level of IL-6 represents the complex
role of this cytokine in disc diseases and pain. IL-6 can be spontaneously produced
in vitro by human herniated and degenerated (i.e., nonherniated) discs 36], 44], and genetic variations in IL-6 are associated with increased risk for IVD disease
and back pain 59]–61]. IL-6 stimulation of human NP cells downregulates ECM gene expression and PG synthesis
62]. In preclinical models, IL-6 increases production of TNF-? by neurons in dorsal root
ganglia and sensitizes dorsal root ganglia to painful stimuli 63], 64]. IL-6 expression levels increase in response to IVD injury in preclinical models
65], 66], and inhibition of IL-6 signaling may be a potential target for analgesic treatment
of DDD 65].
In clinical studies, higher serum levels of IL-6 in participants with DH with reported
higher pain scores were found relative to participants with lower pain scores 48]. The findings of the present clinical study are the first to be based on examination
of circulating cytokine levels in DDD and SS and provide evidence for a more extensive
role of IL-6 in disc disease, where patients with DDD or SS have even higher serum
cytokine levels than those with DH or control participants. While subjects with a
current or past history of OA were not excluded from enrollment in this study, our
findings indicate that OA history does not have an appreciable effect on the findings
of significantly elevated levels of IL-6 in LBP subjects or on the relative differences
observed based on diagnosis of LBP. It is unknown if circulating cytokines are causative
of degenerative changes and pain or whether elevated cytokine levels are a consequence
of the degeneration and painful condition. Elevated serum IL-6 levels at presentation
have also been associated with worse outcomes in recovery from DH at 1-year follow-up
49]. While the effects of elevated serum cytokines in SS or DDD on clinical outcomes
are unknown, we speculate that elevated IL-6 levels may be similarly associated with
poorer outcomes. For example, low-grade systemic inflammation may promote positive
feedback in proinflammatory signaling and degradative enzymes, leading to matrix impairment
and bone remodeling, potentially creating an environment that exacerbates degenerative
changes.
There are multiple sources of IL-6 secretion and activation in disc-related diseases.
In DH, an inflammatory immune response is expected following the protrusion of an
immune-privileged NP tissue. In a pilot study consistent with this theory, Kraychete
et al. found elevated levels of serum IL-6 in participants with DH compared with controls
67]. In the present study, IL-6 levels in the DH group were on average higher than control
levels, though this finding did not achieve statistical significance in a comparison
of three groups (DH, Other Dx, and control). The lack of a significant difference
in IL-6 levels between the control and DH cohorts may be due to the contributions
of covariates, such as age, gender, and OA history, which were controlled for in the
present study but not in the study by Kraychete et al. 67]. Cells from degenerated (i.e., nonherniated) discs secrete increased levels of proinflammatory
cytokines 44]. Stress concentrations and changes in nondisc tissues of the lumbar spine contribute
to the degenerative etiology. In SS, hypertrophy and calcification of spinal ligaments
may compromise the adjacent spinal canal and neural foramina. Vertebral bones enlarge
due to increased bone stress and cause bone spurs, which can encroach on the spinal
canal. Hypertrophy of spinal ligaments, overgrowth of bone, and formation of bone
spurs may all contribute to the increased systemic levels of IL-6 in SS and DDD (Other
Dx). While weakness in outer IVD is a prerequisite for DH to occur, it is not typically
associated with bone spur formation or ligament hypertrophy. Interestingly, some studies
have reported elevated circulating levels of TNF-? in subjects with LBP relative to
control subjects 67], 68]. Our findings suggest that while raw TNF-? levels may be elevated, modeling contributions
of age, gender, and OA history attenuates potential differences in TNF-? levels between
LBP and control subjects.
We observed lower levels of MMP-1 in subjects with LBP, specifically due to DH, than
in control subjects. In disc tissue, expression of MMP-1 is enhanced in degenerated
tissue compared with healthy human disc tissue, and MMP-1 expression by disc cells
increases with increasing disease severity 17]–20], 69]. The lower serum levels of MMP-1 observed in this study may be indicative of misregulation
of MMP-1 levels or activity, confounded by activity of the natural inhibitors of MMP-1
(e.g., tissue inhibitor of metalloproteinases [TIMPs]) or the numerous substrates
on which MMP-1 acts 70], 71]. Misregulated levels of many MMPs are observed in many diseases that are characterized
by or associated with inflammation. For example, serum MMP-1 levels have been found
to be elevated in individuals with disease involving tissue turnover and remodeling,
such as OA and RA, compared with healthy controls 72]. Serum MMP-3 levels did not differ between OA patients and normal sera, however,
but were elevated in sera from patients with RA 72]. While decreased MMP-1 levels may imply reduced degradation compared with subjects
with higher levels, net collagenolytic activity is determined more by the balance
between TIMPs and MMPs. Some studies suggest that systemic levels of MMPs vary in
individuals on the basis of MMP polymorphisms, where polymorphisms in the MMP-1 gene
confer a functional effect on circulating levels of MMP-1 in RA and other diseases
73]–76]. Polymorphisms of MMP-1 have been associated with an increased risk of developing
lumbar disc disease and may contribute to LBP, sciatica, and disability after lumbar
DH 77]–79]. Further studies investigating the relationship between local (i.e., disc) and systemic
MMP and cytokine levels are warranted to more conclusively identify the function of
decreased MMP-1 levels and source of elevated IL-6 levels.
Biomarkers are objective, measurable tools that can predict the incidence or staging
of disease, and they have been validated in the context of several diseases. Pain
is an intensely subjective experience, and depression is a frequent comorbidity of
persistent LBP that is more likely to affect subjective pain measures than serum biochemical
levels 80]. Participants in this study were recruited from multiple clinical sites and were
evaluated for LBP by their treating physicians on the basis of standard clinical practices.
Though validated pain questionnaires were not used in subject recruitment, we observed
a significant correlation between IL-6 levels and symptom duration, suggesting that
IL-6 levels may be associated with pain measurements (e.g., intensity) in study participants.
In many investigations of painful conditions, such as fibromyalgia and OA, investigators
are trying to identify systemic factors that can be used to aid diagnosis, stage severity,
and indicate prognosis. In chronic pain, circulating IL-6 levels have been correlated
with pain levels 81]. In individuals with isolated back pain without radiculopathy, Sowa et al. demonstrated
associations between serum levels of neuropeptide Y or chondroitin sulfate 846 with
pain and pain-related function 82]. The relationship between inflammatory mediators and pain is not exclusive to LBP,
as cytokines and chemokines play a critical role in neuropathic pain 83], 84]. Not surprisingly, patients with other diseases associated with neuropathic pain
have systemic elevations of cytokines as well. Patients with knee OA were found to
have significantly higher serum levels of IL-6 and IL-10 than controls, though levels
of TNF-? and IL-8 were not found to be different in knee OA vs. control subjects 85]. Patients with RA also have elevated systemic IL-6 compared with other patients with
joint pain 86]. Elevations in serum levels of IL-6 have also been described in association with
aging. Numerous studies of healthy older adults show that levels of several cytokines,
especially IL-6 and TNF-?, increase with age in the absence of disease or acute infection.
Though correlated with age, the etiology of elevated inflammatory markers remains
incompletely defined; however, it is attributed to factors such as visceral adiposity,
lower sex steroid hormones, smoking, depression, and periodontal diseases associated
with aging 87]. IL-6 levels also increase with BMI and obesity in the absence of other disease 88]. The correlations we observed with increasing BMI underscore the role of obesity
as a risk factor for LBP 89] and may implicate systemic inflammation in chronic painful conditions. Increased
BMI is associated with disc space narrowing and is a positive risk factor for the
development of disc degeneration 90], 91]. This may be due to moderate changes in spinal loads with increasing body weight
92], acting in concert with in an inflammatory milieu caused by higher amounts of adipose
tissue.
While many physiological or pathological process may alter circulating biochemical
levels (e.g., increases in IL-6), biochemical factors with established roles in disc
diseases and pain may prove useful, possibly in combination with emerging imaging
technology such as T
1?
MRI or with joint metabolism biomarkers measured in urine or serum 93], to define a more specific profile and mechanism of disease. T
1?
MRI provides a measure of functional disc integrity 94]–98], shows promise for more sensitive and early identification of disc pathology, and
may have stronger correlations with clinical status than standard MRI. Combining cytokine
profiles with joint metabolism biomarkers or advanced spinal imaging technology may
be useful in developing biomarker profiles of participants with disc diseases of varying
etiologies and severities. Discography for the diagnosis of the pain generator level
in LBP is now generally in decline due to lack of diagnostic accuracy and sensitivity
and because of its invasive nature. Serum cytokine profiling may represent a less
invasive approach to evaluation of subjects in the absence of specific disc pathology
on clinical imaging. Measures of serum biochemical factors in patients with disc diseases
may assist in refining diagnoses and possibly by defining more uniform cohorts of
patients with reference to circulating inflammatory levels.
ESI is commonly used for treatment of LBP, where nonspecific immunosuppression may
affect inflammatory etiologies of pain. There has been increased use of epidurally
injected medications with increased specificity, including U.S. Food and Drug Administration–approved
agents that target TNF-? (e.g., infliximab, adalimumab, and etanercept) or IL-6 (tocilizumab).
The efficacy of these drugs in treating LBP is still being evaluated, but studies
have shown symptomatic improvement for patients with radiculopathy 99], perhaps due to mitigating the effects of inflammation on the dorsal root ganglia
100]. Ohtori et al. reported reduction of radicular leg pain, numbness, and LBP with tocilizumab
in patients with SS 101]. However, there are few other studies in which researchers have presented clinical
data on LBP due to other diagnoses or in the absence of radiculopathy. The possibility
of using serum cytokine measures of IL-6 to predict response to treatment with progressively
targeted therapies fits the emerging concept of personalized molecular medicine, where
treatments are tailored to an individual’s specific biology through increasingly sensitive
detection of molecular abnormalities, and the use of these inflammatory mediators
as biomarkers for LBP.
Study limitations
While statistical significance was observed for IL-2 and IL-4 levels between the control
and LBP cohorts, these findings are limited by the fact that only 59Â % and 28Â % of
all samples, respectively, had detectable levels. Similarly, IL-13 levels significantly
differed in male subjects by diagnoses; however, IL-13 levels were detectable in only
41Â % of samples. The relatively low percentages of detected levels of IL-2, IL-4,
and IL-13 are consistent with data provided by the assay manufacturer for measurements
in normal human serum. Therefore, caution is warranted in interpreting the findings
of significant differences in these factors.
This study included inhomogeneous distributions of participants across treatment or
diagnosis cohorts. The number of participants undergoing surgery was greater than
those undergoing ESI. Similarly, the diagnoses are not equally represented, with significantly
more participants having DH compared with Other Dx. Participants undergoing surgery
for DDD are particularly underrepresented (?=?2), in part because the indications for surgery with discogenic back pain remain
controversial and participants with a history of discogram were excluded. Additionally,
comorbidities may potentially affect inflammatory mediators observed in the systemic
circulation of control participants.