REVIEW Open Access
The in lamma o y esponse in he
eg ession o lumba disc he nia ion
Ca la Cunha
1,2*
, Ana J. Sil a
1,2
, Paulo Pe ei a
3,4,5
, Rui Vaz
1,3,4,5
, Raquel M. Gonçal es
1,2,6
and Má io A. Ba bosa
1,2,6
Abs ac
Lumba disc he nia ion (LDH) is highly associa ed wi h in lamma ion in he con ex o low back pain. Cu en ly,
in lamma ion is associa ed wi h ad e se symp oms ela ed o he s imula ion o ne e ibe s ha may lead o pain.
Howe e , in lamma ion has also been indica ed as he main ac o esponsible o LDH eg ession. This appa en
con o e sy places in lamma ion as a good p ognos ic indica o o spon aneous eg ession o LDH. This e iew
add esses he molecula and cellula mechanisms in ol ed in LDH eg ession, including ma ix emodeling and
neo ascula iza ion, in he scope o he clinical decision on conse a i e e sus su gical in e en ion. Based on he
e idence, a special ocus on he in lamma o y esponse in he LDH con ex is gi en, pa icula ly in he monocy e/
mac ophage ole. The phenomenon o spon aneous eg ession o LDH, ex ensi ely epo ed in he li e a u e, is
he e o e analyzed he e unde he pe spec i e o he modula o y ole o in lamma ion.
Keywo ds: Low back pain, Spine, In e e eb al disc, Immunomodula ion, Mac ophages
Low back pain and lumba disc he nia ion
Lumba disc he nia ion (LDH) is a majo con ibu o o
low back pain and a ec s a ound 9% o all people wo ld-
wide, wi h a high associa ed economic bu den and a
endency o inc ease as he popula ion ages [1]. LDH has
been associa ed wi h dis up ion o he annulus ib osus
(AF), ex usion o he nucleus pulposus (NP), and s imula-
ion o ne e ibe s, leading o pain. Howe e , mo e e-
cen ly, Rajaseka an e al. [2] sugges ed ha disc he nia ion
is mo e commonly he esul o endpla e junc ion ailu e
han AF up u e. He nia ed discs a e ound in 30–40% o
asymp oma ic people by imaging diagnos ic ools [3].
The cu en ea men s o LDH, as well as o dege-
ne a i e disc disease in gene al, can be di ided in o con-
se a i e e sus su gical app oaches and he decision on
which app oach o use is a iable and pa ien –clinician
dependen . Symp oms o igina ed by LDH may disappea
wi hou any su gical ea men and in some o hese pa-
ien s his is accompanied by a educ ion o he size o
disc he nia ion in imaging s udies. This phenomenon is
known as spon aneous he nia eg ession, which may be
pa ial o comple e. Clea ly, his e idence is indica i e o
he pa amoun need o iden i y he mechanisms behind
LDH eg ession and o de elop p edic i e me hods o
de ec ion o his phenomenon in clinical p ac ice.
Clinical e idence o spon aneous LDH eg ession
Since he i s epo o spon aneous LDH eg ession [4]
and o compu ed omog aphy (CT)-con i med spon aneous
LDH eg ession [5], documen a ion o his phenomenon
has become b oadly a ailable in he li e a u e. Imaging
diagnos ic ools such as magne ic esonance imaging (MRI)
ha e a cen al ole in con i ming LDH eg ession and,
al hough con o e sial o a signi ican numbe o yea s, i
is now widely ecognized ha la ge-sized and seques a ed
LDH end o eg ess mo e han o he LDH sub ypes (Fig. 1).
This eg ession may be pa ial o comple e. The le el mos
commonly a ec ed by his phenomenon is L4–L5, which is
also whe e LDH occu s mo e equen ly [6]. The main
hypo hesis behind he ini ia ion o spon aneous LDH e-
g ession has been desc ibed as he exposu e o he nia ed
disc ma e ial o he epidu al ascula supply h ough he
up u ed pos e io longi udinal ligamen (PLL) (Fig. 1).
* Co espondence: [email p o ec ed]
Ca la Cunha and Ana J. Sil a con ibu ed equally o his wo k.
Raquel M. Gonçal es and Má io A. Ba bosa con ibu ed equally o his wo k.
1
i3S—Ins i u o de In es igação e Ino ação em Saúde, Uni e sidade do Po o,
Rua Al edo Allen 208, 4200-135 Po o, Po ugal
2
INEB—Ins i u o de Engenha ia Biomédica, Uni e sidade do Po o, Rua do
Campo Aleg e 823, 4150-180 Po o, Po ugal
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Cunha e al. A h i is Resea ch & The apy (2018) 20:251
h ps://doi.o g/10.1186/s13075-018-1743-4
MRI analyses ha allow p ecise quan i ica ions o he
dec ease in he nia ion olume ha e shown ha spon an-
eous LDH eg ession is mo e ela ed o he p esence o
ansligamen ous ex ension and no so much o he ini-
ial size o he nia ion. Ahn e al. analyzed longi udinally
in ime 36 symp oma ic he nia ed lumba discs and
showed ha 25 o hem dec eased in size and ha he
mos equen he nia ion ypes ha educed in size we e
seques e ed he nia ions (a e age dec eases o 17%, 48%,
and 82% o he subligamen ous, ansligamen ous, and
seques e ed he nia ion g oups, espec i ely). These e-
sul s sugges ha he PLL up u e is mo e impo an
han he ini ial size o he he nia [7]. In Takada e al.’s
s udy [8], all cases o seques a ed discs we e comple ely
esol ed a e 9 mon hs, whe eas ex uded discs we e
only comple ely esol ed a e 12 mon hs. On he o he
hand, disc p o usions showed li le o no signs o
eg ession a e 12 mon hs, mos p obably because o
he pa ien s’younge age and he abundance o collagen
ibe s and chond ocy e-like cells om he NP in hese
discs. The as e a e o adiog aphic esolu ion seen
wi h seques a ed discs has been adi ionally associa ed
wi h dehyd a ion and sh inkage, as he ee agmen is
no longe supplied wi h nu ien s om he pa en disc
[9,10]. O he au ho s pos ula e ha seques a ed discs,
unlike o he LDH sub ypes, igge an in lamma o y e-
sponse cha ac e ized by neo ascula iza ion and immune
cell-media ed deg ada ion [11–13]. The subsequen
inc ease in blood low a ound ee agmen s explains
why he pe iphe y o disc seques a ions is enhanced
wi h gadolinium (Gd) con as , which is measu ed by
MRI im enhancemen . In ac , Au io e al. [11]
p oposed ha seques a ed discs wi h highe le els o
Gd die hylene iamine pen aace ic acid (Gd-DTPA) en-
hancemen on MRI images se ed as p edic o s o hei
highe eso p ion a e. In his s udy, MRI o he nia ed
pa ien s was epea ed h oughou 12 mon hs showing
ha a signi ican NP eso p ion occu ed un il 2 mon hs
a e diagnosis and was mo e p onounced o e he
1-yea ollow-up pe iod. Highe eso p ion a es we e
associa ed wi h highe baseline sco es o im
enhancemen hickness, highe deg ee o he nia ed
NP displacemen in he Komo i classi ica ion, and an
age ange o 41–50 yea s. The hickness o im
enhancemen was a s onge de e minan o spon-
aneous eso p ion han i s ex en . The ex en o im
enhancemen signi ican ly co ela ed wi h he deg ee
o disc displacemen , being mos p onounced in he
case o seques e s.
Clinical decision on conse a i e e sus su gical
in e en ion o LDH
In o de o unde s and spon aneous LDH eg ession, i s
p ognos ic ac o s, and i s p edic i e ou comes, la ge co-
ho s ha e been conduc ed, namely he Maine Lumba
Spine S udy [14], he Spine Pa ien Ou comes Resea ch
Fig. 1 aSchema ic ep esen a ion o ypical L4–L5 he nia, wi h comp ession and possible up u e o pos e io longi udinal ligamen (PLL). b
Human LDH agmen , ob ained om pa ien who unde wen mic odiscec omy a e in o med consen and e hics commi ee app o al om
Cen o Hospi ala São João. cHis ological s aining o issue collec ed in (b), showing cell clus e s p oducing p o eoglycans (Alcian blue)
embedded in a collagen ma ix (Pic osi ius ed). dLDH is cu en ly di ided in o ou sub ypes, acco ding o MRI, as bulging disc (mildes o m),
p o usion, ex usion, and seques a ion, he se e es o m o LDH. P o eoglycan- ich nucleus pulposus in cen e is su ounded by collagen- ich
concen ic ings o annulus ib osus. Scale ba s: (b) 3 cm, (c) 100 μm. Image c edi s: (a,d) used elemen s om Se ie Medical A ; (b,
c) unpublished
Cunha e al. A h i is Resea ch & The apy (2018) 20:251 Page 2 o 9
T ial (SPORT) [15], and he Hague Spine In e en ion
P ognos ic S udy G oup [16], each one en olling
hund eds o pa ien s su e ing om LDH. In e es ingly,
all o hese s udies indica e ha ea ly su ge y achie es
mo e apid elie o LDH symp oms han conse a i e
ca e bu , in he long un, ou comes g adually become
iden ical o conse a i e ea men . Many o he sys em-
a ic s udies ha e consis en ly ob ained simila esul s
[17–20]. Bu e mann [21] compa ed he LDH symp oms
o pa ien s a e 6 weeks o conse a i e ea men (38
pa ien s) wi h hose who ecei ed epidu al injec ions (20
pa ien s) and ound ha bo h g oups had simila ou -
comes, including he size o he nia ed NP.
Despi e he e idence o spon aneous LDH eg ession,
he e is s ill much deba e among clinicians conce ning
he e icacy o conse a i e s nonconse a i e ea -
men s. Ini ial managemen o pa ien s wi h he seques-
a ed sub ype o LDH may be conse a i e due o he
highe likelihood and as e a e o esolu ion in com-
pa ison o he o he LDH sub ypes ( e iewed in [6,22]).
Pa ien s wi h in ac able pain, neu ological de ici , o
bowel o bladde dys unc ion, among o he associa ed
ac o s, emain candida es o ea lie su gical in e en-
ion. Howe e , i is s ill ha d o p edic which pa ien s
a e mo e likely o bene i om conse a i e ea men
and ha e highe p obabili y o spon aneous eg ession o
he disc he nia ion. I is he e o e ema kably challen-
ging o decide which pa ien s o submi o su ge y. Fu-
u e la ge, p ospec i e, andomized ials a e equi ed o
be e de e mine a se o speci ic clinical c i e ia, such as
hose de ined by Chiu e al. [22], ha may p edic he
su ge y ou come. On a mo e mechanis ic app oach,
inding molecula ma ke s, possibly nonin asi e ones,
such as sys emic ma ke s, o p edic he ou come o
LDH would ha e a majo impac on he clinical decision.
These app oaches a e only s a ing o eme ge, such as
he s udy by Elkan e al. [23] in which high plasminogen
ac i a o inhibi o 1, a ma ke o ib inolysis, analyzed in
blood samples o pa ien s wi h LDH, was ai ly consis -
en ly associa ed wi h poo LDH su ge y ou come.
Mechanisms behind LDH spon aneous eg ession
As al eady desc ibed, he spon aneous eg ession o IVD
he nia ed issue is well documen ed clinically, bu he
unde lying mechanisms emain unclea . To he bes o
ou knowledge, h ee heo ies ha e been p oposed o
explain he eso p ion o he nia ed ma e ial. The i s
heo y p oposes ha he he nia ed disc agmen educes
in size due o g adual dehyd a ion and sh inkage, which
may explain he dec ease o signal in ensi y o he disc
in he ollow-up MRI s udies [9,10]. The second
hypo hesis sugges s ha ension applied by he PLL
leads o e ac ion o he he nia ed disc agmen back
in o he IVD space. This mechanism may explain he
cases whe e he he nia ed disc has an in ac AF, bu no
he cases wi h comple ely ex uded o mig a ed disc
agmen s [5]. Onel e al. [24] showed ha unde a s a ic
ac ion load o 45 kg, he he nia ed issue e ac ed in
21 ou o 30 LDH pa ien s, while in wo o hem he
he nia ed issue has ac ually inc eased. The hi d heo y,
he mos ex ensi ely s udied wi h p eclinical and clinical
e idence o suppo i , is he g adual he nia eso p ion
h ough enzyma ic deg ada ion and phagocy osis
induced by an in lamma o y eac ion and neo ascula i-
za ion [11–13]. This in lamma o y eac ion is supposed
o be igge ed when he disc con en ex udes in o he
epidu al space and is hen ecognized as o eign.
Depending on each indi idual clinical condi ion, i is
possible ha one speci ic mechanism o di e en combi-
na ions o he h ee may ope a e in spon aneous eg es-
sion o he he nia ed disc issue.
The p i ileged immuni y o IVD
The IVD is he la ges a ascula o gan in he human body
and conside ed an immune-p i ileged si e [25]. The NP
appea s o be pa icula ly isola ed om he immune sys em
o he hos , gi en i s posi ion be ween wo ca ilaginous
endpla es and inside he dense collagen ib ous s uc u e o
he AF. Addi ionally o his physiological ba ie , he IVD
cells also ac i ely esis in asion by immune cells, due o
he Fas ligand (FasL) exp ession, which is cha ac e is ic o
immune-p i ileged si es [26]. FasL belongs o he umo
nec osis ac o (TNF) amily and is known o induce apop-
osis by binding o i s ecep o , Fas. While Fas is exp essed
in a wide a ie y o cells, FasL exp ession is es ic ed o he
su ace o cy o oxic T cells, na u al kille (NK) cells, umo
cells, and s omal cells o some immune-p i ileged si es. In
immune-p i ileged si es, FasL o s omal cells binds o Fas
ecep o exp essed on immune cells and in il a ing cells.
This ligand– ecep o binding induces apop osis o he in il-
a ing immune cells, main aining he immune-p i ileged
condi ion o he issue [27]. The immune-p i ileged
en i onmen o he NP is he pilla o he heo y o he in-
lamma o y eac ion behind LDH eso p ion. This heo y
p oposes ha he ex usion o he NP issue in o he
epidu al space e okes an au oimmune eac ion ha leads
o he in il a ion o immune cells which will in e ac wi h
IVD cells and sec e e a a ie y o molecules ini ia ing he
he nia eso p ion p ocess [11,12,28].
Mac ophages as key playe s in LDH eg ession
Mac ophages a e indica ed as he mos impo an im-
mune playe s in he eso p ion p ocess o he nia ed discs.
Nume ous s udies ha e ound by immunohis ochemis y
he p esence o mac ophages in he nia ed IVD issue
specimens [29–32]. These cells ha e he capabili y o
ac i ely phagocy e he he nia ed issue and p ocess i in
hei lysosomes illed wi h collagen-deg ading enzymes.
Cunha e al. A h i is Resea ch & The apy (2018) 20:251 Page 3 o 9
Mac ophages also sec e e lysosomal enzymes by exocy-
osis, which b eak down in e cellula subs ances such as
he disc ma ix componen s p o eoglycans and collagens
[12,33]. Fu he mo e, phagocy ic ac i i y o mac ophages
was obse ed in su gically emo ed samples o he nia ed
NP h ough elec on mic oscopy [12] and hese immune
cells a e known o exp ess sca enge ecep o s, such as
CD36, which ha e been cha ac e ized as he main espon-
sible molecules o he phagocy osis o apop o ic cells,
highligh ing he po en ial ole o mac ophages in he nia
eso p ion [34]. In e es ingly, in LDH his ological samples,
mac ophage phagocy osis was obse ed mo e o en in
seques a ion sub ype LDH han subligamen ous ones
[35], in acco dance wi h he clinical e idence showing ha
seques e ed he nias a e mo e likely o eg ess. Ikeda e al.
[29] also obse ed mo e equen ly he in il a ion o mac-
ophages and neo ascula iza ion along he ma gins o he
ansligamen ous ex uded disc ma e ial han in o he
sub ypes o LDH.
Monocy e ec ui men o IVD issue
The exac mechanism by which monocy es a e ec ui ed
o he IVD emains unclea , as e iewed p e iously [36].
Howe e , i is known ha he IVD endogenously in-
cludes in lamma o y-like cells (i.e., cells wi h phagocy ic
capaci y) and IVD cells a e able o p oduce in lamma-
o y media o s [36], which may hemsel es con ibu e o
ec ui o he immune cells o he he nia si e. In pa icu-
la , monocy e chemoa ac an p o ein (MCP)-1, a CC
chemokine ha con ibu es o he ac i a ion and ec ui -
men o monocy es, has been ex ensi ely demons a ed
o be p oduced by IVD cells [13,37–40]. In pa icula ,
Yoshida e al. [13] de eloped a he nia model in which
IVD cells p oduced p oin lamma o y cy okines as an
ini ial esponse o disc he nia ion. These cy okines
s imula e he p oduc ion o MCP-1 by IVD cells, esul-
ing in mac ophage in il a ion in he nia ed discs. The
in il a ing monocy e-de i ed mac ophages also p oduce
MCP-1, inc easing he monocy e ec ui men o he IVD
[13]. Apa om mac ophages, plasmacy oid dend i ic
cells ha e also been shown o be in ol ed in LDH e-
so p ion [41]. Su gical ma e ial om ansligamen ous
and subligamen ous seques a ions was analyzed by low
cy ome y and plasmacy oid dend i ic cells we e ound
o p edomina e o e mac ophages on ansligamen ous
seques a ions, indica ing ha hese cells may be
in ol ed in he ini ia ion o he immune esponse [41].
In lamma o y cascades in LDH-implica ing mac ophages
Nume ous s udies analyzed immune media o s in LDH
and especially implica ing mac ophages in LDH eg es-
sion (Table 1). Shamji e al. [32] showed high exp ession
le els o mac ophage p oduc s like IL-4, IL-6, IL-12, and
in e e on gamma (IFN-γ) in he nia ed disc issue. O he
s udies demons a ed ha IVD issue is also capable o
spon aneously p oducing o he molecules, such as he
chemokines IL-8 and MCP-1, he main unc ions o
which a e chemo axis o mac ophages and angiogenesis
[37]. Fu he mo e, Kang e al. [42] demons a ed ha
he nia ed discs elease high le els o ma ix me allop o-
einases (MMPs), ni ic oxide (NO), IL-6, and p os a-
glandin E2 (PGE2) and his p oduc ion inc eases when
he discs a e s imula ed wi h IL-1β, e idencing ha IVD
cells a e biologically esponsi e o exogenous s imuli.
Rega ding IL-6, i has al eady been demons a ed in
i o ha i s p oduc ion is induced when IVDs and mac-
ophages a e cocul u ed [43]. Mo e ecen ly, Takada e
al. [44] showed ha cocul u ing IVDs and mac ophages
up egula ed IL-8, PGE2, and cyclooxygenase 2 (COX-2).
In bo h s udies, he men ioned biochemical media o s
a e mainly p oduced by mac ophages. The la e s udy
also showed ha a IVD au og a s induced ex ensi e
mac ophage in il a ion in i o, inc easing he mRNA
le els o TNF-α, IL-6, IL-8, and COX-2. TNF-αis e-
qui ed o IL-6 and PGE2 p oduc ion, bu no o IL-8
p oduc ion, du ing IVD–mac ophage in e ac ion.
Neu aliza ion o TNF-αand IL-8 may be a aluable
he apy o pain ela ed wi h LDH [44]. O he s udies
Table 1 Immune media o s implica ing mac ophages in LDH eg ession
Immune media o Sample Species S udy
IL-6, NO, PGE2, MMP-3, MMP-2/MMP-9 He nia ed IVD issue Human Kang e al., 1997 [42]
IL-8, MCP-1 He nia ed IVD issue Human Bu ke e al., 2002 [37]
IL-4, IL-6, IL-12, IFN-γHe nia ed IVD issue Human Shamji e al., 2010 [32]
MMP-3, MMP-7 Cocul u e o IVD and mac ophages Mouse Ha o e al., 2000 [45]
MMP-7, TNF-αCocul u e o IVD and mac ophages Mouse Ha o e al., 2000 [46]
MMP-3, MMP-7, TNF-α, VEGF Cocul u e o IVD and mac ophages Mouse Ka o e al., 2004 [47]
IL-6 Cocul u e o IVD and mac ophages Ra Takada e al., 2004 [43]
IL-8, PGE2, COX-2 Cocul u e o IVD and mac ophages Ra Takada e al., 2012 [44]
COX-2 cyclooxygenase 2, IFN-γin e e on gamma, IL in e leukin, IVD in e e eb al disc, LDH lumba disc he nia ion, MCP-1 monocy e chemoa ac an p o ein-1,
MMP ma ix me allop o einase, NO ni ic oxide, PGE2 p os aglandin E2, TNF-α umo nec osis ac o alpha, VEGF ascula endo helial g ow h ac o
Cunha e al. A h i is Resea ch & The apy (2018) 20:251 Page 4 o 9
ha e also analyzed he in e ac ion be ween mac ophages
and IVD issue using cocul u es be ween mac ophages
and chond ocy es o whole IVD. Ha o e al. demon-
s a ed ha he p oduc ion o bo h MMP-3 and MMP-7
was s ongly up egula ed in IVD cell/mac ophage cocul-
u e, MMP-3 being p oduced by bo h chond ocy es and
mac ophages while MMP-7 is p oduced p edominan ly
by mac ophages. Mo eo e , he au ho s also e ealed
ha disc-de i ed MMP-3 is equi ed o he physical
deg ada ion o disc issues and o mac ophage in il a-
ion, which ul ima ely leads o he nia eso p ion [45]. In
o he s udies, he same g oup demons a ed ha
MMP-7 eleased by mac ophages con ibu es o he
p ocess o he nia ed disc eso p ion h ough he elease
o soluble TNF-α[46,47]. TNF-αis a po en induce o
many MMPs, such as MMP-3, and also o ascula
endo helial g ow h ac o (VEGF) ha is implica ed in
he neo ascula iza ion o he nia ed discs [47,48]. The
c ucial ole o MMP-7 in he ini ia ion o he nia ed disc
eso p ion esul ed in he de elopmen o a ecombinan
human MMP-7 in adiscal he apy, which is in phase I/II
clinical ials in he Uni ed S a es. This he apy a oids he
side e ec s associa ed wi h su ge y, such as ne e issue
damage [49].
Neo ascula iza ion in LDH eso p ion
Al oge he , hese s udies p o ide e idence o a mecha-
nism o LDH eso p ion associa ed wi h a cascade o in-
lamma ion, ma ix emodeling, and angiogenesis.
Howe e , only ew s udies ha e speci ically add essed
neo ascula iza ion.
Usually, ew blood essels exis in he ma u e IVD.
Howe e , p oli e a ion o new essels has al eady been
demons a ed a he ma gin o he he nia ed issue and
his is hough o be a majo de e minan o spon a-
neous eg ession o LDH [11,50]. Se e al molecula me-
dia o s ha e been sugges ed o be in ol ed in he
neo ascula iza ion o LDH, including TNF-α, VEGF, and
basic ib oblas g ow h ac o (bFGF). As men ioned
ea lie , TNF-αcan p omo e he exp ession o VEGF,
which plays an essen ial ole in he o ma ion o new
blood essels, and some s udies con i med he p esence
o VEGF and VEGF ecep o s in human LDH issue
[48,51]. Also, he in e ac ion be ween mac ophages
and disc issue leads o gene a ion o in lamma o y
cy okines, which a e known o be in ol ed in he
induc ion o angiogenesis. Ha o e al. obse ed an in-
c ease in he mRNA and p o ein le els o VEGF
when mac ophages we e in con ac wi h disc issue
compa a i ely o he exp ession le els when hey we e
cul u ed alone. This enhancemen o he VEGF le els
was appa en ly media ed by a TNF-α-dependen
pa hway since his e ec was ab oga ed by he use o
aTNF-αneu alizing an ibody [48]. Using MRI in
humans, he p esence o capilla ies in ading he
he nia and monocy e-de i ed mac ophages mig a ing
ou o hese capilla ies has been demons a ed [12].
Addi ionally, in an expe imen al model in abbi s,
bFGF, a ac o known o s imula e mi ogenesis and
chemo axis o ib oblas s and capilla y endo helial
cells, as well as o s imula e angiogenesis, p omo ed
he eso p ion o disc ma e ial. In his s udy, ea -
men wi h bFGF led o an inc ease in he numbe o
newly o med essels wi h a consequen highe in il-
a ion o in lamma o y cells (mac ophages, lym-
phocy es, and ib oblas s), which con ibu ed o he
eso p ion p ocess [52].
These s udies ein o ce he impo ance o he c oss alk
be ween angiogenesis and in lamma ion, which ul i-
ma ely leads o LDH eg ession.
Con ibu ion o o he immune esponse media o s
An au oimmune eac ion is a mo e complex han he
me e mac ophage ec ui men and neo ascula iza ion so
a desc ibed. Indeed, apa om mac ophages, o he
immune cells a e p esen in LDH (Fig. 2). The comple e
unde s anding o he immune esponse associa ed wi h
LDH eg ession has bene i ed o a g ea ex en om
in- i o animal models. Mo e han in- i o e idence o
LDH eso p ion, al eady ho oughly clinically demon-
s a ed, animal models ha e been employed o un a el
he unde lying mechanisms o IVD eg ession. We ha e
shown spon aneous eg ession o LDH in a a IVD le-
sion model [53]. In his wo k, IVD lesion was induced
by ei he 21-G o 25-G needle punc u e and we ha e
ound ha he size o he he nia o med was p opo -
ional o he needle gauge used. In bo h cases, he nias
signi ican ly diminish in olume om 2 o 6 weeks pos
inju y. Also, we ound ha he numbe o CD68
+
mac o-
phages wi hin he he nia as well as cell apop osis wi hin
he issue we e bo h p opo ional o he he nia olume.
Using he same model, we u he con i med ha he
numbe o CD68
+
mac ophages in he he nia was p o-
po ional o i s size and hypo hesized ha only a ce ain
numbe o mac ophages will be ec ui ed and ac i a ed
pe a ea o he nia, keeping he issue homeos asis.
Mo eo e , we ha e ound ha he sys emic ansplan -
a ion o a bone ma ow MSC esul ed in a signi ican
educ ion in he size o he he nias o med 2 weeks pos
lesion and ha he numbe o B lymphocy es su oun-
ding he he nia inc eased [54]. In ano he s udy, a abbi
he nia ion model was de eloped, consis ing o he in o-
duc ion o a needle up o pene a ing he PLL and hen
physically comp essing o ex ude he NP o he epidu al
space [13]. The he nia ed discs spon aneously educed
in size g adually up o 12 weeks pos su ge y. In il a ing
cells, mainly composed o mac ophages, we e obse ed
om day 3. Immunohis ochemically, IVD cells in he
Cunha e al. A h i is Resea ch & The apy (2018) 20:251 Page 5 o 9

he nia ed discs p oduced TNF-αand IL-1βon day 1,
ollowed by MCP-1 on day 3.
Ne e heless, mos o he cu en in- i o LDH
models a e au o ansplan a ion models in which IVDs
a e ansplan ed in o subcu aneous o do sal epidu al
spaces o he animals a e laminec omy, wi h he
scope o speci ically analyzing he associa ed immune
esponse. This is because i is s ill con o e sial
whe he he in lamma o y esponse o he hos is
ini ia ed simply by exposu e o s uc u al elemen s
and compounds ha a e p esen in he disc cell
memb ane and ma ix, by di ec con ac o he NP
ma e ial wi h he immune sys em ia an au oimmune
esponse, o whe he i is seconda y o an au o-
immune esponse. I has been shown p e iously ha
subcu aneously ansplan ed NP cell su i al was e-
duced in associa ion wi h an immunological eac ion,
by ansplan ing NP issue in o Lewis a s and in o
NOD mice, and ha NK cells and mac ophages we e
p esen a ound he ou g own NP issues bu no T
cells we e ound [55]. Ano he s udy implan ed
au ologous NP subcu aneously in pigs and ac i a ed T
cells (CD4
+
and CD8
+
) we e ound in he exuda es in
conside able numbe , as well as ac i a ed B cells ex-
p essing immunoglobulin kappa (Igκ). The esul s
showed ha NP a ac s ac i a ed T and B cells [56].
Wha mos o hese s udies show is ha he immune
sys em is able o espond o he in ac heal hy NP issue.
To analyze whe he in lamma ion occu s in esponse o
compounds sec e ed om iable cells in he NP o
whe he in lamma ion simply equi es exposu e o s uc-
u al cell o ma ix componen s, Rand e al. [57] used a
mouse model in which he animal was exposed o disc
issue con aining iable NP and AF cells, o disc issue
con aining iable AF cells, o o disc issue wi h no
iable cells. The h ee issue p epa a ions we e inse ed
in o he igh lowe pe i oneal ca i y. The de i alized is-
sue was in ended o assess a possible e ec o chemical
i i an s ha may be p esen in he disc and ha may in-
duce in lamma ion. Mac ophage ec ui men occu ed
o e he cou se o 1, 2, and 7 days pos inju y and only
a e exposu e o iable disc issue bu no a e expos-
u e o de i alized disc componen s [57], indica ing ha
mac ophage ec ui men occu ed only in esponse o
cell cues.
Mos o he e idence o he in lamma o y eac ion
a ound spon aneous he nia eg ession has been collec ed
om animal s udies. Despi e his, some s udies mos ly
in ol ing immunohis ochemis y analysis o clinical
samples ha e con i med he esul s ob ained in animal
expe imen a ion. The p esence o T and B lymphocy es
on isola ed human he nia ed discs was u he
con i med by he same g oup [58]. Also, his ological
analyses o human he nia ed discs e ealed he p esence
o in il a ed T cells [59]. Fu he mo e, in human he -
nia ed issue, lymphocy es we e ound o be h ee imes
mo e abundan in seques a ed he nias han in ex u-
sions, while no o he in lamma o y cells we e seen in
p o usions apa om mac ophages [60]. In ano he
s udy, he in lamma o y in il a e has been cha ac e ized
by immunos aining in po ions o he nia ed discs which
unde wen su ge y o LDH. None o he 38 samples
exp essed he immunopheno ypic ma ke s o he
lymphocy e (CD20, CD45RO, CD4, CD8, TCR), ma u e
monocy e (CD33), o dend i ic cell (CD1a, CD80, CD86,
S100). Howe e , an abundan in il a ion o CD68
+
cells
ha lacked CD33 wi h a iable amoun s o CD11b,
CD11c, and CD40 was obse ed, likely ep esen ing a
p ocess o di e en ia ion om monocy es o mac o-
phages [61].
Fig. 2 Rep esen a i e p oposed mechanism o LDH eso p ion. Bo h he nia ed IVD issue and mac ophages p oduce umo nec osis ac o alpha
(TNF-α), monocy e chemoa ac an p o ein (MCP)-1, ma ix me allop o einases (MMPs), in e leukin (IL)-6, IL-8, p os aglandin E2 (PGE2),
cyclooxygenase 2 (COX2), and ni ic oxide (NO), which con ibu e o he in lamma o y eac ion and eso p ion o he he nia ed issue. Vascula
endo helial g ow h ac o (VEGF) induces blood essel ing ow h and neo ascula iza ion, which suppo immune cell mobiliza ion o he nia si e.
Inse : in a model o IVD he nia ion, CD68
+
mac ophages localized wi hin he nia (delimi a ed by dashed line), which include a blood essel
(a ow). Scale ba : 100 μm. Image used elemen s om Se ie Medical A ; inse : unpublished
Cunha e al. A h i is Resea ch & The apy (2018) 20:251 Page 6 o 9
Conclusions
LDH spon aneous eso p ion is well documen ed clini-
cally and in p eclinical s udies. Spine su geons a e
becoming inc easingly awa e o his phenomenon and
many ecognize he use ulness o conse a i e ea men
o LDH and ad ise pa ien s acco dingly. Di e en o ms
o nonsu gical ea men s should be exhaus ed be o e
conside ing su ge y in acu e s ages o LDH, unless
conse a i e ea men is con aindica ed o easons
such as neu ological de ici and in ole able pain despi e
adminis a ion o adequa e pain medica ions.
I is clea ha he in lamma o y esponse ha occu s
associa ed wi h LDH is c ucial o i s spon aneous e-
so p ion. The e o e, in lamma ion in his speci ic clinical
con ex is a good p ognos ic indica o and should no be
hal ed. S ill, i is exac ly an in lamma o y esponse ha
causes a ha m ul e ec on he adjacen ne e oo s,
causing pain. The con ol o he in lamma o y eac ion
in his se ing is an impo an challenge when ea ing
pa ien s wi h LDH. The combina ion o knowledge om
he biological mechanisms behind LDH eso p ion and
he de ailed pe sonalized diagnosis will be he de e min-
an o ailo ea men o each indi idual pa ien and
may ul ima ely lead o educ ion in cos s o he heal h
sys em.
Abb e ia ions
AF: Annulus ib osus; IVD: In e e eb al disc; LDH: Lumba disc he nia ion;
MSC: Mesenchymal s em cells; NP: Nucleus pulposus
Acknowledgemen s
The igu es used elemen s om Se ie Medical A , p o ided by Se ie ,
licensed unde a C ea i e Commons A ibu ion 3.0 unpo ed license (h p://
sma .se ie .com). The au ho s decla e no po en ial con lic o in e es s.
Funding
This wo k was inanced by p ojec “Bioenginee ed The apies o in ec ious
diseases and issue egene a ion”(NORTE-01-0145-FEDER-000012), suppo ed
by No e Po ugal Regional Ope a ional P og amme (NORTE 2020), unde he
PORTUGAL 2020 Pa ne ship Ag eemen , h ough he Eu opean Regional
De elopmen Fund (ERDF), by FEDER/COMPETE 2020 (POCI), Po ugal 2020,
and by Po uguese unds h ough FCT/MCTES in he amewo k o he
p ojec "Ins i u e o Resea ch and Inno a ion in Heal h Sciences" (POCI-01-
0145-FEDER-007274). Cunha C and Gonçal es RM acknowledge FCT by hei
pos doc ellowship (SFRH/BDP/87071/2012) and FCT In es iga o G an (IF/
00638/2014), espec i ely. Sil a AJ acknowledges he ellowship unde he
amewo k o he p ojec No e-01-0145-FEDER-000012.
A ailabili y o da a and ma e ials
Da a sha ing no applicable o his a icle as no da ase s we e gene a ed o
analyzed du ing he cu en s udy.
Au ho s’con ibu ions
CC, AJS, RMG, and MAB concei ed he s udy. CC, AJS, PP, RV, RMG, and MAB
w o e he manusc ip . All au ho s ead and app o ed he inal manusc ip .
E hics app o al and consen o pa icipa e
No applicable.
Consen o publica ion
No applicable.
Compe ing in e es s
The au ho s decla e ha hey ha e no compe ing in e es s.
Publishe ’sNo e
Sp inge Na u e emains neu al wi h ega d o ju isdic ional claims in
published maps and ins i u ional a ilia ions.
Au ho de ails
1
i3S—Ins i u o de In es igação e Ino ação em Saúde, Uni e sidade do Po o,
Rua Al edo Allen 208, 4200-135 Po o, Po ugal.
2
INEB—Ins i u o de
Engenha ia Biomédica, Uni e sidade do Po o, Rua do Campo Aleg e 823,
4150-180 Po o, Po ugal.
3
Depa men o Neu osu ge y, Cen o Hospi ala
São João, Po o, Po ugal.
4
Depa men o Clinical Neu osciences and Men al
Heal h, Facul y o Medicine, Uni e si y o Po o, Po o, Po ugal.
5
Neu osciences Cen e , CUF Po o Hospi al, Po o, Po ugal.
6
ICBAS—Ins i u o
de Ciências Biomédicas Abel Salaza , Uni e sidade do Po o, Rua Jo ge
Vi e bo Fe ei a 228, 4050-313 Po o, Po ugal.
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